A peptidogalactomannan isolated from Cladosporium herbarum induces defense-related genes in BY-2 tobacco cells.

Cladosporium herbarum is a plant pathogen associated with passion fruit scab and mild diseases in pea and soybean. In this study, a peptidogalactomannan (pGM) of C. herbarum mycelium was isolated and structurally characterized, and its role in plant-fungus interactions was evaluated. C. herbarum pGM is composed of carbohydrates (76%) and contains mannose, galactose and glucose as its main monosaccharides (molar ratio, 52:36:12). Methylation and 13C-nuclear magnetic resonance (13C-NMR) spectroscopy analysis have shown the presence of a main chain containing (1 → 6)-linked α-D-Manp residues, and ß-D-Galf residues are present as (1 → 5)-interlinked side chains. ß-Galactofuranose containing similar structures were characterized by our group in A. fumigatus, A. versicolor, A. flavus and C. resinae. Tobacco BY-2 cells were used as a model system to address the question of the role of C. herbarum pGM in cell viability and induction of the expression of plant defense-related genes. Native and partially acid hydrolyzed pGMs (lacking galactofuranosyl side-chain residues) were incubated with BY-2 cell suspensions at different concentrations. Cell viability drastically decreased after exposure to more than 400 µg ml-1 pGM; however no cell viability effect was observed after exposure to a partially acid hydrolyzed pGM. BY-2 cell contact with pGM strongly induce the expression of plant defense-related genes, such as phenylalanine ammonia lyase (PAL) and lipoxygenase (LOX), as well as the pathogen-related PR-1a, PR-2 and PR-3 genes, suggesting that pGM activates defense responses in tobacco cells. Interestingly, contact with partially hydrolyzed pGM also induced defense-related gene expression at earlier times than native pGM. These results show that the side chains of the (1 → 5)-linked ß-D-galactofuranosyl units from pGM play an important role in the first line fungus-plant interactions mediating plant responses against C. herbarum. In addition, it was observed that pGM and/or C. herbarum conidia are able to induced HR when in contact with tobacco leaves and in vitro plantlets roots, producing necrotic lesions and peroxidase and NO burst, respectively.