湿地植物根系泌氧及其在自然基质中的扩散效应研究进展

湿地植物根系径向泌氧(ROL)是构造根际氧化-还原异质微生态系统的核心要素,其扩散层为好氧、厌氧微生物提供了良好生境并促进其代谢活动,使湿地植物根际成为有机物降解、物质循环及生命活动最为强烈的场所,已有成果证明湿地植物根系ROL的强弱与污染物的去除效果密切相关。因此,开展湿地植物根系ROL及其在自然基质中的扩散效应研究,对于了解湿地植物根系ROL机理及其根际氧环境特征,进而发挥湿地植物的污染去除功能具有十分重要的意义。基于此,首先归纳了湿地植物根系ROL特征及其受影响机制的研究现状,而后从种属差异、时空分布及对微生物的影响等方面对根系ROL在自然基质中的扩散效应国内外研究成果进行了总结,最终根据研究现状与不足对今后的研究方向进行了简要展望。创新之处在于:1)提出影响根系氧供给及氧输送释放通道的环境、生物等因素,阐述了其对根系ROL的影响机制;2)着重阐述了目前研究较少提及的根系ROL扩散效应测定方法。     

植物根系对根际微环境扰动机制研究进展

根际微环境是构建植物与土壤交流沟通的桥梁,也是植物遭受胁迫时优先作出响应的区域。植物根系作为根际的主要调控者,根构型和根系分泌物种类、数量的改变均可对根际微生物和土壤动物种群分布及其结构造成影响。然而,土壤动物的扰动、微生物的分解作用也可改善根际土壤特性,提高植物抗逆性及养分利用效率,从而促进植物根系生长。可见,植物根系-根际动物-根际微生物之间存在复杂的互作关系。本文从根际内、外微环境出发,分析了根际外植物根系对微环境的物理和化学扰动、根际内植物根系与微生物的互作扰动、根际内植物根系和土壤动物的物理扰动、以植物根系分泌物为介质的化学扰动等方面研究进展,在此基础上,论述了根际微环境主要影响因子之间的互作机制,并对该领域的研究方向进行了展望。     

植物根际微生物群落构建的研究进展

植物根际是指植物根系与土壤的交界面,是根系自身生命活动和代谢对土壤影响最直接、最强烈的区域,其物理、化学和生物性质不同于土体土壤。在这个区域里,与植物发生相互作用的大量微生物,被称为根际微生物。根际微生物在植物的生长发育和植物病虫害的生物防治等方面都具有十分重要的意义。本文总结了根际微生物群落构建的研究现状,介绍了根际微生物的经典和最新的研究方法,包括根箱法、同位素技术以及高通量测序、菌群定量分析、高通量分离培养等方法在根际微生物研究中的应用,讨论了植物根系分泌物(碳水化物、氨基酸、黄酮类、酚类、激素及其信号物质)和土壤物理化学性质对根际微生物群落的影响,概述了根际微生物-植物的互作机制,以及根际微生物群落对植物的促生作用、提高植物抗逆性和抑制作用,并对根际微生物群落研究中存在的问题和未来发展方向进行了展望。     

缺铁敏感度不同的大豆品种对缺铁的适应机制

与供铁处理相比,对缺铁敏感的大豆品种“哈８３”幼苗在缺铁胁迫和上根际没有酸化现象,根系对Ｆｅ（Ⅲ）的还原能力也没有明显增强。但抗缺铁的大豆品种“８７０１”幼苗根际则严重酸化,根系对Ｆｅ（Ⅲ）的还原能力显著增强;加入能抑制根系Ｈ＾＋－ＡＴＰ酶活性、减弱根际酸化作用的Ｈ＾＋－ＡＴＰ酶抑制剂正钒酸钠会降低根系对Ｆｅ（Ⅲ）的还原能力;说明根际酸化与根系还原Ｆｅ（Ⅲ）能力相互联系,初步证实根细胞原生质膜Ｈ＾     

湿地植物根表的铁锰氧化物膜

湿地植物根系具有泌氧能力 ,使其根表及根际微环境呈氧化状态。因而 ,土壤溶液中一些还原性物质被氧化 ,如 Fe2 ,Mn2 ,形成的氧化物呈红色或红棕色胶膜状包裹在根表 ,称为铁锰氧化物膜。铁锰氧化物膜及其根际微环境是湿地植物根系吸收养分和污染物的门户 ,势必会影响这些物质的吸收。主要综述了铁锰氧化物膜的形成和组成 ,以及根表形成的氧化物膜的生态效应 ,也就是氧化物胶膜对植物根系吸收外部介质中的养分及污染物质——重金属离子的影响     

