Plants exhibit daily rhythms in their growth, providing an ideal system for the study of interactions between environmental stimuli such as light and internal regulators such as the circadian clock. We previously found that two basic-look-helix-loop transcription factors, Phytochrome Interacting Factors 4 and 5 (PIF4 and 5), integrate light and circadian clock signaling to generate rhythmic plant growth. Here we use expression profiling and real-time growth assays to identify growth regulatory networks downstream of PIF4 and PIF5. Genome-wide analysis of light, clock, or growth-correlated genes show significant overlap between the transcriptomes of clock, light, and growth-related pathways. Over-representation analysis of growth-correlated genes predicted that the auxin and gibberellic acid (GA) hormone pathways both contribute to diurnal growth control. Indeed, lesions of GA biosynthesis genes retarded rhythmic growth. Surprisingly, GA responsive genes are not enriched among genes regulated by PIF4 and PIF5, whereas auxin pathway and response genes are. Consistent with this finding, auxin response is more severely affected than GA response in pif4 pif5 double mutants and in PIF5 overexpressing lines. We conclude that at least two downstream modules participate in diurnal rhythmic hypocotyl growth; PIF4 and/or 5 modulation of auxin-related pathways and PIF-independent regulation of the GA pathway.

DOI: 10.1104/pp.111.172684
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