BcXYG1, a Secreted Xyloglucanase from Botrytis cinerea, Triggers Both Cell Death and Plant Immune Responses.

In search of Botrytis cinerea cell death-inducing proteins, we found a xyloglucanase (BcXYG1) that induced strong necrosis and a resistance response in dicot plants. Expression of the BcXYG1 gene was strongly induced during the first 12 h post inoculation, and analysis of disease dynamics using PathTrack showed that a B. cinerea strain overexpressing BcXYG1 produced early local necrosis, supporting a role of BcXYG1 as an early cell death-inducing factor. The xyloglucanase activity of BcXYG1 was not necessary for the induction of necrosis and plant resistance, as a mutant of BcXYG1 lacking the xyloglucanase enzymatic activity retained both functions. Residues in two exposed loops on the surface of BcXYG1 were found to be necessary for the induction of cell death but not to induce plant resistance. Further analyses showed that BcXYG1 is apoplastic and possibly interacts with the proteins of the plant cell membrane and also that the BcXYG1 cell death-promoting signal is mediated by the leucine-rich repeat receptor-like kinases BAK1 and SOBIR1. Our findings support the role of cell death-inducing proteins in establishing the infection of necrotrophic pathogens and highlight the recognition of fungal apoplastic proteins by the plant immune system as an important mechanism of resistance against this class of pathogens.

Characterization of Botrytis-plant interactions using PathTrack© -an automated system for dynamic analysis of disease development.

The measurement of disease development is integral in studies on plant-microbe interactions. To address the need for a dynamic and quantitative disease evaluation, we developed PathTrack© , and used it to analyse the interaction of plants with Botrytis cinerea. PathTrack© is composed of an infection chamber, a photography unit and software that produces video files and numerical values of disease progression. We identified a previously unrecognized infection stage and determined numerical parameters of pathogenic development. Using these parameters, we identified differences in disease dynamics between seemingly similar B. cinerea pathogenicity mutants, and revealed new details on plant susceptibility to the fungus. We showed that the difference between the lesion expansion rate on leaves and colony spreading rate on artificial medium reflects the levels of the plant immune system, suggesting that this parameter can be used to quantify plant defence. Our results shed new light and reveal new details of the interaction between the model necrotrophic pathogen B. cinerea and plants. The concept that we present is universal and may be applied to facilitate the study of various types of plant-pathogen association.

Diversity of fungal endophytes in recent and ancient wheat ancestors Triticum dicoccoides and Aegilops sharonensis.

Endophytes have profound impacts on plants, including beneficial effects on agriculturally important traits. We hypothesized that endophytes in wild plants include beneficial endophytes that are absent or underrepresented in domesticated crops. In this work, we studied the structure of endophyte communities in wheat-related grasses, Triticum dicoccoides and Aegilops sharonensis, and compared it to an endophyte community from wheat (T. aeastivum). Endophytes were isolated by cultivation and by cultivation-independent methods. In total, 514 intergenic spacer region sequences from single cultures were analyzed. Categorization at 97% sequence similarity resulted in 67 operational taxonomic units (OTUs) that were evenly distributed between the different plant species. A narrow core community of Alternaria spp. was found in all samples, but each plant species also contained a significant portion of unique endophytes. The cultivation-independent analysis identified a larger number of OTUs than the cultivation method, half of which were singletons or doubletons. For OTUs with a relative abundance >0.5%, similar numbers were obtained by both methods. Collectively, our data show that wild grass relatives of wheat contain a wealth of taxonomically diverse fungal endophytes that are not found in modern wheat, some of which belong to taxa with known beneficial effects.

Anti-apoptotic machinery protects the necrotrophic fungus Botrytis cinerea from host-induced apoptotic-like cell death during plant infection.

Necrotrophic fungi are unable to occupy living plant cells. How such pathogens survive first contact with living host tissue and initiate infection is therefore unclear. Here, we show that the necrotrophic grey mold fungus Botrytis cinerea undergoes massive apoptotic-like programmed cell death (PCD) following germination on the host plant. Manipulation of an anti-apoptotic gene BcBIR1 modified fungal response to PCD-inducing conditions. As a consequence, strains with reduced sensitivity to PCD were hyper virulent, while strains in which PCD was over-stimulated showed reduced pathogenicity. Similarly, reduced levels of PCD in the fungus were recorded following infection of Arabidopsis mutants that show enhanced susceptibility to B. cinerea. When considered together, these results suggest that Botrytis PCD machinery is targeted by plant defense molecules, and that the fungal anti-apoptotic machinery is essential for overcoming this host-induced PCD and hence, for establishment of infection. As such, fungal PCD machinery represents a novel target for fungicides and antifungal drugs.