High ambient temperature antagonizes ethylene-induced exaggerated apical hook formation in etiolated Arabidopsis seedlings.

Ethylene stimulates the exaggerated hook formation in etiolated seedlings. It has been reported that other phytohormones, such as jasmonate or gibberellins, could inhibit or coordinate hook formation, respectively. However, whether any environmental factors participate in this process is unknown. Here, we show that in darkness, high ambient temperature suppresses the ethylene-triggered exaggerated hook formation in wild-type plants and reduces the hook curvatures in constitutively ethylene responsive mutants. Interestingly, high temperature does not abrogate the activity of the central transcription factor EIN3, suggesting that high temperature acts downstream of ethylene signaling. Next, we show that the natural auxin levels in the hook regions are reduced and their asymmetric distributions are disturbed upon high temperature treatment. To explore the mechanisms for reducing auxin accumulation, we monitor the transcription of several genes encoding auxin biosynthesis related enzymes and find that most YUCCA genes are transcriptionally down-regulated. Finally, we show that the currently reported plant thermo-sensory components in light-grown plants (phytochrome or PIF4) are not sufficient for thermo-sensing in etiolated seedlings. We speculate that in darkness, plants sense high ambient temperature through a distinct mechanism. Taken together, we demonstrate that high temperature suppresses ethylene-induced exaggerated hook formation via the inhibition of local auxin activities.