Diel magnesium fluctuations in chloroplasts contribute to photosynthesis in rice

Photosynthesis in plants responds to light fluctuations and displays circadian rhythms in various aspects. Here a new regulator of photosynthetic rhythms is identified in rice and shown to control diurnal magnesium fluctuations in the chloroplast. Further engineering attempts yield enhanced rice growth and photosynthesis. Photosynthesis provides food, fibre and fuel that support our society; understanding the mechanisms controlling dynamic changes in this process helps identify new options to improve photosynthesis. Photosynthesis shows diel changes, which have been largely attributed to external light/dark conditions, as well as internal gene expression and the post-translational modification of critical enzymes. Here we report diel fluctuations of magnesium (Mg) in rice (Oryza sativa) chloroplasts, which may function as a rhythm regulator contributing to the post-translational regulation of photosynthetic CO(2)assimilation in rice. We found that a chloroplast-localized Mg(2+)transporter gene,OsMGT3, which is rhythmically expressed in leaf mesophyll cells, partly modulates Mg fluctuations in rice chloroplasts. Knockout ofOsMGT3substantially reduced Mg(2+)uptake, as well as the amplitude of free Mg(2+)fluctuations in chloroplasts, which was closely associated with a decrease in ribulose 1,5-bisphosphate carboxylase activity in vivo and a consequent decline in the photosynthetic rate. In addition, the mesophyll-specific overexpression ofOsMGT3remarkably improved photosynthetic efficiency and growth performance in rice. Taken together, these observations demonstrate that OsMGT3-dependent diel Mg fluctuations in chloroplasts may contribute to Mg-dependent enzyme activities for photosynthesis over the daily cycle. Enhancing Mg(2+)input to chloroplasts could be a potential approach to improving photosynthetic efficiency in plants.