Juntaro, Negi, Shintaro, Munemasa, Boseok, Song, Ryosuke, Tadakuma, Mayumi, Fujita, Tamar, Azoulay-Shemer, Cawas B, Engineer, Kensuke, Kusumi, Ikuo, Nishida, Julian I, Schroeder, and Koh, Iba
Significance Guard cells have photosynthetically active chloroplasts in most plant species. However, the significance of their existence in guard cells or their developmental mechanisms is unknown. Here, through a forward-genetic approach, we have identified a key feature and a function of guard cell chloroplasts. We observed that a mutation that impaired chloroplast biogenesis in guard cells also disrupted the regulation of stomatal movements by CO2 and light. We demonstrated that guard cell chloroplasts, compared with those in mesophyll cells, display a unique lipid metabolism, in which the prokaryotic pathway is diminished and the eukaryotic pathway gains control. Our findings highlight the importance of the eukaryotic pathway for developing functional chloroplasts in guard cells., Stomatal guard cells develop unique chloroplasts in land plant species. However, the developmental mechanisms and function of chloroplasts in guard cells remain unclear. In seed plants, chloroplast membrane lipids are synthesized via two pathways: the prokaryotic and eukaryotic pathways. Here we report the central contribution of endoplasmic reticulum (ER)-derived chloroplast lipids, which are synthesized through the eukaryotic lipid metabolic pathway, in the development of functional guard cell chloroplasts. We gained insight into this pathway by isolating and examining an Arabidopsis mutant, gles1 (green less stomata 1), which had achlorophyllous stomatal guard cells and impaired stomatal responses to CO2 and light. The GLES1 gene encodes a small glycine-rich protein, which is a putative regulatory component of the trigalactosyldiacylglycerol (TGD) protein complex that mediates ER-to-chloroplast lipid transport via the eukaryotic pathway. Lipidomic analysis revealed that in the wild type, the prokaryotic pathway is dysfunctional, specifically in guard cells, whereas in gles1 guard cells, the eukaryotic pathway is also abrogated. CO2-induced stomatal closing and activation of guard cell S-type anion channels that drive stomatal closure were disrupted in gles1 guard cells. In conclusion, the eukaryotic lipid pathway plays an essential role in the development of a sensing/signaling machinery for CO2 and light in guard cell chloroplasts.