Rab Gene LDS1 Regulates Guard Cell Lipid Droplet Size And Stomatal Function

The main function of stomata is to control the gas exchange between plants and the environment.Fully functional stomata necessitate normal plant growth and defense responses.Lipid droplets(LD)size is an important factor that determines lipolysis efficiency.Accumulating evidence suggests that in the process of stomatal opening,triacylglycerol(TAG)stored in LD is hydrolyzed for ATP production and stomatal movements,however,the underlying mechanistic basis remains largely unkown.Rab GTPases are a group of small G proteins that play a wide range of roles in regulating vesicular transport,signal transduction,lipid metabolism and responses to various stresses in eukaryotes.In a genetic screening,we have identified an Arabidopsis mutant that displayed disturbed stomatal development and function,as well as enlarged lipid droplets in mutant guard cells.The mutant was named lds1(for lipid droplet and stomata 1).In this study,we focused on the characterization of lds1,and have made some interesting findings which are summarized as follows:1.The loss of the function of LDS1 resulted in wizened guard cells,but both the stomatal density and stomatal index of lds1-1 were comparable to those of wild type Arabidopsis(WT).Stomatal conductance of lds1-1 was significantly lower than that of WT under conditions of either light or low concentration of carbon dioxide.In line with this,mutant stomata displayed no appreciable responses to either light-or fusicoccin-induced stomatal opening.2.The loss of the function of LDS1 also led to increased accumulation of lipid droplets in the wizened guard cells,but not in the leaf pavement cells,or in the root,pollen grains or seeds.By contrast to the leaves and seeds,the content of triglyceride in guard cells of lds1-1 was significantly higher than that in WT.3.Map based gene cloning,genetic complementation test and independent allele analysis together revealed that LDS1 gene encodes a member of Rab branch of the Ras superfamily of GTPases.4.By GUS staining and semi-quantitative PCR analysis,we showed wide expression of LDS1 in different organs,tissues,as well as at various developmental stages of Arabidopsis.More interestingly,ectopic over-expression of LDS1 in WT caused no aberrant LDs in guard cells or dysfunctional stomata.5.Homologous LDS1 from the early land plants including Marchantia polymorpha,Physcomitrella patens and Selaginella moellendorffii could rescue LD size and stomatal function in lds1-1 suggesting gene function of LDS1 is conserved across evolution.