Galacturonic acid catabolism in Botrytis cinerea

D-galacturonic acid (GalA) is the major component of pectin, which can be degraded by plant pathogens; GalA potentially is an important carbon source for microorganisms living on decaying plant material. For bacteria, a catabolic pathway of GalA has been described, which consists of five enzymes converting GalA to pyruvate and glyceraldehyde-3-phosphate. A different catabolic pathway is proposed in filamentous fungi. In Hypocrea jecorina, GalA is converted to pyruvate and glycerol via D-galacturonate reductase, Lgalactonate dehydratase, 2-keto-3-deoxy-L-galactonate aldolase, and glycerol dehydrogenase. The Botrytis cinerea genome contains a D-galacturonate reductase gene (BcgaaA), a L-galactonate dehydratase gene (BcgaaB), and a 2-keto-3-deoxy-L-galactonate aldolase gene (BcgaaC). The three genes were cloned into a protein expression vector and the enzymatic activity determined for each gene separately. The heterologous simultaneous expression of BcgaaA, BcgaaB, and BcgaaC in an E. coli ¿uxaC mutant which cannot grow on GalA is performed to determine whether the catabolic pathway from B. cinerea can restore the growth deficiency in E.coli. Targeted gene replacement of BcgaaC or both BcgaaA and BcgaaC resulted in ¿gaaC mutants and ¿gaaAC double knock-out mutants that displayed significantly reduced growth when D-galacturonic acid was used as the sole carbon source. The mutants showed similar virulence as the wild-type stain B05.10 on tomato leaves, indicating that GalA is not the main carbon source for B. cinerea growth during infection on tomato leaves. The virulence will be tested on other pectin-rich plants and tissues