Plant invasions facilitated by suppression of root nutrient acquisition rather than by disruption of mycorrhizal association in the native plant

Invasive species have profound negative impacts on native ranges. Unraveling the mechanisms employed by invasive plant species is crucial to controlling invasions. One important approach that invasive plants use to outcompete native plants is to disrupt mutualistic interactions between native roots and mycorrhizal fungi. However, it remains unclear how differences in the competitive ability of invasive plants affect native plant associations with mycorrhizae. Here, we examined how a native plant, Xanthium strumarium, responds to invasive plants that differed in competitive abilities (i.e., as represented by aboveground plant biomass) by measuring changes in root nitrogen concentration (root nutrient acquisition) and mycorrhizal colonization rate. We found that both root nitrogen concentration and mycorrhizal colonization rate in the native plant were reduced by invasive plants. The change in mycorrhizal colonization rate of the native plant was negatively correlated with both aboveground plant biomass of the invasive plants and the change in aboveground plant biomass of the native plant in monocultures relative to mixed plantings. In contrast, the change in root nitrogen concentration of the native plant was positively correlated with aboveground plant biomass of the invasive plants and the change in aboveground plant biomass of the native plant. When we compared the changes in mycorrhizal colonization rate and root nitrogen concentration in the native plant grown in monocultures with those of native plants grown with invasive plants, we observed a significant tradeoff. Our study shows that invasive plants can suppress native plants by reducing root nutrient acquisition rather than by disrupting symbiotic mycorrhizal associations, a novel finding likely attributable to a low dependence of the native plant on mycorrhizal fungi.     

Mycorrhizal associations of an invasive tree are enhanced by both genetic and environmental mechanisms

Biotic interactions involving exotic plants in their introduced ranges may differ from those of co‐occurring plant species and from interactions in their native ranges. When interactions are less negative, or more positive compared to native plant species, this may increase invasion success, and differences among ranges may cause changes in exotic plant traits. Here, we investigated arbuscular mycorrhizae (AM) associated with Triadica sebifera seedlings from populations in native (China) and introduced ranges (US) and with seedlings from US and China species within three co‐occurring genera (Liquidambar, Ulmus, Celtis) grown in multiple common gardens in both ranges. No general pattern of higher or lower AM colonization was found in the introduced range for China and US Celtis, Liquidambar, or Ulmus species. However, AM colonization was significantly higher for Triadica than for other genera, particularly in the introduced range, suggesting AM may improve Triadica's invasion success. Triadica AM colonization was higher in US than China gardens, decreased with increasing soil nitrogen in China, but was independent of soil nitrogen in the US. This might reflect a different effect of soil fertility on this mutualism among ranges. Introduced Triadica populations had higher AM colonization than native populations, particularly in US gardens, implying a possible advantage from greater AM association in the introduced range. This is the first field study demonstrating post‐introduction changes in mycorrhizal colonization of an invasive species. It indicates that there are ecological and evolutionary components to the effect of positive interactions on plant invasions.     

Arbuscular mycorrhizal colonization of tomato by Gigaspora and Glomus species in the presence of root flavonoids

The effect of flavonoids isolated from arbuscular mycorrhizal (AM) colonized and noncolonized clover roots on the number of entry points and percentage of root colonization of tomato (Lycopersicum esculentum L.) by Gigaspora rosea, Gi margarita, Glomus mosseae and G. intrarradices symbionts was determined. With fungi of both genera, a correlation between the number of entry points and the percentage of root colonization was found in the presence of some of the tested flavonoids. The flavonoids acacetin and rhamnetin, present in AM clover roots, inhibited the formation of AM penetration structures and the AM colonization of tomato roots, whereas the flavonoid 5,6,7,8,9-hydroxy chalcone, which could not be detected in AM clover root, inhibited both parameters. The flavonoid quercetin, which was present in AM clover roots, stimulated the penetration and root colonization of tomato by Gigaspora. However, the flavonoids 5,6,7,8-hydroxy-4'-methoxy flavone and 3,5,6,7,4'-hydroxy flavone, which was not found in AM clover root, increased the number of entry points and the AM colonization of tomato roots by Gigaspora. These results indicated that flavonoids could be imnplicated in the process of regulation of AM colonization in plant root, but its role is highly complex and depend not only on flavonoids, but also on AM fungal genus or even species.     

