Aggregative movement of C mesophyll chloroplasts is promoted by low CO under high intensity blue light.

C plants supply concentrated CO to bundle sheath (BS) cells, improving photosynthetic efficiency by suppressing photorespiration. Mesophyll chloroplasts in C plants are redistributed toward the sides of the BS cells (aggregative movement) in response to environmental stresses under light. Although this chloroplast movement is common in C plants, the significance and mechanisms underlying the aggregative movement remain unknown. Under environmental stresses, such as drought and salt, CO uptake from the atmosphere is suppressed by closing stomata to prevent water loss. We hypothesized that CO limitation may induce the chloroplast aggregative movement. In this study, the mesophyll chloroplast arrangement in a leaf of finger millet, an NAD-malic enzyme type C plant, was examined under different CO concentrations and light conditions. CO limitation around the leaves promoted the aggregative movement, but the aggregative movement was not suppressed, even at the higher CO concentration than in the atmosphere, under high intensity blue light. In addition, mesophyll chloroplasts did not change their arrangement under darkness or red light. From these results, it can be concluded that CO limitation is not a direct inducer of the aggregative movement but would be a promoting factor of the movement under high intensity blue light.