Molecular characterization of nonhost resistance of pepper against phytophthora infestans based on effector-induced cell death

Abstract

Nonhost resistance (NHR) is the resistance of a plant species against the vast majority of potential pathogens. The mechanism of NHR remains poorly understood but it is considered that NHR shares many components with host resistance such as physical/chemical barriers, PAMPs-triggered immunity, and effector-triggered immunity (ETI). Based on the possibility of polygenic control in ETI of NHR, the main hypothesis of this thesis is that multiple interaction between resistance genes and effectors underpin NHR. Pepper is a nonhost plant against Phytophthora infestans which causes severe potato late blight disease in the world. Hypersensitive response (HR) cell death was observed on epidermal cells of pepper infected with P. infestans, which suggests that ETI is a major factor of NHR of pepper against P. infestans because HR is a typical response of ETI induced by the interaction between resistance (R) genes and cognate effectors. To screen cell death induced by P. infestans effectors, 100 pepper accessions were inoculated with 54 RXLR effectors. The findings that multiple effectors induced cell death on pepper accessions imply that multiple putative R genes could be present in nonhost pepper. To investigate how many pepper genes are involved in cell death induced by an effector, inheritance study of effector-induced cell death in F2 population was performed. The segregation ratio of cell death induced by effectors was 15:1, 9:7, and 3:1, which means that multiple pepper genes control cell death by single effector. To identify nonhost R gene(s) which recognize multiple core effectors of P. infestans and confer durable resistance, a total of 445 NB-LRR genes have been cloned based on pepper genome information. Avrblb2, one of core effectors of P. infestans, is critical for virulence of P. infestans. It has seven paralogs in P. infestans genome and induced cell death on all pepper accessions tested. Using agro-coinfiltration assay, cell death induced by the interaction between pepper NB-LRRs and Avrblb2 effectors was screened and CaNBARC114 showed cell death when co-expressed with Avrblb2. Transient expression of CaNBARC114 into N. benthamiana elevated resistance against P. infestans but transgenic potato carrying CaNBARC114 showed no resistance to P. infestans. Taken together, this study provides insight into durable NHR based on multiple interactions between RXLR effectors and nonhost factors.