Biphasic control of cell expansion by auxin coordinates etiolated seedling development

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. This auxin maximum induces PP2C.D1 expression, leading to asymmetrical H+-ATPase activity across the hypocotyl that contributes to the differential cell elongation underlying hook development. Subsequently, auxin concentrations decline acropetally and switch from restraining to promoting elongation, thereby driving hypocotyl elongation. Our findings demonstrate how differential auxin concentrations throughout the hypocotyl coordinate etiolated development, leading to successful soil emergence.

Automated summary

This study reveals the mechanism by which auxin regulates hypocotyl elongation and apical hook formation in etiolated seedlings. Shortly after germination, high levels of auxin inhibit elongation, providing a time window for apical hook development. Gravity-induced auxin maximum on the concave side of the hook induces gene expression that leads to differential cell elongation. Subsequently, auxin concentrations decline and switch from inhibiting to promoting elongation, thereby driving hypocotyl elongation.