AcCNGC2, coding a cyclic nucleotide-gated channel protein from onion, enhances Phytophthora nicotianae resistance in transgenic Nicotiana benthamiana

The cyclic nucleotide-gated channel (CNGC) proteins encode Ca2+ cation channels, which play vital roles in plant biotic and abiotic stress responses. Phytophthora nicotianae has a wide range of host plants and causes severe damage to a wide range of crops including onion (Allium cepa L). Here, an AcCNGC2 gene was isolated from onion based on the transcriptome analysis. AcCNGC2 showed higher expression in resistant onion genotype L17136 compared to susceptible line F419025. AcCNGC2 was a plasma membrane-localized protein and contained conserved regions of plant CNGC protein, such as transmembrane domains and the cyclic nucleotide-binding domain. The transcripts of AcCNGC2 were induced by CaCl2, salicylic acid, H2O2, and cAMP. AcCNGC2 ectopic overexpression in Nicotiana benthamiana lines presented milder disease symptoms than that of wild type (WT) after P. nicotianae inoculation. After inoculation with P.nicotianae, the concentration of Ca2+ and the expression of the NbCAM gene were significantly increased in AcCNGC2 overexpression lines. The overexpression of AcCNGC2 promoted the accumulation of reactive oxygen species (ROS) and upregulated the NbRBOHD transcript level. In addition, AcCNGC2 overexpression promoted the expression of NbCDPK and NbPR1, which related to the pathogen resistance. Moreover, AcCNGC2 interacted with itself to form a calcium channel participating in plant disease resistance. The results indicated that AcCNGC2 played a positive role in P.nicotianae resistance by regulating Ca2+ and ROS accumulation and downstream resistance-related genes expression.