COMPARATIVE HISTOLOGY OF RESISTANT AND SUSCEPTIBLE TOMATO CULTIVARS INFECTED BY PSEUDOMONAS-SOLANACEARUM

The colonization of resistant and susceptible tomato cultivars by Pseudomonas solanacearum was studied by light and electron microscopy, to investigate the nature of the barriers involved in the limitation of bacterial spread in resistant cultivars. In resistant cultivars, tyloses occluded the colonized vessels and the contiguous ones, limiting bacterial spread. In the wilting susceptible cultivars, no tyloses were observed in colonized vessels and bacterial spread was not limited. Tylose production in the susceptible cultivar seemed delayed and less focused compared to the resistant cultivar, because numerous non-colonized vessels were occluded by tyloses. Vascular colonization seemed generalized in the vascular bundle of the susceptible cultivar in contrast to the resistant one. Other reactions involved in resistance or susceptibility were observed, such as gums, cell wall breakdown, and modifications to the primary cell wall. The limitation of bacterial spread associated with the resistance of tomato to bacterial wilt was thus mainly attributed to an induced, non-specific, physical barrier. © 1994 Academic Press Limited.