Early Root Herbivory Impairs Arbuscular Mycorrhizal Fungal Colonization and Shifts Defence Allocation in Establishing Plantago lanceolata

Research into plant-mediated indirect interactions between arbuscular mycorrhizal (AM) fungi and insect herbivores has focussed on those between plant shoots and above-ground herbivores, despite the fact that only below-ground herbivores share the same part of the host plant as AM fungi. Using Plantago lanceolata L., we aimed to characterise how early root herbivory by the vine weevil (Otiorhynchus sulcatus F.) affected subsequent colonization by AM fungi (Glomus spp.) and determine how the two affected plant growth and defensive chemistry. We exposed four week old P. lanceolata to root herbivory and AM fungi using a 2×2 factorial design (and quantified subsequent effects on plant biomass and iridoid glycosides (IGs) concentrations. Otiorhynchus sulcatus reduced root growth by c. 64%, whereas plant growth was unaffected by AM fungi. Root herbivory reduced extent of AM fungal colonization (by c. 61%). O. sulcatus did not influence overall IG concentrations, but caused qualitative shifts in root and shoot IGs, specifically increasing the proportion of the more toxic catalpol. These changes may reflect defensive allocation in the plant against further attack. This study demonstrates that very early root herbivory during plant development can shape future patterns of AM fungal colonization and influence defensive allocation in the plant.     

Contrasting impacts of defoliation on root colonization by arbuscular mycorrhizal and dark septate endophytic fungi of Medicago sativa

Individual plants typically interact with multiple mutualists and enemies simultaneously. Plant roots encounter both arbuscular mycorrhizal (AM) and dark septate endophytic (DSE) fungi, while the leaves are exposed to herbivores. AMF are usually beneficial symbionts, while the functional role of DSE is largely unknown. Leaf herbivory may have a negative effect on root symbiotic fungi due to decreased carbon availability. However, evidence for this is ambiguous and no inoculation-based experiment on joint effects of herbivory on AM and DSE has been done to date. We investigated how artificial defoliation impacts root colonization by AM (Glomus intraradices) and DSE (Phialocephala fortinii) fungi and growth of Medicago sativa host in a factorial laboratory experiment. Defoliation affected fungi differentially, causing a decrease in arbuscular colonization and a slight increase in DSE-type colonization. However, the presence of one fungal species had no effect on colonization by the other or on plant growth. Defoliation reduced plant biomass, with this effect independent of the fungal treatments. Inoculation by either fungal species reduced root/shoot ratios, with this effect independent of the defoliation treatments. These results suggest AM colonization is limited by host carbon availability, while DSE may benefit from root dieback or exudation associated with defoliation. Reductions in root allocation associated with fungal inoculation combined with a lack of effect of fungi on plant biomass suggest DSE and AMF may be functional equivalent to the plant within this study. Combined, our results indicate different controls of colonization, but no apparent functional consequences between AM and DSE association in plant roots in this experimental setup.     

不同水肥条件下AM真菌对丹参幼苗生长和营养成分的影响

利用盆栽接种试验,探讨不同水肥条件下AM真菌摩西球囊霉Glomus mosseae对丹参幼苗生长和微量元素的影响,为丹参水肥合理施用提供理论依据。结果表明,不同水肥条件下,接种AM真菌显著提高了根系侵染率和生物量。40%相对含水量、不同施P水平,接种株丹参酮含量升高,总黄酮、丹参素及地下部总酚酸含量降低,植株Zn及地上部Ca、K、Mn、Fe含量升高,而对植株Mg、Cu和地下部Ca、K、Mn、Fe无显著影响;接种效应随施P量不同而变化。70%相对含水量、不同施P水平,接种株药用成分含量显著升高,植株Ca、Mn和地上部K、Cu及地下部Fe和Zn含量升高,而对植株Mg、地下部K、Cu和地上部Fe和Zn含量无显著影响。不同水分和同一施P水平,接种株丹参酮含量升高,地上部Ca、K和地下部Zn含量升高,接种效应因土壤含水量不同而变化,其中以70%含水量时效果最好。说明AM真菌能促进宿主植物根系对水分和矿质元素的吸收与利用,提高水分和P肥利用率,降低水分和P胁迫对丹参的伤害程度,其中以70%相对含水量,施P量为0.16 gP/kg土时AM真菌对丹参的接种效果最佳。     

