氮有效性增加对细根解剖、形态特征和菌根侵染的影响

氮(N)有效性增加对森林生态系统结构和功能有重要影响.细根作为树木地下最为活跃的组分,其对N有效性增加的响应已成为森林生态学研究的热点.本文对N有效性增加条件下细根解剖构造、形态和菌根侵染的响应趋势及潜在机制进行了综述.N有效性增加导致细根皮层厚度、皮层层数和皮层细胞直径下降,外皮层上通道细胞数量减少,而中柱和木质部横截面积,以及导管(或管胞)直径、数量和壁厚度均增加,根解剖构造的响应与植物激素浓度变化有关;N有效性增加后菌根侵染率下降,比根长(单位根干质量的长度)在外生菌根树种中普遍下降,而在内生菌根树种中增加;根直径和组织密度的变化趋势则表现出较大的种间差异.这些个体根和树种水平上的结果对于理解森林生态系统水平上的碳和养分循环具有重要意义.最后,根据目前研究中存在的不足提出了今后的研究方向与问题.     

海南岛4个热带阔叶树种前5级细根的形态、解剖结构和组织碳氮含量

细根具有复杂的分支系统, 以根序(root order)为取样单元的细根生理生态学研究正在成为根系生态学研究领域的重要内容。该研究以海南岛尖峰岭4个热带阔叶树种海南蕈树(Altingia obovata)、厚壳桂(Cryptocarya chinensis)、山杜英(Elaeocarpus sylvestris)和黄桐(Endospermum chinense)为研究对象, 测定了1-5级细根的形态、解剖结构和组织碳(C)、氮(N)含量, 旨在探讨这些根系特征之间的联系。研究表明: 4个树种的细根形态差异较大, 但在树种水平上直径、根长和组织密度均随着根序的升高而增加, 比根长则随着根序的升高而降低; 低级根(前2级根或前3级根)具有皮层组织, 是典型的吸收根, 而高级根皮层组织消失, 是典型的运输和储藏根; 影响直径大小最重要的因子是皮层厚度, 它可以解释细根直径变异的97%, 而维管束直径仅能解释细根直径变异的70%; 根组织N和C浓度受维管束-根直径比(维根比)的影响, 随着维根比增加, 组织N浓度显著降低, 组织C浓度显著升高。4个树种细根的C/N比的变异受组织N浓度的影响程度为76%, 而受C浓度的影响程度不足10%。上述结果表明, 细根的形态特征、解剖结构和组织化学含量之间存在着紧密联系, 这为我们理解根系结构与功能变异提供了重要依据。     

The platform for European root science,COST action E38: An introduction and overview

Abstract

Globally, forests cover 4 billion ha or 30% of the Earth's land surface and account for more that 75% of carbon stored in terrestrial ecosystem. However, 20 – 40% of the forest biomass is roots. Roots play a key role in acquisition of water and nutrients from the soil, the transfer of carbon to soil, as well as providing physical stabilisation. In temperate forests of Europe, average biomass of trees is estimated to be ca. 220 t ha^?1, of which 52 t ha^?1 are coarse roots and 2.4 t ha^?1 are fine roots. Thus, forests and their soils belong to the planets largest reservoirs of carbon. As an outcome of a recently established European platform for scientists working on woody roots, COST action E38, a series of papers has been initiated in order to review the current knowledge on processes in and of roots of woody plants and to identify possible knowledge gaps. These reviews concentrate on aspects of roots as indicators of environmental change, biomass of fine roots, and modelling of course root systems. The reviews of roots as indicators of environmental change cover a number of aspects including, specific root length, the calcium to aluminium ratio, root electrolyte leakage, and ectomycorrhiza community composition.     

Citrus root responses to localized drying soil: A new approach to studying mycorrhizal effects on the roots of mature trees

