ASPEGILLUS NIGER BETA-GLUCOSIDASE GENE, PROTEIN AND USES THEREOF

Abstract: A polypeptide having beta -glucosidase enzymatic activity, a polynucleotide encoding the polypeptide, a nucleic acid constructs carrying the polynucleotide, transformed or infected cells, such as yeast cells, and transgenic organisms expressing the polynucleotide and various uses of the polypeptide, the polynucleotide, cells and/or organisms, including, producing a recombinant polypeptide having the beta -glucosidase enzymatic activity, increasing the level of aroma compounds in alcoholic beverages, as well as other fermentation products of plant material, hydrolyzing cellobiose and thus increasing the level of fermentable glucose, increasing the production of alcohol, such as ethanol from plant material, increasing the aroma released from a plant or a plant product, and hydrolysis or transglycosylation of glycosides.

Description: ASPEGILLUS NIGER β-GLUCOSIDASE GENE, PROTEIN AND USES THEREOF FIELD AND BACKGROUND OF THE INVENTION The present invention relates to a polypeptide having β-glucosidase enzymatic activity, to a polynucleotide encoding the polypeptide, to nucleic acid constructs carrying the polynucleotide, to transformed or infected cells, such as yeast cells, and organisms expressing the polynucleotide and to various uses of the polypeptide, the polynucleotide, cells and/or organisms, including, but not limited to, producing a recombinant polypeptide having β-glucosidase enzymatic activity, increasing the level of aroma compounds in alcoholic beverages, as well as other fermentation products of plant material, hydrolyzing cellobiose and thus increasing the level of fermentable glucose, to increase production of alcohol, such as ethanol from plant material, increasing the aroma released from a plant or a plant product, and hydrolysis or transglycosylation of glycosides. Abbreviations used herein include: BGL1 - Aspergillus niger Bl β- glucosidase; bgll - a cDNA encoding same; 2FGlcF - 2-deoxy-2-fluoro β- glucosyl fluoride; DNP - 2,4-dinitroρhenol; DNPGlc - 2,4-dinitrophenyl β-D- glucopyranoside; pNP - p-nitrophenol; pNPGlc - p-nitrophenyl β-D- glucopyranoside; MUGlc - 4-methylumbeliferyl-β-D-glucopyranoside; YNB - yeast nitrogen base without amino acids; and X-glu - 5-bromo-4-chloro-3- indolyl β-D-glucopyranoside. β-Glucosidases (EC 3.2.1.21 ; β-D-glucoside glucohydrolase) play a number of different important roles in biology, including the degradation of cellulosic biomass by fungi and bacteria, degradation of glycolipids in mammalian lysosomes and the cleavage of glucosylated flavonoids in plants. These enzymes are therefore of considerable industrial interest, not only as constituents of cellulose-degrading systems, but also in the food industry (2, 3). Aspergillus species are known as a useful source of β-glucosidases (4- 6), and Aspergillus niger is by far the most efficient producer of β-glucosidase among the microorganisms investigated (4). Shoseyov et al. (7) have previously described a β-glucosidase from Aspergillus niger B l (CMI CC 324626) which is active at low pHs, as well as in the presence of high ethanol concentrations. This enzyme effectively hydrolyzes flavor-compound glycosides in certain low-pH products, such as wine and passion fruit juice, thereby enhancing their flavor (8-12), and is particularly attractive for use in the food industry, as A. niger is considered non-toxic (3). In addition, β- glucosidase was found useful in enzymatic synthesis of glycosides (13-15). Other A. niger β-glucosidases have also been purified (16-18), however, differences in their properties have been reported, including ranges of molecular weights (116-137 kDa), isoelectric points (pi values of 3.8-4) and pH optima (3.4-4.5). Indeed, at least two β-glucosidases, with distinct substrate specificities, have been identified in commercial A. niger β- glucosidase preparations (19). Attempts to clear this confusion by cloning and expression of a functional A. niger β-glucosidase gene in S. cerevisiae has been previously reported (20), however the protein was not characterized, and the sequence was not published. Glycosidases have been assigned to families on the basis of sequence similarities, there now being some 77 different such families defined containing over 2,000 different enzymes (21, see also http://afmb.cnrs- mrs.fr/~pedro/CAZY/db.html). With the exception of the glucosylceramidases (Family 30), all simple β-glucosidases belong to either Family 1 or 3. Family 1 contains enzymes from bacteria, plants and mammals, including also 6- phospho-glucosidases and thioglucosidases. Furthermore, most Family 1 enzymes also have significant galactosidase activity. Family 3 contains β- glucosidases and hexosaminidases of fungal, bacterial and plant origin. Enzymes of both families hydrolyze their substrates with net retention of anomeric configuration, presumably via a two-step, double-displacement mechanism, involving two key active site carboxylic acid residues (for reviews of mechanism, see 22-24). In the first step, one of the carboxylic acids (the nucleophile) attacks at the substrate anomeric center, while the other (the acid/base catalyst) protonates the glycosidic oxygen, thereby assisting the departure of the aglycone. This results in the formation of a covalent α- glycosyl-enzyme intermediate. In a second step this intermediate is hydrolyzed by general base-catalyzed attack of water at the anomeric center of the glycosyl-enzyme, to release the β-glucose product and regenerate free enzyme. Both the formation and the hydrolysis of this intermediate proceed via transition states with substantial oxocarbenium ion character. Given that Family 3 contains fungal enzymes of similar mass, including those from other Aspergillus sp., it is likely that the Aspergillus niger β- glucosidase would be a member of this family. Mechanistic information on this family is relatively sparse: the best characterized being the glycosylated 170 kDa β-glucosidase from Aspergillus wentii. By labeling the active site with conduritol B-epoxide, this enzyme was shown to carry out hydrolysis, with net retention of anomeric configuration. This study has demonstrated that the labeled aspartic acid residue was the same as that derivatized by the slow substrate D-glucal (1, 25). Furthermore, it was shown that the 2- deoxyglucosyl-enzyme, trapped by use of D-glucal, was kinetically identical to that formed during the hydrolysis of PNP-2-deoxy-β-D- glucopyranoside (26). Further detailed kinetic analysis of the enzyme was performed by Legler et al. (27), including measurement of Hammett relationships, kinetic isotope effects and studies of the binding of potent reversible inhibitors, such as gluconolactone and nojirimycin. While reducing the present invention to practice, the β-glucosidase protein was isolated from Aspergillus niger, purified, cloned, sequenced, expressed in yeast host cells and its enzymatic function characterized. In addition, the protein as well as signal peptide fused thereto and optionally an endoplasmic reticulum retaining peptide fused thereto were expressed in transgenic plants and the release of aroma substances therefrom following homogenization monitored. The enzyme encoded by the isolated gene, as described above, is of known usefulness in plant and/or plant products, as well as in biotechnological processes, including the food industry. Several unexpected advantages were uncovered, including, but not limited to, pH and temperature stability of the β-glucosidase from Aspergillus niger, requirement for a signal peptide for obtaining catalytic activity when expressed in plants. Advantage for an endoplasmic retaining peptide or for a lack thereof when expressed in plants, depending on the application. SUMMARY OF THE INVENTION According to one aspect of the present invention there is provided an isolated nucleic acid comprising a genomic, complementary or composite polynucleotide preferably being derived from Aspergillus niger, encoding a polypeptide having a β-glucosidase catalytic activity and preferably further encoding, in frame, a signal peptide and an endoplasmic reticulum retaining peptide. According to another aspect of the present invention there is provided a recombinant protein comprising a polypeptide having a β-glucosidase catalytic activity, the polypeptide is preferably derived from Aspergillus niger and it preferably fused to a signal peptide and optionally also to an endoplasmic reticulum retaining peptide. According to yet another aspect of the present invention there is provided a nucleic acid construct comprising the isolated nucleic acid described herein. According to still another aspect of the present invention there is provided host cell or an organism, such as a plant, comprising the nucleic acid or nucleic acid construct described herein. According to further features in preferred embodiments of the invention described below, the polynucleotide is as set forth in SEQ ID NOs:l, 3 or a portion thereof. According to still further features in the described preferred embodiments, the nucleic acid construct further comprising at least one cis acting control element for regulating expression of the polynucleotide. According to still further features in the described preferred embodiments, the host cell is selected from the group consisting of a prokaryotic cell and a eukaryotic cell. According to still further features in the described preferred embodiments the prokaryotic cell is E. coli. According to still further features in the described preferred embodiments the eukaryotic cell is selected from the group consisting of a yeast cell, a fungous cell, a plant cell and an animal cell. According to still further features in the described preferred embodiments the polypeptide is as set forth in SEQ ID NO: 2 or a portion thereof having the β-glucosidase catalytic activity. According to an additional aspect of the present invention there is provided a method of producing recombinant β-glucosidase, the method comprising the step of introducing, in an expressible form, a nucleic acid construct into a host cell, the nucleic acid construct including a genomic, complementary or composite polynucleotide preferably derived from Aspergillus niger, encoding a polypeptide having a β-glucosidase catalytic activity and preferably further encoding, in frame, a signal peptide and an endoplasmic reticulum retaining peptide. According to further features in preferred embodiments of the invention described below, the method further comprising the step of extracting the polypeptide having the β-glucosidase catalytic activity. According to yet an additional aspect of the present invention there is provided a method of producing a recombinant β-glucosidase overexpressing cell, the method comprising the step of introducing, in an overexpressible form, a nucleic acid construct into a host cell, the nucleic acid construct including a genomic, complementary or composite polynucleotide preferably derived from Aspergillus niger, encoding a polypeptide having a β-glucosidase catalytic activity and preferably further encoding, in frame, a signal peptide and an endoplasmic reticulum retaining peptide. According to still an additional aspect of the present invention there is provided a method of increasing a level of at least one fermentation substance in a fermentation product, the method comprising the step of fermenting a glucose containing fermentation starting material by a yeast cell overexpressing a nucleic acid construct including a genomic, complementary or composite polynucleotide being preferably derived from Aspergillus niger, encoding a polypeptide having a β-glucosidase catalytic activity and preferably further encoding, in frame, a signal peptide and an endoplasmic reticulum retaining peptide, thereby increasing the level of the at least one fermentation substance in the fermentation product. According to a further aspect of the present invention there is provided a method of increasing a level of at least one fermentation substance in a fermentation product, the method comprising the step of fermenting a plant derived glucose containing fermentation starting material by a yeast cell, the plant overexpressing a nucleic acid construct including a genomic, complementary or composite polynucleotide preferably derived from Aspergillus niger, encoding a polypeptide having a β-glucosidase catalytic activity and preferably further encoding, in frame, a signal peptide and an endoplasmic reticulum retaining peptide, thereby increasing the level of the at least one fermentation substance in the fermentation product. According to a further aspect of the present invention there is provided a method of increasing a level of at least one aroma substance in a plant derived product, the method comprising the step of incubating a glucose containing plant starting material with a yeast cell overexpressing a nucleic acid construct including a genomic, complementary or composite polynucleotide preferably derived from Aspergillus niger, encoding a polypeptide having a β-glucosidase catalytic activity and preferably further encoding, in frame, a signal peptide and an endoplasmic reticulum retaining peptide, thereby increasing the level of the at least one aroma substance in the plant derived product. According to yet a further aspect of the present invention there is provided a method of increasing a level of at least one aroma substance in a plant derived product, the method comprising the step of incubating a glucose containing plant starting material with a yeast cell, said plant overexpressing a nucleic acid construct including a genomic, complementary or composite polynucleotide preferably derived from Aspergillus niger, encoding a polypeptide having a β-glucosidase catalytic activity and preferably further encoding, in frame, a signal peptide and an endoplasmic reticulum retaining peptide, thereby increasing the level of the at least one aroma substance in the plant derived product. According to still further features in the described preferred embodiments the plant derived product is a fermentation product, such as, but not limited to, an alcoholic beverage. According to still a further aspect of the present invention there is provided a method of increasing a level of free glucose in a glucose containing fermentation starting material, the method comprising the step of fermenting the glucose containing fermentation starting material by a cell overexpressing a nucleic acid construct including a genomic, complementary or composite polynucleotide preferably derived from Aspergillus niger, encoding a polypeptide having a β-glucosidase catalytic activity and preferably further encoding, in frame, a signal peptide and an endoplasmic reticulum retaining peptide, thereby increasing the level of the free glucose in the glucose containing fermentation starting material. According to another aspect of the present invention there is provided a method of increasing a level of free glucose in a plant derived glucose containing fermentation starting material, the method comprising the step of fermenting the plant derived glucose containing fermentation starting material by a cell, the plant overexpressing a nucleic acid construct including a genomic, complementary or composite polynucleotide preferably derived from Aspergillus niger, encoding a polypeptide having a β-glucosidase catalytic activity and preferably further encoding, in frame, a signal peptide and an endoplasmic reticulum retaining peptide, thereby increasing the level of the free glucose in the plant. According to yet another aspect of the present invention there is provided a method of increasing a level of free glucose in a plant, the method comprising the step of overexpressing in the plant a nucleic acid construct including a genomic, complementary or composite polynucleotide preferably derived from Aspergillus niger, encoding a polypeptide having a β-glucosidase catalytic activity and preferably further encoding, in frame, a signal peptide and an endoplasmic reticulum retaining peptide, thereby increasing the level of the free glucose in the plant. According to still another aspect of the present invention there is provided a method of producing an alcohol, the method comprising the step of fermenting a glucose containing fermentation starting material by a cell overexpressing a nucleic acid construct including a genomic, complementary or composite polynucleotide preferably derived from Aspergillus niger, encoding a polypeptide having a β-glucosidase catalytic activity and preferably further encoding, in frame, a signal peptide and an endoplasmic reticulum retaining peptide, and extracting the alcohol