natural-products-in-plant-pest-management

Control of Virus Diseases of Plants 159(TNV) protected the plant systemically against disease caused by the fungusColletotrichum lagenarium (Jenns and Kuc, 1977). SAR expessed in Vigna plantsfollowing inoculation with TNV against challenge by TNV was not expressedagainst challenge by a CMV which infects the host systemically (Pennazzioand Roggero, 1991). Associated with SAR was the stimulation of ethyleneformingenzyme activity. Infection of ecotype Dijon of Arabidopsis thalianawith turnip crinkle virus (TCV) leads to the resistance against further infectionby TCV or Pseudomonas syringae (Uknes et al., 1993). SAR in cucumberplants against powdery mildew disease, caused by Sphaerotheca fuliginea(Schlechtend Fr.) Pollacci, was induced by localized infection in cucumbercotyledons with TNV (Farrag et al., 2007).The SAR phenomenon is observed both in dicotyledonous and monocotyledonousplants; it provides the third and final line of defence againstpathogens. The first line of defence consists of genetically inherited resistancemechanisms that make plants constitutively resistant to the majority ofpathogens present in the environment. The second line of defence is activatedin the immediate vicinity of the infected or wounded site in an attemptto prevent the spread of pathogens throughout the plant. The local resistanceresponse develops more rapidly than SAR and involves cell-wall and cuticlestrengthening, synthesis of toxins, antifeedants and the production ofdefence-related proteins including the PR proteins. In addition to long-distancesignal molecules, local resistance may be partially mediated throughrelatively immobile endogenous elicitors, which include oligogalacturonidefragments of the plant cell wall (Lamb and Dixon, 1990). Several lines of evidencesuggest that endogenous SA is a signal molecule in SAR. Involvementof SA in SAR came from the discovery that endogenous SA increases by atleast 20-fold in the virus-inoculated leaves of tobacco (Malamy et al., 1990).The increase coincides with the appearance of hypersensitive response (HR)lesions on the inoculated leaves. Accumulation of SA increased with theintensity of HR and was proportional to the dose of virus inoculum (Yalpaniet al., 1991). Tissue accumulation of SA in TMV inoculated xanthi-nc tobaccoparalleled or preceded detectable increase in the levels of PR-1 mRNA inboth inoculated and uninoculated leaves (Malamy et al., 1990).7.4 SAR and the Role of PR ProteinsSAR strongly correlates with the coordinate expression of at least nine familiesof genes (sar genes), of which several encode the PR proteins (Wardet al., 1991). Generally, tissues in which a significant amount of PR protein hasbeen induced are more resistant to infection by pathogen than those that lackPR proteins. PR proteins were first discovered in 1970 in tobacco plants reactinghypersensitively to TMV infection (Gianinazzi et al., 1970; Van Loon andKammen, 1970). PR proteins are produced in plants in response to infectionby viruses, bacteria (Metraux and Boller, 1986), fungi (Gianinazi et al., 1980)and viroids (Conejero et al., 1979) and are also synthesized in response tochemical treatments and specific physiological stresses (Stintzi et al., 1993).