High endo-pectin lyase (endo-PL) activity occurred in vascular tissue of susceptible tomato cuttings (cv. Gem S.) 3 days after infection by Verticillium albo-atrum and before symptoms appeared. The enzyme also occurred in a near isogenic resistant cultivar (Gem R.) but at much lower levels. Endo-polygalacturonase (endo-PG) was also found but only at slightly above endogenous levels. Activities of both enzymes decreased markedly as symptoms developed.
In stem segments infiltrated with propagules of V. alto-atrum, endo-PL was the only cell wall-degrading enzyme with increased activity. Similar and high levels were detected after 3 to 4 days in vascular tissue excised from Gem R and Gem S stems, and were related to inoculum density. Calculation of volume of xylem fluids indicated that endo-PL activity in vessels of infected stems was comparable to that attained in cultures containing polygalacturonide substrates.
In root-inoculated plants (cv. Craigella) endo-PG increased slightly at first but later there was no increase in activity of any wall-degrading enzyme, although vessels contained gels and degraded pit membranes.
Disease symptoms, including vascular gels, were reproduced by supplying stems and leaves with partially purified endo-PL and endo-PG so that they contained amounts similar to those detected in xylem of infected plants.
The significance of enzymic degradation or alteration of pit membranes is discussed in relation to the induction of water stress.