Effect of arbuscular mycorrhizal fungi inoculation on root traits and root volatile organic compound emissions of Sorghum bicolor

Volatile organic compounds (VOCs) emitted by plant roots have important functions that can influence the rhizospheric environment. The aim of this study was to examine the effects of arbuscular mycorrhizal (AM) fungi on the profile of root VOCs. Sorghum (Sorghum bicolor) plants were grown in pots inoculated with either Glomus mosseae or Glomus intraradices, which formed mycorrhiza with the roots. Control plants were grown in pots inoculated with sterile inoculum and did not form mycorrhiza. Forty-four VOCs were determined using headspace solid-phase microextraction (HS-SPME) and gas chromatography–mass spectrometry (GC-MS). Alkanes were the most abundant type of VOCs emitted by both mycorrhizal and non-mycorrhizal plants. Both the quantity and type of volatiles were dramatically altered by the presence of AM fungi, and these changes had species specificity. Compared with non-mycorrhizal plants, mycorrhizal plants emitted more alcohols, alkenes, ethers and acids but fewer linear-alkanes. The AM fungi also influenced the morphological traits of the host roots. The total root length and specific root length of mycorrhizal plants were significantly greater than those of non-mycorrhizal plants; however, both the incidence and length of root-hair were dramatically decreased. Our findings confirm that AM fungi can alter the profile of VOCs emitted by roots as well as the root morphology of sorghum plants, indicating that AM fungi have the potential to help plants adapt to and alter soil environments.     

Dual colonization of Eucalyptus urophylla S.T. Blake by arbuscular and ectomycorrhizal fungi affects levels of insect herbivore attack

Abstract 1 Eucalypts are an important part of plantation forestry in Asia but, in south China, productivity is very low. This is due to infertile soils and lack of indigenous symbiotic mycorrhizal fungi. The genus Eucalyptus is unusual because it forms both arbuscular (AM) and ectomycorrhizal (ECM) associations. 2 Eucalyptus urophylla saplings were grown with and without AM (Glomus caledonium) and ECM (Laccaria laccata) fungi in a factorial design. Two experiments were performed: one to simulate nursery conditions and the other to simulate the early stages of plantation establishment. Plant growth was measured over 18 weeks and levels of insect attack were recorded. 3 The AM fungus reduced tree growth in the early stages, but the effect appeared to be transient. No effects of ECM were detected on tree growth, but the ectomycorrhiza reduced colonization by the arbuscular mycorrhiza. AM fungi appear to be rapid invaders of the root system, gradually being replaced by ECM. 4 Both fungal types affected levels of damage by insect herbivores. Most importantly, herbivory by the pest insects Anomala cupripes (Coleoptera) and Strepsicrates spp. (Lepidoptera) was decreased by ECM. 5 It is suggested that mycorrhizal effects on eucalypt insects may be determined by carbon allocation within the plant. Future studies of eucalypt mycorrhizas need to take into account the effects of the fungi on foliar‐feeding insects and also the effects of insect herbivory on mycorrhizal establishment.     

水分胁迫下AM真菌对沙打旺生长和抗旱性的影响

利用盆栽试验研究了水分胁迫条件下接种AM真菌对优良牧草和固沙植物沙打旺(Astragalus adsurgens Pall.)生长和抗旱性的影响。在土壤相对含水量为70%、50%和30%条件下,分别接种摩西球囊霉(Glomus mosseae)和沙打旺根际土著菌,不接种处理作为对照。结果表明,水分胁迫显著降低了沙打旺植株(无论接种AM真菌与否)的株高、分枝数、地上部干重和地下部干重,并显著提高了土著AM真菌的侵染率,对摩西球囊霉的侵染率无显著影响。接种AM真菌可以促进沙打旺生长和提高植株抗旱性,但促进效应因土壤含水量和菌种不同而存在差异。不同水分条件下,接种AM真菌显著提高了植株菌根侵染率、根系活力、地下部全N含量和叶片CAT活性。土壤相对含水量为30%和50%时,接种株地上部全N、叶片叶绿素、可溶性蛋白、脯氨酸含量和POD活性显著高于未接种株;接种AM真菌显著降低了叶片MDA含量;接种土著AM真菌的植株株高、分枝数、地上部和地下部干重显著高于未接种株。土壤相对含水量为30%时,接种AM真菌显著增加了地上部全P含量和叶片相对含水量;接种摩西球囊霉的植株株高、分枝数、地上部和地下部干重显著高于未接种株。水分胁迫40d,接种AM真菌显著提高了叶片可溶性糖含量。水分胁迫80d,接种株叶片SOD活性显著增加。菌根依赖性随水分胁迫程度增加而提高。沙打旺根际土著菌接种效果优于摩西球囊霉。水分胁迫和AM真菌的交互作用对分枝数、菌根侵染率、叶片SOD、CAT和POD活性、叶绿素、脯氨酸、可溶性蛋白、地上部全N和全P、地下部全N和根系活力有极显著影响,对叶片丙二醛和地下部全P有显著影响。AM真菌促进根系对土壤水分和矿质营养的吸收,改善植物生理代谢活动,从而提高沙打旺抗旱性,促进其生长。试验结果为筛选优良抗旱菌种,充分利用AM真菌资源促进荒漠植物生长和植被恢复提供了依据。     

