Plant Root Exudates Research Articles

Allelopathy and Allelochemicals of Leucaenaleucocephala as an Invasive Plant Species.

Leucaena leucocephala (Lam.) de Wit is native to southern Mexico and Central America and is now naturalized in more than 130 countries. The spread of L. leucocephala is probably due to its multipurpose use such as fodder, timber, paper pulp, shade trees, and soil amendment. However, the species is listed in the world’s 100 worst invasive alien species, and an aggressive colonizer. It forms dense monospecific stands and threatens native plant communities, especially in oceanic islands. Phytotoxic chemical interactions such as allelopathy have been reported to play an important role in the invasion of several invasive plant species. Possible evidence for allelopathy of L. leucocephala has also been accumulated in the literature over 30 years. The extracts, leachates, root exudates, litter, decomposing residues, and rhizosphere soil of L. leucocephala increased the mortality and suppressed the germination and growth of several plant species, including weeds and woody plants. Those observations suggest that L. leucocephala is allelopathic and contains certain allelochemicals. Those allelochemicals may release into the rhizosphere soil during decomposition process of the plant residues and root exudation. Several putative allelochemicals such as phenolic acids, flavonoids, and mimosine were identified in L. leucocephala. The species produces a large amount of mimosine and accumulates it in almost all parts of the plants, including leaves, stems, seeds, flowers, roots, and root nodules. The concentrations of mimosine in these parts were 0.11 to 6.4% of their dry weight. Mimosine showed growth inhibitory activity against several plant species, including some woody plants and invasive plants. Mimosine blocked cell division of protoplasts from Petunia hybrida hort. ex E. Vilm. between G1 and S phases, and disturbed the enzyme activity such as peroxidase, catalase, and IAA oxidase. Some of those identified compounds in L. leucocephala may be involved in its allelopathy. Therefore, the allelopathic property of L. leucocephala may support its invasive potential and formation of dense monospecific stands. However, the concentrations of mimosine, phenolic acids, and flavonoids in the vicinity of L. leucocephala, including its rhizosphere soil, have not yet been reported.

Different strategies for colonization and prevalence after inoculation with plant growth-promoting rhizobacteria revealed by a monitoring method

ABSTRACT It is critical to monitor plant growth-promoting rhizobacteria (PGPR) in soils after inoculation. Among common PGPR, the Azospirillum sp. strains TSA2s and TSH100 have the ability to mitigate nitrous oxide (N2O) emissions from agricultural soils; however, their mechanism by which they successfully colonize and achieve beneficial effects in plants remains poorly understood. Here, a simple and robust procedure was developed to design strain-specific primers based on the whole-genome sequences of these two strains. After evaluating their specificity and amplification efficiency, three primer pairs were screened for each Azospirillum sp. strain. Quantification of these two strains in the rhizosphere soils of red clover revealed distinct inoculant dynamics for each strain under greenhouse conditions. Specifically, 22, 34, and 41 days after inoculation with TSA2s, the population size of the inoculant was notably greater than that in the non-inoculated control, and reached a maximum at day 34. In contrast, the population size of the TSH100 inoculant was largest at day 22, then decreased dramatically from 34 days after inoculation. These results suggest that different PGPR may have different strategies for colonization and prevalence after inoculation. Of the 31 rhizosphere carbon sources added to the Biolog Eco Micro plate to simulate plant root exudates, TSA2s utilized 14 whereas TSH100 utilized only one. TSA2s utilized more diverse root exudates of red clover, indicating better colonization and prevalence than TSH100 after inoculation. Therefore, this study presents a method of monitoring PGPR after inoculation. Furthermore, this method can screen PGPR with a better ability to survive and colonize, enabling the development of efficient, stable, and standardized biofertilizers.

Plant root exudates and rhizosphere bacterial communities shift with neighbor context

A plant's neighborhood context can alter its interactions with other organisms, but little is known about how these dynamics occur belowground, especially with soil microbes. Microbial communities in rhizosphere soil are influenced by many factors, including abiotic conditions and root-derived signals. In particular, root exudates have strong effects on rhizosphere assembly, respond to changes in abiotic conditions, and help plants interact with neighbors. Therefore, we predicted that root exudates likely play a central role in neighbor-induced shifts in rhizosphere communities. We conducted a greenhouse experiment to test this and determine how the rhizosphere bacterial community of a focal plant, Panicum virgatum, changed when beside different neighbors, and whether these shifts were mediated by neighbor-induced changes in root exudation. We found that neighbor altered both focal plant exudates and rhizosphere community, and that changes were largest when the focal plant was beside the most competitive neighbor, Rudbeckia hirta, which reduced both focal plant growth and nitrogen uptake. Several factors contributed to neighbor impacts on rhizosphere assembly, including neighbor-induced changes in root exudates during nitrogen-limitation and microbial spillover from roots of larger neighbors. Using an additional soil incubation, we also found that these changes in exudates can have even greater effects on soil nutrients than on microbial assembly. Overall, we show that neighbors influence one another's microbiomes, and highlight neighbor-induced changes in root exudates as one mechanism through which this may occur. This work suggests that rhizosphere assembly may differ in mixed-species communities and thus emphasizes a need for microbiome studies that consider neighborhood context.

