The Botrytis cinerea xylanase Xyn11A contributes to virulence with its necrotizing activity, not with its catalytic activity

Judith Noda,Nélida Brito,Celedonio González

doi:10.1186/1471-2229-10-38

Abstract

BackgroundThe Botrytis cinerea xylanase Xyn11A has been previously shown to be required for full virulence of this organism despite its poor contribution to the secreted xylanase activity and the low xylan content of B. cinerea hosts. Intriguingly, xylanases from other fungi have been shown to have the property, independent of the xylan degrading activity, to induce necrosis when applied to plant tissues, so we decided to test the hypothesis that secreted Xyn11A contributes to virulence by promoting the necrosis of the plant tissue surrounding the infection, therefore facilitating the growth of this necrotroph.ResultsWe show here that Xyn11A has necrotizing activity on plants and that this capacity is conserved in site-directed mutants of the protein lacking the catalytic activity. Besides, Xyn11A contributes to the infection process with the necrotizing and not with the xylan hydrolyzing activity, as the catalytically-impaired Xyn11A variants were able to complement the lower virulence of the xyn11A mutant. The necrotizing activity was mapped to a 30-amino acids peptide in the protein surface, and this region was also shown to mediate binding to tobacco spheroplasts by itself.ConclusionsThe main contribution of the xylanase Xyn11A to the infection process of B. cinerea is to induce necrosis of the infected plant tissue. A conserved 30-amino acids region on the enzyme surface, away from the xylanase active site, is responsible for this effect and mediates binding to plant cells.

Highlights

The Botrytis cinerea xylanase Xyn11A has been previously shown to be required for full virulence of this organism despite its poor contribution to the secreted xylanase activity and the low xylan content of B. cinerea hosts
As part of its invasion strategy, B. cinerea and other necrotrophs are thought to promote programmed cell death (PCD), or apoptosis, in plant cells surrounding the lesion by making use of the plant defence response known as the hypersensitive response (HR) [4]
An alternative hypothesis we proposed at that time was the possibility that Xyn11A was contributing to virulence not with its xylanase activity, but with a putative necrosis inducing activity that had been observed for two xylanases from other fungi, Trichoderma reesei xylanase II [11] and Trichoderma viride EIX [12]

Summary

Introduction

The Botrytis cinerea xylanase Xyn11A has been previously shown to be required for full virulence of this organism despite its poor contribution to the secreted xylanase activity and the low xylan content of B. cinerea hosts. Xylanases from other fungi have been shown to have the property, independent of the xylan degrading activity, to induce necrosis when applied to plant tissues, so we decided to test the hypothesis that secreted Xyn11A contributes to virulence by promoting the necrosis of the plant tissue surrounding the infection, facilitating the growth of this necrotroph. HR comprises a range of effects triggered by pathogens that culminate in PCD of the plant cells around the infected area [5] It is an effective defence against biotrophs, preventing the progression of the infection, but it has been suggested that HR can be exploited by necrotrophs, such as B. cinerea, for its own benefit [1,2,3,4]. In the case of cell wall degrading enzymes causing plant cell death, such as endopolygalacturonases [8], the doubt always arises if the actual inducers of cell death are the enzymes themselves, or the products of their activity. The latter seems to be the case, for example, for the B. cinerea endopolygalacturonase 2, since point mutations in the protein that abolish its enzymatic activity eliminate its necrosis inducing ability [8]

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