DELLA protein function in growth responses to canopy signals

Abstract

SummaryPlants can sense neighbour competitors through light-quality signals and respond with shade-avoidance responses. These include increased shoot elongation, which enhances light capture and thus competitive power. Such plant-plant interactions therefore profoundly affect plant development in crowded populations. Shade-avoidance responses are tightly coordinated by interactions between light signals and hormones, with essential roles for the phytochrome B photoreceptor [sensing the red:far red (R:FR) ratio] and the hormone gibberellin (GA). The family of growth-suppressing DELLA proteins are targets for GA signalling and are proposed to integrate signals from other hormones. However, the importance of these regulators has not been studied in the ecologically relevant, complex realm of plant canopies. Here we show that DELLA abundance is regulated during growth responses to neighbours in dense Arabidopsis stands. This occurs in a R:FR-dependent manner in petioles, depends on GA, and matches the induction kinetics of petiole elongation. Similar interactions were observed in the growth response of seedling hypocotyls and are general for a second canopy signal, reduced blue light. Enhanced DELLA stability in the gai mutant inhibits shade-avoidance responses, indicating that DELLA proteins constrain shade-avoidance. However, using multiple DELLA knockout mutants, we show that the observed DELLA breakdown is not sufficient to induce shade-avoidance in petioles, but plays a more central role in hypocotyls. These data provide novel information on the regulation of shade-avoidance under ecologically important conditions, defining the importance of DELLA proteins and GA and unravelling the existence of GA-and DELLA-independent mechanisms.

Phytochrome‐mediated inhibition of shade avoidance involves degradation of growth‐promoting bHLH transcription factors

“…Functional redundancy between the various members of the PIF family is one possibility. Another possibility is that several distinct mechanisms coordinately control shade avoidance (Djakovic-Petrovic et al, 2007).…”

PIFs: Systems Integrators in Plant Development

“…According to this model, the long hypocotyl of the della mutant is explained by higher PIF activity in the absence of the DELLA repressors and therefore should be suppressed by the pif mutations, a genetic validation of the model that is still lacking. This PIF/DELLA mechanism might also be operating in shade, since DELLAs have also been shown to constrain growth under these conditions (Djakovic-Petrovic et al, 2007).…”

Dynamic Antagonism between Phytochromes and PIF Family Basic Helix-Loop-Helix Factors Induces Selective Reciprocal Responses to Light and Shade in a Rapidly Responsive Transcriptional Network in Arabidopsis

“…As BIM2 and BEE1 have been implicated in the brassinosteroid signaling pathway (Friedrichsen et al, 2002;Yin et al, 2005), these genes are potential regulatory nodes for PIF and brassinosteroid crosstalk (Sorin et al, 2009;Kozuka et al, 2010;Keller et al, 2011;Keuskamp et al, 2011) in response to shade. The presence of CKX5 (a cytokinin oxidase-encoding gene) and GA2OX6 (a gibberellin-2 oxidase) (Figure 2C, top; see Supplemental Data Set 18 online) suggests that cytokinin and gibberellin levels (two other proposed shade-regulatory hormones; Carabelli et al, 2007;Djakovic-Petrovic et al, 2007;Stamm and Kumar, 2010), in addition to auxin levels, may be early targets of PIF-mediated shade regulation. It is notable that GIBBERELLIC ACID INSENSITIVE (GAI), a repressor of the gibberellin pathway proposed to constrain shade avoidance responses (Djakovic-Petrovic et al, 2007), is induced by the shade signal in a robustly PIF-dependent manner.…”

“…The presence of CKX5 (a cytokinin oxidase-encoding gene) and GA2OX6 (a gibberellin-2 oxidase) (Figure 2C, top; see Supplemental Data Set 18 online) suggests that cytokinin and gibberellin levels (two other proposed shade-regulatory hormones; Carabelli et al, 2007;Djakovic-Petrovic et al, 2007;Stamm and Kumar, 2010), in addition to auxin levels, may be early targets of PIF-mediated shade regulation. It is notable that GIBBERELLIC ACID INSENSITIVE (GAI), a repressor of the gibberellin pathway proposed to constrain shade avoidance responses (Djakovic-Petrovic et al, 2007), is induced by the shade signal in a robustly PIF-dependent manner. Since DELLA proteins, including GAI, have been proposed to constrain growth by inhibiting the PIFs (de Lucas et al, 2008;Feng et al, 2008), the data suggest that the PIFs induce a complex negative-feedback regulatory loop, possibly to modulate the response to the shade signal.…”