Plant Root Surface Research Articles

Bacterial Modulation of Plant Ethylene Levels.

A focus on the mechanisms by which ACC deaminase-containing bacteria facilitate plant growth.Bacteria that produce the enzyme 1-aminocyclopropane-1-carboxylate (ACC) deaminase, when present either on the surface of plant roots (rhizospheric) or within plant tissues (endophytic), play an active role in modulating ethylene levels in plants. This enzyme activity facilitates plant growth especially in the presence of various environmental stresses. Thus, plant growth-promoting bacteria that express ACC deaminase activity protect plants from growth inhibition by flooding and anoxia, drought, high salt, the presence of fungal and bacterial pathogens, nematodes, and the presence of metals and organic contaminants. Bacteria that express ACC deaminase activity also decrease the rate of flower wilting, promote the rooting of cuttings, and facilitate the nodulation of legumes. Here, the mechanisms behind bacterial ACC deaminase facilitation of plant growth and development are discussed, and numerous examples of the use of bacteria with this activity are summarized.

Biofilm formation assessment in Sinorhizobium meliloti reveals interlinked control with surface motility.

BackgroundSwarming motility and biofilm formation are opposite, but related surface-associated behaviors that allow various pathogenic bacteria to colonize and invade their hosts. In Sinorhizobium meliloti, the alfalfa endosymbiont, these bacterial processes and their relevance for host plant colonization are largely unexplored. Our previous work demonstrated distinct swarming abilities in two S. meliloti strains (Rm1021 and GR4) and revealed that both environmental cues (iron concentration) and bacterial genes (fadD, rhb, rirA) play crucial roles in the control of surface motility in this rhizobial species. In the current study, we investigate whether these factors have an impact on the ability of S. meliloti to establish biofilms and to colonize host roots.ResultsWe found that strain GR4, which is less prone to translocate on solid surfaces than strain Rm1021, is more efficient in developing biofilms on glass and plant root surfaces. High iron conditions, known to prevent surface motility in a wild-type strain of S. meliloti, promote biofilm development in Rm1021 and GR4 strains by inducing the formation of more structured and thicker biofilms than those formed under low iron levels. Moreover, three different S. meliloti mutants (fadD, rhb, and rirA) that exhibit an altered surface translocation behavior compared with the wild-type strain, establish reduced biofilms on both glass and alfalfa root surfaces. Iron-rich conditions neither rescue the defect in biofilm formation shown by the rhb mutant, which is unable to produce the siderophore rhizobactin 1021 (Rhb1021), nor have any impact on biofilms formed by the iron-response regulator rirA mutant. On the other hand, S. meliloti FadD loss-of-function mutants do not establish normal biofilms irrespective of iron levels.ConclusionsOur studies show that siderophore Rhb1021 is not only required for surface translocation, but also for biofilm formation on glass and root surfaces by strain Rm1021. In addition, we present evidence for the existence of control mechanisms that inversely regulate swarming and biofilm formation in S. meliloti, and that contribute to efficient plant root colonization. One of these mechanisms involves iron levels and the iron global regulator RirA. The other mechanism involves the participation of the fatty acid metabolism-related enzyme FadD.Electronic supplementary materialThe online version of this article (doi:10.1186/s12866-015-0390-z) contains supplementary material, which is available to authorized users.

Study on cellular changes and potential endotrophy of wheat roots due to colonization of Chromium reducing bacteria

Chromium is a highly toxic metal for all living organisms. Industrial use of chromium has resulted in serious widespread pollution. Biological treatment (bioremediation) has proven to be a cost-effective option for cleanup of metal-contaminated sites. Several bacteria and plant species are able to tolerate high levels of chromium compounds that can be used for cleanup. An experiment was designed to study the colonization behavior of two indigenous Cr(VI)-reducing bacterial strain Pseudomonas aeruginosa Rb-1 and Ochrobactrum intermedium Rb-2 that were grown in wheat system amended with and without Cr(VI). Hydroponically grown wheat seedlings were co-inoculated with bacterial cultures to study the root colonization potential by fluorescent and electron microscopy. Bacterial inoculation caused significant increase in the growth of seedlings under Cr(VI) stress. Fluorescent microscopy showed good colonization potential of both bacterial strains with roots of inoculated seedlings. Electron micrographs revealed that Rb-1 tended to accumulate in the form of clusters, while Rb-2 preferred to be attach in groups of two or three cells to the root surface of inoculated seedlings. Chromium stress led to the elongation of bacterial rods along with uneven cell surface due to wrapping of cells in mucilaginous material. Cr(VI) stress also resulted in the damaging of plant root surface. Hence, few cells of Rb-2 entered the damaged root cortex cells and appeared as endophytes. Excessive production of fibrillar material by both bacteria under chromium stress could clearly be observed. Both strains displayed auxin production and Cr(VI) reduction ability, showing promise for bioremediation purposes.

