Ser/Thr Protein Kinase SpkI Affects Photosynthetic Efficiency in Synechocystis sp. PCC 6803 upon Salt Stress

Abstract

High salinity is a common environmental factor that limits productivity and growth for photosynthetic organisms. Here, we identified a mutant defected in gene sll1770, which encodes a Ser/Thr protein kinase SpkI, with a significantly low maximal quantum yield of PSII under high salt condition in Synechocystis sp. PCC 6803. Physiological characterization demonstrated that the ∆spkI mutant had normal growth and photosynthesis under control condition. And a significantly higher NPQ capacity was also observed in ∆spkI when grown under control condition. However, when grown under high salt condition, ∆spkI exhibited apparently slower growth as well as decreased net photosynthesis and PSII activity. Western blot analysis demonstrated that the amount of major photosynthetic proteins declined sharply in ∆spkI when cells grown under high salt condition. Redox kinetics measurement suggested that the activities of PSI and cytochrome b6 f complex were modified in ∆spkI under high salt condition, which resulted in a more reduced PQ pool in ∆spkI. Chlorophyll fluorescence traces suggested that the OA− reoxidation and state transition was also impaired in ∆spkI under high salt condition. Above all, we propose that Ser/Thr protein kinase SpkI plays a role in maintaining high-effective photosynthesis during high-salt acclimation process in Synechocystis.