The Insect Pathogen Photorhabdus luminescens Protects Plants from Phytopathogenic Fusarium graminearum via Chitin Degradation.

Phytopathogens represent a large agricultural challenge. The use of chemical pesticides is harmful to the environment, animals, and humans. Therefore, new sustainable and biological alternatives are urgently needed. The insect-pathogenic bacterium Photorhabdus luminescens, already used in combination with entomopathogenic nematodes (EPNs) as a biocontrol agent, is characterized by two different phenotypic cell forms, called primary (1°) and secondary (2°). The 1° cells are symbiotic with EPNs and are used for biocontrol, and the 2° cells are unable to undergo symbiosis with EPNs, remain in the soil after insect infection, and specifically interact with plant roots. A previous RNA sequencing (RNAseq) analysis showed that genes encoding the exochitinase Chi2A and chitin binding protein (CBP) are highly upregulated in 2° cells exposed to plant root exudates. Here, we investigate Chi2A and CBP functions and demonstrate that both are necessary for P. luminescens 2° cells to inhibit the growth of the phytopathogenic fungus Fusarium graminearum. We provide evidence that Chi2A digests chitin and thereby inhibits fungal growth. Furthermore, we show that 2° cells specifically colonize fungal hyphae as one of the first mechanisms to protect plants from fungal phytopathogens. Finally, soil pot bioassays proved plant protection from F. graminearum by 2° cells, where Chi2A and CPB were essential for this process. This work gives molecular insights into the new applicability of P. luminescens as a plant-growth-promoting and plant-protecting organism in agriculture. IMPORTANCE The enteric enterobacterium Photorhabdus luminescens is already being used as a bioinsecticide since it is highly pathogenic toward a broad range of insects. However, the bacteria exist in two phenotypically different cell types, called 1° and 2° cells. Whereas only 1° cells are symbiotic with their nematode partner to infect insects, 2° cells were shown to remain in the soil after an insect infection cycle. It was demonstrated that 2° cells specifically interact with plant roots. Here, we show that the bacteria are beneficial for the plants by protecting them from phytopathogenic fungi. Specific colonization of the fungus mycelium as well as chitin-degrading activity mediated by the chitin binding protein (CBP) and the chitinase Chi2A are essential for this process. Our data give evidence for the novel future applicability of P. luminescens as a plant-growth-promoting organism and biopesticide.

Host mediated microbiome selection to study the changes in the nutrient cycling, root exudation and microbial population in the rhizosphere of soybean genotypes

The study was conducted with an aim to understand the changes in soil enzyme activity in the plant rhizosphere during host mediated selection of rhizosphere microbiome. Soil samples were collected from three different locations on a farm in the Reading area, UK. Two popular genotypes of Soybean (Glycine max) were chosen for the study, Kenchen and Siverka. Initial microbiome inoculum was prepared by mixing field soil with sterilized water and this was added to the autoclaved coir: sand mix. After four weeks of plant growth, plants were uprooted from the soil, rhizosphere soil collected for soil enzyme estimation and NGS analysis. Root exudates were collected and stored in freezer for analysis. Growth of plants in legume soils was significantly higher. The plants in legume soil produced nodules which confirmed the presence of rhizobia in these soils. Independent analysis within each soil type showed that, the interaction between soil type and genotype is very significant with respect to enzyme activity. The presence of microbiome along with plants had a significant effect on enzymatic activity. GCMS analysis of root exudates from plants in each soil showed variations in individual chromatograms. The difference in enzyme activity with plant growth shows that there are changes taking place in the soil. Further in this study, NGS analysis of rhizosphere soil will shed more light into the changes happening in the soil microbiome and if the changes are related to the enzymatic changes and plant root exudation.

