Plant chitinases: Types, structural classification, antifungal potential and transgenic expression in plants for enhanced disease resistance

Plant chitinases are generally endochitinases that are usually expressed in different parts of a plant like stems, flowers, seeds and tubers. These tissue-specific proteins have molecular weight of 20 to 60 kDa. The plant chitinases, depending upon amino acid sequences, have been classified into seven classes. Class I, II and IV enzymes possess globular domains in their structure while structure of chitinases from class III and V is comprised of 8 alpha- helix and beta strands. Class I, II, IV, VI and VII are grouped as family GH19 chitinases, while class III and V chitinases belong to family GH18. Plant chitinases have been described as pathogenesis related (PR) proteins, as they are induced in response to invasion by phytopathogens. A number of plant chitinases have been investigated for their potential in plant disease protection. Chitinases target and degrade chitin, the main structural part of cell walls of the pathogenic fungi, which render them inactive with no adverse effect on the plants body. Many chitinase genes have been characterized from different plant species and other organisms and over-expressed in economically important plants for increased resistance to phytopathogens. Some researchers have co-transformed chitinase genes with other defense related genes (transgene stacking) for conferring higher resistance to fungal pathogens.