فایل ورد کامل میکوریزا آربوسکولار و کنترل بیولوژیکی پاتوژن های گیاهی خاکزاد – مروری بر مکانیسم های مربوطه

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بخشی از مقاله انگلیسیعنوان انگلیسی:Arbuscular mycorrhizas and biological control of soil-borne plant pathogens – an overview of the mechanisms involved~~en~~

Abstract

Biological control of plant pathogens is currently accepted as a key practice in sustainable agriculture because it is based on the management of a natural resource, i.e. certain rhizosphere organisms, common components of ecosystems, known to develop antagonistic activities against harmful organisms (bacteria, fungi, nematodes etc.). Arbuscular mycorrhizal (AM) associations have been shown to reduce damage caused by soil-borne plant pathogens. Although few AM isolates have been tested in this regard, some appear to be more effective than others. Furthermore, the degree of protection varies with the pathogen involved and can be modified by soil and other environmental conditions. This prophylactic ability of AM fungi could be exploited in cooperation with other rhizospheric microbial angatonists to improve plant growth and health. Despite past achievements on the application of AM in plant protection, further research is needed for a better understanding of both the ecophysiological parameters contributing to effectiveness and of the mechanisms involved. Although the improvement of plant nutrition, compensation for pathogen damage, and competition for photosynthates or colonization/infection sites have been claimed to play a protective role in the AM symbiosis, information is scarce, fragmentary or even controversial, particularly concerning other mechanisms. Such mechanisms include (a) anatomical or morphological AM-induced changes in the root system, (b) microbial changes in rhizosphere populations of AM plants, and (c) local elicitation of plant defence mechanisms by AM fungi. Although compounds typically involved in plant defence reactions are elicited by AM only in low amounts, they could act locally or transiently by making the root more prone to react against pathogens. Current research based on molecular, immunological and histochemical techniques is providing new insights into these mechanisms.

۱ Introduction

A key, universally accepted concept is that natural microbial populations in soil or other “living” substrate are activated to grow around developing plant roots, giving rise to the so-called “rhizosphere” (Azcn-Aguilar and Barea 1992). As a rhizosphere develops at the root-soil interface, microorganisms there interact with both plant roots and soil constituents. Communication among the different components is mainly via chemical or biochemical signals, although physical interaction is also possible. This “dialogue” is modulated by inherent soil characteristics and the prevailing environmental conditions of the ecosystem (Bowen 1980; Lynch 1990). The most important interactions developing in the rhizosphere can be classified into three main groups: (1) plant – plant interactions caused by overlapping rhizospheres, which results in competition for nutrients; (2) root–microorganism interactions, determined by plant activities that stimulate microorganisms to grow around the roots (rhizosphere effect) and by microbial activities that affect plant development, either by benefiting the plant or by inducing disease; and (3) microbe–microbe interactions, which include both synergistic and antagonistic activities (Stoztky 1972; Lynch 1990). Biological control can be defined as the directed, accurate management of common components of ecosystems to protect plants against pathogens. In this regard, microbial diversity is a key natural resource (Kennedy and Smith 1995). Thus, biological control preserves environmental quality by a reduction in chemical inputs, and is characteristic of sustainable management practices (Altieri 1994; Barea and Jeffries 1995). Under natural conditions, plants strictly speaking do not have roots, they have mycorrhizas; the roots of most flowering plants form mutualistic symbioses with certain soil fungi (Harley and Smith 1983). Mycorrhizal associations are found in nearly all ecological situations, with arbuscular mycorrhizas (AM) being the most common type in normal cropping systems and in natural ecosystems (Harley and Smith 1983; Gianinazzi and Schüepp 1994). AM fungi, which belong to the order Glomales of the Zygomycetes (Rosendahl et al. 1994), biotrophically colonize the root cortex and develop an extramatrical mycelium which helps the plant acquire mineral nutrients and water from the soil. AM symbioses play a key role in nutrient cycling in ecosystems (Jeffries and Barea 1994), and the external mycorrhizal mycelium, in association with other soil organisms, forms water-stable aggregates necessary for good soil quality (Bethlenfalvay and Schüepp 1994). It is evident that an increased capacity for nutrient acquisition resulting from mycorrhiza association could help the resulting stronger plants to resist stress. However, AM symbioses may also improve plant health through a more specific increase in protection (improved resistance and/or tolerance against biotic and abiotic stresses) (Bethlenfalvay and Linderman 1992; Barea and Jeffries 1995). The study of a possible role for AM symbiosis in protection against plant pathogens began in the 1970s, and a great deal of information has been published on the subject, however, we still know very little about the underlying mechanisms (Hooker et al. 1994; Linderman 1994). It is not our aim here to discuss all the information in the many published papers. Instead, we will analyse the mechanisms by which AM fungi could control root pathogens and indicate possible fruitful research approaches.

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