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Title: Definition of early transcriptional circuitry involved in light-induced reversal of PIF imposed repression of photomorphogenesis in young Arabidopsis seedlings

Author
item LEIVAR, PABLO - University Of California
item TEPPERMAN, JAMES - University Of California
item MONTE, ELANA - Consejo Superior De Investigaciones Cientificas (CSIC)
item CALDERON, ROBERT - University Of California
item LIU, TIFFANY - University Of California
item QUAIL, PETER - University Of California

Submitted to: The Plant Cell
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/2/2009
Publication Date: 11/17/2009
Citation: Leivar, P., Tepperman, J.M., Monte, E., Calderon, R.H., Liu, T.L., Quail, P.H. 2009. Definition of early transcriptional circuitry involved in light-induced reversal of PIF imposed repression of photomorphogenesis in young Arabidopsis seedlings. The Plant Cell. 21:3535-3553.

Interpretive Summary: Plant seedlings grown in the dark perceive new incoming light signals through proteins called phytochromes. Plants that lack four phytochrome interacting factors (PIFs) are required to sustain the dark-growing seedling state. This work identifies the direct downstream target genes of these four PIF factors at the whole-genome level, and suggests that PIF protein turnover is an important aspect of how phytochromes induce plants make the transition to growing in the light.

Technical Abstract: Light signals perceived by the phytochromes induce the transition from skotomorphogenic to photomorphogenic development (deetiolation) in dark-germinated seedlings. Evidence that a quadruple mutant (pifq) lacking four phytochromeinteracting bHLH transcription factors (PIF1, 3, 4, and 5) is constitutively photomorphogenic in darkness establishes that these factors sustain the skotomorphogenic state. Moreover, photoactivated phytochromes bind to and induce rapid degradation of the PIFs, indicating that the photoreceptor reverses their constitutive activity upon light exposure, initiating photomorphogenesis. Here, to define the modes of transcriptional regulation and cellular development imposed by the PIFs, we performed expression profile and cytological analyses of pifq mutant and wild-type seedlings. Dark-grown mutant seedlings display cellular development that extensively phenocopies wild-type seedlings grown in light. Similarly, 80% of the gene expression changes elicited by the absence of the PIFs in dark-grown pifq seedlings are normally induced by prolonged light in wild-type seedlings. By comparing rapidly light-responsive genes in wild-type seedlings with those responding in darkness in the pifq mutant, we identified a subset, enriched in transcription factor–encoding genes, that are potential primary targets of PIF transcriptional regulation. Collectively, these data suggest that the transcriptional response elicited by light-induced PIF proteolysis is a major component of the mechanism by which the phytochromes pleiotropically regulate deetiolation and that at least some of the rapidly light-responsive genes may comprise a transcriptional network directly regulated by the PIF proteins.