Coordinated regulation of Arabidopsis thaliana development by light and gibberellins

Abstract

Light and gibberellins (GAs) mediate many essential and partially overlapping plant developmental processes. DELLA proteins are GA-signalling repressors that block GA-induced development 1 . GA induces degradation of DELLA proteins via the ubiquitin/ proteasome pathway 2 , but light promotes accumulation of DELLA proteins by reducing GA levels 3 . It was proposed that DELLA proteins restrain plant growth largely through their effect on gene expression 4,5 . 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.Light and GA interact during Arabidopsis thaliana seedling development, regulating hypocotyl elongation, cotyledon opening and light-responsive gene expression; their pathways seem to converge at regulation of the abundance of DELLA proteins (GA pathway repressors) 3,6 . Arabidopsis has five DELLA proteins-RGA, GAI, RGL1, RGL2 and RGL3-defined by their unique DELLA domain and a conserved GRAS domain 4 . To analyse them in vivo, we raised antibodies against endogenous RGA and generated transgenic Arabidopsis expressing each of the five DELLA proteins with tandem affinity purification (TAP) tags ( Supplementary Fig. 1). The response of DELLA protein levels to exogenously applied GA 3 (an active form of GA) or PAC (paclobutrazol, a GA biosynthesis inhibitor) was examined. We found that one-hour-long GA treatment eliminates the majority of DELLA proteins, and this GA effect can be largely prevented by 100 mM MG132 (a 26S proteasome-specific inhibitor). PAC, on the other hand, promotes over-accumulation of DELLA proteins (Fig. 1). These results show for the first time in Arabidopsis that all the DELLA proteins are under negative control by GA and the proteasome. Next, we generated lines expressing TAPtagged RGAD17 and GAID17, which lack a 17 amino acid motif within the DELLA domain that is required for GA-induced degradation 7,8 . As expected, TAP-RGAD17 and TAP-GAID17 are completely resistant to GA and accumulate at higher levels than wild-type proteins, which cannot be further increased by PAC (Fig. 1, and *These authors contributed equally to this work. WTAnti-RPN6Anti-RGA Immunoblot analysis of RGA (by anti-RGA antibody) and TAP-DELLA proteins (by anti-MYC antibody) in various light-grown Ara...

A molecular framework for light and gibberellin control of cell elongation

“…DELLA repressor interaction with PIF3 is in fact described in a companion report 21 . Notably, whereas PIF3 and PIF4 primary function is in hypocotyl elongation control 3,26 , other PIFs have been Western detection was used to assess that similar amounts of the PIF4-HA protein (PIF4-HA) were recovered in both treatments.…”

“…Interestingly, phyB pif4 pif5 mutants remained taller than pif4 pif5 seedlings, indicating that phyBregulated factors other than PIF4 and PIF5 might also participate in hypocotyl growth control (see ref. 21). A positive regulatory function of PIF4 was further supported by chromatin immuno-precipitation (ChIP) assays using lines expressing a fusion of PIF4 to the haemagglutinin (HA) antigen (PIF4-HA).…”

Nuclear factor Y-mediated H3K27me3 demethylation of the SOC1 locus orchestrates flowering responses of Arabidopsis

“…The GA effect on flowering is genetically mediated by five DELLA proteins, GIBBERELLIC ACID INSENSITIVE (GAI), REPRESSOR OF ga1-3 (RGA), RGA-LIKE1 (RGL1), RGL2 and RGL3, which are key GA signalling repressors whose degradation is triggered by GA 22,23 . As transcriptional regulators, DELLA proteins have been shown to exert their function by recruiting other transcription factors rather than directly binding to their target genes [24][25][26][27] .…”

The GATA-type transcription factors GNC and GNL/CGA1 repress gibberellin signaling downstream from DELLA proteins and PHYTOCHROME-INTERACTING FACTORS

“…DELLA proteins repress GA responses, at least in part, by interfering with the DNA-binding activity of PHYTO-CHROME-INTERACTING FACTOR (PIF) basic helixloop-helix (bHLH) transcription factors (de Lucas et al 2008;Feng et al 2008). In Arabidopsis, PIFs constitute a multiprotein family with seven members, and the developmental roles of individual PIFs in isolation and in combination with other PIF family members have been elucidated in recent years; e.g., through the analysis of a pif1 pif3 pif4 pif5 quadruple mutant (Castillon et al 2007;Leivar et al 2008).…”