丛枝菌根真菌对植物繁殖的影响研究进展

丛枝菌根真菌(arbuscular mycorrhizal fungi, AMF)与宿主植物所形成的互惠共生体系是生态系统中广泛分布的共生体系代表类型之一。AMF除能够促进宿主植物生长发育外,也可以对宿主植物的繁殖过程产生多方面影响。研究宿主植物在AMF共生状态下繁殖策略的变化规律,对于深入理解植物繁殖适合度的变化具有重要理论意义。该文综述了AMF对宿主植物繁殖影响的相关研究,包括AMF的侵染对宿主植物繁殖分配、花部特征、虫媒传粉和花期的影响。目前已有研究发现某些AMF能够促进宿主植物增加繁殖资源投入,提高花朵产生的数量或花冠直径,同时增加花粉数量和花蜜量来影响访花昆虫的行为,以及造成开花提前及花期延长,但其作用的具体机制尚不明确,且因宿主植物的差异,并未有完全统一的结论。然而,由于AMF与植物共生的普遍性,其在植物繁殖过程中发挥的重要作用不可忽略。今后除了在以上各方面开展更深入的研究外,还需在AMF对宿主植物繁殖性状的影响机制、AMF共生条件下植物有性繁殖过程中雌雄功能的资源分配,以及对无性繁殖和后代适应性的影响等方面进行更深入的研究。     

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.     

Adult plants facilitate their conspecific seedlings by enhancing arbuscular mycorrhizae in a saline soil

Aims

We tested the hypothesis that adult plants can help their conspecific (i.e. an organism belonging to the same species as another organism) seedlings develop symbiosis with arbuscular mycorrhizal fungi (AMF), thereby increasing seedling nutrient uptake and growth in a saline soil.

Methods

Using the halophytic shrub Tamarix chinensis as a model plant, we conducted two field experiments and a greenhouse experiment. Field experiment 1 assessed the importance of below-ground effects of adult neighbor. Field experiment 2 determined the involvement of AMF in neighbor effects by applying fungicide benomyl to obtain AMF suppressed treatment. The greenhouse experiment tested whether neighbor effects were influenced by AMF hyphal connection between adults and seedlings by using 25 μm and 0.45 μm nylon mesh to allow and prevent the AMF hyphae pass through respectively.

Results

Adult neighbor increased shoot biomass and nutrient of target seedlings and the below-ground effects mediated by AMF was facilitative under high soil salinity level. Field experiment 1 showed that adult neighbors reduced soil salinity, increased soil organic matter, and provided AMF spores for target seedlings via whole plant effects or below-ground effects alone. Field experiment 2 showed that shoot biomass and AMF colonization of target seedlings were greater with an adult neighbor when AMF were not suppressed treatment than in AMF were suppressed or there were no neighbors. In the greenhouse experiment, adult neighbors with AMF hyphal connection increased shoot biomass, AMF colonization, and ¹⁵N content of target seedlings under the high salinity level.

Conclusion

The results support our hypothesis that adult plants can promote the growth and nutrient uptake of their conspecific seedlings in a saline soil by helping them to develop AMF symbiosis. These findings highlight the roles of adult neighbor plants on seedlings regeneration through rhizospheric symbiosis in stressful environments.

     

丛枝菌根真菌数个种的基因组和转录组研究概况

丛枝菌根真菌（AMF）在自然界分布广泛，能与大部分维管植物的根系形成菌根共生体。它们在调节植物群落结构和全球的碳、氮、磷循环等方面发挥着重要的生态功能，也是农林、环境领域最具应用前景的微生物类群。受限于培养方法、研究手段等，长期以来对AMF基因组、转录组特征的认识非常有限。最近10年，AMF基因组和转录组的相关研究在高通量测序技术的推动下取得了较快发展；研究结果也显著提高了对AMF遗传发育、代谢生理、共生机制等的认识。本文综述了目前已完成测序的AMF种类的基因组、转录组信息。结果发现，已测序的AMF种类普遍具有基因组大、转座子丰富、AT碱基含量高、含大量未知功能基因与特异性基因、缺少部分共生相关基因等特点。在转录层面，总结了不同AMF种类、AMF不同共生结构、共生阶段以及与不同寄主植物共生时的转录本特征。结果发现，不同种类AMF的转录本大小差异明显。不同共生阶段或不同共生结构中的AMF转录本也具有较大的差异，且差异表达的基因大部分与养分交易、信号转导等密切相关。相比之下，同种AMF与不同寄主植物共生时的转录本表现出较高的保守性。最后，本文提出了本领域需要重点关注的研究方向，包括AMF纯培养技术的革新、AMF基因功能的解析、非模式AMF类群的研究以及对AMF蛋白组的研究。     