therefrom. According to an additional aspect of the present invention there is provided a method of producing an alcohol, the method comprising the step of fermenting a plant derived glucose containing fermentation starting material by a cell, the plant overexpressing a nucleic acid construct including a genomic, complementary or composite polynucleotide preferably derived from Aspergillus niger, encoding a polypeptide having a β-glucosidase catalytic activity and preferably further encoding, in frame, a signal peptide and an endoplasmic reticulum retaining peptide, and extracting the alcohol therefrom. According to an additional aspect of the present invention there is provided a method of producing an aroma spreading plant, the method comprising the step of overexpressing in the plant a nucleic acid construct including a genomic, complementary or composite polynucleotide preferably derived from Aspergillus niger, encoding a polypeptide having a β-glucosidase catalytic activity and preferably further encoding, in frame, a signal peptide and an endoplasmic reticulum retaining peptide, thereby increasing aroma spread from the plant. According to further features in preferred embodiments of the invention described below, overexpressing the nucleic acid construct is performed in a tissue specific manner. According to still further features in the described preferred embodiments overexpressing the nucleic acid construct is limited to at least one tissue selected from the group consisting of flower, fruit, seed, root, stem, pollen and leaves. The present invention successfully addresses the shortcomings of the presently known configurations by providing a polypeptide having β- glucosidase enzymatic activity, a polynucleotide encoding the polypeptide, a nucleic acid constructs carrying the polynucleotide, transformed or infected cells, such as yeast cells, and organisms expressing the polynucleotide and various uses of the polypeptide, the polynucleotide, cells and/or organisms, including, but not limited to, producing a recombinant polypeptide having β- glucosidase enzymatic activity, increasing the level of aroma compounds in alcoholic beverages, as well as other fermentation products of plant material, hydrolyzing cellobiose and thus increasing the level of fermentable and/or free glucose, to increase production of a fermentation product, such as ethanol from plant material, increasing the aroma released from a plant or a plant product, and hydrolysis or transglycosylation of glycosides. BRIEF DESCRIPTION OF THE DRAWINGS The invention is herein described, by way of example only, with reference to the accompanying drawings. With specific reference now to the drawings in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of the preferred embodiments of the present invention only, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the invention may be embodied in practice. In the drawings: FIGs. la-c demonstrate plasmid maps employed as expression vectors for bgll cDNA. Figure la - __. coli expression vector containing bgll cDNA, inserted into the Ncol/Bam l sites of pET3d. Figure lb - S. cerevisiae expression vector containing bgll cDNA, inserted into the HindUI/BamHl sites of pYES2-bgll plasmid. Figure lc - P. pastoris expression vector containing bgll cDNA, inserted into the EcoRI/BamHI sites of pHIL-Sl. FIGs. 2a-b demonstrates SDS-PAGE analysis of active protein samples eluted from a Mono-Q column, stained with coomassie blue (Figure 2a), or β- glucosidase zymogram (Figure 2b) using MUGlc as a substrate. Lanes (for both Figures 2a and 2b): 1 - Electroeluted band of BGL1 from preparative PAGE-SDS gel stabs; 2, 3, 4, 5 - acetone precipitates from Mono-Q separation of BGL FIG. 3 demonstrates SDS-PAGE analysis of purified β-glucosidase by Mono-Q and Resource-S. Lanes: 1 - crude (27.5 μg protein); 2 - active fraction after Mono-Q (7 μg protein); and 3 - active fraction after Resource-S (10 μg protein). FIG. 4 demonstrates SDS-PAGE analysis of β-glucosidase deglycosylated by N-glycosidase-F. Lanes: 1 - molecular weight marker; 2 - native β-glucosidase; and 3 - deglycosylated protein. FIG. 5a demonstrates the DNA and amino acid sequences of bgll. Amino acid sequences determined by Edman degradation are underlined. DNA sequences of introns are underlined. Signal peptide is indicated by italic letters. FIG 5b. demonstrates bgll gene organization. Exons (El -7) are indicated by filled boxes, introns by solid lines, restriction sites and the stop codon by arrows. FIG. 6a demonstrates a Western blot analysis of recombinant BGL1 expressed in S. cerevisiae. Lanes: 1 - native BGL1 (positive control); 2 - total protein extract of S. cerevisiae expressing recombinant BGL1; 3 - total protein extract of S. cerevisiae without the bgll expression vector (negative control). FIG. 6b demonstrates a Western blot analysis of recombinant BGL1 secreted from P. pastoris. Lanes: 1 - molecular weight marker; 2 - medium supernatant of P. pastoris expressing recombinant BGL1; 3 - medium supernatant of P. pastoris host without the vector (negative control). FIG. 7 demonstrates proton-NMR spectra, illustrating the stereochemical course of pNPGlc hydrolysis by A. niger β-glucosidase. Spectra are for the anomeric proton region of the substrate at different time intervals relative to addition of the enzyme. FIG. 8 demonstrates inactivation of recombinant BGL1 by 2FGlcF. Pure enzyme was incubated in the presence of various concentrations of the inactivator, and residual enzyme activity was determined at different time intervals. Residual activity is presented, semilogarithmically, versus time, in the presence of the indicated concentrations of inactivator. FIG. 9 demonstrates reactivation of 2-deoxy-2-fluoroglucosyl- recombinant BGL1 by linamarin. Activity is plotted versus incubation time in the presence of the indicated concentrations of linamarin. FIG. 10 demonstrates the stability of recombinant A. niger β- glucosidase at various temperatures. Activity is calculated as percent of a recombinant enzyme solution kept at 4 °C. FIGs. l la-c show schematic depictions of expression cassettes used for expression of A. niger β-glucosidase in tobacco plants. Figure 1 la - a cassette encoding BGL1 without a signal peptide (see, SEQ ID NO: 13 for the nucleotide sequence and SEQ ID NO: 14 for the amino acid sequence); Figure l ib - a cassette encoding a BGL1 fused to a Cell signal peptide for secretion into the apoplast (see, SEQ ID NO: 15 for the nucleotide sequence and SEQ ID NO: 16 for the amino acid sequence); and Figure l ie - a cassette encoding a BGL1 fused to Cell signal peptide as in Figure l ib and in addition to HDEL (SEQ ID NO: 17) ER-retaining peptide at the C-terminus for accumulation in the ER (see, SEQ ID NO: 18 for the nucleotide sequence and SEQ ID NO: 19 for the amino acid sequence). FIG. 12 demonstrate PCR amplification results of bgll cDNA indicating the presence of bgll cDNA in transgenic plants. CB10 and CB11 - transgenic plants transformed with bgll and Cell signal peptide without HDEL ER retaining peptide. CBT3, CBT8 and CBT15 - different transgenic lines transformed with bgll, Cell signal peptide and HDEL. Bl - a transgenic plants transformed with bgll . Ikb - 1 kb DNA marker. WT - wild type non transgenic plant. pETBl - bgll plasmid DNA. FIGs. 13a-b show Western blot analyses of transgenic plants containing BGL1 without signal peptide (13a), and BGL1 with Cell signal peptide (13b), with and without HDEL ER retaining peptide. An gluco - purified A. niger beta-glucosidase. WT - nontransgenic control plant. Bl, B15, B16, B20, B27, B33 and B34 - different transgenic lines transformed with bgll. CBT1, CBT 3, CBT 7 and CBT 8 - different transgenic lines transformed with bgll, Cell signal peptide and HDEL. CB10 and CB12 - transgenic plants transformed with bgll and Cell signal peptide without HDEL ER retaining peptide. FIG. 14 show activity gel analysis of transgenic tobacco plant extracts in SDS-PAGE incubated with MUGlu. WT - non-transgenic control plant. CB10 and CB11 - two independent lines of transgenic plants expressing BGLl fused to Cell signal peptide (without HDEL). CBT3, CBT8 and CBT 15 - independent lines of transgenic plants expressing BGLl fused to Cell signal peptide at the N terminus and HDEL ER retaining peptide at the C terminus. Bl and B34 - transgenic plant expressing BGLl without signal peptide or HDEL ER retaining peptide and which were positive for BGLl protein in Western blot analysis. An Glu - control A. niger native beta-glucosidase. FIG. 15 demonstrates level of BGLl activity in different transgenic plants. WT - non-transgenic control plant. Bl and B21 - transgenic plants expressing BGLl without signal peptide or HDEL ER retaining peptide and which were positive for BGLl in Western blot analysis. CBT8, CBT21, CBTO and CBT 15 - independent lines of transgenic plants expressing BGLl fused to Cell signal peptide at the N terminus and HDEL ER retaining peptide at the C terminus. CB12, CB13, CB14 and CB15 - four independent lines of transgenic plants expressing BGLl fused to Cell signal peptide (without HDEL). DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is of a polypeptide having β-glucosidase enzymatic activity, a polynucleotide encoding the polypeptide, a nucleic acid constructs carrying the polynucleotide, transformed or infected cells, such as yeast cells, and organisms expressing the polynucleotide and various uses of the polypeptide, the polynucleotide, cells and/or organisms, including, but not limited to, producing a recombinant polypeptide having the β-glucosidase enzymatic activity, increasing the level of aroma compounds in alcoholic beverages, as well as other fermentation products of plant material, hydrolyzing cellobiose and thus increasing the level of fermentable glucose, increasing the production of alcohol, such as ethanol from plant material, increasing the aroma released from a plant or a plant product, and hydrolysis or transglycosylation of glycosides. The principles and operation of the present invention may be better understood with reference to the drawings and accompanying descriptions. Before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of the components set forth in the following description or exemplified in the examples that follow. The invention is capable of other embodiments or of being practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein is for the purpose of description and should not be regarded as limiting. According to one aspect of the present invention there is provided an isolated nucleic acid comprising a genomic, complementary or composite polynucleotide encoding a polypeptide having a β-glucosidase catalytic activity. Preferably the polynucleotide is derived from Aspergillus niger, however other sources are applicable. These include all isolated polynucleotides encoding polypeptide having β-glucosidase catalytic activity. Such polynucleotides and polypeptides identified by their GenBank Accession Nos. are listed in Table 1 below, all of which can be used while implementing the present invention. TABLE 1 Accession numbers ofcDNA and their encoded beta-glucosidases (EC.3.2.1.21) As used herein in the specification and in the claims section that follows, the term "isolated" refers to a biological component (such as a nucleic acid or protein or organelle) that has been substantially separated or purified away from other biological components in the cell of the organism in which the component naturally occurs, i.e., other chromosomal and extra- chromosomal DNA and RNA, proteins and organelles. Nucleic acids and proteins that have been "isolated" include nucleic acids and proteins purified by standard purification methods. The term also embraces nucleic acids and proteins prepared by recombinant expression in a host cell as well as chemically synthesized nucleic acids. As used herein and in the claims section that follows the terms and phrases "polynucleotide" and "polynucleotide sequence" are used interchangeably and refer to a nucleotide sequence which can be DNA or RNA of, for example, genomic or synthetic origin, which may be single- or double- stranded, and which may represent the sense or antisense strand. Similarly, the terms "polypeptide" and "polypeptide sequence" are interchangeably used herein and refer to an amino acid sequence of any length. As used herein in the specification and in the claims section that follows, the phrase "complementary polynucleotide sequence" includes sequences, which originally result from reverse transcription of messenger RNA using a reverse transcriptase or any other RNA dependent DNA polymerase. Such sequences can be subsequently amplified in vivo or in vitro using a DNA dependent DNA polymerase. As used herein in the specification and in the claims section that follows, the phrase "genomic polynucleotide sequence" includes sequences which originally derive from a chromosome and reflect a contiguous portion of a chromosome. As used herein in the specification and in the claims section that follows, the phrase "composite polynucleotide sequence" includes sequences which are at least partially complementary and at least partially genomic. A composite sequence can include some exonal sequences required to encode the polypeptide having the β-glucosidase catalytic activity, as well as some intronic sequences interposing therebetween. The intronic sequences can be of any source, including of other genes, and typically will include conserved splicing signal sequences. Such intronic sequences may further include cis acting expression regulatory elements, as hereinbelow described. As used herein in the specification and in the claims section that follows, the phrase "having a β-glucosidase catalytic activity" refers to a polypeptide sequence, protein or fragments thereof capable of serving as catalysts to a chemical reaction involving hydrolysis of the O-glycosidic bond of glucosides, the result of which is the release of a β-D-glucose residue(s), or an aglycon, in addition to the β-D-glucose residue. Specifically, hydrolysis by retaining enzymes is performed while maintaining the β -configuration of the anomeric center of the carbohydrate. A wide specificity for β-glucosides exists, thus, some examples also hydrolyze one or more of the following: β-D- galactosides, -L- arabinosides, β-D-xylosides, and β-D-fucosides. As used herein the term "catalyst" refers to a substance that accelerates a chemical reaction, but is not consumed or changed permanently thereby. As used herein the term "glucoside" refers to a compound of at least two monomers, at least one of which is a glucose, including a glycoside bond. Examples of glucosides include, but are not limited to, glucose containing backbones, such as the diglucose cellobiose, and the glucose polymer, cellulose. According to preferred embodiments, the polynucleotide according to this aspect of the present invention encodes a polypeptide as set forth in SEQ ID NO:2 or a portion thereof which retains β-glucosidase catalytic activity. Alternatively or additionally, the polynucleotide according to this aspect of the present invention is as set forth in SEQ ID NO: 1, 3 or a portion thereof, the portion encodes a polypeptide retaining β-glucosidase catalytic activity. In a broader aspect the polynucleotides according to the present invention encode a polypeptide which is at least 75 %, at least 80 %, at least 85 %, at least 90 %, at least 95 % or more, say 95 % - 100 % homologous to SEQ ID NO:2 as determined using the BestFit software of the Wisconsin sequence analysis package, utilizing the Smith and Waterman algorithm, where gap creation penalty equals 8 and gap extension penalty equals 2. According to preferred embodiments, the polynucleotides according to the broader aspect of the present invention encodes a polypeptide as set forth in SEQ ID NOs: 1 or 3 or a portion thereof which retains activity. Alternatively or additionally, the polynucleotides according to this broader aspect of the present invention are hybridizable with SEQ ID NOs: 1 or 3. Hybridization for long nucleic acids (e.g., above 200 bp in length) is effected according to preferred embodiments of the present invention by stringent or moderate hybridization, wherein stringent hybridization is effected by a hybridization solution containing 10 % dextrane sulfate, 1 M NaCl, 1 % SDS and 5 x 10^ cpm 32p labeled probe, at 65 °C, with a final wash solution of 0.2 x SSC and 0.1 % SDS and final wash at 65°C; whereas moderate hybridization is effected by a hybridization solution containing 10 % dextrane sulfate, 1 M NaCl, 1 % SDS and 5 x 10 6 cpm 32 p labeled probe, at 65 °C, with a final wash solution of 1 x SSC and 0.1 % SDS and