Root exudates: a pathway for short-term N transfer from clover and ryegrass

The short-term transfer of nitrogen (N) from legumes to grasses was investigated in two laboratory studies. One study was done in pots where the roots of white clover (Trifolium repens L.) and perennial ryegrass (Lolium perenne L.) were allowed to co-exist, and a second study was performed using a micro-lysimeter system designed to maintain nutrient flow from the clover to the grass, whilst removing direct contact between the root systems. The ¹⁵N-dilution technique was used to quantify the transfer of N between species. Levels of ammonia and amino acids were measured in root exudates. The amounts of N transferred were in the same order of magnitude in both the pot and micro-lysimeter experiments. In the micro-lysimeter experiment, 0.076 mg of N were transferred per plant from clover to ryegrass during the course of the experiment. Ammonium exudation was much higher than amino acid exudation. The most abundant amino acids in both clover and ryegrass root exudates were serine and glycine. However, there was no correlation between the free amino acid profile of root extracts and exudates for both plant species: Asparagine was the major amino acid in clover roots, while glutamine, glutamate and aspartate were the major amino acids in ryegrass roots. Comparison of exudates obtained from plants grown in non-sterile or axenic conditions provides evidence of plant origin of ammonium, serine and glycine.     

柏木根系分泌物对栾树细根形态及N、P含量的影响

为研究混交过程中柏木根系分泌物对栾树细根生长的影响,以一年生栾树盆栽幼苗为研究对象,通过施加1株、2株、4株、8株4个浓度柏木根系分泌物(分别记为G1、G2、G4、G8)于栾树盆栽中,探讨柏木根系分泌物对栾树幼苗1～5级细根形态及N、P含量的影响。结果表明:(1)栾树细根直径随根序的增加而增大,施加根系分泌物显著减小了1～2级细根的直径(P0.05);细根比根长、比表面积均随根序的增加而减小,施加根系分泌物显著增大了1～2级细根的比根长及比表面积(P0.05);随根系分泌物施加浓度的提高,栾树比根长及比表面积先增大,而直径先减小,然后均趋于平缓波动的态势。(2)栾树细根N、P含量均随根序的增加而减小,而N/P在根序间的变化不显著;施加柏木根系分泌物显著增大了栾树1～2级细根的N、P含量(P0.05),但减小了1～5级细根的N/P;随根系分泌物施加浓度的提高,栾树细根P含量增大,N/P减小,而N含量先增加后呈现平缓变化的趋势。(3)栾树细根N、P含量均与其比根长、比表面积和直径等形态特征之间呈显著的相关关系(P0.05)。研究发现,柏木根系分泌物可改善土壤养分的有效性,从而缓解栾树植株的缺P症状,细根通过调整其形态以提高养分利用效率;柏木根系分泌物主要影响栾树1～2级细根的形态及N、P含量;4株柏木根系分泌物的剂量更有利于栾树根系的生长。     

Regulation and function of root exudates

Root-secreted chemicals mediate multi-partite interactions in the rhizosphere, where plant roots continually respond to and alter their immediate environment. Increasing evidence suggests that root exudates initiate and modulate dialogue between roots and soil microbes. For example, root exudates serve as signals that initiate symbiosis with rhizobia and mycorrhizal fungi. In addition, root exudates maintain and support a highly specific diversity of microbes in the rhizosphere of a given particular plant species, thus suggesting a close evolutionary link. In this review, we focus mainly on compiling the information available on the regulation and mechanisms of root exudation processes, and provide some ideas related to the evolutionary role of root exudates in shaping soil microbial communities.     

Transient exposure of root tips to primary and secondary metabolites: Impact on root growth and production of border cells

Here we describe the use of Pisum sativum L. as a model system to measure how short-term treatment of root tips with soluble metabolites can influence root growth and release of root exudates. The results revealed that even a 3-minute exposure of root tips to metabolites normally released from roots into the rhizosphere (e.g. rhamnose, ferulic acid, salicylic acid) can significantly influence root growth without affecting production of border cells and associated exudates. Conversely, products including caffeine, saccharide lactone, and pisatin alter production of border cells, without affecting root growth. Understanding how root-derived and exogenous metabolites can selectively impact root function may yield benefits in crop production, especially in greenhouse agriculture systems where growing roots can be exposed to a significant accumulation of plant exudates.     

