Non-photochemical quenching (NPQ) of excited chlorophyll states is essential for protecting the photosynthetic apparatus (PSA) from the excessive light-induced damage in all groups of oxygenic photosynthetic organisms. The key component of the NPQ mechanism in green algae and some other groups of algae and mosses is the LhcSR protein of the light harvesting complex (LHC) protein superfamily. In vascular plants, LhcSR is replaced by PsbS, another member of the LHC superfamily and a subunit of photosystem II (PSII). PsbS also performs the photoprotective function in mosses. For a long time, PsbS had been believed to be nonfunctional in green algae, although the corresponding gene was discovered in the genome of these organisms. The first evidence of the PsbS accumulation in the model green alga Chlamydomonas reinhardtii in response to the increase in irradiance was obtained only six years ago. However, the observed increase in the PsbS content was short-termed (on an hour-timescale). Here, we report a significant (more than three orders of magnitude) and prolonged (four days) upregulation of PsbS expression in response to the chilling-induced high-light stress followed by a less significant (~ tenfold) increase in the PsbS expression for nine days. This is the first evidence for the long-term upregulation of the PsbS expression in green alga (Chlorophyta) in response to stress. Our data indicate that the role of PsbS in the PSA of Chlorophyta is not limited to the first-line defense against stress, as it was previously assumed, but includes full-scale participation in the photoprotection of PSA from the environmental stress factors.
Keywords: Chlorophyta; LhcSR; PsbS; chilling acclimation; high-light stress; thermal dissipation of light energy.