Phosphate transporters of Medicago truncatula and arbuscular mycorrhizal fungi

Most vascular plants acquire phosphate from their environment either directly, via the roots, or indirectly, via a symbiotic interaction with arbuscular mycorrhizal (AM) fungi. The symbiosis develops in the plant roots where the fungi colonize the cortex of the root to obtain carbon from the plant host, while assisting the plant with acquisition of phosphate and other mineral nutrients from the soil solution. As a first step toward understanding the molecular basis of the symbiosis and phosphate utilization, we have cloned and characterized phosphate transporter genes from the AM fungi Glomus versiforme and Glomus intraradices, and from the roots of a host plant, Medicago truncatula. Expression analyses and localization studies indicate that each of these transporters has a role in phosphate uptake from the soil solution.     

一种改进的丛枝菌根染色方法

研究改进了Vierheilig等描述的AM菌根染色法:将根样于20%KOH溶液中60℃水浴透明40-120 min,5%醋酸酸化5min后,用5%醋酸墨水染色液(派克纯黑书写墨水Quink),于60℃水浴染色30 min,清水浸泡脱色(14h)后即可镜检。根皮层细胞内AM真菌的丛枝结构清晰可见,并且能够明确地分辨AM真菌与其它未知真菌。此外,Quink初染后,再经过SudanⅣ复染(60℃、60 min),70%乙醇脱色5min,暗隔真菌的透明菌丝内所积聚的脂类颗粒被SudanⅣ染上鲜红色,在复式显微镜下能够观察到此类透明菌丝在根皮层组织内的存在状况。采用甘油明胶为封固剂制片,根的染色效果可以保存长久。此项技术可以对同一种植物的多个根样进行同步的透明和染色处理,而且操作简便、低毒性、成本低廉、染色效果极佳,适用于野生和栽培草本植物AM菌根的染色和制片观察。     

Comparison of two test systems for measuring plant phosphorus uptake via arbuscular mycorrhizal fungi

 Plant phosphorus uptake via external hyphae of arbuscular mycorrhizal fungi has been measured using compartmented systems where a hyphal compartment is separated from a rooting compartment by a fine mesh. By labelling the soil within the hyphal compartment with a radioactive phosphorus (P) isotope, hyphal uptake of P into the plant can be traced. The objective of this growth chamber study was to test two hyphal compartments of different design with respect to their suitabilities for measurement of hyphal P uptake. One hyphal compartment was simply a nylon mesh bag filled with ³²P-labelled soil. The labelled soil in the other hyphal compartment was completely surrounded by an 8–10 mm layer of unlabelled soil that served as a buffer zone. Mycorrhizal and non-mycorrhizal subterranean clover plants were grown in pots with a centrally positioned hyphal compartment. Uptake of radioactive P by non-mycorrhizal control plants was 25% of that by mycorrhizal plants with the mesh bag but only 3% when including the buffer zone. Based on this good control of non-mycorrhizal P uptake from within the hyphal compartment and its greater ease of handling once produced, we judged the hyphal compartment including a buffer zone to be superior to the mesh bag. Accepted: 15 September 1998     

Early responses of wild plant seedlings to arbuscular mycorrhizal fungi and pathogens

Many plants form associations with arbuscular mycorrhizal fungi (AMF) because they profit from improved phosphorus nutrition and from protection against pathogens. Whereas mycorrhiza-induced pathogen protection is well understood in agricultural plant species, it is rarely studied in wild plants. As many pathogens infest plants in the first days after germination, mycorrhiza-induced pathogen protection may be especially important in the first few weeks of plant establishment.Here, we investigated interacting effects of AMF and the seedling pathogen Pythium ultimum on the performance of six- to seven-week-old seedlings of six wild plant species of the family Asteraceae in a full factorial experiment.Plant species differed in their response to AMF, the pathogen and their interactions. AMF increased and the pathogen decreased plant biomass in one and three species, respectively. Two plant species were negatively affected by AMF in the absence, but positively or not affected in the presence of the pathogen, indicating protection by AMF. This mycorrhiza-induced pathogen protection is especially surprising as we could not detect mycorrhizal structure in the roots of any of the plants.Our results show that even seedlings without established intraradical hyphal network can profit from AMF, both in terms of growth promotion in the absence of a pathogen and pathogen protection. The function of AMF is highly species-specific, but tends to be similar for more closely related plant species, suggesting a phylogenetic component of mycorrhizal function. Further studies should test a wider range of plant species, as our study was restricted to one plant family, and investigate whether plants profit from early mycorrhizal benefits in the long term.     

