Abstract:
A biotechnological application of the RAV1 gene (related to ABI3 and Viviparous 1) and its homologue RAV2 in relation to their capacity to increase or reduce the development of sylleptic and/or proleptic branches in ligneous species when their expression levels or the activity of the proteins they encode are modified. It is possible to increase the biomass production of a ligneous species plantation or to reduce the number of nodes in the trunk of ligneous species of logging interest by means of modifying the expression of said genes.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a U.S. National-Stage entry under 35 U.S.C. §371 based on International Application No. PCT/ES2012/070471, filed Jun. 26, 2012 which was published under PCT Article 21(2) and which claims priority to Spanish Application No. P201131186, filed Jul. 13, 2011, which are all hereby incorporated in their entirety by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The invention is encompassed in the technical field of biotechnology and forestry practice for energy, chemistry or logging purposes. 
       BACKGROUND 
       [0003]    Short rotation forestry (SRF) of fast growing forestry species under an intensive management system is one of the most interesting sources of biomass. The main species exploited with this model are poplar ( Populus  spp.), eucalyptus ( Eucalyptus  spp.) or willow ( Salix  spp.) with a turn of logging between 2 and 10 years. On one hand, the lignocellulosic biomass is suitable for producing biofuels in the form of liquid fuels or gases for transport, and bioliquids or combustibles liquids intended for energy uses other than transport, such as for producing electricity or heat. On the other hand, such forestry plantations can be established in surplus marginal or agricultural lands, therefore they do not compete directly with the food crop for fertile soils. However, the yield thereof for bioenergetics purposes is still subject to improvement since it remains as an important limiting factor which is far from being economical and commercially viable. 
         [0004]    Lignocellulosic biomass yield is a highly complex genetic trait. It is the integrated and combined result of many other complex traits, each of them is in turn controlled by several genes. These traits include, for example, the height of the tree, the diameter of the trunk, the number and area of the leaves or the number of branches. In recent years, some research equipment have dealt with the problem through identifying the genomic regions involved in the regulation of these traits in ligneous species by means of QTL (Quantitative Trait Loci) mapping. In this sense, the work of Rae et al. (Five QTL hotspots for yield in short rotation coppice bioenergy poplar: the Poplar Biomass Loci. BMC Plant Biology 2009, vol. 9, p. 23) conducted with a family of 320 genotypes of the  Populus trichocarpa×P. deltoides  hybrid cultivated in short rotations is especially interesting. Five QTLs responsible for 20% of the variation among these genotypes in terms of biomass yield are identified in this work. It further describes the positive correlations existing between diameter and basal area of stems and branches from the first year and their biomass yield in that year and in the next five years, and between the number of sylleptic branches growing in the first year and the basal diameter and area of stems and branches from the first year. In most ligneous species in temperate latitudes, the lateral buds do not sprout during the same season in which they are formed. These buds, proleptic, must survive the winter to be able to sprout and form branches the next spring. In contrast, sylleptic buds which appear particularly during the juvenile stage of the tree do not need cold winters to develop into branches. 
         [0005]    Therefore, sylleptic branching does not only significantly increase the general growth of poplar, particularly throughout the first tears, but it also presents itself as a valuable trait which allows an early prediction the final biomass yield of a plantation. 
