Update on Gibberellin Signaling

Bioactive gibberellins (GAs) are plant hormones that control growth and development throughout the life cycle of the plant. Isolation and characterization of mutants that are impaired in GA biosynthesis have been crucial in both understanding the role of GAs in regulating plant development and elucidating the GA biosynthetic pathway (Davies, 1995; Hedden and Phillips, 2000). The broad spectrum of developmental processes controlled by GAs is clearly illustrated by the phenotypes of GA biosynthesis mutants. In general, these GA-deficient mutants exhibit a common phenotype: a dark-green dwarf plant that is defective in leaf expansion and stem elongation (Fig. 1). In many species, GA-deficient mutants also display defects during seed germination, altered floral initiation time, and are impaired in development of flower, fruit, and seed. In recent years our understanding of GA metabolism has advanced rapidly with the cloning of most biosynthetic and catabolic genes (Hedden and Phillips, 2000; Olszewski et al., 2002). In Arabidopsis and other plant species, GAmetabolism is tightly regulated by the GA-response pathway. This homeostatic mechanism involves the feedback and feedforward regulation of the respective biosynthetic and catabolic genes. In contrast to GA metabolism, much less is known about the perception of bioactive GAs, the downstream signaling components, and the changes in gene expression that elicit physiological responses. The cereal aleurone has provided an ideal system to study GA response, with GAs specifically promoting the rapid expression of genes encoding hydrolases, such as a-amylases (Lovegrove and Hooley, 2000; Olszewski et al., 2002). In addition, aleurone studies have provided evidence for a plasma membrane localized GA receptor and have identified GAMYB as an early GA-response gene that is important in activating expression of a-amylase. Although the cereal aleurone system has vastly improved our knowledge of GA response, it has helped to identify only a few genes encoding primary signaling components. The identification of GA-response mutants in many plants has proven instrumental in elucidating the GAsignaling pathway (Richards et al., 2001; Olszewski et al., 2002). The model plant Arabidopsis initially provided the opportunity to use molecular genetics to isolate genes encoding GA-signaling components (Table I). The studies in Arabidopsis not only provided important insights into the GA signal transduction cascade, but also led to the isolation of orthologous genes in other plant species. In fact, many of the components identified so far are highly conserved and have similar roles in GA signaling in their respective species. Once the novel GA-signaling components have been identified, the rapid, transient assay using the cereal aleurone system will provide a powerful tool to define the functional motifs of GA-signaling components. The aim of this update is to focus on recent advances in our understanding of the Arabidopsis GA-signaling pathway, with a particular emphasis on how these studies have interfaced with investigations in other plant species.