Because fine roots tend to be concentrated at the soil surface, exposure to dry surface soil can have a large influence on patterns of root growth, death and respiration. We studied the effects of arbuscular mycorrhizas (AM) formation on specific root length (SRL), respiration and mortality of fine roots of bearing red grapefruit (Citrus paradisi Macf.) trees on Volkamer lemon (C. volkameriana Tan. & Pasq.) rootstock exposed to drying soil. For each tree, the fine roots were removed from two woody lateral roots, the roots were surface sterilized and then each woody root was placed in a separate pair of vertically divided and independently irrigated soil compartments. The two split-pot systems were filled with sterilized soil and one was inoculated with arbuscular mycorrhizal fungi (Glomus etunicatum/G. intraradices). New fine lateral roots that emerged from the woody laterals were permitted to grow inside the pots over a 10-month period. Irrigation was then removed from the top compartment for a 15-week period. At the end of the study, roots inoculated with AM fungi exhibited about 20% incidence of AM formation, whereas the uninoculated roots were completely void of AM fungi. Arbuscular mycorrhizal roots exhibited lower SRL, lower root/soil respiration and about 10% lower fine root mortality than nonmycorrhizal roots after 15 weeks of exposure to dry surface soil. This study demonstrates the feasibility of examining mycorrhizal effects on the fine roots of adult trees in the field using simple inexpensive methods.     

Occurrence of arbuscular mycorrhiza on aged Eucalyptus

Eucalyptus despite the fact that Eucalyptus seedlings do form both endomycorrhiza and ectomycorrhiza early during their developement. In the present study, all the structures of arbuscular mycorrhiza were observed within roots of four Eucalyptus species of 15, 17 and more than 50 years old at three different sites in northern Algeria. Arbuscular mycorrhiza frequency was assessed in roots of 15-years old Eucalyptus camaldulensis species, during two periods in 2 consecutive years (July and November of 1996 and 1997). Intensity of root colonization was dependent on the time of sampling and attained 42% in July 1997. Accepted: 17 September 1999     

Accumulation of secondary compounds in barley and wheat roots in response to inoculation with an arbuscular mycorrhizal fungus and co-inoculation with rhizosphere bacteria

 Colonization of Hordeum vulgare L. cv. Salome (barley)and Triticum aestivum L. cv. Caprimus (wheat) roots by the arbuscular mycorrhizal fungus Glomus intraradices Schenck & Smith leads to de novo synthesis of isoprenoid cyclohexenone derivatives with blumenin [9-O-(2′-O-β-glucuronosyl)-β-glucopyranoside of 6-(3-hydroxybutyl)-1,1,5-trimethyl-4-cyclohexen-3-one] as the major constituent and to transient accumulation of hydroxycinnamate amides (4-coumaroylagmatine and -putrescine). Accumulation of these compounds in mycorrhizal wheat roots started 2 weeks after sowing together with the onset of arbuscule formation and proceeded with mycorrhizal progression. Highest levels were reached in 3- to 4-week-old secondary roots (root branches of first and higher order) characterized by the formation of vesicles. In the final developmental stages, the fungus produced massive amounts of spores, enclosing the stele of older root parts (older than 5 weeks) characterized by cortical death. In these root parts, the secondary compounds were detected in trace amounts only, indicating that they were located in the cortical tissues. Some rhizosphere bacteria tested, i.e. Agrobacterium rhizogenes, Pseudomonas fluorescens, and Rhizobium leguminosarum, markedly stimulated both fungal root colonization and blumenin accumulation, thus, acting as mycorrhiza-helper bacteria (MHB). Application of blumenin itself strongly inhibited fungal colonization and arbuscule formation at early stages of mycorrhiza development. This was associated with a markedly reduced accumulation of the hydroxycinnamate amides 4-coumaroylputrescine and -agmatine. The results suggest that both the isoprenoid and the phenylpropanoid metabolism are closely linked to the developmental stage and the extent of fungal colonization. Their possible involvement in the regulation of mycorrhiza development is discussed. Accepted: 18 September 1998     

Root Anatomy and Mycotrophy (AM) of the Achlorophyllous Voyria truncata (Standley) Standley & Steyermark (Gentianaceae)

Roots of Voyria truncata retain the primary root structure even though they can grow as thick as 2 mm in diameter. These root diameters are due to a retained capability for cell division in the cortex parenchyma. This is explained as a vital adaptation to its life form. Based on the extraradical mycelium, the mode of penetration, the structurally incompatible intraradical phase, the presence of intercellular vesicles in the root cortex, and the occurrence of immediate hyphal bridges from arbuscular mycorrhizal roots of neighbouring plants, the mycorrhiza of V. truncata is described as an arbuscular mycorrhiza (AM), although the characteristic arbuscles are missing. Special features of the AM in V. truncata are interpreted as an improved efficiency in taking advantage of the mycorrhiza. Root connections with roots of neighbouring plants are common and preferred locations for fungal infections. An evolutionary tendency towards parasitism of higher plants is discussed.     

