A natural insertion in melon’s MLO1 gene homologue leads to partial resistance to powdery mildew

Powdery and downy mildew (PM and DM) are two major diseases that severely affect cucurbit crop cultivation. Research for durable resistance is the key for an efficient disease-free crop production. Recessive resistance which is based on loss of functional susceptibility (S) genes offers an important lead in the evolutionary arms race between plant and pathogens. Using re-sequencing data from 100 melon genomes, allele-mining on known S genes for both mildews were performed. We identified accessions carrying loss-of-function mutations in a melon homologue of an already known S gene, TCP14, for DM in Arabidopsis, as well as of one Clade V MLO gene that has been previously associated with PM susceptibility in cucumber, CmMLO1. Using a segregation population, we showed that a two-nucleotide insertion in the coding sequence of the CmMLO1 gene co-segregated with partial resistance to Podosphaera xanthii, which is the causal agent of PM in cucurbits. The findings further confirm earlier speculations regarding the general role of this specific gene in PM susceptibility. At the same time, the mutated allele could be an extra asset utilized in melon breeding programs for the improvement of plant fitness and resilience under high PM pressure field conditions.