The Arbuscular Mycorrhizae

Of all the mycorrhizal associations, AMs are the most widespread and ancient. They are made up by a wide range of land plants, including at least 80% angiosperms, and fungi belonging to glom-eromycota (Smith and Read 2008; Fitter et al. 2011) ; After contact between the symbionts is established (Fig. 11.1a), the fungus forms an appressorium on the root surface and enters the root. Colonization of the root system, by the fungal partner, is restricted to the parenchymatous cells of the cortex, where it grows intercellularly and produces long intercellular hyphae. Lateral branches of the intercellular hyphae penetrate the cortical cells and repeatedly branch dichoto-mously giving rise to the "arbuscules", which give the name to this type of mycorrhiza (Fig. 11.1b, c) . The AM symbiosis is a highly compatible association, and in phosphate-limiting conditions, intraradical development of the fungus can occur over more than 80% of the root length (Harrison 2005).

Arbuscules form inside the cortical cell, but remain separated from the plant cell cytoplasm by the invaginated host plasma membrane, and are surrounded by a metabolically active plant cytoplasm. A new compartment, known as the interfacial compartment, arises and consists of the invaginated host membrane, cell wall-like material, fungal wall and plasma membrane (Fig. 11.1c) (Bonfante and Anca 2009). The development of the AM interaction is, therefore, accompanied by significant alterations of the cellular morphology of both symbionts. Reciprocal exchanges of metabolites are thought to occur at this interface (Harrison 2005; Hodge et al. 2009). Besides these cellular alterations, AM root systems can modify their architecture by increasing the number of lateral roots of different orders, and can change the specific root length, according to the plant species (Scannerini et al. 2001; Hodge et al. 2009).

As in other mycorrhizas, in addition to the intraradical growth phase, the AM fungus maintains a living extraradical mycelium (Fig. 11.1a) that can extend several centimetres from the root. The fungal hyphae within the root are connected to the extraradical mycelium and form a single continuum (Harrison 2005) . These extraradical hyphae absorb nutrients, mainly phosphate (Pi), that are transferred to the host root at the symbiotic interface. Phosphorus is one of the most important elements for plants; however, it is also one of the least available of all essential nutrients in the soil (Vance et al. 2003). A slow diffusion of Pi in the soil solution and rapid absorption results in the development of depletion zones around the roots (Smith and Read 2008). The hyphae of AM fungi extend into soil far beyond the depletion zone. Due to their small diameter, in the range 2-20 mm, they may grow into soil pores that roots cannot penetrate (Smith and Read 2008) . Furthermore, hyphae appear to have the ability to regulate their diameter, depending on the soil pore size (Smith et al. 2010b).

Following AM symbiosis, the P status of the plant tissues is generally higher than that of non-AM plants grown on the same medium (Fusconi

Fig. 11.1 (a) Extraradical hyphae (h) and initiation of colonization (arrow) of a transformed carrot root with Gigaspora rosea. Bar=500 mm. (b) Colonization of the root cortex by the AM fungus Glomus mosseae. Intercellular hyphae (i), arbuscules (a). Bar = 50 mm. (c) Transmission electron micrograph of a Glomus sp. arbuscule inside a cortex cell of Allium porrum. The main trunk of the arbuscule (t) and the thinner branches (tb) are surrounded by the perifungal interfaces (arrows). Nucleus of the cortex cell (n). Bar = 1.5 mm. (d) Typha latifolia plants mycorrhized or not with a mix of Glomus sp. Note the increased growth of AM plants following colonization

Fig. 11.1 (a) Extraradical hyphae (h) and initiation of colonization (arrow) of a transformed carrot root with Gigaspora rosea. Bar=500 mm. (b) Colonization of the root cortex by the AM fungus Glomus mosseae. Intercellular hyphae (i), arbuscules (a). Bar = 50 mm. (c) Transmission electron micrograph of a Glomus sp. arbuscule inside a cortex cell of Allium porrum. The main trunk of the arbuscule (t) and the thinner branches (tb) are surrounded by the perifungal interfaces (arrows). Nucleus of the cortex cell (n). Bar = 1.5 mm. (d) Typha latifolia plants mycorrhized or not with a mix of Glomus sp. Note the increased growth of AM plants following colonization et al. 2005) and a number of plants increase in growth following colonization (Fig. 11.1d), showing a higher shoot-to-root ratio than the non-colonized controls (Scannerini et al. 2001).

