Abstract:
Methods and means for producing xylan structures in plants having a non-native saccharide moiety substitution side chain component, plants and plant cells comprising modified xylan structures, methods of identifying mutant plants comprising xylan structures in plants having a non-native saccharide moiety substitution pattern side chain component, uses thereof, and isolated xylan structures and uses thereof.

Description:
FIELD OF INVENTION 
     The present invention relates to methods for screening for altered xylan saccharide side chain substitution components and altered xylan saccharide side chain substitution enzymic activity and/or patterning in plant cell material, transformed plant cell material and methods for producing modified xylans in plant cell material. 
     In particular, the invention relates to methods for screening for modified xylan saccharide side chain substitution activity in plants, transformed plant cell material comprising xylan molecules having altered 4-O-methyl glucuronic acid and/or glucuronic acid side chain content wherein the activity of xylan glucuronyl transferase (XGAT) enzymes is altered, methods for producing modified xylans comprised in plant cell material, the genetic material required therefor, such as DNA and RNA, vectors, host cells, methods of introduction of genetic material into plant cells, and uses thereof. 
     BACKGROUND OF INVENTION 
     For the purposes of the present invention “glucuronic acid” and “4-O-methyl glucuronic acid” are referred to hereinbelow by the terms “GlcA” and “MeGlcA”, respectively. The term “[Me]GlcA” is used as a collective noun herein to denote both GlcA and MeGlcA. 
     Native xylan is a hemicellulose that is often cross-linked to lignin via [Me]GlcA side chains (e.g. Balakshin et al. 2007, Holzforschung, 61, 1-7). Native xylan is also tightly associated with cellulose microfibrils. Native xylan is of value in certain industries but its extraction is complicated by a range of covalent and non-covalent chemical linkages to other components of the cell wall, such as linkages to [Me]GlcA. Typically, enzymatic and chemical means are used inter alia, to disrupt the side chain linkages, enabling the extraction of xylan. Extraction processes are generally costly and give rise to side products that are undesirable. 
     Enzymatic methods are also used to depolymerise the cellulose and hemicelluloses into soluble hexose and pentose sugars. Depolymerisation (also referred to in the art as “saccharification”) of xylan requires a multiplicity of enzymes to break the backbone and side chain linkages. Some of the products of enzymatic treatments are not able to be used by many organisms used in fermentation or in bio-processing to produce liquid transport fuels such as ethanol or butanol. 
     Extracted native xylan is used inter alia in paper production and modified xylan-containing plant material has potential for use in, inter alia, the production of sugars and indirectly, in the production of liquid transport fuels such as ethanol via fermentation of the sugars. Cellulose fibrils, used inter alia in paper and other materials, are damaged by the processes of extraction of xylan and other hemicellulose from the fibrils. 
     The prior art appears to be silent about the precise role that xylan saccharide side chain modifying enzymes, such as XGAT enzymes, play in secondary cell wall structure, and their function does not appear to have been accurately elucidated. 
     Brown D. M. et al The Plant Cell, Vol. 17, 2281-2295, August 2005 describes a study relating to mutant  Arabidopsis thaliana  plants comprising insertions into a so-called glycosyl transferase 8-like gene, identified as At3g18660, which allegedly resulted in a plant that had a weak stem, a feature known to be a characteristic of known secondary cell wall mutants. However, the elucidation of the function of At3g18660 and the role it plays in secondary cell wall synthesis does not appear to be described by Brown D. M. et al. Indeed, At3g18860 is not described as a xylan glucuronyl transferase and no putative industrial use for At3g18660 is contemplated. 
     Pena M. J. et al The Plant Cell, Vol. 19:549-563, February 2007 describe studies on mutant plants of  Arabidopsis thaliana  in which the presence of “wood-associated GTs”, such as IRX8 are present. The study is confined to looking at IRX8 and IRX9 genes and their effect on glucuronoxylan chemistry and structure.  Arabidopsis thaliana  genes At4g33330 and At3g18660 are mentioned as homologs (sic) of “wood associated GTs” but that appears to be all that Pena M. J. et al supra say about them. 
     Zhong R. et al The Plant Cell Vol. 17, 3390-3408, December 2005 describe studies on inter alia mutant plants of  Arabidopsis thaliana  Fragile Fiber8 which is thought by the authors to encode a glucuronyl transferase that is involved in secondary wall synthesis. Zhong R et al report that the GlcA component of xylan is missing. It is not reported that the level of overall [Me]GlcA substitution is unchanged. The data presented by Zhong R et al indicates that they were unable to distinguish if the level of overall [Me]GlcA substitution had changed. In wild type plants, the xylose in the xylan is about 7% substituted with MeGlcA and 3% with GlcA; 10% overall. Thus, the overall proportion of xylose substitution is no different to wild type plants. The overall substitution pattern is unlike the new xylan structures created using the XGATs of the invention in which the absolute substitution level of [Me]GlcA on the xylans created using At4g33330 and/or At3g18660 can be varied depending on the level of expression or the level of gene silencing that may be generated. 
     SUMMARY OF INVENTION 
     Plants that are produced by methods of the invention have xylans comprising a novel [Me]GlcA composition, that is to say the sum of the MeGlcA and GlcA is different to that of wild type plants or mutant plants described in the prior art. 
     Furthermore, plants comprising modified xylans of the invention may be more amenable to end uses such as paper pulp production; animal feeds for herbivorous animals, such as domesticated animals; improved dietary fibre component of food for human consumption; use as biomass for liquid fuel production; and in the extraction of plant products of interest, such as sugars that may be readily converted to fermentable sugars, since the extraction of plant products may be simpler than that of extraction processes used on conventional plants. These and other advantages of plants of the invention comprising modified xylans will become apparent from the following description and examples. 
     According to the present invention there is provided a transformed plant cell comprising a xylan structure having a non-native saccharide moiety substitution pattern side chain component, such as a [Me]GlcA side chain component. 
     Also encompassed within the ambit of the invention are transformed plants, transformed plant parts, or transformed plant cells that may be derived from a transformed plant tissue of the invention, such as transformed callus, transformed somatic embryo, transformed pre-embryogenic masses, transformed root tip cultures and the like. 
     “Non-native” in the context of the present invention means that the xylan structure has a modified saccharide moiety substitution side chain component, such as a [Me]GlcA side chain component that does not or is not known to occur in a native plant of the same species as that of the transformed plant cell or transformed plant. “Non-native” as applied to mutant plants known in the art, that is to say a mutant plant of the same species that has not been transformed using classical DNA or RNA insertion or deletion techniques common to molecular biology, and especially plant molecular biology, has a similar meaning to that described for a transformed plant (see above). Thus, a mutant plant known in the art refers to naturally-occurring mutants and also to mutants that have been created using techniques, such as chemical mutagenesis, e.g using ethylmethanesulphonate (EMS), or physical procedures e.g. γ-ray irradiation, which typically do not employ classical DNA or RNA insertion or deletion techniques known in the art. Mutant plants may be identified, for example, by employing targeting induced local lesions in genome (“TILLING”) procedures as described by Colbert et al Plant Physiol, June 2001, Vol 126, pp. 480-484. In TILLING procedures DNA is collected from populations of plants which possess random point mutations which may be as a result of natural variation or via artificial induction by man. By selectively pooling the DNA samples and amplifying them with labelled primers, mismatched heteroduplexes form between the wild type DNA strands and the mutant DNA strands. The mismatched heteroduplexes may then be incubated with an endonuclease that is able to cleave the heteroduplex at mismatched sites and the resultant products are identified, for example on a sequencing gel. By analysing the data, plants can be identified which harbour mutations in genes that are known to be involved in saccharide substitution patterning on xylans or are suspected of being involved in such activity. Such plants may then be investigated for their saccharide substitution patterning on xylans using techniques described herein. 
     Preferably, transformed plant cells of the invention are provided wherein the non-native saccharide moiety substitution side chain component pattern is located on up to 50% of the backbone xylose residues of the xylan structure. Such saccharide side chain components, for example the xylan GlcA and MeGlcA side chains of plants of the invention may be found substituting the backbone xylose residues at a proportion that is different from that which may be found in a corresponding plant of the same species of the prior art. Suitable substitution proportions for saccharide moiety substitution patterning (for example [Me]GlcA), that are achievable by the present invention include up to 50%, up to 30%, up to 20%, such as from 0.001% up to 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, or 15% and so on, provided that the actual novel saccharide moiety substitution patterning component (for example the patterning for the [Me]GlcA component) is not known in the relevant plant species of the prior art. 
     In a further aspect of the invention, there is provided a nucleotide sequence encoding an antisense RNA molecule complementary to a sense mRNA molecule encoding for a protein having an enzymic activity in xylan side chain substitution, which nucleotide sequence is under transcriptional control of a promoter and a terminator, both promoter and terminator being capable of functioning in plant cells. 
     The nucleotide sequence encoding the antisense RNA molecule can be of any length provided that the antisense RNA molecule transcribable therefrom is sufficiently long so as to be able to form a complex with a sense mRNA molecule encoding for a protein having an enzymic activity in xylan side chain substitution. Suitable proteins having enzymic activity in xylan side chain substitution reactions include xylan glucuronyl transferases (XGATs), such as the  Arabidopsis thaliana  sequences, At4g33330 and At3g18660, At1g77130, At1g08990, At1g54940 (from  Arabidopsis thaliana ), PttGT8A, PttGT8B and PttGT8C (from poplar), CAK29728 (partial sequence from  Pica abies , conifer), and other XGAT homologues or orthologues thereof from other species, such as Os03g0184300, Os01g0880200, Os05g0426400, OsI — 010047, AAK92624 (from Rice) AK250038 (from Barley), AY110752, (from Maize), and ABE88903 (From  Medicago truncatula ). Thus, it is thought that the antisense RNA molecule or short sequences derived from it in planta, in vivo, or in vitro, such as short interfering RNA (siRNA), form complexes that are capable of interfering with the mRNA of the protein. Thus, the synthesis of functional protein(s), such as XGAT proteins, having enzymic activity in xylan side chain substitution is prevented or substantially inhibited. As a consequence of the interference of the antisense RNA, enzyme activity of XGAT protein(s) involved in xylan side chain substitution is decreased. 
     For the purposes of the present description “nucleotide sequence” will be referred to as DNA unless there is different indication. The DNA encoding the antisense RNA can be from about 20 nucleotides in length up to the length of the relevant mRNA produced by the cell. The length of the DNA encoding the antisense RNA will preferably be from 20 to 1500, more preferably from 20 to 1000 nucleotides in length. When the interfering antisense RNA is interfering siRNA, the length of the siRNA strand is from 20 to 30 nucleotides in length and may be 20, 21, 22, 23, 24, 25, 26, 27, 28, 29 or 30 bases in length. The preferred source of antisense RNA for DNA constructs of the present invention is DNA showing substantial identity or similarity to the genes or fragments thereof of proteins having XGAT enzymic activity. Thus the encoding DNA of constructs of the present invention may be selected from nucleic acid molecules encoding XGATS, such as proteins selected from the group from the group At4g33330, At3g18660, At1g77130, At1g08990, At1g54940 (from  Arabidopsis thaliana ), PttGT8A, PttGT8B and PttGT8C (from poplar), CAK29728 (partial sequence from  Pica abies , conifer), and other XGAT homologues or orthologues thereof from other species, such as Os03g0184300, Os01g0880200, Os05g 0426400, OsI — 010047, AAK92624 (from Rice) AK250038 (from Barley), AY110752, (from Maize), ABE88903 (from  Medicago truncatula ) and fragments thereof such as enzymically active fragments thereof, or orthologues thereof from other plant species or fragments thereof such as enzymically active fragments thereof. 
     In a further aspect of the invention there is provided a nucleotide sequence (nucleotide sequence according to the invention) comprising a transcriptional regulatory sequence, a sequence under the transcriptional control thereof which encodes an RNA which consists of a plurality of subsequences, characterized in that the RNA subsequences are antisense RNAs to mRNAs of proteins having an enzymic activity in xylan side chain substitution in plant cells. 
     The nucleotide sequence may encode in antisense orientation an RNA having any number of subsequences which may comprise more than one siRNA sequence; may comprise at least one siRNA sequence and at least one longer RNA sequence; may comprise at least one longer RNA sequence. Preferably, the number of subsequences is up to 6, and more preferably from 1 to 3. 
     The nucleotide sequence of the invention also includes complementary sense polynucleotide sequences of the anti-sense sequences of the invention, that when transcribed in plant material, leads to an increase in xylan side chain substitution and hence a disproportionately high overall level of xylan side chain substitution relative to the level found in native plants of the same species or of a corresponding species. The skilled addressee will also appreciate that the over-expression of such sense sequences for XGAT proteins may also elicit siRNA mediated XGAT gene silencing, giving rise to xylan side chain substitution patterns wherein the overall level of xylan side chain substitution is lower than that of native plants of the same species. 
     Preferably, the RNA encoded by the contiguous sequence comprises a cleavage site, such as a ribozyme or restriction enzyme site such as XbaI, SalI, KpnI or the like, between two of the subsequences so that the RNA can be cleaved into regions comprising said subsequences, or even into the subsequences per se. Naturally, the skilled addressee will appreciate that the subsequences contained within the RNA encoded by the contiguous sequence resulting from such cleavage will not contain a 5′ cap or a ribozome binding site and will thus not be translated when present in a eukaryotic cell, such as a plant cell. 
     The invention still further provides a nucleotide sequence which is similar to the above disclosed antisense RNA sequences. By “similar” is meant a test sequence which is capable of hybridising to a sequence which is complementary to the inventive nucleotide sequence. When the test and inventive sequences are double stranded the nucleic acid constituting the test sequence preferably has a Tm within 20° C. of that of the inventive sequence. In the case wherein the test and inventive sequences are mixed together and denatured simultaneously, the Tm values of the sequences are preferably within 10° C. of each other. More preferably the hybridization is performed under stringent conditions, with either the test or inventive DNA preferably being supported. Thus either a denatured test or inventive sequence is preferably first bound to a support and hybridization is effected for a specified period of time at a temperature of between 50° and 70° C. in double strength SSC (2×NaCl 17.5 g/l and sodium citrate (SC) at 8.8 g/l) buffered saline containing 0.1% sodium dodecyl sulphate (SDS) followed by rinsing of the support at the same temperature but with a buffer having a reduced SSC concentration. Depending upon the degree of stringency required, and thus the degree of similarity of the sequences, such reduced concentration buffers are typically single strength SSC containing 0.1% SDS, half strength SSC containing 0.1% SDS and one tenth strength SSC containing 0.1% SDS. Sequences having the greatest degree of similarity are those the hybridization of which is least affected by washing in buffers of reduced concentration. It is most preferred that the test and inventive sequences are so similar that the hybridization between them is substantially unaffected by washing or incubation in one tenth strength sodium citrate buffer containing 0.1% SDS. 
     The invention still further provides a nucleotide sequence which is complementary to one which hybridizes under stringent conditions with the above disclosed nucleotide sequences. 
     The invention still further provides the use of the sequence according to the invention, whether “naked” or present in a DNA construct or biological vector in the production of eukaryotic cells, particularly plant cells having a modified xylan content as described herein. 
     The invention still further provides a method of inducing an under expression of an enzymic protein of xylan side chain substitution in plant cells comprising introducing into such cells a nucleotide sequence according to the invention, or a construct or vector containing it. 
     The invention still further provides a method of inhibiting the production of at least one XGAT enzyme in a eukaryotic cell comprising introducing into the said cell a nucleotide sequence comprising a transcriptional regulatory sequence and a sequence contiguous therewith and under the transcriptional control thereof, which contiguous sequence encodes an RNA which consists of a single subsequence or a plurality of subsequences, characterized in that the subsequence or subsequences have the sequences of antisense RNA&#39;s to mRNA&#39;s of proteins having an enzymic activity in xylan side chain substitution in a plant. 
     Examples of the nucleotide sequences of the invention are provided below. These examples relate to the production of plants, such as  Arabidopsis thaliana  plants having an altered xylan component of the invention. 
     1. The nucleotide sequence of the invention may encode an mRNA which consists—in the 5′ to 3′ direction—of (i) a promoter, (ii) at least one cDNA in reverse orientation i.e. 3′ to 5′ orientation, (III) a terminator, (iv) optionally a further promoter, (v) the coding region of a marker gene, such as GFP and (vi) optionally a further stop codon. When such a sequence is introduced into the cells of plants, the sequence encoding the mRNA is transcribed. The region of the thus transcribed mRNA which encodes the marker gene is translated, whilst the region of the mRNA which encodes the cDNA is not.
 
