Metabolic activity of Glomus intraradices in Arum- and Paris-type arbuscular mycorrhizal colonization

Colonization of two plant species by Glomus intraradices was studied to investigate the two morphological types (Arum and Paris), their symbiotic interfaces and metabolic activities. Root pieces and sections were stained to observe the colonization and metabolic activity of all mycorrhizal structures. There were no growth responses observed in the plants caused by mycorrhizal symbiosis. The two morphological types had a similar percentage of root colonized, but the Arum-type had higher metabolic activity. Most of the mycorrhizal structures (88%) showed succinate dehydrogenase activity; about half showed acid phosphatase activity; and a small percentage showed alkaline phosphatase activity. Phosphatase activity was highest in arbuscules and low in intercellular hyphae in the Arum-type colonization. In the Paris-type, hyphal coils and arbusculate coils showed a similar intermediate percentage of phosphatase activity. We conclude that acid phosphatase is more important than alkaline phosphatase in both colonization types. We discuss the possibility that, whereas arbuscules in Arum-type are the main site for phosphorus release to the host plant, both the hyphal and arbusculate coils may be involved in the Paris-type.     

Plant hormones as signals in arbuscular mycorrhizal symbiosis

Abstract

Arbuscular mycorrhizal (AM) fungi are non-specific symbionts developing mutual and beneficial symbiosis with most terrestrial plants. Because of the obligatory nature of the symbiosis, the presence of the host plant during the onset and proceeding of symbiosis is necessary. However, AM fungal spores are able to germinate in the absence of the host plant. The fungi detect the presence of the host plant through some signal communications. Among the signal molecules, which can affect mycorrhizal symbiosis are plant hormones, which may positively or adversely affect the symbiosis. In this review article, some of the most recent findings regarding the signaling effects of plant hormones, on mycorrhizal fungal symbiosis are reviewed. This may be useful for the production of plants, which are more responsive to mycorrhizal symbiosis under stress.     

The interface between the arbuscular mycorrhizal fungus Glomus intraradices and root cells of Panax quinquefolius: a Paris-type mycorrhizal association

Two major types of arbucular mycorrhizal associations, the Arum-type and the Paris-type, have been identified based on morphological features. Although the Paris-type is the most common, it is the Arum-type that has been most intensively studied in terms of structure/function because of its prevalence in agronomically important plant species. In this study, the interface between the host cell cytoplasm and intracellular hyphae (extensive hyphal coils and arbusculate coils), which typify the Paris-type mycorrhiza, was studied. Using immunofluorescence techniques combined with laser scanning confocal microscopy, dramatic changes in the cytoskeleton in colonized cells were observed. Changes in the positioning of both host cell microtubules and actin filaments occurred in colonized plant cells. Both microtubules and actin filaments were associated with the hyphal coils and the arbusculate coils. An interfacial matrix, of host origin, was demonstrated between hyphal coils and arbusculate coils using various affinity techniques. It formed an apoplastic compartment consisting of cellulose and pectins between the fungus and host cell cytoplasm. There was less labelling adjacent to the fine branches of arbusculate coils compared to the hyphal coils. These observations show some similarities to those seen with Arum-type mycorrhizas.     

Chitinase in roots of mycorrhizal Allium porrum: regulation and localization

Chitinase (EC 3.2.1.14) activity was measured in roots of Allium prorrum L. (leek) during development of a vesicular-arbuscular mycorrhizal symbiosis with Glomus versiforme (Karst.) Berch. During the early stages of infection, between 10 and 20 d after inoculation, the specific activity of chitinase was higher in mycorrhizal roots than in the uninfected controls. However, 60–90 d after inoculation, when the symbiosis was fully established, the mycorrhizal roots contained much less chitinase than control roots. Chitinase was purified from A. porrum roots. An antiserum against beanleaf chitinase was found to cross-react specifically with chitinase in the extracts from non-mycorrhizal and mycorrhizal A. porrum roots. This antiserum was used for the immunocytochemical localization of the enzyme with fluorescent and gold-labelled probes. Chitinase was localized in the vacuoles and in the extracellular spaces of non-mycorrhizal and mycorrhizal roots. There was no immunolabelling on the fungal cell walls in the intercellular or the intracellular phases. It is concluded that the chitin in the fungal walls is inaccessible to plant chitinase. This casts doubts on the possible involvement of this hydrolase in the development of the mycorrhizal fungus. However, fungal penetration does appear to cause a typical defense response in the first stages that is later depressed.     

