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Temperature

Heat Cold Frost

Water

Dry air Dry soil Flooding

Gases

Oxygen deficiency Volcanic gases

Minerals

Deficiency

Excess (Toxicity)

Imbalances

Salinity

Acidity

Alkalinity

Mechanical disturbances

Wind

Sulifluction Burial

Snow cover Ice sheets

Biotic

Plants

Crowding Allelopathy Parasitic plants

Microorganisms

Viruses Bacteria Fungi

Predation disturbances

Grazing Trampling

Anthrophogenic activity

Industrial pollution Agrochemicals Soil compaction Fire

Ionization radiation Electromagnetic fields

Environmental Stress Factors

Fig. 11.1 Abiotic and biotic environmental stress factors (adapted from Marschner 1995)

from temporary (reversible) to permanent (irreversible) adaptive responses depending on the relative intensity of the given stress factor. In order to increase their survivorship and reproductive success, plants have developed a remarkable array of stress tolerance (endurance), resistance (acclimation), and (or) avoidance (prevention) mechanisms to circumvent a number of environmental challenges and avoid any permanent associated-stress injuries. Further to these more conventional descriptors of adaptive stress tolerance (e.g., tolerance, resistance, and avoidance), two alternative descriptors have been proposed relating to plant investment (or resource and energy allocation) in either intrinsic (e.g., metabolic) or extrinsic (e.g., symbiotic) tolerance strategies1 (Audet and Charest 2007b, 2008). Here, intrinsic stress tolerance refers to plant investment in inherent (or built-in) metabolic systems that are inducible when subjected to stress. For example, the production of metallothioneins which bind ion-free radicals to prevent metal-induced cellular oxida-tive stress (Chaps. 9, 20, and 21), the production of secondary metabolites to thwart herbivores (Howe and Jander 2008; Mole 1994), the dispatch of heat shock proteins to prevent enzyme denaturing under temperature extremes (Chaps. 5-7), or the production of anti-microbial proteins to inhibit viral, fungal, and (or) bacterial pathogens (Dangl and Jones 2001; Fritig et al. 1998; Ganz and Lehrer 1999) , to name just a few. More broadly, intrinsic stress tolerance can also include constitutive systems such as the processes of cell lignification, the development of trichomes and glandular hairs, or the exudation of resins and waxes which, together, offers mechanical defenses to some of these environmental stressors (Bhuiyan et al. 2009;

1 I n their review of "Heavy metal tolerance in plants," Antonovics et al. (1971), and later Baker and Walker (1990), were first to allude to and distinguish between internal and external tolerance mechanisms. Similarly to the investment of plant resources toward intrinsic versus extrinsic strategies proposed here, the internal and external tolerance mechanisms suggested by Antonovics et al. refer to central (e.g., metabolic) and peripheral (e.g., rhizospheric) processes, respectively, which can impact plant development when faced with critical metal toxicity conditions.

Duke 1994; Langenheim 2003; Wagner 1991). By contrast, extrinsic stress tolerance refers to plant investment in external systems, particularly symbiotic mutualism, to circumvent environmental stress. In this regard, the symbiotic mutualism encompasses an intimately co-operative relationship between two different species (referred to as symbionts) contributing to the mutual benefit of both individuals (Leung and Poulin 2008). Notably, the balance between the benefits of association among the symbionts is critical for defining the symbiotic mutualism since symbiotic relationships are believed to function along a continuum ranging from parasitism to mutualisms (Boucher et al. 1982; Bronstein 2001; Johnson et al. 1997). From this definition, it can be argued that plants have developed the widest assortment of mutualism in the natural world, whereby host plants typically exchange essential resources (e.g., plant carbohydrates, soil nutrients) and (or) ecological services (e.g., pollination services, shelter) with individuals from another species to reciprocally enhance their tolerance to environmental stressors and thereby increase their survivorship (Boucher 1988; Boucher et al. 1982). Examples of plant mutualism include plant-pollinator interactions with insects, birds, and mammals to ensure plant fertilization and sexual reproduction (R0nsted et al. 2005, 2008; Wiebes 1979), plant-fungus (mycorrhizal) interactions to increase the root system's resource acquisition capacity (Douds and Johnson 2007; Marschner and Dell 1994), and plant-rhizobial interactions for the fixation of inorganic soil nitrogen (Long 1996, 2001; Young and Johnston 1989). Among these interactions, the mycorrhizal symbiosis is considered to be one of the most widespread and well-studied ecological associations having key implications at the scale of plant physiological and whole-ecosystem function. In this chapter, focused on the arbuscular mycorrhizal (AM) fungi and their symbiotic association with herbaceous plant species, we examine how some plants invest in mycorrhizal symbiosis as an extrinsic stress tolerance strategy in relation to a number of abiotic environmental stressors, such as macro- and micronutrient deficiency, drought, and metal toxicity. It is also suggested that such an investment can contribute