2. The nucleotide sequence of the invention may encode an mRNA which consists—in the 5′ to 3′ direction—of (i) a promoter, (ii) the coding region of a marker gene, such as GFP, (iii) a translation stop codon, (iv) optionally a further start codon, (v) a region encoding at least one cDNA in reverse orientation i.e. 3′ to 5′ orientation and (vi) optionally a further stop codon. When such a sequence is introduced into the cells of plants, the sequence encoding the mRNA is transcribed. The region of the thus transcribed mRNA which encodes the marker gene is translated, whilst the region of the mRNA which encodes the cDNA in reverse orientation i.e. 3′ to 5′ orientation is not translated.
 
3. The nucleotide sequence of the invention may encode an mRNA which comprises in the 5′ to 3′ direction (i) a promoter, (ii) a cDNA in reverse orientation i.e. 3′ to 5′ orientation, (iii) a terminator, (iv) a promoter, (v) the coding region of a marker gene, such as GFP, (vi) a terminator, (vii) a promoter, (viii) a second cDNA in reverse orientation i.e. 3′ to 5′ orientation, (ix) a terminator. When such a sequence is introduced into the cells of plants, the sequences encoding (ii) and (viii) are transcribed. The region of the thus transcribed mRNA which encodes the marker gene, such as GFP is translated, whilst the regions of the mRNA encoding the cDNA is not.
 