Interactions between microtopography, root exudate analogues and temperature determine CO2 and CH4 production rates in fire-degraded tropical peat

Repeated fires can alter the microtopography, vegetation composition, peat surface temperature and can increase the risk of flooding in tropical peatlands. However, difficult site conditions limit our understanding of these critical factors regulating greenhouse gas (GHG, viz. CO2 and CH4) production and emissions from fire-degraded tropical peatlands. We aimed to relate the complex interactions between peat oxic and anoxic conditions due to changes in microtopography, labile C inputs in the form of plant root exudates (from ferns and sedges), and diurnal temperature change in affecting CO2 and CH4 production from fire-degraded tropical peat. We found that the mesic condition, which reflects the field moisture or water-saturated oxic conditions in hummocks, acted as a strong source of CO2 (230 ± 29 μgCO2 g−1 hr−1) and weak sink for CH4 (−5.6 ± 0.2 ngCH4 g−1 hr−1), while anoxic acted as a weak source of CO2 and strong source of CH4 (61.3 ± 6.2 μgCO2 g−1 hr−1; 592 ± 111 ngCH4 g−1 hr−1). Addition of labile C enhanced both the CO2 and CH4 production across treatments by five and two times for the two gases, respectively. Temperature sensitivity (Q10) for CO2 was higher for peat incubated under mesic conditions (1.21 ± 0.28) whereas for CH4 it was higher in peat under anoxic conditions (1.56 ± 0.35). Collectively, our result highlights how microscale changes in microtopography coupled with the quality and quantity of labile C and temperature variation can regulate GHGs production from fire-degraded tropical peatland areas, which are projected to increase with frequent fire episodes and future climate warming in the region. More importantly, changes in these critical factors may result in a net positive carbon emission with long-term elevated CH4 production and emissions rates from such fire-degraded tropical peatland areas.

MdBAK1 overexpression in apple enhanced resistance to replant disease as well as to the causative pathogen Fusarium oxysporum

Apple replant disease (ARD) is a complex syndrome caused by various biotic and abiotic stresses contained in replanted soil, leading to reduced plant growth and fruit yields and causing serious economic loss. Breeding disease-resistant varieties is an effective and practical method to control ARD. Effective plant defense depends in part on the plant immune responses induced by the recognition of pathogen-associated molecular patterns (PAMPs) by pattern recognition receptors (PRRs). BAK1 participates in the regulation of plant immunity as an important PRR-binding protein. In this study, MdBAK1 overexpression activated indeterminate immune responses in tissue-cultured apple plants. MdBAK1-overexpressing rooted apple plants exhibited enhanced resistance to ARD, as the inhibition of plant growth was significantly alleviated during the replanted soil treatment. In addition, MdBAK1-overexpressing apple plants showed abolished growth inhibition, wilting and root rot induced by Fusarium oxysporum, which is the main pathogen that causes ARD in China. MdBAK1 overexpression changed the microbial community structure in the rhizosphere soil, as reflected by the increase in bacterial content and the decrease in fungal content, and the root exudates of MdBAK1-overexpressing plants inhibited F. oxysporum spore germination compared with that of wild-type plants. Furthermore, the constitutive immunity and cell necrosis induced by the upregulation of MdBAK1 expression were involved in the inhibition of colonization and expansion of F. oxysporum in host plants. In short, MdBAK1 plays an important role in the regulation of apple resistance to ARD, suggesting that MdBAK1 may be a valuable gene for molecular breeding of ARD resistance.

Revealing the underlying mechanisms mediated by endophytic actinobacteria to enhance the rhizobia - chickpea (Cicer arietinum L.) symbiosis

PurposeThe effects of endophytic actinobacterial strains, Microbispora sp. CP56, Actinomadura sp. CP84B, Streptomyces spp. CP200B and CP21A, on the chickpea-Mesorhizobium symbiosis, were investigated in planta, with the aim of revealing the underlying mechanisms of action.MethodsThe actinobacterial endophytes were co-inoculated with Mesorhizobium ciceri onto chickpea seedlings to study the effect on plant growth parameters, nodulation development and grain yield. The role of actinobacterial exudates on rhizobial growth was investigated, as was the role of root exudates of actinobacteria-colonized plants on the expression of rhizobial nod factors and biofilm formation. Changes in expression of plant flavonoids and bacterial N-fixation genes resulting from actinobacterial co-inoculation were assessed using qPCR.ResultsApplication of actinobacterial endophytes, together with M. ciceri, showed growth promotion of chickpea with an increase in root nodule number and weight. Enhanced nodulation was accompanied by increases in total plant nitrogen, larger total plant weight and a 2–3-fold increase in grain yield. Factors associated with this tripartite symbiosis are promotion of rhizobial growth, earlier nodule formation, increased secondary root formation, up-regulated expression of genes related to flavonoid synthesis and nif genes. In addition, exudates of chickpea roots colonised with actinobacteria increased nodulation-related biological processes - rhizobial chemotaxis, biofilm formation and nod gene expression.ConclusionThese endophytic actinobacteria positively affect many aspects of the chickpea-Mesorhizobium symbiosis resulting in increases in grain yield. Similar improvements recorded in chickpea growing in potted field soils, shows the potential to enhance chickpea production in the field.