小麦-蚕豆间作对根系分泌低分子量有机酸的影响

通过盆栽试验收集了不同生育期单作和间作小麦、蚕豆的根系分泌物,用HPLC分析了根系分泌物中低分子量有机酸的含量和种类.结果表明: 小麦-蚕豆间作显著提高了有机酸的分泌量,在小麦分蘖期(57 d)、孕穗期(120 d)和灌浆期(142 d),间作使小麦根系有机酸分泌量分别提高155%、35.6%和92.6%;在蚕豆分枝期(57 d)和籽粒膨大期(142 d),间作使蚕豆根系有机酸分泌量提高87.4%和38.7%.小麦-蚕豆间作改变了根系分泌物中有机酸的种类,与单作小麦相比,在分蘖期,间作小麦根系分泌物中增加了乳酸;在拔节期(98 d),间作小麦根系分泌物中增加了柠檬酸,但未检测到乙酸;在蚕豆分枝期,间作蚕豆根系分泌物中增加了乙酸,但未检测到乳酸;在蚕豆籽粒膨大期,间作蚕豆根系分泌物中增加了乳酸.小麦-蚕豆间作提高了小麦根系有机酸的分泌速率,在小麦孕穗期,间作小麦分泌柠檬酸、富马酸的速率是单作小麦的179和184倍;在小麦灌浆期,间作小麦分泌乳酸的速率是单作的2.53倍.总之,小麦-蚕豆间作增加了有机酸的分泌量,改变了根系分泌物中有机酸的种类,提高了小麦根系有机酸的分泌速率.     

Root exudates of plants

Summary The release of substances from wheat roots was found to be directly related to the growth of the root system. Plants whose root system did not grow released almost no exudates.When exudate concentration in the vicinity of the roots was lowered by frequent replacements of the nutrient solution or by a simultaneous cultivation of exudate-utilizing bacteria, the release of exudates was enhanced. In axenic wheat cultures, the amount of exudates during a 12-day cultivation with 2- or 4-day intervals between medium replacements represented 50% of root dry weight and 12% of whole plant dry weight.Wheat plants cultivated in the presence of the bacteriumPseudomonas putida released up to double the amount of exudates compared with axenic variants.     

Flavonoids from shoots, roots and roots exudates of Brassica alba

Analysis of extracts obtained from shoots, roots and exudates of Brassica alba revealed the presence of 3,5,6,7,8-pentahydroxy-4'-methoxy flavone in shoots, as well as 2',3',4',5',6'-pentahydroxy chalcone and 3,5,6,7,8-pentahydroxy flavone in roots and exudates. Apigenin was also found in the shoots and roots, but not in the root exudates.     

嫁接黄瓜地上部的南瓜根系分泌物对种子萌发的影响

经嫁接黄瓜接穗的南瓜根系分泌物对黄瓜和南瓜的发芽率和胚根、胚轴的伸长均具有明显的抑制作用.分析表明:嫁接黄瓜根系分泌物可以促进黄瓜和南瓜体内吲哚乙酸氧化酶的活性,抑制淀粉酶的活性,从而降低其吲哚乙酸(IAA)水平,影响子叶中贮藏物质的转化和利用,抑制其萌发和生长.     

Role of the modification in root exudation induced by arbuscular mycorrhizal colonization on the intraradical growth of Phytophthora nicotianae in tomato

We studied the role of modification in root exudation induced by colonization with Glomus intraradices and Glomus mosseae in the growth of Phytophthora nicotianae in tomato roots. Plants were grown in a compartmentalized plant growth system and were either inoculated with the AM fungi or received exudates from mycorrhizal plants, with the corresponding controls. Three weeks after planting, the plants were inoculated or not with P. nicotianae growing from an adjacent compartment. At harvest, P. nicotianae biomass was significantly reduced in roots colonized with G. intraradices or G. mosseae in comparison to non-colonized roots. Conversely, pathogen biomass was similar in non-colonized roots supplied with exudates collected from mycorrhizal or non-mycorrhizal roots, or with water. We cannot rule out that a mycorrhiza-mediated modification in root exudation may take place, but our results did not support that a change in pathogen chemotactic responses to host root exudates may be involved in the inhibition of P. nicotianae.     

