Biochemical changes in Glomus fasciculatum colonized roots of Lycopersicon esculentum in presence of Meloidogyne incognita

Glasshouse experiments were conducted to elicit biochemical substantiation for the observed difference in resistance to nematode infection in roots colonized by mycorrhiza, and susceptibility of the fresh flush of roots of the same plant that escaped mycorrhizal colonization. Tomato roots were assayed for their biochemical profiles with respect to total proteins, total phenols, indole acetic acid, activities of polyphenol oxidase, phenylalanine ammonia lyase and indole acetic acid oxidase. The roots of the same plant (one set) received Glomus fasciculatum and G. fasciculatum plus juveniles of Meloidogyne incognita separately; and half the roots of second set of plants received G. fasciculatum while the other half of roots did not receive any treatment. Roots colonized by G. fasciculatum recorded maximum contents of proteins and phenols followed by that of the roots that received G. fasciculatum plus M. incognita. However, IAA content was lowest in the roots that received mycorrhiza or mycorrhiza plus juveniles of root-knot nematode and correspondingly. Roots that received juveniles of root-knot nematode recorded maximum IAA content and per cent increase over healthy check and mycorrhiza-inoculated roots. The comparative assay on the activities of PPO, PAL and IAA oxidase enzymes in treated and healthy roots of tomato, indicated that PAL and IAA oxidase activities were maximum in G. fasciculatum colonized roots followed by the roots that received mycorrhiza plus juveniles of root-knot nematode, while the activity of PPO was minimum in these roots. The roots that received juveniles of root-knot nematode recorded minimum PAL and IAA oxidase activities and maximum PPO activity. Since the roots of same plant that received mycorrhiza and that did not receive mycorrhiza; and the plant that received nematode alone and mycorrhiza plus nematode recorded differential biochemical contents of proteins, total phenols and IAA, and differential activities of enzymes under study, it was evident that the biochemical defense response to mycorrhizal colonization against root-knot nematodes was localized and not systemic. This explained for the response of plant that differed in root galling due to nematode infection in presence of mycorrhizal colonization. The new or fresh roots which missed mycorrhizal colonization, got infected by nematodes and developed root galls.     

Transport processes at the plant-fungus interface in mycorrhizal associations: physiological studies

The roots of most plants form symbiotic associations with mycorrhizal fungi. The net flux of nutrients, particularly phosphorus (P), from the soil into the plant is greater in mycorrhizal than in comparable non-mycorrhizal plants. However despite the widespread occurrence of mycorrhizal associations the processes controlling the transfer of solutes between the symbionts are poorly understood. To understand the mechanisms regulating the transfer of solutes information about conditions at the interface between plant and fungus is needed.Measurements of apoplastic and intracellular electrical potential difference in leek roots colonised by mycorrhizal fungi and estimates of cytosolic pH in fungal hyphae are presented. These and the implications for plant/fungal mineral nutrition in vesicular-arbuscular mycorrhizas are discussed.     

The use of inositol hexaphosphate as a phosphorus source by mycorrhizal and non-mycorrhizal Scots Pine (Pinus sylvestris)

1. The external mycelia of the ectomycorrhizal fungi Thelephora terrestris and Suillus luteus , associated with Pinus sylvestris roots, exhibited a substantial extracellular acid phosphatase activity. The activity was positively correlated with the ergosterol concentration in the growth substratum and decreased with an increasing P nutrition.
2. The pioneer species T. terrestris grew best at a high P_i nutrition level whereas S. luteus , a 'late-stage' mycobiont, produced more active biomass at a low P_i nutrition level.
3. The phytase activity of the external mycelia could not be detected; at the root surface a phytase activity was observed. Mycorrhizas had significantly higher activities than uninfected roots.
4. The addition of a relatively high concentration of a soluble phytate to the growth substratum resulted in an increased relative growth rate (RGR) in both mycorrhizal and non-mycorrhizal plants. The influence of the mycorrhizal fungi on the use of the phytate-P was small, despite the phytase activity of the mycorrhizal feeder roots.
5. The addition of phytate fixed on a HPLC resin did not result in an increase of the RGR and P uptake neither in the non-mycorrhizal nor in the mycorrhizal Pines. The experiment did not support the hypothesis that phytate, which has a low solubility in soils, is a useful P source for ectomycorrhizal plants.     