Development of a model system to assess the impact of genetically modified corn and aubergine plants on arbuscular mycorrhizal fungi

We developed an experimental model system to monitor the impact of generically modified (GM) plants on arbuscular mycorrhizal (AM) fungi, a group of non-target soil microorganisms, fundamental for soil fertility and plant nutrition. The system allowed us to study the effects of root exudates of both commercial Bt corn and aubergine plants expressing Dm-AMP1 defensin on different stages of the life cycle of the AM fungal species G. mosseae. Root exudates of Bt 176 corn significantly reduced pre-symbiotic hyphal growth, compared to Bt 11 and non-transgenic plants. No differences were found in mycelial growth in the presence of Dm-AMP1 and control plant root exudates. Differential hyphal morphogenesis occurred irrespective of the plant line, suggesting that both exuded Bt toxin and defensin do not interfere with fungal host recognition mechanisms. Bt 176 affected the regular development of appressoria, 36% of which failed to produce viable infection pegs. Our experimental model system represents an easy assay for testing the impact of GM plants on non-target soil-borne AM fungi.     

Root Exudates Regulate Soil Fungal Community Composition and Diversity

Plants are in constant contact with a community of soil biota that contains fungi ranging from pathogenic to symbiotic. A few studies have demonstrated a critical role of chemical communication in establishing highly specialized relationships, but the general role for root exudates in structuring the soil fungal community is poorly described. This study demonstrates that two model plant species (Arabidopsis thaliana and Medicago truncatula) are able to maintain resident soil fungal populations but unable to maintain nonresident soil fungal populations. This is mediated largely through root exudates: the effects of adding in vitro-generated root exudates to the soil fungal community were qualitatively and quantitatively similar to the results observed for plants grown in those same soils. This effect is observed for total fungal biomass, phylotype diversity, and overall community similarity to the starting community. Nonresident plants and root exudates influenced the fungal community by both positively and negatively impacting the relative abundance of individual phylotypes. A net increase in fungal biomass was observed when nonresident root exudates were added to resident plant treatments, suggesting that increases in specific carbon substrates and/or signaling compounds support an increased soil fungal population load. This study establishes root exudates as a mechanism through which a plant is able to regulate soil fungal community composition.     

Systemic inhibition of arbuscular mycorrhiza development by root exudates of cucumber plants colonized by<Emphasis Type="Italic"> Glomus mosseae</Emphasis>

The arbuscular mycorrhizal (AM) non-host plants mustard, sugar beet, lupin and the AM host plant cucumber were used as test plants. Cucumber plants were grown either in the absence of the AM fungus (AMF) Glomus mosseae or in a split-root system, with one side mycorrhizal and one side non-mycorrhizal. Root exudates of the AM non-host plants, the non-mycorrhizal cucumber plants and the mycorrhizal and the non-mycorrhizal side of the split-root system of mycorrhizal cucumber plants were collected and applied to cucumber plants inoculated with the AMF. Root exudates of non-mycorrhizal cucumber plants showed a significant stimulatory effect on root colonization, whereas root exudates from the mycorrhizal and the non-mycorrhizal sides of a split-root system of a mycorrhizal cucumber plant did not show this stimulatory effect and were even slightly inhibitory. Root exudates of the two AM non-host plants mustard and sugar beet significantly reduced root colonization in cucumber plants, whereas no such effect was observed when root exudates of the AM non-host plant lupin were applied.     

Invasive warm-season grasses reduce mycorrhizal root colonization and biomass production of native prairie grasses

Soil organisms play important roles in regulating ecosystem-level processes and the association of arbuscular mycorrhizal (AM) fungi with a plant species can be a central force shaping plant species' ecology. Understanding how mycorrhizal associations are affected by plant invasions may be a critical aspect of the conservation and restoration of native ecosystems. We examined the competitive ability of old world bluestem, a non-native grass (Caucasian bluestem [Bothriochloa bladhii]), and the influence of B. bladhii competition on AM root colonization of native warm-season prairie grasses (Andropogon gerardii or Schizachyrium scoparium), using a substitutive design greenhouse competition experiment. Competition by the non-native resulted in significantly reduced biomass production and AM colonization of the native grasses. To assess plant-soil feedbacks of B. bladhii and Bothriochloa ischaemum, we conducted a second greenhouse study which examined soil alterations indirectly by assessing biomass production and AM colonization of native warm-season grasses planted into soil collected beneath Bothriochloa spp. This study was conducted using soil from four replicate prairie sites throughout Kansas and Oklahoma, USA. Our results indicate that a major mechanism in plant growth suppression following invasion by Bothriochloa spp. is the alteration in soil microbial communities. Plant growth was tightly correlated with AM root colonization demonstrating that mycorrhizae play an important role in the invasion of these systems by Bothriochloa spp. and indicating that the restoration of native AM fungal communities may be a fundamental consideration for the successful establishment of native grasses into invaded sites.     

