Characterization of three mRNAs that accumulate in wilted tomato leaves in response to elevated levels of endogenous abscisic acid.

The accumulation of abscisic acid (ABA) has been shown to regulate some of the changes in gene expression which occur during water deficit. In order to characterize these ABA-induced changes, we have identified and isolated three copy DNAs (cDNAs) that represent genes which are expressed in response to ABA during drought stress. The ABA-deficient mutant of tomato, flacca, synthesizes low levels of ABA during water deficit compared to the wild type (WT) (Lycopersicon esculentum Mill. cv. Ailsa Craig). The mutant flacca was used to distinguish cDNAs corresponding to mRNAs which accumulate during water deficit in response to elevated levels of ABA from those mRNAs which are not ABA responsive. A cDNA library representing the mRNA population of wilted WT tomato leaves was constructed and a series of differential screens was used to select cDNAs that represent putative ABA- and drought-induced mRNAs. Three cDNAs were isolated from the screens and were identified as pLE4, pLE16, and pLE25. The corresponding mRNAs were preferentially expressed in wilted WT leaves and were not exessed in wilted ABA-deficient mutant leaves. The inability of the mutant to accumulate these drought-induced transcripts was reversed with exogenously applied ABA. A correlation was observed between the accumulation of the drought-induced mRNAs and the endogenous ABA levels measured in WT leaves throughout increasing periods of water deficit. These results indicate that endogenous ABA regulates the accumulation of pLE4, pLE16, and pLE25 mRNAs in tomato leaves during water deficit.