Genome-wide identification and description of MLO family genes in pumpkin (Cucurbita maxima Duch.)

The Mildew resistance locus o (Mlo) is a plant-specific gene family that encodes a protein with seven-transmembrane (TM) domains that play an important role in plant resistance to powdery mildew (PM). PM caused by Podosphaera xanthii is a widespread plant disease and represents the major fungal threat for many Cucurbits. The recently reported Cucurbita maxima genome sequence data provides an opportunity to identify and characterize the MLO gene family in this species. A total of twenty genes designated CmaMLO1 to CmaMLO20 have been identified using an in silico cloning method with the MLO gene sequences of Cucumis sativus, Cucumis melo, Citrullus lanatus, and Cucurbita pepo as probes. These CmaMLOs were evenly distributed on 15 of the 20 C. maxima chromosomes without any obvious clustering. Multiple sequence alignment showed that common structural features of MLO gene family, such as TM domains, a calmodulin-binding domain, and 30 important amino acid residues for MLO function, were well conserved. Phylogenetic analysis of the CmaMLO genes and other plant species revealed seven different clades (I through VII); however, clade IV is specific to monocots (rice, barley and wheat). Expression analysis showed that four of the five CmaMLO genes that belonged to clade V were up-regulated in pumpkins infected with P. xanthii. Phylogenetic and expression analysis provided preliminary evidence that these five genes could be susceptibility genes that are important for PM resistance. This study is the first comprehensive report on MLO genes in C. maxima to our knowledge. These findings will facilitate the functional analysis of MLOs related to PM susceptibility and are valuable resources for the development of disease resistance in pumpkin.