bZIP transcription factor RSG controls the feedback regulation of NtGA20ox1 via intracellular localization and epigenetic mechanism

Abstract

Gibberellins (GAs) are phytohormones that regulate growth and development throughout the life cycle of plants. Negative feedback contributes to homeostasis of GA levels. DELLA proteins are involved in this process. Since DELLA proteins do not have apparent DNA binding motifs, other DNA binding proteins might act as a mediator downstream of DELLA proteins in the GA feedback regulation. In this review, we highlight the mechanisms of GA feedback regulation, specifically the differential regulation of GA 20-oxidase (GA20ox) and GA 3-oxidase (GA3ox) by transcription factors. RSG (REPRESSION OF SHOOT GROWTH) is a tobacco (Nicotiana tabacum) transcriptional activator with a basic leucine zipper domain that controls the levels of endogenous GAs through the regulation of GA biosynthesis genes. Recently we reported that RSG not only regulates the expression of ent-kaurene oxidase gene but is also involved in the negative feedback of NtGA20ox1 by GAs. RSG plays a role in the homeostasis of GAs through direct binding to the NtGA20ox1 promoter triggered by a decrease in GA levels in the cell. Furthermore, decreases in GA levels promote modifications of active histone marks on the NtGA20ox1 promoter. We have developed a hypothetical model to explain how RSG regulates dual target genes via epigenetic regulation.

Figure 1

Model of feedback regulation of NtGA20ox1 through the regulation of histone modification and the intracellular localization of RSG. (A) Under normal condition, RSG is localized in both the cytoplasm and the nucleus. RSG is phosphorylated by NtCDPK1, which is activated by GAs. The phosphorylated RSG is sequestered into the cytoplasm by 14-3-3 proteins. Histone H3 Lys9 on the NtGA20ox1 promoter is dimethylated under these conditions preventing the binding of RSG to the NtGA20ox1 promoter. (B) Under GA-deficient conditions, RSG is dephosphorylated and accumulates in the nucleus. Histone H3 Lys9 on the NtGA20ox1 promoter is acetylated, allowing the binding of RSG with the NtGA20ox1 promoter, to activate transcription of the gene.