Two defined alleles of the LRR receptor kinase GmNARK in supernodulating soybean govern differing autoregulation of mycorrhization

Plants regulate the extent of nodulation and root colonization by arbuscular mycorrhizal fungi (AMF), a phenomenon named autoregulation of symbiosis. We tested AMF colonization in split roots of various soybean genotypes [ Glycine max (L.) Merr. cv. Bragg, Enrei, Harosoy and Williams], where precolonization of one side of the split-root system by the AMF Glomus mosseae resulted in reduced mycorrhization of the other. AMF precolonization failed to control secondary mycorrhization in the supernodulating Bragg nonsense mutant nts1007 (Q106*), indicating that the GmNARK gene (predicted to encode a leucine-rich repeats (LRR) receptor kinase related to CLAVATA1 in Arabidopsis ) is involved in autoregulation of the AMF symbiosis. Here, we tested whether the allelic En6500 nonsense supernodulating mutant ( GmNARK K606*, derived from cv. Enrei) and supernodulating mutants of cv. Williams ( Nod1-3 and Nod2-4 ) with yet-undefined genetic lesions exhibit a similar symbiotic phenotype in mycorrhizal split-root systems. Surprisingly, these supernodulating mutants retained their ability to autoregulate AMF. To examine possible differences between two allelic mutants, we determined levels of IAA, abscisic acid, coumestrol, daidzein and genistein in mycorrhizal and uninoculated control roots. Compared with wild-type plants, both mutants showed reduced IAA accumulation in mycorrhizal roots. Roots of cv. Enrei and En6500 exhibited high levels of isoflavonoids not seen in Bragg or nts1007 . Taken together, these findings showed that supernodulation mutants, despite a common nodulation phenotype, differ in their ability to autoregulate AMF root colonization. This suggests either that the GmNARK gene product of some mutants is still partially functional (Q106* vs. K606*) or that varietal differences reflected in altered physiological responses suppress the loss of function.     

Auxins in the development of an arbuscular mycorrhizal symbiosis in maize

While the levels of free auxins in maize (Zea mays L.) roots during arbuscular mycorrhiza formation have been previously described in detail, conjugates of indole-3-acetic acid (IAA) and indole-3-butyric acid (IBA) with amino acids and sugars were neglected. In this study, we have therefore determined free, ester and amide bound auxins in roots of maize inoculated with Glomus intraradices during early stages of the colonization process. Ester conjugates of IAA and IBA were found only in low amounts and they did not increase in AM colonized roots. The Levels of IAA and IBA amide conjugates increased 20 and 30 days past inoculation (dpi). The formation of free and conjugated IBA but not IAA was systemically induced during AM colonization in leaves of maize plants. This implicated a role for auxin conjugate synthesis and hydrolysis during AM. We have therefore investigated the in vivo metabolism of 3H-labeled IBA by TLC but only slight differences between control and AM-inoculated roots were observed. The activity of auxin conjugate hydrolase activity measured with three different putative substrates showed a decrease in infected roots compared to controls. The fluorinated IBA analog TFIBA inhibited IBA formation in leaves after application to the root system, but was not transported from roots to shoots. AM hyphae were also not able to transport TFIBA. Our results indicate complex control mechanisms to regulate the levels of free and conjugated auxins, which are locally and systemically induced during early stages of the formation of an arbuscular mycorrhizal symbiosis.     

IAA and ZR content in leek (Allium porrum L.), as influenced by P nutrition and arbuscular mycorrhizae,in relation to plant development

Leek plants (Allium porrum L.), infected or not with the arbuscular mycorrhizal (AM) fungus, Glomus mosseae, were grown in a sand-hydroponic system, fed with a nutrient solution containing 3.2 or 96 M P and analyzed for root IAA and ZR content, to assess the role played by the fungus and P nutrition on host hormonal balance. IAA was analyzed by HPLC-fluorimetry, ZR by HPLC-UV coupled with a bioassay based on the expression of a phytohormone-regulated GUS reporter gene. Shoot and root weights and shoot FW-DW ratio enhancements, as well as root-to-shoot DW ratio decrement in mycorrhizal plants, were related to P nutrition. Shoot P concentration was increased by mycorrhizae at both P levels, but was comparable in AM plants grown at 3.2 M P and non mycorrhizal (NM) plants at 96 M P. Mycorrhizae and P increased IAA at substantially similar values, while P increased ZR much more than mycorrhizae did. These results are discussed in relation to root architecture modifications induced by the AM fungus.     

The dual benefit of arbuscular mycorrhizal fungi under soil zinc deficiency and toxicity: linking plant physiology and gene expression

Plant and Soil - Colonisation of roots by arbuscular mycorrhizal fungi (AMF) can increase plant biomass and nutrition under soil zinc (Zn) deficiency and toxicity conditions, but the genes and...     