Assessment of dynamic microbial community structure and rhizosphere interactions during bioaugmented phytoremediation of petroleum contaminated soil by a newly designed rhizobox system

To understand the plant (Vigna unguiculata) and plant-growth promoting bacteria (PGPB) (Microcococcus luteus WN01) interactions in crude oil contaminated soil, experiments were conducted based on the newly designed rhizobox system. The rhizobox was divided into three main compartments namely the rhizosphere zone, the mid-zone, and the bulk soil zone, in accordance with the distance from the plant. Plants were grown in these three-chambered pots for 30 days under natural conditions. The plant root exudates were determined by analyzing for carbohydrates, amino acids, and phenolic compounds. The degradation of alkane, polycyclic aromatic hydrocarbons (PAHs), and total petroleum hydrocarbons (TPHs) were quantified by GC-FID. Soil catalase, dehydrogenase, and invertase activities were determined. The microbial community structure was assessed using denaturing gradient gel electrophoresis (DGGE). Results showed that the inoculation of M. luteus WN01 significantly enhanced cowpea root biomass and exudates, especially the phenolic compounds. Bioaugmented phytoremediation by cowpea and M. luteus promoted rhizodegradation of TPH. Cowpea stimulated microbial growth, soil dehydrogenase, and invertase activities and enhanced bacterial community diversity in oil contaminated soil. The rhizosphere zone of cowpea inoculated with M. luteus showed the highest removal efficiency, microbial activities, microbial population, and bacterial community diversity indicating the strong synergic interactions between M. luteus and cowpea.

Mineral and Organic Fertilizers Distinctly Affect Fungal Communities in the Crop Rhizosphere

Fungi represent a diverse group of organisms that play an essential role in maintaining soil health and ecosystem functioning. Plant root exudates form nutrient-rich niches that harbor specific fungal communities, or so-called rhizosphere mycobiomes. The long-term application of fertilizers supplies the soil with nutrients that may override the plant-related effects on rhizosphere fungal communities. Here, we assessed the effect of contrasting fertilization regimes on the composition, diversity, and abundance of bulk soil and rhizosphere mycobiomes of potato, white mustard, and maize under NPK (mineral fertilizers) or fresh cattle manure (organic fertilizers). Mineral and organic fertilizers led to distinct fungal communities in the rhizospheres of all studied crops, and the plant-related effects on the mycobiome were overridden by the effect of fertilization. The abundances of Ascomycota and Olpidiomycota were higher under manure, while the abundances of Basidiomycota and Monoblepharomycota increased under NPK. Manure input strongly increased fungal abundance but decreased fungal diversity and the total number of species. NPK had a slight effect on fungal diversity, but significantly increased the relative abundances of fungal phytopathogens, such as Alternaria and Fusarium. Our study shows that that potential plant species effects on the abundance and diversity of the rhizosphere mycobiomes are governed by long-term fertilization. Fertilization management could therefore be used to manipulate rhizosphere fungal communities and soilborne pathogen suppressiveness.

Extending the benefits of PGPR to bioremediation of nitrile pollution in crop lands for enhancing crop productivity

Incessant release of nitrile group of compounds such as cyanides into agricultural land through industrial effluents and excessive use of nitrile pesticides has resulted in increased nitrile pollution. Release of nitrile compounds (NCs) as plant root exudates is also contributing to the problem. The released NCs interact with soil elements and persists for a long time. Persistent higher concentration of NCs in soil cause toxicity to beneficial microflora and affect crop productivity. The NCs can cause more problems to human health if they reach groundwater and enter the food chain. Nitrile degradation by soil bacteria can be a solution to the problem if thoroughly exploited. However, the impact of such bacteria in plant and soil environments is still not properly explored. Plant growth-promoting rhizobacteria (PGPR) with nitrilase activity has recently gained attention as potential solution to address the problem. This paper reviews the core issue of nitrile pollution in soil and the prospects of application of nitrile degrading bacteria for soil remediation, soil health improvement and plant growth promotion in nitrile-polluted soils. The possible mechanisms of PGPR that can be exploited to degrade NCs, converting them into plant useful compounds and synthesis of the phytohormone IAA from degraded NCs are also discussed at length.

