Lumen alkalization under elevated bicarbonate concentration outside isolated thylakoids of higher plants

Illumination causes alkalization of chloroplast stroma, which is accompanied with a substantial increase in stromal bicarbonate concentration there. Whether bicarbonate itself contributes to regulation of thylakoid lumen pH has remained unclear. Here we show that, increasing bicarbonate concentration in the suspension of the isolated pea thylakoids increases luminal pH and stimulates electron transport through the photosynthetic electron transport chain. Addition of bicarbonate stimulated the electron transport in the functionally isolated photosystem I in the case when a pH sensitive electron donor 2,6-dichlorophenolindophenol was used, but not in photosystem II, indicating that under used conditions primary site of bicarbonate action was associated with stromal thylakoid rather than thylakoid grana. Obtained results are consistent with two mechanisms underlying the effect of bicarbonate on luminal pH: (i) stimulation of proton efflux through ATP-synthase, and (ii) bicarbonate transport into the lumen through a potential-independent anion channel in the thylakoid membrane. These findings identify bicarbonate as a potential regulator of thylakoid luminal pH and provide new insight into the regulation of pH-dependent processes during photosynthesis.