Association of highly and weakly mycorrhizal seedlings can promote the extra- and intraradical development of a common mycorrhizal network

Arbuscular mycorrhizal fungi (AMF) are key determinants of plant interactions in ecosystems. Through their effects on competition, they are regulators of the structure of communities. Conversely, the composition of plant assemblages may also influence the AMF colonization dynamics of plant species. Here, we tested under in vitro culture conditions the effects of Medicago truncatula, a highly mycorrhizal plant species, and Silene vulgaris, a weakly mycorrhizal plant species, grown single (monospecies treatments) or in combination (bispecies treatment) on the colonization dynamics of the AMF Rhizophagus irregularis MUCL 43194. The seedlings were placed in a pre-established hyphal network developing from a mature M. truncatula mycorrhizal donor plant. Extraradical mycelium (ERM) and root colonization parameters as well as root morphology were measured over a period of 12 days. An increased ERM length, total root colonization and proportion of arbuscules were noted in the bispecies treatment. Conversely, the bispecies treatment seemed to have no effect on root growth. This study also demonstrated the suitability of the in vitro culture system for studying the interactions between AMF and host plants grown as mono- and bispecies combinations.     

丛枝菌根真菌参与下植物-土壤系统的养分交流及调控

近几年随着有机农业的发展,丛枝菌根的作用受到特别关注。丛枝菌根是由植物根系与丛枝菌根真菌(AMF)形成的一种共生体。在植物-AMF-土壤系统中,AMF为植物提供N、P等营养的同时从根系得到所需的C。概述了植物-AMF-土壤系统中C、N、P等营养物质的交流以及AMF与土壤微生物的互作关系。丛枝菌根的形成可显著提高植物对P的吸收,且在高P条件下多余的P可储存于AMF中。AMF对土壤N循环的影响相当复杂,可能参与调控N循环的多个过程,如硝化作用、反硝化作用和氨氧化作用等。在有机质丰富的土壤中AMF菌丝可快速扩增并吸收其中的N,主要供菌丝自身所需,只有一小部分传递给植物。AMF对土壤C库的影响尚存争议,可能存在时间尺度的差异。短期内可活化土壤C,而在长期尺度上可能有利于土壤C的储存。AMF能够通过改变土壤微生物群落结构而影响植物-土壤体系的物质交流。AMF与解磷菌、根瘤菌和放线菌的协同增效作用可促进土壤有机质的降解或增强其固氮能力;AMF对氨氧化菌的抑制作用可降低氨的氧化减少N2O的释放。AMF与外生共生真菌EMF共存时,表现出协同增效作用,但EMF的优先定殖会限制AMF的侵染。AMF不同类群之间则主要表现为竞争和拮抗关系。AMF与土壤微生物之间的互作关系受土壤无机环境的影响,在养分亏缺条件下微生物之间往往表现为竞争关系。因植物、AMF与土壤微生物之间存在复杂的互作关系,为此AMF并不总是表现出其对植物营养的促进作用。目前关于AMF的作用机理仍以假说为主,需要进一步的实验验证。在植物-AMF-土壤系统中N与C的交流和P与C的交流并未表现出一致性,对N、P循环相互关系的进一步探讨有助于深入理解植物-土壤体系中的养分循环。植物、AMF和土壤微生物的养分来源及其对养分的相对需求强度和吸收效率尚未可知,因此无法深入理解AMF在植物-土壤体系中养分交流和转化的作用。在方法上,传统的土壤学方法在养分动态研究中存在局限性,现代分子生物学手段和化学计量学的结合值得尝试。     

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.     

Benefit, cost and water-use efficiency of arbuscular mycorrhizal durum wheat grown under drought stress

 Arbuscular mycorrhizal fungi (AMF) living symbiotically with host plants enhance plant growth by improving the acquisition of mineral nutrients and water relations. This study determined the effects of AMF inoculation on growth, benefit/cost and water-use efficiency (grams dry matter produced per kilogram water evapotranspired) in two durum wheat genotypes (drought sensitive and drought tolerant) under water-stressed and well-watered conditions. Plants were grown in a low-P silty clay (Typic Xerochrept) soil mix in a greenhouse. Shoot and root dry matter (DM) and root AMF colonization were higher for well-watered than for water-stressed plants. The mycorrhizal plants were more water-use efficient than nonmycorrhizal plants. Shoot DM differences between mycorrhizal and nonmycorrhizal plants represent the benefit derived by plants from AMF-root associations. Shoot DM differences between mycorrhizal and nonmycorrhizal plants under similar conditions of water treatment represent the cost to the plant of AMF-root associations. Values of benefit/cost for AMF-root associations were highest when plants were water-stressed and decreased under well-watered conditions. Genotypic differences in calculated costs and benefits were pronounced. Benefit/cost analysis may be helpful in evaluating host plant genotypes in order to optimize efficiencies of AMF symbiosis under different environmental conditions. Accepted: 4 April 1998     

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).     

