Competitiveness and Aggressiveness of Xanthomonas axonopodis pv. vesicatoria with Mutations in avrBs2 Locus.
No Thumbnail Available
Files
Date
2002-10-10
Authors
Journal Title
Series/Report No.
Journal ISSN
Volume Title
Publisher
Abstract
Strains with mutations in the avrBs2 locus of Xanthomonas axonopodis (syn. campestris) pv. vesicatoria, which causes bacterial spot in pepper (Capsicum spp.), were detected and caused disease in pepper containing the Bs2 resistance gene. Aggressiveness and competitiveness of the mutant strains was evaluated under laboratory and field conditions. Three strains each of races 3, 4, and 6 were separately inoculated on susceptible pepper plants (cv. Camelot) in field plots with one race/plot. Inoculum naturally spread within plots to bordering rows of pepper plants of ECW (bs2/bs2) or ECW-20R (Bs2/Bs2). A specific strain within each race predominated on either ECW or ECW-20R plants. Races 3 (functional avrBs2) and 6 (non-functional avrBs2) had similar AUDPC values, and caused similar disease severity on ECW plants. Races 4 (non-functional avrBs2, but functional avrBs3) and 6, which defeat the Bs2 gene caused severe disease on ECW-20R, but severity was less compared to pepper plants of ECW lacking the Bs2 gene. Frequency of recovery of strains in field plots was associated with bacterial multiplication and lesion efficiency results in the laboratory. Although, expression of the non-functional avrBs2 gene in strains of races 4 and 6 was detected, AvrBs2 protein translation may not occur or be in an inactive form as indicated by no hypersensitive reaction (HR) and disease in plants carrying the Bs2 resistance gene. Loss of avrBs2 activity apparently results in less fitness cost in race 6 than in race 4 strains; however, there is variability in competitiveness among strains within races.
Mutations in the 5-bp short-sequence repeat region of the avirulence gene avrBs2 were detected in field strains that defeat the Bs2 gene. The avrBs2 has both avirulence and virulence activities. We hypothesized that if the avrBs2 gene is required for full virulence and fitness of Xav, in the absence of selection pressure by the Bs2 resistance gene, functional avrBs2 should be favored. Seven strains representing races 4, 5, and 6 with 5-bp mutations (5-bp addition and 5-bp deletion) and resistant to rifampicin were tested for the potential to switch from non-functional to functional avrBs2. This hypothesis was tested in the laboratory using broth cultures and potted pepper plants and in a field experiment. No strains with functional avrBs2 were detected in the laboratory experiments. Although 3 (0.15%) of 1,961 colonies from the field experiment had functional avrBs2, these strains did not result in a detectable population shift. This suggests that the selection pressure for functional avrBs2 in the absence of resistance gene Bs2 is weak or absent.
Results presents here indicate that pathogen strains with loss of avirulence gene functions (i.e., fail to elicit an HR) can be less aggressive under field conditions and that competitiveness varies among strains. However, such strains retain the ability to cause significant, unacceptable disease loss if no other controls are used. Studies have shown that aggressiveness is not the only factor contributing to pathogenic fitness. Other avr genes or other genes and environmental factors may contribute to overall pathogenic fitness.
Description
Keywords
Capsicum, pepper, avirulence, host resistance, Bs2
Citation
Degree
PhD
Discipline
Plant Pathology
