Biphasic Control of Cell Expansion by Auxin Coordinates Etiolated Seedling Development

Abstract

Seedling emergence is critical for food security. It requires rapid hypocotyl elongation and apical hook formation, both of which are mediated by regulated cell expansion. How these events are coordinated in etiolated seedlings is unclear. Here, we show that biphasic control of cell expansion by the phytohormone auxin underlies this process. Shortly after germination, high auxin levels restrain elongation. This provides a temporal window for apical hook formation, involving a gravity-induced auxin maximum on the eventual concave side of the hook, triggering PP2C.D1controlled asymmetrical H+-ATPase activity, resulting in differential cell elongation. Subsequently, auxin concentrations decline acropetally and switch from restraining to promoting elongation, driving hypocotyl elongation. Our findings elucidate how differential auxin concentrations throughout the hypocotyl coordinate etiolated development, leading to successful soil emergence. One-Sentence SummaryAuxin concentration-dependent cell expansion coordinates hypocotyl elongation and apical hook development for soil emergence.