Hrp genes in Xanthomonas campestris pv vesicatoria determine ability to suppress papilla deposition in pepper mesophyll cells

Electron microscopy was used to examine the responses of pepper mesophyll cells to strains of Xanthomonas campestris pv. vesicatoria with insertion mutations in each of the clustered hrp loci hrpA, B, C, D, E, and F, All hrp mutants caused the localized formation of large papillae in adjacent cells, Similar deposits also accumulated after inoculation with the saprophytic strain T55 of X. campestris, which lacks the entire hrp cluster, Immunocytochemistry with antibodies to tomato extensin revealed the accumulation of hydroxyproline-rich glycoproteins in the amorphous electron-dense matrix that encapsulated hrp mutant bacteria onto the plant cell wall and also within paramural deposits, Callose was also detected at reaction sites, By contrast, only minor cell wall alterations were observed in response to inoculation with wild-type strain 85-10 or a mutant deficient in extracellular polysaccharide production (85-10::454), Unlike other hrp mutants, the hrpE(-) strain 85-10::E75, when harboring plasmids overexpressing the avirulence gene avrBs3, caused the hypersensitive reaction in pepper cv, ECW30-R in addition to inducing deposition of large papillae, If cells of strains 85-10 or 85-10::454 were heat-killed or treated with chloramphenicol (Cm) before inoculation they also caused formation of large papillae, Treatment with Cm in the plant, 1 to 4 h after inoculation, also led to papilla deposition adjacent to cells of 85-10 or 85-10::454 but if antibiotic treatment was delayed for 8 h, although bacterial cells were killed, no papillae developed, An hrp also induced papilla formation in Arabidopsis, bean, and lettuce mesophyll cells, Our results suggest that localized cell wall modification and associated papilla deposition represent a defense response of plant cells to strains of X. campestris and that suppression of the reaction by X. c. pv. vesicatoria requires each of the clustered hrp loci.