Shoot induction of ABA-requiring genes in response to soil drying

Plant responses to water deficit are dynamic and varied, requiringco-ordination between the shoot and root. Among these responsesare alterations in gene expression. The expression of four genes,le4, le16, le20, and le25, which require increased ABA contentfor expression, was studied in tomato plants in which the rootsystems were divided between two large pots to impose waterdeficit gradually and to control signals from the root in responseto soil drying without inducing a signal from the shoot. Onegroup of plants had one-half of the roots watered, another grouphad both halves watered, and another group had neither halveswatered. In unwatered plants, the expression of le4 and le25correlated with ABA content, and that of le16 and le20 occurredbefore a detectable increase in leaf ABA content. The contrastingpatterns of expression indicate a difference in sensitivityof these genes to ABA or an additional signalling mechanism.Ample evidence indicates that shoot processes such as stomatalclosure are controlled by signals from the root. This studydemonstrates that genes may also be induced in the shoot bysignals from the root. Shoots of plants in which only half ofthe roots were watered showed no decrease in relative watercontent and no increase in ABA content; however, three of thefour genes, le4, le16, and le20, were induced. Root-to-shootcommunication plays a role in changes in gene expression andin alterations in physiological processes. Key words: Abscisic acid, water deficit, gene expression, split-root plants, long-distance signal