常绿阔叶林外生、内生菌根树种细根化学计量学性状对N添加的响应

为揭示不同菌根类型树种细根化学计量学性状对N添加的塑性响应,在福建省建瓯市万木林自然保护区常绿阔叶林内选择外生菌根树种罗浮栲(Castanopsis faberi)和内生菌根树种木荷(Schima superba)为研究对象,采用根袋法开展N添加试验,细根在根袋内生长半年后测定化学计量学指标(C、N、P、C/N、N/P、C/P)。结果表明:根序对细根化学计量学性状有显著影响,随着根序的增加,罗浮栲与木荷细根C浓度、C/N、C/P明显增加,N浓度与P浓度明显下降。N添加对细根C、N浓度均有极显著的促进作用,但对细根P浓度影响不显著,从而导致细根C/N维持稳定,但N/P、C/P升高,细根受P限制增加。细根化学计量学性状对N添加的塑性响应在不同序级间以及在外生菌根树种罗浮栲和内生菌根树种木荷之间均无显著差异。结论表明,研究所选内生、外生菌根树种细根化学计量学性状对N添加具有基本相似的响应。     

Changes in plastid proteome and structure in arbuscular mycorrhizal roots display a nutrient starvation signature

During arbuscular mycorrhizal symbiosis, arbuscule‐containing root cortex cells display a proliferation of plastids, a feature usually ascribed to an increased plant anabolism despite the lack of studies focusing on purified root plastids. In this study, we investigated mycorrhiza‐induced changes in plastidic pathways by performing a label‐free comparative subcellular quantitative proteomic analysis targeted on plastid‐enriched fractions isolated from Medicago truncatula roots, coupled to a cytological analysis of plastid structure. We identified 490 root plastid protein candidates, among which 79 changed in abundance upon mycorrhization, as inferred from spectral counting. According to cross‐species sequence homology searches, the mycorrhiza‐responsive proteome was enriched in proteins experimentally localized in thylakoids, whereas it was depleted of proteins ascribed predominantly to amyloplasts. Consistently, the analysis of plastid morphology using transmission electron microscopy indicated that starch depletion associated with the proliferation of membrane‐free and tubular membrane‐containing plastids was a feature specific to arbusculated cells. The loss of enzymes involved in carbon/nitrogen assimilation and provision of reducing power, coupled to macromolecule degradation events in the plastid‐enriched fraction of mycorrhizal roots that paralleled lack of starch accumulation in arbusculated cells, lead us to propose that arbuscule functioning elicits a nutrient starvation and an oxidative stress signature that may prime arbuscule breakdown.     

The importance of anatomical structural study of roots for the understanding of physiological processes

Abstract

Besides being species‐specific, the inner structure of the root is influenced by the place and time of origin during the growth period. From the root tip up to the base of a particular root, the zones of cell division, cell elongation, formation of root hairs and root branching can be distinguished. The root tip that is covered by a root cap and mucilage is protected against evaporation and water contact. From the end of the lateral parts of the root cap, the cells become exposed to the surrounding environment. The cells can elongate by water uptake or can shrink by water loss. All processes of geotropic growth take place there. In this study, some differences are illustrated using Zea mays plants. Radicle and roots emerging from several nodes of the shoot as well as lateral roots are compared. The distances from the tip and from the base of the root are also very important for characterization of particular root functions. Distinctive features such as root diameter, size of the stele and of the cortex, ratio of cortex to stele, number and width of the xylem vessels, size of cells, special thickenings and stage of lignification as well as symptoms of maturation are observed.     

Cloning of Pinus sylvestris SCARECROW gene and its expression pattern in the pine root system, mycorrhiza and NPA-treated short roots

The SCARECROW (SCR) gene is central to root radial patterning. Its expression has not been investigated in conifers with morphologically different root types. Additional interest in SCR functions in the Pinus sylvestris root system comes from the effect of ectomycorrhiza formation on the short root apical structure. Here, the P. sylvestris SCR gene (PsySCR) was cloned and its expression investigated by northern blot and in situ hybridization of primary, lateral and short roots and mycorrhiza. Short root dichotomization was induced by auxin transport inhibitor (N-1-naphthylphthalamic acid (NPA)). PsySCR has conserved GRAS family protein motifs at the C-terminus and a variable N-terminus. PsySCR expression occurred in young root tissue and mycorrhiza. In root sections the PsySCR signal runs through the tip in initials for stele and root cap column and becomes upwards-restricted to endodermis in all root types. The PsySCR expression pattern suggests for the first time a regulatory role for SCR in maintaining the endodermal characteristics and radial patterning of roots with open meristem organization. The specific PsySCR localization is also an excellent marker for investigation of the dichotomization process in short roots.     

