EXPRESSION OF XET-RELATED GENES AND ITS RELATION TO ELONGATION IN LEAVES OF BARLEY (HORDEUM-VULGARE L.)

Five cDNA clones were isolated from barley (Hordeum vulgare L.) that e ncoded mRNAs related to xyloglucan endotransglycosylase (XET), One of the clones encoded a protein with XET activity in vitro. Sequence comp arisons revealed five families of XET-related sequences, one of which (containing two of the barley genes) was novel, Hybridization studies using clone-specific probes indicated that the corresponding genes wer e represented once, or possibly twice, in the barley genome, Treatment of dwarf mutants with gibberellic acid (GA(3)), or homozygosity at th e 'slender' (sln1) locus, resulted in a 2.5-fold (approximately) stimu lation of blade elongation rate, Three of the five clones detected mRN As that were: maximally expressed towards the base of the blade, and p resent in greater quantities in GA(3)-treated or slender seedlings, Th e remaining two clones detected mRNAs that were maximally expressed in the middle of the blade, Relative elemental growth rate (REGR) profil es of leaves growing with or without GA(3) treatment revealed similar maximal REGR values despite a 2.5-fold difference in leaf elongation r ate, Segments of GA(3)-treated leaves attained their maximal REGR valu es more rapidly, this being associated with enhanced expression of the three 'basal' XET-related mRNAs, Highest XET activities were detected in the base of the elongation zone, and in GA(3)-treated seedlings a second activity peak was observed near the distal end of the elongatio n zone, We conclude that there are likely to be several XET isoenzymes with different expression patterns, and identify those XFT-related pr oteins potentially involved in leaf elongation.