final wash at 50 °C. Yet alternatively or additionally, the polynucleotides according to this broad aspect of the present invention is preferably at least 70 %, at least 75 %, at least 80 %, at least 85 %, at least 90 %, at least 95 % or more, say 95 % - 100 %, identical with SEQ ID NOs: 1 or 3 as determined using the BestFit software of the Wisconsin sequence analysis package, utilizing the Smith and Waterman algorithm, where gap weight equals 50, length weight equals 3, average match equals 10 and average mismatch equals -9. Thus, this broad aspect of the present invention encompasses (i) polynucleotides as set forth in SEQ ID NOs:l or 3; (ii) fragments thereof; (iii) sequences hybridizable therewith; (iv) sequences homologous thereto; (v) sequences encoding similar polypeptides with different codon usage; (vi) altered sequences characterized by mutations, such as deletion, insertion or substitution of one or more nucleotides, either naturally occurring or man induced, either randomly or in a targeted fashion. According to another aspect of the present invention there is provided a nucleic acid construct comprising the isolated nucleic acid described herein. According to a preferred embodiment, the nucleic acid construct according to this aspect of the present invention further comprising at least one cis acting control (regulatory) element for regulating the expression of the isolated nucleic acid. Such cis acting regulatory elements include, for example, promoters, which are known to be sequence elements required for transcription, as they serve to bind DNA dependent RNA polymerase, which transcribes sequences present downstream thereof. Further details relating to various regulatory elements are described hereinbelow. While the isolated nucleic acid described herein is an essential element of the invention, it is modular and can be used in different contexts. The promoter of choice that is used in conjunction with this invention is of secondary importance, and will comprise any suitable promoter. It will be appreciated by one skilled in the art, however, that it is necessary to make sure that the transcription start site(s) will be located upstream of an open reading frame. In a preferred embodiment of the present invention, the promoter that is selected comprises an element that is active in the particular host cells of interest. These elements may be selected from transcriptional regulators that activate the transcription of genes essential for the survival of these cells in conditions of stress or starvation, including the heat shock proteins. A construct according to the present invention preferably further includes an appropriate selectable marker. In a more preferred embodiment according to the present invention the construct further includes an origin of replication. In another most preferred embodiment according to the present invention the construct is a shuttle vector, which can propagate both in E. coli (wherein the construct comprises an appropriate selectable marker and origin of replication) and be compatible for propagation in cells, or integration in the genome, of an organism of choice, such as a plant. The construct according to this aspect of the present invention can be, for example, a plasmid, a bacmid, a phagemid, a cosmid, a phage, a virus or an artificial chromosome. According to an additional aspect of the present invention there is provided a recombinant protein comprising a polypeptide having a β- glucosidase catalytic activity. The polypeptide is preferably derived from an Aspergillus niger and preferably includes a signal peptide and optionally an endoplasmic reticulum retaining peptide. According to preferred embodiments, the polypeptide according to this aspect of the present invention is as set forth in SEQ ID NO:2 or a portion thereof which retains β-glucosidase catalytic activity. SEQ ID NO:2 of A. niger β-glucosidase is similar to the amino acid sequence of the β-glucosidase of A. kawachii. However, while the former is highly stable at wide range of temperatures and pH treatments, the latter is relatively unstable, and thus has certain disadvantages, rendering its use for the purpose of the present invention as is further detailed and described hereinunder, unfeasible and/or much less attractive. Recently, Iwashita and coworkers have published the sequence of a β- glucosidase (GenBank/EMBL AB003470) obtained from Aspergillus kawachii strain: IFO4308. Sequence comparison between Aspergillus kawachii β- glucosidase and A. niger β-glucosidase revealed that the two share 98 % homology. Enzymes of the two Aspergillus sp. contain seven cysteine residues and identical number of glycosylation sites, while differing in their degree of glycosylation (35). The physical and kinetic properties of three β-glucosidases from Aspergillus kawachii were described (35), and the three were shown to be products of the same gene, differing solely by the degree of glycosylation. The three purified A. kawachii β-glucosidases were, readily inactivated, even at moderate pH and temperature conditions. In sharp distinction, while examining the stability of the recombinant A. niger β-glucosidase according to the present invention under conditions identical to those described by Iwashita et al. and as described hereinbelow in the Examples section, revealed that the enzyme is highly stable, retaining majority of the enzymatic activity even after 1 hour incubation at 60 °C (68 % activity, as defined by percent activity of an enzyme kept at 4 °C). Thus, despite the similarity between the A. kawachii and A. niger β- glucosidases, the A. niger enzyme unexpectedly exhibits significantly higher thermal and pH stability. According to yet another aspect of the present invention there is provided a host cell comprising a nucleic acid construct as described herein. The term "host cell" refers to a recipient of a heterologous nucleic acid, which host cell can be either a prokaryotic cell, such as __. coli, or a eukaryotic cell, such as a yeast cell, a filamentous fungus cell, a plant cell or an animal cell. Examples for a yeast cell include, but not limited to, Pichia sp. such as P. pastoris, and Saccharomyces sp. such as S. cervisiae. As used herein and in the claims section which follows, the term "heterologous" when used in context of a nucleic acid sequence or a protein found within a plant, plant derived tissue or plant cells, or alternatively, within a eukaryotic cell, such as yeast, or a prokaryotic cell such as bacteria, refers to nucleic acid or amino acid sequences typically not native to the plant, plant derived tissue or plant cells, or alternatively, to the eukaryotic cell, such as yeast, or the prokaryotic cell, such as bacteria. Interchangeably, nucleic acid or amino acid sequences typically not native to the plant, plant derived tissue or plant cells, or alternatively, to the eukaryotic cell, such as yeast, or the prokaryotic cell, such as bacteria, are referred to by "recombinant nucleic acid" and "recombinant protein", respectively. Thus, a recombinant nucleic acid is one that has a sequence that is not naturally occurring or has a sequence that is made by an artificial combination of two otherwise separated segments of sequence. This artificial combination is often accomplished by chemical synthesis or, more commonly, by the artificial manipulation of isolated segments of nucleic acids, e.g., by genetic engineering techniques. As used herein in the specification and in the claims section that follows, the term "eukaryotic cell" refers to a cell containing a diploid genome through at least a portion of its life cycle, having membrane-bound nucleus with chromosomes made of DNA, with cell division involving a form of mitosis in which spindles are involved. Possession of a eukaryote type of cell characterizes the four kingdoms, Protoctista, Fungi, Plantae and Animalia. As used herein in the specification and in the claims section that follows, the term "prokaryotic cell" refers to various bacteria and blue-green algae, characterized by the absence of the nuclear organization, mitotic capacities and complex organelles that typify the eukaryote superkingdom. Examples of prokaryotic cell according to the present invention are bacteria, such as, but not limited to, E. coli. According to still another aspect of the present invention there is provided an organism comprising a nucleic acid construct as described herein, such as, but not limited to, a plant. Such an organism is said to be transformed or virally infected. As used herein the term "transformed" and its conjugations such as transformation, transforming and transform, all relate to the process of introducing heterologous nucleic acid sequences into a cell or an organism, which nucleic acid sequences are propagatable to the offspring. The term thus reads on, for example, "genetically modified", "transgenic" and "transfected", which may be used herein to further describe and/or claim the present invention. The term relates both to introduction of a heterologous nucleic acid sequence into the genome of an organism and/or into the genome of a nucleic acid containing organelle thereof, such as into a genome of chloroplast or a mitochondrion. As used herein the phrase "viral infected" includes infection by a virus carrying a heterologous nucleic acid sequence. Such infection typically results in transient expression of the nucleic acid sequence, which nucleic acid sequence is typically not integrated into a genome and therefore not propagatable to offspring, unless further infection of such offspring is experienced. There are various methods of introducing foreign genes into both monocotyledonous and dicotyledenous plants (Potrykus, I., Annu. Rev. Plant. Physiol., Plant. Mol. Biol. (1991) 42:205-225; Shimamoto et al, Nature (1989) 338:274-276). The principle methods of causing stable integration of exogenous DNA into plant genomic DNA include two main approaches: (i) Agrobacterium-mediated gene transfer: Klee et αl. (1987) Annu. Rev. Plant Physiol. 38:467-486; Klee and Rogers in Cell Culture and Somatic Cell Genetics of Plants, Vol. 6, Molecular Biology of Plant Nuclear Genes, eds. Schell, J., and Vasil, L. K., Academic Publishers, San Diego, Calif. (1989) p. 2-25; Gatenby, in Plant Biotechnology, eds. Kung, S. and Arntzen, C. J., Butterworth Publishers, Boston, Mass. (1989) p. 93-112. (ii) direct DNA uptake: Paszkowski et αl., in Cell Culture and Somatic Cell Genetics of Plants, Vol. 6, Molecular Biology of Plant Nuclear Genes eds. Schell, J., and Vasil, L. K., Academic Publishers, San Diego, Calif. (1989) p. 52-68; including methods for direct uptake of DNA into protoplasts, Toriyama, K. et αl. (1988) Bio/Technology 6:1072-1074. DNA uptake induced by brief electric shock of plant cells: Zhang et αl. Plant Cell Rep. (1988) 7:379-384. Fromm et αl. Nature (1986) 319:791-793. DNA injection into plant cells or tissues by particle bombardment, Klein et αl. Bio/Technology (1988) 6:559-563; McCabe et αl. Bio/Technology (1988) 6:923-926; Sanford, Physiol. Plant. (1990) 79:206-209; by the use of micropipette systems: Neuhaus et αl., Theor. Appl. Genet. (1987) 75:30-36; Neuhaus and Spangenberg, Physiol. Plant. (1990) 79:213-217; or by the direct incubation of DNA with germinating pollen, DeWet et αl. in Experimental Manipulation of Ovule Tissue, eds. Chapman, G. P. and Mantell, S. H. and Daniels, W. Longman, London, (1985) p. 197-209; and Ohta, Proc. Natl. Acad. Sci. USA (1986) 83:715-719. The Agrobacterium system includes the use of plasmid vectors that contain defined DNA segments that integrate into the plant genomic DNA. Methods of inoculation of the plant tissue vary depending upon the plant species and the Agrobacterium delivery system. A widely used approach is the leaf disc procedure, which can be performed with any tissue explant that provides a good source for initiation of whole plant differentiation. Horsch et al. in Plant Molecular Biology Manual A5, Kluwer Academic Publishers, Dordrecht (1988) p. 1-9. A supplementary approach employs the Agrobacterium delivery system in combination with vacuum infiltration. The Agrobacterium system is especially viable in the creation of transgenic dicotyledenous plants. There are various methods of direct DNA transfer into plant cells. In electroporation, the protoplasts are briefly exposed to a strong electric field. In microinjection, the DNA is mechanically injected directly into the cells using very small micropipettes. In microparticle bombardment, the DNA is adsorbed on microprojectiles such as magnesium sulfate crystals or tungsten particles, and the microprojectiles are physically accelerated into cells or plant tissues. Following transformation plant propagation is exercised. The most common method of plant propagation is by seed. Regeneration by seed propagation, however, has the deficiency that due to heterozygosity there is a lack of uniformity in the crop, since seeds are produced by plants according to the genetic variances governed by Mendelian rules. Basically, each seed is genetically different and each will grow with its own specific traits. Therefore, it is preferred that the transformed plant be produced such that the regenerated plant has the identical traits and characteristics of the parent transgenic plant. Therefore, it is preferred that the transformed plant be regenerated by micropropagation, which provides a rapid, consistent reproduction of the transformed plants. Micropropagation is a process of growing new generation plants from a single piece of tissue that has been excised from a selected parent plant or cultivar. This process permits the mass reproduction of plants having the preferred tissue expressing the protein. The new generation plants, which are produced, are genetically identical to, and have all of the characteristics of, the original plant. Micropropagation allows mass production of quality plant material in a short period of time and offers a rapid multiplication of selected cultivars in the preservation of the characteristics of the original transgenic or transformed plant. The advantages of cloning plants are the speed of plant multiplication and the quality and uniformity of plants produced. Micropropagation is a multi-stage procedure that requires alteration of culture medium or growth conditions between stages. Thus, the micropropagation process involves four basic stages: Stage one, initial tissue culturing; stage two, tissue culture multiplication; stage three, differentiation and plant formation; and stage four, greenhouse culturing and hardening. During stage one, initial tissue culturing, the tissue culture is established and certified contaminant-free. During stage two, the initial tissue culture is multiplied until a sufficient number of tissue samples are produced to meet production goals. During stage three, the tissue samples grown in stage two are divided and grown into individual plantlets. At stage four, the transformed plantlets are transferred to a greenhouse for hardening where the plants' tolerance to light is gradually increased so that it can be grown in the natural environment. Sequences suitable for permitting integration of the heterologous sequence into the plant genome are recommended. These might include transposon sequences and the like for homologous recombination as well as Ti sequences which permit random insertion of a heterologous expression cassette into a plant genome. Suitable prokaryote selectable markers include resistance toward antibiotics such as ampicillin or tetracycline. Other DNA sequences encoding additional functions may also be present in the vector, as is known in the art. The constructs of the subject invention will include an expression cassette for expression of the protein of interest. Usually, there will be only one expression cassette, although two or more are feasible. The recombinant expression cassette will contain in addition to the heterologous sequence one or more of the following sequence elements, a promoter region, plant 5' untranslated sequences which can include regulatory elements, initiation codon depending upon whether or not the structural gene comes equipped with one, and a transcription and translation termination sequence. Unique restriction enzyme sites at the 5' and 3' ends of the cassette allow for easy insertion into a pre-existing vector. As used herein, the phrase "regulatory element" refers to a nucleotide sequence which are typically included within an expression cassette and function in regulating (i.e., enhancing or depressing) the expression of a coding sequence therefrom. This regulation can be effected either at the transcription or the translation stages. Examples of regulatory elements