Interaction of Xanthomonas campestris with Arabidopsis thaliana: characterization of a gene from X. c. pv. raphani that confers avirulence to most A. thaliana accessions.

Infiltration of leaves of Arabidopsis thaliana accession Columbia with Xanthomonas campestris pathovar campestris leads to bacterial growth and disease symptoms reminiscent of those incited in Brassica plants inoculated under the same conditions. A search among A. thaliana accessions for variation in the reaction phenotype to strains of X. campestris pathovars campestris, aberrans, and raphani showed that there were no clear differential responses between plant accessions to the individual bacterial strains tested. X. c. pv. raphani strain 1067 was avirulent to all A. thaliana accessions tested. A gene was cloned from X. c. pv. raphani 1067 which, when transferred into the virulent X. c. pv. campestris strain 8004, strongly reduced symptom development and bacterial growth in A. thaliana Columbia plants but did not affect virulence to Brassica plants. The gene (denoted avrXca) interacted with all A. thaliana accessions tested except one, Kas-1, which developed disease symptoms and supported growth of the transconjugant to levels similar to those with X. c. pv. campestris 8004 alone. Sequence analysis of avrXca revealed a probable open reading frame encoding a protein of 66,566 Da that has no homology with other known sequences. A sequence motif conserved among hrp genes was identified in the 5' noncoding region of avrXca, and features characteristic of a signal peptide were found in the N-terminal portion of the presumed AvrXca protein. DNA from different phytopathogenic bacteria contained sequences hybridizing with avrXca in related X. campestris pathovars but not in Erwinia or Pseudomonas strains.