Prepenetration apparatus assembly precedes and predicts the colonization patterns of arbuscular mycorrhizal fungi within the root cortex of both Medicago truncatula and Daucus carota

Arbuscular mycorrhizas (AM) are widespread, ancient endosymbiotic associations that contribute significantly to soil nutrient uptake in plants. We have previously shown that initial fungal penetration of the host root is mediated via a specialized cytoplasmic assembly called the prepenetration apparatus (PPA), which directs AM hyphae through the epidermis (Genre et al., 2005). In vivo confocal microscopy studies performed on Medicago truncatula and Daucus carota, host plants with different patterns of AM colonization, now reveal that subsequent intracellular growth across the root outer cortex is also PPA dependent. On the other hand, inner root cortical colonization leading to arbuscule development involves more varied and complex PPA-related mechanisms. In particular, a striking alignment of polarized PPAs can be observed in adjacent inner cortical cells of D. carota, correlating with the intracellular root colonization strategy of this plant. Ultrastructural analysis of these PPA-containing cells reveals intense membrane trafficking coupled with nuclear enlargement and remodeling, typical features of arbusculated cells. Taken together, these findings imply that prepenetration responses are both conserved and modulated throughout the AM symbiosis as a function of the different stages of fungal accommodation and the host-specific pattern of root colonization. We propose a model for intracellular AM fungal accommodation integrating peri-arbuscular interface formation and the regulation of functional arbuscule development.     

Viability of VA-mycorrhizal fungi following soil solarization and fumigation

Two field experiments were conducted to examine the effect of soil solarization on the survival of arbuscular mycorrhizal (AM) fungi and root colonization of three crops. The experiments were carried out in a loamy sand soil (Rehovot) and a silty soil (Bet She'an Valley). For both experiments, assessment of indigenous AM fungal populations by the most probable number (MPN) method indicated that populations were reduced to zero after 2 or 4 weeks of solarization treatment. However, Glomus intraradices inoculum applied to the soil prior to solarization remained viable even after 8 weeks of solarization. After soil fumigation with methyl bromide both indigenous and applied AM fungi were nondetectable. Percentage root colonization by the indigenous AM fungal populations, together with plant-growth parameters, were assessed for three crops: onion and wheat (Rehovot), and carrot (Bet She'an). When sown on solarized field plots, onion and carrot seedlings showed a plant growth retardation, whereas wheat showed an increased growth response. Root colonization by indigenous AM fungi was not evident until 6 weeks after seedling emergence. Fumigation with methyl bromide reduced root colonization by indigenous AM populations, and reduced onion and wheat plant development at early growth stages. In a laboratory experiment, a temperature of 45° C for up to 24 h did not affect AM spore viability, indicating that temperatures reached during the solarization treatment cannot solely account for the reduced AM fungi viability in the field. Apparently, soil solarization temporarily delays root colonization by indigenous AM fungi until 6-8 weeks after plant emergence.     

Soil conditioning effects of Phragmites australis on native wetland plant seedling survival

Interactions between introduced plants and soils they colonize are central to invasive species success in many systems. Belowground biotic and abiotic changes can influence the success of introduced species as well as their native competitors. All plants alter soil properties after colonization but, in the case of many invasive plant species, it is unclear whether the strength and direction of these soil conditioning effects are due to plant traits, plant origin, or local population characteristics and site conditions in the invaded range. Phragmites australis in North America exists as a mix of populations of different evolutionary origin. Populations of endemic native Phragmites australis americanus are declining, while introduced European populations are important wetland invaders. We assessed soil conditioning effects of native and non‐native P. australis populations on early and late seedling survival of native and introduced wetland plants. We further used a soil biocide treatment to assess the role of soil fungi on seedling survival. Survival of seedlings in soils colonized by P. australis was either unaffected or negatively affected; no species showed improved survival in P. australis‐conditioned soils. Population of P. australis was a significant factor explaining the response of seedlings, but origin (native or non‐native) was not a significant factor. Synthesis: Our results highlight the importance of phylogenetic control when assessing impacts of invasive species to avoid conflating general plant traits with mechanisms of invasive success. Both native (noninvasive) and non‐native (invasive) P. australis populations reduced seedling survival of competing plant species. Because soil legacy effects of native and non‐native P. australis are similar, this study suggests that the close phylogenetic relationship between the two populations, and not the invasive status of introduced P. australis, is more relevant to their soil‐mediated impact on other plant species.     