include, but are not limited to, enhancers, suppressers and transcription terminators. As used herein the term "promoter" refers to a nucleotide sequence, which can direct gene expression in cells. Such a promoter can be derived from a plant, a plant virus, or from any other living organism including bacteria and animals. A plant promoter can be a constitutive promoter, such as, but not limited to, CaMV35S and CaMV19S promoters, FMV34S promoter, sugarcane bacilliform badnavirus promoter, CsVMV promoter, Arabidopsis ACT2/ACT8 actin promoter, Arabidopsis ubiquitin UBQ1 promoter, barley leaf thionin BTH6 promoter, and rice actin promoter. The promoter can alternatively be a tissue specific promoter. Examples of plant tissue specific promoters include, without being limited to, bean phaseolin storage protein promoter, DLEC promoter, PHSβ promoter, zein stprotein promoter, conglutin gamma promoter from soybean, AT2S1 gene promoter, ACT11 actin promoter from Arabidopsis, napA promoter from Brassica napus, potato patatin gene promoter and the Tob promoter. The promoter may also be a promoter which is active in a specific developmental stage of a plant's life cycle, for example, a promoter active in late embryogenesis, such as: the LEA promoter; Endosperm-specific expression promoter (the seed storage prolamin from rice is expressed in tobacco seed at the developmental stage about 20 days after flowering) or the promoter controlling the FbL2A gene during fiber wall synthesis stages. In case of a tissue-specific promoter, it ensures that the heterologous protein is expressed only in the desired tissue, for example, only in the flower, the fruit, the root, the seed, etc. Both the tissue-specific and the non-specific promoters may be constitutive, i.e., may cause continuous expression of the heterologous protein. The promoter may also be an inducible promoter, i.e., a promoter which is activated by the presence of an inducing agent, and only upon said activation, causes expression of the heterologous protein. An inducing agent can be for example, light, chemicals, drought, high salinity, osmotic shock, oxidant conditions or in case of pathogenicity and include, without being limited to, the light-inducible promoter derived from the pea rbcS gene, the promoter from the alfalfa rbcS gene, the promoters DRE, MYC and MYB active in drought; the promoters INT, INPS, prxEa, Ha hsρl7.7G4 and RD21 active in high salinity and osmotic stress, the promoters hsr303J and str246C active in pathogenic stress, the copper-controllable gene expression system and the steroid-inducible gene system Alternatively, an inducing agent may be an endogenous agent which is normally present in only certain tissues of the plant, or is produced only at certain time periods of the plant's life cycle, such as ethylene or steroids. By using such an endogenous tissue-specific inducing agent, it is possible to control the expression from such inducible promoters only in those specific tissues. By using an inducing agent produced only during a specific period of the life cycle, it is possible to control the expression from an inducible promoter to the specific phase in the life-cycle in which the inducing agent is produced. Bacterial and yeast derived promoters are well known in the art. Viruses are a unique class of infectious agents whose distinctive features are their simple organization and their mechanism of replication. In fact, a complete viral particle, or virion, may be regarded mainly as a block of genetic material (either DNA or RNA) capable of autonomous replication, surrounded by a protein coat and sometimes by an additional membranous envelope such as in the case of alpha viruses. The coat protects the virus from the environment and serves as a vehicle for transmission from one host cell to another. Viruses that have been shown to be useful for the transformation of plant hosts include CaV, TMV and BV. Transformation of plants using plant viruses is described in U.S. Pat. No. 4,855,237 (BGV), EP-A 67,553 (TMV), Japanese Published Application No. 63-14693 (TMV), EPA 194,809 (BV), EPA 278,667 (BV); and Gluzman, Y. et al, Communications in Molecular Biology: Viral Vectors, Cold Spring Harbor Laboratory, New York, pp. 172- 189 (1988). Pseudovirus particles for use in expressing foreign DNA in many hosts, including plants, is described in WO 87/06261. Construction of plant RNA viruses for the introduction and expression of non-viral foreign genes in plants is demonstrated by the above references as well as by Dawson, W. O. et al., Virology (1989) 172:285-292; Takamatsw et a\. EMBO J. (1987) 6:307-311; Frenc/. et a\. Science (1986) 231 :1294-1297; and Takamatsw et al. FEBS Letters (1990) 269:73-76. When the virus is a DNA virus, the constructions can be made to the virus itself. Alternatively, the virus can first be cloned into a bacterial plasmid for ease of constructing the desired viral vector with the foreign DNA. The virus n then be excised from the plasmid. If the virus is a DNA virus, a bacterial origin of replication can be attached to the viral DNA, which is then replicated by the bacteria. Transcription and translation of this DNA will produce the coat protein which will encapsidate the viral DNA. If the virus is an RNA virus, the virus is generally cloned as a cDNA and inserted into a plasmid. The plasmid is then used to make all of the constructions. The RNA virus is then produced by transcribing the viral sequence of the plasmid and translation of the viral genes to produce the coat protein(s) which encapsidate the viral RNA. Construction of plant RNA viruses for the introduction and expression of non-viral foreign genes in plants is demonstrated by the above references as well as in U.S. Pat. No. 5,316,931. In one embodiment, a plant viral nucleic acid is provided in which the native coat protein coding sequence has been deleted from a viral nucleic acid, a non-native plant viral coat protein coding sequence and a non promoter, preferably the subgenomic promoter of the non-native coat protein coding sequence, capable of expression in the plant host, packaging of the recombinant plant viral nucleic acid, and ensuring a systemic infection of the host by the recombinant plant viral nucleic acid, has been inserted. Alternatively, the coat protein gene may be inactivated by insertion of the non- native nucleic acid sequence within it, such that a protein is produced. The recombinant plant viral nucleic acid may contain one or more additional non- native subgenomic promoters. Each non-native subgenomic promoter is capable of transcribing or expressing adjacent genes or nucleic acid sequences in the plant host and incapable of recombination with each other and with native subgenomic promoters. Non-native (foreign) nucleic acid sequences may be inserted adjacent the native plant viral subgenomic promoter or the native and a non-native plant viral subgenomic promoters if more than one nucleic acid sequence is included. The non-native nucleic acid sequences are transcribed or expressed in the host plant under control of the subgenomic promoter to produce the desired products. In a second embodiment, a recombinant plant viral nucleic acid is provided as in the first embodiment except that the native coat protein coding sequence is placed adjacent one of the non-native coat protein subgenomic promoters instead of a non-native coat protein coding sequence. In a third embodiment, a recombinant plant viral nucleic acid is provided in which the native coat protein gene is adjacent its subgenomic promoter and one or more non-native subgenomic promoters have been inserted into the viral nucleic acid. The inserted non-native subgenomic promoters are capable of transcribing or expressing adjacent genes in a plant host and are incapable of recombination with each other and with native subgenomic promoters. Non-native nucleic acid sequences may be inserted adjacent the non-native subgenomic plant viral promoters such that said sequences are transcribed or expressed in the host plant under control of the subgenomic promoters to produce the desired product. In a fourth embodiment, a recombinant plant viral nucleic acid is provided as in the third embodiment except that the native coat protein coding sequence is replaced by a non-native coat protein coding sequence. The viral vectors are encapsidated by the coat proteins encoded by the recombinant plant viral nucleic acid to produce a recombinant plant virus. The recombinant plant viral nucleic acid or recombinant plant virus is used to infect appropriate host plants. The recombinant plant viral nucleic acid is capable of replication in the host, systemic spread in the host, and transcription or expression of foreign gene(s) in the host to produce the desired protein. In many instances it is desired to target the expression of a recombinant protein. Such targeting can be into a cellular organelle or outside of the cell. This can be effected, as is well known in the art, by appropriate signal peptides, which are fused to the polypeptide to be targeted, typically at the N terminus. Thus, as used herein and in the claims section which follows, the phrase "signal peptide" refers to a stretch of amino acids which is effective in targeting a protein expressed in a cell into a target location. Different signal peptides, which are known in the art, are effective in secreting a protein from bacteria, yeast, plant and animal cells. It should be noted in this respect that signal peptides serve the function of translocation of produced protein across the endoplasmic reticulum membrane. Similarly, transmembrane segments halt translocation and provide anchoring of the protein to the plasma membrane, see, Johnson et al. The Plant Cell (1990) 2:525-532; Sauer et al. EMBO J. (1990) 9:3045-3050; Mueckler et al. Science (1985) 229:941-945. Mitochondrial, nuclear, chloroplast, or vacuolar signals target expressed protein correctly into the corresponding organelle through the secretory pathway, see, Von Heijne, Eur. J. Biochem. (1983) 133: 17-21 ; Yon Heijne, J. Mol. Biol. (1986) 189:239-242; Iturriaga et al. The Plant Cell (1989) 1 :381-390; McKnight et al, Nucl. Acid Res. (1990) 18:4939-4943; Matsuoka and Nakamura, Proc. Natl. Acad. Sci. USA (1991) 88:834-838. A recent book by Cunningham and Porter (Recombinant proteins from plants, Eds. C. Cunningham and A.J.R. Porter, 1998 Humana Press Totowa, N.J.) describe methods for the production of recombinant proteins in plants and methods for targeting the proteins to different compartments in the plant cell. In particular, two chapters therein (14 and 15) describe different methods to introduce targeting sequences that results in accumulation of recombinant proteins in compartments such as ER, vacuole, plastid, nucleus and cytoplasm. The book by Cunningham and Porter is incorporated herein by reference. Presently, the preferred site of accumulation of the fusion protein according to the present invention is the ER using signal peptide such as Cel 1 or the rice amylase signal peptide at the N-terminus and an ER retaining peptide (HDEL, SEQ ID NO: 17; or KDEL, SEQ ID NO:24) at the C-terminus. According to an additional aspect of the present invention there is provided a method of producing recombinant β-glucosidase. The method according to this aspect of the present invention is effected by introducing, in an expressible or overexpressible form, a nucleic acid construct into a host cell. The nucleic acid construct includes a genomic, complementary or composite polynucleotide preferably derived from Aspergillus niger and encoding a polypeptide having a β-glucosidase catalytic activity. The polynucleotide preferably further encodes a signal peptide in frame with the polypeptide. Still preferably, the polynucleotide further encodes an endoplasmic reticulum retaining peptide in frame with the polypeptide. As used herein the term "introducing" refers both to transforming and to virally infecting, as these terms are further defined hereinabove. As used herein the terms "expressible form" and "overexpressible form" refers to a recombinant form which includes the required regulatory elements to effect expression or over expression of a coding region, all as is further detailed hereinabove. According to a preferred embodiment of this aspect of the present invention, after sufficient expression has been detected, the polypeptide having the β-glucosidase catalytic activity is extracted from the expressing host cell. Thus host cells, expressing the polypeptide according to the present invention, provide an immediate, easy and indefinite source of the polypeptide. Any number of well-known liquid or solid culture media may be used for appropriately culturing host cells of the present invention, although growth on liquid media is preferred as the secretion of the polypeptide into the media results in simplification of polypeptide recovery. As is further detailed hereinabove, such secretion can be effected by the incorporation of a suitable signal peptide. The β-glucosidase may be isolated or separated or purified from host cell preparations using techniques well known in the art, such as, but not limited to, centrifugation filtration, chromatography, electrophoresis and dialysis. Further concentration and/or purification of the β-glucosidase may be effected by use of conventional techniques, including, but not limited to, ultrafiltration, further dialysis, ion-exchange chromatography, HPLC, size- exclusion chromatography, cellobiose-sepharose affinity chromatography, and electrophoresis, such as polyacrylamide-gel-electrophoresis (PAGE). Using these techniques, β-glucosidase may be recovered in pure or substantially pure form. According to an additional aspect of the present invention there is provided a method of increasing a level of at least one fermentation substance in a fermentation product. The method according to this aspect of the present invention is effected by fermenting a glucose containing fermentation starting material by a yeast cell overexpressing a nucleic acid construct which includes a genomic, complementary or composite polynucleotide preferably derived from Aspergillus niger and which encodes a polypeptide having a β- glucosidase catalytic activity, thereby increasing the level of the at least one fermentation substance in the fermentation product. The polynucleotide preferably further encodes a signal peptide in frame with the polypeptide. Still preferably, the polynucleotide further encodes an endoplasmic reticulum retaining peptide in frame with the polypeptide. According an alternative aspect of the present invention there is provided a method of increasing a level of at least one fermentation substance in a fermentation product. The method according to this aspect of the present invention is effected by fermenting a plant derived glucose containing fermentation starting material by a yeast cell, the plant overexpressing a nucleic acid construct which includes a genomic, complementary or composite polynucleotide preferably derived from Aspergillus niger and which encodes a polypeptide having a β-glucosidase catalytic activity, thereby increasing the level of the at least one fermentation substance in the fermentation product. The polynucleotide preferably further encodes a signal peptide in frame with the polypeptide. Still preferably, the polynucleotide further encodes an endoplasmic reticulum retaining peptide in frame with the polypeptide. As used herein in the specification and in the claims section that follows, the term "fermentation" refers to a chemical change induced in a complex organic compound by the action of an enzyme, whereby the substance is split into simpler compounds. Specifically, the term "fermentation" includes the anaerobic dissimilation of substrates with the production of energy and reduced compounds, the final products thereof are organic acids, alcohols, such as ethanol, isopropanol, butanol, etc., and Cθ2- Such products, are typically secreted and each of which is referred to herein as a "fermentation substance", i.e., any known fermentation resultant of either microbial or yeast fermentation. As used herein in the specification and in the claims section that follows, the phrase "fermentation product" refers to the resultant material of a fermentation process. Examples include, but are not limited to, alcohol containing fermentation medium and alcoholic beverages, such a, but not limited to, fruit-based alcohol-containing beverages, wines and beers. When used in conjunction with, for example, a β-glucanase, the β- glucosidase is effective for hydrolyzing a variety of cellulose containing materials to glucose. The glucose produced by enzymatic hydrolysis