Genome-Wide Characterization of Stb6 Ortholog Genes and Identification of MdStb6-13 as a Gene Conferring Resistance to Botrytis cinerea in Malus domestica

Apple gray mold is an important disease caused by Botrytis cinerea. Resistance mechanisms against this pathogen in this plant have not been thoroughly revealed. Wall-associated kinases (WAKs) and WAK-like proteins (WAKLs) participate in conferring resistance by stimulating immune-generating receptor complex formation. Stb6 (KY485204) is a WAKL that provides strong resistance against Septoria tritici blotch in wheat. In this study, 15 orthologs of Stb6, named MdStb6-1 to MdStb6-15, were identified in apple by using the Stb6 sequence to conduct BLASTp analysis against the apple genome. These genes were divided into two clades via phylogenetic analysis, with their promoters containing auxin- and biotic and abiotic stress-responsive cis-elements, including pathogen infection-responsive elements. The expression of these genes was differentially induced by plant defensive hormones, MeJA, SA, ethylene and ABA and B. cinerea infection and exhibited differential expression in different organs. Overexpression of MdStb6-13, which was induced by JA and ABA, significantly conferred resistance against B. cinerea. This research highlights the involvement of genes in immunity generation in apple and in plant growth and development and provides a foundation for further elucidating immune signaling in apple and the molecular basis for breeding resistant apple cultivars with relatively high fruit quality and ideal plant stature.