Mildew Resistance Locus O Genes CsMLO1 and CsMLO2 Are Negative Modulators of the Cucumis sativus Defense Response to Corynespora cassiicola

Abstract

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

Cucumber Mildew Resistance Locus O Interacts with Calmodulin and Regulates Plant Cell Death Associated with Plant Immunity

“…CsMLO1 and CsMLO2 were isolated previously and functionally characterized in C. cassiicola -infected cucumber leaves [15]. The sequences of calmodulin-7 (A0A0A0KWT3) were aligned with the cucumber genome database using the service provided by .…”

“…Wang has shown that secondary metabolism and ROS accumulation play important roles in disease resistance during cucumber– C. cassiicola interactions [16]. Moreover, we first found that CsMLO1 is a negative modulator that regulates the defense response of cucumbers to C. cassiicola [15]. Next, the relationships of CsPR and ROS -associated genes with the overexpression and silencing of CsMLO1 in noninfiltrated cucumber plants was investigated, and the results show that CsMLO1 acts as a negative regulator to activate/inhibit the expression of related genes [15].…”

“…Moreover, we first found that CsMLO1 is a negative modulator that regulates the defense response of cucumbers to C. cassiicola [15]. Next, the relationships of CsPR and ROS -associated genes with the overexpression and silencing of CsMLO1 in noninfiltrated cucumber plants was investigated, and the results show that CsMLO1 acts as a negative regulator to activate/inhibit the expression of related genes [15]. Thus, we propose that these genes, including ROS-related genes and defense marker genes, are important in cucumber immunity to C. cassiicola infection.…”

“…Proteins/genes that are mainly involved in the defense response and oxidative stress and calcium signaling pathways during C. cassiicola infection have been identified in cucumber [13,14]. In addition, our research finds that the functions of CsMLO1 and CsMLO2 in C. cassiicola infection act as negative modulators to enhance the expression of reactive oxygen species (ROS)-related genes and defense-related genes for improved cucumber disease resistance [15]. However, few studies have focused on the resistance mechanism of MLO in cucumbers against C. cassiicola infection.…”

“…Arabidopsis MLO2 is a negative regulator of plant ROS responses to biotic stress [19]. In cucumber transgenic cotyledons, the transcription level of HR-associated genes is negatively regulated by CsMLO1 expression [15]. In addition, reports have shown that a calmodulin-binding domain in the cytoplasmic carboxy-terminal region of the MLO protein interacts with Ca 2+ -dependent calmodulin (CaM) [20,21].…”