of the cellulose and other glucose containing saccharides, may be recovered and stored, or it may be subsequently fermented to ethanol using conventional techniques. Many processes for the fermentation of glucose generated from cellulose are well known, and are suitable for use herein. Briefly, the hydrolyzate containing the glucose from the enzymatic reaction is contacted with an appropriate microorganism under conditions effective for the fermentation of the glucose to ethanol. This fermentation may be separate from and follow the enzymatic hydrolysis of the cellulose (sequentially processed), or the hydrolysis and fermentation may be concurrent and conducted in the same vessel (simultaneously processed). Details of the various fermentation techniques, conditions, and suitable microorganisms have been described, for example, by Wyman (1994, Bioresource Technol., 50:3-16) or Olsson and Hahn-Hagerdal (1996, Enzyme Microbial Technol., 18:312-331), the content of each of which is incoφorated herein by reference. Following the completion of a fermentation, the alcohol may be recovered by extraction, and optionally purified e.g., by distillation. Thus, according to still another aspect of the present invention there is provided a method of producing an alcohol. The method according to this aspect of the present invention is effected by fermenting a glucose containing fermentation starting material by a cell overexpressing a nucleic acid construct including a genomic, complementary or composite polynucleotide preferably derived from Aspergillus niger, encoding a polypeptide having a β-glucosidase catalytic activity, and extracting the alcohol therefrom. The polynucleotide preferably further encodes a signal peptide in frame with the polypeptide. Still preferably, the polynucleotide further encodes an endoplasmic reticulum retaining peptide in frame with the polypeptide. According to an additional aspect of the present invention there is provided a method of producing an alcohol. The method according to this aspect of the present invention is effected by fermenting a plant derived glucose containing fermentation starting material by a cell, the plant overexpressing a nucleic acid construct including a genomic, complementary or composite polynucleotide preferably derived from Aspergillus niger, encoding a polypeptide having a β-glucosidase catalytic activity, and extracting the alcohol therefrom. The polynucleotide preferably further encodes a signal peptide in frame with the polypeptide. Still preferably, the polynucleotide further encodes an endoplasmic reticulum retaining peptide in frame with the polypeptide. Plants contain aroma and flavor compounds of glycosidic nature, their inherent aroma property can be released by degrading enzymes, turning a non- volatile aroma compound into its volatile form. Thus, for example, α-L- arabinofuranosidases, assist in the liberation of aroma compounds from substrates such as juices or wines, as described by Gunata et al. (European Patent Application No. 332.281, 1989; and "purification and some properties of an alpha-L-arabinofuranosidase from A. niger action on grape monoterpenyl arabinofuranosylglucosides. J. Agric. Food Chem. 38: 772-776, 1990). This outcome is achieved, for example, in a two step process wherein the first step comprises the use of an α-L-arabinofuranosidase, to catalyze the release of arabinose residues from monoterpenyl α-L-arabinofuranosyl glucosides contained in, for example, the fruit or vegetable juice via the cleavage of the (1 — »6) linkage between a terminal arabinofuranosyl unit and the intermediate glucose of a monoteφenyl α-L-arabinofuranosylglucoside. The α-L- arabinofuranosidase is preferably in a purified form so as to avoid the undesirable degradation of other components of the juice which may be detrimental to its ultimate quality. In the second step, β-glucosidase is required to yield the free teφenol from the resulting desarabinosylated monoteφenyl glucoside. If desired, both reaction steps may be performed in the same reaction vessel without the need to isolate the intermediate product (Gunata et al. (1989), supra). Thus, β-glucosidase is an essential contributor when the liberation of these aroma compounds for improving the flavor of the juice or wine is desired. Moreover, in the case of wine, the control of the liberation of aroma compounds provides wines with a more consistent flavor, thus reducing or eliminating the undesirable effect of "poor vintage years". Additional information is contained in: "Cloning and expression of DNA molecules encoding arabinan degrading enzyme of fungal origin", U.S. Pat. No. 5,863,783; Y. Gueguen, et al. "A Very Efficient β-Glucosidase Catalyst for the Hydrolysis of Flavor Precursors of Wines and Fruit Juices", J. Agric. Food Chem. 44:2336-2340, 1996, each of which is incoφorated herein by reference. Thus, according to a further aspect of the present invention there is provided a method of increasing a level of at least one aroma substance in a plant derived product, such as, but not limited to, an alcoholic beverage. The method according to this aspect of the present invention is effected by incubating a glucose containing plant starting material with a yeast cell overexpressing a nucleic acid construct including a genomic, complementary or composite polynucleotide preferably derived from Aspergillus niger which encodes a polypeptide having a β-glucosidase catalytic activity, thereby increasing the level of the at least one aroma substance in the plant derived product. The polynucleotide preferably further encodes a signal peptide in frame with the polypeptide. Still preferably, the polynucleotide further encodes an endoplasmic reticulum retaining peptide in frame with the polypeptide. While reducing the present invention to practice it was discovered that in order to obtain activity of a β-glucosidase in a transgenic plant, the expression construct should include a signal peptide. In addition, it was found that retaining the enzyme in the endoplasmic reticulum results in higher release of aroma compounds following homogenization and incubation. It is assumed that compartmentalization of the enzyme in for example the ER prevents it from interacting with its substrates which are mainly outside the cells, limiting such interaction following homogenization. Indeed, directing the enzyme to the apoplast resulted in increased release of aroma in vivo. Thus, depending on the specific application, one can chose weather to include in the construct an endoplasmic reticulum retaining peptide or not. According to yet a further aspect of the present invention there is provided a method of increasing a level of at least one aroma substance in a plant derived product, such as, but not limited to, an alcoholic beverage. The method according to this aspect of the present invention is effected by incubating a glucose containing plant starting material with a yeast cell, said plant overexpressing a nucleic acid construct including a genomic, complementary or composite polynucleotide preferably derived from Aspergillus niger which encodes a polypeptide having a β-glucosidase catalytic activity, thereby increasing the level of the at least one aroma substance in the plant derived product. The polynucleotide preferably further encodes a signal peptide in frame with the polypeptide. Still preferably, the polynucleotide further encodes an endoplasmic reticulum retaining peptide in frame with the polypeptide. As used herein in the specification and in the claims section that follows, the phrase "glucose containing starting material" refers to any source of energy, in the form of glucose containing compounds, other than free glucose, including, but not limited to, crushed, minced, diced or extracted plant material, plant, or portions thereof, such as fruits, examples thereof are tropical fruits and grapes. According to an additional aspect of the present invention there is provided a method of producing an aroma spreading plant. As used herein in the specification and in the claims section that follows, the phrase "aroma spreading plant" refers to substantially any part of a plant, in which volatile compounds are generated by the catalytic activity of the β-glucosidase polypeptide of the present invention, release of volatile compounds therefrom is perceived by the olfactory system of an organism, such as a human. The method according to this aspect of the present invention is effected by overexpressing in the plant a nucleic acid construct including a genomic, complementary or composite polynucleotide derived from Aspergillus niger, which encodes a polypeptide having a β-glucosidase catalytic activity, thereby increasing aroma spread from the plant. Such overexpression is preferably performed in a tissue specific manner by, for example, employing a tissue specific promoter, as hereinabove described, to thereby overexpress a heterologous protein in a selected portion of the plant. The tissue in which such overexpression is effected is selected according to the availability of glucose containing non-volatile aroma substrates therein. Thus, such an overexpression will cause the release of a volatile and aroma constituent of the substrate. Thus, according to preferred embodiments overexpressing the nucleic acid construct is limited to at least one tissue, such as a flower, a fruit, a seed, a root, a stem, pollen and leaves. According to still a further aspect of the present invention there is provided a method of increasing a level of free glucose in a glucose containing fermentation starting material. The method according to this aspect of the present invention is effected by fermenting the glucose containing fermentation starting material by a cell overexpressing a nucleic acid construct including a genomic, complementary or composite polynucleotide preferably derived from Aspergillus niger, which encodes a polypeptide having a β- glucosidase catalytic activity, thereby increasing the level of the free glucose in the glucose containing fermentation starting material. The polynucleotide preferably further encodes a signal peptide in frame with the polypeptide. Still preferably, the polynucleotide further encodes an endoplasmic reticulum retaining peptide in frame with the polypeptide. According to another aspect of the present invention there is provided a method of increasing a level of free glucose in a plant derived glucose containing fermentation starting material. The method according to this aspect of the present invention is effected by fermenting the plant derived glucose containing fermentation starting material by a cell, the plant overexpressing a nucleic acid construct including a genomic, complementary or composite polynucleotide preferably derived from Aspergillus niger, which encodes a polypeptide having a β-glucosidase catalytic activity, thereby increasing the level of the free glucose in the plant. The polynucleotide preferably further encodes a signal peptide in frame with the polypeptide. Still preferably, the polynucleotide further encodes an endoplasmic reticulum retaining peptide in frame with the polypeptide. As used herein in the specification and in the claims section that follows, the term "free glucose" refers to glucose residues in the form of a monosaccharide, the levels of which are increased by the catalytic activity of β -glucosidase. As used herein in the specification and in the claims section that follows, the phrase "glucose containing fermentation starting material" refers to any source of energy, in the form of glucose containing compounds, other than free glucose, including, but not limited to, crushed, minced, diced or extracted plant material, plant, or portions thereof, used in industrial fermentation processes. According to yet another aspect of the present invention there is provided a method of increasing a level of extra- or intracellular free glucose in a plant. The method according to this aspect of the present invention is effected by overexpressing in the plant a nucleic acid construct including a genomic, complementary or composite polynucleotide preferably derived from Aspergillus niger, which encodes a polypeptide having a β-glucosidase catalytic activity, thereby increasing the level of the free glucose in the plant. Thus, sweeter fruits can be produced. The polynucleotide preferably further encodes a signal peptide in frame with the polypeptide. Still preferably, the polynucleotide further encodes an endoplasmic reticulum retaining peptide in frame with the polypeptide. Glycosidases, including β-glucosidase, catalyze reactions involving the hydrolysis of O-glycosidic bond of glycosides, and synthesize oligosaccharides when the reaction is run in reverse from the normal direction, a result achieved by, for example, site directed mutagenesis, and Km reversal. As described in the Background section hereinabove, the hydrolysis reaction mechanism of glycosidases involves two catalytic steps, the second of which involves a base catalyzed H 2 0 attack, resulting in the regeneration of the enzyme, and the release of the saccharide residue. Thus, in addition, oligosaccharide synthesis can be achieved by adding a second saccharide to the reaction mixture, which competes with the H 2 0 molecule, and reacts in its place with the first saccharide in, what is known as, a transglycosylation reaction. Hence, as glycosidases are generally available and easy to handle, these enzymes have the potential to catalyze the production of many different products using inexpensive substrates. For further detail see U.S. pat. No. 5,716,812, which is incoφorated herein by reference. Thus, according to yet an additional aspect of the present invention there is provided a method of synthesizing oligosaccharides. The method according to this aspect of the present invention is effected by mixing a polypeptide having a β-glucosidase catalytic activity with first and second saccharide molecules to thereby join the first and second saccharide molecules into an oligosaccharide. Additional objects, advantages, and novel features of the present invention will become apparent to one ordinarily skilled in the art upon examination of the following examples, which are not intended to be limiting. Additionally, each of the various embodiments and aspects of the present invention as delineated hereinabove and as claimed in the claims section below finds experimental support in the following examples. EXAMPLES Reference is now made to the following examples, which together with the above descriptions, illustrate the invention in a non limiting fashion. Generally, the nomenclature used herein and the laboratory procedures utilized in the present invention include molecular, biochemical, microbiological and recombinant DNA techniques. Such techniques are thoroughly explained in the literature. See, for example, "Molecular Cloning: A laboratory Manual" Sambrook et al, (1989); "Current Protocols in Molecular Biology" Volumes I-III Ausubel, R. M., ed. (1994); Ausubel et al, "Current Protocols in Molecular Biology", John Wiley and Sons, Baltimore, Maryland (1989); Perbal, "A Practical Guide to Molecular Cloning", John Wiley & Sons, New York (1988); Watson et al, "Recombinant DNA", Scientific American Books, New York; Birren et al. (eds) "Genome Analysis: A Laboratory Manual Series", Vols. 1-4, Cold Spring Harbor Laboratory Press, New York (1998); methodologies as set forth in U.S. Pat. Nos. 4,666,828; 4,683,202; 4,801,531; 5,192,659 and 5,272,057; "Cell Biology: A Laboratory Handbook", Volumes I-III Cellis, J. E., ed. (1994); "Current Protocols in " Volumes I-III Coligan J. E., ed. (1994); Stites et al. (eds), "Basic and Clinical Immunology" (8th Edition), Appleton & Lange, Norwalk, CT (1994); Mishell and Shiigi (eds), "Selected Methods in Cellular Immunology", W. H. Freeman and Co., New York (1980); available immunoassays are extensively described in the patent and scientific liter, see, for example, U.S. Pat. Nos. 3,791,932; 3,839,153; 3,850,752; 3,850,578; 3,853,987; 3,867,517; 3,879,262; 3,901,654; 3,935,074; 3,984,533; 3,996,345; 4,034,074; 4,098,876; 4,879,219; 5,011,771 and 5,281,521; "Oligonucleotide Synthesis" Gait, M. J., ed. (1984); "Nucleic Acid Hybridization" Hames, B. D., and Higgins S. J., eds. (1985); "Transcription and Translation" Hames, B. D., and Higgins S. J., eds. (1984); "Animal Cell Culture" Freshney, R. I., ed. (1986); "Immobilized Cells and Enzymes" IRL Press, (1986); "A Practical Guide to Molecular Cloning" Perbal, B., (1984) and "Methods in Enzymology" Vol. 1-317, Academic Press; "PCR Protocols: A Guide To Methods And Application", Academic Press, San Diego, CA (1990); Marshak et al, "Strategies for Protein Purification and Characterization - A Laboratory Course Manual" CSHL Press (1996); all of which are incoφorated by reference as if fully set forth herein. Other general references are provided throughout this document. The procedures therein are believed to be well known in the art and are provided for the convenience of the reader. All the information contained therein is incoφorated herein by reference. MATERIALS AND EXPERIMENTAL METHODS Purification of A. niger β-glucosidase: A crude preparation of A. niger Bl (CMI CC 324626) β-glucosidase was obtained from Shaligal Ltd. (Tel- Aviv, Israel). A sample (10 ml) of the crude enzyme (140 Units/ml) was first diafiltered through a 50 kDa cut-off Amicon membrane (Amicon Coφ., Danvers, MA), with 20 mM citrate buffer pH=5. The proteins were then separated on an FPLC equipped with a Mono-Q RH 5/5 column (Amersham Pharmacia Biotech AB, Uppsala, Sweden), equilibrated with the same buffer. The enzyme was eluted with a linear gradient of 0 to 350 mM NaCl. Active fractions (see below, enzyme assays) were monitored and pooled (between 80-110 mM NaCl). The partially purified enzyme was dialyzed against 20 mM citrate buffer pH=3.5, applied to a Resource-S column equilibrated with the same buffer, and eluted with a gradient of 0-1 M NaCl. The purified enzyme (eluted at 155 mM NaCl) was concentrated by ultrafiltration (50 kDa cut-off membrane, Amicon). Enzyme assays: β-glucosidase enzyme activity was monitored using a plate assay as follows. 