Characterising the rhizospheric soil niches of beach plum (Prunus maritima) colonised by arbuscularmycorrhizal fungi and/or phosphate-solubilising fungi when grown under NaCl stress

SummaryThe objective of this work was to study the effects of inoculation with an arbuscular mycorrhizal fungus (AMF), Glomus mosseae, and/or a phosphate-solubilising fungus (PSF), Mortierella sp. SM-1, on the rhizospheric niche of beach plum (Prunus maritima) when grown under 1% (w/v) NaCl stress. Salinisation adversely affected colonisation by AMF, microorganism populations, available-P, and hydrolysable-N concentrations in the soil, as well as phosphatase, urease, and proteinase activities in the rhizospheric niche of beach plum, resulting in a significant reduction (P 0.05) in the dry weight (DW) of the plants. Plants inoculated with AMF and/or PSF avoided these adverse changes. Combined inoculation with AMF and PSF promoted AMF colonisation, increased plant DWs, and significantly increased bacterial and actinomycete numbers on the root surface and in roots, and nitrogen-fixing bacterial numbers on the root surface of beach plum plants. Dual inoculation with AMF plus PSF also resulted in significantly higher phosphatase, urease, and protease activities, and higher concentrations of available-P and hydrolysable-N in rhizosphere soil than following a single inoculation with AMF or PSF. After 90 d of 1% (w/v) NaCl stress, soil pH values in the rhizosphere of beach plum plants co-inoculated with AMF plus PSF were significantly lower than in the rhizosphere of plants inoculated with AMF or PSF alone.We conclude that inoculation with AMF plus the application PSF synergistically improved the micro-ecological environment and increased soil fertility in the rhizosphere of beach plum, which improved the tolerance of these plants to saline stress.

CMEIAS bioimage informatics that define the landscape ecology of immature microbial biofilms developed on plant rhizoplane surfaces

Colonization of the rhizoplane habitat is an important activity that enables certain microorganisms to promote plant growth. Here we describe various types of computer-assisted microscopy that reveal important ecological insights of early microbial colonization behavior within biofilms on plant root surfaces grown in soil. Examples of the primary data are obtained by analysis of processed images of rhizoplane biofilm landscapes analyzed at single-cell resolution using the emerging technology of CMEIAS bioimage informatics software. Included are various quantitative analyses of the in situ biofilm landscape ecology of microbes during their pioneer colonization of white clover roots, and of a rhizobial biofertilizer strain colonized on rice roots where it significantly enhances the productivity of this important crop plant. The results show that spatial patterns of immature biofilms developed on rhizoplanes that interface rhizosphere soil are highly structured (rather than distributed randomly) when analyzed at the appropriate spatial scale, indicating that regionalized microbial cell-cell interactions and the local environment can significantly affect their cooperative and competitive colonization behaviors.

Chromium toxicity tolerance of Solanum nigrum L. and Parthenium hysterophorus L. plants with reference to ion pattern, antioxidation activity and root exudation

