Fungal chitinases and their biological role in the antagonism onto nematode eggs. A review

Chitin, the most abundant aminopolysaccharide in nature, is a rigid and resistant structural component that contributes to the mechanical strength of chitin-containing organisms. Chemically, it is a linear cationic heteropolysaccharide composed of N-acetyl-D-glucosamine and D-glucosamine units. The enzymatic degradation of chitin is performed by a chitinolytic system with synergistic and consecutive action. Diverse organisms (containing chitin or not) produce a great variety of chitinolytic enzymes with different specificities and catalytic properties. Their physiological roles involve nutrition, parasitism, chitin recycling, morphogenesis, and/or defense. Microorganisms, as the main environmental chitin degraders, constitute a very important natural source of chitinolytic enzymes. Nowadays, the most used method for pest and plant diseases control is the utilization of chemical agents, causative of significant environmental pollution. Social concern has generated the search for alternative control systems (i.e., biological control), which contribute to the generation of sustainable agricultural development. Interactions among the different organisms are the natural bases of biological control. Interest in chitinolytic enzymes in the field of biological control has arisen due to their possible involvement in antagonistic activity against pathogenic chitin-containing organisms. The absence of chitin in plants and vertebrate animals allows the consideration of safe and selective target molecules for control of chitin-containing pathogenic organisms. Fungi show appropriate characteristics as potential biological control agents of insects, fungi, and nematodes due to the production of fungal enzymes with antagonistic action. The antagonistic interactions between fungi and plant nematode parasites are among the most studied experimental models because of the high economic relevance. Fungi which target nematodes are known as nematophagous fungi. The nematode egg is the only structural element where the presence of chitin has been demonstrated. In spite of being one of the most resistant biological structures, eggs are susceptible to being attacked by egg-parasitic fungi. A combination of physical and chemical phenomena result in their complete destruction. The contribution of fungal chitinases to the in vitro rupture of the eggshell confirms their role as a pathogenic factor. Chitinases have been produced by traditional fermentation methods, which have been improved by optimizing the culture conditions for industrial processes. Although wild-type microorganisms constitute an alternative source of chitinolytic enzymes, the advances in molecular biology are allowing the genetic transformation of fungi to obtain strains with high capability as biocontrol agents. Simultaneously, a better understanding of rhizosphere interactions, additional to the discovery of new molecular biology tools, will allow the choosing of better alternatives for the biological control of nematodes in order to achieve an integrated management of the soil ecosystem.