GIBBERELLIN-REGULATED PLANT GENES

Gibberellins (GAs) are involved in the control of a number of key deve lopmental processes in plants. including endosperm mobilisation stem e longation and flowering. In many of these systems, GA modulates the tr anscription of specific genes. The aim of this paper is to review curr ent progress in identifying and characterising GA-regulated genes, bot h the control of gene expression and the function of the gene products are discussed. The most well-characterised system in which GA is acti ve in controlling transcription is the aleurone layer of cereal grains , where it induces the synthesis of a range of hydrolytic enzymes, inc luding a-amylase. Analysis of the promoters of a Amyl and a-Amy2 genes by transient expression in aleurone cells and protoplasts together wi th DNase I footprinting and gel-retardation assays, has identified a n umber of cis-acting elements important for high-level, GA-regulated ex pression. In particular a GA-response element (GARE) including the seq uence TAACRRA has been characterised. Recent reports describe cDNA clo nes encoding trans-acting factors that bind to elements in the a-amyla se promoters. Expression of the factor capable of binding to the TAACR RA element is itself induced by GA. In elongating tissues, GA has been shown to control the expression of a number of genes, including the t onoplast intrinsic protein, a water channel which may regulate water f lux into the vacuole during cell expansion. In flower development. exp ression of flavonoid biosynthetic genes, such as chalcone synthase in Petunia corollas, is regulated by GA at the level of transcription. An alysis of GA-response mutants led to the suggestion that one consequen ce of GA action is to regulate its own biosynthesis. Genes encoding GA 20-oxidase and 3 beta-hydroxylase have recently been shown to be down regulated by applied GA, providing a possible mechanism for feedback regulation of the GA biosynthetic pathway. There is evidence that cell s perceive GA at the cell surface, implying the existence of a signal transduction system between plasma membrane and nucleus. This signal t ransduction system has barely begun to be elucidated but is likely to become a major focus of gibberellin research.