Genome-Wide Analysis of Mlo Genes and Functional Characterization of Cm-mlo38 and Cm-mlo44 in Regulating Powdery Mildew Resistance in Melon

The Mildew Resistance Locus (Mlo) gene family is reported in various species as regulators of powdery mildew (PM) resistance. However, the Mlo genes in cucurbit crops remain limited. In this study, a genome-wide investigation of Mlo genes was conducted in eight Cucurbitaceae species and in rice, maize, arabidopsis, and barley, and a total of 202 Mlo genes were identified. The phylogenetic analysis showed that 202 Mlo genes can be classified into six clades, and the Mlo genes from clades I and III are likely pivotal in regulating PM resistance in dicotyledonous and monocotyledonous plants, respectively. The Ka/Ks ratios for these homologous Mlo gene pairs ranged from 0 to 0.6, revealing that they underwent substantial purifying selection during evolution. Among 12 crops, there were the most Mlo genes (22 Cm-mlo) in melon. An expression analysis revealed that six Cm-mlo genes showed expression responses to PM infection in which Cm-mlo38 and Cm-mlo44 were phylogenetically close to Mlo genes that regulated PM resistance. Using the VIGS system for silencing, Cm-mlo38 and Cm-mlo44 enhanced resistance to PM in susceptible material. A protein interaction analysis indicated that Cm-mlo38 might regulate PM resistance through interactions with PR5 and CML proteins. These results provide a comprehensive understanding of the Mlo family in Cucurbitaceae and pave the way for future functional analysis and genetic improvement for improving PM resistance in Cucurbitaceae.