Resumen de Anàlisi mecanística, temporal i espacial de la xarxa reguladora de les respostes de les plantes a la proximitat vegetal
Light is essential for plants, not only for fueling photosynthesis, but also as environmental information that allows them to adapt their development to thrive in different scenarios. In shade-avoider (sun-loving) plants, such as Arabidopsis thaliana, proximity of potentially competing vegetation triggers a group of responses known as Shade Avoidance Syndrome (SAS), including the well-studied hypocotyl elongation. Phytochromes are the photoreceptors that detect vegetation proximity signals that, by binding to PHYTOCHROME INTERACTING FACTORs (PIFs), integrate light signals into acclimation changes in plant development. The current regulatory model states that the shade-induced inactivation of phytochrome B (phyB) releases the repression imposed over PIFs, which results in the rapid activation of gene expression changes and in hypocotyl elongation promotion.
PIFs contain an active phyB-binding (APB) motif and a basic helix-loop-helix (bHLH) domain, responsible of DNA-binding. Among PIFs, the photostable PIF7 has a major role in promoting hypocotyl elongation in shade. However, it is unknown if PIF7 DNA- and phyB-binding activities can modulate independently different aspects of the light-regulated development, as it occurs in the well-studied and founder member PIF3. To address this question, we carried out a structure-function analysis by generating PIF7 derivatives with the bHLH and the APB domains mutated. In the first chapter of this work we show that both the PIF7 DNA- and phyB-binding activities are linked and fundamental to rapidly induce the expression of PIF7 targets and the hypocotyl growth in shade.
Other components classified as positive and negative regulators participate in the modulation of SAS by building a complex regulatory network. Nevertheless, it is still unclear how these components are organized. To refine the architecture of this signaling network and establish the connections between its components, in the second chapter of this thesis we carried out physiological, cell biology and transcriptomic analyses of the shade-induced hypocotyl elongation using defective mutants of the different regulators. Our results demonstrate that the SAS regulatory components are organized in two main branches that act in slightly different moments and modulate the elongation of different cells along the hypocotyl axis. Nevertheless, we also found a signaling convergence between the two branches, as PIFs and ELONGATED HYPOCOTYL 5 (HY5), components that belong to these separate branches, regulate the expression of common target genes.
