Molecular Genetics of Pathogenesis by Soft-Rot Erwinias

Abstract

The soft-rot erwinias are enterobacteria able to cause diseases, a symptom of which is the maceration of host tissues, on a wide range of plants. In the past, this group of pathogens was grouped into a single species, Erwinia caroto­ vora, divided into subspecies (129). Now, the group is split into two main species, Erwinia chrysanthemi and E. carotovora, the latter being divided into the three subspecies, carotovora, atroseptica, and betavasculorum (89). Two other minor pathogens with restricted host range are included in the soft-rot group: Erwinia cypripedii and Erwinia rhapontici (108). The general characteristics and the ecology of these bacteria have been previously re­ viewed (107, 108, 129). The soft-rot erwinias can infect plants in culture as well as in storage. Besides soft rot itself, other symptoms may be observed on the growing plant. For instance, E. chrysanthemi induces a wilt on carnation. E. carotovora subsp. atroseptica, E. carotovora subsp. carotovora, and E. chrysanthemi can cause blackleg on potato, which has leaf wilting as a symptom (107, 108). Infection of chrysanthemu?l by E. chrysanthemi generally induces a canker on the stem, starting from the crown (25). In plants that reproduce vegetative­ ly, the infection can be systemic and, when conditions for development of the disease are not met, latent. Host specificity, although not as strict as that observed in plant-pathogenic pseudomonads and xanthomonads (78, 130), seems to occur in E. chrysanthemi strains. It has been proposed that E. chrysanthemi be divided into five pathotypes (51, 52). Soft-rot erwinias produce several extracellular enzymes that can attack components of the plant