Light Perception: A Matter of Time

Abstract

Optimizing the perception of external cues and regulating physiology accordingly help plants to cope with the constantly changing environmental conditions to which they are exposed. An array of photoreceptors and intricate signaling pathways allow plants to convey the surrounding light information and synchronize an endogenous timekeeping system known as the circadian clock. This biological clock integrates multiple cues to modulate a myriad of downstream responses, timing them to occur at the best moment of the day and the year. Notably, the mechanism underlying entrainment of the light-mediated clock is not clear. This review addresses known interactions between the light-signaling and circadian-clock networks, focusing on the role of light in clock entrainment and known molecular players in this process.

Keywords: Arabidopsis thaliana; circadian clock; light entrainment; light signaling; photoreceptors; signal integration.

Figure 1:. Simplified representation of the molecular network underlying the circadian clock of Arabidopsis thaliana.

Clock components are shown from left to right, according their time of expression throughout the day. Functional groups are enclosed in boxes. Lines with blunt ends and arrows indicate repression and activation, respectively. Broken lines represent regulatory steps not proven to be direct.

Figure 2:. Regulation of core-clock elements by light signaling proteins.

(A) Transcriptional regulation of core-clock genes by photoreceptors or components of light signaling pathways. The presumed regulation is based on various sources of experimental evidence but does not include genetic data. Dot-ended lines represent protein-chromatin associations for which the biological role (i.e. transcriptional regulation) has not been described. Lines with blunt ends and arrows indicate repression and activation, respectively. Cyan lines and boxes indicate clockwork genes. Gray boxes group genes targeted by the same component. (B) Protein-protein interactions between core-clock players and components involved in light perception or signaling. Cyan ovals indicate clockwork elements. Regulatory elements belonging to different light wavelength signaling cascades are represented with distinct colors: Red elements, red light; blue elements, blue light; green elements, regulatory players shared by different light wavelength perception pathways. Ovals represent proteins.

Figure 3:. Conceptual framework highlighting crosstalk among light signaling pathways and their integration to the circadian clock network.

Light signal is sensed by different groups of photoreceptors, which convey the environmental information to molecular signaling hubs. Core-clock components are targets of regulation of photoreceptors, signaling hubs, other clock proteins, and additional molecular players not shown in this figure. Multiple points of crosstalk among photoreceptors, signaling hubs, as well as other components of the network, provide plasticity and accuracy to the system, and allow the entrainment of the endogenous oscillator. Lines with blunt ends and arrows indicate repression and activation, respectively. The biological significance of the interaction between components linked by black, dot ended lines has not been described. Green and orange lines represent photoreceptor- and signaling hubs-mediated regulation, respectively; pink lines indicate PRR7 or ELF3/EC regulatory steps (see text for more detail). FR, far-red; UV-B, ultraviolet B; EC, Evening Complex. Red, blue, and purple arrows: Light signal. Green colored elements: Photoreceptors. Orange colored elements: Signaling hubs. Pink colored elements: Core-clock components.