Nitrogen assimilation in Lolium perenne colonized by the arbuscular mycorrhizal fungus Glomus fasciculatum

To investigate nitrogen assimilation in Lolium perenne L. colonized by the arbuscular mycorrhizal (AM) fungus Glomus fasciculatum (Thax. sensu Gerd.), nitrate uptake, key enzyme activities, and ¹⁵N incorporation into free amino acids were measured. After a 4-h labelling period with [¹⁵N]nitrate, ¹⁵N content was higher in roots and shoots of AM-plants than in those of control plants. Glutamine synthetase (GS) and nitrate reductase (NR) activities were increased in shoots of AM-plants, but not in roots. More label was incorporated into amino acids in shoots of AM plants. Glutamine, glutamate, alanine and γ-aminobutyric acid were the major sinks for ¹⁵N in roots and shoots of control and AM plants. Interactions between mycorrhizal colonization, phosphate and nitrate nutrition and NR activity were investigated in plants which received different amounts of phosphate or nitrate. In shoots of control plants, NR activity was not stimulated by high levels of phosphate nutrition but was stimulated by high levels of nitrate. At 4 m M nitrate in the nutrient solution, NR activity was similar in control and AM plants. We concluded that mycorrhizal effects on nitrate assimilation are not mediated via improved phosphate nutrition, but could be due to improved nitrogen uptake and translocation.     

Arbuscular mycorrhiza enhances auxin levels and alters auxin biosynthesis in Tropaeolum majus during early stages of colonization

Plant hormones, including auxins, might be signals during the establishment of an arbuscular mycorrhizal (AM) symbiosis. Here, we report on the concentrations of three auxins native to nasturtium ( Tropaeolum majus L.) during early AM development. Indole-3-acetic acid (IAA), indole-3-butyric acid (IBA), and phenylacetic acid (PAA) were previously identified as endogenous compounds in this species by full-scan gas chromatography–mass spectrometry. All auxinic compounds were influenced by AM colonization but showed completely different patterns. At very early stage, free IAA and IBA were lower in infected than in control roots, whereas PAA concentration was higher in infected roots than in controls. At later stages, PAA was reduced in colonized roots, whereas, especially, IBA was increased in colonized roots compared with controls. Measurement of total auxins confirmed a complex regulation pattern for the three compounds. In hyphae of Glomus intraradices , none of the auxins was detectable. Biosynthesis of the three auxins was measured using heavy labeled isotopes as precursors in control and AM-inoculated roots. While not much difference was found in the IAA labeling pattern between controls and AM-inoculated roots at both time points, IBA synthesis was slightly higher in AM-inoculated roots. Double labeling experiments showed that two distinct pathways, a tryptophan-dependent and a tryptophan-independent biosynthetic pathway contribute to the synthesis of IAA in T. majus roots. Because T. majus is difficult to genetically manipulate, we have used tobacco plants transformed with the auxin-inducible promoter GH3 fused to the β-glucuronidase (GUS) reporter gene to investigate whether AM structures would co-localize to cells harboring the auxin-inducible promoter. Although the GUS activity increased significantly in AM-inoculated roots, there was no obvious correlation between GH3::GUS expression and fungal structures.     

The mobilisation and fate of soil and rock phosphate in the rhizosphere of ectomycorrhizal Pinus radiata seedlings in an Allophanic soil

A pot experiment was conducted to investigate the uptake of Zn from experimentally contaminated calcareous soil of low nutrient status by maize inoculated with the arbuscular mycorrhizal (AM) fungus Glomus caledonium. EDTA was applied to the soil to mobilize Zn and thus maximize plant Zn uptake. The highest plant dry matter (DM) yields were obtained with a moderate Zn addition level of 300 mg kg^?1. Plant growth was enhanced by mycorrhizal colonization when no Zn was added and under the highest Zn addition level of 600 mg kg^?1, while application of EDTA to the soil generally inhibited plant growth. EDTA application also increased plant Zn concentration, and Zn accumulation in the roots increased with increasing EDTA addition level. The effects of inoculation with G. caledonium on plant Zn uptake varied with Zn addition level. When no Zn was added, Zn translocation from roots to shoots was enhanced by mycorrhizal colonization. In contrast, when Zn was added to the soil, mycorrhizal colonization resulted in lower shoot Zn concentrations in mycorrhizal plants. The P nutrition of the maize was greatly affected by AM inoculation, with mycorrhizal plants showing higher P concentrations and P uptake. The results indicate that application of EDTA mobilized soil Zn, leading to increased Zn accumulation by the roots and subsequent plant toxicity and growth inhibition. Mycorrhizal colonization alleviated both Zn deficiency and Zn contamination, and also increased host plant growth by influencing mineral nutrition. However, neither EDTA application nor arbuscular mycorrhiza stimulated Zn translocation from roots to shoots or metal phytoextraction under the experimental conditions. The results are discussed in relation to the environmental risk associated with chelate-enhanced phytoextraction and the potential role of arbuscular mycorrhiza in soil remediation.     

