natural-products-in-plant-pest-management

250 M. Santos et al.by biotic or abiotic agents’. Induced defence responses are regulated by anetwork of interconnecting signal transduction pathways in which thehormonal signals salicylic acid (SA), jasmonic acid (JA) and ethylene (ET)play a major role (Pieterse and Van Loon, 1999; Glazebrook, 2001), and otherhormones such as brassinosteroids and abscisic acid can also be involved(Nakashita et al., 2003; Ton and Mauch-Mani, 2004). The phenotypic effects ofactivated defences are quite similar when the stimuli are biotic or abioticagents, but the biochemical and mechanistic changes appear to be subtlydifferent (Van Loon et al., 1998; Whipps, 2001). This has resulted in the terminduced systemic resistance (ISR) for bacterially induced resistance andsystemic acquired resistance (SAR) for that induced by abiotic agents ormicroorganisms that cause localized damage (Pieterse et al., 1996).SAR confers long-lasting protection against a broad spectrum of microorganisms.SAR requires the signal molecule SA and is associated with theaccumulation of pathogenesis-related (PR) proteins such as chitinase, β-1-3-glucanases or proteinase inhibitor, which are thought to contribute to resistance(Durrant and Dong, 2004). One of the major differences between SARand ISR is that PR proteins are not universally associated with bacteriallyinduced resistance (ISR) (Hoffland et al., 1995) and SA is not always involvedin the expression of ISR.Several reports suggest that compost and compost-amended soil mayalter the resistance of a plant to disease. This was observed in airborne diseasessuch as powdery mildew of wheat and barley (Tränkner, 1992), earlyblight and bacterial spot of tomato (Lycopersicum esculentum) (Roe et al., 1993)or Anthracnose and Pythium root rot in cucumber (Cucumis sativus) (Zhanget al., 1996). A select few beneficial microorganisms of compost can inducemechanisms of ISR. These microorganisms activate biochemical pathways inplants leading to ISR to root as well as some foliar diseases. This mechanismhelps explain the often heard statement that plants raised on healthy organicsoils are more able to resist diseases (Zhang et al., 1998). Plants produced in compost-amendedmixes harbouring biocontrol agents that induce systemic resistancehave higher concentrations of proteins related to host defence mechanisms(Zhang et al., 1998). The activation of plant defence produces changes in cellwallcomposition, de novo production of PR proteins such as chitinases andglucanases, and synthesis of phytoalexins, although further defensive compoundsare likely to exist but remain to be identified (Heil and Bostock, 2002).Zhang et al. (1996, 1998) showed that composted pine-bark-amendedpotting mix provide Pythium root rot and Anthracnose control in cucumber byinducing systemic resistance (reported as SAR) utilizing split-root techniques.Experiments showed that when only some of the roots of a plant arein compost-amended soil, while the other roots are in diseased soil, the entireplant can still acquire resistance to the disease. On those plants, the systemicprotection induced by compost was accompanied by increased peroxidaseactivity in leaf tissue. Acid peroxidase was previous reported as a putativemolecular marker of SAR in cucumber.ISR plays a role in the suppression of plant pathogens that colonize aerialand soil parts (Pharand et al., 2002). Krause et al. (2003) demonstrated that