AMF colonization and community of a temperate invader and co-occurring natives grown under different CO2 concentrations for 3 years

连续3年不同CO₂浓度下一种温带外来入侵植物和两种共存本地植物丛枝菌根真菌群落及侵染率 大气CO₂浓度升高等全球变化过程不仅能促进外来植物入侵,也能改变土壤丛枝菌根真菌(AMF)的群落结构,但我们并不清楚大气CO₂浓度升高促进外来植物入侵是否与其对外来入侵植物和 本地植物AMF共生的影响有关。为回答这一问题,我们在环境和倍增CO₂浓度下连续3年栽培一年生外来入侵植物瘤突苍耳(Xanthium strumarium)与两种共存的一年生本地近缘植物,比较了AMF侵染率、土 壤养分和土壤AMF群落组成的差异。研究结果表明,大气CO₂浓度升高只在少数情况下提高根系AMF侵染率,并且瘤突苍耳AMF侵染率的提高并不比本地种多。在环境CO₂浓度下,栽培第一年瘤突苍耳的AMF侵染侵染率显著高于两种本地植物;而栽培第二年和第三年与两种本地植物的差异不显著,因为两种本地植物的AMF侵染率随种植时间的增加而增加,而瘤突苍耳AMF侵染率受种植时间的影响较小。物种、CO₂浓度和种植时间对AMF侵染率的影响与它们对土壤养分和AMF群落的影响有关,土壤养分对AMF侵染率的影响可能比AMF群落组成的影响更大,因为后者也受土壤养分的影响。上述结果表明,与本地植物相比,入侵植物能更快地与AMF形成共生关系,有利于其适应和入侵新生境;在探究全球变化如大气CO₂浓度升高等对外来植物入侵的影响时,需要考虑AMF的影响和时间效应。     

Arbuscular mycorrhizal protein mRNA over-expression in bread wheat seedlings by Trichoderma harzianum Raifi (KRL-AG2) elicitation

Association between arbuscular mycorrhizal fungi (AMF) and majority of terrestrial plant species provides many benefits to plants that range from stress alleviation and bioremediation in soils polluted with heavy metals to plant growth promotion and yield quantity. Some non-arbuscular mycorrhizal fungi such as, Trichoderma harzianum, are known to enhance the AMF symbiosis with vascular plants. However, information about their role in AMF symbiosis is still limited. Shoots of (Avocet S) wheat seedlings were sprayed with the fungal culture filtrate and gene expression patterns were analyzed in the treated tissues. An increase in the level of mRNA of arbuscular mycorrhizal protein comparing with control was found. The over-expression of this protein in wheat tissues might contribute in initiation of AMF colonization in wheat tissues. The result of this study can spark future researches to elucidate possible role of this protein in the symbiotic interaction mechanisms between soil AMF and various plant roots.     

Phosphorus addition increased carbon partitioning to autotrophic respiration but not to biomass production in an experiment with Zea mays

Plant carbon (C) partitioning—the relative use of photosynthates for biomass production, respiration, and other plant functions—is a key but poorly understood ecosystem process. In an experiment with Zea mays, with or without arbuscular mycorrhizal fungi (AMF), we investigated the effect of phosphorus (P) fertilization and AMF on plant C partitioning. Based on earlier studies, we expected C partitioning to biomass production (i.e., biomass production efficiency; BPE) to increase with increasing P addition due to reduced C partitioning to AMF. However, although plant growth was clearly stimulated by P addition, BPE did not increase. Instead, C partitioning to autotrophic respiration increased. These results contrasted with our expectations and with a previous experiment in the same set-up where P addition increased BPE while no effect on autotropic respiration was found. The comparison of both experiments suggests a key role for AMF in explaining these contrasts. Whereas in the previous experiment substantial C partitioning to AMF reduced BPE under low P, in the current experiment, C partitioning to AMF was too low to directly influence BPE. Our results illustrate the complex influence of nutrient availability and mycorrhizal symbiosis on plant C partitioning.     

