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 mesoph yll cells to strains of Xanthomonas campestris pv. vesicatoria with in sertion mutations in each of the clustered hrp loci hrpA, B, C, D, E, and F, All hrp mutants caused the localized formation of large papilla e in adjacent cells, Similar deposits also accumulated after inoculati on with the saprophytic strain T55 of X. campestris, which lacks the e ntire hrp cluster, Immunocytochemistry with antibodies to tomato exten sin revealed the accumulation of hydroxyproline-rich glycoproteins in the amorphous electron-dense matrix that encapsulated hrp mutant bacte ria onto the plant cell wall and also within paramural deposits, Callo se was also detected at reaction sites, By contrast, only minor cell w all alterations were observed in response to inoculation with wild-typ e strain 85-10 or a mutant deficient in extracellular polysaccharide p roduction (85-10::454), Unlike other hrp mutants, the hrpE(-) strain 8 5-10::E75, when harboring plasmids overexpressing the avirulence gene avrBs3, caused the hypersensitive reaction in pepper cv, ECW30-R in ad dition to inducing deposition of large papillae, If cells of strains 8 5-10 or 85-10::454 were heat-killed or treated with chloramphenicol (C m) before inoculation they also caused formation of large papillae, Tr eatment 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 ant ibiotic treatment was delayed for 8 h, although bacterial cells were k illed, no papillae developed, An hrp also induced papilla formation in Arabidopsis, bean, and lettuce mesophyll cells, Our results suggest t hat localized cell wall modification and associated papilla deposition represent a defense response of plant cells to strains of X. campestr is and that suppression of the reaction by X. c. pv. vesicatoria requi res each of the clustered hrp loci.