Molecular mechanisms of shade tolerance in plants

Shade tolerance is an ecological concept used in a wide range of disciplines, from plant physiology to landscaping or gardening. It refers to the strategy of some plants to persist and even thrive in environments with low light levels because of the shade produced by the vegetation proximity (e.g. in the understory). Shade tolerance influences the organization, structure, functioning, and dynamics of plant communities. However, little is known about its molecular and genetic basis. By contrast, there is a good understanding on how plants deal with the proximity of other plants, a divergent strategy used by most crops to respond to vegetation proximity. While generally shade-avoiding species strongly elongate in response to the proximity of other plants, shade-tolerant species do not. Here we review the molecular mechanisms that control the regulation of hypocotyl elongation in shade-avoiding species as a reference framework to understand shade tolerance. Comparative studies indicate that shade tolerance is implemented by components also known to regulate hypocotyl elongation in shade-avoiding species. These components, however, show differential molecular properties that explain how, in response to the same stimulus, shade-avoiding species elongate while shade-tolerant ones do not.