Abstract
Arabidopsis thaliana seedlings exhibit longer hypocotyls when they are grown under high ambient temperature, which is defined as thermomorphogenesis. Although it is well established that high temperature triggers auxin biosynthesis to stimulate hypocotyl elongation, the physiological functions of other endogenous phytohormones during thermomorphogenesis are still elusive. Here, we report that exogenous application of abscisic acid (ABA) strongly inhibits hypocotyl elongation under high ambient temperature. Hypocotyl elongations of ABA biosynthesis deficient mutants are more sensitive to high temperature, suggesting that endogenous ABA has a robust inhibition effect. Moreover, blocking ABA perception or signaling impedes the negative effect of ABA. Finally, we show that ABA also suppresses the hypersensitivity to high temperature of an auxin over-accumulation mutant (yuc1D), indicating that activation of auxin signaling is not sufficient to override the repression by ABA. Taken together, we demonstrate that ABA is a negative regulator during plant thermomorphogenesis.