Arbuscular mycorrhizal fungi-enhanced resistance against Phytophthora sojae infection on soybean leaves is mediated by a network involving hydrogen peroxide, jasmonic acid, and the metabolism of carbon and nitrogen

The arbuscular mycorrhizal fungi (AMF) enhance the resistance to pathogen infection in host plant. However, it is unclear how the AMF are involved in the systemic acquired resistance of host plant against pathogen. Here, an experiment was carried out to clarify the role of the AMF in soybean’s defense against the infection from pathogen Phytophthora sojae. It was found that the AMF contributed to the resistance of soybean against Phytophthora sojae by the release of hydrogen peroxide and by the accumulation of jasmonic acid in response to pathogenic invasion. Furthermore, the trade of nitrogen (N) from the fungus for carbon from the host was accelerated in the AM symbiosis in the defense reaction, which was indicated by the increased soluble sugar level, NO content and enzyme activities involved in N metabolism in the AM symbiosis.     

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.     

未来的一种生物肥料:丛枝菌根真菌*

丛枝菌根真菌(Arbuscular Mycorrhizal Fungi,AMF)存在于几乎所有类型的土壤中,可以与绝大多数被子植物的根共生。大多数农作物、果树、蔬菜、观赏植物和花卉等都能形成丛枝菌根。AMF能促进作物吸收利用矿质养分和水分,提高作物抗逆性和抗病性,改良土壤、提高苗木移栽成活率、促进生长、提高产量和改善品质,并且可用于改善退化生态系统的土壤肥力,维持农林业的可持续发展,将成为一种新型的生物肥料被用于农林业生产。本文讨论了影响菌根侵染率的因素、AMF的生态效应和在生态农业中的应用现状和前景。     

Nonlegumes, Legumes, and Root Nodules Harbor Different Arbuscular Mycorrhizal Fungal Communities

Legumes are an important plant functional group since they can form a tripartite symbiosis with nitrogen-fixing Rhizobium bacteria and phosphorus-acquiring arbuscular mycorrhizal fungi (AMF). However, not much is known about AMF community composition in legumes and their root nodules. In this study, we analyzed the AMF community composition in the roots of three nonlegumes and in the roots and root nodules of three legumes growing in a natural dune grassland. We amplified a portion of the small-subunit ribosomal DNA and analyzed it by using restriction fragment length polymorphism and direct sequencing. We found differences in AMF communities between legumes and nonlegumes and between legume roots and root nodules. Different plant species also contained different AMF communities, with different AMF diversity. One AMF sequence type was much more abundant in legumes than in nonlegumes (39 and 13%, respectively). Root nodules contained characteristic AMF communities that were different from those in legume roots, even though the communities were similar in nodules from different legume species. One AMF sequence type was found almost exclusively in root nodules. Legumes and root nodules have relatively high nitrogen concentrations and high phosphorus demands. Accordingly, the presence of legume- and nodule-related AMF can be explained by the specific nutritional requirements of legumes or by host-specific interactions among legumes, root nodules, and AMF. In summary, we found that AMF communities vary between plant functional groups (legumes and nonlegumes), between plant species, and between parts of a root system (roots and root nodules).     

丛枝菌根真菌（AMF）对外来植物入侵反馈机制的研究进展

丛枝菌根真菌（AMF）在植物群落竞争演替、物种多样性的形成及群落空间分布格局、植物群落对全球变化的响应中均起着重要的调节作用;同样也能影响外来植物与本地植物的互作,影响外来植物入侵过程中植物群落演替进程,甚至决定入侵的成败。因此,AMF与外来植物共生及其对外来植物入侵的反馈已成为国际上外来植物入侵机制研究的一个热点。本文基于外来植物的入侵过程,从AMF对外来植物生长、外来植物与本地植物竞争关系的影响,以及外来植物入侵对AMF的影响及AMF对入侵的反馈3个方面综述了AMF对外来植物入侵的反馈机制。外来植物可以通过多种途径改变土著AMF的群落结构和功能,而土著AMF也能直接或间接地改变甚至逆转外来植物与入侵地植物的互作关系。未来的研究不仅需要考虑AMF与外来植物共生的菌根特性和对竞争关系的影响,还需要通过大尺度条件下的野外试验及室内补充试验深入探究影响AMF在外来植物与本地植物竞争演替中的作用的生物和非生物因子,以全面解释AMF影响外来植物入侵的反馈机制。     

