Following AMF colonization, host root tissue P levels are typically enhanced which modify the phospholipid composition and therefore the root membrane permeability resulting in a reduction in the leakage of net amount of sugars, carboxylic acids, and aminoacids into the rhizosphere (Graham and Menge 1982; Ratnayake et al. 1978; Schwab et al. 1983). These alterations arrest the chemotactic effect of pathogens to plant roots and discourage pathogen entry. Prior inoculation of maize plants with G. mosseae decreased the number of Alternaria alternata colony forming units, but when both organisms were inoculated at the same time, there was no effect on pathogen inoculum density in soil (McAllister et al. 1996). It is possible that the G. mosseae symbiont altered membrane permeability of the host roots, thereby reducing the quality and quantity of substances exuded by the roots (Graham et al. 1981), restricting pathogen propagule germination, indicating that the timing of inoculation can enhance biocontrol activity.
Systemic-induced resistance (SIR) is typically the sustained induction of resistance or tolerance to disease in plants by previously inoculating with a pathogen, exposing to an environmental influence or treating with a chemical, which may or may not have antimicrobial activity (Handelsman and Stabb 1996; Kuc 1995). Researchers have suggested that AMF-inoculated plants may employ SIR as a mechanism of biocontrol (Benhamou et al. 1994; Brendan et al. 1996; Trotta et al. 1996). The SIR phenomenon in mycorrhizal plants is demonstrated as localized and systemic resistance to the pathogen (Cordier et al. 1998). An increase in the lignin deposition in plant cell walls following AMF colonization can restrict the spread of pathogens (Dehne and Schonbeck 1979). Using a split root system, Cordier et al. (1998) demonstrated that G. mosseae protected tomato plants against P. parasitica by reducing pathogen development and spread by increasing cell wall appositions containing callose close to the intercellular hyphae and accumulation of phenolic compounds and plant cell defense responses. Root damage was observed in portions of mycorrhizal root systems not containing mycorrhizal structures. The SIR reaction to the pathogen in mycorrhizal plants was further illustrated by host wall thickenings containing nonesterified pectins and pathogenesis related (PR)-1 protein in the nonmycorrhizal areas of the roots. They also noted that the PR-1 protein was found only in the pathogen-invaded tissues of pea. These responses were observed in the nonmycorrhizal pathogen-infected root tissues that ultimately led to cell death. Bodker et al. (1998) reported that the observed increased resistance to A. euteiches in G. intraradices-inoculated pea was probably due to an "induced systemic factor," induced by G. intraradices. The AMF-mediated SIR phenomenon is speculated to play a role in the protection of potatoes against post-harvest suppression of potato dry rot, wherein dry rot in G. intraradix-inoculated potato was reduced by up to 90% compared to uninoculated control (Brendan et al. 1996). This finding suggests that the benefits of AMF inoculation for disease control surpasses the growth and reproduction phase of the host and extends to the storage phase of the product. The area of SIR response in mycorrhizal plants is still developing and several aspects including whether all AMF species can equally elicit a SIR response in the host are not known.
Some researchers have examined the role of PR proteins in the disease control process mediated by AMF (Liu et al. 1995). Enhanced levels of 10 different PR proteins were detected in cotton plants inoculated with G. mosseae, G. versiforme, or Sci. sinuosa challenged with V. dahliae compared with plants not challenged by the pathogen. The PR proteins retarded the hyphal growth of V. dahiiae and killed their conidia. This appears to be a promising field that can be used for the effective control of plant diseases.
Phytoalexins are produced in response to microbial infection (Paxton 1981), whereas phytoanticipins are stored in plant cells in anticipation of or prior to pathogen attack (VanEtten et al. 1995). The level of phytoalexins elicited by pathogens has been shown to be much higher than those elicited by symbiotic organisms (Wyss et al. 1991). The function of an isoflavonoid molecule as a phytoalexin or phytoanticipin can be predicted based on the cellular location of the molecule (Stafford 1992).
An increase in the level of total soluble plant phenolics such as isoflavonoids or flavonoids, lignin, syringic, ferulic or coumaric acids, etc. have been reported as synthesis of phytoalexins following AMF colonization of roots (Harrison and Dixon 1993; Morandi 1989; 1996). Some flavonoids that are not true phytoalexins may also respond to AMF colonization of roots (Harrison and Dixon 1993; Morandi and Le-Quere 1991; Volpin et al. 1995).
The production of phytoalexins as a result of pathogen invasion in mycorrhizal plants has been explored. Tomato plants inoculated with G. mosseae posed greater resistance to the pathogen F. oxysporum and were found to have increased phenylalanine and b-glucosidase activity and total phenol content in their roots compared to plants inoculated with either organism alone (Dehne and Schonbeck 1979). Sundaresan et al. (1993) reported that a purified ethanol fraction of mycorrhizal cowpea root extract inhibited F. oxysporum in vitro. However, the isoflavonoid was not identified. Production of phytoalexins in mycorrhizal plants appears to be independent of the effect of fertilizer addition (Caron et al. 1986b). In general, in the presence or absence of pathogens in plant roots, phytoalexins are induced in mycorrhizal plants that neutralize the negative effects of pathogens.
Differential expression of defense-related genes in mycorrhizal plants has been the recent focus of AMF-mediated biocontrol (Blee and Anderson 1996; Dumas-Gaudot et al. 1996; Lambais and Mehdy 1995; Pozo et al. 2002). Researchers have shown that AMF enter into host (e.g., tomato) roots and induce a local, weak, and transient activation of the host defence mechanism against pathogens such as P. parasitica, which involves the induction of hydrolytic enzymes such as chitinase, chitosanase, b-glucanase, and superoxide dismutase (Pozo et al. 2002). In addition, portions of the mycorrhizal root system not containing mycorrhizal structures appear to have alterations in the constitutive isoforms of the enzymes indicating systemic changes following AMF colonization (Pozo et al. 2002). A high positive correlation between the level of glucanase activity in host tissues and pathogen resistance has been established (Graham and Graham 1991). Further studies examining the role of these glucanases will help in the development of strategies for control of pathogens using AMF.
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