Altered properties and structures of root exudate polysaccharides in a root hairless mutant of barley

Root exudates and rhizosheaths of attached soil are important features of growing roots. To elucidate factors involved in rhizosheath formation, wild-type (WT) barley (Hordeum vulgare L. cv. Pallas) and a root hairless mutant, bald root barley (brb), were investigated with a combination of physiological, biochemical, and immunochemical assays. When grown in soil, WT barley roots bound ∼5-fold more soil than brb per unit root length. High molecular weight (HMW) polysaccharide exudates of brb roots had less soil-binding capacity than those of WT root exudates. Carbohydrate and glycan monoclonal antibody analyses of HMW polysaccharide exudates indicated differing glycan profiles. Relative to WT plants, root exudates of brb had reduced signals for arabinogalactan-protein (AGP), extensin, and heteroxylan epitopes. In contrast, the root exudate of 2-week-old brb plants contained ∼25-fold more detectable xyloglucan epitope relative to WT. Root system immunoprints confirmed the higher levels of release of the xyloglucan epitope from brb root apices and root axes relative to WT. Epitope detection with anion-exchange chromatography indicated that the increased detection of xyloglucan in brb exudates was due to enhanced abundance of a neutral polymer. Conversely, brb root exudates contained decreased amounts of an acidic polymer, with soil-binding properties, containing the xyloglucan epitope and glycoprotein and heteroxylan epitopes relative to WT. We, therefore, propose that, in addition to physically structuring soil particles, root hairs facilitate rhizosheath formation by releasing a soil-binding polysaccharide complex.

The root exudate of a root hairless mutant of barley, relative to wild type, has an altered pattern of polysaccharide epitopes and lesser amounts of an acidic soil-binding polysaccharide complex.     

连作草莓根系分泌物自毒作用的模拟研究

 草莓（Fragaria ananassa）根系分泌物的自毒作用是草莓连作病害发生机理研究的重要内容之一。应用组织培养技术提取草莓根系分泌物,并对其自毒作用进行了测定。结果表明,在含有根系分泌物的生根培养基中定植的草莓组培苗,其生根、根系生长均受到不同程度的抑制,生物量显著下降,而且根系分泌物对草莓幼苗根系生理活性具有抑制作用。主要表现为根系TTC还原活性下降、相对电导率增大、SOD酶活性降低及MDA生成量增多等方面,并导致草莓幼苗生长发育不良、病害加重。说明草莓根系分泌物具有自毒作用,连作条件下田间根系分泌物逐年积累后产生的自毒作用,可能是草莓再植病害发生的重要原因。     

Anisohydric behaviour in grapevines results in better performance under moderate water stress and recovery than isohydric behaviour

Aims

Arbuscular mycorrhizal fungi (AMF) can control root-knot nematode infection, but the mode of action is still unknown. We investigated the effects of AMF and mycorrhizal root exudates on the initial steps of Meloidogyne incognita infection, namely movement towards and penetration of tomato roots.

Methods

M. incognita soil migration and root penetration were evaluated in a twin-chamber set-up consisting of a control and mycorrhizal (Glomus mosseae) plant compartment (Solanum lycopersicum cv. Marmande) connected by a bridge. Penetration into control and mycorrhizal roots was also assessed when non-mycorrhizal or mycorrhizal root exudates were applied and nematode motility in the presence of the root exudates was tested in vitro.

Results

M. incognita penetration was significantly reduced in mycorrhizal roots compared to control roots. In the twin-chamber set-up, equal numbers of nematodes moved to both compartments, but the majority accumulated in the soil of the mycorrhizal plant compartment, while for the control plants the majority penetrated the roots. Application of mycorrhizal root exudates further reduced nematode penetration in mycorrhizal plants and temporarily paralyzed nematodes, compared with application of water or non-mycorrhizal root exudates.

Conclusions

Nematode penetration was reduced in mycorrhizal tomato roots and mycorrhizal root exudates probably contributed at least partially by affecting nematode motility.     

