Microscopic Studies of Intercellular Infection and Protoxylem Invasion of Tomato Roots byPseudomonas solanacearum

The pathogenic interactions between tomato roots and Pseudomonas solanacearum were examined microscopically following inoculation with either a pathogenic strain or a nonpathogenic mutant, both harboring a transposon containing a promoterless Escherichia coli lacZ gene, which is highly expressed. Several stages were distinguished during tomato root infection by the pathogenic strain. After colonization of exudation sites such as root extremities and axils of secondary roots the bacteria intercellularly infect the inner cortex and the vascular parenchyma. These latter tissues are separated by the endodermis which is not yet fully differentiated at root extremities and is reoriented by the development of lateral roots. Following infection, the pathogenic strain invades protoxylem vessels degrading cell walls. The colonization of approximately 25% of xylem vessels in each vascular bundle of the hypocotyl just above the collar zone is sufficient to induce partial wilting of a tomato plant. In contrast, the nonpathogenic strain was able to infect intercellular spaces of the inner cortex of some secondary roots, but was never observed in the vascular cylinder. The application of this approach should enable the characterization of the process of tomato root infection by genetically defined mutant strains of Pseudomonas solanacearum.