Grazing tolerance and mycorrhizal colonization: Effects of resource manipulation and plant size in biennial Gentianella campestris

As herbivory usually leads to loss of photosynthesizing biomass, its consequences for plants are often negative. However, in favorable conditions, effects of herbivory on plants may be neutral or even beneficial. According to the compensatory continuum hypothesis plants can tolerate herbivory best in resource-rich conditions. Besides herbivory, also primarily positive biotic interactions like mycorrhizal symbiosis, bear carbon costs. Tritrophic plant–fungus–herbivore interaction further complicates plant's cost-benefit balance, because herbivory of the host plant is expected to cause decline in mycorrhizal colonization under high availability of soil nutrients when benefits of symbiosis decline in relation to costs. To gain insight into above interactions we tested the effects of plant size and resource manipulation (simulated herbivory and fertilization) on both above-ground performance and on root fungal colonization of the biennial Gentianella campestris.Clipping caused allocation shift from height growth to branches in all groups except in large and fertilized plants. For large plants nutrient addition may have come too late, as the number of meristems was most likely determined already before the fertilization. Clipping decreased the amount of DSE (dark septate endophytic) fungi which generally are not considered to be mycorrhizal. The effect of clipping on total fungal colonization and colonization by arbuscular mycorrhizal (AM) fungal coils were found to depend on host size and resource level. Dissimilar mycorrhizal response to simulated herbivory in small vs. large plants could be due to more intensive light competition in case of small plants. Carbon limited small plants may not be able to maintain high mycorrhizal colonization, whereas large clipped plants allocate extra resources to roots and mycorrhizal fungi at the expense of above-ground parts. Our results suggest that herbivory may increase carbon limitation that leads re-growing shoots and fungal symbionts to function as competing sinks for the limited carbon reserves.     

Widely distributed native and alien plant species differ in arbuscular mycorrhizal associations and related functional trait interactions

It is debated whether alien plants in new environments benefit from being mycorrhizal and whether widely distributed natives and aliens differ in their associations with mycorrhizal fungi. Here, we compared whether species differing in their origin status, i.e. natives, archaeophytes (alien species introduced before the year 1500) and neophytes (introduced after the year 1500), and arbuscular mycorrhizal (AM) status (obligate, facultative, non‐mycorrhizal) differ in their area of occupancy in Germany (i.e. number of occupied grid cells, each ~130 km²). We used generalized linear models, incorporating main effects and up to three‐way interactions combining AM status, origin status and plant functional traits. The latter were chosen to describe the possible trade‐off in carbon allocation either towards the symbiosis or to other plant structures, such as storage organs (significant interactions involving traits were assumed to indicate the existence of such trade‐offs). AM status significantly explained the area of occupancy of natives and neophytes – with facultative mycorrhizal species occupying the largest area in both groups – but was less pronounced among archaeophytes. Archaeophytes may have reduced dependency on AM fungi, as they are generally agricultural weeds and the symbiosis potentially becomes obsolete for plants growing in habitats providing a steady provision of nutrients. Trait interactions between AM status and other functional traits were almost exclusively detected for neophytes. While facultative mycorrhizal neophytes benefit from trade‐offs with other traits related to high C cost in terms of area of occupancy, such trade‐offs were almost absent among natives. This indicates that natives and neophytes benefit differently from the symbiosis and suggests that native AM fungal partners might be less important for neophytic than for native plant species or that more time is required to establish similar relationships between neophytes and native fungal symbionts.     

