Agriculture Reference
In-Depth Information
7.1.1 Life cycle of AM fungi
A generalized life cycle of AM fungi begins in the spring with the germination of a spore
produced the previous autumn. Spores are induced to germinate when (1) any necessary
dormancy period has been met (Tommerup, 1981), and (2) the soil reaches the proper tem-
perature (e.g., 18-25°C; Daniels and Trappe, 1980) and is further stimulated (3) by increased
CO
2
levels in the soil due to microbial or root respiration (Bécard and Piché, 1989a). The
germ tube hyphae grow through the soil in “search” of a host root to colonize. Spores are
capable of regerminating a number of times if initial attempts to find a root to colonize
are unsuccessful. Growth during this “asymbiotic” or presymbiotic phase is supported by
carbon reserves—mainly lipid, trehalose, and glycogen—in the spore (Bago et al., 1999).
Other forms of inocula of AM fungi that can overwinter in agricultural soil are colonized
root pieces containing AM fungus vesicles or spores and the extraradical mycelium (ERM)
of the previous crop host.
Plant roots exude a number of chemical signals into the soil that form a type of com-
munication with the germ tube hyphae (Nagahashi et al., 2010). More of these signals are
released when the root is low in phosphorus (P) nutrition (Nagahashi and Douds, 2000). The
AM fungus hyphae branch in response to these signals in a concentration-dependent man-
ner: The hyphae produce more long branches at low concentrations of the signals and dense
clusters of short branches in response to high concentrations of the signals (Nagahashi and
Douds, 2000). These responses serve to increase the probability of contact with a host root.
Colonization begins with the formation of an appressorium and penetration through
the root epidermis. Subsequent extracellular growth into the cortex is followed by growth
into cortical cells via digestion through the cell wall and envagination of the plasma mem-
brane (Genre and Bonfante, 2010). Once inside the cell, the hyphal tip branches profusely,
forming a small, tree-like structure called the arbuscule. The arbuscule is the most likely
candidate for the site of nutrient transfer between the symbionts due to the high density
of cytoplasm and membrane surface area (Toth and Miller, 1984). Once the fungus begins
to receive fixed carbon from the root, it then grows out into the soil, forming the ERM:
the nutrient-absorbing organ of the mycorrhiza (Bécard and Piché, 1989b). Colonization
of the root spreads, and new spores are produced when a certain colonized root length is
achieved, or on senescence of the host plant, to begin the cycle again (Douds and Chaney,
1982; Gazey et al., 1992).
Colonization of roots is under the regulation of the host plant. There is now abundant
evidence that much of the same genetic regulatory pathway that governs the formation of
nodules in the legume-
Bradyrhizobium
symbiosis also operates to control the formation of
arbuscular mycorrhizas (e.g., Oldroyd and Downie, 2006). Further, and more important for
consideration in agriculture, the formation of the mycorrhiza is regulated by the availabil-
ity of P in the soil and concomitant P status of the host plant. Colonization of roots is inhib-
ited with increasing P status of the root (Jasper et al., 1979). This occurs through reduced
exudation of hyphal branching signals and reduced carbohydrate availability within the
root (Nagahashi and Douds, 2000; Treseder and Allen, 2002).
The most notable benefit plants receive from colonization by AM fungi is enhanced uptake
of mineral nutrients that are immobile in the soil solution (i.e., P, Zn, and Cu). For example,
Barber (1977) estimated that soluble forms of P can only diffuse through the soil 0.5 mm
due to the formation of insoluble salts with cations. The typical zone of uptake for these
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