Responses of Above- and Belowground Carbon Stocks to Degraded and Recovering Wetlands in the Yellow River Delta

Wetlands reserve a large amount of organic carbon (C), playing a key role in contributing global C stocks. It is still uncertain to evaluate wetland C stocks due to wetland disturbance or degradation. In this study, we performed the degraded and recovering wetlands to estimate aboveground C stocks and soil organic C (SOC) stocks at the depth of 1 m in the Yellow River Delta. Our results showed that the recovering wetland sequestered 1.67 Mg C ha–1 aboveground, approximately three times higher than those (0.56 Mg C ha–1) of degraded wetland, and recovering wetland stored more SOC of 51.86 Mg C ha–1 in the top 1 m soils, approximately two times higher than those (26.94 Mg C ha–1) of degraded wetland. These findings indicate that the transformation between degraded and recovering wetlands is associated with the conversion of wetland C sources and sinks. The shifts in aboveground C stocks and SOC stocks were mainly attributed to changed biotic (i.e., aboveground biomass and photosynthetic C) and abiotic (i.e., soil water, salinity, SOC and N contents, and SOC compounds) factors. The improved soil water, salinity, and nutrient enhance C reservoir, sequestering more C in aboveground vegetation and storing more SOC via photosynthetic C input of plant litter and root exudates in recovering wetland than in degraded wetland with poor soil conditions. The relationships among wetland C stocks, plant, and soil properties indicate plant-soil interaction driving wetland ecosystem C stocks in degraded and recovering wetlands. Our research suggests that wetland restoration highlights a positive response to “carbon neutrality” by efficiently sequestering C above- and belowground.

Structure Elucidation and Biosynthesis of Orobanchol.

Strigolactones (SLs), a class of phytohormones that regulate diverse developmental processes, were initially characterized as host-derived germination stimulants for seeds belonging to the genera Striga, Orobanche, and Phelipanche. Orobanchol (1), which is detected in the root exudates of several plants and recognized as a prevalent SL, was first isolated from the root exudates of red clover as a germination stimulant for Orobanche minor in 1998. However, the structure of this stimulant proposed at that time was disputable considering its predicted germination-inducing activity for Striga gesnerioides. The genuine structure of orobanchol was elucidated following a decade-long controversy, which ultimately facilitated the understanding of the importance of SL stereochemistry in Striga seed germination. Recently, studies focusing on clarifying the biosynthesis pathway of orobanchol are being conducted. Cytochrome P450 monooxygenases are involved in orobanchol biosynthesis downstream of carlactonoic acid (CLA) via two pathways: either through 4-deoxyorobanchol or direct conversion from CLA. Substantial progress in the identification of more SL structures and clarification of their biosynthetic mechanisms will further contribute in the comprehension of their structural diversity’s functional importance and agricultural applications. Herein, we have reviewed the history leading to the discovery of the genuine structure of orobanchol and the current understanding of its biosynthetic mechanisms.

Microbiome structure and response to watering in rhizosphere of Nitrosalsola vermiculata and surrounding bulk soil

The plant rhizosphere microbiomes were thought to help the plant stands adverse condition. The study aims at deciphering signatures of rhizosphere soil microbiomes of the medicinal plant Nitrosalsola vermiculata and those of the surrounding bulk soil as well as to detect influence of watering in restructuring soil microbes that can improve the plant’s ability to tolerate drought stress. Amplicon sequencing of partial 16S rRNA gene indicated that alpha diversity indices are higher in rhizosphere than in bulk soils, while no distinctive differences were observed due to the watering. Relative abundance of phylum Cyanobacteria and its descendent unidentified genus is the highest among phyla and genera of bulk soil. Relative abundance of phyla Euryarchaeota, Chloroflexi, Actinobacteria, Proteobacteria, Bacteroidetes, Firmicutes, Acidobacteria and Gemmatimonadetes as well as genera Bacillus, Ammoniphilus, Sphingomonas, Microvirga, Pontibacter, Adhaeribacter and Arthrobacter was significantly higher in rhizosphere soil. The latter taxa were reported to act as plant growth-promoting bacteria (PGPB) through symbiotic associations. We speculate that relative abundance and mutual dominance of these taxa in rhizosphere of N. vermiculata were due to the intensity and type of plant root exudates. Other factors include soil pH where microbes favoring high soil pH can show better growth in rhizosphere soil. Also, co-existence of phyla that promote sustainability of cohabiting phyla in the rhizosphere and have high synergism prevalence in biofilm formation can be one extra factor. Quorum sensing (QS) also mediates bacterial population density in a given environment and elicit specific plant responses. The low abundance of Cyanobacteria in rhizosphere soil can be due to the inhibitory effect of highly abundant members of Firmicutes, especially those of genus Bacillus. The latter conclusion was confirmed by the occurrence of high expression rate of comQ gene triggering QS in genus Bacillus. Highly abundant microbes whose abundance was not changed due to watering are phyla Firmicutes, Proteobacteria, Chloroflexi and Cyanobacteria and their descendent genera Bacillus, Ammoniphilus, Sphingomonas, Microvirga and unidentified genus of Cyanobacteria. We speculate that non-responsive taxa to watering were drought tolerant and can help plants stand adverse conditions of water scarce. In conclusion, insights on the factors involved in shaping microbiome signatures and those eliciting differential plant responses to drought stress are raised and warrant further investigations.

