A functional and evolutionary analysis of avr genes from the bacterial plant pathogens Xanthomonas axonopodis pv. vesicatoria and Xanthomonas vesicatoria

Plants can defend themselves against a variety of pathogenic organisms by several different methods. One method relies on specific recognition of pathogen "elicitors" molecules. Plants detect these pathogen elicitors by resistance (R) genes, and the elicitors are known as avirulence (avr) genes. Many avr genes have been identified in bacterial plant pathogens, but their purpose remains elusive. Because avr genes can alert a host plant to the presence of the invading pathogen (if the host happens to carry a matching R gene), there should be strong negative selection against maintaining avr genes. However avr genes are common in pathogen populations, which indicates that avr genes must also confer counter-acting benefits to the pathogen.; Many avr genes appear to be completely dispensable (the pathogen incurs no fitness loss when the avr gene is removed), which goes against theoretical predictions. We set out to explore this discrepancy by testing a set of bacterial avr gene mutants for loss of fitness under both field and greenhouse conditions. We used the pepper and tomato pathogen Xanthomonas axonopodis pv. vesicatoria (Xav) (or Xanthomonas campestris pv. vesicatoria) and removed up to four avr genes: avrBs1, avrBs2, avrBs3, and avrBs4 (or avrBs3-2). We directly studied the effect of avr gene removal on the ability of the pathogen to transmit and cause new disease in the field. We also tested components of transmission, such as symptom development and epiphytic growth in the field. We further tested the ability of the various avr gene mutants to grow in planta in the greenhouse, and how this correlated with symptom development. We found effects of all avr genes on all aspects of pathogen fitness, although avrBs2 appeared to be the most influential avr gene, and the fitness costs of the loss of the other three avr genes were only observed in a mutant avrBs2 background.; We further studied the molecular evolution of avrBs2 in both Xav and Xanthomonas vesicatoria (Xv). We found that Xav had very low levels of genetic polymorphism throughout the entire genome, consistent with a recent bottleneck or selective sweep, while Xv had substantial genetic variation.