Glassop et al. (2005), Wright et al. (2005), and Nagy et al. (2006)
corroborated by molecular analyses of phosphate transporter activity (Karandashov and Bucher 2005). As in the case of AM-plant N uptake, the ongoing characterization of the mycorrhizal P uptake and translocation pathways provide key evidence as to the active and intricate role of AM fungi in the enhancement of plant nutrient stress tolerance.
2.1.3 Micronutrient (Metal) Uptake
Analogous to the processes of AM-plant N and P acquisition, there is a considerable body of literature describing the beneficial role of the AM symbiosis in micronutrient uptake, particularly soil-metal5 deficiency conditions (Jeffries et al. 2003 ; Koide 1991; Marschner and Dell 1994).
5 Macro- and micronutrients are alternatively classified according to their physicochemical properties to define them, respectively, as either non-metals (nitrogen, sulfur, phosphorus, boron, chlorine) which have a negative valence, or metals (potassium, calcium, magnesium, iron, manganese, zinc, copper, molybdenum, nickel) which have a positive valence (Foy et al. 1978; Larcher 2003).
In this regard, soils of temperate environments (typically classified as alfisols and vertisols) and tropical environments (ultisols and oxisols) can suffer from suboptimal elemental compositions and (or) nutrient imbalances due to long-term weathering and soil erosion resulting in plant zinc (Zn), nickel (Ni), cobalt (Co), copper (Cu), manganese (Mn), and (or) iron (Fe) nutritional deficiencies and (or) mutual antagonisms (Blinkley and Vitousek 1989; White and Zasoski 1999). As described previously, the AM symbiosis contributes in circumventing nutrient deficiency stress by increasing nutrient bioavailability in the mycorrhizosphere due to an increased resource acquisition capability which helps in supplementing the plant nutritional status, especially poorly labile metal nutrients (Eckhard et al. 1994; Liu et al. 2000; Marschner 1998; Rengel et al. 1999; Sharma et al. 1994). A commonality in the mode-of-action seems to exist regarding the general AM-plant metal uptake pathway (Fig. 11.7) as metal nutrients are taken up, translocated, and transferred from the extraradical hyphae to the host roots, as depicted in reviews by Meharg (2003), Gohre and Pazkowski (2006), and González-Guerrero et al. (2009). These recent studies have reported that the exudation of organic chelators by the extraradical hyphae contributes in solubilizing metal ions in the mycorrhizosphere followed by the mobilization of metal-chelator complexes across fungal transporters, such as GintZIP in Glomus intrara-dices (González-Guerrero et al. 2009) . Free metal ions may also be taken up passively across trans-membrane ion channels depending on the soil metal concentration gradient. In the cytosol, metal ions are typically bound and (or) sequestered by metallothionein proteins or glutathione complexes. Notably, it has been shown that this course-of-action can correspond with an upregulation of GintMT1 (encoding for fungal metallothioneins in G. intraradices - González-Guerrero et al. 2005, 2007; López-Pedrosa et al. 2006) as well as an increase in glutathione
S-transferase activity (González-Guerrero et al. 2009) , which could represent critical steps in limiting internal stress due to the production of reactive oxygen species in the fungal cytosol (González-Guerrero et al. 2010a). Subsequently, the production of such less reactive metal complexes enables the AM fungi to store excess metal ions into their vacuolar compartments (via GintZIP, GintABC1, or GintZnT1 membrane transporters), integrate them into their essential metabolic function, or transfer them to the host root across the periarbuscular interface (via MtZIP2 or LeNramp13) (González-Guerrero et al. 2005, 2007, 2009, 2010b). The identification of ion transporters common to both plants and fungi which are putatively involved in the regulation of metal uptake (Burleigh et al. 2003; López-Millán et al. 2004) could support the perspective that this process is actively co-modulated by both symbionts and provides further evidence as to the fundamentally mutual-istic nature of the association, as suggested regarding the symbiotic transfer of P. Altogether, these combined processes characterize the complex role of the mycorrhizosphere in plant mineral nutrition contributing by modulating plant nutrient uptake and thereby enhancing the host plant's physiological status compared to non-AM plants, particularly when subjected to a number of environmental stress factors.
