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.