Genome-wide identification and analysis of the MLO gene family for candidate powdery mildew susceptibility factors in Momordica charantia

Abstract

Powdery mildew (PM) disease is one of the most serious threats to Momordica charantia. Specific mildew resistance locus O (MLO) genes associated with PM susceptibility have been explored in some plant species for mlo-based PM resistance. However, MLO has not been surveyed globally for candidate susceptibility factors in M. charantia. In this study, 15 MLO genes, designated McMLO1∼15, were identified in the recently reported M. charantia genome. Phylogenetic analysis clustered McMLOs into five clades. In clade V, McMLO3, McMLO4, McMLO6, and McMLO11 co-clustered with the known PM susceptibility (PMS) factors of dicotyledonous plants, making them putative PMS factors in M. charantia (McPMS). Three of the four McPMS candidates, McMLO4, McMLO6, and McMLO11, showed much higher sequence identity with PMS-related orthologues than with paralogues. Structure modelling and alignment revealed significant structural similarity between McMLO11 and McMLO4 and the functionally characterized PMS CsMLO8, CmMLO2, and CsMLO11. High structural similarity was also found between McMLO11 and McMLO6 but not between McMLO3 and any other MLO homologs. RNA-seq indicated that the four McPMS candidates were all specifically expressed at the highest level in leaves compared with other tissues. Furthermore, among the expressed McMLOs in leaves, McMLO6 was expressed at the lowest level, McMLO11, McMLO3 and McMLO4 were expressed at the first, second and fourth highest levels, respectively. During PM infection, McMLO11 was moderately induced but highly expressed at the highest level. McMLO4 and McMLO6 were significantly upregulated, but there was no significant differential expression of McMLO3. Consistent with the different expression profiles of the four McPMSs, different types and amounts of cis-acting regulatory elements were found in their promoters. Taken together, we speculate that in M. charantia McMLO4 is a minor PMS factor, and that McMLO11 is the major PMS factor and is functionally redundant with McMLO6. McMLO3 seems to be a unique PMS factor with a different functional mechanism. The results of the study provide valuable information for further study of mlo-based PM resistance improvement in M. charantia.