         [0006]    The RAV1 gene (Related to ABI3 and Viviparous 1) belongs to the family of transcription factors RAV, characterized by the presence of the DNA-binding domains AP2/EREBP (Apetala2/Ethylene Response Element Binding Factor) and B3 in its primary structure (Kagaya et al., RAV1, a novel DNA-binding protein, binds to bipartite recognition sequence through two distinct DNA-binding domains uniquely found in higher plants. Nucleic acids research 1999, vol. 27, p. 470-478; Yamasaki et al., Solution structure of the B3 DNA binding domain of the  Arabidopsis  cold-responsive transcription factor RAV1. The plant cell 2004, vol. 16, p. 3448-3459) According to the literature, in herbaceous plant  Arabidopsis thaliana  the genes RAV1 (AtRAV1; AT1G13260) and RAV2 (AtRAV2/TEM2; AT1G68840) would be involved in abscisic acid-independent cold response (Sakuma et al., DNA-binding specificity of the ERF/AP2 domain of Arabidopsis DREBs, transcription factors involved in dehydration- and cold-inducible gene expression. Biochemical and biophysical research communications 2002, vol. 290, p. 998-1009) and in the response to other external stimuli such as mechanical stimuli (Kagaya and Hattori,  Arabidopsis  transcription factors, RAV1 and RAV2, are regulated by touch-related stimuli in a dose-dependent and biphasic manner. Genes and genetic systems 2009, vol. 84, p. 95-99). AtRAV1 may also act as a negative regulator in the development of lateral roots and rosette leaves and together with the genes TEMPRANILLO1 (TEM1; AT1G25560) and AtRAV2/TEM2, in flowering time (Hu et al.,  Arabidopsis  RAV1 is down-regulated by brassinosteroid and may act as a negative regulator during plant development. Cell research 2004, vol. 61, p. 3947-3947; Castillejo and Pelaz, The balance between CONSTANS and TEMPRANILLO activities determines FT expression to trigger flowering. Current biology 2008, vol. 18, p. 1338-1343). AtRAV1 has also been described as a positive regulator in leaf senescence process (Woo et al., The RAV1 transcription factor positively regulates leaf senescence in  Arabidopsis . Journal of experimental botany 2010, vol. 61, p. 3947-3957). On the other hand, in tomato ( Solanum lycopersicum ), the RAV2 gene modulates the defense response against the pathogenic bacteria  Ralstonia solanacearum  (Li et al., Tomato RAV transcription factor is a pivotal modulator involved in the AP2/EREBP-mediated defense pathway. Plant Physiology 2011, vol. 156, p. 213-227). In these works, the function of  A. thaliana  genes AtRAV1 (Hu et al., 2004), TEM1 and AtRAV2/TEM2 (Castillejo and Pelaz, 2008) has been related with the control of the aspects of growth and the development different from branching regulation, since manipulating their expression in this herbaceous species does not cause a higher number of branches. However, none of these prior art publications relates the RAV1 gene with the regulation of branching development in any ligneous species. 
         [0007]    Application WO 2006/132616 A1 describes the regulatory function of genes RAV1 and RAV2 in plant gene silencing. This patent has a generic interest in plant genetic manipulation technology, but its protected subject-matter does not relate to the development processes controlled by said RAV genes nor does it relate to plant productivity. 
         [0008]    On the other hand, the suppression of genes RAV1 and RAV2 expression should inhibit the appearance of branches and lead to less nodes in the tree trunk, whereby high quality woods would be obtained. The inhibition of sylleptic and/or proleptic branching is thus revealed as a possible tool with great commercial value. 
         [0009]    Therefore, the problem that arises in the art effectively is to get a modification in the amount or quality of wood production from a ligneous plant, as needed, to increase the profit of its derivative products. The solution proposed by the present invention is to modify the expression of genes RAV1 and RAV2 in said ligneous plant and thus influence its sylleptic and/or proleptic branching production by technical methods. In addition, other objects, desirable features and characteristics will become apparent from the subsequent summary, and detailed description, and the appended claims, taken in conjunction with the accompanying drawings, sequence listing, and this background. 
       SUMMARY 
       [0010]    The present invention describes a gene modification applicable to ligneous species which allows modifying the development of sylleptic branches since the plants are very young. 
         [0011]    It consists of a biotechnological application of the RAV1 gene (Related to ABI3 and Viviparous 1) and its homologue RAV2 in relation to their capacity for increasing or reducing the degree of development of sylleptic and/or proleptic branching in a ligneous species when their expression levels, or the activity of the proteins encoded by them are modified. 
         [0012]    Therefore, a first embodiment of the invention is a method for increasing or reducing the development of sylleptic and/or proleptic branching in a ligneous plant with respect to its wild variety, which comprises modifying the expression of genes RAV1 and/or RAV2, or of their functional derivatives. This expression modification can be performed in the whole plant or specifically in a tissue or organ and can be permanent or temporary. 
