Growth inhibitors associated with fusarium wilt of tomato

The localization of vascular infections by a process of vascular occlusion is a primary basis for single dominant gene resistance of tomato to fusarium wilt. Failure of this response process to prevent systemic spread of fusarium in susceptible tomato plants is associated with an inhibition of tylose development, a key factor in the overall walling off process. This research was designed to detect, isolate and characterize inhibitory metabolites that accumulate during the course of the tomato/ Fusarium interaction which could inhibit tylose development. Isolation of inhibitors from chloroform extracts of systemically infected tissues was achieved by a series of successive chromatographic separations and bioassays. R_FValues of inhibitors in the several solvent systems employed and their respective colour reactions with chromogenic reagents were recorded. The accumulation of a highly inhibitory compound in chloroform extracts during the early stages of Fusarium infection was determined quantitatively by colourimetric assay in a subsequent experiment. This compound, tentatively identified as rishitin, was isolated from the region of initial infection in stem vascular tissues 1–11 days after inoculation. This inhibitor accumulated rapidly in near-isolines of both resistant and susceptible tomato cultivars in 1–4 days and then continued to accumulate in susceptible, but declined in resistant, plants from 5 to 11 days. The relative phytotoxicity of this compound at low concentrations was determined by assessing its effects on the viability of cultured tomato cells. These data, together with measurements of its accumulation and determinations of its distribution in infected tissues, indicates that rishitin, a host product that accumulates in response to infection, could account for the inhibition of tylose development observed in susceptible tomato plants following infection by Fusarium. Thus we are faced with a paradox in that a substance that presumably functions in defence can, under certain circumstances, interfere with the defence process. A likely resolution of this paradox is discussed.