HTT1, a Stearoyl-Acyl Carrier Protein Desaturase Involved Unsaturated Fatty Acid Biosynthesis, Affects Rice Heat Tolerance

Elucidating the mechanisms underlying heat tolerance in rice (Oryza Sativa. L) is vital for adapting this crop to rising global temperature while increasing yields. Here, we identified a rice mutant, high temperature tolerance 1 (htt1), with high survival rates under heat stress. HTT1 encodes a chloroplast-localized stearoyl-acyl carrier protein (ACP) desaturase involved in the biosynthesis of unsaturated fatty acids, converting C18:0 to C18:1 fatty acid. Overexpression and knockout rice lines provided evidence that HTT1 negatively regulates the response to heat stress. In the htt1 mutant, a G-to-A base substitution in HTT1 impairs unsaturated fatty acid biosynthesis, remodelling the lipid content of cellular membranes and in particular increasing diglyceride contents, which improves membrane stability under heat stress. HTT1 was differentially expressed in all tissues analyzed and was inhibited by heat. Yeast one-hybrid and dual-luciferase reporter assays showed that OsHsfA2d binds to the promoter of HTT1, inhibiting its expression. Different HTT1 alleles were identified between the two Asian cultivated rice subspecies, indica and japonica, potentially facilitating their adaptation to different environmental temperature. Taken together, these findings demonstrate that HTT1 is a previously unidentified negative regulator of heat tolerance and a potential target gene for the improvement of heat adaptability in rice.