Ethylene as a toxin synergist in Verticillium wilt of tomato

Abstract

Culture filtrates of Verticillium albo-atrum produced two major components toxigenic to tomato shoots. Aqueous extracts of acetone powders derived from 14-day culture filtrates were chromatographed on Bio-Gel P60 polyacrylamide gel and separated into a protein peak possessing strong polygalacturonase (PG) and pectin transeliminase (PTE) activities. A second peak consisted entirely of carbohydrate and was tentatively identified as a keto-polysaccharide, probably a fructosan mol. wt 3000. Petioles and leaves of healthy tomato plants cv. Craigella susceptible (S) and resistant (R) to V. albo-atrum treated with aqueous eluates of both components only developed symptoms with the protein, pectolytic enzyme fractions. The sequence of loss of turgor, chlorosis and necrosis was identical with the natural infection syndrome in older leaves. Leaves of the S isoline responded initially more severely than R, but by 60 h no differences between isolines could be observed. Leaves treated with the polysaccharide fractions remained symptomless up to 72 h. Endogenous ethylene production from detached leaves was increased only in leaves treated with fractions 6 to 15 containing protein and pectolytic enzymes. S leaves produced more in the initial (0 to 20 h) period but thereafter, from 45 to 92 h R leaves, produced variable but higher levels. At 140 h ethylene output fell in R leaves as tissue became severely necrotic. Pre-treatment of leaves with 10·0 parts/10 6 ethylene prior to uptake of the culture filtrate fractions resulted in increased wilting and necrosis in leaves treated with fractions 6 to 15, compared with non-ethylene-treated leaves in similar fractions. Symptoms appeared by 18 h and were severe by 42 h. Leakage experiments with 86Rb revealed no differences between healthy, ethylene-treated and control, S and R leaves. Polysaccharide-treated leaves previously exposed to ethylene showed a marked increase in ionic permeability comparable to the response of ungassed leaves to the protein, pectolytic enzyme component. The hypothesis is advanced for the rôle of ethylene as a toxin synergist in V. albo-atrum wilt of tomato. The implications of the in vivo interacton of ethylene and undetectable concentrations of fungal toxins in the early stages of pathogenesis are discussed in relation to the interpretation of symptoms.