缺铁敏感度不同的大豆品种对缺铁的适应机制

与供铁处理相比,对缺铁敏感的大豆品种“哈８３”幼苗在缺铁胁迫条件下根际没有酸化现象,根系对Ｆｅ（Ⅲ）的还原能力也没有明显增强。但抗缺铁的大豆品种“８７０１”幼苗根际则严重酸化,根系对Ｆｅ（Ⅲ）的还原能力显著增强;加入能抑制根系Ｈ＋－ＡＴＰ酶活性、减弱根际酸化作用的Ｈ＋－ＡＴＰ酶抑制剂正钒酸钠会降低根系对Ｆｅ（Ⅲ）的还原能力;说明根际酸化与根系还原Ｆｅ（Ⅲ）能力相互联系,初步证实根细胞原生质膜Ｈ＋－ＡＴＰ酶和缺铁诱导的还原酶相互偶联的假说。     

湿地植物根表铁膜研究进展

为了适应渍水环境,许多湿地植物都具有根系泌氧、形成铁膜的能力。因铁膜具有特殊的物理或化学结构,可以通过吸附和共沉淀作用影响元素在土壤中的化学行为和生物有效性,在植物吸收营养元素和重金属中起重要作用。综述了湿地植物根表铁膜的形成、影响因素以及根表铁膜对营养元素和重金属的生态环境效应,从表征技术方面阐述了根表铁膜的作用机制。对今后的研究方向给出如下建议:(1)扩大研究领域;(2)铁膜形成的动态变化过程;(3)铁膜对植物生理形态的影响;(4)利用先进的表征技术以确定铁膜的作用机制。     

现代生物技术在根际微生物群落研究中的应用

根际微生物的定义为植物的根表及受根系直接影响的土壤区域中的微生物群落.根际微生物与植物根系相互作用,是植物生态系统物质交换的主要媒介.根际微生物群落的研究对于微生物生态学与植物生态学均具有重要意义.近年来现代生物学技术发展迅速,对根际自然群落中不可培养微生物的研究取得了突破.综述了目前已取得的研究进展,对现代生物技术在根际微生物群落研究中的应用做了讨论.     

重金属不同积累型植物品种的根际生态效应分析

植物对重金属的吸收积累与其根际环境密切相关, 因此对于根际生态效应的研究有助于理解不同品种积累重金属差异的机理。从根系形态、根际土壤化学特性、根系分泌物及微生物特性等方面对根际生态效应和植物积累重金属的关系进行了综述, 旨在从根际方面诠释植物积累重金属差异的机理, 并为重金属积累典型品种的选育提供理论基础。     

植物根系分泌物主要生态功能研究进展

根系分泌物在植物根系-土壤-微生物互作过程及其生态反馈机制中发挥重要作用。在植物根际复杂网络互作过程中, 根系分泌物被认为是“根际对话”的媒介, 其在调控植物适应微生境、缓解根际养分竞争及构建根际微生物群落结构方面意义重大。该文结合国内外该领域主要研究成果, 综述了根系分泌物对植物生长、土壤微生物特性及土壤养分循环的影响, 并展望了未来根系分泌物的研究方向。     

Red mud (RM)-Induced enhancement of iron plaque formation reduces arsenic and metal accumulation in two wetland plant species

Human activities have resulted in arsenic (As) and heavy metals accumulation in paddy soils in China. Phytoremediation has been suggested as an effective and low-cost method to clean up contaminated soils. A combined soil-sand pot experiment was conducted to investigate the influence of red mud (RM) supply on iron plaque formation and As and heavy metal accumulation in two wetland plant species (Cyperus alternifolius Rottb., Echinodorus amazonicus Rataj), using As and heavy metals polluted paddy soil combined with three rates of RM application (0, 2%, 5%). The results showed that RM supply significantly decreased As and heavy metals accumulation in shoots of the two plants due to the decrease of As and heavy metal availability and the enhancement of the formation of iron plaque on the root surface and in the rhizosphere. Both wetland plants supplied with RM tended to have more Fe plaque, higher As and heavy metals on roots and in their rhizospheres, and were more tolerant of As and heavy metal toxicity. The results suggest that RM-induced enhancement of the formation of iron plaque on the root surface and in the rhizosphere of wetland plants may be significant for remediation of soils contaminated with As and heavy metals.     

