Relating foliar dehydration tolerance of mycorrhizal Phaseolus vulgaris to soil and root colonization by hyphae

Mycorrhizal symbiosis can modify plant response to drying soil, but little is known about the relative contribution of soil vs. root hyphal colonization to drought resistance of mycorrhizal plants. Foliar dehydration tolerance, characterized as leaf and soil water potential at the end of a lethal drying episode, was measured in bean plants (Phaseolus vulgaris) colonized by Glomus intraradices or by a mix of arbuscular mycorrhizal fungi collected from a semi-arid grassland. Path analysis modeling was used to evaluate how colonization rates and other variables affected these lethal values. Of several plant and soil characteristics tested, variation in dehydration tolerance was best explained by soil hyphal density. Soil hyphal colonization had larger direct and total effects on both lethal leaf water potential and soil water potential than did root hyphal colonization, root density, soil aggregation, soil glomalin concentration, leaf phosphorus concentration or leaf osmotic potential. Plants colonized by the semi-arid mix of mycorrhizal fungi had lower lethal leaf water potential and soil water potential than plants colonized by G. intraradices. Our findings support the assertion that external, soil hyphae may play an important role in mycorrhizal influence on the water relations of host plants.     

Contribution of six arbuscular mycorrhizal fungal isolates to water uptake by Lactuca sativa plants under drought stress

It is currently accepted that, along with nutrients, arbuscular mycorrhizal (AM) fungi also transport water to their host plant. However, the quantity of water supplied and its significance for plant water relations remain controversial. The objective of this work was to evaluate and compare the ability of six AM fungi to alter rates of root water uptake under drought stress conditions. Soil drying rates of uninoculated control plants of comparable size and nutritional status and mycorrhizal plants were recorded daily. Lactuca sativa plants colonized by Glomus coronatum , G. intraradices , G. claroideum and G. mosseae depleted soil water to a higher extent than comparably sized uninoculated control plants or plants colonized by G. constrictum or G. geosporum . The differences ranged from 0.6% volumetric soil moisture for G. mosseae -colonized plants to 0.95% volumetric soil moisture for G. intraradices -colonized plants. These differences in soil moisture were equivalent to 3–4.75 ml plant⁻¹ day⁻¹, respectively, and could not be ascribed to differences in plant size, but to the activity of AM fungi. The AM fungi tested in this study differed in their effectiveness to enhance plant water uptake from soil. This ability seems to be related to the amount of external mycelium produced by each AM fungus and to the frequency of root colonization in terms of live and active fungal structures.     

Cadmium accumulation in sunflower plants influenced by arbuscular mycorrhiza

In order to investigate the cadmium (Cd) accumulation patterns and possible alleviation of Cd stress by mycorrhization, sunflower plants (Helianthus annuus L.) were grown in the presence or absence of Cd (20 micromol L(-1)) and inoculated or not inoculated with the arbuscular mycorrhizal fungus (AMF) Glomus intraradices. No visual symptoms of Cd phytotoxicity were observed; nevertheless, in non-mycorrhizal plants the presence of Cd decreased plant growth. The addition of Cd had no significant effect on either mycorrhizal colonization or the amount of extra-radical mycelia that was produced by the AMF. Cd accumulated mainly in roots; only 22% of the total Cd absorbed was translocated to the shoots, where it accumulated to an average of 228 mg Cd kg(-1). Although the shoot-to-root ratio of Cd was similar in both the AMF inoculated and non-inoculated plants, the total absorbed Cd was 23% higher in mycorrhizal plants. Cd concentration in AMF extra-radical mycelium was 728 microg g(-1) dry weight. Despite the greater absorption of Cd, mycorrhizal plants showed higher photosynthetic pigment concentrations and shoot P contents. Cd also influenced mineral nutrition, leading to decreased Ca and Cu shoot concentrations; N, Fe and Cu shoot contents; and increased S and K shoot concentrations. Cd induced guaiacol peroxidase activity in roots in both mycorrhizal and non-mycorrhizal plants, but this increase was much more accentuated in non-mycorrhizal roots. In conclusion, sunflower plants associated with G. intraradices were less sensitive to Cd stress than non-mycorrhizal plants. Mycorrhizal sunflowers showed enhanced Cd accumulation and some tolerance to excessive Cd concentrations in plant tissues.     