根系分泌物收集技术研究进展

根系分泌物在调控陆地生态系统根际微环境间的物质、能量和信息交流中具有重要作用.构建准确、适用的根系分泌物收集方法,对根系分泌物的种类、含量及其对环境变化的响应等信息的精准获取是理解植物根系-土壤界面生态过程与信息交流的关键环节和前提.目前,传统或新型的根系分泌物收集技术都致力于认知根系分泌物中化合物种类多样性和含量变化,但根系的生长高度依赖周围介质和生长环境,使根系分泌物的收集很难避开对根系本身的损伤、土壤颗粒对根系分泌物的吸附和释放,以及微生物代谢等因素的干扰,导致不同根系分泌物收集方法都存在各自的优缺点.本研究从室内收集和野外原位收集两方面系统综述了当前应用较广的一些传统和新型根系分泌物收集技术,并总结和比较了每种收集方法的优缺点;在此基础上,基于森林根际生态学过程野外原位研究的重要性和代表性,结合当前根系分泌物研究的不足,展望了未来森林根系分泌物野外原位收集技术构建中值得关注的3个重点方向,旨在为相关研究者开展根系分泌物收集与作用研究提供参考.     

Inhibition of maize root growth by high nitrate supply is correlated with reduced IAA levels in roots

The plant root system is highly sensitive to nutrient availability and distribution in the soil. For instance, root elongation is inhibited when grown in high nitrate concentrations. To decipher the mechanism underlying the nitrate-induced inhibition of root elongation, the involvement of the plant hormone auxin in nitrate-dependent root elongation of maize was investigated. Root growth, nitrogen and nitrate concentrations, and indole-3-acetic acid (IAA) concentrations in roots and in phloem exudates of maize grown under varying nitrate concentrations were analyzed. Total N and nitrate concentrations in shoots and roots increased and elongation of primary, seminal and crown roots were inhibited with increasing external nitrate from 0.05 to 5 mM. High nitrate-inhibited root growth resulted primarily from the reduced cell elongation and not from changes in meristem length. IAA concentrations in phloem exudates reduced with higher nitrate supply. Inhibition of root growth by high nitrate was closely related to the reduction of IAA levels in roots, especially in the sections close to root tips. Exogenous NAA and IAA restored primary root growth in high nitrate concentrations. It is concluded that the inhibitory effect of high nitrate concentrations on root growth may be partly attributed to the decrease in auxin concentrations of roots.     

Manipulating exudate composition from root apices shapes the microbiome throughout the root system

Certain soil microorganisms can improve plant growth, and practices that encourage their proliferation around the roots can boost production and reduce reliance on agrochemicals. The beneficial effects of the microbial inoculants currently used in agriculture are inconsistent or short-lived because their persistence in soil and on roots is often poor. A complementary approach could use root exudates to recruit beneficial microbes directly from the soil and encourage inoculant proliferation. However, it is unclear whether the release of common organic metabolites can alter the root microbiome in a consistent manner and if so, how those changes vary throughout the whole root system. In this study, we altered the expression of transporters from the ALUMINUM-ACTIVATED MALATE TRANSPORTER and the MULTIDRUG AND TOXIC COMPOUND EXTRUSION families in rice (Oryza sativa L.) and wheat (Triticum aestivum L.) and tested how the subsequent release of their substrates (simple organic anions, including malate, citrate, and γ-amino butyric acid) from root apices affected the root microbiomes. We demonstrate that these exudate compounds, separately and in combination, significantly altered microbiome composition throughout the root system. However, the root type (seminal or nodal), position along the roots (apex or base), and soil type had a greater influence on microbiome structure than the exudates. These results reveal that the root microbiomes of important cereal species can be manipulated by altering the composition of root exudates, and support ongoing attempts to improve plant production by manipulating the root microbiome.

One-sentence summary: The root microbiome of rice and wheat can be manipulated by altering the activity of root transporters and exudates.     

Herbivore-induced changes in plant carbon allocation: assessment of below-ground C fluxes using carbon-14

Effects of above-ground herbivory on short-term plant carbon allocation were studied using maize (Zea mays) and a generalist lubber grasshopper (Romalea guttata). We hypothesized that above-ground herbivory stimulates current net carbon assimilate allocation to below-ground components, such as roots, root exudation and root and soil respiration. Maize plants 24 days old were grazed (c. 25–50% leaf area removed) by caging grasshoppers around individual plants and 18 h later pulse-labelled with¹⁴CO₂. During the next 8 h,¹⁴C assimilates were traced to shoots, roots, root plus soil respiration, root exudates, rhizosphere soil, and bulk soil using carbon-14 techniques. Significant positive relationships were observed between herbivory and carbon allocated to roots, root exudates, and root and soil respiration, and a significant negative relationship between herbivory and carbon allocated to shoots. No relationship was observed between herbivory and¹⁴C recovered from soil. While herbivory increased root and soil respiration, the peak time for¹⁴CO₂ evolved as respiration was not altered, thereby suggesting that herbivory only increases the magnitude of respiration, not patterns of translocation through time. Although there was a trend for lower photosynthetic rates of grazed plants than photosynthetic rates of ungrazed plants, no significant differences were observed among grazed and ungrazed plants. We conclude that above-ground herbivory can increase plant carbon fluxes below ground (roots, root exudates, and rhizosphere respiration), thus increasing resources (e.g., root exudates) available to soil organisms, especially microbial populations.     

