Powdery mildew (PM) is a widespread plant disease that causes significant economic losses in thousands crops of temperate climates, including Lamiaceae species. Multiple scientific studies describe a peculiar form of PM-resistance associated at the inactivation of specific members of the Mildew Locus O (MLO) gene family, referred to as mlo-resistance. The characterization of Lamiaceae MLO genes, at the genomic level, would be a first step toward their potential use in breeding programs. We carried out a genome-wide characterization of the MLO gene family in 11 Lamiaceae species, providing a manual curated catalog of 324 MLO proteins. Evolutionary history and phylogenetic relationships were studied through maximum likelihood analysis and motif patter reconstruction. Our approach highlighted seven different clades diversified starting from an ancestral MLO domain pattern organized in 18 highly conserved motifs. In addition, 74 Lamiaceae putative PM susceptibility genes, clustering in clade V, were identified. Finally, we performed a codon-based evolutionary analysis, revealing a general high level of purifying selection in the eleven Lamiaceae MLO gene families, and the occurrence of few regions under diversifying selection in candidate susceptibility factors. The results of this work may help to address further biological questions concerning MLOs involved in PM susceptibility. In follow-up studies, it could be investigated whether the silencing or loss-of-function mutations in one or more of these candidate genes may lead to PM resistance.
Keywords: MLO; disease resistance; functional inference; medicinal plants; powdery mildew.