Internal sugar allocation in response to a shade signal is regulated by concerted action of auxin and sucrose

Plants detect proximity to neighboring vegetation by light spectral signatures which are sensed by phytochrome photoreceptors. As a response, many species grow taller to out-compete their neighbors. In seedlings, the rapid elongation of hypocotyls requires enhanced supply of carbon resources from the cotyledons, transported as sucrose. The mechanisms of how phytochrome signaling regulates carbon allocation are unknown. We show that sucrose biosynthesis, particularly the step catalyzed by sucrose phosphate synthase (SPS), is a key determinant of hypocotyl elongation. Moreover, we show that auxin directs resource allocation to the elongating hypocotyl. Increased sucrose availability enhances elongation only when auxin levels are sufficient, highlighting the interdependence between sugar supply and auxin. In contrast, our data reveal that reduced sucrose availability does not impair neighbor-proximity-induced auxin synthesis and signaling in hypocotyls and cotyledons. Our findings shed light on the regulation of carbon allocation -- a process which is poorly understood, despite its importance for crop yield.