Root Anatomy and Mycotrophy of the Achlorophyllous Voyria tenella Hook. (Gentianaceae)

The central cylinder of the root of Voynet tenella consists of up to ten central, non-lignified, tracheidal xylem elements surrounded by some parenchymatic tissue and 5–7 groups of phloem. A pericycle could not be discerned. Even though the endodermis carries a faint suberin lamella it cannot be discerned anatomically without special staining. The cells of the 1–3 cortex layers next to the endodermis are elongated longitudinally, the subsequent cortex parenchyma is multi-layered and consists of isodiametric cells. The cells of the 2–3 layered outer dermal tissue are smaller than those of the adjacent cortex, their walls carry a suberin lamella and the outermost of them constantly scale off. The dermal tissue is interpreted as a multilayered exodermis. The fungal colonization in roots of Voyria tenella remarkably differs from any known mycorrhizal pattern. After having penetrated the dermal tissue, the always intracellularly-growing hyphae head straight towards the inner cortex layers, where they spread along the central cylinder. Ramifications from these inner-spreading hyphae then colonize the cortex parenchyma from the inside and they develop dense hyphal coils. Eventually, the coiled hyphae swell and collapse, resulting in amorphous clumps of fungal material. This mycorrhizal pattern is referred to as an intraradical fungus garden. Arguments are given to call the mycorrhiza in Voyria tenella a specialized arbuscular mycorrhiza. Phylogenetic and ecological implications of the observations and the results are discussed.     

Effects of Co-Inoculation with Arbuscular Mycorrhizal Fungi and Rhizobia on Fungal Occupancy in Chickpea Root and Nodule Determined by Real-Time PCR

Legume roots in nature are usually colonized with rhizobia and different arbuscular mycorrhizal fungi (AMF) species. Light microscopy that visualizes the presence of AMF in roots is not able to differentiate the ratio of each AMF species in the root and nodule tissues in mixed fungal inoculation. The purpose of this study was to characterize the dominant species of mycorrhiza in roots and nodules of plants co-inoculated with mycorrhizal fungi and rhizobial strains. Glomus intraradices (GI), Glomus mosseae (GM), their mix (GI + GM), and six Mesorhizobium ciceri strains were used to inoculate chickpea. Quantitative real-time polymerase chain reaction (qRT-PCR) was used to assess occupancy of these fungal species in roots and nodules. Results showed that GI molecular ratio and relative density were higher than GM in both roots and nodules. These differences in molecular ratio and density between GI and GM in nodules were three folds higher than roots. The results suggested that M. ciceri strains have different effects on nodulation and mycorrhizal colonization pattern. Plants with bacterial S3 and S1 strains produced the highest root nodulation and higher fungal density in both the roots and nodules.     

华北山地6个外生菌根树种的根属性及其与菌根侵染率的关系

该研究以共存于同一暖温带森林的6个外生菌根(ECM)树种为研究对象,测定分析不同根序(1～5级)和功能根系(吸收细根和运输细根)的主要形态和构型属性及ECM侵染率,探究不同外生菌根树种的根属性变异模式及其与菌根真菌侵染程度的关系。结果表明:(1)随着根序的增加,不同树种根直径和单根长度均增加,而比根长和根分支强度均降低;根属性在同一根序下均存在显著的种间差异,尤其是2个裸子植物(落叶松和油松)的根直径较其他4个被子植物大。(2)同一树种的所有根属性在吸收细根和运输细根之间均有显著差异;吸收细根和运输细根的根直径、比根长和根组织密度在树种间均存在显著差异,而其单根长度和根分支强度在树种间无显著差异。(3)ECM侵染率以落叶松最高,千金榆和白桦最低,且与根尖直径呈显著正相关关系,与根尖比根长呈显著负相关关系。研究发现,基于根序或者功能根系,根属性在种间的变异模式不完全一致,单根长度和根分支强度在两个功能根系中均没有表现出显著的种间差异;吸收细根的比根长和根分支强度的变异系数较大,对环境变化有较敏感的响应;古老树种的根直径相对较粗,对菌根真菌的依赖性更高。     

Arbuscular mycorrhizal colonization outcompetes root hairs in maize under low phosphorus availability

