The D-galacturonic acid catabolic pathway in Botrytis cinerea

PO-53 The D-galacturonic acid catabolic pathway in Botrytis cinerea Lisha Zhang and Jan A. L. van Kan Laboratory of Phytopathology, Wageningen University, Wageningen, The Netherlands, E-mail: [email protected]. D-galacturonic acid is the major component of pectin, which can be degraded by saprotrophic and pathogenic fungi; galacturonic acid potentially is an important carbon source for microorganisms living on decaying plant material. A catabolic pathway was proposed in filamentous fungi, comprising three enzymatic steps, involving D-galacturonate reductase, L-galactonate dehydratase, and 2-keto-3-deoxy-L-galactonate aldolase. The Botrytis cinerea genome contains two non-homologous galacturonate reductase genes (Bcgar1 and Bcgar2), a galactonate dehydratase gene (Bclgd1), and a 2-keto-3-deoxy-L-galactonate aldolase gene (Bclga1). Their expression levels were highly induced in cultures containing galacturonic acid, pectate, or pectin as the sole carbon source. The four proteins were expressed in Escherichia coli and their enzymatic activity was characterized. Targeted gene replacement of all four genes in B. cinerea, either separately or in combinations, yielded mutants that were affected in growth on galacturonic acid, pectate, or pectin as the sole carbon source. The virulence of B. cinerea mutants in the D-galacturonic acid catabolic pathway was unaltered on tomato leaves, apple fruit and bell pepper. However, the mutants showed reduced virulence on leaves of Nicotiana benthamiana and Arabidopsis thaliana. Cell wall composition analysis revealed that the amount of uronic acid was 2-fold higher in N. benthamiana and A. thaliana leaves than in tomato leaves. Currently, we are investigating the relationship between the galacturonic acid content of different plant leaves and the virulence of B. cinerea galacturonic acid catabolic deficient mutants on these plants.