         [0013]    In the scope of the present application, “expression of a gene” is understood as the protein product resulting from the array of mechanisms decoding the information contained in said gene processed by means of transcription, translation and post-translational modifications to the final form of the protein. 
         [0014]    The inventors have observed that, upon expressing the coding region of the RAV1 gene isolated from  Castanea sativa  Miller (CsRAV1) under the control of a constitutive promoter in the  Populus tremula×P. alba  (clone NRA 717 1-B4) hybrid, all the individuals of the ten transformation events which have been analyzed develop sylleptic branches from the lateral buds in a short time period. This time period is approximately one month after their transplant to soil from in vitro cultivation. In contrast, the development described does not occur in the wild type (WT) untransformed control poplars. It is deduced from this observation that the continuous overexpression of the CsRAV1 gene is responsible for the early formation of sylleptic branches in poplar. 
         [0015]    On the other hand, given the high sequence identity existing between the proteins RAV1 and RAV2 of  Populus trichocarpa  (91.9%) and therefore, their possible functional redundancy, the increase in the expression of a RAV2 gene may have a similar effect on branching as the overexpression of CsRAV 1. 
         [0016]    The genome of the genus  Populus  and specifically of the species  P. trichocarpa , the sequence of which is known and can be accessed freely through the NCBI or Phytozome public database, contains five genes encoding RAV transcription factors, two of which known as PtRAV1 and PtRAV2 are homologous to the genes AtRAV1, AtRAV2/TEM2 and TEM1 mentioned above. Other ligneous species such as  Eucalyptus grandis, Vitis vinifera, Citrus clementina, C. sinensis  or  Prunus persica , the genomic sequences of which are available in public databases, also contain genes homologous to PtRAV1, PtRAV2, AtRAV1, AtRAV2/TEM2 and TEM1. 
         [0017]    These are examples of polynucleotides originating from other plant species sharing a common evolutionary origin with CsRAV1, i.e., polynucleotides homologous to CsRAV1 encoding proteins with completely or partially equal or equivalent functions and which could also be used entirely or partially in the method of the invention. Therefore, a new aspect of the invention relates to the nucleotide sequences of other genes encoding polypeptides which, like CsRAV1, simultaneously contain the AP2 and B3 domains (Kagaya et al., RAV 1, a novel DNA-binding protein, binds to bipartite recognition sequence through two distinct DNA-binding domains uniquely found in higher plants. Nucleic acids research 1999, vol. 27, p. 470-478). The AP2 and B3 domains will be identified by means of bioinformatics applications such as Basic Local Alignment Search Tool (BLAST; Altschul et al., Gapped BLAST and PSI-BLAST: a new generation of protein database search programs. Nucleic Acid Research 1997, vol. 25, p. 3389-3402) or Conserved Domain Database (CCD; Marchler-Bauer et al., CDD: a Conserved Domain Database for the functional annotation of proteins. Nucleic Acid Research 2011, vol. 39, p. D225-D229). 
         [0018]    The first aspect of the invention is a tool for increasing the biomass production of a forestry plantation genetically modified in this manner, the biotechnological application of which is of great interest in various industrial sectors. 
         [0019]    Therefore, one embodiment is the method of the invention, wherein said modification of the expression of RAV1 and/or RAV2 is an overexpression. Said overexpression preferably comprises introducing a gene construct in the plant. In a more preferred embodiment, said gene construct comprises the fusion of a gene encoding a polypeptide simultaneously containing the AP2 and B3 domains, or their functional derivative to a promoter; said gene is preferably RAV 1 and/or RAV2 and is more preferably CsRAV1. In another preferred embodiment, the promoter used is a constitutive promoter, preferably CaMV35S, cauliflower mosaic virus 35S, or its functional derivative. In another preferred embodiment, said promoter is an inducible promoter. Another embodiment complementary to the foregoing is that said promoter is a tissue- or organ-specific promoter. 