高磷土壤中丛枝菌根真菌的分离鉴定

【背景】丛枝菌根(arbuscular mycorrhiza,AM)真菌具有广泛的寄主范围、环境适应性和优良的植物促生能力。然而,土壤的高磷水平严重抑制了AM真菌生长及AM形成。【目的】分离鉴定出耐较高有效磷含量的华南土著AM真菌菌株,为菌根学研究工作提供新颖材料。【方法】采用经典形态学和分子系统学方法鉴定高磷土壤中AM真菌。【结果】从有效磷含量为53-131 (平均值±标准差为88.2±17.6) mg/kg的根区土壤中鉴定出7属25种AM真菌,包括无梗囊霉属(Acaulospora) 12种、球囊霉属(Glomus) 7种、隔球囊霉属(Septoglomus) 2种、近明球囊霉属(Claroideoglomus) 1种、根孢囊霉属(Rhizophagus) 1种、硬囊霉属(Sclerocystis) 1种和类球囊霉属(Paraglomus) 1种,其中幼套近明球囊霉(Claroideoglomus etunicatum)和蜜色无梗囊霉(Acaulospora mellea)是优势种。在(87.7±8.0) mg/kg的高磷水平下,AM真菌仍能形成丛枝和泡囊。但当有效磷含量达到(99.7±1.2) mg/kg时,菌根侵染率和丛枝丰度显著下降,但仍能够形成泡囊。【结论】从广州市南沙区有效磷含量为(88.2±17.6) mg/kg的耕地植物根区土壤中,鉴定出具有耐高磷潜力的7属25种AM真菌,幼套近明球囊霉和蜜色无梗囊霉等分离株可作为后续高磷抑制机制解析及耐高磷AM真菌菌剂研发工作的试验菌株。     

培养容器容积对AM真菌生长发育的影响

研究宿主植物栽培容器对丛枝菌根(Arbuscularmycorrhizae,AM)真菌Glomusmosseae生长发育的影响。结果表明:小容积容器的根系密度相对较大,在菌根共生体建立初期,菌根真菌繁殖体与根接触的机会增大,对于菌根真菌的迅速侵染及共生体的迅速建立非常有利,同时还增大了根外菌丝二次侵染的机会,从而使菌根真菌生长发育形成了一个良性循环,最终有利于根外孢子的形成。容器对共生体的影响决不是简单的盆的体积问题,而与其面积和体积之比有关,也和种植密度有密切关系。     

Putative sites for nutrient uptake in arbuscular mycorrhizal fungi

Nutrition of the arbuscular mycorrhiza (AM) is addressed from a fungal point of view. Intraradical and extraradical structures proposed as preferential sites for nutrient acquisition in arbuscular mycorrhizal (AM) fungi are considered, and their main features compared. This comparison includes the formation and function of branched structures (either intra- or extraradical) as putative nutrient uptake sites with unique morphological and physiological features in the AM fungal colony. The morphology and functioning of these structures are further affected by intra- or extraradical environmental factors. A model is presented which portrays the intrinsic developmental and physiological duality of the AM fungus. This revised version was published online in June 2006 with corrections to the Cover Date.     

Temperature constraints on the growth and functioning of root organ cultures with arbuscular mycorrhizal fungi

In this study we investigated the effects of temperature on fungal growth and tested whether the differences in fungal growth were related to the effects of temperature on carbon movement to, or within, the fungus. Growth curves and C uptake-transfer-translocation measurements were obtained for three arbuscular mycorrhizal fungi (AMF) isolates cultured within a 6-30 degrees C temperature range. A series of experiments with a model fungal isolate, Glomus intraradices, was used to examine the effects of temperature on lipid body and 33P movement, and to investigate the role of acclimation and incubation time. Temperature effects on AMF growth were both direct and indirect because, despite clear independent root and AMF growth responses in some cases, the uptake and translocation of 13C was also affected within the temperature range tested. Root C uptake and, to a lesser extent, C translocation in the fungus, were reduced by low temperatures (< 18 degrees C). Uptake and translocation of 33P by fungal hyphae were, by contrast, similar between 10 and 25 degrees C. We conclude that temperature, between 6 and 18 degrees C, reduces AMF growth, and that C movement to the fungus is involved in this response.     

Rapid and sensitive bioassay to study signals between root exudates and arbuscular mycorrhizal fungi**

A sensitive bioassay was developed to provide a way to detect chemical signals from host plants which induce changes in hyphal growth patterns of germinated spores of arbuscular mycorrhizal (AM) fungi. The assay can be used to test host root exudates, as well as particulate fractions (root cap border cells and root mucilage), for their ability to affect AM fungal growth. Hyphal branching, induced by various host root components, can be detected as early as 4 h although results of the bioassay were usually determined after 16 to 24 h. The type of branching pattern observed was dose-dependent.     

丛枝菌根真菌对植物耐旱性的影响研究进展

丛枝菌根真菌(arbuscular mycorrhizal fungi,AMF)能与植物根系形成互惠共生体,对植物的生长发育和抗逆性有积极的影响,在改善植物水分代谢和提高植物耐旱性中发挥了重要作用.本文综述了近年来AMF与植物水分代谢关系的研究进展,从植物的光合作用、蒸腾与气孔导度、水分利用效率、水力导度、渗透调节、内源激素和抗氧化系统等方面说明AMF对植物水分代谢的影响.从4个方面介绍了AMF提高植物耐旱性的机理:1)菌丝网络增加植物根系吸收范围;2)增强植物保水能力和抗氧化能力;3)稳定和改善土壤团聚体;4)促进植物养分吸收.并提出今后研究需注意的问题和建议.     

