natural-products-in-plant-pest-management

Proteinaceous and Polyketide Compounds in Plant Protection 123Arabidopsis cells during pathogen invasion must be folded with the plantcyclophylin ROC1 to form an active conformation able to cleave plantproteins. Along with other proteins, the activated bacterial protease cleavesprotein RIN4, which is combined in Arabidopsis in an inactive complex witha signal protein RPS2. Proteolysis of RIN4 releases RPS2 that triggers defenceresponses (Mackey et al., 2003). PPIases of one organism have been reportedto inhibit the growth of another organism by means of competitive bindingwith a receptor, and probably some PPIases can suppress growth of culturedplant pathogens. For instance, cyclophylin C-CyP isolated from Chinesecabbage (Brassica campestris L. ssp. pekinensis) inhibits in vitro growth ofseveral fungi including Rhizoctonia solani, B. cinerea, F. solani and F. oxysporum(Lee et al., 2007).Various experiments have shown that MF3 induced resistance in variousmonocotyledonous and dicotyledonous plants against several fungal andviral pathogens. MF3 was found to protect tobacco against TMV, potato Yvirus and A. longipes, and barley against Bipolaris sorokiniana. At the sametime, this protein did not influence TMV infectivity, was not fungitoxic andhad no phytotoxicity towards tobacco plants and cereals. Moreover, treatmentof barley and wheat seeds, naturally infected by B. sorokiniana andF. culmorum, with MF3 promoted formation of a well-developed root systemin the diseased barley and wheat seedlings, that conferred plant tolerance toroot rots and was consistent with growth-stimulatory functions for PPIasesin plants (Dzhavakhiya et al., 2005; Shumilina et al., 2006).Comparative analysis of MF3 with 45 proteins of different homology levelsrevealed two conserved sequences in the MF3 polypeptide chain. Proteolysiswith trypsin inside one of these conserved sequences producedfragments that did not induce resistance to TMV in tobacco leaves. Thesedata allowed the assumption that the analysed sequence contained a motifresponsible for the resistance-inducing activity of MF3. Such a motif,IIPGLEKALE GKAVGDDLEVAVEPEDAYG, was detected and named MF3-29 because it was found to consist of 29 amino acid residues. This fragmentwas necessary and sufficient for induction of tobacco resistance to TMV. Biologicaltests on isolated tobacco leaves showed that treatments of tobaccoleaves with chemically synthesized oligopeptide MF3-29 at concentrations0.5, 5 and 50 nM were as effective against the virus as the whole protein at thesame concentrations.In order for products based on protein elicitors to effectively controlplant pathogens, they have to access plant receptors recognizing PAMPs. Thelarge size of the molecules or hydrophobic barriers on the plant surface (suchas cuticle) can impede physical contact or chemically mediated recognitionof elicitor proteins and the subsequent induction of defence responses inplants. To solve this problem, special molecular carriers facilitating elicitortransport to plant cells should be developed. Various polycationic molecules,especially chitosan, which are used now for the delivery of large biologicalmolecules (DNA or proteins) to their outer or intracellular receptors, lookpromising as putative carriers of proteinaceous elicitors. Experiments withthe wheat leaf spot agent S. nodorum and turnip mosaic virus (TuMV)