Plant Cell Wall Degrading Enzymes of Fungi and its Role in Pathogenesis
Authors: A. Kandan, J. Akhtar, Pardeep Kumar and Z. Khan
Division of Plant Quarantine, ICAR-National Bureau of Plant Genetic Resources (NBPGR), Pusa Campus, New Delhi - 110012


Plant cell walls are heterogeneous structures in nature which composed of polysaccharides, proteins, and aromatic polymers. The composition and structure of the cell wall differ significantly among different plant lineages however the share basic principles in their structural components like containing cellulose microfibrils embedded in a matrix of pectin, hemicellulose, lignin, and structural proteins. In addition to the different physical complexity of the plant cell wall, its structure variably changes as the plant grows and develops which differ at different phase. For example, in both monocot and dicot species, during the maturation of the cell wall from a primary to secondary wall, the amount of pectin, xyloglucan and structural proteins decreases, whereas the amount of xylan and lignin increases. For fungal pathogenesis, phytopathogenic fungi produce various enzymes which disrupt the cellulose, xylan, and pectin, that are capable of degrading cell wall polymers usually called as cell wall degrading enzymes (CWDEs).

Some of the CWDEs are as follows which plays major role in the pathogenesis of plant.

1. Cellulose-degrading enzymes
2. Hemicellulose-degrading enzymes
3. Pectin-degrading enzymes

The complexity of plant cell wall degraded by the above mentioned enzymes so that it paves way for the cause of infection and disease to the plant.

Cellulose-degrading enzymes

Fungi degrade the cellulose by discrete noncomplexed cellulases and complexed cellulases. The primary role of these enzymes is to degrade the cellulose structure by hydrolysis process which mainly involves the synergistic action of two cellulases namely exo and endocellulases followed by a Beta-glucosidase that hydrolyzes the soluble cellodextrin oligomers to glucose. Cellulases have evolved series of overlapping modes of action mainly from totally random endoglucanases through processive endoglucanases and highly processive cellobiohydrolases. Exocellulases (cellobiohydrolases) and endocellulases (endo-Beta-1,4-glucanases) both are widely distributed throughout the glycoside hydrolases (GH) family.

Hemicellulose-degrading enzymes

The plant cell wall which contains hemicellulose describes as the noncellulosic polysaccharides that comprise xyloglucans, xylans, and galactomannan. Fungi use both specific and nonspecific groups of endo-Beta-(1,4)-glucanases for thorough hydrolysis of the backbone chain of xyloglucan. This endo-Beta-1,4-glucanases with effective xyloglucanase activity can be found in GH5, GH12, GH16, and GH44 families. The specific and effective xyloglucanases completely lack or very low activities toward Beta-glucan and carboxymethyl-cellulose of the plant. Fungal endo-1,4-Beta-xylanases cleave the glycosidic bonds of backbone chain of xylan. Other important mannan-degrading enzymes comprise Beta-mannanase and Beta-mannosidase in which Beta-mannosidase contains enzymes with Beta-1,3-xylanase activity, whereas these enzymes belonging to this family characterized from fungi viz., Penicillium chrysogenum and Aspergillus spp. were exclusively Beta-mannanases play major role in degradation of plant galactomannan.

Pectin-Degrading Enzymes

Pectin lyases and pectate lyases effectively cleave building block of pectin (sugar acid) by hydrolysis (polygalacturonases) and a nonhydrolytic reaction. Fungi like Rhizopus oryzae, Aspergillus niger and few phytopathogenic fungi used in food and feed industries which effectively use polygalacturonidases comprises endo- and exoacting enzymes for degradation of pectin of the plant. Fungal polysaccharide lyases cleave uronic acid-containing polysaccharide chains to generate an unsaturated hexenuronic acid residue and a new reducing end of the plant cell wall, whereas fungal pectin lyases and pectate lyases also target the homogalacturonan backbone of pectin of the plant.

Role of CWDEs in pathogenesis

Scientists conducted several experiments to prove the role of CWDEs in fungal pathogenesis. Some of the examples are as follows: Expression of several CWDEs gene(s) were upregulated in the corn smut disease, Ustilago maydis was observed. In Arabidopsis model, degradation of cellulose and hemicellulose by endoglucanase and arabinofuranosidase with xylase upregulated respectively. High level of gene expression mainly cellulases, hemicellulases, and pectate lyases was noted in rice when inoculated with blast fungal pathogen Magnaporthe oryzae. Endopolygalacturonase gene is one of the important CWDEs of fungal pathogen as a virulence factor. Isshiki and his co-workers (2001) compared the effect of endopolygalacturonase gene in Alternaria alternata rough lemon pathotype, the cause of Alternaria brown spot and Alternaria citri, the cause of Alternaria black rot. These two Alternaria species are morphologically indistinguishable fungal pathogens of citrus, whereas A. citri causes citrus rot by macerating tissues of the plant, but A. alternata causes specific necrotic spots in citrus by producing a host-selective toxin. Eventhough the endopolygalacturonase gene produced by these two fungi have similar biochemical properties, and the genes are highly similar, an A. citri strain carrying a endopolygalacturonase gene knockout was reduced in its pathogenesis ability to cause black rot symptoms on citrus, whereas similar endopolygalacturonase gene knockout mutants in A. alternata were unchanged in pathogenicity on citrus.

Recently RNAi technology added more strong evidence for the involvement of these CWDEs in fungal pathogenesis. In Magnaporthe oryzae, RNAi was used to knock down gene expression of xylanases in two GH families (GH10 and -11) as well as endo- and exocellulases in GH6 and GH7 families. An effective decrease in the production of numerous enzymes (xylanases, endo- and exocellulases) was associated with a parallel decrease in virulence. Fascinatingly, knockdowns of the CWDEs (xylanases) from GH10 and GH11 families yielded greater defects in virulence as compared with knockdowns of CWDEs (endo- and exocellulases) belongs to GH6 and GH7 families (Van Vu et al. 2012). Thus, all these observations were very clearly showed the importance of CWDEs in fungal pathogenesis for different hosts.

References:

1. Isshiki A, Akimitsu K, Yamamoto M, Yamamoto H. (2001). Endopolygalacturonase is essential for citrus black rot caused by Alternaria citri but not brown spot caused by Alternaria alternata. Molecular Plant-Microbe Interactions 14:749-57.
2. VanVuB, Itoh K, Nguyen QB, TosaY, Nakayashiki H. (2012). Cellulases belonging to glycoside hydrolase families 6 and 7 contribute to the virulence of Magnaporthe oryzae. Molecular Plant-Microbe Interactions 25:1135-41.


About Author / Additional Info:
Senior Scientist (Plant Pathology), Division of Plant Quarantine, Indian Council of Agricultural Research (ICAR)-National Bureau of Plant Genetic Resources (NBPGR), Pusa Campus, New Delhi-110012.