         [0020]    Another embodiment of the invention is the CsRAV1 gene isolated from  Castanea sativa  Miller which regulates the sylleptic and proleptic production of the species and is identified by SEQ ID NO: 1. This SEQ ID NO: 1 as is reported includes an untranslated 5′ region prior to the start codon, such that a preferred embodiment is the CsRAV1 identified by a sequence with at least 91.7% of identity with respect to SEQ ID NO: 1. Another preferred embodiment is the protein obtained in the expression of CsRAV1 identified by SEQ ID NO: 2. The gene and the resulting protein can be used entirely or partially in the methods of the present invention. 
         [0021]    Another preferred embodiment is an expression vector comprising the CsRAV1 gene and the promoter CaMV35S, or their functional derivatives. Another very preferred embodiment is a ligneous plant with at least one CsRAV1-encoding polynucleotide, or its functional analogs, stably integrated in the genome of the cells. 
         [0022]    Another embodiment of the invention is the method of the invention wherein said overexpression is obtained at least by a mutation or an insertion of T-DNA in a gene encoding a polypeptide simultaneously containing the AP2 and B3 domains, said gene is preferably RAV 1 and/or RAV2, or their functional derivatives. 
         [0023]    In contrast, a reduction in the expression of the RAV1 gene in a ligneous species in which the sylleptic branching is frequent must result in the reduction thereof or in the reduction of proleptic branching the following year. 
         [0024]    This second aspect of the present invention does not only include the individual reduction in the expression of the genes RAV1 or RAV2 but it can include the joint reduction in the expression of both genes. That is to say, it would reduce the sylleptic and/or proleptic branching in a ligneous species thank of the reduction in the expression of genes RAV1 and/or RAV2 by gene silencing techniques, through the endogenous production of antibodies specific for the proteins RAV1 and/or RAV2 or by means of introducing mutations or T-DNA insertions. 
         [0025]    According to the foregoing, another embodiment is the method of the invention in which said modification of the expression of RAV 1 and/or RAV2 is a reduction in expression. 
         [0026]    This reduction in expression preferably comprises gene silencing, more preferably by interference RNAs, microRNAs or antisense messengers. Another embodiment is that said reduction comprises introducing a gene construct to the plant, and that preferably said gene construct comprises the fusion to a promoter of a gene encoding a polypeptide simultaneously containing the AP2 and B3 domains or their functional derivative. Said gene is preferably RAV1 and/or RAV2. A more preferred embodiment is that said promoter is a constitutive promoter, preferably CaMV35S, cauliflower mosaic virus 35S, or its functional derivative. Another preferred embodiment is that said promoter is an inducible promoter. Another embodiment complementary to the foregoing is that the promoter is a tissue- or organ-specific promoter. 
         [0027]    Another preferred embodiment of the invention is that the reduction in expression comprises the endogenous production of at least one antibody specific for a protein simultaneously containing the AP2 and B3 domains; said protein is preferably RAV1 and/or RAV2. 
         [0028]    Another preferred embodiment is that the reduction in expression comprises at least one mutation or an insertion of T-DNA in a gene encoding a polypeptide simultaneously containing the AP2 and B3 domains; said gene is preferably RAV1 and/or RAV2 or their functional derivatives. 
         [0029]    This second aspect of the invention involves a tool for reducing the number of nodes in the trunk in a ligneous species of logging interest genetically modified in this manner. 
         [0030]    The techniques for obtaining transgenic constructs and plants, as well as for handling the bioinformatics analysis tools referred to in the present application belong to the scope of genetic engineering, in vitro cultivation and bioinformatics and they are widely known by the persons skilled in the art. 
         [0031]    In one way or another, the genetic modification of the invention is potentially applicable to any genotype of a ligneous species, it therefore allows taking advantage of the adaptive characteristics of said genotype to a specific habitat. 
         [0032]    The present invention is therefore applicable in various industrial sectors. The increase in the biomass yield of short rotation forestry plantations may be interesting to the energy industry interested in using biomass for biofuel production, the chemical industry interested in preparing chemical products from this biomass, as well as the silvicultural industries and paper industries. 