Phosphorus effect on phosphatase activity in endomycorrhizal maize

Success of a mycorrhizal symbiosis is influenced by the availability of phosphorus (P) in the soil. Maize ( Zea mays L. cv. Great Lakes 586) plants were grown under five different levels of soil P, either in the presence or absence of formononetin or the vesicular‐arbuscular mycorrhizal (VAM) fungus Glomus intraradices Schenck and Smith. We detected physiological differences in mycorrhizal roots very early in the development of symbiosis, before the onset of nutrient‐dependent responses. Under low P levels, VAM roots accumulated a greater shoot dry weight (13%), root P concentration (15%) and protein concentration (30%) than non-VAM roots, although root growth was not statistically significantly different. At higher P levels, mycorrhizal roots weighed less than non-VAM roots (10%) without a concomitant host alteration of growth or root P concentration. Mycorrhizal colonization decreased as soil P increased. Formononetin-treatment enhanced colonization of the root by G. intraradices and partially overcame inhibition of VAM colonization by high soil P concentrations. This is the first report that formononetin improves root colonization under high levels of soil P. Acid phosphatase (ACP) and alkaline phosphatase (ALP) activities were closely related to the level of fungal colonization in corn roots. ACP activity in corn roots responded more to soil P availability than did ALP activity (38% more). These results suggest that ACP was involved in the increased uptake of P from the soil, while ALP may be linked to active phosphate assimilation or transport in mycorrhizal roots. Thus, soil P directly affected a number of enzymes essential in host-endophyte interplay, while formononetin enhanced fungal colonization.     

AM fungal communities inhabiting the roots of submerged aquatic plant <Emphasis Type="Italic">Lobelia dortmanna</Emphasis> are diverse and include a high proportion of novel taxa

While the arbuscular mycorrhizal (AM) symbiosis is known to be widespread in terrestrial ecosystems, there is growing evidence that aquatic plants also form the symbiosis. It has been suggested that symbiosis with AM fungi may represent an important adaptation for isoëtid plants growing on nutrient-poor sediments in oligotrophic lakes. In this study, we address AM fungal root colonization intensity, richness and community composition (based on small subunit (SSU) ribosomal RNA (rRNA) gene sequencing) in five populations of the isoëtid plant species Lobelia dortmanna inhabiting oligotrophic lakes in Southern Sweden. We found that the roots of L. dortmanna hosted rich AM fungal communities and about 15 % of the detected molecular taxa were previously unrecorded. AM fungal root colonization intensity and taxon richness varied along an environmental gradient, being higher in oligotrophic and lower in mesotrophic lakes. The overall phylogenetic structure of this aquatic fungal community differed from that described in terrestrial systems: The roots of L. dortmanna hosted more Archaeosporaceae and fewer Glomeraceae taxa than would be expected based on global data from terrestrial AM fungal communities.     

Phosphate in the arbuscular mycorrhizal symbiosis: transport properties and regulatory roles

In response to the colonization by arbuscular mycorrhizal (AM) fungi, plants reprioritize their phosphate (Pi)-uptake strategies to take advantage of nutrient transfer via the fungus. The mechanisms underlying Pi transport are beginning to be understood, and recently, details of the regulation of plant and fungal Pi transporters in the AM symbiosis have been revealed. This review summarizes recent advances in this area and explores current data and hypotheses of how the plant Pi status affects the symbiosis. Finally, suggestions of an interrelationship of Pi and nitrogen (N) in the AM symbiosis are discussed.     

The Ecology of Arbuscular Mycorrhizal Fungi

Arbuscular mycorrhiza is a mutually beneficial biological association between species in the fungal phylum Glomeromycota and higher plants roots. The symbiosis is thought to have afforded green plants the opportunity to invade dry land ca 450 Ma ago and the vast majority of extant terrestrial plants retain this association. Arbuscular mycorrhizal (AM) fungi perform various ecological functions in exchange for host photosynthetic carbon that almost always contribute to the fitness of hosts from an individual to community level. Recent AM fungal research, increasingly delving into the ‘Black Box’, suggests that species in this phylum may play a key facilitative role in below-ground micro- and meso-organism community dynamics, even more perhaps, that of a bioengineer. The ubiquitous nature of the symbiosis in extant flora and the fact that variations from the AM symbiosis are recent events suggest that Glomeromycota and plant roots coevolved. This review considers aspects of AM fungal ecology emphasizing past and present importance of the phylum in niche to global ecosystem function. Nutrient exchange, evolution, taxonomy, phenology, below-ground microbial interaction, propagule dissemination, invasive plants interactions, the potential role in phytoremediation and some of the factors affecting AM fungal biology are discussed. We conclude that it is essential to include AM association in any study of higher plants in natural environments in order to provide an holistic understanding of ecosystems.     