Aboveground resource allocation in response to root herbivory as affected by the arbuscular mycorrhizal symbiosis

Aims

Arbuscular mycorrhizal (AM) fungi associate with the majority of terrestrial plants, influencing their growth, nutrient uptake and defence chemistry. Consequently, AM fungi can significantly impact plant-herbivore interactions, yet surprisingly few studies have investigated how AM fungi affect plant responses to root herbivores. This study aimed to investigate how AM fungi affect plant tolerance mechanisms to belowground herbivory.

Methods

We examined how AM fungi affect plant (Saccharum spp. hybrid) growth, nutrient dynamics and secondary chemistry (phenolics) in response to attack from a root-feeding insect (Dermolepida albohirtum).

Results

Root herbivory reduced root mass by almost 27%. In response, plants augmented investment in aboveground biomass by 25%, as well as increasing carbon concentrations. The AM fungi increased aboveground biomass, phosphorus and carbon. Meanwhile, root herbivory increased foliar phenolics by 31% in mycorrhizal plants, and increased arbuscular colonisation of roots by 75% overall. AM fungi also decreased herbivore performance, potentially via increasing root silicon concentrations.

Conclusions

Our results suggest that AM fungi may be able to augment plant tolerance to root herbivory via resource allocation aboveground and, at the same time, enhance plant root resistance by increasing root silicon. The ability of AM fungi to facilitate resource allocation aboveground in this way may be a more widespread strategy for plants to cope with belowground herbivory.

     

Influences of Arbuscular Mycorrhizal Fungus Glomus mosseae on Growth and Nutrition of Lentil Irrigated with Arsenic Contaminated Water

Arsenic (As) contamination of irrigation water represents a major constraint to Bangladesh agriculture. While arbuscular mycorrhizal (AM) fungi have their most significant effect on P uptake, they have also been shown to alleviate metal toxicity to the host plant. This study examined the effects of As and inoculation with an AM fungus, Glomus mosseae, on lentil (Lens culinaris L. cv. Titore). Plants were grown with and without AM inoculum for 9 weeks in a sand and terra green mixture 50:50 v/v and watered with five levels of As (0, 1, 2, 5, 10 mg As L⁻¹ arsenate). Inoculum of Rhizobium leguminosarum b.v. Viceae strain 3841 was applied to all plants. Plants were fed with modified Hoagland solution (1/10 N of a full-strength solution and without P). Plant height, leaf number, pod number, plant biomass and shoot and root P concentration/offtake increased significantly due to mycorrhizal infection. Plant height, leaf/ pod number, plant biomass, root length, shoot P concentration/offtake, root P offtake and mycorrhizal infection decreased significantly with increasing As concentration. However, mycorrhizal inoculation reduced As concentration in roots and shoots. This study shows that growing lentil with compatible AM inoculum can minimise As toxicity and increase growth and P uptake.     