Fig. 11.2 Defining the mycorrhizosphere and its zone of influence (adapted from Beare et al. 1995)

in shaping plant development by influencing edaphic conditions within the proximal growth environment.

1.2 Mycorrhizaeandthe AM Symbiosis

Aptly referred to as "fungus roots" (Frank 1885), the mycorrhizae are mostly nonpathogenic soil fungi living intimately with terrestrial plant roots that, together, form a symbiotic mutualism characterized by the direct exchange of plant carbohydrates for soil resources, such as mineral nutrients and water (Allen 1991). The mycor-rhizae are ubiquitous organisms having adapted to and successfully colonized nearly all known terrestrial ecosystems by forming symbiotic associations with the broad majority of all plant families (Peterson et al. 2004) . For this reason, the mycorrhizal fungi are classified into three primary assemblages depending on their respective morphologies and specific plant hosts: the ecto-mycorrhizae (primarily associated with Pinaceae, Fagaceae, Betulaceae, and Salicaceae species), the endomycorrhizae (associated with the majority of angiosperms and some gymnosperms), and the ectendomycorrhizae (primarily associated with

Orchidaceae and Ericaceae species). A common feature among all mycorrhizae is the development of the mycorrhizosphere2 (Fig. 11.2) consisting in the combined zones of influence of the roots (rhizosphere) and extraradical hyphae (hyphosphere), and encompassing a highly active and multilateral interface between the host plants, mycorrhizal fungi, and proximal soil environment (Duponnois et al. 2008; Garbaye 1991). In line with the notion of plant investment in extrinsic systems for the purpose of stress tolerance, plant investment toward the development of the myc-orrhizospheric network involves a considerable plant carbon allocation (occasionally representing up to and possibly well over 20% of the plant's total carbon budget) which is required for

2Fitzpatrick's (1984) characterization of the micromor-phology of soils indicates that the pedosphere (e.g., soil realm) is constituted of four essential "spheres": atmosphere (e.g., soil air), biosphere (e.g., litter and microorganisms), lithosphere (e.g., rocks and minerals), and hydrosphere (e.g., soil water). Accordingly, Beare et al. (1995) have subclassified arenas of interaction to identify further interfaces within the pedospheric framework, such as the drilosphere (e.g., worm castings), detritusphere (e.g., saprotrophs), rhizosphere (e.g., plant roots), mycor-rhizosphere (e.g., combined roots and extraradical hyphae), etc.

actively sustaining the symbiotic infrastructure and maintaining the functional viability of the mycorrhizal symbiont (Douds et al. 2000; Tinker et al. 1994) . In exchange, this extrinsic investment provides the host plant with a number of ecological services typically pertaining to the enhancement of the plant's resource acquisition capability and the stabilization of the proximal soil environment.

