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 s everal genes during water deficit. The expression patterns of the ABA-requi ring genes, le4, le16, le25 and his1-s, differed during a 4 d period of wat er deficit and a 24 h recovery period in intact tomato plants. During water deficit, a strong correlation between ABA content and mRNA accumulation wa s observed for le16, with only a moderate or poor correlation observed betw een 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 comp onents 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 expressio n of the four genes. However, the mRNA content of le25 increased in the pre sence of translation inhibitors plus ABA, implicating inhibition of a trans criptional repressor. The regulation of gene expression in response to stre ss 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.