Production of glomus intraradices propagules, an arbuscular mycorrhizal fungus, in an airlift bioreactor

This work addresses the symbiotic culture of the arbuscular mycorrhizal (AM) fungus Glomus intraradices with Daucus carota hairy roots transformed by Agrobacterium rhizogenes, in two submerged culture systems: Petri dish and airlift bioreactor. AM fungi play an active role in plant nutrition and protection against plant pathogens. These fungi are obligate biotrophs as they depend on a host plant for their needs in carbohydrates. The effect of the mycorrhizal roots inoculum-to-medium volume ratio on the growth of both symbionts was studied. A critical inoculating condition was observed at approximately 0.6 g dry biomass (DW). L-1 medium, above which root growth was significantly reduced when using a low-salt minimal (M) liquid medium previously developed for hairy root-AM fungi co-culture. Below critical inoculum conditions the maximum specific root growth and specific G. intraradices spore production rates of 0.021 and 0.035 d-1, respectively, were observed for Petri dish cultures. Maximum spore production in the airlift bioreactor was ten times lower than that of Petri dish cultures and obtained with the lowest inoculum assessed (0.13 g DW. L-1 medium) with 1.82 x 10(5) +/- 4.05 x 10(4) (SEM) spores (g DW inoculum)-1 (L medium)-1 in 107 d. This work proposes a second-generation bioprocess for AM fungi propagule production in bioreactors. Copyright 1999 John Wiley & Sons, Inc.     

Effects of simultaneous infections of endophytic fungi and arbuscular mycorrhizal fungi on the growth of their shared host grass Achnatherum sibiricum under varying N and P supply

Grasses can be infected by endophytic fungi and arbuscular mycorrhizal (AM) fungi simultaneously. Here, we investigated the interactions of a native grass, Achnatherum sibiricum, with both endophytic and AM fungi (Glomus mosseae, GM and Glomus etunicatum, GE) at different nitrogen (N) and phosphorus (P) levels. The results showed that endophyte infection significantly suppressed the colonization rates and spore density of GE, but had no effect on those of GM. Endophyte infection increased shoot biomass regardless of the nutrient conditions. The effects of AM fungi on host growth were dependent on mycorrhizal species. There was no significant interaction between endophytic fungi and GE on host growth; however, a significant interaction between endophytic fungi and GE occurred in total phenolic concentrations and P uptake. As for GM, a significant interaction among endophytic fungi, AM fungi and nutrient availability occurred in shoot growth. Under sufficient N and P conditions, endophyte infection alleviated the detrimental effects of GM colonization on host growth.     

丛枝菌根真菌对柑橘嫁接苗枳/红肉脐橙抗旱性的影响

采用盆栽试验,研究了自然水分胁迫和胁迫解除复水条件下接种AM真菌摩西球囊霉对柑橘嫁接苗枳/红肉脐橙生长和保护系统能力的影响.结果表明,接种AM真菌的柑橘嫁接苗的株高、穗粗、叶面积和新梢生长量显著或极显著地高于未接种植株.在胁迫解除复水第4天,接种AM真菌的根系可溶性蛋白质含量、超氧化物歧化酶(SOD)和过氧化氢酶(CAT)活性显著或极显著高于未接种植株.在自然水分胁迫和胁迫解除复水过程中,接种AM真菌较未接种处理降低叶片丙二醛(MDA)含量,提高可溶性糖和可溶性蛋白质含量,增强SOD、过氧化物酶(POD)、CAT活性,从而增强柑橘嫁接苗的渗透调节和保护防御能力,提高柑橘嫁接苗的抗旱能力.水分和菌根显著交互影响叶片SOD活性.AM真菌提高寄主植物的抗旱性机制可能与寄主植物的保护系统能力的改变有关.     

Improved growth of salinity-stressed soybean after inoculation with salt pre-treated mycorrhizal fungi

Two sets of experiments to determine the effect of mycorrhiza on soybean (Glycine max) growth under saline conditions and to investigate the salt acclimation of mycorrhizal fungi were conducted. In the first experiment, the effect of an arbuscular mycorrhizal (AM) fungus Glomus etunicatum on mineral nutrient, proline and carbohydrate concentrations and growth of soybean. Under different NaCl concentrations (0, 50, 100, 150 and 200mM) was evaluated. Salinity decreased AM colonization. In both the M and nonAM plants shoot and root proline and shoot Na and Zn concentrations were increased under salinity. Soybean plants inoculated with the AM fungus had significantly higher fresh and dry weight, root proline, P, K and Zn but lower shoot proline and Na concentrations compared to the non inoculated plants. In the second experiment, the AM fungus was pre-treated with NaCl (salt acclimation) then was used as inoculum for soybean plants subjected to 100mM NaCl. Root colonization, fresh and dry weight, root proline, P, K and Zn concentrations were greater in soybean plants inoculated with the salt pre-treated fungus, compared to those inoculated with the nonsalt pre-treated fungus. However, for Na, the situation was the opposite. Based on these results, the AM inoculation helps the growth of soybean plants grown in saline conditions. When the AM fungus was pre-treated with NaCl with a gradual increase of concentration, and then exposed to a sudden salt stress, their efficiency was increased. This may be due to the acclimation of the AM fungus to salinity.     

