Abstract
BackgroundThe powdery mildew disease affects thousands of plant species and arguably represents the major fungal threat for many Cucurbitaceae crops, including melon (Cucumis melo L.), watermelon (Citrullus lanatus L.) and zucchini (Cucurbita pepo L.). Several studies revealed that specific members of the Mildew Locus O (MLO) gene family act as powdery mildew susceptibility factors. Indeed, their inactivation, as the result of gene knock-out or knock-down, is associated with a peculiar form of resistance, referred to as mlo resistance.ResultsWe exploited recently available genomic information to provide a comprehensive overview of the MLO gene family in Cucurbitaceae. We report the identification of 16 MLO homologs in C. melo, 14 in C. lanatus and 18 in C. pepo genomes. Bioinformatic treatment of data allowed phylogenetic inference and the prediction of several ortholog pairs and groups. Comparison with functionally characterized MLO genes and, in C. lanatus, gene expression analysis, resulted in the detection of candidate powdery mildew susceptibility factors. We identified a series of conserved amino acid residues and motifs that are likely to play a major role for the function of MLO proteins. Finally, we performed a codon-based evolutionary analysis indicating a general high level of purifying selection in the three Cucurbitaceae MLO gene families, and the occurrence of regions under diversifying selection in candidate susceptibility factors.ConclusionsResults of this study may help to address further biological questions concerning the evolution and function of MLO genes. Moreover, data reported here could be conveniently used by breeding research, aiming to select powdery mildew resistant cultivars in Cucurbitaceae.Electronic supplementary materialThe online version of this article (doi:10.1186/s12864-015-2325-3) contains supplementary material, which is available to authorized users.
Highlights
The powdery mildew disease affects thousands of plant species and arguably represents the major fungal threat for many Cucurbitaceae crops, including melon (Cucumis melo L.), watermelon (Citrullus lanatus L.) and zucchini (Cucurbita pepo L.)
The Mildew Locus O (MLO) gene family encodes for plant-specific proteins structurally related to metazoan G-protein coupled receptors (GPCRs), harbouring seven transmembrane domains and a calmodulin-binding domain that is likely implicated in the perception of calcium-dependent stimuli [1, 2]
In contrast with other kind of immunities, such as those conferred by plant resistance genes (R-genes), a body of evidence indicates that mlo resistance is characterized by broad-spectrum effectiveness and durability
Summary
The powdery mildew disease affects thousands of plant species and arguably represents the major fungal threat for many Cucurbitaceae crops, including melon (Cucumis melo L.), watermelon (Citrullus lanatus L.) and zucchini (Cucurbita pepo L.). Several studies revealed that specific members of the Mildew Locus O (MLO) gene family act as powdery mildew susceptibility factors Their inactivation, as the result of gene knock-out or knock-down, is associated with a peculiar form of resistance, referred to as mlo resistance. Specific MLO homologs act as susceptibility factors towards fungi causing the powdery mildew (PM) disease, affecting thousands of plant species and representing a major threat in many agricultural settings [7] Their inactivation, resulting from gene knock-out or knock-down, leads to a peculiar form of resistance, referred to as mlo resistance, based on the enhancement of pre-penetrative defence responses [2, 8, 9]. Pea er PM resistance, due to loss of function mutations of the PsMLO1 gene, was first reported in 1948 and is the only resistance source worldwide used for breeding purposes [19, 20]
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