SPAs promote thermomorphogenesis via regulating the phyB-PIF4 module in Arabidopsis

High ambient temperature due to global warming has a profound influence on plant growth and development at all stages of life cycle. Plant response to high ambient temperature termed thermomorphogenesis is characterized by hypocotyl and petiole elongation, and hyponastic growth at seedling stage. However, the molecular mechanism of thermomorphogenesis is still rudimentary. Here, we show that a set of four SUPPRESSOR OF PHYA-105 (SPA) genes are required for thermomorphogenesis. Consistently, SPAs are necessary for global gene expression changes in response to high ambient temperature. SPA1 level is unaffected, while the thermosensor phyB is stabilized in the spaQ mutant at high ambient temperature. Furthermore, in the absence of four SPA genes, the pivotal transcription factor PIF4 fails to accumulate, indicating a role of SPAs in regulating the phyB-PIF4 module at high ambient temperature. SPA1 directly phosphorylates PIF4 in vitro, and a mutant SPA1 affecting the kinase activity fails to rescue the PIF4 level as well as the thermo-insensitive phenotype of spaQ, suggesting that the SPA1 kinase activity is necessary for thermomorphogenesis. Taken together, these data suggest that SPAs integrate light and temperature signaling via fine tuning the phyB-PIF4 module.