Determining where E. chrysanthemi cells multiply and how they migrate in the plant is essential for further understanding the molecular events involved in the disease process. In natural infections, E. chrysanthemi enters the aerial parts of a host plant through wounds and natural openings. Experimental infections are performed by leaf infiltration using E. chrysanthemi 3937, a strain isolated from Saintpaulia ionantha H. Wendl. (African violet), and widely recognized by plant pathologists as a valuable model for the analysis of phytopathogenicity determinants at the molecular level. Cytological examination of leaf and petiole of Saintpaulia plants inoculated with strain 3937 revealed the occurrence of a symptom-less phase that may last several days, during which bacterial cells remain clustered in intercellular spaces of the cortical parenchyma or migrate intercellularly without severe injury of cellular structures. The symptomatic phase is consecutive to the production of pectinases, especially pectate lyase isozymes (PelA to PelE) that, by hydrolysing the pectic components of the middle lamella, progressively dissolve plant cell walls and enable the bacteria to multiply and to disseminate within the leaf and petiole (Murdoch et al., 1999). If favourable conditions of humidity and temperature are met, a total collapse of the infected plant is observed. Bacterial cells do not migrate in the vascular tissue. During this infectious process, the bacteria encounter various environmental conditions, of which Fe availability and production of reactive oxygen species by the plant are two factors that can limit their spread.
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