     Suitable cDNAs that may be used in plants and constructs of the invention include XGAT cDNAs, that encode proteins selected from the group At4g33330 and At3g18660 or At1g77130, At1g08990, At1g54940 (from  Arabidopsis thaliana ), PttGT8A, PttGT8B and PttGT8C (from poplar), CAK29728 (partial sequence from  Pica abies , conifer), and other XGAT homologues or orthologues thereof from other species, such as Os03g0184300, Os01g0880200, Os05g 0426400, OsI — 010047, AAK92624 (from Rice) AK250038 (from Barley), AY110752, (from Maize), and ABE88903 (From  Medic ago truncatula ) that are capable of altering saccharide side chain substitution on xylan when introduced into a plant cell comprised in a plant. The anti-sense sequences of At4g33330 and/or At3g18660 may be placed in other plant species such as members of the Brassicaceae, for example, curly kale, cabbages, cauliflowers, broccolis and the like. 
     The cDNA&#39;s encoding a protein of use in the invention, such as, At4g33330 and/or At3g18660, in a vector containing at least one type of promoter that is operable in a plant cell, for example, an inducible or a constitutive promoter operatively linked to a first and/or second nucleic acid sequence or nucleic acid sequence component as herein defined and as provided by the present invention. As discussed, this enables control of expression of the polynucleotide of the invention. The invention also provides plants transformed with polynucleotide sequences or constructs and methods including introduction of such polynucleotide nucleic acid sequences or constructs into a plant cell and/or induction of expression of said first or second nucleic acid sequence or construct within a plant cell, e.g. by application of a suitable stimulus, such as an effective exogenous inducer. 
     The term “inducible” as applied to a promoter is well understood by those skilled in the art. In essence, expression under the control of an inducible promoter is “switched on” or increased in response to an applied stimulus (which may be generated within a cell or provided exogenously). The nature of the stimulus varies between promoters. Some inducible promoters cause little or undetectable levels of expression (or no expression) in the absence of the appropriate stimulus. Other inducible promoters cause detectable constitutive expression in the absence of the stimulus. Whatever the level of expression is in the absence of the stimulus, expression from any inducible promoter is increased in the presence of the correct stimulus. The preferable situation is where the level of expression increases upon application of the relevant stimulus by an amount effective to alter a phenotypic characteristic. Thus an inducible (or “switchable”) promoter may be used which causes a basic level of expression in the absence of the stimulus which level is too low to bring about a desired phenotype (and may in fact be zero). Upon application of the stimulus, expression is increased (or switched on) to a level, which brings about the desired phenotype. One example of an inducible promoter is the ethanol inducible gene switch disclosed in Caddick et al (1998) Nature Biotechnology 16: 177-180. A number of inducible promoters are known in the art. 
     Chemically regulated promoters can be used to modulate the expression of a gene or a polynucleotide sequence of the invention in a plant through the application of an exogenous chemical regulator. Depending upon the objective, the promoter may be a chemically inducible promoter, where application of the chemical induces gene expression, or a chemical-repressible promoter, where application of the chemical represses gene expression. Chemically inducible promoters are known in the art and include, but are not limited to, the maize In2-2 promoter, which is activated by benzenesulfonamide herbicide safeners, the maize GST promoter, which is activated by hydrophobic electrophilic compounds that are used as pre-emergent herbicides, and the tobacco PR-1a promoter, which is activated by salicylic acid. Other chemically regulated promoters of interest include steroid-responsive promoters (see, for example, the glucocorticoid-inducible promoter in Schena et al. (1991)  Proc. Natl. Acad. Sci. USA  88:10421-10425 and McNellis et al. (1998)  Plant J.  14(2):247-257) and tetracycline-inducible and tetracycline-repressible promoters (see, for example, Gatz et al. (1991)  Mol. Gen. Genet.  227:229-237, and U.S. Pat. Nos. 5,814,618 and 5,789,156), herein incorporated by reference. 
     Where enhanced expression of XGAT sequences of the invention (either in antisense orientation or in sense orientation) in particular tissues is desired, tissue-specific promoters can be utilized. Tissue-specific promoters include those described by Yamamoto et al. (1997)  Plant J.  12(2)255-265; Kawamata et al. (1997)  Plant Cell Physiol.  38(7):792-803; Hansen et al. (1997)  Mol. Gen. Genet.  254(3):337-343; Russell et al. (1997)  Transgenic Res.  6(2):157-168; Rinehart et al. (1996)  Plant Physiol.  112(3):1331-1341; Van Camp et al. (1996)  Plant Physiol.  112(2):525-535; Canevascini et al. (1996)  Plant Physiol.  112(2):513-524; Yamamoto et al. (1994)  Plant Cell Physiol.  35(5):773-778; Lam (1994)  Results Probl. Cell Differ.  20:181-196; Orozco et al. (1993)  Plant Mol. Biol.  23(6):1129-1138; Matsuoka et al. (1993)  Proc Natl. Acad. Sci. USA  90(20):9586-9590; and Guevara-Garcia et al. (1993)  Plant J.  4(3):495-505. 
     So-called constitutive promoters may also be used in the methods of the present invention. Constitutive promoters include, for example, CaMV 35S promoter (Odell et al. (1985)  Nature  313:810-812); rice actin (McElroy et al. (1990)  Plant Cell  2:163-171); ubiquitin (Christensen et al. (1989)  Plant Mol. Biol.  12:619-632 and Christensen et al. (1992)  Plant Mol. Biol.  18:675-689); pEMU (Last et al. (1991)  Theor. Appl. Genet.  81:581-588); MAS (Velten et al. (1984)  EMBO J.  3:2723-2730); ALS promoter (U.S. application Ser. No. 08/409,297), and the like. Other constitutive promoters include those in U.S. Pat. Nos. 5,608,149; 5,608,144; 5,604,121; 5,569,597; 5,466,785; 5,399,680; 5,268,463; and 5,608,142. 
     Naturally, the man skilled in the art will appreciate that terminator DNA sequences will be present in constructs used in the invention. A terminator is contemplated as a DNA sequence at the end of a transcriptional unit which signals termination of transcription. These elements are 3′-non-translated sequences containing polyadenylation signals, which act to cause the addition of polyadenylate sequences to the 3′ end of primary transcripts. For expression in plant cells the nopaline synthase transcriptional terminator (A. Depicker et al., 1982, J. of Mol. &amp; Applied Gen. 1:561-573) sequence serves as a transcriptional termination signal. 
     Those skilled in the art are well able to construct vectors and design protocols for recombinant nucleic acid sequences or gene expression. Suitable vectors can be chosen or constructed, containing appropriate regulatory sequences, including promoter sequences, terminator fragments, polyadenylation sequences, enhancer sequences, marker genes and other sequences as appropriate. For further details see, for example,  Molecular Cloning: a Laboratory Manual:  2nd edition, Sambrook et al, 1989, Cold Spring Harbor Laboratory Press. Many known techniques and protocols for manipulation of nucleic acid, for example in preparation of nucleic acid constructs, mutagenesis, sequencing, introduction of DNA into cells and gene expression, and analysis of proteins, are described in detail in  Current Protocols in Molecular Biology , Second Edition, Ausubel et al. eds., John Wiley &amp; Sons, 1992. The disclosures of Sambrook et al. and Ausubel et al. are incorporated herein by reference. Specific procedures and vectors previously used with wide success upon plants are described by Bevan (Nucl. Acids Res. 12, 8711-8721 (1984)) and Guerineau and Mullineaux (1993) (Plant transformation and expression vectors. In: Plant Molecular Biology Labfax (Croy R R D ed.) Oxford, BIOS Scientific Publishers, pp 121-148). 
     Naturally, the skilled addressee will appreciate that each introduced nucleic acid sequence, such as a genomic DNA sequence or a cDNA sequence coding for at least one saccharide moiety side chain component modifying protein, such as an XGAT sense sequence, and designed to over-express XGAT and thereby initiate activation of the siRNA gene silencing mechanisms of a plant cell, that is to say a sequence oriented in the 5′ to 3′ direction behind its promoter (e.g. sense At4g33330 and/or sense At3g18660), will be under regulatory control of its own exogenous promoter and terminator. 
     Selectable genetic markers may facilitate the selection of transgenic plants and these may consist of chimaeric genes that confer selectable phenotypes such as resistance to antibiotics such as kanamycin, neomycin, hygromycin, puramycin, phosphinotricin, chlorsulfuron, methotrexate, gentamycin, spectinomycin, imidazolinones and glyphosate or they may consist of other markers, such as proteins capable of fluorescence, such as green fluorescent protein (GFP). 
     When introducing selected nucleic acid sequences according to the present invention into a cell, certain considerations must be taken into account, well known to those skilled in the art. The nucleic acid to be inserted should be assembled within a construct, which contains effective regulatory elements, which will drive transcription. There must be available a method of transporting the construct into the cell. Once the construct is within the cell membrane, integration into the endogenous chromosomal material either will or will not occur. Finally, as far as plants are concerned the target cell type must be such that cells can be regenerated into whole plants. 
     Plants transformed with DNA segments containing sequences of interest as provided herein may be produced by standard techniques, which are already known for the genetic manipulation of plants. DNA can be transformed into plant cells using any suitable technology, such as a disarmed Ti-plasmid vector carried by  Agrobacterium  exploiting its natural gene transfer ability (EP-A-270355, EP-A-0116718, NAR 12(22) 8711-87215 1984), particle or micro projectile bombardment (U.S. Pat. No. 5,100,792, EP-A-444882, EP-A-434616) microinjection (WO 92/09696, WO 94/00583, EP 331083, EP 175966, Green et al. (1987)  Plant Tissue and Cell Culture , Academic Press), electroporation (EP 290395, WO 8706614) other forms of direct DNA uptake (DE 4005152, WO 9012096, U.S. Pat. No. 4,684,611), liposome mediated DNA uptake (e.g. Freeman et al.  Plant Cell Physiol.  29: 1353 (1984)), or the vortexing method (e.g. Kindle,  PNAS U.S.A.  87: 1228 (1990d) Physical methods for the transformation of plant cells are reviewed in Oard, 1991,  Biotech. Adv.  9: 1-11. 
     Thus once a nucleic acid sequence or gene has been identified, it may be reintroduced into plant cells using techniques well known to those skilled in the art to produce transgenic plants of the appropriate phenotype. 
       Agrobacterium  transformation is widely used by those skilled in the art to transform dicotyledonous species. 