Root fungal symbionts interact with mammalian herbivory, soil nutrient availability and specific habitat conditions

Herbivory, competition and soil fertility interactively shape plant communities and exhibit an important role in modifying conditions for host-dependent fungal symbionts. However, field studies on the combined impacts of natural herbivory, competition and soil fertility on root fungal symbionts are rare. We asked how mammalian herbivory, fertilization, liming and plant–plant competition affect the root colonization of arbuscular mycorrhizal fungi (AMF) and dark septate endophytic (DSE) fungi of the dicot herb, Solidago virgaurea. The 2-year full-factorial experiment was conducted in two contrasting habitats: non-acidic and acidic mountain tundra. We found that herbivory increased arbuscular colonization (i.e. the site of resource exchange) at fertile non-acidic sites, where vegetation was rich in species having AMF symbionts, whereas at infertile acidic sites, where plants having AMF symbiont are scarce, the response was the opposite. Herbivory of the host plant negatively affected DSE hyphal and sclerotial colonization in unfertilized plots, possibly due to reduced carbon flow from the host plant while there was no effect of herbivory in fertilized plots. DSE colonization was highest in unfertilized exclosures where soil nutrient concentrations were also lowest. Liming had a negative effect on DSE hyphal colonization, and its effect also interacted with herbivory and the habitat. Biomass removal of the neighboring plants did not affect the root colonization percent of either arbuscules or DSE. Our results show that the impacts of aboveground mammalian herbivory, soil nutrient availability and specific habitat conditions on belowground root fungal symbionts are highly dependent on each other. Arbuscule response to herbivory appeared to be regulated by specific habitat conditions possibly caused by differences in the AMF availability in the soil while DSE response was associated with availability of host-derived carbon. Our result of the relationship between herbivory and soil nutrients suggests an important role of DSE in ecosystem processes.     

Fire effects on mycorrhizal symbiosis and root system architecture in southern African savanna grasses

Mycorrhizal symbiosis is a key factor influencing aspects of grassland and savanna structure and functioning including plant growth, competition, population and community dynamics, and responses to fire and herbivory. This study assessed the effects of fire on mycorrhizal symbiosis and root system architecture (RSA) in South African savanna grasses. Eighteen grass species were sampled across contrasting fire frequency treatments in the Kruger National Park experimental burn plots. All eighteen species studied were highly colonized by arbuscular mycorrhizal fungi (AMF). Both mycorrhizal symbiosis and RSA were strongly affected by fire, with an increase in AMF colonization and a decrease in root branching and fine root development with decreasing fire frequency. Greater water limitation in frequently burned savanna may result in greater fine root development, thus reducing plant dependency on AMF for acquisition of soil resources. Reduced mycorrhizal colonization in frequently burned savanna may also be driven by higher phosphorus : nitrogen ratios, or indirect effects related to higher grazing intensities in frequently burned sites.     

Nitrogen limitation impairs plant control over the arbuscular mycorrhizal symbiosis in response to phosphorus and shading in two European sand dune species