Endogenous auxins and branching of wheat roots gaining nutrients from isolated compartments

The auxin content in roots of hydroponically grown wheat (Triticum durum Desf.) plants was affected by imbalanced distribution of nutrients when the root medium fed to plants from isolated compartments. One day after the transfer of seedlings on the nutrient medium with uneven ion distribution, the IAA content in roots contacting concentrated nutrient solution became significantly higher than in roots bathed with a dilute solution. The IAA content reached the peak on the second day and remained steadily high later on. The lateral root primordia developed in these roots were more numerous; the largest difference in this parameter was observed in 1–2 days after the increase in root content of auxin. One day later, numerous lateral roots appeared on the parent roots contacting the concentrated nutrient solution. Thus, the increase in concentration of the nutrient solution bathing a part of root system raised the IAA content in the affected roots prior to the enhanced root branching. This hormonal response of plants might play an important role in changes of root growth rate and root branching, thereby improving plant nutrition.     

The significance of ectomycorrhizal fungi for sulfur nutrition of trees

Sulfur nutrition of plants is largely determined by sulfate uptake of the roots, the allocation of sulfate to the sites of sulfate reduction and assimilation, the reduction of sulfate to sulfide and its assimilation into reduced sulfur-containing amino acids and peptides, and the allocation of reduced sulfur to growing tissues that are unable to fulfill their own demand for reduced sulfur in growth and development. Association of the roots of pedunculate oak (Quercus robur L.) and beech (Fagus sylvatica L.) trees with ectomycorrhizal fungi seems to interact with these processes of sulfur nutrition in different ways, but the result of these interactions is dependent on both the plant and the fungal partners. Mycorrhizal colonisation of the roots can alter the response of sulfate uptake to sulfate availability in the soil and enhances xylem loading and, hence, xylem transport of sulfate to the leaves. As a consequence, sulfate reduction in the leaves may increase. Simultaneously, sulfate reduction in the roots seems to be stimulated by ectomycorrhizal association. Increased sulfate reduction in the leaves of mycorrhizal trees can result in enhanced phloem transport of reduced sulfur from the leaves to the roots. Different from herbaceous plants, enhanced phloem allocation of reduced sulfur does not negatively affect sulfate uptake by the roots of trees. These interactions between mycorrhizal association and the processes involved in sulfur nutrition are required to provide sufficient amounts of reduced sulfur for increased protein synthesis that is used for the enhanced growth of trees frequently observed in response to ectomycorrhizal association. This revised version was published online in June 2006 with corrections to the Cover Date.     

丛枝菌根真菌对枳根净离子流及锌污染下枳苗矿质营养的影响

盆栽实验研究了不同施Zn水平(0、300 mg/kg和600 mg/kg)下,接种丛枝菌根真菌Glomus intraradices对枳苗生长、Zn、Cu、P、K、Ca、Mg分布的影响,并采用非损伤微测技术测定分析了菌根化与非菌根化枳根净Ca²⁺、H⁺、NO₃^-离子流动态。结果表明:(1)在不同施Zn水平下,接种菌根真菌显著提高了枳苗地上部及根部鲜重;随着施Zn水平的提高,菌根侵染率呈降低趋势,枳苗地上部与根部Zn含量呈增加趋势,且接种株根部Zn含量显著高于未接种株。(2)接种株未施Zn处理的地上部Cu、P、K、Mg和根部Cu含量、施600 mg/kg Zn处理的根部Cu及施300 mg/kg Zn处理的根部P含量均显著高于对照,而菌根真菌侵染对枳苗Ca含量并无显著性影响。(3)接种株未施Zn处理的根部距根尖端0 μm和600 μm处净Ca²⁺流出速率、600 μm处净H⁺流入速率、2400 μm处净NO₃^-流入速率均显著高于未接种株。     

Mycorrhizal types differ in ecophysiology and alter plant nutrition and soil processes