     Production of stable, fertile transgenic plants in almost all economically relevant monocot plants is also now routine: (Toriyama, et al. (1988)  Bio/Technology  6, 1072-1074; Zhang, et al. (1988)  Plant Cell Rep.  7, 379-384; Zhang, et al. (1988)  Theor. Appl. Genet.  76, 835-840; Shimamoto, et al. (1989)  Nature  338, 274-276; Datta, et al. (1990)  Bio/Technology  8, 736-740; Christou, et al. (1991)  Bio/Technology  9, 957-962; Peng, et al. (1991) International Rice Research Institute, Manila, Philippines 563-574; Cao, et al. (1992)  Plant Cell Rep.  11, 585-591; Li, et al. (1993)  Plant Cell Rep.  12, 250-255; Rathore, et al. (1993)  Plant Molecular Biology  21, 871-884; Fromm, et al. (1990)  Bio/Technology  8, 833-839; Gordon-Kamm, et al. (1990)  Plant Cell  2, 603-618; D&#39;Halluin, et al. (1992)  Plant Cell  4, 1495-1505; Walters, et al. (1992)  Plant Molecular Biology  18, 189-200; Koziel, et al. (1993)  Biotechnology  11, 194-200; Vasil, I. K. (1994)  Plant Molecular Biology  25, 925-937; Weeks, et al. (1993)  Plant Physiology  102, 1077-1084; Somers, et al. (1992)  Bio/Technology  10, 1589-1594; WO92/14828). In particular,  Agrobacterium  mediated transformation is now a highly efficient alternative transformation method in monocots (Hiei et al. (1994)  The Plant Journal  6, 271-282). 
     The generation of fertile transgenic plants has been achieved in the cereals rice, maize, wheat, oat, and barley (reviewed in Shimamoto, K. (1994)  Current Opinion in Biotechnology  5, 158-162.; Vasil, et al. (1992)  Bio/Technology  10, 667-674; Vain et al., 1995,  Biotechnology Advances  13 (4): 653-671; Vasil, 1996,  Nature Biotechnology  14 page 702). Wan and Lemaux (1994)  Plant Physiol.  104: 37-48 describe techniques for generation of large numbers of independently transformed fertile barley plants. 
     The generation of fertile transgenic trees has been achieved in poplar (Halpin, C et al. TREE GENETICS &amp; GENOMES 3 (2): 101-110 APR 2007, Song J Y, PLANT AND CELL PHYSIOLOGY 47 (11): 1582-1589 NOV 2006), loblolly pine (reviewed in Boerjan W CURRENT OPINION IN BIOTECHNOLOGY 16 (2): 159-166 APR 2005), eastern white pine ( Pinus strobus  L.) Tang W PLANT CELL REPORTS 26 (5): 673-682 MAY 2007) 
     Micro projectile bombardment, electroporation and direct DNA uptake are preferred where  Agrobacterium  is inefficient or ineffective. Alternatively, a combination of different techniques may be employed to enhance the efficiency of the transformation process, e.g. bombardment with  Agrobacterium  coated micro particles (EP-A-486234) or micro projectile bombardment to induce wounding followed by co-cultivation with  Agrobacterium  (EP-A-486233). 
     Following transformation, a plant may be regenerated, e.g. from single cells, callus tissue or leaf discs, as is standard in the art. Almost any plant can be entirely regenerated from cells, tissues and organs of the plant. Available techniques are reviewed in Vasil et al.,  Cell Culture and Somatic Cell Genetics of Plants, Vol. I, II and III, Laboratory Procedures and Their Applications , Academic Press, 1984, and Weiss Bach and Weiss Bach,  Methods for Plant Molecular Biology , Academic Press, 1989. 
     The particular choice of a transformation technology will be determined by its efficiency to transform certain plant species as well as the experience and preference of the person practising the invention with a particular methodology of choice. It will be apparent to the skilled person that the particular choice of a transformation system to introduce nucleic acid into plant cells is not essential to or a limitation of the invention, nor is the choice of technique for plant regeneration. 
     The invention further encompasses a host cell transformed with vectors or constructs as set forth above, especially a plant or a microbial cell. Thus, a host cell, such as a plant cell, including nucleotide sequences of the invention as herein indicated is provided. Within the cell, the nucleotide sequence may be incorporated within the chromosome. 
     Also according to the invention there is provided a plant cell having incorporated into its genome at least a nucleotide sequence, particularly heterologous nucleotide sequences, as provided by the present invention under operative control of regulatory sequences for control of expression as herein described. The coding sequence may be operably linked to one or more regulatory sequences which may be heterologous or foreign to the nucleic acid sequences employed in the invention, such as not naturally associated with the nucleic acid sequence(s) for its (their) expression. The nucleotide sequence according to the invention may be placed under the control of an externally inducible promoter to place expression under the control of the user. A further aspect of the present invention provides a method of making such a plant cell involving introduction of nucleic acid sequence(s) contemplated for use in the invention or a suitable vector including the sequence(s) contemplated for use in the invention into a plant cell and causing or allowing recombination between the vector and the plant cell genome to introduce the said sequences into the genome. The invention extends to plant cells containing a nucleotide sequence according to the invention as a result of introduction of the nucleotide sequence into an ancestor cell. 
     The term “heterologous” may be used to indicate that the gene/sequence of nucleotides in question have been introduced into said cells of the plant or an ancestor thereof, using genetic engineering, ie by human intervention. A transgenic plant cell, i.e. transgenic for the nucleotide sequence in question, may be provided. The transgene may be on an extra-genomic vector or incorporated, preferably stably, into the genome. A heterologous gene may replace an endogenous equivalent gene, ie one that normally performs the same or a similar function, or the inserted sequence may be additional to the endogenous gene or other sequence. An advantage of introduction of a heterologous gene is the ability to place expression of a sequence under the control of a promoter of choice, in order to be able to influence expression according to preference. Furthermore, mutants, variants and derivatives of the wild-type gene, e.g. with higher or lower activity than wild type, may be used in place of the endogenous gene. Nucleotide sequences heterologous, or exogenous or foreign, to a plant cell may be non-naturally occurring in cells of that type, variety or species. Thus, a nucleotide sequence may include a coding sequence of or be derived from a particular type of plant cell or species or variety of plant, placed within the context of a plant cell of a different type or species or variety of plant. A further possibility is for a nucleotide sequence to be placed within a cell in which it or a homologue is found naturally, but wherein the nucleotide sequence is linked and/or adjacent to nucleic acid which does not occur naturally within the cell, or cells of that type or species or variety of plant, such as operably linked to one or more regulatory sequences, such as a promoter sequence, for control of expression. A sequence within a plant or other host cell may be identifiably heterologous, exogenous or foreign. 
     Plants which include a plant cell according to the invention are also provided, along with any part or propagule thereof, seed, selfed or hybrid progeny and descendants. Particularly provided are transgenic field crop plants and transgenic tree species, which have been engineered to carry genes identified as stated above. Examples of suitable plants include  Nicotania tabacum  and other  Nicotiana  species, sugar beet, sugar cane, wheat, barley, (corn) maize, rice, Miscanthus, Switch grass ( Panicum virgatum ) sorghum, and cotton. Examples of tree species amenable to transformation according to the teaching of the invention include poplar, loblolly pine, Hybrid Aspen:  Populus tremula×Populus tremuloides , Hybrid poplar:  P. tremula×P. alba, Eucalyptus  species such as  Eucalyptus globulus  (Southern Blue gum),  Eucalyptus camaldulensis×Eucalyptus globulus, Eucalyptus grandis, Eucalyptus gunnii, pinus  species, silver fir, balsam fir, Japanese fir, Siberian fir, Japanese Cyprus, European larch, Western larch, Siberian larch, European spruce, White spruce, Sitka spruce, Western white pine, European Black pine, Longleaf pine, Ponderosa pine, Radiata pine, Red pine, Pitch pine, Eastern white pine, Scots pine, Matai, Douglas fir, European white birch, paper birch, yellow poplar, white willow, black willow, American elm, and mountain elm. Especially preferred transformed plants and/or transformed plant cells of the invention are selected from poplar, loblolly pine,  pinus  species selected from those listed herein, eucalyptus species selected from  Eucalyptus globulus  (Southern Blue gum),  Eucalyptus camaldulensis×Eucalyptus globulus, Eucalyptus grandis, Eucalyptus gunnii , wheat, barley, (corn) maize, rice, Miscanthus, Switch grass ( Panicum virgatum ), sugar cane. 
     In addition to a plant, the present invention provides any clone of such a plant, seed, selfed or hybrid progeny and descendants, and any part of any of these, such as cuttings, seed. The invention provides any plant propagule, that is, any part which may be used in reproduction or propagation, sexual or asexual, including cuttings, seed and so on. Also encompassed by the invention is a plant which is a sexually or asexually propagated off-spring, clone or descendant of such a plant, or any part or propagule of said plant, off-spring, clone or descendant. 
     Native, non-transformed plants can also be screened and analysed for naturally occurring mutations in nucleic acid sequences of interest that are employed in saccharide moiety substitution of xylans in native populations of wild type plants. Furthermore, plants in which mutations have been induced, for example via conventional mutagenesis or via TDNA insertion as outlined herein may also be screened for alterations in nucleotide sequences known to be or suspected of being employed in the saccharide moiety substitution of xylans, that is to say, in populations of such conventionally mutagenised plants. Furthermore, the level of saccharide substitution that may be present in such plants comprising mutations in nucleic acid sequences may be screened using procedures similar to those outlined herein and the level of saccharide moiety substitution of xylan can be determined as outlined herein. Initially, plant populations of interest may be screened using appropriate nucleic acid sequences of interest that are known to be employed in saccharide moiety substitution of xylans, such as, nucleic acid sequences encoding XGAT proteins of the group selected from At4g33330, At3g18660, At1g77130, At1g08990, At1g54940 (from  Arabidopsis thaliana ), PttGT8A, PttGT8B and PttGT8C (from poplar), CAK29728 (partial sequence from  Pica abies , conifer), and other XGAT homologues or orthologues thereof from other species, such as Os03g0184300, Os01g0880200, Os05g 0426400, OsI — 010047, AAK92624 (from Rice) AK250038 (from Barley), AY110752, (from Maize), and ABE88903 (From  Medicago truncatula ). 
     Thus, in a further aspect of the invention there is provided a method for screening plants in a given plant population for mutant alleles involved in the saccharide moiety substitution of xylans that comprises:
         i) obtaining nucleic acid samples from the plants;   ii) screening the nucleic acid samples with at least one known marker sequence of a nucleic acid sequence that is employed in saccharide substitution of xylan;   iii) identifying plants that comprise at least one mutant allele relative to the known marker sequence of step ii).       