Falling within the grouping of the endomycor-rhizae, the AM fungi and their symbiosis with herbaceous plants are particularly well studied in the field of plant physiology and mycology, and widely recognized for benefiting host plants when subjected to various environmental stressors (Table 11.1). Having originated an estimated 450 million years ago, the AM fungi comprise species of the Glomeromycota phylum which are believed to form associations with up to 90% of all herbaceous plants (Redecker et al. 2000; Remy et al. 1994; SchüBler et al. 2001). As characterized by a unique morphology consisting of intra- and extraradical hyphae, arbuscules, vesicles, and spores (Fig. 11.3), the AM fungi behave in a peculiar manner compared to other mycorrhizal phyla since they penetrate between the cortical cells of vascular plant roots in order to develop an intracellular exchange network (Garcia-Garrido et al. 2009): a behavior believed to be rooted in ancient parasitic origins (Purin and Rillig 2008). Following reciprocal signaling processes between the symbionts resulting in the successful colonization of host roots (Harrison 1999, 2005; Vierheilig and Piché 2002), the intraradical hyphae proliferate within the root architecture to interact with roots cells across a slender periar-buscular zone formed between the fungal arbuscular structures and plant cell membranes. It is across the periarbuscular zone where soil resources (e.g., phosphorus, nitrogen, or mineral nutrients) and plant carbohydrates (e.g., glucose, hexose) are actively exchanged between the sym-bionts (Hahn and Mendgen 2001). Meanwhile, the extraradical hyphae typically scavenge beyond the root depletion zone to form an expansive mycorrhizospheric network, thereby increasing the host roots' resource acquisition capabilities and zone of influence compared to the rhizosphere alone (Koide 1991, 2000; Koide and Elliott 1989). With the development of this active and bidirectional symbiotic exchange network, the AM fungi then shift their developmental allocation to the production of extraradical spores and vesicles which are involved, respectively, in fungal reproduction and lipid storage (Bago et al. 2000; Dalpe et al. 2005). Under these circumstances, the AM fungi are generally considered to be "true" mutu-alists due to their host obligate status which requires that they maintain an active symbiosis in order to ensure an influx of plant carbon allocations for the completion their life cycle (Johnson et al. 1997; Jones and Smith 2004).

As stated previously, numerous advances have been made over the past decades demonstrating the beneficial role of the AM fungi in plant physiology and soil ecology; this being attributed especially to the dynamic function of the mycor-rhizosphere in relation to various edaphic processes (Fig. 11.4). In order to accurately depict such dynamic interactions, the classification of the mycorrhizospheric processes presented here distinguishes specifically between two types of interaction depending on the nature of the benefits of association being either direct or indirect. In the present context, the direct benefits of interaction refer to processes that directly enhance the plant health status as mediated by the dynamics of bidirectional exchange between the symbionts described above (Cushman and Beattie 1991; Schwartz and Hoeksema 1998). For example, the process of mycorrhizal enhanced uptake in which the extraradical hyphae increase the uptake of limiting soil resources in exchange for plant carbohydrates, then enabling the host plant to supplement its nutrient status when subjected to deficiency conditions. Alternatively and occasionally overlooked from a plant physiological perspective, the indirect benefits3 of interaction refer to processes that indirectly enhance the plant growth or survival status by altering the proximal growth environment thereby providing

3 The notion of "indirect benefits" derived from mutualism has previously been used within the context of species community structure.

Table 11.1 Summary of the impact of AM symbiosis on plant physiology and soil ecology

Mechanism

Target

Description

Reference

Direct benefit

Enhanced resource acquisition capability

Essential soil resources (nonmetals, metals, and water)

Preferential uptake of nitrogen (N03/NH4) and phosphorus (P.)

Bolan (1991)a, Chapman et al. (2006)a, Schachtman et al. (1998)a, Smith et al. (2003)a, and George et al. (1995)a

Mobilization and uptake of trace essential elements having low bioavailability (e.g., Zn, Ni, Co, Cu, Mn, Fe) particularly under nutrient-deficiency conditions

Jeffries et al. (2003)a, Koide (1991)a. and Marschner and Dell (1994)a

Enhanced water use efficiency and drought recovery

Auge et al. (2001), Auge (2001)a Montaño et al. (2007)a and

Indirect benefit Soil structure stabilization Metal bioavailability

Soil retention capacity

Metal binding due to negatively charged surface constituents of extraradical hyphae (e.g., carboxyls, hydroxides, oxy-hydroxides, sulfhydryls); Reduction of plant metal uptake to delay phytotoxicity, particularly at high soil exposure levels (e.g., Zn Pb, Cd, Ni)

