Modulation of Scion Immunity by Resistant Rootstocks in the Pepper–Ralstonia solanacearum Pathosystem

Grafting is a common strategy to control bacterial wilt (Ralstonia solanacearum) in susceptible crops, including pepper. However, the contribution of resistant rootstocks to scion immunity, beyond physical pathogen exclusion, requires further elucidation. In this study, susceptible ‘HF8’ scions were grafted onto six rootstock genotypes, and their resistance was evaluated via leaf-clip inoculation, which bypasses root-mediated physical barriers. Scions grafted onto resistant rootstocks exhibited restricted pathogen spread and higher survival rates; specifically, the NK4-HF8 combination demonstrated a 100% survival rate, compared to 11% in susceptible-rootstock combinations and 46% in self-grafted controls. Comparative transcriptomic profiling revealed distinct transcriptional reprogramming in scions on the resistant NK4 compared to the susceptible JD2 following infection. These changes involved the modulation of genes associated with pathogen perception (NDR1-like, Xa21), defense-associated transcription factors (CaNAC90, CaMYB13), and specific antioxidant isoforms (CaAPX3, CaAPX6). Physiological analyses indicated that resistant rootstocks are associated with localized hydrogen peroxide accumulation in the scion, alongside the coordinated activation of an antioxidant gene network including CaSOD, CaCAT, CaPOD, CaAPX, and CaGST. These findings suggest that resistant rootstocks modulate scion immunity by regulating transcriptional networks linked to oxidative defense and redox homeostasis, offering potential physiological markers for rootstock selection in pepper.