Genome-Wide Analysis of the MLO Family Reveals Their Involvement in Abiotic and Powdery Mildew Stress Responses in Momordica charantia L.

Abstract

Although loss-of-function mutations in mildew resistance locus o (MLO) genes confer broad-spectrum and durable powdery mildew (PM) resistance in various crops, the MLO family in bitter gourd (Momordica charantia L.) has not been systematically characterized. Here, we identified 15 McMLO genes unevenly distributed across 11 chromosomes, which were phylogenetically grouped into five clades with conserved motifs and similar gene structures. Segmentally duplicated McMLO genes showed strong purifying selection (Ka/Ks = 0.187), and comparative synteny analyses revealed more conserved collinearity with Cucurbitaceae species than with Arabidopsis, supporting their conserved evolution within the Cucurbitaceae. Cis-regulatory element analysis further revealed that the promoter regions of McMLO genes contain stress and hormone responsive cis-elements. Most McMLO genes showed dynamic expression under PM infection, with eight genes (McMLO1, 5, 8, 9, 10, 11, 12 and 13) peaking at early time points and then declining. Cold, NaCl, and mannitol stresses significantly induced a subset of McMLO genes, while McMLO1 and McMLO14 were significantly suppressed exclusively under mannitol stress. Exogenous Methyl Jasmonate (MeJA) and Salicylic acid (SA) significantly induced most McMLO genes, with McMLO9 and McMLO5 showing the strongest induction by MeJA and SA, respectively. Conversely, abscisic acid treatment predominantly suppressed the majority of McMLO genes. Collectively, this study identifies candidate McMLO genes potentially mediating PM susceptibility and abiotic stress responses, providing a valuable foundation for functional studies and resistance breeding in bitter gourd.