Mildew Resistance Locus O Genes and Are Negative Modulators of the Defense Response to .

Corynespora leaf spot caused by is one of the major diseases in cucumber ( L.). However, the resistance mechanisms and signals of cucumber to are unclear. Here, we report that the mildew resistance locus O (MLO) genes, and , are both negative modulators of the cucumber defense response to . Subcellular localization analysis showed that CsMLO1 and CsMLO2 are localized in the plasma membrane. Expression analysis indicated that the transcript levels of and are linked to the defense response to . Transient overexpression of either or in cucumber cotyledons reduced resistance to , whereas silencing of either or enhanced resistance to . The relationships of pathogenesis-related proteins, reactive oxygen species (ROS)-associated genes, and abscisic acid (ABA)-related genes to the overexpression and silencing of / in non-infested cucumber plants were investigated. The results indicated that mediated resistance against by regulating the expression of pathogenesis-related proteins and ROS-associated genes, as well as through ABA signaling pathway-associated genes. The -mediated resistance against primarily involves regulation of the expression of pathogenesis-related proteins. Our findings will guide strategies to enhance the resistance of cucumber to corynespora leaf spot.