Allelopathic Potential of Sorghum Residues in Control of Two Weeds, the Great-Brome and Milk-Thistle

Biological weed control through the use of the ‘natural herbicide’ potential of certain allelopathic crops could be an alternative to conventional herbicidal methods. This work was carried out to investigate the allelopathic potential of a ‘Super Graze II’ hybrid of forage sorghum (Sorghum bicolor L.), used as a cover crop by growers who practice conservation agriculture in Tunisia, to control two endemic weeds, the great-brome (Bromus diandrus) and milk-thistle (Silybum marianum), that infest the fields where winter straw cereals are grown in the semi-arid zone of Tunisia, causing yield losses. A bioassay was used to evaluate the phytotoxicity of sorghum plant components and root exudates. Water extracts of the sorghum expressed an inhibitory effect on the germination and seedlings’ growth of the weeds. Radicle growth was more affected than coleoptiles growth. Stem-extract was the most phytotoxic on radicle growth of great-brome and milk-thistle with inhibition of 66% and 86%, respectively. Regardless of the weed species, the effect of root exudates of sorghum was significant (p=0.05). Radicle length was more affected than coleoptiles length which was reduced by 42% and 71%, for great-brome and milk-thistle, respectively. Sorghum was more efficient for milk-thistle control. The results showed a potential use of sorghum as a natural herbicide, either alone or in combination with classical control methods. This practice should consider the choice of a cereal species/variety in succession tolerant to the phytotoxicity expressed by the sorghum hybrid.

Plant Growth Promoting (PGP) Activities of Rhizobial Isolates from Sesbania bispinosa in Response to Pesticides

The aim of this study was to investigate the impact of some pesticides on the plant growth promoting activities of Rhizobium spp. isolated from Sesbania bispinosa. Sustainability and environmental safety of agricultural production relies on eco-friendly approaches like use of biofertilizers, biopesticides and crop residue return. The plant growth promoters (PGP) emphasize the need for further strengthening the research and their use in modern agriculture. PGPs inhabit the rhizosphere for nutrients from plant root exudates. On return, they help to increase plant growth through soil nutrient enrichment by nitrogen fixation, phosphate solubilization, siderophore and phytohormone production, etc. Using pesticide/insecticide in the field may protect the plants from undesirable organisms, however, if accumulated in the habitat may adversely affect the growth and development of non-target lives like the rhizobacteria. The present study was performed to demonstrate the influence of pesticides on plant growth promoting activities of rhizobial isolates from Sesbania bispinosa known as green manure among farmers of Bangladesh. It was observed that two pesticides/insecticides (Imitaf 20SL and Tilt 250EC) in vitro shows variable effect on the plant growth promoting activities of the rhizobial isolates.
 Bangladesh J Microbiol, Volume 38, Number 2, December 2021, pp 31-37

Diagnostic significance of the synthesis of phenolic compounds and proline in the leaves of Schisandra chinensis and Actinidia arguta for the indication of the stress levels of plants under conditions of mixed plantings

