Adjustment of light-responsive NADP dynamics in chloroplasts by stromal pH

Cyclic electron transfer (CET) predominates when NADP⁺ is at basal levels, early in photosynthetic induction; however, the mechanism underlying the subsequent supply of NADP⁺ to fully drive steady-state linear electron transfer remains unclear. Here, we investigated whether CET is involved in de novo NADP⁺ supply in Arabidopsis thaliana and measured chloroplastic NADP dynamics to evaluate responsiveness to variable light, photochemical inhibitors, darkness, and CET activity. The sum of oxidized and reduced forms shows that levels of NADP and NAD increase and decrease, respectively, in response to light; levels of NADP and NAD decrease and increase in the dark, respectively. Moreover, consistent with the pH change in the stroma, the pH preference of chloroplast NAD⁺ phosphorylation and NADP⁺ dephosphorylation is alkaline and weakly acidic, respectively. Furthermore, CET is correlated with upregulation of light-responsive NADP level increases and downregulation of dark-responsive NADP level reductions. These findings are consistent with CET helping to regulate NADP pool size via stromal pH regulation under fluctuating light conditions.