Bioprotection from AM fungi-colonized plants is the outcome of complex interactions between plant, pathogen and AM fungi. Various mechanisms are proposed for conferring bioprotection, but generally effective bioprotection is a cumulative result of all mechanisms working either separately or together. The challenges to obtain biocontrol through AM fungi include the obligate nature of AM fungi, poor understanding of the mechanisms involved and the role of environmental factors in these interactions. Moreover, improved understanding of agricultural practices on AM colonization is required using new techniques like confocal laser scanning microscopy. These techniques may reveal the processes involved in root colonization and also in the biocontrol process. Furthermore, these techniques may provide new ways for increasing benefits of AM fungi by their use with other beneficial microorganisms.
The potential of AM fungi to enhance plant growth is well recognized but not exploited to the fullest extent. These organisms are rarely found in nurseries due to the use of composted soil-less media, high levels of fertilizer and regular application of fungicide drenches. The potential advantages of AM fungi in horticulture, agriculture, and forestry are not perceived by these industries as significant. This may be due in part to inadequate methods for large-scale inoculum production. Monoxenic root-organ in vitro culture methods for AMF inocula production have also been attempted by various workers (Mohammad and Khan, 2002; Fortin et al., 2002) but these techniques, although useful for the study of physiological, biochemical, and genetic relationships, have limitations in terms of producing inocula of AM fungi for commercial purposes. Pot cultures in pasteurized soils have been the most widely used method for producing AM fungi inocula but are time-consuming, bulky, and often not pathogen-free. To overcome these difficulties, soil-free methods such as soil-less growth media, aeroponics, hydroponics and axenic cultures of AM fungi have been used successfully to produce AMF-colonized root inocula (Sylvia and Jarstfer, 1994a, b; Mohammad and Khan, 2002). Substrate-free colonized roots produced by these methods can be sheared and used for large-scale inoculation purposes.
Although AMF are ubiquitous, natural associations of AM fungi are not efficient in increasing plant growth (Fitter, 1985). Cropping sequences, fertilization, and plant pathogen management practices affect both AM fungi propagules in soil and their effects on plants (Bethlenfalvay and Linderman, 1992). The propagation system used for horticultural fruit and micropropagated plants can benefit most from AM biotechnology. Micropropagated plants can withstand transplant stress from in vitro to in vivo systems if they are inoculated with appropriate AM fungi (Lovato et al, 1996; Azcon-Aguilar et al., 2002). In order to use AM fungi in sustainable agriculture, knowledge of factors such as fertilizer inputs, pesticide use, and soil management practices which influence AM fungi is essential (Bethlenfalvay and Linderman, 1992; Allen, 1991, 1992). In addition efficient inoculants should be identified and used as biofertilizers, bioprotectants, and biostimulants for sustainable agriculture.
In general, a single biocontrol agent is used for biocontrol of plant disease against a single pathogen (Wilson and Backman, 1999). This protocol may account for the inconsistent performance by the biocontrol agent, because a single agent is not active in all soil environments or against all pathogens that attack the host plant. On the other hand, mixtures of biocontrol agents with different plant colonization patterns may be useful for the biocontrol of different plant pathogens via different mechanisms of disease suppression. Moreover, mixtures of biocontrol agents with taxonomically different organisms that require different optimum temperature, pH, and moisture conditions may colonize roots more aggressively, and improve plant growth and efficacy of biocontrol. Greater suppression and enhanced consistency against pathogens was observed using mixtures of biocontrol agents (Akhtar and Siddiqui, 2007a). Consortia of biocontrol agents may better adapt to the environmental changes that occur throughout the growing season and protect against a broader range of pathogens. Mixtures of micro-organisms increase the genetic diversity of biocontrol systems that may persist longer in the rhizosphere and utilize a wider array of biocontrol mechanisms (Pierson and Weller, 1994). Multiple organisms may enhance the level and consistency of biocontrol via a more stable rhizosphere community and effectiveness over a wide range of environmental conditions.
AM fungi, root nodule bacteria, plant growth promoting rhizo-bacteria, antagonistic fungi and their use with composted manure may provide protection at different times, under different conditions, and occupy different or complementary niches (Siddiqui and Akhtar, 2008). These mixtures may coexist without exhibiting adverse effects on each other (Akhtar and Siddiqui, 2007a, b) and suitable combinations of these bio-control agents may increase plant growth and resistance to pathogens. More detailed investigations of the relationships in various pathosystems and interactions between these microorganisms and the host plant are needed for developing suitable biocontrol of plant diseases.
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