Erwinia herbicola pv. gypsophilae (Ehg), which induces galls on Gypsophila paniculata, harbors two major pathways for indole-3-acetic acid (IAA) synthesis, the indole-3-acetamide (IAM) and indole-3-pyruvate (IPyA) routes, as well as cytokinin biosynthetic genes. Mutants were generated in which the various biosynthetic routes were disrupted separately or jointly in order to assess the contribution of IAA of various origins and cytokinins to pathogenicity and epiphytic fitness. Inactivation of the IAM pathway or cytokinin biosynthesis caused the largest reduction in gall size. Inactivation of the IPyA pathway caused a minor, nonsignificant decrease in pathogenicity. No further reduction in gall size was observed by the simultaneous inactivation of both IAA pathways only or in combination with that of cytokinin production. However, inactivation of the IPyA pathway caused a 14-fold reduction in the population of Ehg on bean plants. Inactivation of the IAM pathway or cytokinin production did not affect epiphytic fitness. While the apparent transcriptional activity of iaaM-inaZ fusion increased slightly in cells of Ehg on bean and gypsophila leaves, compared with that in culture, very high levels of induction were observed in cells injected into gypsophila stems. In contrast, moderate levels of induction of ipdC-inaZ in Ehg were observed on leaves of these plants and in gypsophila stems, when compared with that in culture. These results suggest that the IAM pathway is involved primarily in gall formation and support the main contribution of the IpyA pathway to the epiphytic fitness of this bacterial species.

Clavibacter michiganensis subsp. michiganensis (Cmm) is a gram-positive actinomycete, causing bacterial wilt and canker disease in tomato (Solanum lycopersicum). Host responses to gram-positive bacteria and molecular mechanisms associated with the development of disease symptoms caused by Cmm in tomato are largely unexplored. To investigate plant responses activated during this compatible interaction, we used microarray analysis to monitor changes in host gene expression during disease development. This analysis was performed at 4 d postinoculation, when bacteria were actively multiplying and no wilt symptoms were yet visible; and at 8 d postinoculation, when bacterial growth approached saturation and typical wilt symptoms were observed. Of the 9,254 tomato genes represented on the array, 122 were differentially expressed in Cmminfected plants, compared with mock-inoculated plants. Functional classification of Cmm-responsive genes revealed that Cmm activated typical basal defense responses in the host, including induction of defense-related genes, production and scavenging of free oxygen radicals, enhanced protein turnover, and hormone synthesis. Cmm infection also induced a subset of host genes involved in ethylene biosynthesis and response. After inoculation with Cmm, Never ripe (Nr) mutant plants, impaired in ethylene perception, and transgenic plants with reduced ethylene synthesis showed significant delay in the appearance of wilt symptoms, compared with wild-type plants. The retarded wilting in Nr plants was a specific effect of ethylene insensitivity, and was not due to altered expression of defense-related genes, reduced bacterial populations, or decreased ethylene synthesis. Taken together, our results indicate that host-derived ethylene plays an important role in regulation of the tomato susceptible response to Cmm.