Temporal and spatial frameworks supporting plant responses to vegetation proximity.

After perception of vegetation proximity by phytochrome photoreceptors, shade-avoider plants initiate a set of responses known as the Shade Avoidance Syndrome (SAS). Shade perception by the phytochrome B (phyB) photoreceptor unleashes the PHYTOCHROME INTERACTING FACTORs (PIFs) and initiates SAS responses. In Arabidopsis (Arabidopsis thaliana) seedlings, shade perception involves rapid and massive changes in gene expression, increases auxin production, and promotes hypocotyl elongation. Other components, such as phyA and ELONGATED HYPOCOTYL 5 (HY5), also participate in the shade regulation of the hypocotyl elongation response by repressing it. However, why and how so many regulators with either positive or negative activities modulate the same response remain unclear. Our physiological, genetic, cellular, and transcriptomic analyses showed that (1) these components are organized into two main branches or modules and (2) the connection between them is dynamic and changes with the time of shade exposure. We propose a model for the regulation of shade-induced hypocotyl elongation in which the temporal and spatial functional importance of the various SAS regulators analyzed here helps to explain the co-existence of differentiated regulatory branches with overlapping activities.

Photoreceptor Activity Contributes to Contrasting Responses to Shade in Cardamine and Arabidopsis Seedlings.

Plants have evolved two major ways to deal with nearby vegetation or shade: avoidance and tolerance. Moreover, some plants respond to shade in different ways; for example, Arabidopsis (Arabidopsis thaliana) undergoes an avoidance response to shade produced by vegetation, but its close relative Cardamine hirsuta tolerates shade. How plants adopt opposite strategies to respond to the same environmental challenge is unknown. Here, using a genetic strategy, we identified the C. hirsuta slender in shade1 mutants, which produce strongly elongated hypocotyls in response to shade. These mutants lack the phytochrome A (phyA) photoreceptor. Our findings suggest that C. hirsuta has evolved a highly efficient phyA-dependent pathway that suppresses hypocotyl elongation when challenged by shade from nearby vegetation. This suppression relies, at least in part, on stronger phyA activity in C. hirsuta; this is achieved by increased ChPHYA expression and protein accumulation combined with a stronger specific intrinsic repressor activity. We suggest that modulation of photoreceptor activity is a powerful mechanism in nature to achieve physiological variation (shade tolerance versus avoidance) for species to colonize different habitats.