Chromium (Cr), being a highly toxic metal, adversely affects the mineral uptake and metabolic processes in plants when present in excess. The current study was aimed at investigating the Cr accumulation in various plant tissues and its relation to the antioxidation activity and root exudation. Plants were grown in soil spiked with different concentrations of Cr for three weeks in pots and analysed for different growth, antioxidants and ion attributes. Furthermore, plants treated with different concentrations of Cr in pots were shifted to rhizobox-like system for 48h and organic acids were monitored in the mucilage dissolved from the plant root surface, mirroring rhizospheric solution. The results revealed that the Cr application at 1mM increased the shoot fresh and dry weight and root dry weight of Solanum nigrum, whereas the opposite was observed for Parthenium hysterophorus when compared with lower levels of Cr (0.5mM) or control treatment. In both plant species, Cr and Cl concentrations were increased while Ca, Mg and K concentrations in root, shoot and root exudates were decreased with increasing levels of Cr. Higher levels of Cr treatments enhanced the activities of SOD, POD and proline content in leaves of S. nigrum, whereas lower levels of Cr treatment were found to have stimulatory effects in P. hysterophorus. P. hysterophorus exhibited highest exudation of organic acid contents. With increasing levels of Cr treatments, citric acid concentration in root exudates increased by 35% and 44% in S. nigrum, whereas 20% and 76% in P. hysterophorus. Cr toxicity was responsible for the shoot growth reduction of S. nigrum and P. hysterophorus, however, shoot growth response was different at different levels of applied Cr. Consequently, Cr stress negatively altered the plant physiology and biochemistry. However, the enhanced antioxidant production, Cl uptake and root exudation are the physiological and biochemical indicators for the plant adaptations in biotic systems polluted with Cr.

Reducing Strength Prevailing at Root Surface of Plants Promotes Reduction of Ag+ and Generation of Ag0/Ag2O Nanoparticles Exogenously in Aqueous Phase

Potential of root system of plants from wide range of families to effectively reduce membrane impermeable ferricyanide to ferrocyanide and blue coloured 2,6-dichlorophenol indophenol (DCPIP) to colourless DCPIPH2 both under non-sterile and sterile conditions, revealed prevalence of immense reducing strength at root surface. As generation of silver nanoparticles (NPs) from Ag+ involves reduction, present investigations were carried to evaluate if reducing strength prevailing at surface of root system can be exploited for reduction of Ag+ and exogenous generation of silver-NPs. Root system of intact plants of 16 species from 11 diverse families of angiosperms turned clear colorless AgNO3 solutions, turbid brown. Absorption spectra of these turbid brown solutions showed silver-NPs specific surface plasmon resonance peak. Transmission electron microscope coupled with energy dispersive X-ray confirmed the presence of distinct NPs in the range of 5–50 nm containing Ag. Selected area electron diffraction and powder X-ray diffraction patterns of the silver NPs showed Bragg reflections, characteristic of crystalline face-centered cubic structure of Ag0 and cubic structure of Ag2O. Root system of intact plants raised under sterile conditions also generated Ag0/Ag2O-NPs under strict sterile conditions in a manner similar to that recorded under non-sterile conditions. This revealed the inbuilt potential of root system to generate Ag0/Ag2O-NPs independent of any microorganism. Roots of intact plants reduced triphenyltetrazolium to triphenylformazon and impermeable ferricyanide to ferrocyanide, suggesting involvement of plasma membrane bound dehydrogenases in reduction of Ag+ and formation of Ag0/Ag2O-NPs. Root enzyme extract reduced triphenyltetrazolium to triphenylformazon and Ag+ to Ag0 in presence of NADH, clearly establishing potential of dehydrogenases to reduce Ag+ to Ag0, which generate Ag0/Ag2O-NPs. Findings presented in this manuscript put forth a novel, simple, economically viable and green protocol for synthesis of silver-NPs under ambient conditions in aqueous phase, using root system of intact plants.

Effects of Fusarium oxysporum on rhizosphere microbial communities of two cucumber genotypes with contrasting Fusarium wilt resistance under hydroponic condition

Two cucumber genotypes were chosen that differ in Fusarium wilt resistance and inoculated with Fusarium oxysporum (Schlechtend, Fr) f. sp. cucumerinum (Owen) Snyder & Hansen (FO) in a hydroponic nutrient solution system to determine whether differences in plant genotype affected rhizosphere microbial community and its involvement in resistance to root-borne pathogens. Response of the FO population and microbial communities were compared in the nutrient solution and on root surfaces. Results demonstrated that FO inoculation resulted in a higher FO population on root surfaces and a lower population in the nutrient solution for the susceptible genotype JinYan NO. 4 (JYan); however, an inverse pattern was observed in the resistant JinYou NO. 1 (JYou). Similarly, promotive effects on FO spore germination and germ-tube elongation were only observed in root exudates from FO-inoculated susceptible JYan. FO inoculation resulted in overall increases in cultivable fungi and oomycetes, and actinomycete populations on root surfaces of the resistant JYou compared to the JYan counterpart. PCR-DGGE analysis of 16S rDNA fragments indicated that following FO inoculation, significant changes in the bacterial structure isolated from the root surfaces of susceptible plants were observed, such as an increased diversity index, a decreased evenness index, and the occurrence of several types of bacteria that decomposed organic substances. These results suggest that a complex interaction between plant genotypes and pathogen affected the rhizosphere microbial community.