Effects of preplant inoculation of apple (Malus domestica Borkh.) with arbuscular mycorrhizal fungi on population growth of the root-lesion nematode,Pratylenchus penetrans

Six species of arbuscular mycorrhizal (AM) fungi (Glomus aggregatum, G. clarum, G. etunicatum, G. intraradices, G. mosseae and G. versiforme) were evaluated, in three greenhouse experiments, for their effects on reproduction of the root-lesion nematode, Pratylenchus penetrans, and growth of Ottawa 3 apple rootstock. Glomus mosseae increased total dry weights of nematode-inoculated and non-inoculated rootstock in all three greenhouse experiments, and G. intraradices increased dry weights in two of three greenhouse experiments. Plants inoculated with G. mosseae generally supported fewer P. penetrans per gram of root than plants inoculated with other AM fungi, but did not differ significantly from the controls in any greenhouse experiment. Colonization of roots by AM fungi was reduced by P. penetrans at initial inoculum densities greater than 250 nematodes/L soil. In field trials, preplant inoculation with either G. intraradices or G. mosseae increased rootstock growth and leaf concentrations of P, Mg, Zn and Cu in fumigated plots but not in non-fumigated plots, indicating that colonization by native AM fungi in non-fumigated plots may have been sufficient for adequate nutrient acquisition. The abundance of vesicles and arbuscules was greater in roots of plants inoculated with AM fungi before planting than in roots of non-inoculated plants, in both fumigated and non-fumigated plots. P. penetrans per gram of root and per 50 ml soil were significantly lower for G. mosseae- inoculated plants than for non-inoculated plants in fumigated soil but not in non-fumigated soil.     

Community of arbuscular mycorrhizal fungi in a coastal vegetation on Okinawa island and effect of the isolated fungi on growth of sorghum under salt-treated conditions

Community of arbuscular mycorrhizal (AM) fungi in a coastal vegetation on Okinawa island in Japan was examined. A sampling plot was established in a colony of Ipomoea pes-caprae (Convolvulaceae) on the beach in Tamagusuku, Okinawa Pref, in which eight root samples of I. pes-caprae and three root samples each of Vigna marina (Leguminosae) and Paspalum distichum (Poaceae) were collected. Partial 18S rDNA of AM fungi was amplified from the root samples by polymerase chain reaction (PCR) with primers NS31 and AM1. Restriction fragment length polymorphism analysis with HinfI and RsaI for cloned PCR products revealed that two types of Glomus sp., type A and type B, were dominant in the colony. Among them, the fungi of type A were especially dominant near the edge of the colony facing the sea. A phylogenetic analysis showed that the AM fungi of type B are closely related to Glomus intraradices and those of type A are nearly related to type B. From the sequence data, it was also found that type A was further divided into two types, type A1 and A2. One representative strain each of the three types, type A1, A2, and B, propagated from single spore each, was examined for the growth of sorghum (Sorghum bicolor) at three different salinity levels, 0, 100, and 200 mM NaCl. At the non-salt-treated condition, the type B fungus was the most effective on shoot growth enhancement of the host plant, whereas at the salt-treated conditions, the type A2 fungus was the most effective. An efficient suppression of Na + translocation into the shoot by the examined AM fungi was found. These results suggested that the AM fungi dominant near the sea are adapted to salt-stressed environment to alleviate the salt stress of host plants.     

