natural-products-in-plant-pest-management

116 L.A. Shcherbakovapathogenic fungi in vitro, halt their growth in planta and play a role inmycoparasitism (Elad et al., 1982; Benitez et al., 1998; Lorito, 1998; Cherninand Chet, 2002). Oligosaccharides or chitosan derived from fungal cell wallsexposed to microbial glucanases and chinases elicit a cascade of defenceresponses in plants: generation of ROS, induction of pathogenesis-relatedproteins (PR proteins) (including plant chitinases and glucanases),phytoalexins and lignification (Dyakov and Ozeratskovskaya, 2007).The potential of lytic enzymes for plant-disease management was welldemonstrated by studying chinolytic systems in the biocontrol bacteria Bacilluscereus, Pantoea agglomerans, Pantoea dispersa, and fungi, especially in thewidely used biocontrol fungus Trichoderma. Chitinases produced by Trichodermaare effective on virtually all chitinous pathogens, non-toxic for plantsand possess higher antifungal activity than such enzymes isolated from othersources. The antifungal activity of chitinases from Trichoderma can reach thelevel of some chemical pesticides (Lorito, 1998; Bonaterra et al., 2003; Changet al., 2003; Gohel et al., 2004).Along with use of enzyme-producing biocontrol agents, there are severalother application strategies for chinolytic enzymes. The most conventionalapproach consists of conferring resistance via engineering transgenic plantscontaining heterologous chitinase and glucanase genes. Overexpression ofthese genes in response to pathogen invasion can cause higher levels of theenzymes in the plant cells followed by a faster and effective neutralization ofthe pathogen. Indeed, transgenic broccoli, potato and tobacco plants expressingthe T. harzianum endochitinase gene have been found to show resistanceagainst A. alternata, A. solani, B. cinerea and R. solani. Transgenic tobacco andcabbage, carrying a bean chitinase gene were protected against R. solani.Transgenic cucumber, rice, grapevine, strawberry and wheat transformedwith chitinase genes from rice (Oryza sativa) were resistant to B. cinerea,R. solani, M. grisea, Sphaerotheca humuli and F. graminearum, respectively(Gohel et al., 2006). Expression of exochitinase genes in transgenic apple treesconfers resistance to apple scab (Venturia inaequalis), a pathogen which is controlledby multiple applications of chemical fungicides during the growingseason (Bolar et al., 2000). These results show the broad potential for themicrobial chitinase transgenesis into plants for controlling fungal phytopathogens.The additional strategies are related to fermentation and differentways of improving the enzyme producers (Gohel et al., 2006).Other cases suggesting feasibility of crop protection with enzymes canbe illustrated by the examples of constructing transgenic potato carryingglucose oxidase gene from A. niger or apple, potato and tobacco plantsexpressing the bacteriophage T4 lysozyme gene (Wu et al., 1997). Glucoseoxidase is an enzyme involved in generating plant ROS. Expression of theglucose oxidase gene led to accumulation of peroxide ions in plant tissuesthat increased resistance to fungal diseases, e.g. to late blight (P. infestans),wilt (Verticillium dahliae) and early blight (A. solani). Lysozymes are widespreadenzymes that hydrolyse peptidoglycan of bacterial cell walls. Appleplants with the T4L gene showed significant resistance to the fire blight agentE. amylovora (Ko et al., 2000), while potato and tobacco was resistant to