Mycorrhizal fungi benefit plants by improved mineral nutrition and protection against stress, yet information about fundamental differences among mycorrhizal types in fungi and trees and their relative importance in biogeochemical processes is only beginning to accumulate. We critically review and synthesize the ecophysiological differences in ectomycorrhizal, ericoid mycorrhizal and arbuscular mycorrhizal symbioses and the effect of these mycorrhizal types on soil processes from local to global scales. We demonstrate that guilds of mycorrhizal fungi display substantial differences in genome‐encoded capacity for mineral nutrition, particularly acquisition of nitrogen and phosphorus from organic material. Mycorrhizal associations alter the trade‐off between allocation to roots or mycelium, ecophysiological traits such as root exudation, weathering, enzyme production, plant protection, and community assembly as well as response to climate change. Mycorrhizal types exhibit differential effects on ecosystem carbon and nutrient cycling that affect global elemental fluxes and may mediate biome shifts in response to global change. We also note that most studies performed to date have not been properly replicated and collectively suffer from strong geographical sampling bias towards temperate biomes. We advocate that combining carefully replicated field experiments and controlled laboratory experiments with isotope labelling and ‐omics techniques offers great promise towards understanding differences in ecophysiology and ecosystem services among mycorrhizal types.     

Beech carbon productivity as driver of ectomycorrhizal abundance and diversity

We tested the hypothesis that carbon productivity of beech ( Fagus sylvatica ) controls ectomycorrhizal colonization, diversity and community structures. Carbon productivity was limited by long-term shading or by girdling. The trees were grown in compost soil to avoid nutrient deficiencies. Despite severe limitation in photosynthesis and biomass production by shading, the concentrations of carbohydrates in roots were unaffected by the light level. Shade-acclimated plants were only 10% and sun-acclimated plants were 74% colonized by ectomycorrhiza. EM diversity was higher on roots with high than at roots with low mycorrhizal colonization. Evenness was unaffected by any treatment. Low mycorrhizal colonization had no negative effects on plant mineral nutrition. In girdled plants mycorrhizal colonization and diversity were retained although ¹⁴C-leaf feeding showed almost complete disruption of carbon transport from leaves to roots. Carbohydrate storage pools in roots decreased upon girdling. Our results show that plant carbon productivity was the reason for and not the result of high ectomycorrhizal diversity. We suggest that ectomycorrhiza can be supplied by two carbon routes: recent photosynthate and stored carbohydrates. Storage pools may be important for ectomycorrhizal survival when photoassimilates were unavailable, probably feeding preferentially less carbon demanding EM species as shifts in community composition were found.     

Influence of Mycorrhizal Fungus, Phosphorus,and Burrowing Nematode Interactions on Growth of Rough Lemon Citrus Seedlings

Rough lemon seedlings were grown in mycorrhizal-infested or phosphorus-amended soil (25 and 300 mg P/kg) in greenhouse experiments. Plants Were inoculated with the citrus burrowing nematode, Radopholus citrophilus (0, 50, 100, or 200 nematodes per pot). Six months later, mycorrhizal plants and nonmycorrhizal, high-P plants had larger shoot and root weights than did non-mycorrhizal, low-P plants. Burrowing nematode population densities were lower in roots of mycorrhizal or nonmycorrhizal, high-P plants than in roots of nonmycorrhizal, low-P plants; however, differences in plant growth between mycorrhizal and nonmycorrhizal plants were not significant with respect to initial nematode inoculum densities. Phosphorus content in leaf tissue was significantly greater in mycorrhizal and nonmycorrhizal, high-P plants compared with nonmycorrhizal, low-P plants. Nutrient concentrations of K, Mg, and Zn were unaffected by nematode parasitism, whereas P, Ca, Fe, and Mn were less in nematode-infected plants. Enhanced growth associated with root colonization by the mycorrhizal fungus appeared to result from improved P nutrition and not antagonism between the fungus and the nematode.     

濒危药用植物桃儿七根的显微结构及其菌根真菌分布研究

本文研究了桃儿七Sinopodophyllum hexandrum根的显微结构及其真菌分布。结果表明,桃儿七的根为根状茎,节状,不定根形成的须根系发达。根的结构主要由表皮、皮层、维管柱三部分构成,其中,皮层所占比例最大,超过80%。根的木质部有四原型和五原型两种类型,五原型较为常见;四原型的根和五原型的根在皮层细胞形态上存在一定差异。在桃儿七的不定根和其上的侧根观察到真菌菌丝分布,其数量和种类与根的直径有关,在不定根较细(先端)的部位真菌以暗色有隔内生真菌(DSE真菌)为主,侵染率为77.9%;而较粗根中真菌菌丝为无隔菌丝为主,分布很少且仅存在于皮层细胞的一至二层,不侵染皮层深部和维管柱。不定根侧根中真菌以丛枝菌根真菌为主,丛枝菌根常常占据大部分的皮层细胞,侵染率高达90%以上。桃儿七根中没有发现根毛存在,因此,侧根中共生的丛枝菌根真菌可能是桃儿七养分和水分吸收的主要途径。