Defense-Related Responses in Fruit of the Nonhost Chili Pepper against Xan-thomonas axonopodis pv. glycines Infection

Xanthomonas axonopodis pv. glycines (Xag) is a nec-rotrophic bacterial pathogen of the soybean that causes bacterial pustules and is a nonhost pathogen of the chili pepper. In the current study, chili pepper fruit wound inoculated in planta with Xag 8ra formed necrotic lesions on the fruit surface and induced sev-eral structural and chemical barriers systemically in the fruit tissue. The initial defense response included programmed cell death of necrotizing and necrotized cells, which was characterized by nuclear DNA cleav-age, as detected by TUNEL-confocal laser scanning microscopy (CLSM), and phosphatidylserine exposure on cell walls distal to the infection site, as detected by Annexin V FLUOS-CLSM. These two responses may facilitate cell killing and enhance transportation of cell wall materials used for cell wall thickening, respective-ly. The cells beneath the necrotic tissue were enlarged and divided to form periclinal cell walls, resulting in extensive formation of several parallel boundary lay-ers at the later stages of infection, accompanying the deposition of wall fortificationmaterials for strength-ening structural defenses. These results suggest that nonhost resistance of chili pepper fruit against the nonhost necrotrophic pathogen Xag 8ra is activated systematically from the initial infection until termi-nation of the infection cycle, resulting in complete inhibition of bacterial pathogenesis by utilizing organ-specific in situ physiological events governed by the expression of genes in the plant fruit organ. KCI Citation Count: 1