Induced resistance defense mechanism

Toth et al. 2005), induced resistance was found only in the top blades of S.filipendula (Taylor et al. 2002). The authors concluded that valuable basal stipes are mechanically and constitutively defended, while the valuable meristematic tissues have inducible chemical defenses (Taylor et al. 2002). 

In addition to its role as a preformed resistance factor, Hijwegen (5) has also proposed active induction of lignification as a defense mechanism of cucumber against Cladosporium. Subsequently, in a number of host-pathogen interactions, induced lignification has been proposed as the active... 

Local infection of a plant will stimulate the development of natural defense mechanisms often resulting in an immune reaction toward a variety of pathogens. Certain chemicals can also trigger the same biochemical reactions in plants. Among the chemicals found to induce this systemic acquired resistance (SAR) are thieno[2,3-rf [ 1,2,3]-thiadiazoles 195 <1999JPR341>. 

Induced resistance is the qualitative or quantitative enhancement of a plant s defense mechanisms against pests in response to extrinsic physical or chemical stimuli. These extrinsic stimuli are known as inducers or elicitors. 

A better understanding of the role of phytoalexins in plant defenses and of the mechanisms of induced resistance may potentially open a powerful new approach to the control of insect pests of cultivated plants. If indeed, in light of the hypothesis of optimal defense strategies (3), a post-attack response is a more efficient line of defense than the attack-independent accumulation of allelochemics, the exploitation of phytoalexin-producing mechanisms may represent a fertile field for future investigations. Several uses of induced resistance may be conceived. Four of these approaches are briefly discussed. 

Immunization of cucumbers by (L lagenarium, C. cucumerinum, P. 1achrymans or TNV generates a systemic increase in peroxidase activities (. TJ, ] 9, 8U) > Like 1 i gni f ic a t ion and phytoalexin induction, peroxidase activities also rise more quickly in response to infection in leaves of immunized plants, even though total activity eventually may be highest in infected susceptible leaves (77). Several other stimuli can induce local (mechanical and chemical injury) or systemic (senescence, ethylene) peroxidase increases that are not accompanied by increased disease resistance. Thus, enhanced peroxidase activity per se may not be a defense mechanism, but may be a necessary adjunct with appropriate chemical substrates for processes important in disease resistance, e.g., lignification, suberization, and me 1anization. 

In contrast to the above-mentioned constitutive types of resistance, plants also possess adaptive defense mechanisms to defend themselves against pathogen or insect attack. Following an appropriate stimulation, they are capable of developing an enhanced defensive capacity, commonly referred to as induced resistance [5,6,7,8]. 

In vivo, interferon inducers exert a wide variety of effects on host defense mechanisms interferon production, pyrogenicity, stimulation of cellular and humoral immunity and stimulation of reticulo-endothelial activity. It is not surprising, therefore, that interferon inducers increase the host s resistance to both viral and non-viral (bacterial, fungal, protozoal) infections and tumor growth. A major obstacle confronting the clinical usefulness of interferon inducers is their toxicity. Are interferon induction and toxicity necessarily coupled or can they be disconnected Preliminary evidence suggests that, at least with poly(I) poly(C), toxicity and activity could be uncoupled. 

Th + or AP" " induced a precipitate to form in all Bradyrhizobium and Sinorhi-zobium cultures tested, which suggested a defense mechanism based on metal precipitation by extracellular polymers (Santamaria et al., 2003). Among the metals tested, only Fe " ", Ap+, and Th were able to induce the formation of precipitate. AP+ is probably the natural soil component against which this defence mechanism could be directed, and a different defence mechanism based on extracellular aluminium precipitation within a gelatinous residue has been described for P. fluorescens (Appanna and St. Pierre, 1996). However, tliis polymer was composed mainly of phosphatidylethanolamine. While metal binding to extracellular polymers and bacterial surfaces have been proposed as the reason for increased metal resistance of biofilm-growing bacteria, this proposed defense mechanism involved the physical removal of the capsule after metal binding (Santamaria et al., 2003). 

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