Abscisic acid metabolism in tomato plants infected with tobacco mosaic virus: relationships with growth, symptoms and the Tm-1 gene for TMV resistance

Abstract

Abscisic acid, an endogenous inhibitor of plant growth, has previously been shown to be important in the mechanism by which tobacco mosaic virus inhibits the growth of tobacco plants. The objectives of the present paper were to study the problem of whether abscisic acid was also involved in the control of growth inhibition in virus-infected tomato plants, and to study further aspects of abscisic acid metabolism in healthy and infected plants of the two species. Abscisic acid concentrations in healthy tomato plants were higher than they were in healthy tobacco plants. Tomato plants metabolized abscisic acid to form small amounts of phaseic acid, but no detectable dihydrophaseic acid. However, tomato plants contained high levels of free abscisic acid under all conditions tested. In contrast, tobacco plants metabolized abscisic acid rapidly and accumulated high concentrations of phaseic acid. Infection by tobacco mosaic virus did not cause increased concentrations of abscisic acid or phaseic acid in tomato plants. This is in contrast to the situation in tobacco, where virus infection increased both abscisic acid and its rate of metabolism. It was not possible, because of the instability of isolated chloroplasts, to discover whether virus infection of tomato caused a change in compartmentation of abscisic acid, as occurs in tobacco. Growth of healthy tomato plants was inhibited by the application of abscisic acid, but only to a small extent, and only when the compound was applied at high concentrations. These results suggest that abscisic acid is unlikely to be involved in the inhibition of growth of tomato plants by tobacco mosaic virus. This contrasts with the situation in tobacco. Exogenous abscisic acid had no effect on the multiplication of tobacco mosaic virus in either tomato or tobacco, but did strongly suppress formation of mosaic symptoms in both hosts. Tomato genotypes with the Tm-1 gene for resistance to tobacco mosaic virus were shown to contain higher concentrations of abscisic acid than were susceptible plants. This might be involved in the mechanism by which the gene inhibits the formation of mosaic symptoms.