PAR1 and PAR2 integrate shade and hormone transcriptional networks

PHYTOCHROME RAPIDLY REGULATED1 (PAR1) and PAR2 are two negative regulators of shade avoidance syndrome (SAS) responses in Arabidopsis. PAR1 and PAR2 belong to the bHLH family of transcription factors and act as direct transcriptional repressors of auxin- and brassinosteroid-responsive genes. These observations led us to propose that PAR1 and PAR2 might integrate shade and hormone signals. After plant proximity perception by the phytochrome photoreceptors, the expression of PAR1, PAR2 and dozens of additional PAR genes is affected, initiating a complex web of transcriptional events instrumental for the establishment of the SAS responses. Studying the organization of this complex transcriptional network, that is, the interactions amongst the different PAR factors involved and how they are connected with the endogenous hormone-regulated transcriptional networks, seems therefore fundamental to understand how SAS is modulated.

Integration of light and hormone signals

After germination, seedlings follow one of two developmental programs, photo- and skotomorphogenesis, and the choice is determined by the interplay between environmental signals (light) and endogenous cues (the plant hormones gibberellins among others). In the December issue of Plant Journal we describe a molecular mechanism that allows the integration of light and gibberellin signaling to tightly control the switch between skoto- and photomorphogenesis. On one hand, the stability of HY5, a transcription factor required by light to promote photomorphogenesis, is enhanced in the light and in situations with compromised GA biosynthesis. And, on the other hand, the promotion of growth during etiolation is exerted by the PIF family of transcription factors, whose abundance is enhanced by the absence of light, and whose activity is regulated by functional interaction with gibberellin signaling. In this addendum we propose that the control of the activity of light-dependent transcription factors by gibberellins is a common theme in other developmental processes, such as shade avoidance and photoperiodic regulation of cell expansion.

Interaction of shade avoidance and auxin responses: a role for two novel atypical bHLH proteins

Plants sense the presence of potentially competing nearby individuals as a reduction in the red to far-red ratio of the incoming light. In anticipation of eventual shading, a set of plant responses known as the shade avoidance syndrome (SAS) is initiated soon after detection of this signal by the phytochrome photoreceptors. Here we analyze the function of PHYTOCHROME RAPIDLY REGULATED1 (PAR1) and PAR2, two Arabidopsis thaliana genes rapidly upregulated after simulated shade perception. These genes encode two closely related atypical basic helix-loop-helix proteins with no previously assigned function in plant development. Using reverse genetic approaches, we show that PAR1 and PAR2 act in the nucleus to broadly control plant development, acting as negative regulators of a variety of SAS responses, including seedling elongation and photosynthetic pigment accumulation. Molecularly, PAR1 and PAR2 act as direct transcriptional repressors of two auxin-responsive genes, SMALL AUXIN UPREGULATED15 (SAUR15) and SAUR68. Additional results support that PAR1 and PAR2 function in integrating shade and hormone transcriptional networks, rapidly connecting phytochrome-sensed light changes with auxin responsiveness.

Struggling for light: della regulation during plant-plant interactions

We recently described how DELLA proteins are involved in plant growth responses to neighbors in dense stands. These responses that are called shade avoidance include enhanced stem and petiole elongation and are a classic example of adaptive phenotypic plasticity. Although much is known about neighbor detection, much less is known about the signal transduction network downstream of these signals. We will discuss here how a group of growth-supressors, called DELLA proteins, are functionally regulated upon the detection of neighbors. DELLA proteins are degraded upon binding of gibberellin (GA) to its receptor, thus releasing the restraint of GA responses. We discuss here that GA positively regulates shade avoidance by reducing DELLA protein levels. Furthermore, we will show that this is an essential step in shade avoidance, but also that reduced DELLA abundance alone is not sufficient to induce these growth responses. It is concluded that GA-dependent DELLA degradation is one essential step in the signal transduction network from light-mediated neighbor detection towards adaptive shoot elongation responses.

The role of leaf lobation in elongation responses to shade in the rosette-forming forb Serratula tinctoria (Asteraceae)

BACKGROUND AND AIMS

Lobed leaves are considered selectively advantageous in conditions of high irradiance. However, most studies have involved woody species, with only a few considering the role of leaf lobation in herbaceous rosette species. In this study, it is hypothesized that, in addition to its adaptive value in high light, leaf lobation may add to the function of petioles as vertical spacers in herbaceous species in conditions of strong competition for light.

METHODS

To test this hypothesis, leaf development was examined under seasonally changing natural light conditions and a field experiment was conducted in which light climate was manipulated in a wooded meadow population of Serratula tinctoria.