4-methylumbelife β-D-glucopyranoside (MUGlc, Sigma Chemical Inc. St. Louis, Missouri) to a final concentration of 0.5 mM, was dissolved in PC buffer (50 mM phosphate, 12 mM citric acid, pH=3.4) at 45 °C. The solution was mixed with 3 % agar in water, previously boiled and then cooled to 45 °C. The resulting solution (20 ml) was poured into a petri dish and after solidification, 10 μl enzyme samples were spotted. The plate was incubated at 50 °C for one hour, and then illuminated with long UV. An intense fluorescence was indicative of β-glucosidase activity. Detection of β-glucosidase in polyacrylamide gels was carried out by washing the SDS-polyacrylamide gel with 1 :1 isopropanol :PC buffer to remove SDS and renature the enzyme. The gel was washed once in PC buffer and incubated in a thin layer of a solution of 0.5 mM MUGlc. After incubation at 50 °C for one hour, the active protein band was visualized by UV light. Quantitative assays were performed using pNPGlc as a substrate according to Shoseyov (7). Determination of thermal stability of A. niger β-glucosidase: Recombinant enzyme (40 μg/ml) was dissolved in 20 mM citrate phosphate buffer, pH=5. Each tested sample (8 μl) was covered by 15 μl mineral oil. The activity was determined by the standard pNPGlc assay (7). Deglycosylation of A. niger β-glucosidase by N-glycosidase-F: A N-glycosidase-F (Boehringer Mannheim, Mannheim, Germany) reaction mixture, containing 0.125 μg pure β-glucosidase (previously denatured by boiling for 3 minutes in 1 % SDS and 5 % β-mercaptoethanol), 0.2 units of the N-glycosidase-F, sodium phosphate buffer (50 mM, pH=7.5), EDTA (25 mM), 1 % Triton X-100 and 0.02 % sodium azide, in a total volume of 12.5 μl, was incubated for 4 hours at 37 °C. Reaction was stopped by addition of PAGE sample application buffer followed by 3 minutes of boiling. Proteolysis and N-terminal sequences of A. niger Bl ^-glucosidase: Partial enzymatic proteolysis with Staphylococcus aureus V8 protease was carried out as described by Cleveland (28). Briefly, FPLC-purified β- glucosidase (5 μg), was concentrated by acetone precipitation. The protein was separated on a preparative 10 % SDS-PAGE. The gel was stained with coomassie blue, destained and rinsed with cold water, and the β-glucosidase protein band was excised. The resulting gel slice was applied to a second SDS-PAGE gel (15 % acrylamide) and overlaid with Staphylococcus aurous V8 protease. Digestion was carried out within the stacking gel by turning off the current for 30 min. As the bromophenol blue dye neared the bottom of the stacking gel, the current was restored. The electrophoresed cleavage products were electroblotted to PVDF membranes. The native protein was transferred to PVDF in parallel. The N-terminal sequence of the native protein and two of the numerous cleavage products were analyzed by Edman degradation using a gas-phase protein sequencer (Applied Biosystems model 475A microsequencer). Cloning of bgll cDNA and genomic gene: Total RNA isolation: Total RNA was isolated from Aspergillus niger Bl as follows: A. niger Bl was grown in liquid culture consisting of mineral media (NH 4 ) 2 S0 4 -3H 2 0 (0.5 g/1), KH2PO4 (0.2 g/1), MgS0 4 (0.2 g/1), CaCl H 2 0 (0.1 g/1), FeS0 4 -6H 0 (0.001 g/1), ZnS0 4 7H 2 0 (0.001 g/1), and 2 mM citric acid, at pH=3.5 with 1 % w/v bran as a carbon source. The medium was autoclaved, cooled and inoculated with A. niger Bl (10^ spores/ml). Baffled flasks were used with constant shaking (200 RPM) at 37 ° C. The appearance of β-glucosidase activity was monitored by placing 5 μl of growth medium on 1 % agar plates containing 0.5 mM MUGlc, as described above. Activity was detected following 15 hours incubation. The mycelium was harvested following 24 hours growth period, and the medium removed by filtering through GFA glass microfibre (Whatman Inter. Ltd., Maidstone, England). The mycelium was then frozen with liquid nitrogen and ground to fine powder with mortar and pestle. Total RNA was produced from this powder by the Guanidine thiocyanate (TriReagent™) method (Molecular Research Center, Inc.). RNA reverse-transcription reaction: cDNA was obtained by reverse transcribing total RNA (10 μg) using Stratagene RT-PCR kit (Stratagene, La Jolla, CA). The reaction mixture (50 μl) additionally consisted of: Oligo dT18 (1 μg), RNase Block Ribonuclease Inhibitor (20 units), lx buffer (50 mM Tris- HC1, pH=8.3, 75 mM KC1, 10 mM DTT, 3 mM MgCl 2 ), dNTPs (500 μM each) and reverse transcriptase (300 units). Total RNA was initially denatured at 70 °C, allowed to cool to room temperature (for primers annealing), and added to the reaction mixture. The reaction mixture was incubated for 1 hour at 37 °C, followed by heating (95 °C, 5 minutes) and stored at -70 °C until further use. DNA amplification: Degenerate primers for DNA amplification reaction by PCR methods were synthesized, based on part of the amino acid N- terminal sequence and an internal sequence, as determined by the Edman degradation, following V8 proteolysis (hereinbelow, experimental results). The partial sequence from β-glucosidase N-terminal derived amino acid sequence was Ser-Pro-Pro-Tyr-Tyr-Pro (SEQ ID NO:4), yielding the following primer: 5'-(C/G)(A/C/G/T)CC(A/C/G/T) CC(A/C/G/T)TA(C/T)TA(C/T)CC-3' (SEQ ID NO:5). The partial sequence from E2 internal cleavage product amino acid sequence was Gln-Pro-Ile-Leu-Pro-Ala-Gly-Gly (SEQ ID NO: 6), yielding the following primer: 5'-TCCIGC(T/G/C/A)GG(TG/C/A)A(G/A) (T/G/A)AT(T/G/C/A)GG(T/C)TG-3* (SEQ ID NO: 7). DNA amplification reaction mixture (25 μl) contained: reverse transcriptase reaction product (1 μl), 10 x PCR buffer (2.5 μl, Promega Coφ., Madison, WI), dNTPs (250 μM each), MgCl 2 (2.0 mM), degenerate primers (250 pmol each), DNA polymerase (3 units, Stratagene, La Jolla, CA) and overlaid with mineral oil (25 μl). The reaction was performed in an automated heating block (Programmable thermal controller - MJ Research, Inc.). PCR cycling conditions were 30 seconds denaturing at 94 °C, 60 seconds annealing at 50 °C, and 150 seconds elongation at 72 °C, repeated 36 times. The resulting amplified product was electrophoresed on a 1.2 % (w/v) agarose/TBE gel, resulting in a 2.2 kb cDNA gene fragment, which was further isolated using Gel Extraction Kit (QIAGEN, Hilden, Germany) and cloned directly into the single 3'-T PCR insertion site of pGEM-T cloning vector (Promega Coφ., Madison, WI). Probe preparation: The 2.2 kb partial cDNA was digested with Pstl to produce a 1.2 kb fragment DNA probe. A sample (25 ng) of the fragment was labeled with [32p]dCTP, using the random sequence nanonucleotide rediprime DNA labeling system (Amersham Pharmacia Biotech AB, Buckinghamshire, England). Preparation of genomic DNA plasmid library: An A. niger Bl genomic library was constructed in the pYEAUra3 yeasi/E. coli shuttle vector (Clontech Lab. Inc. Palo Alto, CA). A. niger Bl was grown in liquid culture as described above, the mycelium harvested following 48 hours of growth, frozen in liquid nitrogen and grounded. The mycelium ground was used to produce genomic DNA by the CTAB method of Murray and Thompson (29). The library was constructed from partially digested Sau3A genomic DNA, cloned into the BamHI site of the pYEUra3 yeast shuttle vector (Clontech Lab. Inc. Palo Alto, CA). pYEAUra3 yeast/£. coli shuttle vector was digested with BamYfl and dephosphorylated with CIP to prevent self ligation. The partially digested genomic DNA was cloned into the shuttle vector with T4 ligase and used to transform TOP 10 E. coli electro-competent cells, which were then plated on LB-agar containing ampicillin (50 μg/ml). A total of 4 x 10^ colonies were grown on LB-agar plates, blotted to Hybond-N membranes (Amersham Pharmacia Biotech AB, Buckinghamshire, England) and screened using the above described 1.2 kb probe. Positive clones were subcloned in pUC18 and sequenced (Biological Services, The Weizmann Institute of Science, Rehovot, Israel). Expression of bgll cDNA in E. coli: Two specific primers were designed according to the 5' and the 3' sequences, corresponding to the N-terminal and C-terminal region of the mature protein: sense primer: 5'-' (SEQ ID NO:8). Antisense primer: 5'- AAAGGATCCTTAGTGAACAGTAGGCAGAGACGC-3* (SEQ ID NO:9). The isolated cDNA was digested with Ncol and BamHl and cloned into a pET3d expression vector (Figure 1A, Novagen Inc., Madison, WI). Positive E. coli BL21(DE3) pLysS colonies, containing the bgll cDNA, were confirmed by enzyme restriction and sequence analysis. Recombinant BGLl was expressed according to the manufacturer's protocol. Expression of bgll cDNA in Saccharomyces cerevisiae and Pichia pastoris: The pYES2 vector (Invitrogen Inc., San Diego, CA) was used to successfully clone the bgll cDNA gene into the HindllllBamXΩ. of pYES2- bgll plasmid (Figure lb), and transform Saccharomyces cerevisiae using the lithium acetate method (30). The BGLl was expressed by inducing the Gall promoter according to the manufacturer's protocol. Saccharomyces cerevisiae strain INVSc2 (MATa, his3-D200, ura3-167) was used as the host. Pichia pastoris strain GS115 (his4 mutant) was used as the host for shuttle and expression vector plasmid pHIL-Sl (Invitrogen Inc., San Diego, CA). The bgll cDNA was cloned into the EcoRl/BamHl sites of pHIL-Sl, yielding the pHIL-Sl-bgll expression and secretion vector (Figure lc). Expression in P. pastoris was carried out according to the manufacturer's protocol. Screening of β-glucosidase-expressing clones was facilitated by top-agar, containing 50 mg X-Glc, 30 ml methanol and 1 % agar per liter. Blue color indicated a colony producing active β-glucosidase. Western blot analysis: Antibodies were produced from rabbit serum 36 days following a second injection of 100 μg purified protein and adjuvant (AniLab Biological Services, Tal-Sachar, Israel). High molecular weight ladder was from Sigma Chemical Inc. St. Louis, Missouri. Western blot conditions were as described in reference 36. Determination of the Stereochemical Course of Hydrolysis: The method was essentially as described by Wong et al. (31). PNPGlc (10 μmols) was dissolved in 0.5 ml of 25 mM acetate buffer pH=3.5 in D 2 0 in an NMR tube. β-Glucosidase was lyophilized and redissolved in 100 μl D 0 (35 units/ml). The 1H-NMR spectrum of the substrate was recorded, enzyme added (10 μl), and spectra recorded at specified time intervals on a Bruker AMX400 at 25 °C. Inactivation and reactivation studies: Pure A. niger β-Glucosidase enzyme (0.47 mg/ml) was incubated in the presence of various concentrations of 2-deoxy-2-fluoro-β-glucosyl fluoride (2FGlcF, 0.5-6 mM) in 30 mM citrate buffer pH=4.8 at 50 °C. Residual enzyme activity was determined at different time intervals by addition of an aliquot (10 μl) of the inactivation mixture, to a solution containing citrate buffer (30 mM, pH=4.8), BSA (8 μg) and 2,4-dinitrophenyl β-D- glucopyranoside (DNPGlc, 0.625 mM, 830 μl). Release of DNP was determined spectrophotometrically by measuring the absorbance at 400 nm one minute after the addition of the substrate. Reactivation rates were determined as follows: pure A. niger β- glucosidase (0.34 mg/ml) was preincubated with 2FGlcF (5 mM) for 15 min, after which the excess of the inactivator was diafiltered by 20-kDa nominal molecular mass cutoff centrifugal concentrators (Sartorius Inc., Goettingen, Germany). Samples of the purified, inactivated enzyme were incubated in the presence linamarin (0-16 mM) in citrate buffer (30 mM, pH=4.8) at 50 °C for 0, 10, 20 and 30 minutes, and the activity of each sample was determined using p-nitrophenyl β-D-glucopyranoside (pNPGlc) as a substrate. Expression of bgll cDNA in tobacco plants: Genetic constructs: Bgll cDNA was cloned in pETBl (37). pJD330 and pBINPlus (38) « were used as an intermediate and binary vector, respectively. Cell signal sequence as well as 35S plus Ω fragment were retrieved from pB21, modified pBluescript SK (39). Nicotiana tabacum cv. Samson was used as a model plant for gene transformation. Three gene constructs were employed (Figures l la-c): (i) bgll without any signal peptide which served for cytoplasmic expression (Figure 11a, plasmid pJDBl); (ii) bgll including a cell signal peptide at the N terminus for secretion into the apoplast (Figure l ib, plasmid pJDCBl); and (iii) bgll including the cell signal peptide and the KDEL (SEQ ID NO:24) ER-retaining peptide at the C-terminus for accumulation in the ER (Figure 1 lc, plasmid pJDCBIT). To this end, bgll cDNA (2.5 kb) was released from pETBl (37) with Nco and BamH and inserted into pJD330 between the 35S promoter Ω fragment and the nos terminator, eliminating the gus gene, resulting in plasmid pJDBl . Endoplasmic reticulum retaining signal tetrapeptide HDEL (SEQ ID NO: 17) was synthesized and fused with bgll at the C-terminal in pJDBl by a fidelity PCR reaction with the following pair of primers: Forward primer (23 mer), starting from nucleotide 1248 of bgll cDNA 5' - (1248) - CAGTGACCGTGGATGCGACAATG - (1270') - 3' (SEQ ID NO:20); Reverse primer (40 mer), starting at nucleotide 2506 of bgll cDNA encoding also for the HDEL (SEQ ID NO: 17) peptide 5' - (2506) - AGAGACGGATGACAAGTACTACTTGAAATTGGGCCCAAAA-3 ' (SEQ ID NO:21). For pJDCBIT (35S Ω + Cell + bgll + HDEL), the 35S Ω fragment of pJDBl was replaced by a 35S Ω + Cell fragment digested from pB21 with BarnYfl and Xbal. For pJDCBl (35S Ω + Cell + bgll), the fragment containing 35S Ω and Cell as well as part of bgll was cut from pJDCBIT with Hindlϊl and Nrul and ligated with the vector of pJDBl digested with the same pair of restriction enzymes. The nucleotide sequence of all of the genetic constructs was confirmed by DNA sequencing. Gene cassettes in the intermediate vectors of pJDBl, pJDCBl and pJDCBIT were further isolated with Hindlϊl and EcoBl and inserted into multiple cloning sites of the binary vector pBINPlus. Disarmed Agrobateruim LB4404 was transformed with pBINPlus containing bgll gene cassettes. Tobacco plant transformation: The young leaves of in vitro grown plantlets were excised and cut into 0.5 cm pieces and then immersed for 5 minutes in an overnight grown culture of Agrobacterium. After blotted with sterile Whatman filter paper, the infected leaves were co-cultured for 2 days with Agrobacterium on MS medium plus 2.0 mg/L of Zeatin and 0.1 mg/L of IAA as well as 0.35 % (w/v) phytagel and then transferred to the same medium but with 300 mg/L kanamycin and 300 mg/L carbenicillin. Regenerates were then transferred to the rooting media, containing only MS salts, vitamins and the same antibiotics. Rooted plants were transferred to greenhouse after PCR screening. Screening for transgenic plants: DNA and protein of plants were extracted according to Nagy et al. (40). PCR verification of gene insertion into plant genome was done with the following pairs of primers, which cover the DNA fragment from position 1248 to the end of bgll : 5'-CAGTGACCGTGGATGCGACAATG-3' (SEQ ID NO:22) and 5*-AAAGGATCCTTAGTGAACAGTAGGCAGAGACGC-3' (SEQ ID NO:23). Identifying transgenic plants expressing BGLl protein and activity: Western blot (40) and SDS-PAGE activity gel staining (37) were employed to screen successful transgenic lines, using the purified A. niger BGLl protein as positive controls and non-transgenic plant as negative control. SPMI-GC/MS analysis: The effect of bgll on flavor compound evolution and composition was studied. Fresh leaves of transgenic plants and of wild type control plants were excised and ground in liquid nitrogen. Ice-cold extraction buffer, containing 10 mM EDTA, 4 mM DTT in 50 mM phosphate buffer, pH 4.3, was added in a ratio of 1 :3 w/w. The mixture was then shaken for 0.5 hours. 