Angela Hodge

An arbuscular mycorrhizal fungus accelerates decomposition and acquires nitrogen directly from organic material

Arbuscular mycorrhizal fungi (order Glomales), which form mycorrhizal symbioses with two out of three of all plant species, are believed to be obligate biotrophs that are wholly dependent on the plant partner for their carbon supply. It is thought that they possess no degradative capability and that they are unable to decompose complex organic molecules, the form in which most soil nutrients occur. Earlier suggestions that they could exist saprotrophically were based on observation of hyphal proliferation on organic materials. In contrast, other mycorrhizal types have been shown to acquire nitrogen directly from organic sources. Here we show that the arbuscular mycorrhizal symbiosis can both enhance decomposition of and increase nitrogen capture from complex organic material (grass leaves) in soil. Hyphal growth of the fungal partner was increased in the presence of the organic material, independently of the host plant.

Plant root growth, architecture and function.

Without roots there would be no rhizosphere and no rhizodeposition to fuel microbial activity. Although micro-organisms may view roots merely as a source of carbon supply this belies the fascinating complexity and diversity of root systems that occurs despite their common function. Here, we examine the physiological and genetic determinants of root growth and the complex, yet varied and flexible, root architecture that results. The main functions of root systems are also explored including how roots cope with nutrient acquisition from the heterogeneous soil environment and their ability to form mutualistic associations with key soil micro-organisms (such as nitrogen fixing bacteria and mycorrhizal fungi) to aid them in their quest for nutrients. Finally, some key biotic and abiotic constraints on root development and function in the soil environment are examined and some of the adaptations roots have evolved to counter such stresses discussed.

Substantial nitrogen acquisition by arbuscular mycorrhizal fungi from organic material has implications for N cycling

Arbuscular mycorrhizal (AM) fungi are obligate biotrophs that acquire carbon (C) solely from host plants. AM fungi can proliferate hyphae in, and acquire nitrogen (N) from, organic matter. Although they can transfer some of that N to plants, we tested the hypothesis that organic matter is an important N source for the AM fungi themselves. We grew pairs of plants with and without the AM fungus Glomus hoi in microcosms that allowed only the fungus access to a 15N/13C-labeled organic patch; in some cases, one plant was shaded to reduce C supply to the fungus. The fungal hyphae proliferated vigorously in the patch, irrespective of shading, and increased plant growth and N content; ∼3% of plant N came from the patch. The extraradical mycelium of the fungus was N-rich (3–5% N) and up to 31% of fungal N came from the patch, confirming the hypothesis. The fungus acquired N as decomposition products, because hyphae were not 13C-enriched. In a second experiment, hyphae of both G. hoi and Glomus mosseae that exploited an organic material patch were also better able to colonize a new host plant, demonstrating a fungal growth response. These findings show that AM fungi can obtain substantial amounts of N from decomposing organic materials and can enhance their fitness as a result. The large biomass and high N demand of AM fungi means that they represent a global N pool equivalent in magnitude to fine roots and play a substantial and hitherto overlooked role in the nitrogen cycle.

Plant root proliferation in nitrogen–rich patches confers competitive advantage

Plants respond strongly to environmental heterogeneity, particularly below ground, where spectacular root proliferations in nutrient–rich patches may occur. Such ‘foraging’ responses apparently maximize nutrient uptake and are now prominent in plant ecological theory. Proliferations in nitrogen–rich patches are difficult to explain adaptively, however. The high mobility of soil nitrate should limit the contribution of proliferation to N capture. Many experiments on isolated plants show only a weak relation between proliferation and N uptake. We show that N capture is associated strongly with proliferation during interspecific competition for finite, locally available, mixed N sources, precisely the conditions under which N becomes available to plants on generally infertile soils. This explains why N–induced root proliferation is an important resource–capture mechanism in N–limited plant communities and suggests that increasing proliferation by crop breeding or genetic manipulation will have a limited impact on N capture by well–fertilized monocultures.

