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