Alien plants associate with widespread generalist arbuscular mycorrhizal fungal taxa: evidence from a continental‐scale study using massively parallel 454 sequencing

Aim The biogeography of arbuscular mycorrhizal (AM) fungi is poorly understood, and consequently the potential of AM fungi to determine plant distribution has been largely overlooked. We aimed to describe AM fungal communities associating with a single host‐plant species across a wide geographical area, including the plant’s native, invasive and experimentally introduced ranges. We hypothesized that an alien AM plant associates primarily with the geographically widespread generalist AM fungal taxa present in a novel range. Location Europe, China. Methods We transplanted the palm Trachycarpus fortunei into nine European sites where it does not occur as a native species, into one site where it is naturalized (Switzerland), and into one glasshouse site. We harvested plant roots after two seasons. In addition, we sampled palms at three sites in the plant’s native range (China). Roots were subjected to DNA extraction, polymerase chain reaction (PCR) and 454 sequencing of AM fungal sequences. We analysed fungal communities with non‐metric multidimensional scaling (NMDS) ordination and cluster analysis and studied the frequency of geographically widespread fungal taxa with log‐linear analysis. We compared fungal communities in the roots of the palm with those in resident plants at one site in the introduced range (Estonia) where natural AM fungal communities had previously been studied. Results We recorded a total of 73 AM fungal taxa. AM fungal communities in the native and introduced ranges differed from one another, while those in the invasive range contained taxa present in both other ranges. Geographically widespread AM fungal taxa were over‐represented in palm roots in all regions, but especially in the introduced range. At the Estonian site, the palm was colonized by the same community of widespread AM fungal taxa as associate with resident habitat‐generalist plants; by contrast, resident forest‐specialist plants were colonized by a diverse community of widespread and other AM fungal taxa. Main conclusions AM fungal communities in the native, invasive and experimentally introduced ranges varied in taxonomic composition and richness, but they shared a pool of geographically widespread, non‐host‐specific taxa that might support the invasion of a generalist alien plant. Our dataset provides the first geographical overview of AM taxon distributions obtained using a single host‐plant species.     

Flavonoids in seed and root exudates of Lotus pedunculatus and their biotransformation by Mesorhizobium loti

Analysis of the flavonoid content of seed and root exudates of Lotus pedunculatus was undertaken using multiple coupled analytical techniques: capillary zone electrophoresis coupled to a UV spectral array detector (CZE-UV), high performance thin-layer chromatography with densitometry (HPTLC-UV) and gas chromatography with mass spectrometry (GC-MS). These procedures provided separation, identification and structural confirmation of the previously unidentified flavonoids in this plant's seed and root exudates and were particularly applicable to samples from a small seeded legume. Catechin, naringenin, kaempferol, quercetin aglycone and 3 different glycosides of quercetin were detected in seed exudate. Sterile root exudates contained catechin, naringenin and quercetin in addition to apigenin, kaempferol and other flavones and flavanones for which partial identifications were obtained. When sterile root exudate was incubated with Mesorhizobium loti , changes in its flavonoid content were detected. Analysis of bacterial cells after incubation revealed the presence of quercetin, kaempferol and one other flavone. The monocyclic aromatics protocatechuic acid and phloroglucinol were detected in both the incubated root exudate and its bacterial cells.     

Arbuscular mycorrhizal colonization of giant sequoia (Sequoiadendron giganteum) in response to restoration practices

Interactions with soil microbiota determine the success of restoring plants to their native habitats. The goal of our study was to understand the effects of restoration practices on interactions of giant sequoia Sequoiadendron giganteum with arbuscular mycorrhizal (AM) fungi (Glomeromycota). Natural regeneration of Sequoiadendron is threatened by the absence of severe fires that create forest canopy gaps. Generating artificial canopy gaps offers an alternative tool for giant sequoia restoration. We investigated the effect of regeneration practices, including (i) sapling location within gaps, (ii) gap size and (iii) soil substrate, on AM fungal colonization of giant sequoia sapling roots in a native giant sequoia grove of the Sierra Nevada, California. We found that the extent of AM fungal root colonization was positively correlated with sapling height and light availability, which were related to the location of the sapling within the gap and the gap size. While colonization frequency by arbuscules in saplings on ash substrate was higher relative to saplings in mineral soil, the total AM fungal root colonization was similar between the substrates. A negative correlation between root colonization by Glomeromycota and non-AM fungal species indicated antagonistic interactions between different classes of root-associated fungi. Using DNA genotyping, we identified six AM fungal taxa representing genera Glomus and Ambispora present in Sequoiadendron roots. Overall, we found that AM fungal colonization of giant sequoia roots was associated with availability of plant-assimilated carbon to the fungus rather than with the AM fungal supply of mineral nutrients to the roots. We conclude that restoration practices affecting light availability and carbon assimilation alter feedbacks between sapling growth and activity of AM fungi in the roots.     

