Photosensory Cues in Plant–Plant Interactions: Regulation and Functional Significance of Shade Avoidance Responses

Abstract

Plants growing in dense vegetations compete with their neighbors for resources such as water, nutrients, and light. Particularly, competition for light has been thoroughly studied, both for 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 through 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 chapter, we 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 discuss how the SAS functions in the context of the complex multifacetted environments that plants usually grow in. Special attention is paid to trade-offs between SAS and defense against attackers, since recent breakthrough papers have elucidated some of the mechanisms behind this interaction.