         [0033]    Therefore, another additional embodiment of the invention is a cell of ligneous plant with modified RAV1 and/or RAV2 expression. Another preferred embodiment is a transgenic ligneous plant with modified RAV1 and/or RAV2 expression. Another embodiment is the plant product obtained from that transgenic ligneous plant, preferably a bioliquid and more preferably a biofuel. 
         [0034]    On the other hand, the reduction in sylleptic and/or proleptic branching and, therefore in the number of nodes in the trunk of a ligneous species can also be interesting for silvicultural and logging industries interested in utilizing harvestable forestry. 
         [0035]    Therefore, another very preferred embodiment is the wood obtained from a transgenic ligneous plant with modified RAV 1 and/or RAV2 expression. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0036]      FIG. 1  shows a relative abundance of the CsRAV1 transgene in ten independent transformants of the  Populus tremula×P. alba  (clone INRA 717 1-B4) hybrid. The independent transformants or events are designated as CsRAV1 OE (overexpressor) followed by the symbol “#” and a number. The values represent the average of three technical replicates ±SD (standard deviation). 
           [0037]      FIG. 2  shows images of (A) the CsRAV1 OE #59 event of  FIG. 1  of  Populus tremula×P. alba  (clone INRA 717 1-B4), where the development of sylleptic branches from the lateral buds shown with arrows accompanied by a general image of said event is seen; and (B) a wild type  Populus tremula×P. alba  (clone INRA 717 1-B4) individual in which this development does not occurs. The plants are 35-day old, counted from the transplant to soil from the in vitro cultivation. 
           [0038]      FIG. 3  shows relative abundance of the genes PtaRAV1 (bars with diagonal stripes) and PtaRAV2 (bars with horizontal stripes) in eight independent transformants of the  Populus tremula×P. alba  (clone INRA 717 1-B4) hybrid. The independent transformants or events are designated as PtaRAV1&amp;2 KD (knock-down) followed by the symbol “#” and a number. The values represent the average of three technical replicates ±SD (standard deviation). 
       
    
    
     DETAILED DESCRIPTION 
       [0039]    The following detailed description is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding background of the invention or the following detailed description. 
       EXAMPLES 
       [0040]    The following examples are provided to the present invention in an illustrative but non-limiting manner. 
       Example 1 
     Overexpression of Poplar RAV1 and RAV2 
       [0041]    The coding region of the RAV1 gene isolated from stems of  Castanea sativa  Miller (CsRAV1) was cloned in the binary vector pGWB2 (Nakagawa et al., Development of series of gateway binary vectors, pGWBs, for realizing efficient construct of fusion genes for plant transformation. Journal of bioscience and bioengineering 2007, vol. 104, p. 34-41) carrier of the constitutive promoter CaMV35S (cauliflower mosaic virus 35S). This construct was used for transforming the explants obtained from 4-week old young seedlings of the  Populus tremula×P. alba  (clone NRA 717 1-B4) hybrid cultivated in vitro via infection with  Agrobacterium tumefaciens  (strain GV3101/pMP90). In all the stages of the in vitro cultivation, the media were prepared with 1× Murashige &amp; Skoog 1B (Duchefa), 2% sucrose (Merck), 0.7 or 0.8% agar (explants and calluses or shoots and seedlings, respectively; BD Bacto), and were adjusted to pH 5.8 with 0.1 N sodium hydroxide. After two days of cultivation in a medium supplemented with 0.01 mg/L thidiazuron (Sigma-Aldrich) and 1 mg/L 2,4-dichlorophenoxyacetic acid (Sigma-Aldrich) to induce cell dedifferentiation in these explants, they were infected by submerging them for 15 minutes in a 2YT  A. tumefaciens  culture (16 g/L BD Bacto tryptone, 10 g/L BD Bacto yeast extract, 5 g/L sodium chloride Merck) with an optical density of 0.05 (λ=660). The explants thus infected