The symbiosis of plants with arbuscular mycorrhizal fungi (AMF) may become parasitic if the cost:benefit ratio (carbon:phosphorus ratio) increases. In case of mycorrhizal parasitism, a plant may prevent growth depression through the reduction of root colonization as a form of control over the symbiosis. In this greenhouse study, we attempted to manipulate the cost:benefit ratio of the arbuscular mycorrhizal symbiosis by shading and/or phosphorus (P) fertilization in the differentially mycotrophic plant species Hieracium pilosella and Corynephorus canescens. By repeated sampling of soil cores, we assessed the temporal progress of plant investment towards mycorrhizal structures as a measure of plant control over the AMF. Unexpectedly, we found no obvious treatment effects on mycorrhizal growth dependency (MGD), most likely caused by constant N-limitation in AM plants being enhanced by P-fertilization and shade probably not exacerbating plant C-budget for AMF. This highlights the importance of N:P:C stoichiometry for the outcome of the symbiosis. Nevertheless, we found possible control mechanisms in shaded H. pilosella, with considerably higher resource investments into root than into hyphal growth, while root colonization was only marginally suppressed. This control only manifested after 4 weeks of growth under potentially detrimental conditions, emphasizing the importance of time in plant control over the arbuscular mycorrhizal symbiosis. In contrast, the less mycotrophic C. canescens did not exhibit obvious changes in mycorrhizal investments in reaction to shading and P-fertilization, possibly because the low mycotrophy and AMF colonization already imposes a functioning control mechanism in this species. Our study suggests that highly mycotrophic plants may have a stronger need to keep AMF in check than less mycotrophic plants, which may have implications for the role of mycotrophy in the outcome of symbiotic interactions in natural situations.     

Influence of arbuscular mycorrhizal colonization on whole‐plant respiration and thermal acclimation of tropical tree seedlings

Symbiotic arbuscular mycorrhizal fungi (AMF) are ubiquitous in tropical forests. AMF play a role in the forest carbon cycle because they can increase nutrient acquisition and biomass of host plants, but also incur a carbon cost to the plant. Through their interactions with their host plants they have the potential to affect how plants respond to environmental perturbation such as global warming. Our objective was to experimentally determine how plant respiration rates and responses to warmer environment are affected by AMF colonization in seedlings of five tropical tree species at the whole plant level. We evaluated the interaction between AMF colonization and temperature on plant respiration against four possible outcomes; acclimation does or does not occur regardless of AMF, or AMF can increase or decrease respiratory acclimation. Seedlings were inoculated with AMF spores or sterilized inoculum and grown at ambient or elevated nighttime temperature. We measured whole plant and belowground respiration rates, as well as plant growth and biomass allocation. There was an overall increase in whole plant, root, and shoot respiration rate with AMF colonization, whereas temperature acclimation varied among species, showing support for three of the four possible responses. The influence of AMF colonization on growth and allocation also varied among plant species. This study shows that the effect of AMF colonization on acclimation differs among plant species. Given the cosmopolitan nature of AMF and the importance of plant acclimation for predicting climate feedbacks a better understanding of the patterns and mechanisms of acclimation is essential for improving predictions of how climate warming may influence vegetation feedbacks.     

禾草内生真菌对其它微生物的影响研究进展

禾草内生真菌在宿主植物的茎叶等地上组织中普遍存在,不仅能够提高禾草对生物与非生物逆境的抗性,而且能够对周围环境中的不同微生物类群产生影响。主要总结了禾草Neotyphodium/Epichlo内生真菌对病原真菌、丛枝菌根真菌和土壤微生物的影响及其作用机理。发现禾草内生真菌普遍存在对病原真菌的抑制作用,而对丛枝菌根真菌存在不对称的竞争作用,且因种类而异。禾草内生真菌对土壤微生物群落的作用则会随着土壤类型和时间等外界因素发生变化。禾草内生真菌对不同类群微生物的影响机制主要包括:通过生态位竞争、抑菌物质分泌、诱导抗病性等对病原真菌造成影响;通过根系化学物质释放、营养元素调节、侵染条件差异等对丛枝菌根真菌造成影响;通过根际沉积物和凋落物等对土壤微生物群落造成影响。禾草内生真菌产生的生物碱能提高宿主植物对包括昆虫在内草食动物采食的抗性,影响病原菌的侵入、定殖和扩展;根组织分泌物中包含次生代谢产物能够抑制菌根真菌、土传病原真菌及其它土壤微生物的侵染与群落组成;也可能通过次生代谢物影响禾草的其它抗性。因此,禾草内生真菌在植物-微生物系统中的作用应该给予更多的关注和深入研究。     

Chemical defense,mycorrhizal colonization and growth responses in <Emphasis Type="Italic">Plantago lanceolata</Emphasis> L.