Arbuscular mycorrhizal fungi can transfer substantial amounts of nitrogen to their host plant from organic material

Nitrogen (N) capture by arbuscular mycorrhizal (AM) fungi from organic material is a recently discovered phenomenon. This study investigated the ability of two Glomus species to transfer N from organic material to host plants and examined whether the ability to capture N is related to fungal hyphal growth. Experimental microcosms had two compartments; these contained either a single plant of Plantago lanceolata inoculated with Glomus hoi or Glomus intraradices, or a patch of dried shoot material labelled with (15)N and (13)carbon (C). In one treatment, hyphae, but not roots, were allowed access to the patch; in the other treatment, access by both hyphae and roots was prevented. When allowed, fungi proliferated in the patch and captured N but not C, although G. intraradices transferred more N than G. hoi to the plant. Plants colonized with G. intraradices had a higher concentration of N than controls. Up to one-third of the patch N was captured by the AM fungi and transferred to the plant, while c. 20% of plant N may have been patch derived. These findings indicate that uptake from organic N could be important in AM symbiosis for both plant and fungal partners and that some AM fungi may acquire inorganic N from organic sources.

Arbuscular mycorrhiza and nitrogen: implications for individual plants through to ecosystems

BackgroundArbuscular mycorrhizal fungi (AMF) form mutualistic symbioses with c. two-thirds of all land plants. Traditionally, it was thought that they played no role in nitrogen (N) acquisition for their host, despite early evidence to the contrary. More recently, this perception has changed radically, with the demonstration that AMF can acquire N from both inorganic and organic N sources and transfer some of this N to their host plant.ScopeThis review discusses the current evidence for AMF N uptake, transport and plant transfer under different experimental conditions and highlights key questions that remain to be resolved. The relevance of this AMF N acquisition pathway is discussed both in relation to host plant and fungal N nutrition. The importance of interactions with the soil community and subsequent implications for soil N cycling are also highlighted.ConclusionsReported AMF contribution to plant N varies widely, but the reasons for this variability are unclear. In low N systems even small amounts of ‘extra’ N may confer the plant with a competitive advantage, but it is also likely that competition for N between symbionts occurs. To advance this area, a more mechanistic approach is required that treats the fungus as a Darwinian organism rather than a mere extension of the plant. Application of genomics and metabolomics technologies to this topic should enable resolution of some of the key questions outlined in this review.

An arbuscular mycorrhizal fungus significantly modifies the soil bacterial community and nitrogen cycling during litter decomposition

Arbuscular mycorrhizal fungi (AMF) perform an important ecosystem service by improving plant nutrient capture from soil, yet little is known about how AMF influence soil microbial communities during nutrient uptake. We tested whether an AMF modifies the soil microbial community and nitrogen cycling during litter decomposition. A two-chamber microcosm system was employed to create a root-free soil environment to control AMF access to (13) C- and (15) N-labelled root litter. Using a 16S rRNA gene microarray, we documented that approximately 10% of the bacterial community responded to the AMF, Glomus hoi. Taxa from the Firmicutes responded positively to AMF, while taxa from the Actinobacteria and Comamonadaceae responded negatively to AMF. Phylogenetic analyses indicate that AMF may influence bacterial community assembly processes. Using nanometre-scale secondary ion mass spectrometry (NanoSIMS) we visualized the location of AMF-transported (13) C and (15) N in plant roots. Bulk isotope ratio mass spectrometry revealed that the AMF exported 4.9% of the litter (15) N to the host plant (Plantago lanceolata L.), and litter-derived (15) N was preferentially exported relative to litter-derived (13) C. Our results suggest that the AMF primarily took up N in the inorganic form, and N export is one mechanism by which AMF could modify the soil microbial community and decomposition processes.

Microbial mediation of plant competition and community structure

Summary The drivers behind plant competition and diversity have long been debated, and there is acceptance that soil micro-organisms may act as key drivers in plant interactions and community structure. The evidence for a microbial role in shaping plant interactions and communities will be considered here with emphasis on symbionts and pathogens. Microbial populations are themselves strongly influenced by the plant community via the ‘rhizosphere’ effect. The rhizosphere community includes microbes both beneficial and detrimental to the plant. Both the ability of plants to cultivate different rhizosphere microbial populations and the resulting impact upon other plant species have largely been studied via ‘plant–soil’ feedback studies, a proxy necessitated by the fact the majority of soil micro-organisms are unculturable, but which nevertheless has rarely been used in conjunction with modern techniques to identify and quantify the micro-organisms involved. Both microbial symbionts and pathogens can affect plant diversity and productivity, but the direct evidence for impacts on competitive interactions is surprisingly scarce. Evidence comes from biological invasions, the unintentional introduction of microbial pathogens to native plant communities and manipulative experiments under both field and controlled conditions. Pathogens generally have direct effects on plants, reducing their growth and so rendering them less competitive, whereas other symbionts may act by altering the availability of resources, with more subtle effects on competitive interactions. Some of the best evidence for indirect effects comes from studies on arbuscular mycorrhizal (AM) fungi. New developments in our understanding of nutrient exchange in the arbuscular mycorrhizal symbiosis emphasize the need to view the fungal partners not as mere extensions of the plant. The suggestion that AM fungi may act to share resources among connected plants however remains to be proved. Although plant competitive interactions can be driven by key microbial groups including symbionts and pathogens, knowledge gaps in the basic biology of these micro-organisms has hindered a full mechanistic understanding of these processes. If ecologists now embrace new technologies, significant advances in this area should be forthcoming.