     The marker sequence may be selected from nucleic acid sequences that are known to be employed in saccharide substitution of xylans such as XGAT sequences encoding proteins selected from the group At4g33330, At3g18660, At1g77130, At1g08990, At1g54940 (from  Arabidopsis thaliana ), PttGT8A, PttGT8B and PttGT8C (from poplar), CAK29728 (partial sequence from  Pica abies , conifer), and other XGAT homologues or orthologues thereof from other species, such as Os03g0184300, Os01g0880200, Os05g 0426400, OsI — 010047, AAK92624 (from Rice) AK250038 (from Barley), AY110752, (from Maize), ABE88903 (From  Medicago truncatula ), depending on the species of plant under investigation. 
     Once plants possessing mutant alleles have been identified their xylan content, and in particular, the saccharide substitution pattern of their xylans may be determined, for example, in accordance with methods as outlined herein (Goubet et al; Vicky Wong, PhD thesis, University of Cambridge 2005) 
     Thus, the present invention further encompasses the isolated, modified xylan product of novel mutagenised plants and/or the isolated, modified xylan product of naturally-occurring mutant plants that may be identified in plant populations as outlined herein. 
     In addition to the above outlined methods of screening for and/or identifying plants comprising novel saccharide moiety substitution patterns on their xylans that where a TDNA library may be used in the creation of plant lines with a view to obtaining novel saccharide moiety substitution patterns on their xylans, techniques for generating mutant plants comprising T-DNA inserts in nucleic acid sequences or of locating mutant plants comprising T-DNA inserts may be employed. Accordingly, as a further aspect of the invention there is provided a method of identifying mutant plants that comprises
         i) extracting nucleic acid;   ii) screening the extracted nucleic acid for native DNA and T-DNA inserts using primers;   iii) amplifying the screened nucleic acid of step ii) via PCR; and   iv) comparing the PCR products against a reference standard.       

     The skilled addressee will appreciate that once a mutant plant has been identified according to this embodiment of the invention, the saccharide moiety substitution pattern on the xylan structure may be investigated and confirmed using methods such as PACE, as outlined herein. 
     In a further embodiment of the invention there is provided a method of generating a mutant plant comprising a modified saccharide substitution pattern on its xylan structure that comprises:
         i) inserting a DNA sequence into a nucleic acid sequence encoding an enzyme that has enzyme activity in saccharide moiety substitution patterning on xylan into a viable plant cell; and   ii) generating a plant from said plant cell.       

     The inserted DNA sequence may be a T-DNA sequence or a nonsense DNA sequence that either renders the targeted nucleic acid sequence (“gene” sequence) at least partially dysfunctional, or substantially dysfunctional, that is to say, incapable of giving rise to a fully functional enzyme capable of giving rise to a native saccharide substitution pattern on the xylan structure. Such incapacitated nucleic acids may be fully dysfunctional. Preferably, the incapacitated nucleic acids are fully dysfunctional, that is to say, incapable of giving rise to a native saccharide substitution pattern on a xylan structure. 
     Such plants may be generated using standard protocols and procedures as outlined herein and are applicable to the provision of plants comprising anti-sense nucleic acid sequences of interest or sequences that give rise to “co-suppression”, such sequences typically being linked to the generation of siRNA species. 
     The present invention also encompasses the modified xylan product of a transformed plant according to the invention as disclosed herein or obtainable in accordance with the information and suggestions herein. Those skilled in the art are well able to construct vectors and design protocols and systems suitable for the carrying out of the invention. 
     Use of either of the terms “homology” and “homologous” herein does not imply any necessary evolutionary relationship between compared sequences, in keeping for example with standard use of terms such as “homologous recombination” which merely requires that two nucleotide sequences are sufficiently similar to recombine under the appropriate conditions. 
     The teaching of all references cited herein is incorporated in its entirety into the present description. 
    