喜旱莲子草在青藏高原对模拟增温的可塑性: 引入地和原产地种群的比较

气候变暖背景下植物可通过关键性状的表型可塑性来适应环境温度的增加。表型可塑性增强进化假说预测定植到新环境中的入侵植物种群具有演化出更强表型可塑性的潜力。此前对可塑性进化的研究涵盖了外来植物性状对水分条件、光照变化、土壤养分、邻体根系以及天敌防御等的响应, 而较少有研究关注增温条件下植物重要性状的可塑性进化。已有的部分研究多集中在温带和热带地区, 而较少关注入侵植物在高寒地区对增温的响应; 且研究多集中在植物生长相关性状, 较少关注功能性状和防御性状。本研究采用同质园实验比较了喜旱莲子草6个引入地(中国)种群和6个原产地(阿根廷)种群, 在西藏拉萨模拟全天增温2℃处理下的适合度性状、功能性状和防御性状的响应差异。结果表明: (1)高寒地区模拟全天增温显著提高了喜旱莲子草总生物量(+36.4%)、地上生物量(+34.5%)、贮藏根生物量(+51.4%)和毛根生物量(+33.6%), 降低了分枝强度(-19.8%)和比茎长(-30.2%); (2)模拟全天增温使引入地种群的比叶面积和黄酮含量增加, 而原产地种群则相反。这些结果表明高寒地区全天增温2℃对喜旱莲子草可能是一种有利条件。引入地种群的适合度性状对模拟全天增温2℃的响应比原产地种群更强, 而其光能利用相关性状和防御性状的响应可能提升了其在高寒地区的适合度。因此, 在未来全球气候变暖的背景下, 高寒地区温度升高可能更有利于喜旱莲子草引入地种群的定植和扩散。     

Constitutive overexpression of the sucrose transporter SoSUT1 in potato plants increases arbuscular mycorrhiza fungal root colonization under high, but not under low, soil phosphorus availability

The sucrose transporter SUT1 functions in phloem loading of photoassimilates in solanaceous plant species. In the present study, wildtype and transgenic potato plants with either constitutive overexpression or antisense inhibition of SUT1 were grown under high or low phosphorus (P) fertilization levels in the presence or absence of the arbuscular mycorrhizal (AM) fungus Glomus intraradices. At a low soil P fertilization level, the extent of AM fungal root colonization was not different among the genotypes. In all plants, the AM symbiosis contributed significantly to P uptake under these conditions. In response to a high soil P fertilization level, all genotypes showed a decrease in AM fungal root colonization, indicating that the expression level of SUT1 does not constitute a major mechanism of control over AM development in response to the soil P availability. However, plants with overexpression of SUT1 showed a higher extent of AM fungal root colonization compared with the other genotypes when the soil P availability was high. Whether an increased symbiotic C supply, alterations in the phytohormonal balance, or a decreased synthesis of antimicrobial compounds was the major cause for this effect requires further investigation. In plants with impaired phloem loading, a low C status of plant sink tissues did apparently not negatively affect plant C supply to the AM symbiosis. It is possible that, at least during vegetative and early generative growth, source rather than sink tissues exert control over amounts of C supplied to AM fungi.     

Arbuscular mycorrhizas in cycads of southern India

Root and soil samples of three potted or ground-grown cycads ( Cycas circinalis, C. revoluta, Zamiasp.) were collected between November 1999 and June 2000 and surveyed for arbuscular mycorrhizal (AM) colonization and spore populations. AM fungi were associated with all root systems and rhizosphere samples examined. Root colonization was of a typical Arum type and AM colonization levels differed significantly between species and between potted and ground-grown cycads. Mycorrhizal colonization levels were inversely related to root hair number and length. Spores of nine morphotypes belonging to three genera ( Acaulospora, Glomus, Scutellospora) were extracted from soil. The percentage root length colonized by AM fungi was not related to soil factors, but total AM fungal spore numbers in the rhizosphere soil were inversely related to soil nitrogen and phosphorus levels. AM fungal spore numbers in the soil were linearly related to root length colonized. The co-occurrence of septate non-mycorrhizal fungi was recorded for the first time in cycads. These observations and the relationship between plant mycorrhizal status and soil nutrients are discussed.     

