Regulation of Photosynthetic Electron Transport Is Affected Differently in the Arabidopsis thaliana Manganese Transporter Mutants pam71 and cmt1.

Manganese (Mn) is an essential element in photosynthesis as a catalyst for water splitting in photosystem II (PSII). Previous studies have shown that Mn-deficient plants are more susceptible to photoinhibition and have reduced PSII and PSI contents and activities. Here, we examined the impact of Mn deficiency on the regulation of photosynthetic electron transport, using the two previously characterized A. thaliana Mn transporter mutants pam71 and cmt1. PAM71 is localized in the thylakoid membrane, while CMT1 is found in the chloroplast inner envelope membrane. We quantified the metal content in thylakoids and leaves to clarify the roles of these proteins in Mn translocation. While the Mn content is decreased in thylakoids, the content of other cations (Ca, Cu, Mg, and Fe) is increased. Chlorophyll fluorescence and induction curves demonstrate that PSII is impaired in both mutants, while state transitions and NPQ are only reduced in pam71, indicating differences in regulation between the mutants. Moreover, quantification of the P700-to-plastocyanin ratio revealed that the relative amount of PSI compared to plastocyanin is reduced in pam71, but remains unaltered in cmt1. We hypothesize that the altered stoichiometry between the photosynthetic reaction centers and electron carriers restricts electron transport in pam71. Meanwhile, cmt1 might adjust to Mn deficiency, retaining pH and redox control through stoichiometric balance.