Coordinated regulation of Arabidopsis thaliana development by light and gibberellins

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Abstract
Many developmental processes in plants are regulated jointly by light and by the gibberellins, yet the molecular basis of cross-talk between the two is not fully understood. Two groups now report results that reveal a signalling cascade that contributes to coordinated plant growth regulation by light and gibberellins. In the absence of gibberellins, DELLA proteins inhibit the binding of the transcription factor PIF3 (a phytochrome-interacting protein) to gene promoters. Gibberellins trigger the degradation of DELLA proteins, thereby allowing PIFs to bind to their target promoters and regulate gene expression. Light acts via a photoreceptor to destabilize PIF4. Thus the DELLA proteins, and competitive interactions between members of the PIF family, appear to key components linking light to the gibberellins. Light and gibberellins (GA) regulate multiple aspects of plant development, and this paper is one of two studies that provide molecular insights into the connection between these pathways. Without GA, DELLA proteins (GA-signalling repressors) interact with PIFs (phytochrome-interacting proteins) and inhibit their binding to gene promoters. GA triggers degradation of DELLA proteins, thereby allowing PIFs to bind to their target promoters and regulate gene expression. These results reveal a signalling cascade that contributes to coordinated plant growth regulation by light and gibberellins. Light and gibberellins (GAs) mediate many essential and partially overlapping plant developmental processes. DELLA proteins are GA-signalling repressors that block GA-induced development1. GA induces degradation of DELLA proteins via the ubiquitin/proteasome pathway2, but light promotes accumulation of DELLA proteins by reducing GA levels3. It was proposed that DELLA proteins restrain plant growth largely through their effect on gene expression4,5. However, the precise mechanism of their function in coordinating GA signalling and gene expression remains unknown. Here we characterize a nuclear protein interaction cascade mediating transduction of GA signals to the activity regulation of a light-responsive transcription factor. In the absence of GA, nuclear-localized DELLA proteins accumulate to higher levels, interact with phytochrome-interacting factor 3 (PIF3, a bHLH-type transcription factor) and prevent PIF3 from binding to its target gene promoters and regulating gene expression, and therefore abrogate PIF3-mediated light control of hypocotyl elongation. In the presence of GA, GID1 proteins (GA receptors) elevate their direct interaction with DELLA proteins in the nucleus, trigger DELLA protein’s ubiquitination and proteasome-mediated degradation, and thus release PIF3 from the negative effect of DELLA proteins.