MICROSCOPIC STUDIES OF INTERCELLULAR INFECTION AND PROTOXYLEM INVASION OF TOMATO ROOTS BY PSEUDOMONAS-SOLANACEARUM

The pathogenic interactions between tomato roots and Pseudomonas solan acearum were examined microscopically following inoculation with eithe r a pathogenic strain or a nonpathogenic mutant, both harboring a tran sposon 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 s ites such as root extremities and axils of secondary roots the bacteri a 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 dev elopment of lateral roots. Following infection, the pathogenic strain invades protoxylem vessels degrading cell walls. The colonization of a pproximately 25% of xylem vessels in each vascular bundle of the hypoc otyl just above the collar zone is sufficient to induce partial wiltin g of a tomato plant. In contrast, the nonpathogenic strain was able to infect intercellular spaces of the inner cortex of some secondary roo ts, but was never observed in the vascular cylinder. The application o f this approach should enable the characterization of the process of t omato root infection by genetically defined mutant strains of Pseudomo nas solanacearum.