Arabidopsis Phytochrome A Directly Targets Numerous Promoters for Individualized Modulation of Genes in a Wide Range of Pathways.

The far-red light (FR) photoreceptor phytochrome A (phyA) contains no DNA binding domain but associates with the CHALCONE SYNTHASE promoter through its chaperone FAR-RED ELONGATED HYPOCOTYL1 and transcription factors. Here, we performed a genome-wide identification of phyA targets using a combination of phyA chromatin immunoprecipitation and RNA sequencing methods in Arabidopsis thaliana. Our results indicate that phyA signaling widely affects gene promoters involved in multiple FR-modulated aspects of plant growth. Furthermore, we observed an enrichment of hormone- and stress-responsive elements in the phyA direct target promoters, indicating that a much broader than expected range of transcription factors is involved in the phyA signaling pathway. To verify our hypothesis that phyA regulates genes other than light-responsive ones through the interaction with corresponding transcription factors, we examined the action of phyA on one of its direct target genes, NAC019, which encodes an abscisic acid-dependent transcription factor. The phyA signaling cascade not only targets two G-boxes on the NAC019 promoter for subsequent transcriptional regulation but also positively coordinates with the abscisic acid signaling response for root elongation inhibition under FR. Our study provides new insight into how plants rapidly fine-tune their growth strategy upon changes in the light environment by escorting photoreceptors to the promoters of hormone- or stress-responsive genes for individualized modulation.

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