The susceptibility of ripe tomato fruit to Botrytis cinerea is influenced by the structure and composition of the extracellular plant cell wall

ANN L.T. POWELL, DARIO CANTU, ARIEL R. VICENTE, MOLLY DEWEY, ALAN B. BENNETT AND JOHN M. LABAVITCH

Sudafrica

Conferencia; 14th International Botrytis Symposium.; 2007

Fruit ripening includes processes that result in a soft, ripe plant organ that is palatable to numerous organisms, but this developmental program also coincides with increased susceptibility to pathogens. While the release of mature seeds is facilitated by the consumption of ripe fruit by seed dispersing animals and pathogens, healthy human diets include generous portions of fully ripened intact fruit. Therefore, understanding what aspects of ripening can be modified so that fruit ripen optimally to contribute to the human diet while not retaining an increased susceptibility to pathogens, is an important goal. Fruit softening is a significant process during ripening and is associated largely with the disassembly of the major structural components of the plant extracellular compartment, the cell wall polysaccharides (CWs). The roles of plant extracellular enzymes and proteins in the breakdown and relaxation of the structural network of wall polysaccharides have been described for many fruit, including tomato. In addition to their structural roles for the textural properties of fruit, plant CWs are an obstacle that necrotrophic pathogens encounter during host tissue penetration and colonization. CW degrading enzymes (CWDEs) are produced by many of these pathogens and the plant CWs are the targets of these virulence functions. To determine if CW disassembly by endogenous fruit enzymes and proteins influences susceptibility to the necrotrophic fungus, Botrytis cinerea, transgenic tomato fruit with suppressed expression of the ripening-associated polygalacturonase (PG) and/or expansin (LeExp1) were evaluated. The simultaneous suppression of PG and LeExp1 (-PG-Exp) resulted in uninfected fruit that were firmer with reduced pectin solubilization and depolymerization compared to the control or the -PG or the -LeExp1 fruit.  Cell wall swelling, typically noted in ripening fruit, was reduced in -PG-Exp fruit as observed in vitro and by electron microscopy. The fully ripe -PG-Exp fruit had an ca. 70% reduction in susceptibility to B. cinerea, whereas the suppression of either PG or LeExp1 alone did not alter susceptibility as measured by tissue maceration and fungal biomass accumulation. When CWs were added as carbon sources for the in vitro liquid culture of B. cinerea, growth was 3-fold greater on CWs from ripe control fruit than on CWs from ripe -PG-Exp fruit, suggesting a direct effect of the fruit CW composition and integrity on the vigor of fungal growth. The observation that the fruit ripening program assists pathogen development is interesting because the pathogen itself produces a substantial array of CWDEs to enable its colonization of plant hosts.   Phenolic compounds and lignin accumulate near inoculation sites in ripe -PG-Exp tomato fruit, and thus, indicate that induced plant responses are involved in the restriction of B. cinerea infection development. Fruit gene expression changes in green fruit inoculated with B. cinerea and thus suggests that the pathogen is capable of inducing responses in green fruit before the ripening-associated softening functions are expressed. Comparisons of induced responses in green and red fruit in the presence and absence of PG and LeExp1 will demonstrate how the ripening-associated fruit softening processes influence the responses of fruit to B. cinerea infections.