Control of Fungal Diseases in Agricultural Crops by Chitinase and Glucanase Transgenes

Abstract

In agriculture, pathogens cause fungal diseases leading to loss in crop yield and quality. These diseases have been controlled by chemical fungicides. But their effects are often non-specific, targeting beneficial organisms as well as pathogens. The actual crop cultivars have disease responses and in-built genetic resistance against diseases for their defense against fungal pathogens. For instance, chitinases and beta-glucanases have been proposed to have a role in fungal cell wall lysis by targeting key macromolecular components of the cell walls, i.e. chitin and beta-glucan microfibrils. Chitin does not occur in plants, whereas in many plant-pathogenic fungi chitin comprises of 22-44 % cell wall material and maintains the structural integrity of hyphae. The glucans are structural polymers for maintaining rigidity and conferring protection. In this chapter we review the targeting mechanism of chitinase, beta-glucanase genes with focus on domains and amino acid sequences responsible for antifungal action. The chitinases from Trichoderma spp. are more effective as compared to the corresponding enzymes produced by plants, and against a wider range of pathogens. Site-directed mutagenesis and crystallographic studies led to identification of conserved glutamate residue involved in the catalytic mechanism of chitinases, responsible for hydrolysis of the beta-1,4-N-acetyl-D-glucosamine linkages in chitin polymers. Likewise, the amino acid alignment of plant glucanses showed that the conserved region of these enzymes contain a conserved tryptophan residue, which could be involved in the interaction with the glucan substrates and the highly conserved active site located between two glutamate residues participates in cleaving beta-1,3-and beta-1,4-glycosidic bonds. Cloning and characterization of antifungal genes from plant, non-plant sources encoding the hydrolytic enzymes have confirmed their function in antifungal activities. The progress made in utilizing these genes individually or synergistically for combating fungal diseases in agriculture through transgenesis is summarized.