Expression Analysis of Five Arabidopsis PDLP5 Homologous in Watermelon Subjected to Biotic and Abiotic Stresses

Watermelon (Citrullus lanatus (Thunb.) Matsum. & Nakai var. lanatus) is an economically important crop in South Korea. Although efforts have been made to develop disease-resistant cultivars, defense signal pathways in watermelon are still unknown. GNK2 domain containing proteins in plants are involved in oxidative stress and salt stress, and in defense signaling responses to pathogens and viruses. Arabidopsis thaliana plasmodesmata-located protein 5 (PDLP5) controls cell-to-cell communication and defense signaling. We identified five Arabidopsis PDLP5 (AtPDLP5) homologous in watermelon, using the Cucurbit Genomics Database. The five putative watermelon PDLPs had two GNK2 domains and showed high sequence identities to AtPDLP5. Their mRNAs were expressed in three watermelon cultivars, PI189225, Au-Producer, and 920533. Each mRNA was differently expressed after infection of Colletotrichum orbiculare, Didymella bryoniae, and Pseudomonas syringae. In addition to pathogen infection, their expression patterns were altered when subjected to abiotic stresses (drought, salt, and cold) or salicylic acid treatment. Cla006974 mRNA was absent in the 920533 cultivar infected with D. bryoniae. Based on these findings, we propose that five putative watermelon PDLPs are involved in defense against fungal pathogens and abiotic stresses. In addition, Cla006974 could be useful as a marker for selecting Gummy stem blight-resistant cultivars via molecular screening.