丛枝菌根真菌对植物根尖分生区和根冠细胞的侵染*

一般说来,从枝菌根（ＡＭ）真菌大多数是从植物根系根毛区（成熟区）侵入和扩展的,在显微镜下往往看不到根尖分生区和根冠表皮细胞被ＡＭ真菌侵染的特征。这就很容易给人们造成一种假象,似乎ＡＭ真菌不能侵染根尖分生区和根冠表皮细胞,即它们对ＡＭ真菌是免疫的。然而笔者多次于显微镜下看到ＡＭ真菌侵染根尖分生区和根冠表皮细胞,并形成典型的泡囊、丛枝、菌丝等结构。这一现象导致作者在温室盆栽和大田条件下研究了玫瑰红巨孢囊霉（ Ｇｉｇａｓｐｏｒａ　ｒｏｓｅａ Ｎｉｃｏｌ　＆ Ｓｃｈｅｎｃｋ）、珠状巨孢囊霉（Ｇｉｇａｓｐｏｒａ　ｍａｒｇａｒｉｔａ　Ｂｅｃｋｅｒ　＆　Ｈａｌｌ）、根内球囊霉（Ｇｌｏｍｕｓ　ｏｍｔｒａｒａｄｉｃｅｓ　ｓｃｈｅｎｃｋ　＆　Ｓｍｉｔｈ、摩西球囊霉（Ｇｌｏｍｕｓ ｍｏｓｓｅａｅ （Ｎｉｃｏｌ　＆ Ｇｅｒｄ．） Ｇｅｒｄｅｍａｎｎ　＆ Ｔｒａｐｐｅ）、地表球囊霉（ Ｇｌｏｍｕｓ ｖｅｒｓｉｆｏｒｍｅ（ Ｋａｒｓｔｅｎ）Ｂｅｒｃｈ）和弯丝硬囊霉（ Ｓｃｌｅｒｏｃｙｓｔｉｓ　ｓｉｎｕｏｓａ　Ｇｅｒｄｅｍａｎｎ　＆ Ｂａｋｈｉ）对棉花（Ｇｏｓｓｙｐｉｕｍ　ｈｉｒｓｕｔｕｍ　Ｌ．）、烟草（Ｎｉｃｏｔｉａｎａ　 ｔａｂａｃｕｍ Ｌ．）和白     

INFLUENCE OF SUPPLEMENTAL INORGANIC NUTRIENTS ON GROWTH,SURVIVORSHIP, AND MYCORRHIZAL RELATIONSHIPS OF SCHIZACHYRIUM SCOPARIUM (POACEAE) GROWN IN FUMIGATED AND UNFUMIGATED SOIL

Little bluestem grass Schizachyrium scoparium ([Michx.] Nash) plants were grown under field conditions for 2 years in soils fumigated with methyl bromide and chloropicrin, or in unfumigated soil, and treated with supplemental inorganic nutrients (bases calcium and magnesium) phosphorus, nitrogen, and potassium. Most differences in measured plant responses were due to interactions between fumigation and nutrient treatments. These included biomass production, root mass per unit length (μg/cm), root lengths, flowering culm production, percent colonization, colonized root length, and spore production in rhizosphere soil. Plants generally responded to mycorrhizal fungal colonization by reducing total root length and producing thicker roots. Treatment of plants with bases appeared to profoundly affect the mycorrhizal association by reducing sporulation of vesicular-arbuscular mycorrhizal fungi and increasing colonization. When fumigated or unfumigated soils were considered separately, base-treated plants produced more biomass than other treatments. Base-treated plants grown on unfumigated soil had more flowering culms and longer colonized root lengths than all other plants. Percent colonization by mycorrhizal fungi and colonized root length were positively correlated with phosphorus/nitrogen ratios, but the ratio was not correlated with plant biomass production. This suggests that phosphorus is not a limiting nutrient in our soil and investment in a mycorrhizal association may not result in enhanced plant growth. The base-nutrient effects may indicate a need to reevaluate earlier studies of macro nutrient effects that did not take into account the role played by calcium and magnesium in assessing fungus-host plant interactions.     

