Role of PSBS and LHCSR in Physcomitrella patens acclimation to high light and low temperature.

Photosynthetic organisms respond to strong illumination by activating several photoprotection mechanisms. One of them, non-photochemical quenching (NPQ), consists in the thermal dissipation of energy absorbed in excess. In vascular plants NPQ relies on the activity of PSBS, whereas in the green algae Chlamydomonas reinhardtii it requires a different protein, LHCSR. The moss Physcomitrella patens is the only known organism in which both proteins are present and active in triggering NPQ, making this organism particularly interesting for the characterization of this protection mechanism. We analysed the acclimation of Physcomitrella to high light and low temperature, finding that these conditions induce an increase in NPQ correlated to overexpression of both PSBS and LHCSR. Mutants depleted of PSBS and/or LHCSR showed that modulation of their accumulation indeed determines NPQ amplitude. All mutants with impaired NPQ also showed enhanced photosensitivity when exposed to high light or low temperature, indicating that in this moss the fast-responding NPQ mechanism is also involved in long-term acclimation.