Genome-wide identification and a comparative transcriptomics approach reveal FaSAD3 as a strawberry fruit ripening regulator

Fatty acid desaturases (FADs) play a crucial role in plant growth as well as in the regulation of biotic and abiotic stresses. However, their function has not been well understood in the ripening of fruit. In this study, we identified 73 FADs in the octoploid strawberry genome and analyzed their physicochemical characteristics, chromosomal localization, phylogeny, gene structures, covariance, cis-acting elements, and transcript levels during fruit ripening stages. According to the predicted subcellular localization, FADs are mostly located in the plasma membrane, with a lower abundance in the cell wall, cytoplasm, and nucleus. Numerous cis-elements in the FADs promoter are responsive to light, hormones, and stress. Here, combining comparative transcriptomics, we identified a Δ9 stearoyl-acyl carrier protein (ACP) desaturase gene (FxaC_8 g24100, namely FaSAD3) that is negatively regulated by both key glycolysis genes, cytoplasmic glyceraldehyde-3-phosphate dehydrogenase (FaGAPC2) and pyruvate kinase (FaPKc2.2). The spatiotemporal expression analysis revealed that FaSAD3 is expressed in both vegetative and reproductive organs. Subcellular localization analysis confirmed that FaSAD3 is expressed in both the cytoplasm and nucleus. Meanwhile, the transient overexpression of FaSAD3 reduced anthocyanin, sugar, organic acid, total phenol, and flavonoid contents, while increased fruit firmness. Furthermore, the yeast one hybridization and dual-luciferase assay revealed that the ripening regulator FaGAMYB negatively regulates the transcriptional activity of FaSAD3 promoter, promoting strawberry fruit ripening. These findings indicated that FaSAD3 acts as a negative regulator in strawberry fruits ripening, providing a novel model for comprehending the mechanisms involved in strawberry fruit ripening.