Ley val et al. (1997)a, Galli et al. (1994)a, Gadd (1993)a, Meharg (2003 )a, and González-Guerrero et al. (2009)a

Enhanced soil aggregation properties; increased water and nutrient retention capacity, decrease of nutrient leaching

Auge et al. (2001), Auge (2001)a, Bearden (2001), Bearden and Petersen (2000), Miller and Jastrow (1990)

Promotion of beneficial bacteria (i.e., nitrogen fixation), competitive exclusion of soil pathogens

Metal bioavailability Precipitation of metal-ligands, modulation of soil pH

Ley val et al. (1997)a, Galli et al. (1994)a, and Gadd (1993)a a Denotes review publications

Fig. 11.3 Defining structures of an arbuscular mycorrhizal fungus (Glomus intraradices Schenck & Smith) in association with Ri T-DNA carrot roots (Daucus carota L.) grown under aseptic conditions. Shown from (a) to (e) are: vesicles

(Ve), arbuscules (Ar), host roots (HR), spores (Sp), spore clusters (Sc), extraradical hyphae (Eh), and intraradical hyphae (Ih). Roots are stained with an aniline blue 0.02% dye solution and observed under a compound microscrope

Fig. 11.3 Defining structures of an arbuscular mycorrhizal fungus (Glomus intraradices Schenck & Smith) in association with Ri T-DNA carrot roots (Daucus carota L.) grown under aseptic conditions. Shown from (a) to (e) are: vesicles

(Ve), arbuscules (Ar), host roots (HR), spores (Sp), spore clusters (Sc), extraradical hyphae (Eh), and intraradical hyphae (Ih). Roots are stained with an aniline blue 0.02% dye solution and observed under a compound microscrope more favorable developmental conditions (Bertness and Callaway 1994; Stachowicz 2001; Müller and Krauss 2005) . For instance, the process of mycorrhizospheric-enhanced soil aggregation which contributes in stabilizing the proximal growth environment to increase its resource retention capacity. A key aspect of the indirect benefits of interaction is the notion that such processes can benefit host plants as well as nonassociated species within the mycorrhizo-sphere's zone of influence, unlike the direct benefits which suggest an intimate exchange occurring exclusively between the symbionts. By distinguishing between the direct and indirect benefits of interaction, it is intriguing that a combination of such AM-induced mycorrhizospheric processes can complement many intrinsic tolerance mechanisms when subjected to a broad

Fig. 11.4 Summary of potential mycorrhizospheric interactions

range of environmental conditions and abiotic stress. For this reason, it is considered that these processes likely play a key role in enhancing plant stress tolerance, as well as shaping the proximal growth environment to increase the soil's resilience, in relation to a number of potential ecological stressors.

2 Direct Benefits of Association 2.1 Macro- and Micronutrient Uptake 2.1.1 N Acquisition

Plant productivity in temperate agro-ecosystems is most commonly limited by soil nitrogen (N) bioavailability (Vitousek and Howarth 1991; Chapin et al. 2002) . Due to its principal role in protein biosynthesis and nucleic acid metabolism, N deficiency typically results in stunted plant growth and increased leaf senescence thereby detrimentally affecting the plant's photo-synthetic potential and overall growth yield. A particular environmental challenge regarding plant N assimilation exists with regard to the source of N in soils, whether it is in the form of nitrate (NO3-) which is readily assimilated in roots and (or) shoots but easily leached from the rhizosphere or, instead, in the form of ammonium (NH4+) which can be more abundant than the former but requires detoxification prior to its assimilation (Gutschick 1981; Bloom 1997). The primary benefit of mycorrhizal associations pertains to the increase in belowground surface area (e.g., roots and extraradical hyphae) which enhances the host plant's soil resource acquisition capability compared to the rhizosphere alone. In this regard, two complementary mechanisms describing the role of AM fungi in plant N assimilation have been presented suggesting that the

GS-GOGAT Cycle

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

GS-GOGAT Cycle

Exudation of Organic Chelators

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