The potential for nitrification and nitrate uptake in the rhizosphere of wetland plants: a modelling study

BACKGROUND AND AIMS: It has recently found that lowland rice grown hydroponically is exceptionally efficient in absorbing NO3-, raising the possibility that rice and other wetland plants growing in flooded soil may absorb significant amounts of NO3- formed by nitrification of NH4+ in the rhizosphere. This is important because (a) this NO3- is otherwise lost through denitrification in the soil bulk; and (b) plant growth and yield are generally improved when plants absorb their nitrogen as a mixture of NO3- and NH4+ compared with growth on either N source on its own. A mathematical model is developed here with which to assess the extent of NO3- absorption from the rhizosphere by wetland plants growing in flooded soil, considering the important plant and soil processes operating. METHODS: The model considers rates of O2 transport away from an individual root and simultaneous O2 consumption in microbial and non-microbial processes; transport of NH4+ towards the root and its consumption in nitrification and uptake at the root surface; and transport of NO3- formed from NH4+ towards the root and its consumption in denitrification and uptake by the root. The sensitivity of the model's predictions to its input parameters is tested over the range of conditions in which wetland plants grow. KEY RESULTS: The model calculations show that substantial quantities of NO3- can be produced in the rhizosphere of wetland plants through nitrification and taken up by the roots under field conditions. The rates of NO3- uptake can be comparable with those of NH4+. The model also shows that rates of denitrification and subsequent loss of N from the soil remain small even where NO3- production and uptake are considerable. CONCLUSIONS: Nitrate uptake by wetland plants may be far more important than thought hitherto. This has implications for managing wetland soils and water, as discussed in this paper.     

Rhizosphere processes induce changes in dissimilatory iron reduction in a tidal marsh soil: a rhizobox study

Background and aims

Although the role of microbial iron respiration in tidal marshes has been recognized for decades, the effect of rhizosphere processes on dissimilatory ferric iron reduction (FeR) is poorly known. Herein, we examined the FeR surrounding the root zone of three tidal marsh plants.

Methods

Using in situ rhizoboxes, we accurately separated rhizobox soil as one rhizosphere zone, and three bulk soil zones. Dissimilatory and sulfidic-mediated FeR were quantified by accumulation of non-sulfidic Fe(II) and Fe sulfides over time, respectively.

Results

The rates of dissimilatory FeR attained 42.5 μmol Fe g^?1 d^?1 in the rhizosphere, and logarithmically declined by up to 19.1 μmol Fe g^?1 d^?1 in the outer bulk soil. The rates of sulfidic-mediated FeR were less than 2 μmol Fe g^?1 d^?1 among all zones. Poorly crystalline Fe(III), DOC and DON, porewater Fe²⁺, and SO₄^2? were all enriched in the rhizosphere, whereas non-sulfidic Fe(II) and Fe sulfides gradually accumulated away from the roots. Iron reducers (Geobacter, Bacillus, Shewanella, and Clostridium) had higher populations in the rhizosphere than in the bulk soil. Higher rates of dissimilatory FeR were observed in the Phragmites australis and Spartina alterniflora rhizoboxes than in the Cyperus malaccensis rhizoboxes.

Conclusions

The radial change pattern of dissimilatory FeR rates were determined by allocation of poorly crystalline Fe(III) and dissolved organic carbon. The interspecies difference of rhizosphere dissimilatory FeR was associated with the root porosity and aerenchyma of the tidal marsh plants.

     

Root porosity, radial oxygen loss and iron plaque on roots of wetland plants in relation to zinc tolerance and accumulation

Background and aims

Wetland plants have been widely used in constructed wetlands for the clean-up of metal-contaminated waters. This study investigated the relationship between rate of radial oxygen loss (ROL), root porosity, Zn uptake and tolerance, Fe plaque formation in wetland plants.

Methods

A hydroponic experiment and a pot trial with Zn-contaminated soil were conducted to apply different Zn level treatments to various emergent wetland plants.

Results

Significant differences were found between plants in their root porosities, rates of ROL, Zn uptake and Zn tolerance indices in the hydroponic experiment, and concentrations of Fe and Mn on roots and in the rhizosphere in the pot trial. There were significant positive correlations between root porosities, ROL rates, Zn tolerance, Zn, Fe and Mn concentrations on roots and in the rhizosphere. Wetland plants with higher root porosities and ROL tended to have more Fe plaque, higher Zn concentrations on roots and in their rhizospheres, and were more tolerant of Zn toxicity.

Conclusions

Our results suggest that ROL and root porosity play very important roles in Fe plaque formation, Zn uptake and tolerance, and are useful criteria for selecting wetland plants for the phytoremediation of Zn-contaminated waters and soils/sediments.     

