A light-dependent molecular link between competition cues and defence responses in plants.

Growth responses to competition and defence responses to the attack of consumer organisms are two classic examples of adaptive phenotypic plasticity in plants. However, the mechanistic and functional links between these responses are not well understood. Jasmonates, a family of lipid-derived signals, are potent growth inhibitors and central regulators of plant immunity to herbivores and pathogens, with both roles being evolutionarily conserved from bryophytes to angiosperms. When shade-intolerant plants perceive the proximity of competitors using the photoreceptor phytochrome B, they activate the shade-avoidance syndrome and downregulate jasmonate responses. Despite the central implications of this light-mediated change in the growth/defence balance for plant adaptation and crop yield, the mechanisms by which photoreceptors relay light cues to the jasmonate signalling pathway remain poorly understood. Here, we identify a sulfotransferase (ST2a) that is strongly upregulated by plant proximity perceived by phytochrome B via the phytochrome B-phytochrome interacting factor signalling module. By catalysing the formation of a sulfated jasmonate derivative, ST2a acts to reduce the pool of precursors of active forms of jasmonates and represents a direct molecular link between photoreceptors and hormone signalling in plants. The metabolic step defined by this enzyme provides a molecular mechanism for prioritizing shade avoidance over defence under intense plant competition.