Influence of liming,inoculum level and inoculum placement on root colonization of subterranean clover

Arbuscular mycorrhizal (AM) fungi differ in their response to soil pH. Thus, change in soil pH may influence the relative abundance of mycorrhizal fungi inside roots. Root colonization by two AM fungi was studied in relation to addition of lime (CaCO3), quantity of inoculum and inoculum placement. Addition of CaCO3 to an acid soil decreased the colonization of roots by Acaulospora laevis but increased colonization by Glomus invermaium when both fungi were present. In acid soil (pH 4.7), almost all roots were colonized by A. laevis, while G. invermaium was dominant when soil pH was increased to pH 7.3. This occurred regardless of whether the inoculum was banded or mixed throughout the soil. There was no effect of CaCO3 on the relative abundance of fungi inside roots at intermediate rates of CaCO3 application (pH 5.3-6.3) when both fungi were inoculated together. In this experiment, both fungi colonized roots at all levels of CaCO3 when inoculated alone, except for A. laevis at the highest level of CaCO3. We conclude that soil pH affects the competitive ability of these two AM fungi during mycorrhiza formation primarily by affecting hyphae growth in soil and thus the relative abundance of hyphae at the root surface and subsequently inside the root.     

The dark side of the mycorrhiza

Plant association with arbuscular mycorrhizal (AM) fungi is usually regarded as mutualistic. However, this positive effect could disappear if the benefit of the fungal-plant association changes with colonization density. In order to test the conditionality of this interaction, we evaluated plant performance and tolerance to defoliation across five levels of commercial AM fungal inoculum concentrations. Additionally, we evaluated if plant performance and tolerance were similarly affected by a whole soil community collected under a native congener. Along the gradient of inoculation, plant performance exhibited a peak at intermediate inoculum concentration, indicating the presence of an optimum level of AM fungal concentration that maximized AM fungal benefit. Root colonization by fungal hyphae increased linearly across the experimental inoculation gradient. Paralleling root colonization, plant tolerance to defoliation decreased linearly along the inoculum gradient. Plant performance was similar under the whole soil and commercial treatments. Our results show a negative correlation between tolerance to defoliation and AM fungal inoculum concentration, indicating that AM fungi colonization could constrain the evolution of plant tolerance to herbivory.Key words: compensation, defences, ecological interactions, herbivory, multitrophic interactions, mycorrhizal fungi, toleranceArbuscular mycorrhizal (AM) fungi occur in all ecosystems of the world and associate with the roots of about 70% of all vascular plants.¹ This association is typically regarded as mutualistic, because there is a bidirectional transfer of nutrients between the host plant and its fungal partners. Carbon compounds are passed from the plant to the fungus and, in return, there is a transfer of mineral nutrients, principally nitrate and phosphate.² However, this association also entails costs. The amount of carbon allocated to AM fungi is estimated to range from 4% to 20% of a plant''s total carbon budget.² Throughout the literature, there are examples of the conditionality of this relationship exemplified by a continuum of the effects of AM fungal colonization on hosts from positive, through null to negative.^3–5 Moreover, it has been suggested that the benefit of a plant associating with fungal symbionts depends not only on the identity of AM fungi⁴ and plant genotypes⁶ but also on hyphal colonization density in roots.⁷ In a recent greenhouse study, we examined components of the conditionality of plant interactions with soil biota.⁸ We were interested in knowing how the performance and tolerance to defoliation of the annual plant Datura stramonium varied along a concentration gradient of commercial AM fungal inoculum containing four Glomus species (Mycorrhizal Applications, Grants Pass, OR USA).We found a curvilinear relationship between AM fungal inoculum concentration and plant performance, as predicted by previous models.