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Title: Gibberllin driven growth in elf3 mutants requires PIF4 and PIF5

Author
item Harmon, Frank
item THINES, BRYAN - Claremont Colleges
item WU, AUSTIN - Keck Graduate Institute
item FILO, JULIE - Keck Graduate Institute
item ELIASON, ERICA - Keck Graduate Institute

Submitted to: Plant Signaling and Behavior
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/24/2014
Publication Date: 3/4/2015
Citation: Harmon, F.G., Thines, B., Wu, A., Filo, J., Eliason, E. 2015. Gibberllin driven growth in elf3 mutants requires PIF4 and PIF5. Plant Signaling and Behavior. 10(3):e992707). doi: 10.4161/15592324.2014.992707.

Interpretive Summary: Plants have a circadian clock that synchronizes biological processes with daily and seasonal changes in the environment. Gibberellins (GA) are plant hormones that are important in many growth processes including seed germination, development, and flowering. The activities of the circadian clock directly influence GA synthesis pathways and the networks that respond to the hormone (i.e., signaling pathways). The EARLY FLOWERING 3 (ELF3) protein from the model plant Arabidopsis thaliana acts together with two other proteins to form the so-called Evening Complex (EC). EC activity is fundamental to generation and maintenance of the primary circadian rhythm, as well as direct control of gene expression involved in growth regulation. The findings here describe chemical treatments that show excessive GA synthesis likely underlies the enhanced stem and petiole (the stalk that connects leaves to the stem) elongation common to Arabidopsis elf3 mutants. In addition, a null elf3 mutant has high levels of the GA20ox1 and GA20ox2 genes that promote GA synthesis, indicating that the EC controls these genes and, likely, hormone synthesis. The transcription factors PHYTOCHROME INTERACTING FACTOR 4 (PIF4) and PIF5 are also needed for the two growth phenotypes, indicating these proteins function in a signaling pathway with the EC. This work in Arabidopsis agrees with comparable studies in barley that show elf3 mutants have enhanced growth phenotypes because of increased GA production. Thus, a regulatory network reliant on the EC, PIF4 and PIF5 has very similar, if not identical, control GA signaling in both eudicots and monocots and is, therefore, highly conserved.

Technical Abstract: The regulatory connections between the circadian clock and hormone signaling are essential to understand, as these two regulatory processes work together to time growth processes relative to predictable environmental events. Gibberellins (GAs) are phytohormones that control many growth processes throughout all stages of the plant life cycle, including germination and flowering. An increasing number of examples demonstrate that the circadian clock directly influences GA biosynthesis and signaling. EARLY FLOWERING 3 (ELF3) participates in a tripartite transcriptional complex known as the Evening Complex (EC). In this capacity, ELF3 is fundamental to core circadian clock activity, as well as time-of-day specific regulation of genes directly responsible for growth control, namely the PHYTOCHROME INTERACTING FACTOR 4 (PIF4) and PIF5 genes. Here we show that the GA biosynthesis inhibitor paclobutrazol substantially reduces the long hypocotyl and petiole phenotypes of Arabidopsis elf3 mutants. In addition, loss of ELF3 activity causes upregulation of the key GA biosynthesis genes GA20ox1 and GA20ox2. Moreover, GA20ox1 and GA20ox2 expression depends strongly on the redundant activities of PIF4 and PIF5. These findings indicate that the defining growth phenotypes of elf3 mutants arise from altered GA biosynthesis due to misregulation of PIF4 and PIF5. These observations agree with recent work linking increased GA production with the elongated growth phenotypes of the barley elf3 mutant. Thus, the role of the EC in regulation of GA biosynthesis and signaling in eudicots is shared with monocots and, therefore, is a highly conserved mechanism for growth control.