Sigma factor FaSigE positively regulates strawberry fruit ripening by ABA

Bacterial sigma factor E (SigE) is a positive regulator of sugar catabolism via its interaction with the H subunit of Mg-chelatase (CHLH), which serves as both putative abscisic acid (ABA) receptor (ABAR) in Arabidopsis thaliana and positive regulator of strawberry fruit ripening. However, whether SigE influences strawberry fruit ripening has not been determined. We used RNA sequencing and a qPCR to confirm that the expression of the strawberry SigE gene (FaSigE) is rapidly upregulated in fruits turning red, suggesting FaSigE might mediate strawberry fruit ripening. Silencing FaSigE by intron-spliced hairpin RNA-mediated RNA interference significantly inhibited fruit ripening. This observation was confirmed by analyses of fruit firmness, soluble sugar, ABA, and anthocyanin contents, as well as transcript levels of genes related to fruit ripening and ABA signaling. Interestingly, a firefly luciferase complementation assay revealed that FaSigE can interact with FaABAR, while an in vitro fruit disc incubation test indicated ABA induces FaSigE expression. Moreover, a surface plasmon resonance assay proved that FaABAR produced in yeast cells can bind to ABA, with a binding dissociation constant of 50 µM. In conclusion, FaSigE can interact with FaABAR and positively regulates strawberry fruit ripening via ABA.