SPAs promote thermomorphogenesis by regulating the phyB-PIF4 module in Arabidopsis.

High ambient temperature attributable to global warming has a profound influence on plant growth and development at all stages of the life cycle. The response of plants to high ambient temperature, termed thermomorphogenesis, is characterized by hypocotyl and petiole elongation and hyponastic growth at the seedling stage. However, our understanding of the molecular mechanism of thermomorphogenesis is still rudimentary. Here, we show that a set of four SUPPRESSOR OF PHYA-105 (SPA) genes is required for thermomorphogenesis. Consistently, SPAs are necessary for global changes in gene expression in response to high ambient temperature. In the spaQ mutant at high ambient temperature, the level of SPA1 is unaffected, whereas the thermosensor phytochrome B (phyB) is stabilized. 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 in addition to the thermo-insensitive phenotype of spaQ, suggesting that the SPA1 kinase activity is necessary for thermomorphogenesis. Taken together, these data suggest that SPAs are new components that integrate light and temperature signaling by fine-tuning the phyB-PIF4 module.

Direct phosphorylation of HY5 by SPA kinases to regulate photomorphogenesis in Arabidopsis.

Elongated hypocotyl5 (HY5) is a key transcription factor that promotes photomorphogenesis. Constitutive photomorphogenic1 (COP1)-Suppressor of phytochrome A-105 (SPA) E3 ubiquitin ligase complex promotes ubiquitination and degradation of HY5 to repress photomorphogenesis in darkness. HY5 is also regulated by phosphorylation at serine 36 residue. However, the kinase responsible for phosphorylation of HY5 remains unknown. Here, using extensive in vitro and in vivo biochemical, genetic, and photobiological techniques, we have identified a new kinase that phosphorylates HY5 and demonstrated the significance of phosphorylation of HY5 in Arabidopsis thaliana. We show that SPA proteins are the missing kinases necessary for HY5 phosphorylation. SPAs can directly phosphorylate HY5 in vitro, and the phosphorylated HY5 is absent in the spaQ background in vivo. We also demonstrate that the unphosphorylated HY5 interacts strongly with both COP1 and SPA1 and is the preferred substrate for degradation, whereas the phosphorylated HY5 is more stable in the dark. In addition, the unphosphorylated HY5 actively binds to the target promoters and is the physiologically more active form. Consistently, the transgenic plants expressing the unphosphorylated form of HY5 display enhanced photomorphogenesis. Collectively, our study revealed the missing kinase responsible for direct phosphorylation of HY5 that fine-tunes its stability and activity to regulate photomorphogenesis.