New Citrate-Bicarbonate-Ethylenediaminetetraacetate (CBE) Method for Chemical Extraction of Hydrous Iron Oxides from Plant Root Surfaces

In the present study, the sodium citrate, sodium bicarbonate, and ethylenediaminetetraacetate (CBE) method was evaluated for iron (Fe) extraction from plant root surfaces and compared with the dithionite-citrate-bicarbonate (DCB) method. Iron plaque on root surfaces was induced by growing rice seedlings in soil with 1.8 mM Fe2+. Iron plaque was extracted following CBE and DCB methods. The effects of pH, temperature, and incubation time of these methods on Fe extraction from root surfaces were also examined. Iron extraction of CBE and DCB methods did not differ significantly (P < 0.05) at pH between 6 and 8, whereas Fe extraction decreased substantially for further increase of the pH of CBE and DCB solution. In some instances, there were significant differences between CBE and DCB methods in extracellular Fe extraction for temperature and incubation time. The average Fe extraction of CBE and DCB methods were 94% and 81%, respectively, indicating that CBE method would be a better choice for Fe extraction from plant roots. The recommended optimal conditions for CBE method are pH 8, volume of the solution 30 mL, incubation time 30 min, and solution temperature 22 ± 2 °C.

Robust in vitro assay system for quantitative analysis of parasitic root-knot nematode infestation using Lotus japonicus

Root-knot nematodes are sedentary endoparasites that induce permanent infestation sites inside the roots of a broad range of crop plants. The development of effective control strategies require understanding the root-knot nematode parasitic process, however, the key molecular determinants for host manipulation during infestation remain elusive. One limiting factor has been the lack of a standardized conventional method for quantitative measurement of host parasitism by root-knot nematodes, particularly one that enables efficient downstream analyses and is free from other biological sources of variability. We report here a robust, highly reproducible system for quantitative analysis of all stages of root-knot nematode infestation using the legume Lotus japonicus as the plant host. This system provides a high quality nematode inoculum that maintains consistency in juvenile age and viability even between independently prepared populations. An optimized root transformation protocol was also developed for L.japonicus to facilitate downstream molecular studies in conjunction with the quantitative assay. Hairy root transformation efficiencies up to 91% were achieved. Root-knot nematodes formed egg masses at the root surface of both intact plants and transgenic hairy root cultures within eight weeks, confirming the assay conditions support an efficient completion of the infestation cycle. The in vitro assay system described here is compatible with other plant hosts and will benefit agricultural biotechnology research as it now enables specific high-throughput screening of nematode resistance traits together with subsequent mechanistic elucidation of the causative factors.

Filter strip as a method of choice for apoplastic fluid extraction from maize roots

Apoplastic fluid was extracted from maize (Zea mays L.) roots using two procedures: collection from the surface of intact plant roots by filter paper strips (AF) or vacuum infiltration and/or centrifugation from excised root segments (AWF). The content of cytoplasmic marker (glucose-6-phosphate, G-6-P) and antioxidative components (enzymes, organic acids, phenolics, sugars, ROS) were compared in the extracts. The results obtained demonstrate that AF was completely free of G-6-P, as opposed to AWF where the cytoplasmic constituent was detected even at mildest centrifugation (200×g). Isoelectric focusing of POD and SOD shows the presence of cytoplasmic isoforms in AWF, and HPLC of sugars and phenolics a much more complex composition of AWF, due to cytoplasmic contamination. Organic acid composition differed in the two extracts, much higher concentrations of malic acid being registered in AF, while oxalic acid due to intracellular contamination being present only in AWF. EPR spectroscopy of DEPMPO spin trap in the extracts showed persistent generation of hydroxyl radical adduct in AF. The results obtained argue in favor of the filter strip method for the root apoplastic fluid extraction, avoiding the problems of cytoplasmic contamination and dilution and enabling concentration measurements in minute regions of the root.

