Plant Chitinases: Genetic Diversity and Physiological Roles

Chitinase proteins are widely distributed across diverse biological systems. Chitinases hydrolyze chitin, chitosan, lipochitooligosaccharides, peptidoglycan, arabinogalactan and glycoproteins containing N-acetylglucosamine. Analyses of genome-wide sequence and microarray expression profilings show that chitinase genes are represented by large families and the individual member genes are expressed in diverse conditions. Chitinase proteins are members in the group of the pathogenesis-related proteins that are strongly induced when host plant cells are challenged by pathogen stress and thus chitinases constitute an important arsenal of plants against fungal pathogens. Transgenic plants have been produced that overexpress chitinases alone or in conjunction with other defense-related proteins. The phenotype analyses of such plants have shown enhanced disease resistance in large number of cases. Apart from defense against pathogen stress, chitinases are implicated in relationships between plant cells and fungi (e.g., mycorrhizae associations) and bacteria (e.g., legume/Rhizobium associations). Chitinases are also involved in plant abiotic stress responses as noted for osmotic, salt, cold, wounding and heavy metal stresses. Chitinases play a role in developmental aspects of plants too (i.e., regulation of plant embryogenesis process). A detailed account of the genetic diversity and functional aspects of plant chitinases is presented in this review.