Arabidopsis JMJD5/JMJ30 Acts Independently of LUX ARRHYTHMO Within the Plant Circadian Clock to Enable Temperature Compensation

The circadian system ensures that plants respond appropriately to environmental change by predicting regular transitions that occur during diel cycles. In order to be most useful, the circadian system needs to be compensated against daily and seasonal changes in temperature that would otherwise alter the pace of this biological oscillator. We demonstrate that an evening-phased protein, the putative histone demethylase JMJD5, contributes to temperature compensation. JMJD5 is co-expressed with components of the Evening Complex, an agglomeration of proteins including EARLY FLOWERING3 (ELF3), ELF4, and LUX ARRHYTHYMO (LUX), which also integrates temperature changes into the molecular clockwork. One role of the Evening Complex is to regulate expression of PSEUDORESPONSE REGULATOR9 (PRR9) and PRR7, important components of the temperature compensation mechanism. Surprisingly we find that LUX, but not other Evening Complex components, is dispensable for clock function at low temperatures. Further genetic analysis suggests JMJD5 acts in a parallel pathway to LUX within the circadian system. Although an intact JMJD5 catalytic domain is required for its function within the clock, our findings suggest JMJD5 does not directly regulate H3K36 methylation at circadian loci. Such data refine our understanding of how JMDJ5 acts within the Arabidopsis circadian system

Multiple feedback loops of the Arabidopsis circadian clock provide rhythmic robustness across environmental conditions.

Although circadian oscillators in diverse eukaryotes all depend on interlinked transcriptional feedback loops, specific components are not conserved across higher taxa. Moreover, the circadian network in the model plant Arabidopsis thaliana is notably more complex than those found in animals and fungi. Here, we combine mathematical modeling and experimental approaches to investigate the functions of two classes of Myb-like transcription factors that antagonistically regulate common target genes. Both CCA1/LHY- and RVE8-clade factors bind directly to the same cis-element, but the former proteins act primarily as repressors, while the latter act primarily as activators of gene expression. We find that simulation of either type of loss-of-function mutant recapitulates clock phenotypes previously reported in mutant plants, while simulated simultaneous loss of both type of factors largely rescues circadian phase at the expense of rhythmic amplitude. In accord with this prediction, we find that plants mutant for both activator- and repressor-type Mybs have near-normal circadian phase and period but reduced rhythmic amplitude. Although these mutants exhibit robust rhythms when grown at mild temperatures, they are largely arrhythmic at physiologically relevant but nonoptimal temperatures. LHY- and RVE8-type Mybs are found in separate clades across the land plant lineage and even in some unicellular green algae, suggesting that they both may have functioned in even the earliest arising plant circadian oscillators. Our data suggest that the complexity of the plant circadian network may have arisen to provide rhythmic robustness across the range of environmental extremes to which plants, as sessile organisms, are regularly subjected

Multiple feedback loops of the Arabidopsis circadian clock provide rhythmic robustness across environmental conditions

Although circadian oscillators in diverse eukaryotes all depend on interlinked transcriptional feedback loops, specific components are not conserved across higher taxa. Moreover, the circadian network in the model plant Arabidopsis thaliana is notably more complex than those found in animals and fungi. Here, we combine mathematical modeling and experimental approaches to investigate the functions of two classes of Myb-like transcription factors that antagonistically regulate common target genes. Both CCA1/LHY- and RVE8-clade factors bind directly to the same cis-element, but the former proteins act primarily as repressors, while the latter act primarily as activators of gene expression. We find that simulation of either type of loss-of-function mutant recapitulates clock phenotypes previously reported in mutant plants, while simulated simultaneous loss of both type of factors largely rescues circadian phase at the expense of rhythmic amplitude. In accord with this prediction, we find that plants mutant for both activator- and repressor-type Mybs have near-normal circadian phase and period but reduced rhythmic amplitude. Although these mutants exhibit robust rhythms when grown at mild temperatures, they are largely arrhythmic at physiologically relevant but nonoptimal temperatures. LHY- and RVE8-type Mybs are found in separate clades across the land plant lineage and even in some unicellular green algae, suggesting that they both may have functioned in even the earliest arising plant circadian oscillators. Our data suggest that the complexity of the plant circadian network may have arisen to provide rhythmic robustness across the range of environmental extremes to which plants, as sessile organisms, are regularly subjected