    
     
       DETAILED DESCRIPTION OF THE INVENTION 
       There now follow non-limiting examples and figures illustrating the invention. 
         FIG. 1 : PACE gel showing reduction in substitution of xylan in xgat mutants. In xgat1-2 and the double mutant xgat1/xgat2, (Xyl) 3  and (Xyl) 4  increase as [Me]GlcA(Xyl) 4  decreases. Xgat2-1 shows a small reduction in [Me]GlcA(Xyl) 4 . (*) unspecific band. 
         FIG. 2 : Quantity of xylan backbone in xgat mutants is unchanged in relation to wild type plants. N=2 to 4 biological replicates. 
         FIG. 3 : Substitution of xylose with [Me]GlcA is reduced in xgat1, and missing in xgat1/xgat2 double mutants, as determined by PACE, in two independent experiments (WT n=3). 
         FIG. 4 : MALDI-TOF MS of xylanase Xyl11-digested cell walls confirms that [Me]GlcA(Xyl) 4  is detected in xgat single mutants, but missing in the double mutant. Both GlcA(Xyl) 4  and [Me]GlcA(Xyl) 4  are missing in the double mutant. (A) Wild type; (B) xgat1-2; (C) xgat2-1; (D) xgat1-2 xgat2-1. 
         FIG. 5 : HPLC analysis of monosaccharides in the de-pectinated cell wall shows the absence of GlcA in the mutant. Polysaccharides in the walls were hydrolysed by 2 M trifluoroacetic acid at 120° C. for three hours. 
         FIG. 6 : [Me]GlcA substitution strongly influences the extractability of xylan from walls. Most xylan is extracted by 1M NaOH in the xgat double mutant. Walls were successively extracted by CDTA, Na2CO3, 1M KOH, 4M KOH. The xylan in the extracts and the insoluble residue were then analysed by PACE. 
         FIG. 7 : RT-PCR shows xgat1-1, xgat1-2, xgat2-1 and xgat2-2 are transcriptional knockouts. Histone H1 was used to confirm cDNA quality. 
     
    
    