Allelochemicals defend plants against herbivore and pathogen attack aboveground and belowground. Whether such plant defenses incur ecological costs by reducing benefits from plant mutualistic symbionts is largely unknown. We explored a potential trade-off between inherent plant chemical defense and belowground mutualism with arbuscular mycorrhizal fungi (AMF) in Plantago lanceolata L., using plant genotypes from lines selected for low and high constitutive levels of the iridoid glycosides (IG) aucubin and catalpol. As selection was based on IG concentrations in leaves, we first examined whether IG concentrations covaried in roots. Root and leaf IG concentrations were strongly positively correlated among genotypes, indicating genetic interdependence of leaf and root defense. We then found that root AMF arbuscule colonization was negatively correlated with root aucubin concentration. This negative correlation was observed both in plants grown with monocultures of Glomus intraradices and in plants colonized from whole-field soil inoculum. Overall, AMF did not affect total biomass of plants; an enhancement of initial shoot biomass was offset by a lower root biomass and reduced regrowth after defoliation. Although the precise effects of AMF on plant biomass varied among genotypes, plants with high IG levels and low AMF arbuscule colonization in roots did not produce less biomass than plants with low IG and high AMF arbuscule colonization. Therefore, although an apparent trade-off was observed between high root chemical defense and AMF arbuscule colonization, this did not negatively affect the growth responses of the plants to AMF. Interestingly, AMF induced an increase in root aucubin concentration in the high root IG genotype of P. lanceolata. We conclude that AMF does not necessarily stimulate plant growth, that direct plant defense by secondary metabolites does not necessarily reduce potential benefits from AMF, and that AMF can enhance concentrations of root chemical defenses, but that these responses are plant genotype-dependent.     

接种AM真菌对玉米和油菜种间竞争及土壤无机磷组分的影响

在温室盆栽条件下,分别模拟单作、间作和尼龙网分隔种植,比较接种丛枝菌根(arbuscular mycorrhizal, AM)真菌Glomus intraradices和Glomus mosseae对菌根植物玉米和非菌根植物油菜生长和磷吸收状况的影响,并分析土壤中各无机磷组分的变化。结果发现,接种AM真菌可以促进土壤中难溶性磷(Ca₁₀-P和O-P)向有效态磷转化,并显著降低总无机磷含量 (P<0.05),显著提高菌根植物玉米的生物量和磷吸收量(P<0.05),特别是在间作体系中使玉米的磷营养竞争比率显著提高了45.0%-104.1% (P<0.05),显著降低了油菜的生物量和磷吸收量(P<0.05),从而增强了了菌根植物的竞争优势,降低了非菌根植物与菌根植物的共存能力。揭示了石灰性土壤中AM真菌对植物物种多样性的影响,有助于更加全面地理解AM真菌在农业生态系统中的作用。     

接种AMF对菌根植物和非菌根植物竞争的影响

为了研究丛枝菌根真菌(arbuscular mycorrhizal fungus, AMF)对菌根植物与非菌根植物种间竞争的影响,以玉米(菌根植物)和油菜(非菌根植物)作为供试植物,分别进行间作、尼龙网分隔和单作,模拟这两种植物之间不同的竞争状态,接种丛枝菌根真菌Glomus intraradices和Glomus mosseae,比较菌根植物和非菌根植物的生长和磷营养状况,分析AMF侵染对植物种间竞争作用的影响。结果显示,与单作相比,间作模式下玉米的生物量及磷营养状况有所降低,但其菌根依赖性却有所提高。与不接种相比,接种处理显著降低了间作体系油菜根系的磷含量和磷吸收量,但趋于改善菌根植物玉米的磷营养状况。因此,接种AMF可以降低非菌根植物的磷营养状况及生物量,使得菌根植物的相对竞争能力明显提高,说明AMF在维持物种多样性方面有着重要的作用。     

Effects of co-inoculation with arbuscular mycorrhizal fungi and rhizobia on soybean growth as related to root architecture and availability of N and P

