Circadian rhythm as a key regulator of fitness

All organisms possess an endogenous clock. It is presumed that the ability to anticipate light/dark cycles gives organisms a fitness advantage. In bacteria, competitive ability depends on the correspondence between an accession's free-running period (FRP) and ambient day length, but the adaptive significance of FRP and plasticity to circadian inputs has not been explicitly tested in plants. In addition, it is not clear how “real-world” field conditions influence the entrainment and phasing of circadian rhythms. Perhaps unique to plants is the extensive entanglement of the clock with photosignaling cascades: in Arabidopsis , multiple photorecptors can entrain the clock, and the transcript level of these photoreceptors is itself circadian controlled.

Many shade-avoidance traits are under clock control including hypocotyl and stem elongation, leaf angle movement, and flowering time. Furthermore, circadian rhythms gate, or control the effectiveness of, light signaling. These close connections suggest that there may be trade-offs between competitive responses and circadian entrainment or function, and that variation in circadian rhythms could be a key component of variation in competitive responses.

We are using a reporter construct to evaluate the relationship between circadian traits (see adjacent figure illustrating period and phase) and both the expression of phenotypic traits (crowding responses and floral morphologies) and fitness. The reporter gene ( LUCIFERASE ) is driven by the promoter of the native circadian gene ( CCR2 ). As CCR2 transcription increases, LUC expression correspondingly increases. Plants bioluminesce when the substrate, luciferin, is present, enabling quantification of circadian rhythm.

Illustration of potential circadian phenotypes estimated from reporter expression ( CCR2 :: LUC ). LUC levels vary as a function of expression at CCR2 ( COLD CIRCADIAN REGULATED 2 ), a clock-regulated locus. In the figure, A) vs. B) depict hypothetical individuals with similar period (near 24 hrs) and different phase; peak expression occurs earlier in A) than in B). By contrast, A) vs. C) depict individuals with different periods, but similar phase; peak expression is attained halfway through the period.