     EXAMPLE 1 
     Analysis of  Arabidopsis thaliana  Plants for the Presence of Insertion in the Genes At4g33330 or At3g18660 
     To isolate mutant plants lacking the activity of the xgat genes, insertion lines were identified. DNA was extracted and screened by PCR for T-DNA insertions. 
       Arabidopsis thaliana  cv Columbia plants of all genotypes were stratified by incubating in water at 4° C. in the dark for 72 hours and subsequently sowed in soil and allowed to grow under controlled environmental conditions (25/20° C., 16-h-light/8-h-dark cycle). 
     Available T-DNA insertion mutants were identified from the SIGnAL “T-DNA Express”  Arabidopsis  Gene Mapping Tool located in the SIGnAL website. 
     The following plant insertion lines were identified for the genes of interest, At3g18660 (xgat1) and At4g33330 (xgat2): (xgat1-1, SALK_063763 (NASC stock number N563763); xgat1-2, SALK — 046841 (NASC stock number 546841) and xgat2-1, GK-722F09 (NASC stock number N469285); xgat2-2, SM_3.16768 (NASC stock number N104457). 
     For DNA extraction, a single rosette leaf from four-week-old plants of all putative T-DNA lines was collected and frozen in liquid nitrogen and ground to a fine powder. The ground leaf tissue was then incubated with pre-heated DNA extraction buffer (20% w/w CTAB, 1.4 M NaCl, 0.02 M EDTA, 0.1 M Tris-HCL pH 8.0) p-mercaptoethanol for 30 min at 60° C. This was followed with chloroform:isoamyl alcohol 24:1 incubation and samples were inverted, centrifuged at 10,000 g for 30 min, and the aqueous layer collected. Cold isopropanol was added, mixed, and subsequently centrifuged at 10,000 g for 30 min and the supernatant removed. The DNA pellets were washed with 70% ethanol, centrifuged at 10,000 g for 5 min, the supernatant was removed, and the DNA re-suspended in DNA suspension buffer (containing 0.1 mM Tris-HCL and 0.02 μM EDTA). 
     DNA samples representing the various putative mutants were screened for both the wild-type gene and the T-DNA insert. The following primer sets were used to amplify the wild-type gene (open reading frame) of xgat1-1: (R-primer) 5′-CAATGCCGCAGCATACTTTTC-3′ (Seq. Id. No. 1) and (L-primer) 5′-GCAAGAGGAGATTCCGGAGAA-3′ (Seq. Id. No. 2) (amplification product=2.5 kb) and to amplify the T-DNA insert: (L-primer) 5′-GCAAGAGGAGATTCCGGAGAA-3′ (Seq. Id. No. 2) and (L-border primer) 5′-TTTTTCGCCCTTTGACGTTGGAG-3′ (Seq. Id. No. 3) (amplification product=2 kb). xgat1-2: (R-primer) 5′-CAATGCCGCAGCATACTTTTC-3′ (Seq. Id. No. 1) and (L-primer) 5′-GCAAGAGGAGATTCCGGAGAA-3′ (Seq. Id. No. 2) (amplification product=2.5 kb) and to amplify the T-DNA insert: (L-primer) 5′-GCAAGAGGAGATTCCGGAGAA-3′ (Seq. Id. No. 2) and (L-border primer) 5′-TTTTTCGCCCTTTGACGTTGGAG-3′ (Seq. Id. No. 3) (amplification product L-border primer/L-primer=0.9 kb). xgat2-1: (R-primer) 5′-TATGATGTCTAAATACAAGGA-3′ (Seq. Id. No. 4) and (L-primer) TACGCTTTAATCTAGTCTTGTT-3′ (Seq. Id. No. 5) (amplification product=2.9 kb) and to amplify the T-DNA insert: (R-primer) 5′-TATGATGTCTAAATACAAGGA-3′ (Seq. Id. No. 4) and (L-border primer2) 5′-ATATTGACCATCATACTCATTGC-3′ (Seq. Id. No. 6) (amplification product=0.9 kb). xgat2-2: (R-primer) 5′-TATGATGTCTAAATACAAGGA-3′ (Seq. Id. No. 4) and (L-primer) TACGCTTTAATCTAGTCTTGTT-3′ (Seq. Id. No. 5) (amplification product=2.9 kb) and to amplify the T-DNA insert: (R-primer) 5′-TATGATGTCTAAATACAAGGA-3′ (Seq. Id. No. 4) and (L-border primer3) 5′-GGTGCAGCAAAACCCACACTTTTACTTC-3′ (Seq. Id. No. 7) (amplification products L-border primer=1.2 kb). 
     DNA samples (2 μl) were used for PCR reactions and were aliquoted into PCR tubes containing 10 μl Sigma REDTaq ready mix with MgCl 2  (Cat #R2523), 1 μl primer (L, R or Left border) adjusted to a final volume of 20 μl using H 2 O. For loading control and positive control, Histone primers were used in place of the gene/gene or gene/Left border primers. For negative control, 5 μl of sterile water was used instead of DNA. The following PCR program was used with the annealing time adjusted longer or shorter depending on the length of PCR product: 94° C. for 2 min (1 cycle), followed by 94° C. for 15 sec, 55° C. for 30 sec, 68° C. for 3 min (15 cycles), 94° C. for 15 sec, 55° C. sec, 68° C. for 3 min (25 cycles), 68° C. for 10 min (1 cycle), and finally the reaction was held at 4° C. PCR products, 8-10 μl per sample and 5 μl hyperladder, were then loaded onto a 0.8% agarose gel in 1×TAE buffer (0.04 M Tris acetate, 0.001 M EDTA) containing ethidium bromide (5 μl/100 ml). Samples were separated at 100 volts at room temperature for approximately 45 min. The gels were then visualized under UV and imaged using a digital camera. 
     EXAMPLE 2 
     Analysis of  Arabidopsis thaliana  Plants for The Presence of Modified Xylan 
     The mutant plants were and for the quantity of [Me]GlcA side chains on the xylan PACE, which involves hydrolysis of the xylan with a xylanase enzyme, derivatisation of oligosaccharides with a fluorophore, and separation of the oligosaccharides by polyacrylamide gel electrophoresis (Goubet at al. 2002). 
       Arabidopsis thaliana  plants were grown at 22′C in controlled environment cabinets under a 16 h day light regime of 150 to 180 μmol m −2  s −1 . Stem fractions were incubated for 30 mm in 95% (v/v) ethanol at 65° C. to inactivate enzymes, and then were ground in a Mixer Mill MM200 (Glen Creston, Middlesex, UK). The homogenate was centrifuged at 4,000 g for 15 min. The pellet was washed with 60% (v/v) ethanol (3-4 times), methanol/chloroform (2:3 (v/v); overnight), 100% acetone, ethanol/water [6:4 (v/v)] and ethanol/water [9:1 (v/v)]. The remaining pellet, containing the cell wall, was dried overnight at 80° C. 
     Dried cell wall material (50 μg) was treated with 4 M NaOH (20 μL) for 1 h at room temperature before adjusting to pH 5-6 with HCl (1 M). The xylan hydrolysis was performed in 0.1 M ammonium acetate pH 6 with 20 mU of xylanase overnight. Endo-β-1, 4-xylanase, Xyl10A (glycosylhydrolase family 10 from  Cellvibrio japonicus ) or Xyl11 (glycosylhydrolase family 11 from  Nocallimastix patriciarum ) was a gift from Harry Gilbert (University of Newcastle, UK), Controls without substrates or enzymes were performed under the same conditions to identify any unspecific compounds in the enzymes, polysaccharides/cell walls or labelling reagents. The reactions were stopped by boiling for 30 min and the samples dried. 
     Derivatisation of the sugars with ANTS (8-aminonaphthalene-1,3,6-trisulfonic acid) was in 10 μL of buffer (DMSO:water:acetic acid, 20:17:3). ANTS was purchased from Molecular Probes (Leiden, The Netherlands). Derivatization was carried out in tubes containing dried polysaccharides, oligosaccharides or monosaccharides. For monosaccharide or oligosaccharide standards, 5 μl of 1 mM sugars were added to a tube and dried before derivatization. ANTS was prepared in acetic acid/water (3/17, v/v) at 0.2 M as final concentration (made freshly or stored at: −20° C.). NaCNBH 3  (1 M, made freshly and used immediately) was solubilized in DMS for ANTS derivatisation. To each dry sample 5 μl of ANTS solution and 5 μl of the appropriate NaCNBH 3  solution were added. The reagents were mixed, centrifuged, and incubated at 37° C. overnight. The solution was lyophilized in a centrifugal vacuum evaporator for 3 h at 40° C. The derivatised sugars were re-suspended in 100 μL of 3M urea and stored before use at −20° C. 
     Separation of ANTS-derivatised sugars, using 1 μL of the sample per gel lane, was performed using an Hoefer SE 660 vertical slab gel electrophoresis apparatus (Amersham, Bucks, UK) with 24 cm. plates, 0.75 mm spacer and well of width 0.25 cm. Standard glass or low-fluorescence pyrex plates were used. Electrophoresis was performed at; 10° C. in all cases. The 20% (v/v) polyacrylamide gel contained 0.5% (w/v) N,N′-methylenebisacrylamide with a stacking gel (2 cm) of 8% (w/v) polyacrylamide and 0.2% (w/v) N,N′-methylenebisacrylamide. Polyacrylamide containing a ratio of acrylamide/N,N′-methylenebisacrylamide (29:1) was obtained from Severn Biotech Ltd. (Worcs, UK). The electrophoresis buffer system was 0.1 M Tris adjusted to pH 8.2 with boric acid (Tris-borate). The samples were electrophoresed initially at 200 V for 20 ruin and then at 1,000 V for 90 min. 
     Gels were scanned using a MasterImager CCD camera system (Amersham, Bucks, UK) with an excitation filter at 400 nm and a detection filter at 530 nm. The exposure time was optimised to increase sensitivity without saturating the intense bands. An image of the gel (resolution, 100 microns) was obtained and exported in a 16 bit file to be quantified. The gel was also visualised using a standard UV transilluminator (wavelength, 360 nm). Quantitation was performed using GeneTools software (Syngene, Cambridge, UK), using rolling ball background detection. 
     Standards (single or multiple) were run in each gel to obtain a standard curve for quantitating sugars in the samples. Standards for quantification [Xylose, (Man) 2  and (Man) 3 ] were separated alongside samples in each gel to obtain a standard curve of pmol quantity of fluorophore-labelled oligosaccharide. For digests with Xyl11, the quantity of Xyl, (Xyl) 2  (Xyl) 3  and [Me]GlcUA(Xyl) 4  in. 1 ul, of sample was calculated using this standard curve. The ratio of Xyl to Glc/Me-Glc was calculated by summing the relative contribution of the Xyl containing bands=(Xyl) 1 x1+(Xyl) 2 x2+(Xyl) 3 x3+([Me]GlcUA (Xyl) 4 )x4 compared to GlcUA/MeGlcUA=([Me]GlcUA(Xyl) 4 )x1. 
     The structure of the xylan was studied in stems of single mutants, xgat1-2 and xgat2-1 and the double mutant.  FIG. 1  snows PACE gels of digests of stem xylan with Xyl11 which yields mostly Xyl, (Xyl) 2  (Xyl) 3  and [Me]GlcAXyyl 4 , MeGlcAXyl 4  and GlcAXyl 4  were not well distinguished by the PACE technique. In the single mutants, the intensity of the [Me]GlcAXyl 4  band was reduced. In xgat1-2 xgat2-1 double mutant plant lines, [Me]GlcAXyl 4  was absent. The total quantity of xylan backbone was measured in xgat mutants and wild type plants, and found to be identical ( FIG. 2 ). The proportion of xylose residues in the xylan backbone substituted with [Me]GlcA was measured in xgat mutants and wild type plants, and shown to be reduced in the single mutants, and essentially absent in the double mutants ( FIG. 3 ). Together, this indicates that manipulation of XGAT activity can be used to reduce the substitution of xylan by [Me]GlcA, without altering substantially the quantity of xylan. 
     EXAMPLE 3 
     Xylan Structure Fingerprinting of Modified Plants by Xylanase Digest Ion and Mass Spectrometry 
     The presence or GlcA or MeGlcA on xylan was investigated by studying xylanase-released oligosaccharides by mass spectrometry. 
     Cell wall material (500 μg) prepared as Example 2 was treated with 4M NaOH (50 μL) for 1 h at room ten before adjustment to pH. 5-6 with HCl (1M). The xylan hydrolysis was performed in 0.1M ammonium acetate pH6 with 100 mU of xylanase eg Xyl10A or Xyl11 overnight. The reactions were stopped by boiling for 30 min. The samples were filtered using a Nanosep system (molecular weight cut-off of 10 kDa, Pall, New York, USA) and dried. The resulting oligosaccharides were purified using HyperSep Hypercarb cartridges (ThermoHypersil-Keystone, Runcorn, Cheshire, UK) and subsequently analysed by MALDI-TOF-MS. Due to the presence of contaminant signals complicating these native spectra, the remainder of each sample was perdeuteromethylated (using the NaOH slurry method described In Dell et al., 1989) prior to re-analysis by MALDI-TOF-MS All mass spectra were recorded in the positive ion mode on a 4700 Proteomics Analyzer (Applied Biosystems, Foster City, Calif.). This MALDI tandem mass spectrometer uses a 200 Hz frequency-triple Nd-YAG laser operating at a wavelength of 355 nm. 2,5-Dihydroxybenzoic acid (DHB) (Fluka), dissolved in 50% aqueous methanol, was used as the matrix and averages of 2500 shots were used to obtain all MS spectra. 
     The result of MS analysis of xylanase-released oligosaccharides in xgat single and double mutants is shown ( FIG. 4 ). The 963 and 966 Da [M+Na] corresponds to MeGlcA Xyl 4  and GlcAXyl 4 , respectively and they differ in mass by 3 Da after deuteropermethylation. In the single mutants, both oligosaccharides are still present, but there was a small increase in proportion of MeGlcA Xyl 4 , over GlcA Xyl 4 . A much larger increase in proportion of MeGlcA over GlcA has previously been found in xylan synthesis mutants, irx7/fra8, irx3 and irx9 (Pena at al. 2007, Zhong et al., 2005). It can be seen that both MeGlcA and GlcA substitutions disappear in the double mutants, and were not detectable above background. Taken together with the quantitative PACE analysis of substitution level, the MS indicates that manipulation of XGAT activity leads to xylan without [Me] GlcA side chains, and can be used to manipulate both MeGlcA and GlcA substitution of xylan. 
     EXAMPLE 4 
     Sugar Composition of Hemicelluloses of Modified  Arabidopsis thaliana  Plants 
     To detect GlcA in the hemicelluloses, cell walls were prepared as above. Pectin was removed, leaving the hemicelluloses including xylan in the insoluble material (“hemicellulosic material”), Cell wall material (50 μg) was suspended in 1 mL of 0.05 M 1,2-cyclohexanediaminetetraacetic acid (CDTA) (pH 6.5) for 24 h at room temperature. The suspension was centrifuged and the pellet washed once with distilled water. The residue was subsequently extracted using 0.05M Na 2 CO 3  containing 0.01M NaBH 4  for 24 h at 4° C. The residue was adjusted to pH 5 with glacial acetic acid, and then dialysed extensively against de-ionised water for 5 d and then lyophilised. 
     Hemicellulosic material was acid hydrolysed in 400 μl of 2M trifluoroacetic acid at 120° C. for three hours, dried and suspended in 100 L distilled water. 
     The monosaccharide analysis was performed a Dionex DX-500 BioLC system composed of an electrochemical detector (ED40), gradient pump (GP50), injector system (IC 30 and UV/VIS detector (UVD 170U), using a CarboPac™ PA20 analytical column (3×150 mm) in combination with a CarboPac™ PA20 guard column (3×30 mm), Dionex Corp., CA, USA, Data was interpreted by Chromeleon software. HPAEC-PAD was performed at 30° C. with a flow rate of 0.5 ml/min using an isocratic gradient of three eluents prepared from deionised water degassed by helium gas: eluent A: 100 mM NaOH, 5.23 ml from 46/48% (w/w) NaOH (Fisher Scientific, UK) stock to minimise the carbonate content; eluent B, 1 M NaOH, 52.3 mL of NaOH stock; eluent D, deionised-degassed water. The column was washed with 200 mM NaOH for 10 min and re-equilibrated with 1.5 mM NaOH for 10 min before the next injection. A 20 μL sample was injected and monitored by pulsed-amperometric detector with a disposable gold working electrode and an Ag/AgCl 2  reference electrode (Dionex, CA, USA). GlcA was detected by reference to a standard. 
     Analysis of wild type and double xgat mutant plants revealed that GlcA was reduced to trace levels in hemicellulose of the modified plants ( FIG. 5 ). This shows that GlcA is missing from the xylan in the modified plants. 
     EXAMPLE 5 
     Chemical Extractability of Modified Xylan in  Arabidopsis thaliana    
     The quantity of xylan extracted by 0.05 M CDTA (ph 6.5), 1 M NaOH (mild) or 4 M NaOH (strong) base solution was measured by PACE as described in Brown et al (2007). 
     Dried cell wall material (500 mg) was first extracted with 0.05 M CDTA (pH 6.5) for 24 h at room temperature. The suspension was centrifuged (48 000 g), and the pellet washed once with distilled H 2 O. The supernatants were combined as the CDTA-soluble fraction. The AIR was subsequently extracted under oxygen-free conditions using 0.05 M Na2CO3 containing 0.01 M NaBH4 for 24 h at 4_C (Na2CO3-soluble fraction), 1 M KOH containing 0.01 M NaBH4 for 24 at room temperature (1 M KOH-soluble fraction) and then 4 M KOH containing 0.01 M NaBH4 for 24 h at room temperature (4 M KOH soluble fraction). All fractions were filtered through a GF/C glass fibre filter (Whatman). The Na2CO3 and KOH fractions were also chilled on ice and adjusted to pH 5 with glacial acetic acid. All cell-wall fractions were then dialysed extensively against deionized water for 5 days, and then lyophilized The xylan in 1/20 th  of the samples (extract and residue) was hydrolysed in 0.1 M ammonium acetate pH 6 with 20 mU of xylanase overnight. The samples were derivatised with ANTS, and mono- and oligosaccharides were separated polyacrylamide gel electrophoresis as described for Example 2. 
     In the wild type plants, some of the xylan was solubilised by 1 M NaOH, and some remained attached to the cellulosic residue ( FIG. 6 ). In the xgat double mutant plants, most of the xylan was extracted by 1 M NaOH extraction, in indicating that the interaction with lignin or cellulose in the wall is altered. The absence of the [Me]GlcA in the modified plants improves the extractability of the xylan in mild base solution. This indicates that the manipulation of [Me]GlcA substitution can be used to alter solubility of xylans. 
     EXAMPLE 6 
     Xylan Modification in Poplar Species 
     Xylan is modified in trees such as poplar species by generating transgenic plants with increased expression of XGAT genes, or reduction in XGAT activity using antisense approaches as described herein. 
     XGAT genes are cloned using primers specific for genes encoding XGATs using procedures as described herein. For poplar, the sequences of PttGTb and pttGT8C are amplified by PCR using cDNAs from hybrid aspen the primers for example: 
     The text file, “Replacement Sequence Listing Jan 2013” created on Tuesday Jan. 29, 2013, with a size of 76.7 KB and 78,615 Bytes is herein incorporated by reference in its entirety. 
     PttGT8C, using sequence AY935503, 
                             F   5′-GTGCAACCCTTGTTGCTAAGA-3′   (Seq Id No. 8)               R   5′-GCCTCTTTAGTCAAATGAAACAGAAC-3′   (Seq Id No. 9)            
PttGT8B using sequence AY935502 B
 
