Circadian clock intersections with metabolism and stress signalling

Terrestrial plants are confronted with daily oscillations of light, temperature, and humidity. A circadian clock exists primarily as a predictor of these impending perturbations to the plant’s physiology; a clock that resonates with the earth's rotation leads to fitness gains of growth and reproduction, which drives yield benefits for crop plants and sets the ecological balance in wilderness settings. Rhythmic environmental cues create a periodic life-strategy that starts from oscillations in metabolism, primarily photosynthesis. Light absorption during the warm day leads to carbon fixation. The assembly of carbon thus occurs in the context of an assault on biological molecules from irradiation. The clock functions to prime resistance to the oxidation reactions that occur during light harvesting. As the day warms, the clock controls abscisic acid signalling and stomatal aperture to maximise water-use efficiency from the molecular to the whole-leaf level. Towards the end of the light period, a plant can expect a cooler night and the depletion of starch depots and consumption of soluble carbohydrates, which are timed by the clock. These are coincident processes with a preparation for a cold-stress response. Added to these issues of rhythmic carbon metabolism and abiotic stress actions, other aspects of plant nutrition are under clock control, including nitrogen metabolism and mineral nutrition. A relationship between root uptake of required elements is associated with a reciprocal connection of the clocks in the leaf and the root. Such a connection is associated with a homeostatic relationship of tissues, which work in source-sink connections to drive growth in a turgor and hormonally driven fashion. Water trafficking through aquaporins and cell division and elongation intervals are thus also clock-controlled processes. A variety of hormonal pathways driving growth, including gibberellins, brassinosteroids, auxins and jasmonates are markedly under direct clock control. Many of these hormones also function in disease resistance, and as microbial and herbivore pathogens are themselves often clock-controlled, it is perhaps not surprising that much of disease-resistance signalling is also a diurnally-driven reaction coordinated by the circadian clock. Together, the molecular oscillator sets a prevalent transcriptional landscape that potentiates homeostasis from the cellular to the whole-plant level. In this a coordination of rhythmic metabolism overlays plant nutrition and stress resistance to biotic and abiotic cues.