⁷ The quadratic decelerating function between inoculum concentration and plant performance indicates an optimum level of AM fungal concentration (1/24^th total pot volume) that maximizes AM fungal benefit (Fig. 1A). This result suggests that, in D. stramonium, positive associations between AM fungi and plant fitness may not be proportional and, that at high colonization densities, mycorrhizae may have detrimental effects, perhaps by competing with plants for nutrients, or by interfering with other essential interactions.^4,5 We also found, from root examination, that hyphal colonization of roots increased linearly with AM fungi inoculum concentration. Moreover, we found that tolerance to herbivory decreased linearly with increasing AM fungal inoculum concentration (r² = −0.40; F_1,27 = 5.89; p = 0.0222; Fig. 1B), suggesting that, in our system, at high densities, mycorrhizae may become parasitic and may compete for resources (e.g., carbon) with the induced host plant response to leaf damage.Open in a separate windowFigure 1Effect of a gradient in AM fungi inoculum concentration on D. stramonium performance. (A) Non-linear relation between seed production and inoculum concentration. In general, plants achieved their maximal performance at an inoculum concentration of 1/24^th total pot volume. (B) Tolerance to defoliation decreased linearly with inoculum concentration. Tolerance was calculated as the difference in standardized seed production between related damaged and undamaged genetically related plants corresponding to six genetic full-sib families.In order to know whether the effects we found in the greenhouse using commercial inoculum could be expected in the field, we addressed whether or not D. stramonium performance and tolerance were similarly influenced by whole soil field communities; including AM fungi, pathogens, root herbivores, etc. Unfortunately, D. stramonium is not native to the area where this research was undertaken, so we collected soil immediately below plants of a native congener Datura wrightii, a perennial herb that grows at the Putah Creek Reserve (UC, Davis). Pots were inoculated at a 1/12^th total pot volume with this live soil and plants were grown concurrently with those in the previous experiment. We compared plant performance and tolerance under the live soil treatment and the last level of the commercial AMF inoculum gradient (both inoculated at a 1/12^th total pot volume). Results indicated no differences in foliar area (F_1,94 = 1.18; p = 0.2782), root mass (F_1,94 = 0.99; p = 0.3222), flowering day (χ² = 0.31; p = 0.5804) and fitness (χ² = 0.03; p = 0.8691). Moreover, root colonization levels were (F_1,94 = 0.75; p = 0.3877) in both 1/12^th volume vs. live soil, as well as in the 0 AMF and sterilized soil (F_1,94 = 2.56; p = 0.1130). Despite these similarities, plant tolerance did differ significantly between AMF and live soil treatments (F_1,94 = 5.49; p = 0.0411), tolerance being greater under the live soil treatment (0.3755 ± 0.0311 tolerance) relative to the 1/12^th AM fungal treatment (−0.5744 ± 0.2714 tolerance). This result suggests that the expression of plant tolerance may also depend on the identity of AMF colonizing roots or the number and identities of soil bacteria. We did not know which microbial species were in the soils we collected.We show that, when inoculated over a gradient of abundance, Glomus AM fungal colonization consistently decreased tolerance to herbivory. The presence of mycorrhizae could therefore decrease the adaptive value of traits increasing tolerance. We also show here that though live soil inoculum had similar effects in magnitude and direction to those of commercial AMF incoculum on growth and fitness, live soil biota collected under a congener of D. stramonium increased tolerance to herbivory at the same levels of root colonization. Overall, the results of this study indicate that the interaction between soil biotic components and the response of D. stramonium to leaf damage is highly conditional; and can depend on amounts of root colonization, as well as perhaps identities of AM fungi and bacteria. In both cases, soil biota affected the impact of damage to leaves aboveground. AM fungi may mediate the efficacy of tolerance as a defense, and this effect may be especially important in light of herbivore adaptation, when tolerance may be favored over resistance as a plant defense strategy.¹⁰     