Assessment of the nutrient removal effectiveness of floating treatment wetlands applied to urban retention ponds

The application of floating treatment wetlands (FTWs) in point and non-point source pollution control has received much attention recently. Although the potential of this emerging technology is supported by various studies, quantifying FTW performance in urban retention ponds remains elusive due to significant research gaps. Actual urban retention pond water was utilized in this mesocosm study to evaluate phosphorus and nitrogen removal efficiency of FTWs. Multiple treatments were used to investigate the contribution of each component in the FTW system with a seven-day retention time. The four treatments included a control, floating mat, pickerelweed (Pontederia cordata L.), and softstem bulrush (Schoenoplectus tabernaemontani). The water samples collected on Day 0 (initial) and 7 were analyzed for total phosphorus (TP), total particulate phosphorus, orthophosphate, total nitrogen (TN), organic nitrogen, ammonia nitrogen, nitrate-nitrite nitrogen, and chlorophyll-a. Statistical tests were used to evaluate the differences between the four treatments. The effects of temperature on TP and TN removal rates of the FTWs were described by the modified Arrhenius equation. Our results indicated that all three FTW designs, planted and unplanted floating mats, could significantly improve phosphorus and nitrogen removal efficiency (%, E-TP and E-TN) compared to the control treatment during the growing season, i.e., May through August. The E-TP and E-TN was enhanced by 8.2% and 18.2% in the FTW treatments planted with the pickerelweed and softstem bulrush, respectively. Organic matter decomposition was likely to be the primary contributor of nutrient removal by FTWs in urban retention ponds. Such a mechanism is fostered by microbes within the attached biofilms on the floating mats and plant root surfaces. Among the results of the four treatments, the FTWs planted with pickerelweed had the highest E-TP, and behaved similarly with the other two FTW treatments for nitrogen removal during the growth period. The temperature effects described by the modified Arrhenius equation revealed that pickerelweed is sensitive to temperature and provides considerable phosphorus removal when water temperature is greater than 25 °C. However, the nutrient removal effectiveness of this plant species may be negligible for water temperatures below 15 °C. The study also assessed potential effects of shading from the FTW mats on water temperature, DO, pH, and attached-to-substrate periphyton/vegetation.

Coupled pot and lysimeter experiments assessing plant performance in microbially assisted phytoremediation

We performed an experiment at pot scale to assess the effect of plant growth-promoting bacteria (PGPB) on the development of five plant species grown on a tailing dam substrate. None of the species even germinated on inoculated unamended tailing material, prompting use of compost amendment. The effect of inoculation on the amended material was to increase soil respiration, and promote elements immobilisation at plant root surface. This was associated with a decrease in the concentrations of elements in the leaching water and an increase of plant biomass, statistically significant in the case of two species: Agrostis capillaris and Festuca rubra. The experiment was repeated at lysimeter scale with the species showing the best development at pot scale, A. capillaris, and the significant total biomass increase as a result of inoculation was confirmed. The patterns of element distribution in plants also changed (the concentrations of metals in the roots of A. capillaris and F. rubra significantly decreased in inoculated treatments, while phosphorus concentration significantly increased in roots of A. capillaris in inoculated treatment at lysimeter scale). Measured variables for plant oxidative stress did not change after inoculations. There were differences of A. capillaris plant-soil system response between experimental scales as a result of different substrate column structure and plant age at the sampling moment. Soil respiration was significantly larger at lysimeter scale than at pot scale. Leachate concentrations of As, Mn and Ni had significantly larger concentrations at lysimeter scale than at pot scale, while Zn concentrations were significantly smaller. Concentrations of several metals were significantly smaller in A. capillaris at lysimeter scale than at pot scale. From an applied perspective, a system A. capillaris-compost-PGPB selected from the rhizosphere of the tailing dam native plants can be an option for the phytostabilisation of tailing dams. Results should be confirmed by investigation at field plot scale.

Reduced root anchorage of freshwater plants in sandy sediments enriched with fine organic matter