Besides the significant role of the AM symbiosis in plant mineral nutrition, there is a considerable body of literature describing the beneficial effects of the AM mycorrhizosphere in plant water relations across a broad range of water stress, for instance, from amply watered to droughted conditions (Augé 2001,2004). Yet, unlike the dynamics of AM-plant nutrient acquisition described above, the specific mechanisms underlying the direct impact of the mycorrhizosphere on plant stress tolerance under such environmental conditions remain slightly ambiguous. A central question in this regard considers whether the AM symbiosis benefits plants more by enhancing their intrinsic drought resistance (i.e., survival at low internal water content) or, rather, by increasing their drought avoidance (i.e., maintenance of high internal water content) when subjected to a low external water potential, such as drought and drought recovery conditions. As outlined in recent reviews by Auge (2001, 2004), a wide array of AM-plants in association with a number of Glomus and allied AM species (refer to Auge 2001 for a more comprehensive list of plant and AM fungal species interactions) have been shown to develop a variety of beneficial physiological responses compared to non-AM plants, with such responses ranging from relatively higher stomatal conductance, leaf transpiration, and (or) osmotic potential (Allen et al. 1981, Auge 2000; Auge et al. 2003, 2007, 2008). Among AM-plants, such enhanced metabolic and physiological functions under strained water conditions have subsequently been linked to an increased photosynthetic potential due to a generally larger leaf area, relatively greater water potential (i.e., water content) in roots and shoots, and an increased overall growth status observed under both greenhouse and field conditions (Cho et al. 2006; Khalvati et al. 2005). Altogether, these physiological responses contribute to an increased AM-plant stress tolerance occurring especially, but not exclusively, during both drought and drought recovery conditions. In addition to imposing a direct physiological stress toward host plants as symptomatically expressed by a general loss of turgidity and down regulation of various essential metabolisms (Hsiao 1973; Nautiyal et al. 1994), strained water relations often also cause alterations in nutrient bioavailability in the soil solution which leads to significant nutrient imbalances and potential antagonisms between metal ions. This potential correlationality between environmental stressors (e.g., both water and nutrient deficiency) represents an important challenge faced by experimental investigators in determining the role(s) of AM fungi in plant water relations, particularly when attempting to distinguish the specific mechanisms of AM-enhanced plant stress tolerance (Koide 1993). Similar to the notion of enhanced mycorrhizospheric uptake regarding AM-plant nutrient acquisition, it has been suggested that the mycorrhizosphere should play a fundamental role in enhancing both the nutrient and water acquisition capabilities of host plants by increasing their resource acquisition pool compared to the rhizosphere alone; this, again through the general mechanism of actively scavenging the proximal soil environment for essential resources (Koide 1993). More specifically, extraradical hyphae could contribute directly in circumventing water deficiency conditions by penetrating soil micropores to improve hydraulic conductivity due to the expansive myc-orrhizosphere, and thereby enhance plant stress avoidance during drought stress and drought recovery (Miller and Jastrow 1990). Accordingly, analyses comparing AM and non-AM root conductivity among plants under drought and amply watered conditions have also shown improvements in AM-plant water uptake that often coincide with increases in the host plant's mineral nutrition. Here, an improved N, P, and (or) micro-nutrient status could further benefit AM-plants by circumventing internal mineral deficiencies that may otherwise detrimentally affect their intrinsic drought resistance mechanisms, such as the accumulation and maintenance of high foliar concentrations of soluble sugars and free polyamines (Augé 2001, 2004) . This improved mineral status has also been linked with lower amino acid accumulation in AM than non-AM plants which suggests that the former are generally less metabolically strained under these conditions (Augé 2001) . The consensus from these overall findings suggests that AM colonization primarily contributes by bolstering intrinsic plant resistance mechanisms by circumventing internal deficiencies. In addition, water conductance and nutrient uptake are improved due to an increased bioavailable pool of soil resources within the mycorrhizosphere (Cho et al. 2006; Khalvati et al. 2005). Together, these mycorrhizospheric processes can increase AM-plant resilience in relation to stressful water relations by enabling a relatively more stable (or less strained) metabolic function, which is typically manifested through an increased overall photosynthetic potential and relative growth potential during drought stress and drought recovery. While these physiological effects provide a significant environmental advantage to AM vs. non-AM plants when subjected to adverse water conditions, plants having overall increased photosynthetic activities and stomatal conductances also tend to have higher rates of evapo-transpiration. Ironically, this effect can impose further stress on the plants themselves by increasing the rate of soil-drying in the proximal soil environment. In adding further complexity to the role of the AM symbiosis in plant water relations, these plants could be more vulnerable compared to less photosynthetically active plants under such environmental conditions due to complications stemming from accelerated soil-drying. Nevertheless, investigations into the role of the mycorrhizosphere in stabilizing the proximal soil environment could shed light into these matters, especially regarding the impact of extraradical hyphae in soil aggregation and biosorption processes which can buffer a number of edaphic factors such as the water and nutrient retention capacities. In fact, such "indirect" mycorrhizo-spheric processes could represent equally important components of plant stress tolerance and ecosystem function compared to the "direct" processes presented here.
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