长期施肥对砂姜黑土丛枝菌根真菌群落的影响

在农业生态系统中,丛枝菌根真菌(AM真菌)与很多作物的根系都存在互惠互利的共生关系,与作物的生长和健康密切相关,同时,这类特殊的真菌群落也会受到施肥等农业措施的影响.本研究依托长期定位试验4个试验处理(不施肥、单施化肥、化肥配施秸秆、化肥配施粪肥),研究砂姜黑土AM真菌群落对不同施肥措施的响应及其影响因素,探索不同处理AM真菌指示种的存在.结果表明: 砂姜黑土中的主要AM真菌类群为原囊霉科、多孢囊霉科、巨孢囊霉科、近明球囊霉科、球囊霉科和类球囊霉科;其中类球囊霉属在化肥和有机物料配施中具有显著指示作用.与对照相比,长期单施化肥显著改变AM真菌群落结构并降低其多样性,配施秸秆处理进一步降低AM真菌群落多样性,而配施粪肥明显缓解因施用化肥而造成的多样性减少现象.检验发现,导致AM真菌群落变化最主要的影响因素是土壤pH和可溶性碳.总之,长期不同有机物料和化肥配施对砂姜黑土AM真菌群落结构和多样性会产生不同影响,其中化肥配施粪肥更有利于土壤AM真菌群落多样性的维持.     

Soil nutritional status, not inoculum identity, primarily determines the effect of arbuscular mycorrhizal fungi on the growth of Knautia arvensis plants

Arbuscular mycorrhizal (AM) symbiosis is among the factors contributing to plant survival in serpentine soils characterised by unfavourable physicochemical properties. However, AM fungi show a considerable functional diversity, which is further modified by host plant identity and edaphic conditions. To determine the variability among serpentine AM fungal isolates in their effects on plant growth and nutrition, a greenhouse experiment was conducted involving two serpentine and two non-serpentine populations of Knautia arvensis plants grown in their native substrates. The plants were inoculated with one of the four serpentine AM fungal isolates or with a complex AM fungal community native to the respective plant population. At harvest after 6-month cultivation, intraradical fungal development was assessed, AM fungal taxa established from native fungal communities were determined and plant growth and element uptake evaluated. AM symbiosis significantly improved the performance of all the K. arvensis populations. The extent of mycorrhizal growth promotion was mainly governed by nutritional status of the substrate, while the effect of AM fungal identity was negligible. Inoculation with the native AM fungal communities was not more efficient than inoculation with single AM fungal isolates in any plant population. Contrary to the growth effects, a certain variation among AM fungal isolates was revealed in terms of their effects on plant nutrient uptake, especially P, Mg and Ca, with none of the AM fungi being generally superior in this respect. Regardless of AM symbiosis, K. arvensis populations significantly differed in their relative nutrient accumulation ratios, clearly showing the plant’s ability to adapt to nutrient deficiency/excess.     

Plant potassium content modifies the effects of arbuscular mycorrhizal symbiosis on root hydraulic properties in maize plants

It is well known that the arbuscular mycorrhizal (AM) symbiosis helps the host plant to overcome several abiotic stresses including drought. One of the mechanisms for this drought tolerance enhancement is the higher water uptake capacity of the mycorrhizal plants. However, the effects of the AM symbiosis on processes regulating root hydraulic properties of the host plant, such as root hydraulic conductivity and plasma membrane aquaporin gene expression, and protein abundance, are not well defined. Since it is known that K(+) status is modified by AM and that it regulates root hydraulic properties, it has been tested how plant K(+) status could modify the effects of the symbiosis on root hydraulic conductivity and plasma membrane aquaporin gene expression and protein abundance, using maize (Zea mays L.) plants and Glomus intraradices as a model. It was observed that the supply of extra K(+) increased root hydraulic conductivity only in AM plants. Also, the different pattern of plasma membrane aquaporin gene expression and protein abundance between AM and non-AM plants changed with the application of extra K(+). Thus, plant K(+) status could be one of the causes of the different observed effects of the AM symbiosis on root hydraulic properties. The present study also highlights the critical importance of AM fungal aquaporins in regulating root hydraulic properties of the host plant.     