Production of Vesicular-Arbuscular Mycorrhizal Fungus Inoculum in Aeroponic Culture

Bahia grass (Paspalum notatum) and industrial sweet potato (Ipomoea batatas) colonized by Glomus deserticola, G. etunicatum, and G. intraradices were grown in aeroponic cultures. After 12 to 14 weeks, all roots were colonized by the inoculated vesicular-arbuscular mycorrhizal fungi. Abundant vesicles and arbuscules formed in the roots, and profuse sporulation was detected intra-and extraradically. Within each fungal species, industrial sweet potato contained significantly more roots and spores per plant than bahia grass did, although the percent root colonization was similar for both hosts. Mean percent root colonization and sporulation per centimeter of colonized root generally increased with time, although with some treatments colonization declined by week 14. Spore production ranged from 4 spores per cm of colonized root for G. etunicatum to 51 spores per cm for G. intraradices. Infectivity trials with root inocula resulted in a mean of 38, 45, and 28% of bahia grass roots colonized by G. deserticola, G. etunicatum, and G. intraradices, respectively. The germination rate of G. etunicatum spores produced in soil was significantly higher than that produced in aeroponic cultures (64% versus 46%) after a 2-week incubation at 28°C. However, infectivity studies comparing G. etunicatum spores from soil and aeroponic culture indicated no biological differences between the spore sources. Aeroponically produced G. deserticola and G. etunicatum inocula retained their infectivity after cold storage (4°C) in either sterile water or moist vermiculite for at least 4 and 9 months, respectively.     

Comparing contributions of soil versus root colonization to variations in stomatal behavior and soil drying in mycorrhizal Sorghum bicolor and Cucurbita pepo

In prior studies we learned that colonization of soil can be as important as colonization of roots in determining mycorrhizal influence on the water relations of host plants. Here we use a path analysis modeling approach to test (a) whether quantity of hyphae in soil contributes to variations in stomatal behavior and soil drying, and (b) whether soil colonization or root colonization has a stronger influence on these stomatal and soil drying responses. Experiments were performed on Sorghum bicolor and Cucurbita pepo, with soils and roots colonized by a mixture of Glomus intraradices and Gigaspora margarita. Soil colonization generally made more significant contributions to stomatal conductance than did root colonization. Soil colonization did not make significant direct contributions to soil water potential measures (soil water potential at stomatal closure or soil drying rate), whereas root colonization did contribute a potentially important path to each. The findings further support a role for mycorrhization of the soil itself in contributing to the regulation of stomatal behavior of host plants.     

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.     

Regulation of arbuscular mycorrhizal development by plant host and fungus species in alfalfa

Two cvs of alfalfa ( Medicago sativa L.), Gilboa and Moapa 69, were inoculated in glasshouse pots with three arbuscular mycorrhizal (AM) fungi to investigate the efficacy of mycorrhizas with respect to the extent of colonization and sporulation. Paspalum notatum Flugge also was inoculated to describe fungal parameters on a routine pot culture host. Percentage root length of P. notatum colonized by Glomus mosseae (Nicol. & Gerd.) Gerdemann & Trappe, Glomus intraradices Schenck & Smith, and Gigaspora margarita Becker & Hall increased from 10 to 21 wk, and all fungi sporulated during that period. In alfalfa, only colonization by G. intraradices increased over that time period, and it was the only fungus to sporulate in association with alfalfa at 10 wk. Glomus mosseae did not sporulate after 16–21 wk despite having colonized 30–35% of the root length of both alfalfa cvs. In vitro experiments in which Ri T-DNA-transformed roots of alfalfa were inoculated with AM fungi showed normal mycorrhizal formation by G. intraradices and a hypersensitivity-like response to Gi. margarita . Colonized cells became necrotic, and HPLC analysis indicated increased concentrations of phenolics and isoflavonoids in these root segments. These data strongly support the existence of a degree of specificity between AM fungi and host that might rely on specific biochemical regulatory processes initiated in the host as a result of the attempts at colonization by the fungus.     