However, the AM pathway through extraradical hyphae also plays a major role in the Pi uptake in non-responsive AM plants (Smith and Read 2008) . Recently, it has been demonstrated that the development of an AM association changes the pathway of plant phosphate uptake. In functional AM symbiosis, in fact, the direct plant Pi influx via root epidermis and root hairs is reduced, while genes involved in the AM P uptake pathway, through the plant-fungus interfaces, are up-regulated (Grace et al. 2009).

Although usually considered important primarily because of Pi uptake, there is now evidence that AM fungi may also increase the efficiency of the uptake of other nutrients. The hyphae of AM fungi are involved in the uptake and transfer of inorganic N, although it is not clear whether this always occurs in amounts that are significant for whole plant nutrition (Smith and Read 2008; Fitter et al. 2011). It has been calculated that up to 42 and 30% of plant N is absorbed via the AM pathway in tomato and carrot, respectively (Smith and Read 2008). AM fungi are especially important with respect to the uptake of relatively immobile nutrients that form depletion zones around roots (Cavagnaro 2008): the uptake of Cu has been confirmed for a number of plant-fungus combinations, and Zn uptake, via the AM pathway, has inequivocally been demonstrated. However, the uptake of other micronutri-ents, such as Mn and K, via external hyphae is not so well established (Smith and Read 2008).

The positive effects of AM fungi on plant nutrition are of particular significance in low nutrient status soils, and where the distribution of the soil nutrients is heterogeneous (Cavagnaro 2008). Moreover, the positive effects of AM fungi on host plant nutrient uptake are noticeable in drought environments, since nutrient mobility is limited under drought conditions (Boomsma and Vjn 2008).

Improvements in the nutrition of plants colonized by AM fungi can be attributed not only to the uptake of nutrients via the mycorrhizal pathway, but also to indirect effects brought about by morphological and physiological changes in roots due to colonization. AM fungi may also influence nutrient availability via their effects on soil phys-icochemical properties, nutrient cycling and microbial communities (Cavagnaro 2008 and references therein).

In AM symbiosis, fungal growth, spore production and mycelial transport require high amounts of energy in the form of hexoses and a significant proportion of the photosynthesis products are therefore delivered to the fungus, which leads to an increase in the sink strength of the root (Feddermann et al. 2010) . The cost for the plant to sustain the associated mycelium, in terms of carbon loss, varies according to the plant and fungal species, plant age, and AM developmental stage. Moreover, the respiratory cost of AM symbiosis can vary according to the environmental conditions. It has been calculated that AM can consume between 2 and 20% of the daily photo-synthate production of the host (Boomsma and Vjn 2008). Therefore, AM colonization is generally advantageous in poor-nutrient soils, while colonization by AM in fertile agricultural soils can actually reduce crop productivity, because the carbon costs associated with AM colonization may exceed the carbon-production benefits that are derived from the symbiotic relationship (Boomsma and Vjn 2008).

In the last few years, several publication have dealt with the contribution of AM symbiosis to the tolerance of plants growing under one or more different types of environmental stress (Hildebrandt et al. 2007; Evelin et al. 2009; Gamalero et al. 2009; Garg and Chandel 2010; Koltai and Kapulnik 2010; Miransari 2010; Smith et al. 2010b). Water deficit, salinity and elevated temperatures that cause dehydration in plant tissues are the most common environmental stress factors experienced by plants. Another important form of environmental stress is represented by contamination, which can be caused by essential elements in excessive concentrations, by toxic elements and ions or by other organic or inorganic pollutants, which may result from either human activities or natural processes. Many researchers have shown that AM promotes plant fitness under a variety of stress conditions and through various mechanisms. Some of these mechanisms are non-specific, and include enhanced nutrient acquisition and, frequently, enhanced plant growth, regulation of the plant hormone balance, and improving rhizospheric and soil conditions. In addition, AM improves plant growth tolerance with more specific mechanisms in relation to the kind of stress.

In this chapter, we provide an overview of the effects of AM colonization on host plants subjected to drought stress and soil pollution, and of the possible mechanisms involved in the beneficial effects of AM fungi.

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