0.75 ml of supernatant from each of the centrifuged mixtures was taken into a glass vial. All manipulations were at 4 °C. After 9 hours of incubation at 37 °C, the volatiles in the vial were analyzed according to Clark et al. (41) using a Saturn Varian 3800 SPMI-GC-MS apparatus, equipped with a DB-5 capillary column. The temperature of splitless injections was 250 °C and the transfer line was maintained at 280 °C. Helium was used as a carrier gas. The oven was programmed as follows: 1 minute at 40 °C with gradually heating to 250 °C at a rate of 5 °C/minute. EXPERIMENTAL RESULTS Purification of wild type A. niger β-glucosidase: A. niger β-glucosidase enzyme preparation was purified by Mono-Q FPLC. Active protein samples eluted from the Mono-Q column were separated on a 10 % SDS-PAGE gel, stained with coomassie blue, and incubated in the presence of MUGlc to demonstrate activity of the enzyme. At this stage of purification, a discrete band, having an apparent molecular mass of approximately 160 kDa and β-glucosidase activity could be detected (Figure 2b, lanes 1-5: 1 - electroeluted band of BGLl from preparative PAGE-SDS gel stabs; 2-5 - acetone precipitates from Mono-Q separation of BGLl). However, the apparent mass of the denatured enzyme (boiled for 10 min in the presence of β-mercaptoethanol), was shown to be 120 kDa on 10 % SDS-PAGE (Figure 2a). The enzyme was designated BGLl was further purified to homogeneity on a Resource-S column (Figure 3). Deglycosylation of A. niger β-glucosidase was performed by N-glycosidase-F. As demonstrated in Figure 4, SDS-PAGE analysis indicated that approximately 20 kDa of the A. niger β-glucosidase mass can be attributed to N-linked carbohydrates. Proteolysis and N-terminal sequences of BGLl: Partial enzymatic proteolysis with Staphylococcus aureus V8 protease of purified BGLl was conducted. The undigested protein and cleavage products were separated by SDS-PAGE, followed by electroblotting onto PVDF membranes and determination of the N-terminal sequence of the native protein and two of the cleavage products. Amino acid sequences obtained were as follows: N-terminal native protein: Asp-Glu-Leu-Ala-Tyr-Ser-Pro-Pro-Tyr-Tyr- Pro-Ser-Pro-Tφ-Ala-Asn-Gly-Gln-Gly-Asp (SEQ ID NO: 10). Underlined portion represents SEQ ID NO:4. Internal cleavage product - El polypeptide: Val-Leu-Lys-His-Lys-Asn- Gly-Val-Phe-Thr-Ala-Thr-Asp-Asn-Tφ-Ala-Ile-Asp-Gln-Ile-Glu-Ala-Leu- Ala-Lys (SEQ ID NO: 11). Internal cleavage product - E2 polypeptide: Gly-Ala-Thr-Asp-Gly-Ser- Ala-Gln-Pro-Ile-Leu-Pro-Ala-Gly-Gly-Glv-Pro-Glv-Gly-Asn-Pro (SEQ ID NO: 12). Underlined portion represents SEQ ID NO:6. FastA analysis (32) indicated that the N-terminal sequence, as well as the internal sequences, have sequence similarity with sequences of β- glucosidase from the yeast Saccharomycopsis fibuligera which belonging to Family 3 of the glycosyl hydrolases. Isolation and characterization of bgll cDNA and genomic DNA: In order to clone the A. niger β-glucosidase gene, degenerate primers were designed according to the sequence of digest fragments of the polypeptide. These oligonucleotides were used to amplify a cDNA fragment of the β-glucosidase gene by RT-PCR. A 1.2 kb probe was excised from the resultant 2.2 kb amplification product and was used to screen a genomic library, constructed in pYEUra3 yeast/E. coli shuttle vector. Positive clones were successfully subcloned and sequenced, resulting in full length bgll genomic sequence (SEQ ID NO:3, Figure 5a). Amplification primers were then generated, according to the genomic DNA sequence, corresponding to the N- and C-terminal of the mature protein. RT-PCR was thereafter used for amplifying the full length β-glucosidase cDNA sequence (SEQ ID NO:l, Figure 5a, GenBank Accession No. AJ 132386). The cDNA sequence perfectly matched the DNA sequence of the combined exons. The open reading frame was found to encode a polypeptide with a predicted molecular weight of 92 kDa. The gene includes 7 exons intercepted by 6 introns (Figure 5b). Analysis of the DNA sequence upstream to the sequence encoding for the mature protein revealed a putative leader sequence, intercepted by an 82 bp intron. Production ofrBGLl in E. coli: Recombinant BGLl was overexpressed in __. coli. No apparent β- glucosidase activity could be detected in the E. coli extracts, however SDS- PAGE analysis revealed a relatively intense protein band expressed at the expected molecular weight. Western blot analysis using rabbit polyclonal anti- native BGLl antibodies (AniLab Biological Services, Tal-Sachar, Israel), positively identified the 90 kDa protein band (not shown). Further analysis revealed that the protein was accumulated in inclusion bodies. Several refolding experiments were conducted, however, these efforts to produce active protein from E. coli failed (not shown). Expression of recombinant BGLl in S. cerevisiae and P. pastoris: Recombinant BGLl was successfully expressed both in S. cerevisiae and P. pastoris. In S. cerevisiae a relatively low level of expression was found. The recombinant protein was detected by a Western blot analysis (Figure 6a). The total protein extract of S. cerevisiae expressing bgll cDNA had a β-glucosidase activity of 1.9 units/mg protein. No β-glucosidase activity was detected in control S. cerevisiae, transformed with vector only, under the same assay conditions. However, no protein band corresponding to recombinant BGLl could be detected by coomassie blue staining. P. pastoris transformed with bgll secreted relatively high levels of recombinant BGLl to the medium (about 0.5 g/1) appearing as an almost pure protein in the culture supernatant (Figure 6b). This recombinant enzyme was highly active (124 units/mg protein) and without further purification, yielded specific activity similar to that of the pure native enzyme. 1H-NMR determination of stereochemical outcome: 1H-NMR spectra of a reaction mixture containing pNPGlc and BGLl revealed that the beta anomer of glucose was formed first (H-l = 4.95 ppm), with delayed appearance of the alpha anomer (H-l 5.59 ppm), the consequence of mutarotation (Figure 7). BGLl is indeed, therefore, a retaining glycosidase, as has been observed for other family members (33, 34). Inactivation and reactivation of A. niger ^-glucosidase: Enzyme was incubated in the presence of various concentrations of 2FGlcF and residual enzyme activity was monitored at different time intervals. Enzyme activity decreased in a time-dependent manner, according to pseudo- first order kinetics, allowing the determination of pseudo-first order rate constants: K ; = 4.5 min "1 and K j = 35.4 mM, for inactivation at each inactivator concentration (0, 0.5, 1, 2, 4, and 6 mM, Figure 8). Rates of reactivation of 2-deoxy-2-fluoroglucosyl-BGLl were determined in the presence of different concentrations of linamarin by monitoring activity regain after 0, 10, 20 and 30 min (Figure 9). The regain of activity followed a first order process at each linamarin concentration. Thermal stability of A. niger β-glucosidase: Thermal stability of the recombinant enzyme was evaluated at different temperatures, presented as percent enzymatic activity relative to an enzyme solution kept at 4 °C. Results obtained are summarized in Table 2 and illustrated in Figure 10. The purified enzyme exhibits high thermal stability, as majority (above 50 %) of the activity is maintained at a temperature ranging from 4-60 °C. TABLE 2 Expression of BGLl in tobacco plants: Agrobacterium mediated leaf disc transformation resulted in transgenic tobacco plants as was proved by PCR (Figure 12) for the presence of the transgene, Western blotting (Figures 13a-b) for presence of the protein and activity assays (Figures 14 and 15) for presence of protein activity. Table 3 below summarizes the results. TABLE 3 Of the 29 PCR positive regenerates transformed with cDNA encoding BGLl, which fails to encode a signal peptide, only in 4 the BGLl protein was detectable via Western blotting, however no BGLl activity was measurable in any of which. The BGLl was found smaller in molecular weight compared to wild type A. niger beta-glucosidase and of processed recombinant BGLl containing a signal peptide. Its apparent size of about 95 kDa is very close to 92 kDa which is the calculated molecular weight of the un-glycosylated A. niger beta-glucosidase. This result coincides with the fact that a protein with no signal peptide is expected to be released from the ribosomes and remain in the cytoplast (42) un-glycosylated, as protein glycosylation is conducted in the lumen of the endoplasmic reticulum (43). Of the 9 PCR positive regenerates transformed with a cDNA encoding the BGLl and a Cell signal peptide and in addition encodes the HDEL ER retaining peptide, all plants expressed detectable amounts of BGLl protein and activity. Of the 23 PCR positive regenerates transformed with a cDNA which encodes the BGLl protein and the Cell signal peptide but not the HDEL ER retaining peptide, 18 plants expressed detectable amounts of BGLl protein and activity. The effect of BGLl on flavor compound evolution and composition in transgenic tobacco plants: Extracts of transgenic plants (CB14 and CBT21 containing similar BGLl activity, see Figure 15) were incubated for 9 hours at 37 °C, and flavor compounds were analyzed by SPMI-GC/MS. The results, which are summarized in Table 4 below, show that with the exception of oleyl alcohol, the concentration of different flavor compounds is increased in transgenic plants expressing active BGLl compared with the control. Furthermore, it seems that compartmentalization of BGLl in the ER (or for that matter, any other subcellular organelle), rather then its secretion to the apoplast, results in higher release of flavor compounds. It is likely that this is resulted from the localization many flavor compounds in the apoplast, thus, secretion of BGLl to the apoplast cause in vivo release of flavor compounds, while compartmentalization of BGLl in the ER results in release of flavor compounds only in the event of cell disruption and decompartmentalization. TABLE 4 Retention Time Scan Name CB14 CBT 21 (minutes) 3.917 419 Hexanal 4.749 508 3-methyl-ρentanoic acid 4.863 520 2-Hexenal 5.167 552 ? + 6.564 702 1-Heptanol 7.1 752 ? ++ d 8.085 865 2-ethyl-1-pexanol + 8.132 870 Limonene ++ + 8.194 877 2-methyl-p enol + 10.653 1139 Menthol + 11.757 1258 Nerol + 12.039 1288 6-Quinolinol + 12.1 1294 2-butyl-1-octanol + 13.0 1458 ? + 13.7 1466 ? + 14.091 1507 Vitispirane + 14.094 1516 4-[2,6,6-trimethyl-1 -cyclohexen-1 -yl] 3-Buteπ-1-one ++ 15.985 1710 ? 19.327 2069 Oleyl alcohol CB14 - transgenic plant containing Cell signal peptide + BGLl; CBT 21 - transgenic plant containing Cell signal peptide + BGLl + HDEL ER retaining peptide. a - "-" means no significant difference in concentration compared with wile type, b - "+" means significant increase compared with the wild type, c - "— " means significant decrease compared with the wild type, d - "++" means significant increase compared with a respective mark "+". ? - unknown compound. Although the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and broad scope of the appended claims. All publications, patents, patent applications and sequences identified by GenBank accession numbers mentioned in this specification are herein incoφorated in their entirety by reference into the specification, to the same extent as if each individual publication, patent, patent application or sequence was specifically and individually indicated to be incoφorated herein by reference. In addition, citation or identification of any reference in this application shall not be construed as an admission that such reference is available as prior art to the present invention. REFERENCES: 1. Bause, E., and Legler, G. (1974) Hoppe-Seyler's Z. Physiol. Chem. 355, 438-442. 2. Beguin, P., and Aubert, J. P. (1994) FEMS Microbiol. Rev. 13, 25-58. 3. Rombouts, F. M. and Pilnik, W. (1978), Proc. Biochem. 8, 9-13. 4. Sternberg, D., Vijayakumar, P. and Reese, E. T. (1977), Can. J. Microbiol. 23, 139-147. 5. Woodward, J. and Wiseman, A. (1982), Enz. Microbiol. Technol. 4, 73-79. 6. Kerns, G., Dalchow, E., Klappach, G. and Meyer, D. (1986), Acta. Biotechnol. 6(4), 355-359. 7. Shoseyov, O., Bravdo, B., Ikan, R. and Chet, I. (1988), Phytochem. 27(7), 1973-1976. 8. Shoseyov, O., Bravdo, B., Siegel, D., Goldman, A., Cohen, S. and Shoseyov, L. (1990), J. Agric. Food. Chem. 39, 1387-1390. 9. Dekker, R. F. H. (1986), Biotechnol. Bioengin. 26, 1438-1442. 10. Kitpreechavanich, V. M., Hayashi, M. and Nagai, S. (1986), Agric. Biol. Chem. 50, 1703-1711. l l . Yeoh, H. H., Tan, T. K. and Koh, S. K. (1986), Appl. Microbiol. Biotechnol. 25, 25-28. 12. Crouzet, J. and Chassagne, D. (1999) in Naturally Occurring Glycosides (Ikan. R., ed.) pp.225-274, Wiley Press. 13. Prade, H., L.F. Mackenzie and S.G. Withers (1998) Carbohyd. Res. 305, 371-381. 14. Zarevucka M., Vacek, M., Wimmer, Z., Demnerova, K. and Mackova, M. (1998) Chirality 10, 676-68. 15. Yi, Q., Sarney, D.B., Khan, J.A. and Vulfson, E.N. (1998) Biotechnol. Bioeng. 60, 385-390. 16. McCleary, B. V. and Harrington, J. (1988), Methods Enzymology, 160, 575-583. 17. Watanabe, T., Sato, T., Yoshioka, S., Koshijima, T. and Kuwahara, M. (1992), Eur. J. Biochem. 209, 651-659. 18. Unno, T., Ide, K., Yazaki, T., Tanaka, Y., Nakakuki, T. and Okada, G. (1993), Biosci. Biotech. Biochem. 57(12), 2172-2173. 19. Le Traon, M. M. P., and Pellerin P., (1998) Enz. Micro. Technol., 22 (5) 374-382. 20. Penttila, M. E., Nevalainen, H. K. M., Raynal, A., and Knowles, J. K. C., (1984), Mol. Gen. Genet. 194, 494-499. 21. Henrissat, B., and Bairoch, A. (1996) Biochem. J. Lett. 316, 695-696 22. Sinnott, M. L. (1990) Chem. Rev. 90, 1171-1202. 23. McCarter, J., and Withers, S. G. (1994) Curr. Opin. Struct. Biol. 4, 885-892. 24. Davies, G., Sinnott, M. L., and Withers, S. G. (1998) in CBiological Catalysis (Sinnott, M. L., ed.) Vol. 1, pp. 119-208, Academic Press. 25. Legler, G., Roeser, K. R., and Illig, H. K. (1979) Eur. J. Biochem. 101, 85-92. 26. Roeser, K. R., and Legler, G. (1981) Biochem. Biophys. Acta. 657, 321-333. 27. Legler, G., Sinnott, M. L., and Withers, S. G. (1980) J. Chem. Soc, Perkin Trans. 11 , 1376-1383. 28. Cleveland, D. W., Fischer, S. G., Kirschner, M. W. and Laemmli, U.K. (1977), J. Biol. Chem. 252(3), 1102-1106. 29. Murray, M. G. and Thompson, W. F. (1980), Nucl. Acids Res. 8, 4321- 4325. 30. Ito, H., Fukuda, Y., Murata, K. and Kimura, A. (1983), J. Bacteriol. 153(1), 163-168. 31. Wong, A.W., He, S., Grubb, J. H., Sly, W. S. and Withers S. G. (1998), J. Biol. Chem. 273, 34057-34062. 32. Lipman, D. J. and Pearson, W. R. (1985) Science, 227, 1435-1441. 33. Legler, G. (1968) Hoppe-Zeyler's Z. Physiol. Chem. 349, 767-774. 34. Hrmova, M. and Fincher, G. B. (1996) J. Biol. Chem. 271, 5277-5286. 35. Iwashita K., Todoroki, ML, Kimura, H., Shimoi, H. and Ito, K. (1998) Biosci. Biotechnol. and Biochem. 62 (10), 1938-1946. 36. Rlow E. and Lane D. (1988) Antibodies, a laboratory manual. Cold Spring Harbor Laboratory, Cold Spring Harbor, New York. 37. Siegel D., Marton I., Dekel M., Bravdo B.A., He S.M., Wither S.G., Shoseyov O., 2000, Cloning, expression, characterization, and nucleophile identification of family 3, Aspergillus niger beta-glucosidase. J. Biol. Chem. 275(7),4973-4980. 38. Engelen F.A.V., Molthoff J. W., Conner A. J., Nap J. P., Pereira A and Stiekema W., 1995, pBINPLUS: an improved plant transformation vector based on ρBIN19. Transgenic Res. 4, 288-290. 39. Shani Z., Dekel M., Tsabary G., Shoseyov., 1997, Cloning and characterization of elongation specific endo-1,4- beta-glucanase (Cell) from Arabidopsis thaliana, Plant Molecular Biol. 34: 837-842. 40. Nagy F., Kay S.A.K., Chua N. H., 1989, Analysis of gene expression in transgenic plants. In Plant Molecular Biology: manual edited by Gelvin et al., Kluwer Academic Publishers. 41. Clark T. J., Bunch J. E., 1997, Qualitative and quantitative analysis of flavor additives on tobacco products using SPME-GC-Mass Spectroscopy. J. Aagric. Food Chem. 45 (3),844-849. 42. Lewin B., 1994, The apparatus for protein localization, In Gene V, Oxford University Press. Pρ:279-314. 43. Prodi J. A., 2000, The role of N-oligosaccharide endoplasmic reticulum processing reactions in glycoprotein folding and degradation. Biochem. J., 348, 1-13.

