natural-products-in-plant-pest-management

Proteinaceous and Polyketide Compounds in Plant Protection 115Cladosporium sphaerospermum, Colletotrichum lindemuthianum, Diploidia maydis,Magnaporthe grisea, Mycosphaerella arachidicola, Mycosphaerella fijinesis,Nectria haematococca, Penicillium digitatum, Penicillium expansum, Phaeoisariopsispersonata, Physalospora piricola, Rhizoctonia solani, Septoria tritici, Verticiliumalbo-atrum, V. dahliae, and the toxigenic species Fusarium culmorum, F. decemcellulare,F. graminearum, F. oxysporum, F. verticillioides and Aspergillus niger);two species of oomycetes (Phytophthora infestans and P. parasitica) and alsobacteria, viz. C. michiganensis and Ralstonia solanacearum, are among them.Effective concentrations of plant defensins differ depending on the testedpeptides and the targeted pathogens (values of IC 50, a protein concentrationthat is required for 50% growth inhibition, vary from 1–100 μg/ml). The levelof antimicrobial activity may be regulated with bivalent ions ( Terras et al.,1992; 1993; Osborn et al., 1995; Segura et al., 1998; Wong and Ng, 2005). Alongwith the growth inhibitory effect, some plant defensins cause morphologicalchanges in fungal mycelia (Carvalho and Gomes, 2009).Studies on the mode of action of antimicrobial plant defensins are inprogress. There are confirmed hypotheses that an interaction with the cellmembrane of microorganisms resulting in ion efflux and reactive oxygenspecies (ROS) generation significantly contributes to the mechanisms responsiblefor antifungal properties of plant defensins. Besides antimicrobial activity,plant defensins possess a range of biological functions (Lay and Anderson,2005; Carvalho and Gomes, 2009).The availability of antimicrobial peptides produced by plants or microorganismsthrough ribosomal synthesis for crop protection has been demonstratedby an increased disease resistance of transgenic plants expressingfungal and plant defensin genes (Montesinos, 2007; Carvalho and Gomes,2009). Several agricultural crops, e.g. tobacco, tomato, rice, aubergine, papayaand canola, which are transformed with these genes and produce the correspondingpeptides, have little or no disease development in laboratory,greenhouse or field experiments. A number of peptides produced by microorganismsare insecticidal or nematicidal. The ability of plant defensins toinhibit α-amylase and proteases can contribute to plant defence against pestinsects (Lin et al., 2007).Enzymes, proteinase inhibitors, lectins and PR proteinsLytic enzymesTo obtain nutrients, microorganisms synthesize various lytic enzymes thatcan attack polymeric compounds of different origin. Biocontrol agents canuse these enzymatic activities on plant pathogens. Microbial chitinases, glucanasesand proteases are lytic enzymes of most importance for the biocontrolof phytopathogens. These enzymes hydrolyse chitin, β-glucans andproteins, which can result in direct suppression of pathogen development orgenerate products that function as resistance inducers. For instance, biocontrolisolates of Trichoderma harzianum and Trichoderma atroviride produceendochitinase, β-1,3-glucanase and alkaline proteinase, which degrade plant