Assessment of the Role of Chemotaxis and Biofilm Formation as Requirements for Colonization of Roots and Seeds of Soybean Plants by <Emphasis Type="Italic">Bacillus amyloliquefaciens</Emphasis> BNM339

This article correlates colonization with parameters, such as chemotaxis, biofilm formation, and bacterial growth, that are believed to be connected. We show here, by using two varieties of soybean plants that seeds axenically produced exudates, induced a chemotactic response in Bacillus amyloliquefaciens, whereas root exudates did not, even when the exudates, also collected under axenic conditions, were concentrated up to 200-fold. Root exudates did not support bacterial cell division, whereas seed exudates contain compounds that support active cell division and high cell biomass at stationary phase. Seed exudates of the two soybean varieties also induced biofilm formation. B. amyloliquefaciens colonized both seeds and roots, and plant variety significantly affected bacterial root colonization, whereas it did not affect seed colonization. Colonization of roots in B. amyloliquefaciens occurred despite the lack of chemotaxis and growth stimulation by root exudates. The data presented in this article suggest that soybean seed colonization, but not root colonization, by B. amyloliquefaciens is influenced by chemotaxis, growth, and biofilm formation and that this may be caused by qualitative changes of the composition of root exudates.     

根系分泌物研究方法(综述)

根系分泌物是植物与土壤进行物质、能量与信息交流的重要载体物质.研究根系分泌物的种类、数量对了解植物与土壤及根际微生物相互作用机理具有重要指导意义,根系分泌物研究方法的选择对研究结果有重要的影响.本文着重讨论根系分泌物收集、分离纯化及鉴定的常用方法,并列举特定根系分泌物的研究方法,旨在为根系分泌物的研究提供借鉴和参考.     

Spatial distribution of bacterial communities and phenanthrene degradation in the rhizosphere of Lolium perenne L

Rhizodegradation of organic pollutants, such as polycyclic aromatic hydrocarbons, is based on the effect of root-produced compounds, known as exudates. These exudates constitute an important and constant carbon source that selects microbial populations in the plant rhizosphere, modifying global as well as specific microbial activities. We conducted an experiment in two-compartment devices to show the selection of bacterial communities by root exudates and phenanthrene as a function of distance to roots. Using direct DNA extraction, PCR amplification, and thermal gradient gel electrophoresis screening, bacterial population profiles were analyzed in parallel to bacterial counts and quantification of phenanthrene biodegradation in three layers (0 to 3, 3 to 6, and 6 to 9 mm from root mat) of unplanted-polluted (phenanthrene), planted-polluted, and planted-unpolluted treatments. Bacterial community differed as a function of the distance to roots, in both the presence and the absence of phenanthrene. In the planted and polluted treatment, biodegradation rates showed a strong gradient with higher values near the roots. In the nonplanted treatment, bacterial communities were comparable in the three layers and phenanthrene biodegradation was high. Surprisingly, no biodegradation was detected in the section of planted polluted treatment farthest from the roots, where the bacterial community structure was similar to those of the nonplanted treatment. We conclude that root exudates and phenanthrene induce modifications of bacterial communities in polluted environments and spatially modify the activity of degrading bacteria.     

Amino acids in exudates of healthy and fungus-affected pea roots

Summary Amino acids in exudates of uninoculated pea roots were compared quantitatively and qualitatively with exudates of roots inoculated with Gliocladium catenulatum. This fungus has the potential of causing severe root necrosis. Twenty-one amino acids were found in exudates of healthy roots and apparently some of these were utilized by the fungus. A relatively high concentration of ammonia was detected in exudates of inoculated pea roots, indicating an intense deamination by the fungus. No other imbalance in amino acids was found which could be related to known toxic effects of amino acids on plant tissues.