The peculiarities of the accumulation of polyphenolic compounds and free proline were investigated in the leaves of Actinidia arguta and Shisandra chinensis during their cultivation in vegetation containers with different ratios of the number of plants, namely 1 : 1, 2 : 1, 1 : 2. Monocultural (single-species) planting was used as a control. The content of free proline in plant leaves was carried out according to the method, which is based on the interaction of proline with a ninhydrin reagent, forming a pink-red color. The amount of polyphenolic compounds was determined by the Folin-Ciocalteu method. It was found that the ratio of plants grown together significantly affects the accumulation of primary and secondary metabolites in their leaves. Under conditions of mixed planting, more proline and phenolic substances are accumulated in plant leaves compared to monoculture. The maximum proline content, 19.44 ± 0.91 mg/g of dry weight (DW), was observed in the leaves of A. arguta in the experiment combination with a prevailing number of schisandra plants at a ratio of S. chinensis and A. arguta plants of 2:1. In the same combination, the leaves of actinidia contained the highest amount of phenolic compounds (36.87 ± 2.22 mg/g DW). The studied root exudates of the experimental plants had an allelopathic inhibitory effect on the test culture. The exudates caused 12.0 % average decrease in root growth of A. arguta test objects, and 30.0 % average decrease in root growth of S. chinensis test objects compared with the control. This allows us to conclude about the high activity of schisandra’s allelochemicals, which negatively affect the development of actinidia plants. The optimal ratio of plants A. arguta and S. chinensis when grown together is 2 : 1, since a higher concentration of schisandra plants in a container more stress in actinidia plants, expressed as an increased accumulation of phenols and prolines in its leaves. The result of a comparative analysis of the amount of proline and phenolic compounds can be used to assess the mutual influence of plants in mixed plantings to optimize their growing conditions, which confirms the diagnostic significance of these metabolites for indicating the stress state of the studied plants.

Trophic niche but not abundance of Collembola and Oribatida changes with drought and farming system.

Higher frequencies of summer droughts are predicted to change soil conditions in the future affecting soil fauna communities and their biotic interactions. In agroecosystems drought effects on soil biota may be modulated by different management practices that alter the availability of different food resources. Recent studies on the effect of drought on soil microarthropods focused on measures of abundance and diversity. We here additionally investigated shifts in trophic niches of Collembola and Oribatida as indicated by stable isotope analysis (13C and 15N). We simulated short-term summer drought by excluding 65% of the ambient precipitation in conventionally and organically managed winter wheat fields on the DOK trial in Switzerland. Stable isotope values suggest that plant litter and root exudates were the most important resources for Collembola (Isotoma caerulea, Isotomurus maculatus and Orchesella villosa) and older plant material and microorganisms for Oribatida (Scheloribates laevigatus and Tectocepheus sarekensis). Drought treatment and farming systems did not affect abundances of the studied species. However, isotope values of some species increased in organically managed fields indicating a higher proportion of microorganisms in their diet. Trophic niche size, a measure of both isotope values combined, decreased with drought and under organic farming in some species presumably due to favored use of plants as basal resource instead of algae and microorganisms. Overall, our results suggest that the flexible usage of resources may buffer effects of drought and management practices on the abundance of microarthropods in agricultural systems.