AM真菌物种多样性:生态功能、影响因素及维持机制

AM真菌物种多样性是土壤生态系统生物多样性的重要组分之一。尽管对AM真菌多样性已有多年研究,但是,已有研究绝大多数仅停留在对AM真菌群落种属解析层面上,对AM真菌物种多样性生态功能及维持机制方面的认识较浅。从生态功能、影响因素及维持机制3个方面系统地综述了近年来AM真菌多样性领域的研究进展。认为AM真菌多样性对植物群落生产力的调控机制及结合理论与实践解析AM真菌多样性维持机制是该领域未来的重点研究方向。     

Isolate Identity Determines Plant Tolerance to Pathogen Attack in Assembled Mycorrhizal Communities

Arbuscular mycorrhizal fungi (AMF) are widespread soil microorganisms that associate mutualistically with plant hosts. AMF receive photosynthates from the host in return for various benefits. One of such benefits is in the form of enhanced pathogen tolerance. However, this aspect of the symbiosis has been understudied compared to effects on plant growth and its ability to acquire nutrients. While it is known that increased AMF species richness positively correlates with plant productivity, the relationship between AMF diversity and host responses to pathogen attack remains obscure. The objective of this study was to test whether AMF isolates can differentially attenuate the deleterious effects of a root pathogen on plant growth, whether the richest assemblage of AMF isolates provides the most tolerance against the pathogen, and whether AMF-induced changes to root architecture serve as a mechanism for improved plant disease tolerance. In a growth chamber study, we exposed the plant oxeye daisy (Leucanthemum vulgare) to all combinations of three AMF isolates and to the plant root pathogen Rhizoctonia solani. We found that the pathogen caused an 81% reduction in shoot and a 70% reduction in root biomass. AMF significantly reduced the highly deleterious effect of the pathogen. Mycorrhizal plants infected with the pathogen produced 91% more dry shoot biomass and 72% more dry root biomass relative to plants solely infected with R. solani. AMF isolate identity was a better predictor of AMF-mediated host tolerance to the pathogen than AMF richness. However, the enhanced tolerance response did not result from AMF-mediated changes to root architecture. Our data indicate that AMF communities can play a major role in alleviating host pathogen attack but this depends primarily on the capacity of individual AMF isolates to provide this benefit.     

Resolving the ‘nitrogen paradox’ of arbuscular mycorrhizas: fertilization with organic matter brings considerable benefits for plant nutrition and growth

Arbuscular mycorrhizal fungi (AMF) can transfer nitrogen (N) to host plants, but the ecological relevance is debated, as total plant N and biomass do not generally increase. The extent to which the symbiosis is mutually beneficial is thought to rely on the stoichiometry of N, phosphorus (P) and carbon (C) availability. While inorganic N fertilization has been shown to elicit strong mutualism, characterized by improved plant and fungal growth and mineral nutrition, similar responses following organic N addition are lacking. Using a compartmented microcosm experiment, we determined the significance to a mycorrhizal plant of placing a ¹⁵N‐labelled, nitrogen‐rich patch of organic matter in a compartment to which only AMF hyphae had access. Control microcosms denied AMF hyphal access to the patch compartment. When permitted access to the patch compartment, the fungus proliferated extensively in the patch and transferred substantial quantities of N to the plant. Moreover, our data demonstrate that allowing hyphal access to an organic matter patch enhanced total plant N and P contents, with a simultaneous and substantial increase in plant biomass. Furthermore, we demonstrate that organic matter fertilization of arbuscular mycorrhizal plants can foster a mutually beneficial symbiosis based on nitrogen transfer, a phenomenon previously thought irrelevant.     

Aquaporin genes GintAQPF1 and GintAQPF2 from Glomus intraradices contribute to plant drought tolerance

Arbuscular mycorrhizal (AM) symbiosis, established between AM fungi (AMF) and roots of higher plants, occurs in most terrestrial ecosystems. It has been well demonstrated that AM symbiosis can improve plant performance under various environmental stresses, including drought stress. However, the molecular basis for the direct involvement of AMF in plant drought tolerance has not yet been established. Most recently, we cloned two functional aquaporin genes, GintAQPF1 and GintAQPF2, from AM fungus Glomus intraradices. By heterologous gene expression in yeast, aquaporin localization, activities and water permeability were examined. Gene expressions during symbiosis in expose to drought stress were also analyzed. Our data strongly supported potential water transport via AMF to host plants. As a complement, here we adopted the monoxenic culture system for AMF, in which carrot roots transformed by Ri-T DNA were cultured with Glomus intraradices in two-compartment Petri dishes, to verify the aquaporin gene functions in assisting AMF survival under polyethylene glycol (PEG) treatment. Our results showed that 25% PEG significantly upregulated the expression of two aquaporin genes, which was in line with the gene functions examined in yeast. We therefore concluded that the aquaporins function similarly in AMF as in yeast subjected to osmotic stress. The study provided further evidence to the direct involvement of AMF in improving plant water relations under drought stresses.     

