B otrytis cinerea mutants deficient in d ‐galacturonic acid catabolism have a perturbed virulence on N icotiana benthamiana and A rabidopsis , but not on tomato

Summary d ‐Galacturonic acid is the most abundant monosaccharide component of pectic polysaccharides that comprise a significant part of most plant cell walls. Therefore, it is potentially an important nutritional factor for B otrytis cinerea when it grows in and through plant cell walls. The d ‐galacturonic acid catabolic pathway in B . cinerea consists of three catalytic steps converting d ‐galacturonic acid to pyruvate and l ‐glyceraldehyde, involving two nonhomologous galacturonate reductase genes ( Bcgar 1 and Bcgar 2), a galactonate dehydratase gene ( Bclgd 1) and a 2‐keto‐3‐deoxy‐ l ‐galactonate aldolase gene ( Bclga 1). Knockout mutants in each step of the pathway ( Δ Bcgar 1/ Δ Bcgar 2, Δ Bclgd 1 and Δ Bclga 1) showed reduced virulence on N icotiana benthamiana and A rabidopsis thaliana leaves, but not on S olanum lycopersicum leaves. The cell walls of N . benthamiana and A . thaliana leaves were shown to have a higher d ‐galacturonic acid content relative to those of S . lycopersicum. The observation that mutants displayed a reduction in virulence, especially on plants with a high d ‐galacturonic acid content in the cell walls, suggests that, in these hosts, d ‐galacturonic acid has an important role as a carbon nutrient for B . cinerea . However, additional in vitro growth assays with the knockout mutants revealed that B . cinerea growth is reduced when d ‐galacturonic acid catabolic intermediates cannot proceed through the entire pathway, even when fructose is present as the major, alternative carbon source. These data suggest that the reduced virulence of d ‐galacturonic acid catabolism‐deficient mutants on N . benthamiana and A . thaliana is not only a result of the inability of the mutants to utilize an abundant carbon source as nutrient, but also a result of the growth inhibition by catabolic intermediates.