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Most plants grow in dense vegetation with the risk of being out-competed by neighboring plants. These neighbors can be detected not only through the depletion in light quantity that they cause, but also through the change in light quality, which plants perceive using specific photoreceptors. Both the reduction of the red:far-red ratio and the depletion of blue light are signals that induce a set of phenotypic traits, such as shoot elongation and leaf hyponasty, which increase the likelihood of light capture in dense plant stands. This set of phenotypic responses are part of the so called shade avoidance syndrome (SAS). This addendum discusses recent findings on the regulation of the SAS of Arabidopsis thaliana upon blue light depletion. Keller et al. and Keuskamp et al. show that the low blue light attenuation induced shade avoidance response of seedling and rosette-stage A. thaliana plants differ in their hormonal regulation. These studies also show there is a regulatory overlap with the R:FR-regulated SAS.

The shade avoidance syndrome serves to improve the competitive power of plants growing in crowded plant communities. An important element of avoiding shade is to rapidly elongate shoots and outgrow competing neighbours. We investigated the role of cell wall modifying proteins expansins and xyloglucan endotransglucosylase/hydrolases (XTHs) in mediating this vital elongation growth in Arabidopsis thaliana. These proteins act on the cell wall and modify it to make it more extensible thereby facilitating cellular expansion. We found that XTHs are essential for shade-induced growth in Arabidopsis. Expansin activity on the other hand was not regulated in plants exposed to shade. Shade also resulted in rapid apoplastic acidification which is necessary for the optimal activity of cell wall modifying proteins such as XTHs and expansins.

Plants growing in dense vegetations compete with their neighbors for resources such as water, nutrients and light. The competition for light has been particularly well studied, both for its fitness consequences as well as the adaptive behaviors that plants display to win the battle for light interception. Aboveground, plants detect their competitors through photosensory cues, notably the red:far-red light ratio (R:FR). The R:FR is a very reliable indicator of future competition as it decreases in a plant-specific manner though red light absorption for photosynthesis and is sensed with the phytochrome photoreceptors. In addition, also blue light depletion is perceived for neighbor detection. As a response to these light signals plants display a suite of phenotypic traits defined as the shade avoidance syndrome (SAS). The SAS helps to position the photosynthesizing leaves in the higher zones of a canopy where light conditions are more favorable. In this review we will discuss the physiological control mechanisms through which the photosensory signals are transduced into the adaptive phenotypic responses that make up the SAS. Using this mechanistic knowledge as a starting point, we will discuss how the SAS functions in the context of the complex multi-facetted environments that plants usually grow in.

Using time-lapse photography, we studied the response kinetics of low light intensity-induced upward leaf-movement, called hyponastic growth, in Arabidopsis thaliana. This response is one of the traits of shade avoidance and directs plant organs to more favorable light conditions. Based on mutant- and pharmacological data we demonstrated that among other factors, functional auxin perception and polar auxin transport (PAT) are required for the amplitude of hyponastic growth and for maintenance of the high leaf angle, upon low light treatment. Here, we present additional data suggesting that auxin and PAT antagonize the hyponastic growth response induced by ethylene treatment. We conclude that ethylene- and low light-induced hyponastic growth occurs at least partly via separate signaling routes, despite their strong similarities in response kinetics.

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.