Plant, soil fauna and microbial responses to N-rich organic patches of contrasting temporal availability

Abstract A simple ( L -lysine) 15 N / 13 C dual-labelled organic patch was added to soil microcosm units with or without Lolium perenne L. plants either as a single addition of 5 ml of 200 mM L -lysine (‘patch’ treatment) or as a series of 1 ml aliquots of 200 mM L -lysine added at 7 day intervals over 28 days (‘pulse’ treatment) thus both treatments supplied the same amount of nitrogen (N). Controls were added as 1 ml H 2 O over 28 days (control pulse treatment) or as a single addition of 5 ml H 2 O (control patch treatment) to planted tubes. Decomposition of the added L -lysine was rapid as shown by amounts of 13 C detected in the soil atmosphere and were greatest from the planted L -lysine pulse treatment indicating the presence of plant roots was influencing decomposition of the pulse. Plant uptake of N, as 15 N , from the added L -lysine was also rapid and detected in the shoots by day 4. However, the mean rate at which 15 N appeared in the shoots did not differ between patch or pulse treatments. No 13 C levels greater than background were detected in the plant material. Root production and mortality in the patch or pulse addition zone was measured in situ using minirhizotron tubes. Cumulative root births in the L -lysine patch treatment were greater than controls in the latter part of the experiment while instantaneous root births were greater in the L -lysine pulse treatment compared to all others at 21 d. Root death rate was faster in the L -lysine treatments than in the controls. Root and shoot dry weights at final harvest (35 d) were greater in the pulse treatments. Shoot, but not root, dry weights were also significantly greater in the L -Lysine treatments compared to controls. Total root and shoot nitrogen contents were greater in the L -lysine treatments than controls. Total N capture by the L. perenne plants from the added L -lysine was 57% (patch) and 61% (pulse) of the N added and did not differ significantly between treatments. Protozoan biomass measured at harvest was greater in the L -lysine treatments (planted and unplanted) than the planted controls. The physiological profile of the microbial community did not vary because of patch or pulse treatments although significant differences between planted and unplanted L -lysine treatments and planted and H 2 O controls occurred. The response of roots and microorganisms in relation to N capture is discussed.

Growth and symbiotic effectiveness of an arbuscular mycorrhizal fungus in organic matter in competition with soil bacteria

Arbuscular mycorrhizal (AM) fungi can enhance the rate of decomposition of organic material, and can acquire nitrogen (N) from organic sources, although they are not saprotrophs. These fungi may instead indirectly influence decomposition through interactions with other soil microorganisms. We investigated the impact of both AM hyphae and a bacterial filtrate on N capture by a host plant from sterilized organic material (Lolium perenne shoots dual labelled with (15) N and (13) C), using compartmented microcosms. The addition of a bacterial filtrate considerably suppressed AM hyphal growth in the patch and reduced the root phosphorus content, demonstrating that bacterial populations can reduce symbiotic effectiveness. In contrast, AM hyphae had only a limited impact on bacterial community structure. Uptake of (15) N greatly exceeded that of (13) C, demonstrating that fungi acquired N in an inorganic form. We also examined the ability of AM fungi in gnotobiotic hairy root culture to acquire N directly from organic materials of varying complexities (glutamic acid, urea, bacterial lysate and L. perenne shoots). AM colonization did not enhance root N capture from these materials, although the bacterial lysate reduced both total AM colonization and arbuscule frequency. Collectively, these data demonstrate antagonistic interactions between AM fungi and bacteria that reflect resource competition for decomposition products.