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.     

The developmental ecology of mycorrhizal associations in mayapple, Podophyllum peltatum, Berberidaceae

Associations between plants and arbuscular mycorrhizal (AM) fungi are widespread and well-studied. Yet little is known about the pattern of association between clonal plants and AM fungi. Here we report on the pattern of mycorrhizal association within the rhizome systems of mayapple, Podophyllum peltatum. Mayapple is a long-lived understory clonal herb that is classified as obligately mycorrhizal. We found that while all mayapple rhizome systems maintained mycorrhizal associations, the percent colonization of roots by AM fungi differed among ramets of different age. The highest concentrations of AM fungi were in the roots of intermediate-aged ramets, while roots beneath the youngest ramet were not colonized. This pattern of ramet age or position-dependent colonization was observed in two separate studies; each conducted in a different year and at a different site. The pattern of AM fungal colonization of mayapple rhizome systems suggests that the mycorrhizal relationship is facultative at the ramet level. This conclusion is reinforced by our observation that augmentation of soil phosphate lowers root colonization by AM fungi. We also found that soil phosphate concentrations were depleted by ca. 1% under the same ramet positions where roots bore the highest AM fungal loads. Three non-exclusive hypotheses are proposed regarding the mechanisms that might cause this developmentally dependent pattern of mycorrhizal association.     

Stimulated saprotrophic fungi in arable soil extend their activity to the rhizosphere and root microbiomes of crop seedlings

Saprotrophic fungi play an important role in ecosystem functioning and plant performance, but their abundance in intensively managed arable soils is low. Saprotrophic fungal biomass in arable soils can be enhanced with amendments of cellulose-rich materials. Here, we examined if sawdust-stimulated saprotrophic fungi extend their activity to the rhizosphere of crop seedlings and influence the composition and activity of other rhizosphere and root inhabitants. After growing carrot seedlings in sawdust-amended arable soil, we determined fungal and bacterial biomass and community structure in roots, rhizosphere and soil. Utilization of root exudates was assessed by stable isotope probing (SIP) following ¹³CO₂-pulse-labelling of seedlings. This was combined with analysis of lipid fatty acids (PLFA/NLFA-SIP) and nucleic acids (DNA-SIP). Sawdust-stimulated Sordariomycetes colonized the seedling's rhizosphere and roots and actively consumed root exudates. This did not reduce the abundance and activity of bacteria, yet higher proportions of α-Proteobacteria and Bacteroidia were seen. Biomass and activity of mycorrhizal fungi increased with sawdust amendments, whereas exudate consumption and root colonization by functional groups containing plant pathogens did not change. Sawdust amendment of arable soil enhanced abundance and exudate-consuming activity of saprotrophic fungi in the rhizosphere of crop seedlings and promoted potential beneficial microbial groups in root-associated microbiomes.     

Arbuscular mycorrhizal adaptation,spore germination,root colonization,and host plant growth and mineral acquisition at low pH

Arbuscular mycorrhizal (AM) fungi colonize plant roots and often enhance host plant growth and mineral acquisition, particularly for plants grown under low nutrient and mineral stress conditions. Information about AM fungi and mycorrhizal ( +AM) host plant responses at low pH ( < 5) is limited. Acaulospora are widely reported in acid soil, and Gigaspora sp. appear to be more common in acid soils than Glomus sp. Spores of some AM fungi are more tolerant to acid conditions and high Al than others; t Acaulospora sp., Gigaspora sp., and Glomus manihotis are particularly tolerant. Root colonization is generally less in low than in high pH soils. Percentage root colonization is generally not related to dry matter (DM) produced. Maximum enhancement of plant growth in acid soil varies with AM fungal isolate and soil pH, indicating adaptation of AM isolates to edaphic conditions. Acquisition of many mineral nutrients other than P and Zn is enhanced by +AM plants in acid soil, and the minerals whose concentration is enhanced are those commonly deficient in acid soils (Ca, Mg, and K). Some AM fungal isolates are effective in overcoming soil acidity factors, especially Al toxicity, that restrict plant growth at low pH.