The plant cell wall is one of the first physical interfaces encountered by plant pathogens and consists of
polysaccharides, of which
arabinan is an important constituent. During
infection, the necrotrophic plant pathogen Botrytis cinerea secretes a cocktail of plant cell-wall-degrading
enzymes, including endo-arabinanase activity, which carries out the breakdown of
arabinan. The roles of
arabinan and endo-arabinanases during microbial
infection were thus far elusive. In this study, the gene Bcara1 encoding for a novel α-1,5-L-endo-arabinanase was identified and the heterologously expressed BcAra1
protein was shown to hydrolyze linear
arabinan with high efficiency whereas little or no activity was observed against the other oligo- and
polysaccharides tested. The Bcara1 knockout mutants displayed reduced arabinanase activity in vitro and severe retardation in secondary lesion formation during
infection of Arabidopsis leaves. These results indicate that BcAra1 is a novel endo-arabinanase and plays an important role during the
infection of Arabidopsis. Interestingly, the level of Bcara1 transcript was considerably lower during the
infection of Nicotiana benthamiana compared with Arabidopsis and, consequently, the ΔBcara1 mutants showed the wild-type level of virulence on N. benthamiana leaves. These results support the conclusion that the expression of Bcara1 is host dependent and is a key determinant of the disease outcome.