Plant photoreceptors: Multi-functional sensory proteins and their signaling networks

Abstract

Light is a crucial environmental cue not only for photosynthetic energy production but also for plant growth and development. Plants employ sophisticated methods to detect and interpret information from incoming light. Five classes of photoreceptors have been discovered in the model plant Arabidopsis thaliana. These photoreceptors act either distinctly and/or redundantly in fine-tuning many aspects of plant life cycle. Unlike mobile animals, sessile plants have developed an enormous plasticity to adapt and survive in changing environment. By monitoring different information arising from ambient light, plants precisely regulate downstream signaling pathways to adapt accordingly. Given that changes in the light environment is typically synchronized with other environmental cues such as temperature, abiotic stresses, and seasonal changes, it is not surprising that light signaling pathways are interconnected with multiple pathways to regulate plant physiology and development. Indeed, recent advances in plant photobiology revealed a large network of co-regulation among different photoreceptor signaling pathways as well as other internal signaling pathways (e.g., hormone signaling). In addition, some photoreceptors are directly involved in perception of non-light stimuli (e.g., temperature). Therefore, understanding highly inter-connected signaling networks is essential to explore the photoreceptor functions in plants. Here, we summarize how plants co-ordinate multiple photoreceptors and their internal signaling pathways to regulate a myriad of downstream responses at molecular and physiological levels.

Keywords: E3 Ubiquitin ligase; Photomorphogenesis; Photoreceptors; Sensory proteins; Signal integration.

Fig. 1.

Spectral wavelength-specificity in plant photoreceptors to regulate multiple physiological pathways. Plants utilize wavelength-specific photoreceptors to perceive and interpret incoming light signals to regulate their physiology and development. Five phytochromes (phyA-E) present in Arabidopsis perceive red (650–670 nm) and far-red (705–740 nm) lights. Three cryptochromes (cry1, cry2, and cry3), two phototropins (phot1, phot2), and three LOV domain F-box proteins (ZTL, FKF1 and LKP2) identified in Arabidopsis perceive blue light. UVR8 perceives UV-B light.

Fig. 2.

Convergence of multiple environmental cues on photoreceptors. Phytochromes directly perceive ambient light as well as temperature information. Through co-ordination between photoreceptors, they can modulate myriad of plants’ responses including biotic and abiotic stress responses, gravitropism, as well as multiple developmental transitions. At molecular level, photoreceptors share some of the critical signaling components such as E3 ligase COP1-SPA complex as well as transcription factors called PIFs (PHYTOCHROME INTERACTINF FACTORS), HY5, and HFR1. By inactivating negative regulators of light signaling, COP1-SPA and PIFs, photoreceptors can initiate massive gene expression changes in response to light signal. Photoreceptors also stabilize master transcription factors such as HY5 and HFR1 to target a large number of genes in Arabidopsis genome. These photoreceptor-mediated regulations eventually lead to optimized growth and fitness of plants resulting in enhanced grain and biomass yield in agriculture.