Distinctive mechanism of LHCSR3 expression and function under osmotic stress in Chlamydomonas reinhardtii

Light-harvesting complex stress-related protein 3 (LHCSR3) expression is observed in various protoxidizing conditions like high light and nutrient starvation. LHCSR3 expression is essential for energy-dependent quenching (qE), whereas its role under nutrient starvation is elusive. It is also unclear how nutrient starvation can induce LHCSR3 expression under subsaturating light intensities. To study the role of LHCSR3 under nutrient starvation, the C. reinhardtii cells are grown under osmotic stress that would prevent water uptake; therefore same holds true for soluble nutrients in the medium. In this work, we have shown that LHCSR3 expression can occur under osmotic stress and subsaturating light intensities, whereas it does not elicit qE. Further examination of thylakoid membrane architecture from wild-type and npq4 mutant grown under nutrient starvation revealed that LHCSR3 expression affects the interaction between the PSII core with its peripheral LHCII antenna and possibly can prevent excitation energy transfer. Thylakoid lumen acidification is essential for the expression and function of LHCSR3. Under saturating light intensities, this is achieved by the increased rate of photosynthetic electron flow coupled with proton translocation into the thylakoid lumen. Whereas, under nutrient starvation, the reports of LHCSR3 expression also showed reduced photosynthetic electron flow. Therefore, an alternative mechanism should exist for developing the proton gradient. We observed the downregulation of chloroplast (cp) ATP synthase activity and its abundance under osmotic stress, suggesting the role of (cp) ATP synthase in thylakoid lumen acidification under reduced photosynthetic electron flow. This observation is supported by the expression of LHCSR3 in (cp) ATP-synthase mutant atpF upon exposure to moderate light intensity. This study proposes that the mechanism of LHCSR3 expression and its functionality can vary with the type of photooxidizing stress.