Identification of resistance gene analogs involved in Phytophthora capsici recognition in black pepper (Piper nigrum L.)

Black pepper (Piper nigrum L.) is an important spice crop with high economic value. However, its production is severely hampered by the oomycete pathogen, Phytophthora capsici. Integrative disease management strategies have been developed to control the pathogen, but the pathogen is in the phase of evolving its virulence. Absolute resistance against Phytophthora rot was not reported in black pepper germplasm. However, Piper colubrinum, a wild species is reported as resistant. Resistance proteins are involved in continuous surveillance of pathogen entry and activation of plant defense signalling pathways for an effective hypersensitive response to prevent pathogen invasion. In this study, a sequence-based homology approach using the conserved nucleotide-binding site (NBS) of known plant resistance genes was used to isolate Resistance Gene Analogs (RGA) and assess their transcript level during Phytophthora infection. The RGA transcript level was evaluated in resistant wild species (P. colubrinum), two moderately resistant black pepper genotypes (IISR Sakthi and 04-P24-1), and one susceptible genotype (Subhakara). The identified RGAs of black pepper were found to be of non-TIR R gene class with NBS motifs. The expressions of six PnRGAs were assessed employing qRT-PCR at different time points after challenging with highly aggressive Phytophthora isolate. The kinetics of differential expression post-infection with P. capsici indicates the differential timing and magnitude of pathogen recognition in resistant P. colubrinum and moderately resistant black pepper genotypes compared to susceptible genotype. In silico analysis revealed that differentially expressed P. nigrum RGAs function through ADP phosphorylation, which is a key process in pathogen recognition. The identification of P. nigrum RGAs induced by P. capsici should provide valuable information for cloning and characterization of resistance genes.