Background and AimsAn increase in root hair length and density and the development of arbuscular mycorrhiza symbiosis are two alternative strategies of most plants to increase the root–soil surface area under phosphorus (P) deficiency. Across many plant species, root hair length and mycorrhization density are inversely correlated. Root architecture, rooting density and physiology also differ between species. This study aims to understand the relationship among root hairs, arbuscular mycorrhizal fungi (AMF) colonization, plant growth, P acquisition and mycorrhizal-specific P_i transporter gene expression in maize.MethodsUsing nearly isogenic maize lines, the B73 wild type and the rth3 root hairless mutant, we quantified the effect of root hairs and AMF infection in a calcareous soil under P deficiency through a combined analysis of morphological, physiological and molecular factors.Key ResultsWild-type root hairs extended the rhizosphere for acid phosphatase activity by 0.5 mm compared with the rth3 hairless mutant, as measured by in situ zymography. Total root length of the wild type was longer than that of rth3 under P deficiency. Higher AMF colonization and mycorrhiza-induced phosphate transporter gene expression were identified in the mutant under P deficiency, but plant growth and P acquisition were similar between mutant and the wild type. The mycorrhizal dependency of maize was 33 % higher than the root hair dependency.ConclusionsThe results identified larger mycorrhizal dependency than root hair dependency under P deficiency in maize. Root hairs and AMF inoculation are two alternative ways to increase P_i acquisition under P deficiency, but these two strategies compete with each other.     

亚热带典型树种根毛特征及其与共生真菌的关系

根毛和共生真菌增加了吸收面积,提高了植物获取磷等土壤资源的能力。由于野外原位观测根表微观结构较为困难,吸收细根、根毛、共生真菌如何相互作用并适应土壤资源供应,缺乏相应的数据和理论。该研究以受磷限制的亚热带森林为对象,选取了21种典型树种,定量了根毛存在情况、属性变异,分析了根毛形态特征与共生真菌侵染率、吸收细根功能属性之间的关系,探讨了根表结构对低磷土壤的响应和适应格局。结果表明:1)在亚热带森林根毛不是普遍存在的, 21个树种中仅发现7个树种存有根毛, 4个为丛枝菌根(AM)树种, 3个为外生菌根(ECM)树种。其中,马尾松(Pinus massoniana)根毛出现率最高,为86%;2)菌根类型是理解根-根毛-共生真菌关系的关键,AM树种根毛密度与共生真菌侵染率正相关,但ECM树种根毛直径与共生真菌侵染率负相关; 3) AM树种根毛长度和根毛直径、ECM树种根毛出现率与土壤有效磷含量呈负相关关系。该研究揭示了不同菌根类型树种根毛-共生真菌-根属性的格局及相互作用,为精细理解养分获取策略奠定了基础。     

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.     

Colonisation patterns of root tissues byPhytophthora nicotianae var.parasitica related to reduced disease in mycorrhizal tomato

Tomato plants pre-colonised by the arbuscular mycorrhizal fungusGlomus mosseae showed decreased root damage by the pathogenPhytophthora nicotianae var.parasitica. In analyses of the cellular bases of their bioprotective effect, a prerequisite for cytological investigations of tissue interactions betweenG. mosseae andP. nicotianae v.parasitica was to discriminate between the hyphae of the two fungi within root tissues. We report the use of antibodies as useful tools, in the absence of an appropriate stain for distinguishing hyphae ofP. nicotianae v.parasitica from those ofG. mosseae inside roots, and present observations on the colonisation patterns by the pathogenic fungus alone or during interactions in mycorrhizal roots. Infection intensity of the pathogen, estimated using an immunoenzyme labelling technique on whole root fragments, was lower in mycorrhizal roots. Immunogold labelling ofP. nicotianae v.parasitica on cross-sections of infected tomato roots showed that inter or intracellular hyphae developed mainly in the cortex, and their presence induced necrosis of host cells, the wall and contents of which showed a strong autofluorescence in reaction to the pathogen. In dual fungal infections of tomato root systems, hyphae of the symbiont and the pathogen were in most cases in different root regions, but they could also be observed in the same root tissues. The number ofP. nicotianae v.parasitica hyphae growing in the root cortex was greatly reduced in mycorrhizal root systems, and in mycorrhizal tissues infected by the pathogen, arbuscule-containing cells surrounded by intercellularP. nicotianae v.parasitica hyphae did not necrose and only a weak autofluorescence was associated with the host cells. Results are discussed in relation to possible processes involved in the phenomenon of bioprotection in arbuscular mycorrhizal plants.