KEY RESULTS

No changes in leaf lobation were observed in response to experimental shading or different natural light conditions. However, in tall herbaceous vegetation, plants with highly lobed leaves achieved significantly greater vertical elongation than plants with less-lobed leaves. In contrast to herbaceous shade, tree shade had no effect on leaf elongation, suggesting differential responsiveness to competition from neighbouring herbs versus overhead shade. In shading treatments, imposed shade could only be responded to by the elongation of leaves that were produced late in development.

CONCLUSIONS

The results show that extensive leaf lobation can enable greater leaf elongation in response to shade from surrounding herbaceous vegetation. The different morphological responses displayed by Serratula tinctoria to different types of shade demonstrate the importance of critically assessing experimental designs when investigating phenotypic plasticity in response to shade.

The effects of mechanical stress and spectral shading on the growth and allocation of ten genotypes of a stoloniferous plant

BACKGROUND AND AIMS

Because plants protect each other from wind, stand density affects both the light climate and the amount of mechanical stress experienced by plants. But the potential interactive effects of mechanical stress and canopy shading on plant growth have rarely been investigated and never in stoloniferous plants which, due to their creeping growth form, can be expected to respond differently to these factors than erect plants.

METHODS

Plants of ten genotypes of the stoloniferous species Potentilla reptans were subjected to two levels of mechanical stress (0 or 40 daily flexures) and two levels of spectral shading (15 % of daylight with a red:far red ratio of 0.3 vs. 50 % daylight and a red:far red ratio of 1.2).

KEY RESULTS

Mechanically stressed plants produced more leaves with shorter more flexible petioles, more roots, and more but less massive stolons. Responses to spectral shading were mostly in the opposite direction to thigmomorphogenesis, including the production of thinner, taller petioles made of more rigid tissue. The degree of thigmomorphogenesis was either independent of light climate or stimulated by spectral shading. At the genotypic level there were no clear correlations between responses to shade and mechanical stress.

CONCLUSIONS

These results suggest that in stoloniferous plants mechanical stress results in clones with a more compact, shorter shoot structure and more roots. This response does not appear to be suppressed by canopy shading, which suggests that wind shielding (reduced mechanical stress) by neighbours in dense vegetation serves as a cue that induces shade avoidance responses such as increased petiole elongation.

Remote sensing of future competitors: impacts on plant defenses

Far-red radiation (FR) reflected by green tissues is a key signal that plants use to detect the proximity of future competitors. Perception of increased levels of FR elicits a suite of responses collectively known as the shade-avoidance syndrome, which includes increased stem elongation, production of erect leaves, and reduced lateral branching. These responses improve the access to light for plants that occur in crowded populations. Responses to the proximity of competitors are known to affect the susceptibility to disease and predation in several organisms, including social animals. However, the impacts of warning signals of competition on the expression of defenses have not been explicitly investigated in plants. In the experiments reported here, we show that reflected FR induced a dramatic down-regulation of chemical defenses in wild tobacco (Nicotiana longiflora). FR altered the expression of several defense-related genes, inhibited the accumulation of herbivore-induced phenolic compounds, and augmented the performance of the specialist herbivore Manduca sexta. Complementary studies with tomato suggested that the effects of FR on defenses are mediated by the photoreceptor phytochrome B. The central implication of these results is that shade-intolerant species such as wild tobacco and tomato activate functional changes that affect their ability to cope with herbivore attack in response to phytochrome signals of future competition, even in the absence of real competition for resources. These findings suggest that competition overshadowed herbivory during the evolution of this group of species and add a new axis to the definition of the shade-avoidance syndrome.

New perspectives in flooding research: the use of shade avoidance and Arabidopsis thaliana

BACKGROUND

Complete submergence of Rumex palustris leads to hyponastic (upward) petiole growth followed by enhanced petiole elongation. Previous pharmacological experiments have provided insights into the signal transduction pathway leading to this combined 'escape' response. It will, however, be difficult to gain further knowledge using these methods. Consequently, new approaches are required.

SCOPE

Here we propose that different environmental signals resulting in similar phenotypes can help to understand better the submergence response. In this review, we show that both ethylene and shade induce similar growth responses in R. palustris and Arabidopsis thaliana. We illustrate how this can be exploited to unravel novel signalling components in submergence-induced elongation growth. Furthermore, we illustrate the potential of arabidopsis as a useful model in submergence research based on similarities with submergence-tolerant species such as R. palustris and the molecular opportunities it presents. This is illustrated by examples of current work exploring this concept.

CONCLUSIONS

Incorporating different model systems, such as arabidopsis and shade avoidance, into submergence research can be expected to create powerful tools to unravel signal transduction routes determining submergence tolerance.