Genome-wide identification and expression analysis of the MLO gene family reveal a candidate gene associated with powdery mildew susceptibility in bitter gourd (Momordica charantia)

Powdery mildew (PM), caused by Podosphaera xanthii, is one of the most serious disease in bitter gourd (Momordica charantia). Specific members of the Mildew Locus O (MLO) gene family act as susceptibility factors by recognizing PM pathogens and the corresponding loss-of-function mutations confer broad-spectrum durable host resistance. However, the potential PM-susceptibility genes in bitter gourd have not been identified yet. Therefore, genome-wide characterization and expression analysis of the bitter gourd MLO gene family were carried out in the present study. The results suggested that 15 MLO homologs were present in the bitter gourd genome. Gene organization, cis-elements, conserved amino acid residues and motifs were also investigated and analyzed. Phylogenetic analysis revealed that these identified MLO proteins clustered in six well-defined clades. It was noteworthy that McMLO7b, a bitter gourd MLO homolog falling in clade V which clustered all the known PM susceptibility factors of dicots, was up-regulated more than 6-fold at an early infection stage (6 h) and was inferred to be a candidate gene of PM susceptibility factor. In addition, exogenous salicylic acid (SA) treatment enhanced the PM resistance of bitter gourd, accompanied by a significantly elevated level of endogenous SA. However, McMLO7b showed nearly equivalent expression in SA-treated plants compared to the mock-treated seedlings at the early PM infection stage. MLO proteins were directly targeted by PM to trigger pathogenesis, therefore, a weak effect of exogenous SA on interaction between bitter gourd and PM could be inferred. To conclude, the results will be helpful in proposing better strategies to control PM disease with either resistant cultivars or cultivation management.