continued to be cultivated in the same medium for two more days. They were then transferred to a medium supplemented with 0.02 mg/L thidiazuron, 1 mg/L 2,4-dichlorophenoxyacetic acid, 50 mg/L kanamycin sulfate (Roche), 20 mg/L hygromycin B (Duchefa) and 250 mg/L cefotaxime (Duchefa), and cultivated until obtaining calluses of 0.5 cm3 in size. These calluses were then transferred to a medium supplemented with 0.004 mg/L thidiazuron, 0.05 mg/L naphthaleneacetic acid (Sigma-Aldrich), 50 mg/L kanamycin sulfate, 20 mg/L hygromycin B and 250 mg/L cefotaxime so that their cells started to differentiate again and developed into shoots. Finally, when these shoots reached a height of 1 cm, they were dissected and rooted in a medium supplemented with 0.5 mg/L indolaecetic acid (Sigma-Aldrich), 50 mg/L kanamycin sulfate, 20 mg/L hygromycin B and 125 mg/L cefotaxime for generating whole poplar seedlings. Once analyzed, the seedlings expressing the transgene ( FIG. 1 ) were selected, transplanted to soil and cultivated in a greenhouse under long day conditions, 16 hours of light for 8 hours of darkness, at a temperature of 20±2° C. In these conditions and in the period between 30 and 40 days, when the plants reached a height between 30 and 40 cm, they started to develop sylleptic branching. In contrast, the non-transgenic control plants did not do so ( FIG. 2 ). 
       Example 2 
     Analysis of Transgene Expression by qRT-PCR 
       [0042]    Before transferring them to ground, the seedlings of Example 1 were analyzed by means of quantitative real-time PCR (qRT-PCR) to detect and quantify the expression of the transgene integrated in their nuclear genome and to dismiss those events in which the transgene was not expressed. The methods used for extracting total RNA for cDNA synthesis, as well as the composition and the conditions in which the qRT-PCR reactions were performed are detailed by Ibañez et al., Overall alteration of circadian clock gene expression in the chestnut cold response. PLoS ONE 2008, vol. 3, e3567 (doi: 10.1371/journal.pone.0003567). The RNA used for carrying out said analysis was obtained from 2-month old young plant leaves cultivated in vitro. The primers used for detecting the expression of CsRAV1 were those identified by SEQ ID NO: 3 and SEQ ID NO: 4. The analysis of the relative abundance of the transgene CsRAV1 in wild type individuals represents the negative control. 18s ribosomal RNA was used as a reference using the primers identified by SEQ ID NO: 5 and SEQ ID NO: 6 described in Böhlenius et al. (CO/FT regulatory module controls timing of flowering and seasonal growth cessation in trees. Science 2006, vol. 312, p. 1040-1043). 
       Example 3 
     Reduction in the Expression of Poplar RAV1 and/or RAV2 
       [0043]    In order to reduce the expression of the genes RAV 1 and RAV2, a hairpin interfering RNA (hpiRNA) construct from the RAV1 sequence of  Populus alba —homologue to CsRAV1 Gene—was generated in the binary vector pHELLSGATE12 (Helliwell and Waterhouse, Constructs and methods for high-throughput gene silencing in plants. Methods 2003, vol. 30, p. 289-295), carrier of the constitutive promoter CaMV35S. Specifically, to make this construct the region comprised between the DNA-binding domains AP2/EREBP and B3 of the RAV1 gene of  P. alba , the sequence of which is specified in SEQ ID NO: 7, was used Given the high overall degree of homology existing between the DNA coding sequences of the genes RAV1 (NCBI Reference Sequence XM — 002315922.1) and RAV2 (XM — 002311402.1) of  P. trichocarpa  (91.1%), the construct was designed for silencing the joint expression of the endogenous genes RAV1 and RAV2 and thus preventing a possible partial effect on branch development inhibition. This construct was used for transforming explants obtained from 4-week old young seedlings of the  Populus tremula×P. alba  (clone NRA 717 1-B4) hybrid cultivated in vitro through infection with  Agrobacterium tumefaciens  (cepa GV3101/pMP90). In all the stages of the in vitro cultivation, the