Rhythmic growth explained by coincidence between internal and external cues

Abstract

Most organisms use circadian oscillators to coordinate physiological and developmental processes such as growth with predictable daily environmental changes like sunrise and sunset. The importance of such coordination is highlighted by studies showing that circadian dysfunction causes reduced fitness in bacteria and plants, as well as sleep and psychological disorders in humans. Plant cell growth requires energy and water-factors that oscillate owing to diurnal environmental changes. Indeed, two important factors controlling stem growth are the internal circadian oscillator and external light levels. However, most circadian studies have been performed in constant conditions, precluding mechanistic study of interactions between the clock and diurnal variation in the environment. Studies of stem elongation in diurnal conditions have revealed complex growth patterns, but no mechanism has been described. Here we show that the growth phase of Arabidopsis seedlings in diurnal light conditions is shifted 8-12 h relative to plants in continuous light, and we describe a mechanism underlying this environmental response. We find that the clock regulates transcript levels of two basic helix-loop-helix genes, phytochrome-interacting factor 4 (PIF4) and PIF5, whereas light regulates their protein abundance. These genes function as positive growth regulators; the coincidence of high transcript levels (by the clock) and protein accumulation (in the dark) allows them to promote plant growth at the end of the night. Thus, these two genes integrate clock and light signalling, and their coordinated regulation explains the observed diurnal growth rhythms. This interaction may serve as a paradigm for understanding how endogenous and environmental signals cooperate to control other processes.

Phytochrome‐mediated inhibition of shade avoidance involves degradation of growth‐promoting bHLH transcription factors

“…In particular, they promote hypocotyl growth during the night while their protein abundance is high. At dawn, growth is stopped as the result of their light-triggered degradation (Nozue et al, 2007). Taking these results into account, we performed our shade-avoidance experiments in continuous light to distinguish between the effects of diurnal cycles versus light quality.…”

“…PIF4 and PIF5 protein levels are specifically reduced in response to R light Recently, it has been shown that PIF4 and PIF5 protein stability depends on light (Nozue et al, 2007), with an Seedlings were grown for 12 days under high R/FR (12 h light, 12 h dark), then transferred to continuous high or low R/FR at dawn on day 13. The expression of all samples relative to Col-0 at dawn was determined according to the method described by Pfaffl (2001), using ACTIN2 as a reference gene.…”

The Energy-Signaling Hub SnRK1 Is Important for Sucrose-Induced Hypocotyl Elongation

“…The circadian oscillator regulates hypocotyl elongation because the accumulation of PIF proteins is restricted to the end of the night (Nozue et al, 2007;Nusinow et al, 2011). Because the circadian oscillator responds to exogenous and endogenous sugars (Dalchau et al, 2011;Haydon et al, 2013) and KIN10 overexpression can lengthen circadian period (Shin et al, 2017), we investigated whether Suc-induced increases in hypocotyl length under short photoperiods involve the circadian oscillator.…”

Abiotic stress and the plant circadian clock

“…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. In summary, the plethora of genes under circadian control and the capacity the clock has on modulating multiple responses through a gating mechanism, highlights the importance of the circadian clock in adjusting the plant to its environment.…”