Further reading in geomicrobiology

In the wetland rhizosphere, high densities of lithotrophic Fe(II)-oxidizing bacteria (FeOB) and a favorable environment (i.e., high Fe(II) availability and microaerobic conditions) suggest that these organisms are actively contributing to the formation of Fe plaque on plant roots. We manipulated the presence/absence of an Fe(II)-oxidizing bacterium (Sideroxydans paludicola, strain BrT) in axenic hydroponic microcosms containing the roots of intact Juncus effusus (soft rush) plants to determine if FeOB affected total rates of rhizosphere Fe(II) oxidation and Fe plaque accumulation. Our experimental data highlight the importance of both FeOB and plants in influencing short-term rates of rhizosphere Fe oxidation. Over time scales ca. 1 wk, the FeOB increased Fe(II) oxidation rates by 1.3 to 1.7 times relative to FeOB-free microcosms. Across multiple experimental trials, Fe(II) oxidation rates were significantly correlated with root biomass, reflecting the importance of radial O ₂ loss in supporting rhizosphere Fe(II) oxidation. Rates of root Fe(III) plaque accumulation (time scales: 3 to 6 wk) were ～ 70 to 83% lower than expected based on the short-term Fe(II) oxidation rates and were unaffected by the presence/absence of FeOB. Decreasing rates of Fe(II) oxidation and Fe(III) plaque accumulation with increasing time scales indicate changes in rates of Fe(II) diffusion and radial O ₂ loss, shifts in the location of Fe oxide accumulation, or temporal changes in the microbial community within the microcosms. The microcosms used herein replicated many of the environmental characteristics of wetland systems and allowed us to demonstrate that FeOB can stimulate rates of Fe(II) oxidation in the wetland rhizosphere, a finding that has implications for the biogeochemical cycling of carbon, metals, and nutrients in wetland ecosystems.     

Enumeration of Fe(II)-oxidizing and Fe(III)-reducing bacteria in the root zone of wetland plants: Implications for a rhizosphere iron cycle

Iron plaque occurs on the roots of most wetland and submersed aquatic plant species and is a large pool of oxidized Fe(III) in some environments. Because plaque formation in wetlands with circumneutral pH has been largely assumed to be an abiotic process, no systematic effort has been made to describe plaque-associated microbial communities or their role in plaque deposition. We hypothesized that Fe(II)-oxidizing bacteria (FeOB) and Fe(III)-reducing bacteria (FeRB) are abundant in the rhizosphere of wetland plants across a wide range of biogeochemical environments. In a survey of 13 wetland and aquatic habitats in the Mid-Atlantic region, FeOB were present in the rhizosphere of 92% of the plant specimens collected (n = 37), representing 25 plant species. In a subsequent study at six of these sites, bacterial abundances were determined in the rhizosphere and bulk soil using the most probable number technique. The soil had significantly more total bacteria than the roots on a dry mass basis (1.4 × 10⁹ cells/g soil vs. 8.6 × 10⁷ cells/g root; p < 0.05). The absolute abundance of aerobic, lithotrophic FeOB was higher in the soil than in the rhizosphere (3.7 × 10⁶/g soil vs. 5.9 × 10⁵/g root; p < 0.05), but there was no statistical difference between these habitats in terms of relative abundance (1% of the total cell number). In the rhizosphere, FeRB accounted for an average of 12% of all bacterial cells while in the soil they accounted for < 1% of the total bacteria. We concluded that FeOB are ubiquitous and abundant in wetland ecosystems, and that FeRB are dominant members of the rhizosphere microbial community. These observations provide a strong rationale for quantifying the contribution of FeOB to rhizosphere Fe(II) oxidation rates, and investigating the combined role of FeOB and FeRB in a rhizosphere iron cycle.     

Role of dissimilatory fermentative iron-reducing bacteria in Fe uptake by common bean (Phaseolus vulgaris L.) plants grown in alkaline soil

Iron (Fe) is an essential element for plant growth and development. Some plant growth-promoting rhizobacteria can increase Fe uptake by plants through reduction of Fe(III) to Fe(II) at the root surface. The aim of this work was to identify novel bacterial strains with high Fe(III) reduction ability and to evaluate their role in plant Fe uptake. Four bacterial strains (UMCV1 to UMCV4) showing dissimilatory Fe-reducing activity were isolated from the rhizosphere of bean and maize plants and further identified by 16S rDNA amplification and sequence analysis. From these analyses, UMCV1 and UMCV2 isolates were identified as Bacillus megaterium and Arthrobacter spp., respectively, whereas UMCV3 and UMCV4 were identified as Stenotrophomonas maltophilia. All four isolates showed Fe reduction in a nonflooded soil and when associated with roots of bean plants grown in alkaline soil or in mineral medium. In addition, the bacterial isolates were able to stimulate plant growth in vitro and on a broad level, plants grown in inoculated soil were generally bigger and with higher Fe content than those grown in sterilized soil. These results indicate that bacterial species isolated from the rhizosphere of bean and maize plants contribute significantly to Fe uptake by plants likely through increased Fe(III) reduction in the rhizosphere.