media were prepared with 1× Murashige &amp; Skoog 1B (Duchefa), 2% sucrose (Merck), 0.7 or 0.8% agar (explants and calluses or shoots and seedlings, respectively; BD Bacto), and were adjusted to pH 5.8 with 0.1 N sodium hydroxide. After two days of cultivation in a medium supplemented with 0.01 mg/L thidiazuron (Sigma-Aldrich) and 1 mg/L 2,4-dichlorophenoxyacetic acid (Sigma-Aldrich) to induce cell dedifferentiation in these explants, they were infected by submerging them for 15 minutes in a 2YT  A. tumefaciens  culture (16 g/L BD Bacto tryptone, 10 g/L BD Bacto yeast extract, 5 g/L sodium chloride Merck) with an optical density of 0.05 (λ=660). The explants thus infected continued to be cultivated in the same medium for two more days. They were then transferred to a medium supplemented with 0.02 mg/L thidiazuron, 1 mg/L 2,4-dichlorophenoxyacetic acid, 50 mg/L kanamycin sulfate (Roche), 20 mg/L hygromycin B (Duchefa) and 250 mg/L cefotaxime (Duchefa), and cultivated until obtaining calluses of 0.5 cm3 in size. These calluses were then transferred to a medium supplemented with 0.004 mg/L thidiazuron, 0.05 mg/L naphthaleneacetic acid (Sigma-Aldrich), 50 mg/L kanamycin sulfate, 20 mg/L hygromycin B and 250 mg/L cefotaxime so that their cells started to differentiate again and develop into shoots. Finally, when these shoots reached a height of 1 cm, they were dissected and rooted in a medium supplemented with 0.5 mg/L indolaecetic acid (Sigma-Aldrich), 50 mg/L kanamycin sulfate, 20 mg/L hygromycin B and 125 mg/L cefotaxime for generating whole poplar seedlings. Once analyzed, the seedlings in which the expression of the endogenous genes PtaRAV1 and PtaRAV2 had been diminished with respect to the expression of said genes in wild type seedlings ( FIG. 3 ) were selected. These seedlings transplanted to soil and cultivated in a greenhouse under long day conditions, 16 hours of light for 8 hours of darkness, at a temperature of 20±2° C. The results of the proleptic branching can be obtained after the winter of 2011-2012. The logical hypothesis is that these plants must develop a lower number of branches and, therefore, the number of nodes in the wood obtained from their stems will also be less, thus increasing the commercial value of the wood. 
       Example 4 
     Analysis of PtaRAV1 and PtaRAV2 Expression by qRT-PCR 
       [0044]    Before transferring them to soil, the seedlings obtained from Example 3 were analyzed by means of quantitative real-time PCR (qRT-PCR) to quantify the expression of the endogenous genes PtaRAV1 and PtaRAV2 and to select those in which the expression thereof had been diminished with respect to their expression in wild type seedlings. The methods used for extracting total RNA for cDNA synthesis, as well as the composition and the conditions in which the qRT-PCR reactions were performed are detailed by Ibañez et al., Overall alteration of circadian clock gene expression in the chestnut cold response. PLoS ONE 2008, vol. 3, e3567 (doi: 10.1371/journal.pone.0003567). The RNA used for carrying out said analysis was obtained from 2-month old young plant leaves cultivated in vitro subjected to a temperature of 4° C. for 3 hours. The primers used for detecting the expression of PtaRAV1 were those identified with SEQ ID NO: 8 and SEQ ID NO: 9; for PtaRAV2 the primers were those identified with SEQ ID NO: 10 and SEQ ID NO: 11. The analysis of the relative abundance of the genes PtaRAV1 and PtaRAV2 in wild type individuals represents the positive control and the calibrator. 18s ribosomal RNA was used as a reference using the primers identified by SEQ ID NO: 5 and SEQ ID NO: 6 described in Böhlenius et al. (CO/FT regulatory module controls timing of flowering and seasonal growth cessation in trees. Science 2006, vol. 312, p. 1040-1043). 
         [0045]    While at least one exemplary embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment, it being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the invention as set forth in the appended claims and their legal equivalents.