Soybean plants can form tripartite symbiotic associations with rhizobia and arbuscular mycorrhizal (AM) fungi, but little is known about effects of co-inoculation with rhizobia and AM fungi on plant growth, or their relationships to root architecture as well as nitrogen (N) and phosphorus (P) availability. In the present study, two soybean genotypes contrasting in root architecture were grown in a field experiment to evaluate relationships among soybean root architecture, AMF colonization, and nodulation under natural conditions. Additionally, a soil pot experiment in greenhouse was conducted to investigate the effects of co-inoculation with rhizobia and AM fungi on soybean growth, and uptake of N and P. Our results indicated that there was a complementary relationship between root architecture and AMF colonization in the field. The deep root soybean genotype had greater AMF colonization at low P, but better nodulation with high P supply than the shallow root genotype. A synergistic relationship dependent on N and P status exists between rhizobia and AM fungi on soybean growth. Co-inoculation with rhizobia and AM fungi significantly increased soybean growth under low P and/or low N conditions as indicated by increased shoot dry weight, along with plant N and P content. There were no significant effects of inoculation under adequate N and P conditions. Furthermore, the effects of co-inoculation were related to root architecture. The deep root genotype, HN112, benefited more from co-inoculation than the shallow root genotype, HN89. Our results elucidate new insights into the relationship between rhizobia, AM fungi, and plant growth under limitation of multiple nutrients, and thereby provides a theoretical basis for application of co-inoculation in field-grown soybean.     

菌根真菌与植物共生营养交换机制研究进展

菌根是陆地生态系统普遍存在的、由土壤中的菌根真菌侵染宿主植物根系形成的联合共生体.菌根的建立是以共生体双方的营养交换为基础的:菌根真菌从土壤中吸收氮、磷等营养物质并转运给宿主植物,供其生长;作为交换,植物则以脂质或糖的形式向菌根真菌提供其生长所必需的碳水化合物.近年来,菌根真菌与宿主植物间的营养交换机制一直是研究的热点,国内外对菌根真菌介导的植物营养物质吸收和转运机制的研究也取得了巨大进展.本文综述了丛枝和外生两种菌根真菌与宿主植物间营养交换的最新研究进展,尤其是碳、氮、磷等几种重要营养物质的吸收与双向转运机制,以及营养交换在菌根形成中的潜在调控作用,并对目前存在的关键问题和未来研究方向进行了分析和展望,这对菌根模型的建立及菌根效益的优化具有重要意义.     

Independent effects of arbuscular mycorrhiza and earthworms on plant diversity and newcomer plant establishment

Questions: How do arbuscular mycorrhiza and earthworms affect the structure and diversity of a ruderal plant community? Is the establishment success of newcomer plants enhanced by these soil organisms and their interactions? Methods: We grew a native ruderal plant community composed of different functional groups (grasses, legumes and forbs) in the presence and absence of arbuscular mycorrhizal fungi (AMF) and endogeic earthworms in mesocosms. We introduced seeds of five, mainly exotic, plant species from the same functional groups after a disturbance simulating mowing. The effects of the soil organisms on the native ruderal plant community and seedling establishment of the newcomer plants were assessed. Results: After disturbance, the total above‐ground regrowth of the native plant community was not affected by the soil organisms. However, AMF increased plant diversity and shoot biomass of forbs, but decreased shoot biomass of grasses of the native plant community. Earthworms led to a reduction in total root biomass. Establishment of the introduced newcomer plants increased in the presence of AMF and earthworms. Especially, seedling establishment of the introduced non‐native legume Lupinus polyphyllus and the native forb Plantago lanceolata was promoted in the presence of AMF and earthworms, respectively. The endogeic earthworms gained more weight in the presence of AMF and led to increased extraradical AMF hyphal length in soil. However, earthworms did not seem to modify the effect of AMF on the plant community. Conclusion: The present study shows the importance of mutualistic soil organisms in mediating the establishment success of newcomer plants in a native plant community. Mutualistic soil organisms lead to changes in the structure and diversity of the native plant community and might promote newcomer plants, including exotic species.     

Growth of mycorrhizal tomato and mineral acquisition under salt stress

 High salt levels in soil and water can limit agricultural production and land development in arid and semiarid regions. Arbuscular mycorrhizal fungi (AMF) have been shown to decrease plant yield losses in saline soils. The objective of this study was to examine the growth and mineral acquisition responses of greenhouse-grown tomato to colonization by the AMF Glomus mosseae [(Nicol. And Gerd.) Gerd. and Trappe] under varied levels of salt. NaCl was added to soil in the irrigation water to give an EC_e of 1.4 (control), 4.7 (medium) and 7.4 dS m^–1 (high salt stress). Plants were grown in a sterilized, low P (silty clay) soil-sand mix. Mycorrhizal colonization was higher in the control than in saline soil conditions. Shoot and root dry matter yields and leaf area were higher in mycorrhizal than in nonmycorrhizal plants. Total accumulation of P, Zn, Cu, and Fe was higher in mycorrhizal than in nonmycorrhizal plants under both control and medium salt stress conditions. Shoot Na concentrations were lower in mycorrhizal than in nonmycorrhizal plants grown under saline soil conditions. The improved growth and nutrient acquisition in tomato demonstrate the potential of AMF colonization for protecting plants against salt stress in arid and semiarid areas. Accepted: 21 February 2000     

