Circadian Rhythm - in Plants

In Plants

Plant circadian rhythms tell the plant what season it is in and when to flower for the best chance of attracting insects to pollinate them and can include leaf movement, growth, germination, stomatal/gas exchange, enzyme activity, photosynthetic activity, and fragrance emission. Circadian rhythms occur as a biological rhythm with light, are endogenously generated and self-sustaining, and are relatively constant over a range of ambient temperatures. Circadian rhythms feature a transcriptional feedback loop, a presence of PAS proteins, and several photoreceptors that fine-tune the clock to different light conditions. Anticipation of changes in the environment changes the physiological state that provides plants with an adaptive advantage. A better understanding of plant circadian rhythms has applications in agriculture such as helping farmers stagger crop harvests thus extending crop availability, and to secure against massive losses due to weather.

Clocks are set through signals such as light, temperature, and nutrient availability, so that the internal time matches the local time. Light is the signal and is sensed by a wide variety of photoreceptors. Red and blue light are absorbed through several phytochromes and cryptochromes. One phytochrome, phyA, is the main phytochrome in dark-grown seedlings, but rapidly degrades in light to produce Cry1. Phytochromes B–E are more stable with phyB the main phytochrome in light-grown seedlings. The cryptochrome (cry) gene is also a light-sensitive component of the circadian clock. Cryptochromes 1–2 (involved in blue–UVA) help to maintain the period length in the clock through a whole range of light conditions.

The central oscillator generates a self-sustaining rhythm and is made of two genes: CCA1 (Circadian and Clock Associated 1) and LHY (Late Elongated Hypocotyl) that encode closely related MYB transcription factors that regulate circadian rhythms in Arabidopsis. When CCA1 and LHY are overexpressed (under constant light or dark conditions) plants become arrhythimcal and mRNA signals reduce contributing to a negative feedback loop. CCA1 and LHY expression oscillates and peaks in early morning while TOC1 oscillates and peaks in early evening. From past observations and studies, it is hypothesised that these three components model a negative feedback loop in which over-expressed CCA1 and LHY repress TOC1 and over-expressed TOC1 is a positive regulator CCA1 and LHY.