Physiological regulation and functional significance of shade avoidance responses to neighbors

Abstract

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.

Figure 1

Shade avoidance responses in Petunia X hybrida, Nicotiana tabacum CV Samsun NN, Rumex palustris and Arabidopsis thaliana accession Col-0. For each species, the plant on the left represents a control plant and the plant on the right represents a plant under low R:FR conditions. Note that low R:FR-exposed plants display more vertical leaf angles, elongated stems and/or petioles and early flowering.

Figure 2

Schematic representation of some of the signal transduction steps in the process of SAS induction upon detection of neighbors in dense stands.

Figure 3

Ecotypic variation in Stellaria longipes (alpine versus prairie). (A and B) Native habitat for the alpine (rocky, virtually no competitors) and prairie (dense grassland) ecotype. (C) Shoot elongation responses in the alpine and prairie ecotype to low R:FR and true shade. Copyright American Society of Plant Biologists, www.plantphysiol.org, reproduced with permission from Sasidharan et al. (2008). (D) Acid-induced extension (µm/min) of young internodes as a measure for expansin activity in the two ecotypes in control, low R:FR and true shade conditions. Modified with permission from Sasidharan et al. (2008), copyright American Society of Plant Biologists, www.plantphysiol.org.