Summary The decline of submerged macrophytes in nutrient‐enriched lakes has been explained by shading from phytoplankton. Reduced anchorage in softer organic sediments may, however, enhance the loss of submerged macrophytes; therefore, we tested whether increased water content results in a decline of anchorage of artificial root systems and isoetid species in sandy sediments enriched with fine organic particles, and compared anchorage and hydraulic drag for short isoetid and tall elodeid species to evaluate the risk of uprooting. Constructed and natural sandy sediments followed the same hyperbolic relationship of higher water content with increasing organic content. Anchorage of artificial root systems peaked in sediments containing small amounts of organic matter (0.7–1.2% dry weight) and water (20–27% WW) and was up to six times higher compared with more organic‐rich sediments. Anchorage of the isoetid, Lobelia dortmanna, in natural sandy sediments increased linearly with plant weight and root surface area and declined with higher organic content of sediments. Multiple regression models could account for the positive influence of plant size on root anchorage and the negative influence of organic matter and water in sediments. Hydraulic drag on rigid shoots of three isoetid species increased with the second power of water velocity. The species‐specific drag coefficient was consistently high for these isoetids and markedly lower and declining with increasing velocity for two elodeids with flexible tissue. Anchorage of two isoetid species in mineral and more organic sediments from shallow water markedly exceeded estimated hydraulic drag under storm events inducing 0.6 m s−1 currents, while elodeid species were only safely rooted in firm mineral sediments. These findings are in agreement with the growth of isoetids in wave‐exposed shallow waters, while elodeids prefer deeper or sheltered sites. The multiple regression models predicted that L. dortmanna could attain critically low anchorage on highly organic sediments in accordance with observations of plant‐free littoral sediments of colloidal organic texture. Because it is well established that sediment anoxia severely impairs root development of L. dortmanna, enrichment of sediments with labile organic matter may represent a double jeopardy by reducing root development and anchorage.

Volatile Organic Compound Mediated Interactions at the Plant-Microbe Interface

Microorganisms colonize the surfaces of plant roots, leaves, and flowers known as the rhizosphere, phyllosphere, and anthosphere. These spheres differ largely in a number of factors that may determine the ability of microbes to establish themselves and to grow in these habitats. In this article, we focus on volatile organic compounds (VOCs) emitted by plants, and we discuss their effects on microbial colonizers, with an emphasis on bacteria. We present examples of how growth-inhibiting properties and mechanisms of VOCs such as terpenoids, benzenoid compounds, aliphatics, and sulfur containing compounds prevent bacterial colonization at different spheres, in antagonism with their role as carbon-sources that support the growth of different bacterial taxa. The notion that VOCs represent important factors that define bacterial niches is further supported by results for representatives of two bacterial genera that occupy strongly diverging niches based on scent emissions of different plant species and organs. Bacteria are known to either positively or negatively affect plant fitness and to interfere with plant-animal interactions. Thus, bacteria and other microbes may select for VOCs, enabling plants to control microbial colonizers on their surfaces, thereby promoting the growth of mutualists and preventing the establishment of detrimental microbes.

Bacterial Traits Involved in Colonization of Arabidopsis thaliana Roots by Bacillus amyloliquefaciens FZB42.

Colonization studies previously performed with a green-fluorescent-protein, GFP, labeled derivative of Bacillus amyloliquefaciens FZB42 revealed that the bacterium behaved different in colonizing surfaces of plant roots of different species (Fan et al., 2012). In order to extend these studies and to elucidate which genes are crucial for root colonization, we applied targeted mutant strains to Arabidopsis seedlings. The fates of root colonization in mutant strains impaired in synthesis of alternative sigma factors, non-ribosomal synthesis of lipopeptides and polyketides, biofilm formation, swarming motility, and plant growth promoting activity were analyzed by confocal laser scanning microscopy. Whilst the wild-type strain heavily colonized surfaces of root tips and lateral roots, the mutant strains were impaired in their ability to colonize root tips and most of them were unable to colonize lateral roots. Ability to colonize plant roots is not only dependent on the ability to form biofilms or swarming motility. Six mutants, deficient in abrB-, sigH-, sigD-, nrfA-, yusV and RBAM017410, but not affected in biofilm formation, displayed significantly reduced root colonization. The nrfA- and yusV-mutant strains colonized border cells and, partly, root surfaces but did not colonize root tips or lateral roots.