Arbuscular mycorrhizal contribution to heavy metal uptake by maize (Zea mays L.) in pot culture with contaminated soil

In two pot-culture experiments with maize in a silty loam (P2 soil) contaminated by atmospheric deposition from a metal smelter, root colonization with indigenous or introduced arbuscular mycorrhizal (AM) fungi and their influence on plant metal uptake (Cd, Zn, Cu, Pb, Mn) were investigated. Soil was -irradiated for the nonmycorrhizal control. In experiment 1, nonirradiated soil provided the mycorrhizal treatment, whereas in experiment 2 the irradiated soil was inoculated with spores of a fungal culture from P2 soil or a laboratory reference culture, Glomus mosseae. Light intensity was considerably higher in experiment 2 and resulted in a fourfold higher shoot and tenfold higher root biomass. Under the conditions of experiment 1, biomass was significantly higher and Cd, Cu, Zn and Mn concentrations significantly lower in the mycorrhizal plants than in the nonmycorrhizal plants, suggesting a protection against metal toxicity. In contrast, in experiment 2, biomass did not differ between treatments and only Cu root concentration was decreased with G. mosseae-inoculated plants, whereas Cu shoot concentration was significantly increased with the indigenous P2 fungal culture. The latter achieved a significantly higher root colonization than G. mosseae (31.7 and 19.1%, respectively) suggesting its higher metal tolerance. Zn shoot concentration was higher in both mycorrhizal treatments and Pb concentrations, particularly in the roots, also tended to increase with mycorrhizal colonization. Cd concentrations were not altered between treatments. Cu and Zn, but not Pb and Cd root-shoot translocation increased with mycorrhizal colonization. The results show that the influence of AM on plant metal uptake depends on plant growth conditions, on the fungal partner and on the metal, and cannot be generalized. It is suggested that metal-tolerant mycorrhizal inoculants might be considered for soil reclamation, since under adverse conditions AM may be more important for plant metal resistance. Under the optimized conditions of normal agricultural practice, however, AM colonization even may increase plant metal absorption from polluted soils.     

Thiols of Cu-treated maize plants inoculated with the arbuscular-mycorrhizal fungus Glomus intraradices

Mycorrhizal colonization of roots, fresh weight, content of cysteine, γ-glutamylcysteine (γEC). glutathione (GSH), thiol groups in Cu-binding peptides (CuBP), and the uptake of Cu were measured in roots and shoots of maize ( Zea mays L., cv. Honeycomb F-1) grown in quartz sand, with Cu at 0, 4.5, 9, 15 and 30 μg g⁻¹ added with or without inoculum of the arbuscular-mycorrhizal fungus (AMF) Glomus intraradices . In control plants (no Cu added) AMF significantly reduced shoot growth, but did not affect root growth. At an external Cu supply of 9 μg (g quartz sand)⁻¹ or higher, both mycorrhizal colonization and growth of roots and shoots of mycorrhizal and non-mycorrhizal plants were significantly reduced.
With up to 9 μg Cu g⁻¹, mycorrhizal colonization increased the content of cysteine, γEC and GSH in the roots. However, the amount of thiols in CuBPs was not increased by mycorrhizal colonization in Cu-treated plants and no differences in Cu uptake were detected between non-mycorrhizal and mycorrhizal plants. A CuBP-complex with a relative molecular mass of 7300 and a SH:Cu ratio of 1.77:1 was separated on a Sephadex G-50 column from both non-inoculated and inoculated roots of Cu-treated plants. HPLC chromatography of the CuBPs of both non-inoculated and inoculated roots resulted in a similar peak pattern, indicating that no additional CuBPs were formed by the fungus. In conclusion, our results do not support the idea that AMF protects maize from Cu-toxicity.     

Effectiveness of autochthonous bacterium and mycorrhizal fungus on Trifolium growth, symbiotic development and soil enzymatic activities in Zn contaminated soil

AIMS: This study investigates how autochthonous micro-organisms [bacterium and/or arbuscular mycorrhizal (AM) fungi] affected plant tolerance to Zn contamination. METHODS AND RESULTS: Zinc-adapted and -nonadapted Glomus mosseae strains protected the host plant against the detrimental effect of Zn (600 microg g(-1)). Zn-adapted bacteria increased root growth and N, P nutrition in plants colonized by adapted G. mosseae and decreased the specific absorption rate (SAR) of Cd, Cu, Mo or Fe in plants colonized by Zn-nonadapted G. mosseae. Symbiotic structures (nodule number and extraradical mycelium) were best developed in plants colonized by those Zn-adapted isolates that were the most effective in increasing plant Zn tolerance. The bacterium also increased the quantity and quality (metabolic characteristics) of mycorrhizal colonization, with the highest improvement for arbuscular vitality and activity. Inocula also enhanced soil enzymatic activities (dehydrogenase, beta-glucosidase and phosphatase) and indol acetic acid (IAA) accumulation, particularly in the rhizosphere of plants inoculated with Zn-adapted isolates. CONCLUSIONS: Glomus mosseae strains have a different inherent potential for improving plant growth and nutrition in Zn-contaminated soil. The bacterium increased the potential of mycorrhizal mycelium as inoculum. SIGNIFICANCE AND IMPACT OF THE STUDY: Mycorrhizal performance, particularly that of the autochthonous strain, was increased by the bacterium and both contributed to better plant growth and establishment in Zn-contaminated soils.     