Roots and Their Symbiotic Microbes: Strategies to Obtain Nitrogen and Phosphorus in a Nutrient-Limiting Environment

The association between Rhizobium and legumes and that between arbuscular mycorrhizal (AM) fungi and most land plants display a remarkable degree of similarity. Both events involve the recognition of, entrance into, and coexistence within the plant root, with the development of a specialized interface that always separates the two partners and at which nutrient exchange occurs. Molecules produced by rhizobia during the early stages of the symbiosis are related to fungal chitin, and the plant responds to both microbes with an increase in the production of flavonoids, which may assist in recognition and development of the symbioses. Many of the same plant genes are up-regulated in the two symbiotic pathways, and notably plants that are Nod^? are often defective in the AM association as well. However, there are a number of differences between the associations, and these are important for understanding the relationship between the two symbioses. The Rhizobium and AM symbioses will be compared and the question of whether the nitrogen-fixing association evolved from the much more ancient AM symbiosis will be discussed.     

The regulation of arbuscular mycorrhizal symbiosis by phosphate in pea involves early and systemic signalling events

Most plants form root symbioses with arbuscular mycorrhizal (AM) fungi, which provide them with phosphate and other nutrients. High soil phosphate levels are known to affect AM symbiosis negatively, but the underlying mechanisms are not understood. This report describes experimental conditions which triggered a novel mycorrhizal phenotype under high phosphate supply: the interaction between pea and two different AM fungi was almost completely abolished at a very early stage, prior to the formation of hyphopodia. As demonstrated by split-root experiments, down-regulation of AM symbiosis occurred at least partly in response to plant-derived signals. Early signalling events were examined with a focus on strigolactones, compounds which stimulate pre-symbiotic fungal growth and metabolism. Strigolactones were also recently identified as novel plant hormones contributing to the control of shoot branching. Root exudates of plants grown under high phosphate lost their ability to stimulate AM fungi and lacked strigolactones. In addition, a systemic down-regulation of strigolactone release by high phosphate supply was demonstrated using split-root systems. Nevertheless, supplementation with exogenous strigolactones failed to restore root colonization under high phosphate. This observation does not exclude a contribution of strigolactones to the regulation of AM symbiosis by phosphate, but indicates that they are not the only factor involved. Together, the results suggest the existence of additional early signals that may control the differentiation of hyphopodia.     

Pectin localization in the Mediterranean orchid Limodorum abortivum reveals modulation of the plant interface in response to different mycorrhizal fungi

In most mycorrhizal symbioses, phylogenetically distinct fungi colonize simultaneously the roots of individual host plants. A matter of debate is whether plants can distinguish among these fungal partners and differentiate their cellular responses. We have addressed this question in the orchid mycorrhizal symbiosis, where individual roots of the Mediterranean species Limodorum abortivum can be colonized by a dominant unculturable fungal symbiont belonging to the genus Russula and by more sporadic mycelia in the genus Ceratobasidium (form-genus Rhizoctonia). The phylogenetic position of the Ceratobasidium symbionts was further investigated in this work. Both Russula and Ceratobasidium symbionts form intracellular coils in the cortical roots of L. abortivum, but hyphae are very different in size and morphology, making the two fungi easily distinguishable. We have used John Innes Monoclonal 5, a widely used monoclonal antibody against pectin, to investigate the composition of the symbiotic plant interface around the intracellular coils formed by the two fungal partners. Immunolabelling experiments showed that pectin is exclusively found in the interface formed around the Ceratobasidium, and not around the Russula symbiont. These data indicate that the plant responses towards distinct mycorrhizal fungal partners can vary at a cellular level.     

Chemotropism in the arbuscular mycorrhizal fungus Glomus mosseae

In this work, we report the occurrence of chemotropism in the arbuscular mycorrhizal (AM) fungus Glomus mosseae. Fungal hyphae were able to respond to host-derived signals by reorienting their growth towards roots and to perceive chemotropic signals at a distance of at least 910 microm from roots. In order to reach the source of chemotropic signals, hyphal tips crossed interposed membranes emerging within 1 mm from roots, eventually establishing mycorrhizal symbiosis. The specificity of chemotropic growth was evidenced by hyphal growth reorientation and membrane penetration occurring only in experimental systems set up with host plants. Since pre-symbiotic growth is a critical stage in the life cycle of obligate AM fungal symbionts, chemotropic guidance may represent an important mechanism functional to host root location, appressorium formation and symbiosis establishment.