HMS1 Interacts with HMS1I to Regulate Very-long-chain Fatty Acid Biosynthesis and the Humidity-sensitive Genic Male Sterility in Rice (Oryza sativa).

Environment-sensitive male sterility (EGMS) lines are widely used in two-line hybrid breeding in rice (Oryza sativa). At present, photoperiod-sensitive genic male sterility (PGMS) lines and thermo-sensitive genic male sterility (TGMS) lines are predominantly used in two-line hybrid rice, with humidity-sensitive genic male sterility (HGMS) lines rarely being reported. Here, we showed that HUMIDITY-SENSITIVE GENIC MALE STERILITY 1 (HMS1), encoding a β-ketoacyl-CoA synthase, plays key roles in the biosynthesis of very-long-chain fatty acids (VLCFAs) and HGMS in rice. The hms1 mutant displayed decreased seed setting at low-humidity, but normal seed setting at high-humidity. HMS1 catalyzed the biosynthesis of the C26 and C28 VLCFAs, contributing to the formation of bacula and tryphine in the pollen wall, which protect the pollen from dehydration. Under low-humidity, hms1 pollen showed poor adhesion and reduced germination on the stigmas, which could be rescued by increasing humidity. HMS1-INTERACTING PROTEIN (HMS1I) interacted with HMS1 to co-regulate HGMS. Furthermore, both japonica and indica rice varieties with defective HMS1 exhibited HGMS, suggesting that hms1 could potentially be used in hybrid breeding. Our results reveal the novel mechanism of VLCFA-mediated pollen wall formation, which protects pollen from low-humidity stress in rice, and has a potential use in hybrid crop breeding.