Different growth response of five co-existing stoloniferous plant species to inoculation with native arbuscular mycorrhizal fungi

Five species of stoloniferous plants originating from the same field site (Galeobdolon montanum, Glechoma hederacea, Potentilla anserina, Ranunculus repens and Trifolium repens) were studied with respect to their interaction with arbuscular mycorrhizal (AM) fungi. More specifically, the question was addressed whether mycorrhizal growth response of host plant species could be related to their vegetative mobility. The roots of all the species examined were colonised with AM fungi in the field, with the percentage of colonisation varying among species from approximately 40% to 90%. In a subsequent pot experiment, plants of all the species were either left non-inoculated or were inoculated with a mixture of three native AM fungi isolated from the site of plant origin (Glomus mosseae, G. intraradices and G. microaggregatum). AM fungi increased phosphorus uptake in all the plant species; however, plant growth response to inoculation varied widely from negative to positive. In addition to the biomass response, AM inoculation led to a change in clonal growth traits such as stolon number and length or ramet number in some species. Possible causes of the observed differences in mycorrhizal growth response of various stoloniferous plants are discussed.     

Wheat responses to aggressive and non-aggressive arbuscular mycorrhizal fungi

In southwestern Australia fields, colonization of wheat roots by arbuscular mycorrhizal fungi (AMF) is reduced due to repeated use of phosphate (P) fertilizers. We predicted AMF that aggressively colonize wheat roots at low P supply would also aggressively colonize at high P supply, but provide no additional P uptake benefit and reduce growth. Wheat (cv. Kulin) seedlings were non-mycorrhizal (NM) or inoculated separately with 10 isolates of AMF from wheat-belt soils in a glasshouse experiment. Kojonup loamy sand was supplied with P to provide suboptimal and supraoptimal P for growth of NM wheat in this soil. At low P supply, wheat growth was limited by P availability. All AMF isolates colonized wheat roots at 14 days after emergence of seedlings. At 42 days, percentage root length colonized (%RLC) was highest for two isolates of Scutellospora calospora, WUM 12(2) and WUM 12(3), followed by Glomus sp. WUM 51, G. invermaium WUM 10(1), Acaulospora laevis WUM 11(4) and Gigaspora decipiens WUM 6(1). These isolates, designated as `aggressive colonizers', ranged from 50 to 89%RLC. A second group of AMF ranged from 1 to 19%RLC at 42 days. This group, termed `non-aggressive colonizers', included Acaulospora spp. WUM 11(1), WUM 46, and WUM 49 and Glomus sp. WUM 44. High soil P supply increased seedling growth 2–3 fold, but reduced%RLC. Grouping of aggressive and non-aggressive AMF based on colonization rate at high P supply was similar to that at low P. At low P supply, only the two isolates of S. calospora increased wheat growth compared to the NM plant. The remaining aggressive and non-aggressive AMF reduced growth of wheat at low P, while aggressive colonizers reduced growth at high P. At low P supply, the aggressive colonizers increased shoot P concentration, while at high P, shoot P was not affected by AMF. Growth depression by aggressive colonizers was associated with reduced sucrose concentration in roots. Based on the negative growth response under low and high P fertility in the glasshouse, AMF could be expected to produce non-beneficial effects on wheat in the field depending on the P status of the soil and the aggressiveness of AMF in the community. This revised version was published online in June 2006 with corrections to the Cover Date.     

A petunia mutant affected in intracellular accommodation and morphogenesis of arbuscular mycorrhizal fungi

The regulation of the arbuscular mycorrhizal (AM) symbiosis is largely under the control of a genetic programme of the plant host. This programme includes a common symbiosis signalling pathway that is shared with the root nodule symbiosis. Whereas this common pathway has been investigated in detail, little is known about the mycorrhiza-specific regulatory steps upstream and downstream of the common pathway. To get further insight in the regulation of the AM symbiosis, a transposon-mutagenized population of Petunia hybrida was screened for mutants with defects in AM development. Here, we describe a petunia mutant, penetration and arbuscule morphogenesis1 (pam1), which is characterized by a strong decrease in colonization by three different AM fungi. Penetrating hyphae are frequently aborted in epidermal cells. Occasionally the fungus can progress to the cortex, but fails to develop arbuscules. The resulting hyphal colonization of the cortex in mutant plants does not support symbiotic acquisition of phosphate and copper by the plant. Expression analysis of three petunia orthologues of the common SYM genes LjPOLLUX, LjSYMRK and MtDMI3 indicates that pam1 is not mutated in these genes. We conclude that the PAM1 gene may play a specific role in intracellular accommodation and morphogenesis of the fungal endosymbiont.