丛枝菌根真菌和磷水平对番茄幼苗侧根形成的影响

为了探讨丛枝菌根真菌(AMF)和磷水平对植物根系构型的影响,在两个磷水平下对番茄幼苗接种AMF菌株Rhizophagus irregularis BGC JX04B,研究AMF和磷水平对番茄幼苗侧根形成的影响.结果表明: AMF对植株生物量的促进效应不明显,但显著降低了植株的根冠比;显著增加了主根长而减少了1级侧根长,并与侵染时期存在互作;显著降低了2～3级侧根数量及2级侧根数与1级侧根数的比值,对1~2级侧根密度无显著影响.高磷(50 mg·kg^-1P)显著促进了植株生长,降低了植株的根冠比;对主根长和1级侧根长无显著影响,显著增加了1~3级侧根数量及2级侧根数与1级侧根数的比值,提高了1~2级侧根密度.表明AMF和磷水平对番茄侧根形成的影响机制不同,高磷的影响可能基于对养分吸收和生长的促进效应,而AMF的影响则更为复杂,且AMF与侵染时期的互作效应预示着碳素分配(糖信号)可能参与了AMF对根系构型的调控.     

Mycorrhizal fungi as mediators of defence against insect pests in agricultural systems

• 1 Below‐ground organisms influence above‐ground interactions in both natural and agricultural ecosystems. Among the most important below‐ground organisms are mycorrhizal fungi, comprising ubiquitous and ancient plant mutualists that have significant effects on plant growth and fitness mediated by resource exchange with plants. In the present study, we focus on the effects of arbuscular mycorrhizal fungi (AMF) on crop defence against insect pests.
• 2 AMF alter the availability of resources used by crop plants to manufacture defences against pests and to compensate for pest damage. However, AMF also provide plants with nutrients that are known to increase insect performance. Through potentially opposing effects on plant nutritional quality and defence, mycorrhizal fungi can positively or negatively affect pest performance.
• 3 Additionally, AMF may directly affect gene expression and plant defence signalling pathways involved in the construction and induction of plant defences, and these effects are apparently independent of those caused by nutrient availability. In this way, AMF may still influence plant defences in the fertilized and highly managed systems typical of agribusiness.
• 4 Because AMF can affect plant tolerance to pest damage, they may have a significant impact on the shape of damage–yield relationships in crops. Potential mechanisms for this effect are suggested.
• 5 We highlight the need for continuing research on the effects of AMF identity and the abundance on crop defences and tolerance to pest attack. Much work is needed on the potential effects of mycorrhizal colonization on plant signalling and the induction of direct and indirect defences that may protect against pest damage.

     

Evidence of a mycorrhizal mechanism for the adaptation of Andropogon gerardii (Poaceae) to high- and low-nutrient prairies

Andropogon gerardii seed obtained from Kansas and Illinois was grown in a controlled environment in their own and each other's soils, with and without arbuscular mycorrhizal fungi (AMF). Each ecotype grew comparatively better in its own soil indicating adaptation to its soil of origin. Overall, A. gerardii benefited more from AMF in low-nutrient Kansas soil than Illinois soil. The two ecotypes, however, did not benefit equally from mycorrhizal infection. The Kansas ecotype was three times more responsive to mycorrhizal infection in the Kansas soil than was the Illinois ecotype. Our results indicate that plant adaptation to the nutrient levels of their local soils is likely to be due, at least in part, to a shift in their dependence on mycorrhizal fungi. The Illinois ecotype of A. gerardii has evolved a reduced dependence upon these fungi and greater reliance on a more highly branched root system. In contrast, the Kansas ecotype had a significantly coarser root system and invested proportionately greater carbon in the symbiotic association with AMF as measured by spore production. This study provides the first demonstration that plants can adapt to changing soil nutrient levels by shifting their dependence on AMF. This result has broad implications for our understanding of the role of these fungi in agricultural systems.