SHOOT INDUCTION OF ABA-REQUIRING GENES IN RESPONSE TO SOIL DRYING

Plant responses to water deficit are dynamic and varied, requiring co- ordination between the shoot and root. Among these responses are alter ations in gene expression. The expression of four genes, le4, le16, le 20, and le25, which require increased ABA content for expression, was studied in tomato plants in which the root systems were divided betwee n two large pots to impose water deficit gradually and to control sign als from the root in response to soil drying without inducing a signal from the shoot. One group of plants had one-half of the roots watered , another group had both halves watered, and another group had neither halves watered. In unwatered plants, the expression of le4 and le25 c orrelated with ABA content, and that of le16 and le20 occurred before a detectable increase in leaf ABA content. The contrasting patterns of expression indicate a difference in sensitivity of these genes to ABA or an additional signalling mechanism. Ample evidence indicates that shoot processes such as stomatal closure are controlled by signals fro m the root. This study demonstrates that genes may also be induced in the shoot by signals from the root. Shoots of plants in which only hal f of the roots were watered showed no decrease in relative water conte nt and no increase in ABA content; however, three of the four genes, l e4, le16, and le20, were induced. Root-to-shoot communication plays a role in changes in gene expression and in alterations in physiological processes.