The Circadian Clock Acts Through PIF4 and PIF5 to Regulate Gibberellin Signaling in Arabidopsis (LB251)

Abstract

The circadian clock confers 24‐hour rhythmicity to biological events and phases these processes a particular time of day. Plants, being sessile, use the clock to anticipate predictable environmental conditions, such as night and day, and they restrict growth to specific parts of the 24‐hour cycle. The Arabidopsis early flowering 3‐1 (elf3‐1) mutant has an arrested oscillator and shows excessive petiole and hypocotyl growth. Introducing a second gene mutation, phytochrome interacting factor 4 (pif4), to the elf3‐1 background restores a normal growth phenotype to the elf3‐1 mutant. PIF4, and closely related protein PIF5, are bHLH transcription factors that impact plant growth. We hypothesize that the clock acts through PIF4 and PIF5 to influence hormone pathways, such as auxin (IAA) and gibberellin (GA), and this ultimately regulates hypocotyl and petiole elongation. Analysis of transcript levels with quantitative PCR (qPCR) shows genes in both IAA and GA pathways are up‐regulated in the elf3‐1 mutant, but are restored to normal levels in the elf3‐1 pif4‐2 and elf3‐1 pif5‐3 double mutants. Growth of elf3‐1 on paclobutrazol (PAC), a GA biosynthesis inhibitor, also restores wild type like growth. Collectively, these results demonstrate that aberrant growth in elf3‐1 is dependent upon excessive GA signaling, and further implies that GA is responsible for time‐of‐day dependent growth in Arabidopsis.