Shade avoidance: phytochrome signalling and other aboveground neighbour detection cues

Plants compete with neighbouring vegetation for limited resources. In competition for light, plants adjust their architecture
to bring the leaves higher in the vegetation where more light is available than in the lower strata. These architectural
responses include accelerated elongation of the hypocotyl, internodes and petioles, upward leaf movement
(hyponasty), and reduced shoot branching and are collectively referred to as the shade avoidance syndrome. This
review discusses various cues that plants use to detect the presence and proximity of neighbouring competitors and
respond to with the shade avoidance syndrome. These cues include light quality and quantity signals, mechanical
stimulation, and plant-emitted volatile chemicals. We will outline current knowledge about each of these signals individually
and discuss their possible interactions. In conclusion, we will make a case for a whole-plant, ecophysiology
approach to identify the relative importance of the various neighbour detection cues and their possible interactions in
determining plant performance during competition.