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.     

Arbuscular mycorrhiza infection enhances the growth response of Lolium perenne to elevated atmospheric pCO(2)

Elevated atmospheric pCO(2) increases the C-availability for plants and thus leads to a comparable increase in plant biomass production and nutrient demand. Arbuscular mycorrhizal fungi (AMF) are considered to play an important role in the nutrient uptake of plants as well as to be a significant C-sink. Therefore, an increased colonization of plant roots by AMF is expected under elevated atmospheric pCO(2). To test these hypotheses, Lolium perenne L. plants were grown from seeds in a growth chamber in pots containing a silica sand/soil mixture for 9 weeks with and without inoculation with Glomus intraradices (Schenck and Smith). The growth response of plants at two different levels of N fertilization (1.5 or 4.5 mM) combined with ambient (35 Pa) and elevated atmospheric pCO(2) (60 Pa) was compared. The inoculation with G. intraradices, the elevated atmospheric pCO(2) and the high N fertilization treatment all led to an increased plant biomass production of 16%, 20% and 49%, respectively. AMF colonization and high N fertilization increased the plant growth response to elevated atmospheric pCO(2); the plant growth response to high N fertilization was also increased by AMF colonization. The root/shoot ratio was reduced by high N fertilization or elevated atmospheric pCO(2), but was not affected by AMF colonization. The unchanged specific leaf area indicated that if AMF colonization represented an increased C-sink, this was fully covered by the plant. Elevated atmospheric pCO(2) strongly increased AMF colonization (60%) while the high N fertilization had a slightly negative effect. AMF colonization neither improved the N nor P nutrition status, but led to an improved total P uptake. The results underline the importance of AMF for the response of grassland ecosystems to elevated atmospheric pCO(2).     

Agrobacterium rhizogenes-transformed roots of Medicago truncatula for the study of nitrogen-fixing and endomycorrhizal symbiotic associations

Medicago truncatula, a diploid autogamous legume, is currently being developed as a model plant for the study of root endosymbiotic associations, including nodulation and mycorrhizal colonization. An important requirement for such a plant is the possibility of rapidly introducing and analyzing chimeric gene constructs in root tissues. For this reason, we developed and optimized a convenient protocol for Agrobacterium rhizogenes-mediated transformation of M. truncatula. This unusual protocol, which involves the inoculation of sectioned seedling radicles, results in rapid and efficient hairy root organogenesis and the subsequent development of vigorous "composite plants." In addition, we found that kanamycin can be used to select for the cotransformation of hairy roots directly with gene constructs of interest. M. truncatula composite plant hairy roots have a similar morphology to normal roots and can be nodulated successfully by their nitrogen-fixing symbiotic partner, Sinorhizobium meliloti. Furthermore, spatiotemporal expression of the Nod factor-responsive reporter pMtENOD11-gusA in hairy root epidermal tissues is indistinguishable from that observed in Agrobacterium tumefaciens-transformed lines. M. truncatula hairy root explants can be propagated in vitro, and we demonstrate that these clonal lines can be colonized by endomycorrhizal fungi such as Glomus intraradices with the formation of arbuscules within cortical cells. Our results suggest that M. truncatula hairy roots represent a particularly attractive system with which to study endosymbiotic associations in transgenically modified roots.     

