AAL TOXIN-INDUCED PHYSIOLOGICAL-CHANGES IN LYCOPERSICON-ESCULENTUM MILL - DIFFERENTIAL SUCROSE TRANSPORT IN TOMATO LINES ISOGENIC FOR THE ASC LOCUS

Susceptibility to Alternaria alternata f. sp. lycopersici and sensitivity to AAL toxin are regulated by the Asc locus in tomato. Toxin-induced patterns in leaflet necrosis, defined by cell location and relative tissue age, correspond to developmental patterns in capacity to retrieve exogenous sucrose in leaves. Uptake of AAL toxin by detached leaflets of sensitive asc/asc and resistant Asc/Asc lines induced interveinal necrosis within 48 h, the magnitude of which was dependent on the age of the tissue. The youngest leaflets were the most sensitive. In the absence of the toxin, the youngest asc/asc leaflets tested exhibited a three-fold higher endogenous sucrose transport capacity compared to equivalent Asc/Asc leaflets. Exposure to toxin resulted in a strong temporal decrease in sucrose influx within 4 h in both lines, although the magnitude of this effect was greater in asc/asc than in Asc/Asc. Recovery in sucrose influx was observed in Asc/Asc by 8 h but not in asc/asc. In intermediate-aged leaflets, differential genotype-specific changes in sucrose influx were observed within 1 h following toxin treatment. Treatment of leaflets of each genotype with an inhibitor of sucrose transport, p-chloromercuribenzene-sulphonic acid, or with lithium, an inhibitor of sucrose transport via perturbation of signal transduction pathways, caused genotype-specific necrosis similar to AAL toxin. These inhibitors enhanced necrosis in the presence of AAL toxin in both genotypes. These observations suggest: (1) a relationship between sucrose transport capacity, initial response to the toxin, and the Asc locus, and (2) a correlation between high capacity for sucrose influx in the asc/asc isoline and sensitivity to AAL toxin. © 1993 Academic Press Limited.