Agriculture Reference
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nutrients for a nonmycorrhizal root is 1 or 2 mm, about the length of a root hair (Li et al.,
1991). The extraradical hyphae of the mycorrhizal fungi extend upward of 14 cm away
from the root and act, in effect, as extensions of the root system (Mozafar et al., 2001).
The same forms of P that are normally available to roots are taken up by the ERM, where
they are synthesized into polyphosphate granules and translocated into the root. There
P is released to the interfacial apoplastic space between the membranes of both partners
(Balestrini et al., 2007).
Other benefits plants receive from the symbiosis are enhanced water relations and
disease resistance. Mycorrhizal plants have been found to continue to photosynthesize
at lower soil moisture levels than paired nonmycorrhizal plants due to a number of rea-
sons, including increases in overall plant health, better exploration of the soil volume;
enhanced soil structure, which retains more water; increased stomatal conductance; and
lower osmotic potential in leaf tissues due to increased solute concentrations (Augé, 2000).
Enhanced disease resistance can occur not only due to nutritive effects but also due to
selection of the ERM for soil bacteria that are antagonistic to fungal plant pathogens
(Linderman, 2000).
The receipt of fixed carbon, in the form of hexose, from the host plant is essential
to the fungus due to its obligate symbiotic nature. There is a metabolic division of labor
among the structures of these coenocytic organisms (Shachar-Hill et al., 1995). Only the
hyphae inside the root express the genes necessary for the uptake of hexose and synthesis
of fatty acids. Extraradical hyphae are incapable of these metabolic events, and germinat-
ing spores have only limited carbon uptake and no de novo synthesis of lipids (Bago et al.,
1999; Pfeffer et al., 1999).
Although AM fungi are generally considered to be mutualists, the cost of the symbio-
sis to the host plant, in the form of fixed carbon, can exceed benefits to net photosynthesis
due to any impact of the symbiosis on nutrient uptake or water relations (Johnson et al.,
1997). The metabolic demands of AM fungi can require 4-20% of host photosynthate (see
Douds et al., 2000, for review). Experimentation with fungicide application to reduce AM
fungus colonization has demonstrated mycorrhiza-mediated growth suppression of citrus
in the field under P-sufficient conditions (Graham and Eissenstat, 1998). On the far nega-
tive end of the spectrum is tobacco stunt disease caused by the AM fungus
Glomus macro-
carpum
(Hendrix et al., 1992).
Although a common viewpoint is that the over 150 described species of AM fungi
more or less occupy the same niche (Dodd et al., 2000), there is mounting experimental
evidence of functional diversity among these fungi. AM fungi have been shown to differ
in competitiveness in colonization of roots (Bennett and Bever, 2009), in the distance away
from the root in which the ERM forages for nutrients (Smith et al., 2000), and ramification
of ERM in localized areas of nutrient availability (Gavito and Olsson, 2008). Functional
complementarity achieved through colonization of a host root system by a community of
AM fungi can result in enhanced plant growth over that occurring with colonization by
only one AM fungus (Jansa et al., 2008).
with communities of AM fungi
The goals of agriculture have progressed from subsistence to economies of scale to renewed
emphasis on smaller farms and sustainability. With the move toward sustainability, many
agricultural practices have been modified to better manage soil nutrients, organic matter,
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