Solution equilibrium behind the room-temperature synthesis of nanocrystalline titanium dioxide

Formation of nanocrystalline and monodisperse TiO2 from a water soluble and stable precursor, ammonium oxo-lactato-titanate, (NH4)8Ti4O4(Lactate)8·4H2O, often referred to as TiBALDH or TALH, is demonstrated to be due to a coordination equilibrium. This compound, individual in the solid state, exists in solution in equilibrium with ammonium tris-lactato-titanate, (NH4)2Ti(Lactate)3 and uniform crystalline TiO2 nanoparticles (anatase) stabilized by surface-capping with lactate ligands. This equilibrium can be shifted towards nano-TiO2via application of a less polar solvent like methanol or ethanol, dilution of the solution, introduction of salts or raising the temperature, and reverted on addition of polar and strongly solvating media such as dimethyl sulfoxide, according to NMR. Aggregation and precipitation of the particles were followed by DLS and could be achieved by a decrease in their surface charge by adsorption of strongly hydrogen-bonding cations, e.g. in solutions of ammonia, ethanolamine or amino acid arginine or by addition of ethanol. The observed equilibrium may be involved in formation of nano-titania on the surface of plant roots exerting chelating organic carboxylate ligands and thus potentially influencing plant interactions.

Phytotoxicity and uptake of nanoscale zero-valent iron (nZVI) by two plant species

Use of nano-scale zero valent iron (nZVI) for the treatment of various environmental pollutants has been proven successful. However, large scale introduction of engineered nanomaterials such as nZVI into the environment has recently attracted serious concerns. There is an urgent need to investigate the environmental fate and impact of nZVI due to the scope of its application. The goal of this study was to evaluate the toxicity and accumulation of bare nZVI by two commonly encountered plant species: cattail (Typha latifolia) and hybrid poplars (Populous deltoids×Populous nigra). Plant seedlings were grown hydroponically in a greenhouse and dosed with different concentrations of nZVI (0–1000mg/L) for four weeks. The nZVI exhibited strong toxic effect on Typha at higher concentrations (>200mg/L) but enhanced plant growth at lower concentrations. nZVI also significantly reduced the transpiration and growth of hybrid poplars at higher concentrations. Microscopic images indicated that large amount of nZVI coated on plant root surface as irregular aggregates and some nZVI penetrated into several layers of epidermal cells. Transmission electron microscope (TEM) and scanning transmission electron microscope (STEM) confirmed the internalization of nZVI by poplar root cells but similar internalization was not observed for Typha root cells. The upward transport to shoots was minimal for both plant species.

Biocontrol of tomato wilt disease by Bacillus subtilis isolates from natural environments depends on conserved genes mediating biofilm formation.

Bacillus subtilis and other Bacilli have long been used as biological control agents against plant bacterial diseases but the mechanisms by which the bacteria confer protection are not well understood. Our goal in this study was to isolate strains of B. subtilis that exhibit high levels of biocontrol efficacy from natural environments and to investigate the mechanisms by which these strains confer plant protection. We screened a total of 60 isolates collected from various locations across China and obtained six strains that exhibited above 50% biocontrol efficacy on tomato plants against the plant pathogen Ralstonia solanacearum under greenhouse conditions. These wild strains were able to form robust biofilms both in defined medium and on tomato plant roots and exhibited strong antagonistic activities against various plant pathogens in plate assays. We show that plant protection by those strains depended on widely conserved genes required for biofilm formation, including regulatory genes and genes for matrix production. We provide evidence suggesting that matrix production is critical for bacterial colonization on plant root surfaces. Finally, we have established a model system for studies of B. subtilis-tomato plant interactions in protection against a plant pathogen.

Effects of a low-voltage electric pulse charged to culture soil on plant growth and variations of the bacterial community

This study was conducted to verify the effect of an electric pulse on growth of crops (lettuce and hot pepper) that were cultivated in lab-scale soil. The electric pulse generated from direct-circuited 2, 4, 6, 8, and 10 V of electricity by periodic exchange of the anode and cathode was charged to a culture soil that is an electrically pulsed culture soil (EPCS) but not charged to a conventional culture soil (CCS). Growth of lettuce increased and growth duration of hot pepper plants was more prolonged at 4, 6, 8, and 10 V of EPCS than at 2 V of EPCS and CCS. The fruiting duration and yield of hot pepper fruits were proportional to the growth duration of the hot pepper plants. Temperature gradient gel electrophoresis (TGGE) patterns of 16S-rDNA obtained from the bacterial community inhabiting the CCS and EPCS were identical at the initial time and did not change significantly at days 28 and 56 of cultivation. The bacterial communities inhabiting the surface of lettuce roots were not influenced by the electric pulse but were significantly different from those inhabiting the culture soil based on the TGGE patterns. Growth of lettuce and hot pepper plants that were cultivated in 4 - 10 V of EPCS may increase; however, the bacterial community inhabiting the soil and the surface of plant roots may not be influenced by an electric pulse.