REVEILLE1, a Myb-like transcription factor, integrates the circadian clock and auxin pathways

Abstract

The circadian clock modulates expression of a large fraction of the Arabidopsis genome and affects many aspects of plant growth and development. We have discovered one way in which the circadian system regulates hormone signaling, identifying a node that links the clock and auxin networks. Auxin plays key roles in development and responses to environmental cues, in part through regulation of plant growth. We have characterized REVEILLE1 (RVE1), a Myb-like, clockregulated transcription factor that is homologous to the central clock genes CIRCADIAN CLOCK ASSOCIATED 1 (CCA1) and LATE ELON-GATED HYPOCOTYL (LHY). Despite this homology, inactivation of RVE1 does not affect circadian rhythmicity but instead causes a growth phenotype, indicating this factor is a clock output affecting plant development. CCA1 regulates growth via the bHLH transcription factors PHYTOCHROME INTERACTING FACTOR4 (PIF4) and PIF5, but RVE1 acts independently of these genes. RVE1 instead controls auxin levels, promoting free auxin production during the day but having no effect during the night. RVE1 positively regulates the expression of the auxin biosynthetic gene YUCCA8 (YUC8), providing a mechanism for its growth-promoting effects. RVE1 is therefore a node that connects two important signaling networks that coordinate plant growth with rhythmic changes in the environment.growth control ͉ hypocotyl ͉ yucca C ircadian rhythms are approximately 24-h rhythms in physiology or behavior that are generated by an endogenous clock. Circadian rhythms persist in constant environmental conditions; however they can be entrained or set by environmental cues like light/dark or temperature cycles (1). In plants, these rhythms regulate myriad processes including leaf and cotyledon movement, growth, photosynthesis, and timing of the transition to flowering (2). A functional circadian clock provides an adaptive advantage, perhaps by predicting fluctuations in the external environment (3, 4). In all model systems studied, these self-sustained rhythms are generated by a cell-autonomous central oscillator which regulates the expression of many genes involved in metabolic and physiological functions (1). Microarray studies show that nearly one third of Arabidopsis genes are circadian regulated, with peak expression at different times of day (5, 6).In plants, the central oscillator is composed of interlocking feedback loops with both positive and negative transcriptional regulators (1). The central loop is thought to consist of three genes, CIRCADIAN CLOCK ASSOCIATED 1 (CCA1), LATE ELON-GATED HYPOCOTYL (LHY), and TIMING OF CAB EXPRES-SION (TOC1). CCA1 and LHY are morning-phased transcription factors with a single Myb-like domain containing a distinctive SHAQKYF motif (7,8). They bind a motif, termed the evening element (EE; AAAATATCT), in the TOC1 promoter to negatively regulate TOC1 expression. TOC1, an evening-phased gene, in turn positively regulates the expression of CCA1 and LHY through an unknown mechanism, thus forming the core clock negative feedback loop (7...

PIF4-controlled auxin pathway contributes to hybrid vigor in Arabidopsis thaliana

“…The PIF4 target gene YUC8 showed no change of expression in the 5-d and 7-d-old-seedlings of F1 hybrids compared with the MPV. PIF4 is expressed mainly in the aerial tissues of young seedlings (22,23), YUC8 is mainly expressed in the roots (24), so change in expression in the cotyledons may not have been detected.…”

Abiotic stress and the plant circadian clock

“…58 Additionally, many auxin genes are clock regulated and the later gates the sensitivity to the hormone depending on the time of day. 59,60 The gated response to auxin explains the observed growth of plants around dawn in a diurnal photoperiod. 61 At this time of the day, auxin levels and plant responsiveness coincide with increased water turgor pressure and renewal of carbon supply, promoting growth.…”

MYB-related transcription factors function as regulators of the circadian clock and anthocyanin biosynthesis in Arabidopsis

“…Transcription factors such as REVEILLE 2 (RVE2), RVE7, RVE8, and MYB HYPO-COTYL ELONGATION-RELATED (MYBH) ( Table 1) act on the circadian clock in ways similar to that of CCA1 and LHY1. 1,3,4,6,[9][10][11][12][13][14] The RVE7/EARLY-PHYTOCHROME-RESPONSIVE1 (EPR1) was found to be regulated by PHYTO-CHROME A and PHYTOCHROME B, and it functions as a component of the slave oscillator. 9 Additionally, MYBH/KUA1 expression is regulated by the circadian clock and this MYBH/ KUA1 transcription factor regulates cell expansion, hypocotyl elongation, and leaf senescence in the plant.…”