Enhanced phytoremediation of soils contaminated with PAHs by arbuscular mycorrhiza and rhizobium

Greenhouse experiment was conducted to evaluate the potential effectiveness of a legume (Sesbania cannabina), arbuscular mycorrhizal fungi (AMF) (Glomus mosseae), and rhizobia (Ensifer sp.) symbiosis for remediation of Polycyclic aromatic hydrocarbons (PAHs) in spiked soil. AMF and rhizobia had a beneficial impact on each other in the triple symbiosis. AMF and/or rhizobia significantly increased plant biomass and PAHs accumulation in plants. The highest PAHs dissipation was observed in plant + AMF + rhizobia treated soil, in which >97 and 85–87% of phenanthrene and pyrene, respectively, had been degraded, whereas 81–85 and 72–75% had been degraded in plant-treated soil. During the experiment, a relatively large amount of water-soluble phenolic compounds was detected in soils of AMF and/or rhizobia treatment. It matches well with the high microbial activity and soil enzymes activity. These results suggest that the mutual interactions in the triple symbiosis enhanced PAHs degradation via stimulating both microbial development and soil enzyme activity. The mutual interactions between rhizobia and AMF help to improve phytoremediation efficiency of PAHs by S. cannabina.     

Mycorrhiza in floriculture: difficulties and opportunities

Arbuscular mycorrhizal fungi (AMF) establish symbiotic relationships with the roots of about 80% of plant species. The arbuscular mycorrhizal (AM) symbiosis benefits the growth and development of plants, among them a variety of ornamental horticulture (floriculture) plants. AMF have been shown to benefit plants that suffer from stunted growth, including that stemming from abiotic stress. Many floriculture crops are grown in the semi-arid environment of the Mediterranean region, and therefore AMF application may be implemented in floriculture practices to significantly promote crop growth and yield. However, for successful commercial utilization of the AM symbiosis and its introduction into ornamental floriculture practices, several considerations must be taken into account. These are highlighted and examined in the present review. They include the prerequisite of availability of high quantities of good-quality AMF inocula, the need to use different types and dosages of AMF inocula, at different times of inoculation, and the agricultural crop growth practices that are best-suited for integration with AMF inoculation. The difficulties and opportunities in AMF usage in floriculture are discussed and suggestions on different ways to solve the encountered difficulties are made, to promote cost-effective use of AMF in floriculture.     

Intra and Inter-Spore Variability in Rhizophagus irregularis AOX Gene

Arbuscular mycorrhizal fungi (AMF) are root-inhabiting fungi that form mutualistic symbioses with their host plants. AMF symbiosis improves nutrient uptake and buffers the plant against a diversity of stresses. Rhizophagus irregularis is one of the most widespread AMF species in the world, and its application in agricultural systems for yield improvement has increased over the last years. Still, from the inoculum production perspective, a lack of consistency of inoculum quality is referred to, which partially may be due to a high genetic variability of the fungus. The alternative oxidase (AOX) is an enzyme of the alternative respiratory chain already described in different taxa, including various fungi, which decreases the damage caused by oxidative stress. Nevertheless, virtually nothing is known on the involvement of AMF AOX on symbiosis establishment, as well on the existence of AOX variability that could affect AMF effectiveness and consequently plant performance. Here, we report the isolation and characterisation of the AOX gene of R. irregularis (RiAOX), and show that it is highly expressed during early phases of the symbiosis with plant roots. Phylogenetic analysis clustered RiAOX sequence with ancient fungi, and multiple sequence alignment revealed the lack of several regulatory motifs which are present in plant AOX. The analysis of RiAOX polymorphisms in single spores of three different isolates showed a reduced variability in one spore relatively to a group of spores. A high number of polymorphisms occurred in introns; nevertheless, some putative amino acid changes resulting from non-synonymous variants were found, offering a basis for selective pressure to occur within the populations. Given the AOX relatedness with stress responses, differences in gene variants amongst R. irregularis isolates are likely to be related with its origin and environmental constraints and might have a potential impact on inoculum production.