Each of the chloroplast potassium efflux antiporters affects photosynthesis and growth of fully developed Arabidopsis rosettes under short-day photoperiod.

In Arabidopsis thaliana, the chloroplast harbors three potassium efflux antiporters (KEAs), namely KEA1 and KEA2 in the inner envelope and KEA3 in the thylakoid membrane. They may play redundant physiological roles as in our previous analyses of young developing Arabidopsis rosettes under long-day photoperiod (16 h light per day), chloroplast kea single mutants resembled the wild-type plants, whereas kea1kea2 and kea1kea2kea3 mutants were impaired in chloroplast development and photosynthesis resulting in stunted growth. Here, we aimed to study whether chloroplast KEAs play redundant roles in chloroplast function of older Arabidopsis plants with fully developed rosettes grown under short-day photoperiod (8 h light per day). Under these conditions, we found defects in photosynthesis and growth in the chloroplast kea single mutants, and most dramatic defects in the kea1kea2 double mutant. The mechanism behind these defects in the single mutants involves reduction in the electron transport rate (kea1 and kea3), and stomata conductance (kea1, kea2 and kea3), which in turn affect CO2 fixation rates. The kea1kea2 mutant, in addition to these alterations, displayed reduced levels of photosynthetic machinery. Taken together, our data suggest that, in addition to the previously reported roles in chloroplast development in young rosettes, each chloroplast KEA affects photosynthesis and growth of Arabidopsis fully developed rosettes.