Contribution of the S-type anion channel SLAC1 to stomatal control and its dependence on developmental stage in rice

Rice production depends on water availability and carbon fixation by photosynthesis. Therefore, optimal control of stomata, which regulate leaf transpiration and CO₂ absorption, is important for high productivity. SLOW ANION CHANNEL-ASSOCIATED 1 (SLAC1) is an S-type anion channel protein that controls stomatal closure in response to elevated CO₂. Rice slac1 mutants showed significantly increased stomatal conductance (g_s) and enhanced CO₂ assimilation. To discern the contribution of stomatal regulation to rice growth, we compared g_s in the wild type (WT) and two mutants, slac1 and the dominant-positive mutant SLAC1-F461A, which expresses a point mutation causing an amino acid substitution (F461A) in SLAC1, at different growth stages. Because the side group of F461 is estimated to function as the channel gate, stomata in the SLAC1-F461A mutant are expected to close constitutively. All three lines had maximum g_s during the tillering stage, when the g_s values were 50% higher in slac1 and 70% lower in SLAC1-F461A, compared with the WT. At the tillering stage, the g_s values were highest in the first leaves at the top of the stem and lower in the second and third leaves in all three lines. Both slac1 and SLAC1-F461A retained the ability to change g_s in response to the day–night cycle, and showed differences in tillering rate and plant height compared with the WT, and lower grain yield. These observations show that SLAC1 plays a crucial role in regulating stomata in rice at the tillering stage.