Electric signaling and Pin2 gene expression on different abiotic stimuli depend on a distinct threshold level of endogenous abscisic acid in several abscisic acid-deficient tomato mutants

Experiments were performed on three abscisic acid (ABA)deficient tomato (Ly copersicon esculentum Mill.) mutants, notabilis, flacca, and sitiens, to in vestigate the role of ABA and jasmonic acid (JA) in the generation of elect rical signals and Pin2 ((p) under bar roteinase <(in)under bar>inhibitor <( II)under bar>) gene expression. We selected these mutants because they cont ain different levels of endogenous ABA. ABA levels in the mutant sitiens we re reduced to 8% of the wild type, in notabilis they were reduced to 47%, a nd in flacca they were reduced to 21%. In wild-type and notabilis tomato pl ants the induction of Pin2 gene expression could be elicited by heat treatm ent, current application, or mechanical wounding. In flacca and sitiens onl y heat stimulation induced Pin2 gene expression. JA levels in flacca and si tiens plants also accumulated strongly upon heat stimulation but not upon m echanical wounding or current application. Characteristic electrical signal s evolved in the wild type and in the notabilis and flacca mutants consisti ng of a fast action potential and a slow variation potential. However, in s itiens only heat evoked electrical signals; mechanical wounding and current application did not change the membrane potential. In addition, exogenous application of ABA to wild-type tomato plants induced transient changes in membrane potentials, indicating the involvement of ABA in the generation of electrical signals. Our data strongly suggest the presence of a minimum th reshold value of ABA within the plant that is essential for the early event s in electrical signaling and mediation of Pin2 gene expression upon woundi ng. In contrast, heat-induced Pin2 gene expression and membrane potential c hanges were not dependent on the ABA level but, rather, on the accumulation of JA.