Multiple mechanisms control the expression of abscisic acid (ABA)-requiring genes in tomato plants exposed to soil water deficit

Elevated levels of abscisic acid (ABA) are required for the expression of several genes during water deficit. The expression patterns of the ABA-requiring genes, le4, le16, le25 and his1-s, differed during a 4 d period of water deficit and a 24 h recovery period in intact tomato plants. During water deficit, a strong correlation between ABA content and mRNA accumulation was observed for le16, with only a moderate or poor correlation observed between the accumulation of ABA and the of her mRNAs, The regulation of le4 and le16 can be accounted for by an increase in transcription rate, while components of post-transcriptional regulation are implicated in the regulation of the le25 and his1-s genes. A strong correlation between leaf ABA content and the transcription rate of le4 and le16 during the stress and recovery periods was observed. Protein synthesis was not necessary for the expression of the four genes. However, the mRNA content of le25 increased in the presence of translation inhibitors plus ABA, implicating inhibition of a transcriptional repressor. The regulation of gene expression in response to stress is complex; although ABA was required for the regulation of the genes in this study, there must be additional control mechanisms that interact with components of the ABA regulatory machinery.