CIRCADIAN CLOCK ASSOCIATED 1 Expression Responds to High Temperature Precisely Through Alternative Splicing in Arabidopsis

ABSTRACT

The circadian clock enables plants to adapt to ambient temperature fluctuations. In Arabidopsis thaliana , CIRCADIAN CLOCK ASSOCIATED 1 ( CCA1 ) generates distinct isoforms via alternative splicing (AS), but how this mediates clock stability under high temperature (HT) remains unclear. Here, we show that CCA1 precisely discriminates HT gradients through AS: under moderate HT (28°C–37°C), CCA1βab isoforms ( CCA1βa / βb ) are upregulated, with CCA1βb specifically accumulating at 37°C, while CCA1α remains stable; at extreme HT (42°C), CCA1 is globally upregulated without isoform specificity. The HT response of CCA1 is time‐dependent, CCA1βab is transiently induced around dawn, whereas CCA1α exhibits delayed, AS‐dependent upregulation with thermal memory to anticipate subsequent HT. Tissue‐specific AS was observed, with CCA1α upregulated in roots/stems but downregulated in shoots/flowers under HT. Notably, the temperature‐dependent shift in the CCA1βa ‐to‐ CCA1βb isoform ratio is involved in fine‐tuning plant defense against biotic stresses and coordinating reactive oxygen species homeostasis. Our findings uncover a sophisticated regulatory network where CCA1 integrates temperature signals via AS, ensuring plant adaptation to thermal fluctuations while preserving circadian stability.