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

Growth responses to competition <sup>1</sup> and defence responses to the attack of consumer organisms <sup>2</sup> 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 <sup>3,4</sup> , with both roles being evolutionarily conserved from bryophytes <sup>5</sup> to angiosperms <sup>6</sup> . When shade-intolerant plants perceive the proximity of competitors using the photoreceptor phytochrome B, they activate the shade-avoidance syndrome and downregulate jasmonate responses <sup>7</sup> . Despite the central implications of this light-mediated change in the growth/defence balance for plant adaptation and crop yield <sup>8,9</sup> , the mechanisms by which photoreceptors relay light cues to the jasmonate signalling pathway remain poorly understood <sup>10</sup> . 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