CONTROL OF OSMOTIN GENE-EXPRESSION BY ABA AND OSMOTIC-STRESS IN VEGETATIVE TISSUES OF WILD-TYPE AND ABA-DEFICIENT MUTANTS OF TOMATO

Osmotin gene expression and protein synthesis were up-regulated in you ng tomato (Lycopersicon esculentum cv. Rheinlands Ruhm) plants after a short exposure (24 h) to 150 mM NaCl or 100 mu M abscisic acid (ABA) or after dehydration to 80% of original plant fresh weight. Osmoticall y induced accumulation of osmotin mRNA was accompanied by a large incr ease in endogenous ABA levels. Increasing accumulation of osmotin prot ein was also observed during a longer exposure (7 days) to salt. Upon treatment with NaCl, osmotin mRNA levels increased in both root and le af tissues, with an additional longer transcript induced in roots. No induction of osmotin mRNA was observed upon salt or water stress of th e tomato ABA-deficient mutant sitiens. Treatment of sitiens with exoge nous ABA induced osmotin mRNA accumulation to the level normally found in salt-treated wild-type plants. However, salt stress alone enhanced accumulation of osmotin mRNA in plants of another tomato mutant (flac ca) which is also impaired in ABA synthesis, In tobacco plants carryin g an osmotin promoter/beta-glucuronidase (GUS) fusion gene, NaCl induc tion of GUS could be only partially blocked by the ABA inhibitor fluor idone. In flacca plants simultaneous treatment with NaCl and ABA resul ted in higher levels of osmotin transcript compared to those following treatment with NaCl alone, No accumulation of osmotin protein was obs erved after short- or long-term osmotic treatments of the mutants. Our results support previous evidence that osmotin gene expression may be triggered in part through ABA and in part through a separate pathway of gene activation.