ATP-CITRATE LYASEB1 supplies materials for sporopollenin biosynthesis and microspore development in Arabidopsis

Acetyl-CoA is the main substrate of lipid metabolism and functions as an energy source for plant development. In the cytoplasm, acetyl-CoA is mainly produced by ATP-citrate lyase (ACL), which is composed of ACLA and ACLB subunits. In this study, we isolated the restorer-4 (res4) of the thermo-sensitive genic male sterile mutant reversible male sterile-2 (rvms-2) in Arabidopsis (Arabidopsis thaliana). RES4 encodes ACLB1, and res4 harbors a point mutation (Gly584 to Arg) in the citryl-CoA lyase domain. Both the ACLA and ACLB subunits are expressed in the tapetal layer of anthers. RES4 is regulated by MS188, and the res4 point mutation leads to pollen with a defective exine structure. In res4, lipid accumulation was significantly reduced within the tapetum and locules. These results indicate that acetyl-CoA synthesized by ACL is used for sporopollenin biosynthesis in the tapetum. Microspore diameter was significantly smaller in res4 than in wild type, indicating that acetyl-CoA from the tapetum supplies microspore development. Previous studies have shown that delayed degradation of the tetrad wall in res2 and res3 provides additional protection for rvms-2 microspores. The reduced volume of res4 microspores may lessen the requirement for cell wall protection to restore rvms-2 fertility. This study reveals the function of ACL in anther development and the mechanisms of fertility restoration in photoperiod- and thermo-sensitive genic male sterile lines.