Defense-related gene induction in Brassica campestris in response to defined mutants of Xanthomonas campestris with altered pathogenicity.

We have studied the induction of beta-1,3-glucanase (BGL) in turnip following inoculation with pathovars of Xanthomonas campestris and derived mutants. BGL transcript accumulated more rapidly in leaves in the incompatible interactions with X. c. pv. armoraciae and X. c. pv. raphani than in the compatible interaction with X. c. pv. campestris. No accumulation was seen in response to wounding or inoculation with water, salicylic acid, or Escherichia coli. Deletion of the hrp cluster from the X. campestris pathovars caused a reduction in the level of transcript accumulation; these effects were much more pronounced in the incompatible than in the compatible interaction, in which bacterial growth was also affected. In the compatible interaction, bacterial growth and BGL transcript accumulation were not altered by mutation of bacterial genes involved in the regulation of the synthesis of extracellular enzymes or their export from the cell, or by mutation of the structural genes for extracellular endoglucanase and serine protease. Mutation of genes involved in the synthesis of extracellular polysaccharide or lipopolysaccharide reduced bacterial survival in planta, so that the numbers were between two and three orders of magnitude lower than the number of wild-type bacteria. However, total BGL transcript accumulation after inoculation with these mutants was about 80% of that seen after inoculation with the wild-type bacteria, suggesting that one aspect of the role of extracellular polysaccharide and lipopolysaccharide in pathogenesis is to mask the presence of bacteria in the plant. Our results are discussed in the context of work on other plant-microbe interactions.