Trade-offs between arbuscular mycorrhizal fungal competitive ability and host growth promotion in Plantago lanceolata

In this study we tested for trade-offs between the benefit arbuscular mycorrhizal (AM) fungi provide for hosts and their competitive ability in host roots, and whether this potential trade-off shifts in the presence of a plant stress (herbivory). We used three species of AM fungi previously determined to vary in host growth promotion and spore production in association with host plants. We found that these AM fungal species competed for root space, and the best competitor, Scutellospora calospora, was the worst mutualist. In addition, the worst competitor, Glomus white, was the best mutualist. Competition proved to have stronger effects on fungal infection patterns than herbivory, and competitive dominance was not altered by herbivory. We found a similar pattern in a previous test of competition among AM fungi, and we discuss the implications of these results for the persistence of the mutualism and feedbacks between AM fungi and their plant hosts.     

Species of plants and associated arbuscular mycorrhizal fungi mediate mycorrhizal responses to CO2 enrichment

It has been suggested that enrichment of atmospheric CO₂ should alter mycorrhizal function by simultaneously increasing nutrient‐uptake benefits and decreasing net C costs for host plants. However, this hypothesis has not been sufficiently tested. We conducted three experiments to examine the impacts of CO₂ enrichment on the function of different combinations of plants and arbuscular mycorrhizal (AM) fungi grown under high and low soil nutrient availability. Across the three experiments, AM function was measured in 14 plant species, including forbs, C₃ and C₄ grasses, and plant species that are typically nonmycorrhizal. Five different AM fungal communities were used for inoculum, including mixtures of Glomus spp. and mixtures of Gigasporaceae (i.e. Gigaspora and Scutellospora spp.). Our results do not support the hypothesis that CO₂ enrichment should consistently increase plant growth benefits from AM fungi, but rather, we found CO₂ enrichment frequently reduced AM benefits. Furthermore, we did not find consistent evidence that enrichment of soil nutrients increases plant growth responses to CO₂ enrichment and decreases plant growth responses to AM fungi. Our results show that the strength of AM mutualisms vary significantly among fungal and plant taxa, and that CO₂ levels further mediate AM function. In general, when CO₂ enrichment interacted with AM fungal taxa to affect host plant dry weight, it increased the beneficial effects of Gigasporaceae and reduced the benefits of Glomus spp. Future studies are necessary to assess the importance of temperature, irradiance, and ambient soil fertility in this response. We conclude that the affects of CO₂ enrichment on AM function varies with plant and fungal taxa, and when making predictions about mycorrhizal function, it is unwise to generalize findings based on a narrow range of plant hosts, AM fungi, and environmental conditions.     

Impact of antifungals producing rhizobacteria on the performance of Vigna radiata in the presence of arbuscular mycorrhizal fungi

Plant growth-promoting rhizobacteria (PGPR) that produce antifungal metabolites are potential threats for the arbuscular mycorrhizal (AM) fungi known for their beneficial symbiosis with plants that is crucially important for low-input sustainable agriculture. To address this issue, we used a compartmented container system where test plants, Vigna radiata, could only reach a separate nutrient-rich compartment indirectly via the hyphae of AM fungi associated with their roots. In this system, where plants depended on nutrient uptake via AM symbiosis, we explored the impact of various PGPR. Plants were inoculated with or without a consortium of four species of AM fungi (Glomus coronatum, Glomus etunicatum, Glomus constrictum, and Glomus intraradices), and one or more of the following PGPR strains: phenazine producing (P⁺) and phenazine-less mutant (P⁻), diacetylphloroglucinol (DAPG) producing (G⁺) and DAPG-less mutant (G⁻) strains of Pseudomonas fluorescens, and an unknown antifungal metabolite-producing Alcaligenes faecalis strain, SLHRE425 (D). PGPR exerted only a small if any effect on the performance of AM symbiosis. G⁺ enhanced AM root colonization and had positive effects on shoot growth and nitrogen content when added alone, but not in combination with P⁺. D negatively influenced AM root colonization, but did not affect nutrient acquisition. Principal component analysis of all treatments indicated correlation between root weight, shoot weight, and nutrient uptake by AM fungus. The results indicate that antifungal metabolites producing PGPR do not necessarily interfere with AM symbiosis and may even promote it thus carefully chosen combinations of such bioinoculants could lead to better plant growth.     

Differential effects of AM fungal isolates on Medicago truncatula growth and metal uptake in a multimetallic (Cd, Zn, Pb) contaminated agricultural soil

Toxic metal accumulation in soils of agricultural interest is a serious problem needing more attention, and investigations on soil–plant metal transfer must be pursued to better understand the processes involved in metal uptake. Arbuscular mycorrhizal (AM) fungi are known to influence metal transfer in plants by increasing plant biomass and reducing metal toxicity to plants even if diverging results were reported. The effects of five AM fungi isolated from metal contaminated or non-contaminated soils on metal (Cd, Zn) uptake by plant and transfer to leachates was assessed with Medicago truncatula grown in a multimetallic contaminated agricultural soil. Fungi isolated from metal-contaminated soils were more effective to reduce shoot Cd concentration. Metal uptake capacity differed between AM fungi and depended on the origin of the isolate. Not only fungal tolerance and ability to reduce metal concentrations in plant but also interactions with rhizobacteria affected heavy metal transfer and plant growth. Indeed, thanks to association with nodulating rhizobacteria, one Glomus intraradices inoculum increased particularly plant biomass which allowed exporting twofold more Cd and Zn in shoots as compared to non-mycorrhizal treatment. Cd concentrations in leachates were variable among fungal treatments, but can be significantly influenced by AM inoculation. The differential strategies of AM fungal colonisation in metal stress conditions are also discussed.