     
       
         
               
               
               
             
           
               
                 F 
                 5′-ACGGAAGCGGAAGAAGATAA-3′ 
                 (Seq Id No. 10) 
               
               
                   
               
               
                 R 
                 5′-TCATTTCCCATTAGTCTCACCATAT-3′ 
                 (Seq Id No. 11) 
               
             
          
         
       
     
     The sequences are inserted into a cloning vector, such as the pBIN cloning vector under the control of a suitable promoter such as the enhanced tandem CaMV 35S constitutive promoter, or under a promoter specifically active in cells synthesising secondary cell walls, such as the 2 kb of the promoter 5′ to an XGAT coding sequence of a dicot plant, such as  Arabidopsis thaliana . To increase XGAT activity, the sequence is cloned in the sense orientation. To reduce XGAT activity, the sequence is cloned in the reverse (antisense) orientation. The sequence may also be used to generate other constructs that cause the production of double stranded RNA to suppress expression of XGAT genes, using methods well known to those skilled in the art. To confirm the insertion of the promoter and gene into the binary vector, the nucleotide sequence of the construct is determined. 
     Hybrid poplar is transformed using virulent  Agrobacterium tumefaciens  using standard techniques, such as leaf disc inoculation as described herein. For example, poplar leaf discs are cut and co-cultured with  Agrobacterium tumefaciens  for 1 h at room temperature, blotted dry, and plated abaxially onto suitable agar solidified medium supplemented with 0.1 μM each naphthalene acetic acid (NAA), 6-benzylaminopurine (BA), and thiadiazuron (TDZ). After three days the discs are transferred to agar plates supplemented with carbenicillin disodium (500 mg l −1 ) and cefotaxime sodium salt (250 mg l −1 ). After three further days, the discs are transferred to agar plates with medium containing carbenicillin, cefotaxime, and kanamycin (25 mg l −1 ). After 5 weeks, shoots and callus material are transferred to medium supplemented as above plus 0.01 μM BA. Once individual shoots are visible, plantlets are transferred to solidified medium with 0.01 μM NAA and antibiotic selection to induce rooting. After two consecutive 5-week periods on this medium, shoot tips are isolated to solidified antibiotic-free medium with 0.01 μM NAA. 
     Plants are confirmed as transformants by PCR screening of genomic DNA employing gene and promoter-specific oligonucleotides as described above. 
     Plantlets in tissue culture are transferred into 7.5 l pots containing a peat, fine bark, and pumice soil mixture, and grown in a greenhouse until planting in the field. 
     Clones of plants with improved xylan properties are identified by extracting the xylan with 1M NaOH and comparing the quantity to untransformed plants, or by analysing the xylan branching by PACE as described herein. 
     References 
     
         
         Goubet, F, P Jackson, M Deery and P Dupree (2002) Polysaccharide Analysis using Carbohydrate gel Electrophoresis (PACE): a method to study plant cell wall polysaccharides and polysaccharide hydrolases. Analytical Biochemistry, 300, 53-68.