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Soil Environment

Fig. 11.5 AM fungal nitrogen uptake and transfer pathway (adapted from Govindarajulu et al. 2005; modified according to Jin et al. 2005, Chalot et al. 2006, and Cruz et al. 2007). Bidirectional transfer is indicated by solid and dashed lines. Refer to text for abbreviations mycorrhizosphere contributes first by generally increasing the plant N acquisition capability (both NO3- and NH4+ sources), and second by specifically increasing the uptake of NH,+ as a result of fungal pre-assimilation and detoxification processes (Chapman et al. 2005; Marschner and Dell 1994). Together, these mechanisms are believed to increase the bioavailability of N within and often beyond the rhizosphere's resource depletion zone to improve plant stress tolerance when subjected to N-deficiency conditions. Among others, Haystead et al. (1988), Faure et al. (1998), and Subramanian and Charest (1997,1998,1999) investigated these hypotheses using greenhouse experimental systems in the objective of assessing the nutritional status of plants in relation to the amendment of NO3- and NH4+ fertilizers, and then comparing the N assimilation pathways of AM vs. non-AM plants by measuring the activity of key assimilation enzymes: namely, nitrate reductase (NR), glu-tamine synthase (GS), glutamate dehydrogenase

(GDH), and glutamate synthase (GOGAT). Collectively, these studies have shown that AM-colonized rye grasses (Lolium perenne), "field" clover (Trifolium repens), and maize (Zea mays) all gained considerable increases in total nitrogen uptake leading to an overall greater amino acid composition compared to non-AM plants. Notably, these physiological effects coincided with increases in the activity of NR, GDH, and GS-GOGAT assimilation enzymes measured in the AM roots and shoots. As predicted, the AM-plant nutritional status was supplemented by increasing the overall uptake of N, especially by increasing the uptake of its less labile form, [NH4+]. More recently, the studies of Toussaint et al. (2004), Govindarajulu et al. (2005), Jin et al. 2005; Chalot et al. (2006) , and Cruz et al. (2007) have corroborated these general findings using in vitro culture tools to further characterize the AM fungal uptake, assimilation, and translocation pathways as soil-N travels from the extraradical hyphae to the host root (Fig. 11.5).

Accordingly, it has been elucidated that NO3- and NH4+ are actively taken up by extraradical hyphae via transporters of the AMT/Mep/Rh protein superfamily (Khademi et al. 2004). Having reached the cytosol of the extraradical mycelium, NO3- and NH4+ are converted to glutamine and then arginine via the fungal NR, GDH, and GS-GOGAT enzyme cycles. In this form, argin-ine travels to the intraradical mycelium through cytoplasmic streaming via coenocytic channels to be further broken down through the action of ornithine aminotransferase and urease, thereby releasing ornithine, urea, and ultimately NH4 +. Finally, the NH4+ is either catabolized via AM fungal amino acid synthesis or transferred to roots across the periarbuscular interface apparently via ammonium transport proteins. To complete the bidirectional exchange between the symbionts, plant carbohydrates in the form of hexose are transferred to the AM fungus via putative transporters (HXT1, potentially among others - Hahn and Mendgen 2001) and likely mediated via plasma membrane H+-ATPases (GmHA1-5 - Ferrol et al. 2000). Once in the fungal cytosol, the hexose is converted to trehalose, glycogen, and glucose for usage in various fungal metabolisms. As such, the characterization of the AM nitrogen uptake pathway provides key evidence as to the active role of AM fungi in supplementing the plant N nutritional status in the enhancement of plant nutrient stress tolerance.

2.1.2 P Acquisition

After soil-N, plant productivity in temperate agro-ecosystems is limited by phosphorus (P) bioavail-ability4: a key bioenergetic constituent (e.g., ATP) and cellular structural component (e.g., phospho-lipids, DNA, RNA). In this regard, P deficiency is prevalent in areas of high rainfall due to extensive

4 Unlike temperate environments, plant productivity in tropical agro-ecosystems is primarily limited by phosphorus bioavailability followed by less labile soil micronutri-ents due to their slow diffusion rates and subsequently low bioavailability to plants. In addition, the high rates of plant photosynthesis and evapo-transpiration in this ecosystem typically cause the bioavailable nutrient pool to be rapidly assimilated (Brams 1973; Baligar and Bennett 1986; Ewel 1986).