Claims: WHAT IS CLAIMED IS: 1. An isolated nucleic acid comprising a genomic, complementary or composite polynucleotide being derived from Aspergillus niger, encoding a polypeptide having a β-glucosidase catalytic activity. 2. The isolated nucleic acid of claim 1, wherein said polynucleotide is as set forth in SEQ ID NOs:l, 3 or a portion thereof. 3. A nucleic acid construct comprising the isolated nucleic acid of claim 1. 4. The nucleic acid construct of claim 3, further comprising at least one cis acting control element for regulating expression of said polynucleotide. 5. A host cell comprising the nucleic acid construct of claim 3. 6. The host cell of claim 5, wherein the host cell is selected from the group consisting of a prokaryotic cell and a eukaryotic cell. 7. The host cell of claim 6, wherein said prokaryotic cell is E. coli. 8. The host cell of claim 6, wherein said eukaryotic cell is selected from the group consisting of a yeast cell, a fungous cell, a plant cell and an animal cell. 9. An organism comprising the nucleic acid construct of claim 3. 10. The organism of claim 9, wherein the organism is a plant. 11. A recombinant protein comprising an Aspergillus niger derived polypeptide having a β-glucosidase catalytic activity. 12. The recombinant protein of claim 11, wherein said polypeptide is as set forth in SEQ ID NO: 2 or a portion thereof having said β-glucosidase catalytic activity. 13. A method of producing recombinant β-glucosidase, the method comprising the step of introducing, in an expressible form, a nucleic acid construct into a host cell, said nucleic acid construct including a genomic, complementary or composite polynucleotide being derived from Aspergillus niger, encoding a polypeptide having a β-glucosidase catalytic activity. 14. The method of claim 13, further comprising the step of extracting said polypeptide having said β-glucosidase catalytic activity. 15. The method of claim 13, wherein said host cell is selected from the group consisting of a prokaryotic cell and a eukaryotic cell. 16. The method of claim 15, wherein said prokaryotic cell is E. coli. 17. The method of claim 15, wherein said eukaryotic cell is selected from the group consisting of a yeast cell, a fungous cell, a plant cell and an animal cell. 18. The method of claim 13, wherein said polypeptide is as set forth in SEQ ID NO: 2 or a portion thereof having said β-glucosidase catalytic activity. 19. The method of claim 13, wherein said polynucleotide is as set forth in SEQ ID NOs: 1, 3 or a portion thereof. 20. A method of producing a recombinant β-glucosidase overexpressing cell, the method comprising the step of introducing, in an overexpressible form, a nucleic acid construct into a host cell, said nucleic acid construct including a genomic, complementary or composite polynucleotide being derived from Aspergillus niger, encoding a polypeptide having a β- glucosidase catalytic activity. 21. The method of claim 20, wherein said host cell is selected from the group consisting of a prokaryotic cell and a eukaryotic cell. 22. The method of claim 21 , wherein said prokaryotic cell is E. coli. 23. The method of claim 21, wherein said eukaryotic cell is selected from the group consisting of a yeast cell, a fungous cell, a plant cell and an animal cell. 24. The method of claim 20, wherein said polypeptide is as set forth in SEQ ID NO: 2 or a portion thereof having said β-glucosidase catalytic activity. 25. The method of claim 20, wherein said polynucleotide is as set forth in SEQ ID NOs: 1, 3 or a portion thereof. 26. A method of increasing a level of at least one fermentation substance in a fermentation product, the method comprising the step of fermenting a glucose containing fermentation starting material by a yeast cell overexpressing a nucleic acid construct including a genomic, complementary or composite polynucleotide being derived from Aspergillus niger, encoding a polypeptide having a β-glucosidase catalytic activity, thereby increasing the level of the at least one fermentation substance in the fermentation product. 27. The method of claim 26, wherein said polypeptide is as set forth in SEQ ID NO: 2 or a portion thereof having said β-glucosidase catalytic activity. 28. The method of claim 26, wherein said polynucleotide is as set forth in SEQ ID NOs: 1, 3 or a portion thereof. 29. A method of increasing a level of at least one fermentation substance in a fermentation product, the method comprising the step of fermenting a plant derived glucose containing fermentation starting material by a yeast cell, said plant overexpressing a nucleic acid construct including a genomic, complementary or composite polynucleotide being derived from Aspergillus niger, encoding a polypeptide having a β-glucosidase catalytic activity, thereby increasing the level of the at least one fermentation substance in the fermentation product. 30. The method of claim 29, wherein said polypeptide as set forth in SEQ ID NO: 2 or a portion thereof having said β-glucosidase catalytic activity. 31. The method of claim 29, wherein said polynucleotide is as set forth in SEQ ID NOs: 1, 3 or a portion thereof. 32. A method of increasing a level of at least one aroma substance in a plant derived product, the method comprising the step of incubating a glucose containing plant starting material with a yeast cell overexpressing a nucleic acid construct including a genomic, complementary or composite polynucleotide being derived from Aspergillus niger, encoding a polypeptide having a β-glucosidase catalytic activity, thereby increasing the level of the at least one aroma substance in the plant derived product. 33. The method of claim 32, wherein said plant derived product is a fermentation product. 34. The method of claim 32, wherein said polypeptide is as set forth in SEQ ID NO: 2 or a portion thereof having said β-glucosidase catalytic activity. 35. The method of claim 32, wherein said polynucleotide is as set forth in SEQ ID NOs: 1, 3 or a portion thereof. 36. A method of increasing a level of at least one aroma substance in a plant derived product, the method comprising the step of incubating a glucose containing plant starting material with a yeast cell, said plant overexpressing a nucleic acid construct including a genomic, complementary or composite polynucleotide being derived from Aspergillus niger, encoding a polypeptide having a β-glucosidase catalytic activity, thereby increasing the level of the at least one aroma substance in the plant derived product. 37. The method of claim 36, wherein said plant derived product is a fermentation product. 38. The method of claim 36, wherein said polypeptide is as set forth in SEQ ID NO: 2 or a portion thereof having said β-glucosidase catalytic activity. 39. The method of claim 36, wherein said polynucleotide is as set forth in SEQ ID NOs: 1, 3 or a portion thereof. 40. A method of increasing a level of free glucose in a glucose containing fermentation starting material, the method comprising the step of fermenting the glucose containing fermentation starting material by a cell overexpressing a nucleic acid construct including a genomic, complementary or composite polynucleotide being derived from Aspergillus niger, encoding a polypeptide having a β-glucosidase catalytic activity, thereby increasing the level of the free glucose in the glucose containing fermentation starting material. 41. The method of claim 40, wherein said cell is selected from the group consisting of a prokaryotic cell and a eukaryotic cell. 42. The method of claim 41, wherein said prokaryotic cell is E. coli. 43. The method of claim 41, wherein said eukaryotic cell is selected from the group consisting of a yeast cell, a fungous cell, a plant cell and an animal cell. 44. The method of claim 40, wherein said polypeptide is as set forth in SEQ ID NO: 2 or a portion thereof having said β-glucosidase catalytic activity. 45. The method of claim 40, wherein said polynucleotide is as set forth in SEQ ID NOs: 1, 3 or a portion thereof. 46. A method of increasing a level of free glucose in a plant derived glucose containing fermentation starting material, the method comprising the step of fermenting the plant derived glucose containing fermentation starting material by a cell, said plant overexpressing a nucleic acid construct including a genomic, complementary or composite polynucleotide being derived from Aspergillus niger, encoding a polypeptide having a β-glucosidase catalytic activity, thereby increasing the level of the free glucose in the plant. 47. The method of claim 46, wherein said polypeptide is as set forth in SEQ ID NO: 2 or a portion thereof having said β-glucosidase catalytic activity. 48. The method of claim 46, wherein said polynucleotide is as set forth in SEQ ID NOs: 1, 3 or a portion thereof. 49. A method of increasing a level of free glucose in a plant, the method comprising the step of overexpressing in the plant a nucleic acid construct including a genomic, complementary or composite polynucleotide being derived from Aspergillus niger, encoding a polypeptide having a β- glucosidase catalytic activity, thereby increasing the level of the free glucose in the plant. 50. The method of claim 49, wherein said polypeptide is as set forth in SEQ ID NO: 2 or a portion thereof having said β-glucosidase catalytic activity. 51. The method of claim 49, wherein said polynucleotide is as set forth in SEQ ID NOs: 1, 3 or a portion thereof. 52. A method of producing an alcohol, the method comprising the step of fermenting a glucose containing fermentation starting material by a cell overexpressing a nucleic acid construct including a genomic, complementary or composite polynucleotide being derived from Aspergillus niger, encoding a polypeptide having a β-glucosidase catalytic activity, and extracting the alcohol therefrom. 53. The method of claim 52, wherein said cell is selected from the group consisting of a prokaryotic cell and a eukaryotic cell. 54. The method of claim 53, wherein said prokaryotic cell is E. coli. 55. The method of claim 53, wherein said eukaryotic cell is selected from the group consisting of a yeast cell, a fungous cell, a plant cell and an animal cell. 56. The method of claim 52, wherein said polypeptide is as set forth in SEQ ID NO: 2 or a portion thereof having said β-glucosidase catalytic activity. 57. The method of claim 52, wherein said polynucleotide is as set forth in SEQ ID NOs: 1, 3 or a portion thereof. 58. A method of producing an alcohol, the method comprising the step of fermenting a plant derived glucose containing fermentation starting material by a cell, said plant overexpressing a nucleic acid construct including a genomic, complementary or composite polynucleotide being derived from Aspergillus niger, encoding a polypeptide having a β-glucosidase catalytic activity, and extracting the alcohol therefrom. 59. The method of claim 58, wherein said polypeptide is as set forth in SEQ ID NO: 2 or a portion thereof having said β-glucosidase catalytic activity. 60. The method of claim 58, wherein said polynucleotide is as set forth in SEQ ID NOs: 1, 3 or a portion thereof. 61. A method of producing an aroma spreading plant, the method comprising the step of overexpressing in the plant a nucleic acid construct including a genomic, complementary or composite polynucleotide being derived from Aspergillus niger, encoding a polypeptide having a β-glucosidase catalytic activity, thereby increasing aroma spread from the plant. 62. The method of claim 61, wherein overexpressing said nucleic acid construct is performed in a tissue specific manner. 63. The method of claim 61, wherein overexpressing said nucleic acid construct is limited to at least one tissue selected from the group consisting of flower, fruit, seed, root, stem, pollen and leaves. 64. The method of claim 61, wherein said polypeptide is as set forth in SEQ ID NO: 2 or a portion thereof having said β-glucosidase catalytic activity. 65. The method of claim 61, wherein said polynucleotide is as set forth in SEQ ID NOs: 1, 3 or a portion thereof. 66. An isolated nucleic acid comprising a genomic, complementary or composite polynucleotide encoding a polypeptide having a β-glucosidase catalytic activity and further encoding a signal peptide in frame with said polypeptide. 67. The isolated nucleic acid of claim 66, further encoding an endoplasmic retaining sequence in frame with said polypeptide. 68. A nucleic acid construct comprising the isolated nucleic acid of claim 66. 69. The nucleic acid construct of claim 68, further comprising at least one cis acting control element for regulating expression of said polynucleotide. 70. A host cell comprising the nucleic acid construct of claim 68. 71. The host cell of claim 70, wherein the host cell is of a plant. 72. The host cell of claim 70, wherein the host cell is of a yeast. 73. The host cell of claim 70, wherein the host cell is of an animal. 74. An organism comprising the isolated nucleic acid of claim 66 as an inter or intra chromosomal genetic element. 75. The organism of claim 74, wherein the organism is a plant. 76. A recombinant protein comprising a polypeptide having a β- glucosidase catalytic activity and a signal peptide fused thereto. 77. The recombinant protein of claim 76, further comprising an endoplasmic reticulum retaining peptide fused thereto. 78. A method of producing recombinant β-glucosidase, the method comprising the step of introducing, in an expressible form, a nucleic acid construct into a host cell, said nucleic acid construct including a genomic, complementary or composite polynucleotide encoding a polypeptide having a β-glucosidase catalytic activity and a signal peptide being in frame with said polypeptide. 79. The method of claim 78, wherein said polynucleotide further encodes an endoplasmic reticulum retaining peptide being in frame with said polypeptide. 80. The method of claim 78, further comprising the step of extracting said polypeptide having said β-glucosidase catalytic activity. 81. The method of claim 78, wherein said host cell is a plant cell. 82. The method of claim 78, wherein said host cell is an animal cell. 83. The method of claim 78, wherein said host cell is a yeast cell. 84. A method of producing a recombinant β-glucosidase overexpressing cell, the method comprising the step of introducing, in an overexpressible form, a nucleic acid construct into a host cell, said nucleic acid construct including a genomic, complementary or composite polynucleotide encoding a polypeptide having a β-glucosidase catalytic activity and further encoding a signal peptide being in frame with said polypeptide. 85. The method of claim 84, wherein said polynucleotide further encodes an endoplasmic reticulum retaining peptide being in frame with said polypeptide. 86. The method of claim 84, wherein said host cell is a plant cell. 87. The method of claim 84, wherein said host cell is a yeast cell. 88. The method of claim 84, wherein said host cell is an animal cell. 89. A method of increasing a level of at least one fermentation substance in a fermentation product, the method comprising the step of fermenting a plant derived glucose containing fermentation starting material by a yeast cell, said plant overexpressing a nucleic acid construct including a genomic, complementary or composite polynucleotide encoding a polypeptide having a β-glucosidase catalytic activity and further encoding a signal peptide being in frame with said polypeptide, thereby increasing the level of the at least one fermentation substance in the fermentation product. 90. The method of claim 89, wherein said polynucleotide further encodes an endoplasmic reticulum retaining peptide being in frame with said polypeptide. 91. A method of increasing a level of at least one aroma substance in a plant derived product, the method comprising the step of incubating a glucose containing plant starting material with a yeast cell, said plant overexpressing a nucleic acid construct including a genomic, complementary or composite polynucleotide encoding a polypeptide having a β-glucosidase catalytic activity and further encoding a signal peptide being in frame with said polypeptide, thereby increasing the level of the at least one aroma substance in the plant derived product. 92. The method of claim 91, wherein said polynucleotide further encodes an endoplasmic reticulum retaining peptide being in frame with said polypeptide. 93. The method of claim 91, wherein said plant derived product is a fermentation product. 94. A method of increasing a level of free glucose in a glucose containing fermentation starting material, the method comprising the step of fermenting the glucose containing fermentation starting material by a cell overexpressing a nucleic acid construct including a genomic, complementary or composite polynucleotide encoding a polypeptide having a β-glucosidase catalytic activity and further encoding a signal peptide being in frame with said polypeptide, thereby increasing the level of the free glucose in the glucose containing fermentation starting material. 95. The method of claim 94, wherein said polynucleotide further encodes an endoplasmic reticulum retaining peptide being in frame with said polypeptide. 96. A method of increasing a level of free glucose in a plant derived glucose containing fermentation starting material, the method comprising the step of fermenting the plant derived glucose containing fermentation starting material by a cell, said plant overexpressing a nucleic acid construct including a genomic, complementary or composite polynucleotide encoding a polypeptide having a β-glucosidase catalytic activity and further encoding a signal peptide being in frame with said polypeptide, thereby increasing the level of the free glucose in the plant. 97. The method of claim 96, wherein said polynucleotide further encodes an endoplasmic reticulum retaining peptide being in frame with said polypeptide. 98. A method of increasing a level of free glucose in a plant, the method comprising the step of overexpressing in the plant a nucleic acid construct including a genomic, complementary or composite polynucleotide encoding a polypeptide having a β-glucosidase catalytic activity and further encoding a signal peptide being in frame with said polypeptide, thereby increasing the level of the free glucose in the plant. 99. The method of claim 98, wherein said polynucleotide further encodes an endoplasmic reticulum retaining peptide being in frame with said polypeptide. 100. A method of producing an alcohol, the method comprising the step of fermenting a glucose containing fermentation starting material by a cell overexpressing a nucleic acid construct including a genomic, complementary or composite polynucleotide encoding a polypeptide having a β-glucosidase catalytic activity and further encoding a signal peptide being in frame with said polypeptide, and extracting the alcohol therefrom. 101. The method of claim 100, wherein said polynucleotide further encodes an endoplasmic reticulum retaining peptide being in frame with said polypeptide. 102. A method of producing an alcohol, the method comprising the step of fermenting a plant derived glucose containing fermentation starting material by a cell, said plant overexpressing a nucleic acid construct including a genomic, complementary or composite polynucleotide encoding a polypeptide having a β-glucosidase catalytic activity and further encoding a signal peptide being in frame with said polypeptide, and extracting the alcohol therefrom. 103. The method of claim 102, wherein said polynucleotide further encodes an endoplasmic reticulum retaining peptide being in frame with said polypeptide. 104. A method of producing an aroma spreading plant, the method comprising the step of overexpressing in the plant a nucleic acid construct including a genomic, complementary or composite polynucleotide encoding a polypeptide having a β-glucosidase catalytic activity and further encoding a signal peptide being in frame with said polypeptide, thereby increasing aroma spread from the plant. 105. The method of claim 104, wherein said polynucleotide further encodes an endoplasmic reticulum retaining peptide being in frame with said polypeptide. 106. The method of claim 104, wherein overexpressing said nucleic acid construct is performed in a tissue specific manner. 107. The method of claim 104, wherein overexpressing said nucleic acid construct is limited to at least one tissue selected from the group consisting of flower, fruit, seed, root, stem, pollen and leaves.

Inventors: BEN-AMI BRAVDO, DEKEL MARA, MARTON IRA, SHOSEYOV ODED, SHU WEI, SIEGEL DANIEL L, BEN-AMI, BRAVDO, DEKEL, MARA, MARTON, IRA, SHOSEYOV, ODED, SHU, WEI, SIEGEL, DANIEL, L.
Applicants: BEN AMI BRAVDO, DEKEL MARA, MARTON IRA, SHOSEYOV ODED, SHU WEI, SIEGEL DANIEL L, YISSUM RES DEV CO, BEN-AMI, BRAVDO, DEKEL, MARA, MARTON, IRA, SHOSEYOV, ODED, SHU, WEI, SIEGEL, DANIEL, L., YISSUM RESEARCH DEVELOPMENT COMPANY OF THE HEBREW
Application year: 2000
Main classification: C12N
IPC codes: C12N9, C12N15, C12P19