A PIF-SAUR module safeguards hypocotyl elongation from ABA inhibition in the dark.

Abstract

Driven by cell elongation, hypocotyl growth is tightly controlled by light and responds to the stress signaling hormone abscisic acid (ABA). However, the molecular connections between ABA and light to control cell elongation are poorly understood. Here, we show that, in Arabidopsis, ABA inhibits hypocotyl elongation in the light but not in the dark. In the dark, hypocotyl sensitivity to ABA is restored in <i>pifq</i> and <i>cop1-4</i> mutants, suggesting that an active light signaling pathway is necessary for hypocotyl responsiveness to ABA. Through an RNA sequencing subtractive approach, we identified ABA differentially expressed genes that correlate with ABA inhibition of hypocotyl elongation, including several <i>SAUR</i> genes. The abrogation of PP2C.D2-SAUR interaction in the <i>pp2c.d2</i>/<i>PP2C.D2^M331K-GFP</i> line restored ABA sensitivity in the dark, suggesting that SAUR proteins are key to maintain hypocotyls insensitive to ABA. This hypocotyl-specific mechanism enables growth toward the light, overriding ABA inhibition of cell elongation, ensuring subsequent seedling establishment and survival.