nutrient leaching and (or) acidic soil conditions conducive to reciprocal antagonisms (also known as phosphorus-induced micronutrient deficiencies) which can cause leaf chlorosis and stunted growth (Cleveland et al. 2002; Mengel and Kirkby 2001). Similarly to the case of N assimilation described above, it has been hypothesized that the AM fungi hold a significant role in plant P acquisition by increasing the plant's soil resource uptake capacity, particularly by enhancing the uptake of phosphates and inorganic P which typically have slow soil diffusion rates (Marschner 1995; Picone et al. 2003; Saito 2000). In this regard, these physiological mechanisms are among the most thoroughly investigated in the study of the AM symbiosis and have been well reviewed by Bolan (1991), Koide (1991), George et al. (1995), Schachtman et al. (1998), and Smith et al. (2003) . The consensus from these studies is that host plants benefit from an investment in AM symbiosis for the supplementation of their P nutritional status, especially when subjected to soil-P deficiency, due to the expansive mycor-rhizosphere's ability to increase soil-surface contact and reduce soil-P diffusion distances. Consistent with the notion of plant investment in extrinsic systems to circumvent environmental stress, the mycorrhizal investment is often inversely correlated with the bioavailability of soil-P such that AM root colonization and symbiotic activity are believed to be highest under low P conditions (Smith et al. 2003, 2004). Consequently, this relationship would suggest that P bioavailability is a key factor dictating the plant's relative symbiotic investment (or mycor-rhizal responsiveness) in order to maximize the reciprocal benefits of association (Graham et al. 1991; Janos 2007; Tawaraya 2003). Under such environmental conditions, soil-P can be actively taken up by the extraradical hyphae and efficiently transferred to host roots (Fig. 11.6 - Schachtman et al. 1998; Smith et al. 2003; Javot et al. 2007). This process is characterized by the activity of extraradical hyphae which increase the solubility of soil-P due to the exudation of organic chelators to then facilitate the uptake of both organic (Porg) and inorganic (Pi) forms of P. Coinciding with these events, a number of AM fungal phosphate

Soil Environment

Fig. 11.6 AM fungal phosphorus uptake and transfer pathway (adapted from Schachtman et al. 1998; modified according to Smith et al. 2003 and Javot et al. 2007) . Bidirectional transfer is indicated by solid and dashed lines. Refer to text for abbreviations

Soil Environment

Fig. 11.6 AM fungal phosphorus uptake and transfer pathway (adapted from Schachtman et al. 1998; modified according to Smith et al. 2003 and Javot et al. 2007) . Bidirectional transfer is indicated by solid and dashed lines. Refer to text for abbreviations transporters have been isolated from the mycelium of a number of AM fungi and found to be upregulated under P-deficiency conditions, such as: Glomus intraradices (GiPT - Maldonado-Mendoza et al. 2001), G. mosseae (GmosPT -Benedetto et al. 2005), and G. versiforme (GvPT -Harrison and van Buuren 1995). Once taken up into the cytosol of the extraradical mycelium, the Porg and Pi are converted to polyphosphate complexes (poly-P - e.g., glucose-6-phosphate) through the enzymatic activity of polyphosphate glucokinase (Capaccio and Callow 1982; Cox et al. 1980) . In this more stable cytosolic form, the poly-P complexes are either stored in fungal vacuoles or transferred to the intraradical mycelium through cytoplasmic streaming via coeno-cytic channels: a process which is putatively linked with H+-ATPase co-transport (Gauthier and Turpin 1994). Here, the polyphosphate complexes can be broken down by polyphosphatases for use in AM fungal ATP, DNA, RNA, and

(or) phospholipid syntheses, or transferred to the host plant across species-specific phosphate transporters in exchange for plant carbohydrates in the form of hexose. To date, advanced molecular analyses have identified an array of phosphate transporters (Table 11.2) isolated especially in the periarbuscular interface region of various model study organisms, such as barley (Hordeum vul-gare), deervetches (Lotus japonica), tomato (Lycopersicon esculentum), alfalfa (Medicago truncatula), rice (Oryza sativa), potato (Solanum tuberosum), and maize (Zea mays). Accordingly, it has been reported that these species also experience an increase in phosphorus assimilation activity (e.g., acid phosphatase, alkaline phosphatase, and H+-ATPase) in their roots which can contribute in increasing P assimilation and P nutritional status following the enhanced uptake and transfer of soil-P from the extraradical hyphae to the host plant (Capaccio and Callow 1982; Dexheimer et al. 1982; Schwab et al. 1991), which was later

Table 11.2 Summary of known AM fungal and plant phosphate transporters (from Javot et al. 2007) Taxon Nomenclature Reference

Arbuscular mycorrhizal fungi

Table 11.2 Summary of known AM fungal and plant phosphate transporters (from Javot et al. 2007) Taxon Nomenclature Reference

Arbuscular mycorrhizal fungi

Glomus intraradices

GiPT

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