Figure 1. Signaling pathways involved in the response to light stress.
Light stress in plants can be caused by excess light, prolongation of the photoperiod, fluctuating light or UV light. (A) Excess light stress results in photoinhibition and ROS production which are counteracted by the D1 repair cycle and ROS scavenging enzymes. Upon perception of excess light, high light (HL)-responsive genes are induced resulting in the accumulation of anthocyanins but also resulting in an upregulation of genes involved in D1 repair and ROS scavenging. A specific role for blue light and UV-B, through CRY1 and UVR8 via COP1/HY5 has been shown in the regulation of HL-responsive genes. Moreover, CRY1 itself, upon light perception, produces ROS by the interconversion of the flavin redox states caused by photoexcitation. Chloroplast avoidance movement mediated through PHOT2 also contributes to enhanced high light tolerance. (B) Fluctuating light results in PSI photoinhibition. To deal with fluctuating light, plants developed alternative cyclic electron transport forces (CEF), of which the PGR5-dependent CEF has a bigger role as the NDH-dependent CEF. Together with the Mehler and water-water cycle, these CEFs act as alternative electron sinks. All these mechanisms avoid PSI photoinhibition causing tolerance to fluctuating light. Only FR light is known to ameliorate the energy dissipation via NPQ in PSII. (C) Photoperiod stress is caused by a prolongation of the light period resulting in a stress syndrome characterized by ROS production, jasmonic acid (JA) accumulation and eventually programmed cell death (PCD). Both cytokinin (CK) and CCA1/LHY are negative regulators of photoperiod stress. Recurrent photoperiod stress events reduce the stress response to subsequent stresses. (D) To overcome UV stress, plants induce UV-B-responsive genes, including genes encoding ROS scavenging enzymes, flavonols and CDP photolyases. The UV-B-responsive genes can be regulated via UVR8 and CRY1 in a COP1/HY5 dependent manner. The CPD photolyases can also be induced through phyB and phyA. Also, UV-C irradiance, perceived by phyA or phyB, results in an inhibition of PCD. For more detailed information about the different pathways, please refer to section 2. Abbreviations: B, blue light; R, red light; FR, far-red light; UV, ultraviolet light; ROS, reactive oxygen species; CPD, cyclobutene pyrimidine dimers; PS, photosystem; ASC, ascorbate; MDA, monodehydroascorbate; SOD, superoxide dismutase; PQ, plastoquinone; PC, plastocyanin; Fd, ferredoxin; cytb6f, cytochrome b6f; NPQ; non-photochemical quenching.