A mycorrhiza-responsive protein in wheat roots

A small protein, designated Myk15, was found to be strongly induced in wheat ( Triticum aestivum) roots colonized by the arbuscular mycorrhizal fungus Glomus intraradices. This protein, which is most abundant in root fractions characterized by strong mycorrhizal colonization, has been characterized using two-dimensional polyacrylamide gel electrophoresis and microsequencing. It has an apparent molecular mass of 15 kDa and an isoelectric point of 4.5. The N-terminal sequence has high similarity to a peptide sequence deduced from an expressed sequence tag (EST) clone derived from Medicago truncatula roots colonized by G. intraradices. This EST clone is predicted to code for a protein with a similar size and isoelectric point as Myk15. The N-terminus of the deduced M. truncatula protein contains a highly hydrophobic stretch of 24 amino acid residues preceding the region with high similarity to the Myk15 N-terminus. This hydrophobic stretch is predicted to form a transmembrane alpha-helix and may correspond to a cleavable targeting domain.     

Mycorrhizal fungi and nonhydraulic root signals of soil drying

We propose that mycorrhizal colonization of roots alters nonhydraulic root to shoot communication of soil drying. Split-root rose (Rosa hybrida L. cv Samantha) plants—one side of the root system colonized by Glomus intraradices Schenck & Smith, the other side nonmycorrhizal—displayed different stomatal conductances upon partial drying, depending upon whether mycorrhizal or nonmycorrhizal roots were dried. No differences in leaf water status were observed among control plants and those whose mycorrhizal or nonmycorrhizal roots were dried.     

Fatty Acid Methyl Ester Profiles for Characterization of Glomalean Fungi and Their Endomycorrhizae

Arbuscule-forming fungi in the order Glomales form obligate endomycorrhizal associations with plants that make them difficult to quantify, and taxonomy of the group is only beginning to be objectively understood. Fatty acid methyl ester (FAME) profiles were analyzed to assess the diversity and quantity of fatty acids in 53 isolates of 24 glomalean species. Spores and endomycorrhizal roots of sudan grass (Sorghum sudanense) and the citrus rootstock Carrizo citrange (Poncirus trifoliata x Citrus sinensis) were examined. Spores yielded reproducible FAME profiles from replicate spore collections extracted from soil pot cultures despite being grown in association with a host plant and with contaminating microorganisms present. Unweighted pair group analysis revealed relatively tight clusters of groups at the intraspecific, specific, and generic levels; however, lipid profiles at the family level were convergent. Thus, FAME profile comparisons provided a robust measure of similarity below the family level. FAME profiles in sudan grass roots containing vesicles and/or spores of Glomus intraradices were more similar to spore profiles than to profiles from nonmycorrhizal roots. The FAME profiles for Gigaspora species, which do not form vesicles or spores in roots, were less distinct from nonmycorrhizal roots. G. intraradices and G. rosea produced fatty acids in roots that were distinguishable from each other as well as from the host root. Production in citrus roots of the fatty acid 16:1(inf(omega)5) cis by two Glomus species was correlated with the development of mycorrhizal colonization as measured by clearing and staining procedures and by estimates of total incidence and vesicle intensity. FAME analysis of roots not only provided a measure of colonization development but also served as an index of carbon allocated to intraradical fungal growth and lipid storage.     

Endo- and ectomycorrhizas in Quercus agrifolia Nee. (Fagaceae): patterns of root colonization and effects on seedling growth

We documented the patterns of root occupancy by Glomalean and ectomycorrhizal (EM) fungi in Quercus agrifolia, and host plant responses to inoculation with each mycorrhizal type alone or in combination. Glomalean hyphae, coils and vesicles, and EM root tips were recorded. Colonization patterns conformed to a succession from Glomalean and EM fungi in 1-year-old seedlings to predominantly EM in saplings (>11 years old); both mycorrhizal types were rarely detected within the same root segment. Inoculation of Q. agrifolia seedlings with EM or Glomalean fungi (AM) alone or in combination (EM+AM) altered the cost:benefit relationship of mycorrhizas to the host plant. Seedling survival, plant biomass, foliar nitrogen (N), and phosphorus (P) status were greatest in EM- or AM-only inoculated seedlings. Seedlings inoculated with both mycorrhizal types (AM+EM) exhibited the lowest survival rates, biomass, foliar N, and P levels. Roots of these plants were highly colonized by both EM (38% root length colonized) and Glomalean fungi (34%). Because these levels of colonization were similar to those detected in 1-year-old field seedlings, the presence of both mycorrhizal types may be a carbon cost and, in turn, less beneficial to oaks during establishment in the field. However, the shift to EM colonization in older plants suggests that mycorrhizal effects may become positive with time.