Indi an 1 0u rn al of Experimental Bi ology Vol. 37, October 1999, pp. 1001-1004 Effect of GA, ABA and water stress on leaf elongation and XET activity in barley Hordeum vulgare L. * M. Maheswa ri Nucl ear Resea rch Laboratory, Indian Agricultu ral Re search Institu te, New Delhi-liDO 12 , In dia Received 18 January 1999; rel·ised 12 JIII.I' 1999 The re lationship between xyloglucan e ll do transglycosyiase (XET) and leaf elongation was investigated in barley (H. vulgare L.) . Im posing of treatmen ts of gibberellic acid (G A), abscisic ac id (A BA) and watcr dctic it we re used to atlain stimJlation and inh ib iti on o f growth r es pec tive ly. In GA-respo nsive dwa rf mut ant M489, ledt elongation rat e a nd XET ac tivity in creased fo llowi ng trea tment with GA). Temporal aspects of GA sti mul ation indicated tha t XET is prohahl y involved in GA-s ti mulated leaf elongation. Feeding ABA decreased leaf length to 50% by 3 and 4 da ys aft er tr eatment. XET activit y assayed on day 3 wa s fou nd to be signific ant ly reduced in ABA fed leaves as compa red to disti ll ed wat er controls. Water stress redu ced the len gth of bo th 3,,1 and 4th leaves compa red to well-watered co ntrols from 4days aft er withholding wat er. XET activi ty decreased considerab ly under water stress in both leaves a nd foll owed similar trend as th at of leaf elongation. Ove rall, th e res ul ts po int to a link between ext r ac tabl e XET ac ti vi ty and .-nle s of leaf elongation , whic h indicates the pote nti al ro le of XET in leaf elongatio n. Xy lo glucan endo transglycosyl ase (XET) whi ch catal yses cleavage of the hemicellul ose xyloglucan and transfe rs th e newly formed poten tially reducing terminu s to a new XG9 oli gomer is be li eved to be a key enzy me in cell wa ll rec on stmc ti on XET has been suggested to allow ce ll ex pansion by cleaving xy log lu can molec ul es that tether adjace nt cellulose microfibril s and rejoini ng th e cut e nd s of cleaved tethers l . XET is thu s beli eved to be important in control of cell expans ion and organ growth. A wide va riety of plan t spec ies in cluding bryop hytes, dic()ts and monocots ha ve been shown to exhibit XET ac tivit / . Moreove r, th e hi ghest levels of activity have been demonstrated to be associated with regions of ac tive elongation in both dicots and mon ocots l A-8. A ge ne famil y encodes XET enzymes and five CDNA clones related to th e barley XET were characterized and spat ia l di stribution of the corresponding mR NAs we re determined throu ghollt th e elonga tin g leaf us in g clo ne specific hybridization probes 6 . High levels of XET acti VIty and it s as ;ociation with regions of active elongation in monocots is particularl y in teres ting, as unlike dicots wh erein xyloglucan is t he major cell wall hemicellu lose, monocots co nt ain glucurono arabinoxylans as the ir predo min ant hemicellulose with mixed linkage glucans and *Wo rk was done al : Div isi on of Plant In dustry, CSIRO, Canberm,ACT 2601. Aus tralia xylogl uc an each contributing appr ox imately 20 % on a mol e % ba sis 9. In view of the above th e prese nt study was undertaken to inv es ti gate th e relationship be tween leaf elongation and XET acti vit y In barl ey. Treatme nt s lik e us e of gibbere lli c ac id (GA ), abscisic acid (ABA) and water defic it we re imposed to attain stimulation and inhibition of grow th re spect iv ely. Materials and Methods Plant culture-Ba rl ey (Hordeum vulgare L) was grown in 20x 12x I 0 cm plasti c co nt aine rs wit h I : I mixture (v/v) of perl it e and ve rm iculi te in artificiall y lit growth cabinets ma intained at 18/ I JOC day/night and 12 h photoperiod in Ph ytotron at Ca nb erra. Plant s were irri ga ted with deio ni zed water a nd Ho ag land V2 N nutri ent solu t io n on each morning · and afternoon respectively. T he following experi me nt s we re co ndu cted using th e pian t culture protocol. Experiment I : T ime course study of induction of XET activity and leaf elonga ti on in response to g ib berel lic acid (GA) ap plicat io n: GA responsive dwarf mutant M489, whi ch had been isolated from var. Himalaya fo llow ing sodium azi de mut agenesis subjected to ba ck cros ' in g with the parent Hima lay a by Dr. P.M.Cha nd le r, Division of Pl ant Industry, CSlRO, Canberra was used in this experiment. As th e leaf 2 emerged ,