Uncoupling DELLAs Function in Growth Repression and Abiotic Stress Signalling

Résumé

DELLA proteins play a master role in the control of gibberellin (GA) signalling. In the absence of GAs, these GRAS repressors act as growth inhibitors through the interaction with several transcription factors (TFs) or signalling regulators, like the PHYTOCHROME INTERACTING FACTORS (PIFs), and prevent them for binding to DNA, thus interfering with the expression of their target genes. DELLA proteins can also co-occupy promoter regions in a complex with TFs by binding a protein region different than the DNA-binding domain, thus acting as co-activators or co-repressors of their transcriptional partners (Davière and Achard, 2015). Moreover, GA-deficient/-insensitive mutants which accumulate DELLAs are more tolerant to abiotic stresses, although the molecular mechanisms driving this response are little understood (Colebrook et al., 2014). In this work, we postulate that DELLAs lead to growth inhibition and protection against abiotic stresses via each of these alternative sequestration or co-activation mechanisms. Our aim has been to uncouple both functions by identifying allelic mutations that interfere with DELLA-PIFs interaction, and therefore cancel their blocking effects on these growth promoting factors. To do so, we have screened an allelic GAI library for mutations that compromise the interaction of this protein with the PIF DNA binding domain but preserve its binding to the WUSCHEL-related factor WOX9, conferring tolerance to salinity. Mutations A325V, D315G and A325T identified by Y2H locate into the second leucine heptad repeat (LHRII) of the GRAS DELLAs domain and are exposed to the protein surface, revealing this external region as a PIF-interacting domain. Plants expressing a stable form of the GAI A325T allele show longer floral stems, develop more branches and generate a greater number of siliques and seeds than those expressing a stable form the wild-type protein, whereas both lines display a similar behaviour upon salt stress. In addition, we have in vivo validated a role of several DELLA-interacting TFs in enhancing plant tolerance to salinity in greenhouse assays. New TFs found to confer tolerance to salt stress include SEPALLATA 2 (SEP2), AINTEGUMENTA (ANT) and TRASPARENT TESTA 2 (TT2), therefore identifying these TFs as novel loci to the selection of genotypes that better cope with the osmotic and ionic toxicity effects of saline irrigation water.