短期牦牛放牧强度对川西北高原高寒草甸土壤细菌群落的影响

PDF HTML阅读 XML下载 导出引用 引用提醒 短期牦牛放牧强度对川西北高原高寒草甸土壤细菌群落的影响 DOI: 10.5846/stxb202011273042 作者: 作者单位: 作者简介: 通讯作者: 中图分类号: 基金项目: 国家自然科学基金（42001055）；中央引导地方-自由探索项目（20ZYZYTS0020）；西南民族大学中央高校基本科研业务费专项（2020NQN27）和研究生创新型科研项目（CX2020SZ22） Effects of short-term yak grazing intensity on soil bacterial communities in an alpine meadow of the Northwest Sichuan Plateau Author: Affiliation: Fund Project: 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:气候变化和人类干扰使青藏高原的草地退化日趋严重，特别是过度放牧导致的草地退化现象尤为突出；然而，牦牛作为青藏高原分布广且数量多的重要家畜，其不同放牧强度对高寒草甸土壤理化性质与细菌群落的影响仍不明晰。在川西北高原典型高寒草甸开展牦牛放牧强度试验，设置了对照组（禁牧）、轻度放牧（1头牦牛/hm2）、中度放牧（2头牦牛/hm2）和重度放牧（3头牦牛/hm2）4个放牧强度，每个强度设置3个重复。两年放牧实验后的结果表明：短期的重度放牧导致土壤有效磷显著增加，而其它土壤性质在各放牧强度变化不显著；随着放牧强度的增加，土壤细菌α多样性呈先增加后减少的趋势，其主要原因是牦牛活动为细菌的生长繁殖提供了有利的营养条件，但由于放牧的持续时间较短，变化不显著；就优势菌而言，土壤绿弯菌门中存在光合自养细菌，在重度放牧下显著高于对照组，其它各菌门在不同放牧强度下无显著差异；土壤氮与硝化螺旋菌门呈正相关，速效磷与酸杆菌门也呈显著正相关，均说明放牧强度对土壤细菌类群的影响是通过土壤性质间接实现的。本实验通过研究不同牦牛放牧强度下的土壤细菌群落结构，为放牧策略的制定提供了基础数据支持，为草地退化的防治提供理论依据。 Abstract:As an important part of China, the Qinghai-Tibetan Plateau plays a vital role in the balance of global ecosystems. Climate change and human activities have led to grassland degradation on the Qinghai-Tibetan Plateau, especially overgrazing-induced grassland degradation. However, as one of the most dominated livestock on the Qinghai-Tibetan Plateau, the effects of different yak grazing intensities on soil physicochemical properties and microbial communities and structures still remain unclear. To address yak grazing effect on soil bacterial communities, we conducted a yak grazing intensity experiment in a typical alpine meadow of the Northwest Sichuan Plateau. According to local investigations and previous experiment, we set four grazing intensities, including control group (no grazing), light grazing (1 yak/hm2), moderate grazing (2 yak/hm2), and heavy grazing (3 yak/hm2). After two years of grazing experiments, we found that heavy grazing significantly increased soil available phosphorus and short term grazing reduced the bulk density in the 0-10cm soils, while other soil properties showed no responses to different grazing intensities. The possible reason may be that the effect of grazing on soil physicochemical properties was long-term, and the short-term grazing intensity of yak was difficult to change soil properties. The α diversity of soil bacteria increased firstly and then decreased with the increasing grazing intensity, but the changes were not statistically significant due to the short duration of grazing experiment. On the one hand, yak grazing could provide available nutrients for the growth of bacteria through dung and urine. On the other hand, high grazing intensity changed the root distribution and root exudates of plants, and increased ultraviolet radiation reduced the diversity of bacteria. In terms of the dominant bacteria, the Chloroflexi was significantly higher under no-grazing than that under heavy grazing, which has photosynthetic autotrophic bacteria. In contrast, other phylum had no significant differences among grazing intensities. With the increase of grazing intensity, the relative abundance of Actinobacteri and Firmicutes increased firstly and then decreased, while Acidobacteria and Gemmatimonadetes showed reverse trends. The β diversity of soil bacteria showed that grazing intensity had no significant effect on bacterial community structure. Soil total nitrogen is positively correlated with Nitrospira. There was a significantly negative correlation between soil total phosphorus and Acidobacteria, but soil available phosphorus was also significantly positive related to Acidobacteria, indicating that grazing intensity indirectly affect soil bacteria may be through changing soil properties. This study provides basic data to grazing managements and a theoretical basis to the restoration of degraded grasslands. 参考文献 相似文献 引证文献

植物根系分泌物对土壤污染修复的作用及影响机理

生物修复是一种经济环保的土壤修复技术。根系分泌物是利用生物修复污染土壤过程中的关键物质，也是植物与土壤微生物进行物质交换和信息传递的重要载体，在植物响应污染物胁迫中扮演重要角色。研究植物根系分泌物对土壤污染修复的作用和影响机理，是深入理解植物和微生物环境适应机制的重要途径，对促进生物修复污染土壤有重要指导意义。从污染物胁迫对根系分泌物的影响、根系分泌物对土壤污染物环境行为的影响、根系分泌物在调控污染土壤中根际微生物群落结构和多样性中发挥的作用等几个方面综述了根系分泌物对土壤污染修复的影响及内在机制。研究结果表明，根系分泌物在降低重金属对植物的毒性、加速有机污染物降解等方面有非常重要的作用。根系分泌物对土壤微生物的丰度和多样性均有显著影响，其与根际微生物互作在土壤污染物的消减中发挥了重要的调控作用。在此基础上，提出了以往研究中的不足，并对污染物胁迫下根系分泌物未来研究的方向和趋势进行了展望。;Bioremediation is an environment friendly and economically feasible remediation technology. Plant root exudates, playing an active role in root-microbe communications, are the key substances in bioremediation processes of soil contamination. Understanding the effects and mechanisms of plant root exudates on soil remediation can provide a better guidance with bioremediation techniques. Moreover, it is also helpful to understand the adapted mechanisms of plants and microbial communities in contaminated soils. In this paper, the effects of contaminants on the characteristics of root exudates, the effects of root exudates on the environmental behaviors of contaminants in soils were studied. Furthermore, the roles and mechanisms of root exudates in regulating the diversity of the rhizosphere microbial communities were also reviewed to reveal the effects and mechanisms of plant root exudates in the remediation of soil contaminants. The results indicated that root exudates played an important role in reducing the toxicity of metal(loid)s to plants, and also in accelerating the degradation of organic pollutants. Besides, root exudates had significant effects on the abundance and diversity of soil microorganisms. Interactions of root exudates with rhizosphere microorganisms played an important role in the reduction of soil contaminants. On this basis, current knowledge gaps and future research directions of root exudates under the stress of soil contaminants are also identified.