Organic Acids In Root Exudates Research Articles

Nitrogen enrichment stimulates rhizosphere multi-element cycling genes via mediating plant biomass and root exudates

Global nitrogen (N) deposition has a significant impact on the structure and function of above-ground plant communities; however, the impact of N enrichment on below-ground microbial communities remains insufficiently elucidated, especially regarding the functional gene structure. An 18-month pot experiment was conducted to examine the effects of N addition rate (0, 5, 10, and 15 g N m−2 yr−1) on the functional gene composition of rhizosphere microorganisms of Bothriochloa ischaemum, a native grass species of the semiarid area, and to evaluate the soil and plant variables connected with the observed variation. Using microarray GeoChip analysis, 2676, 744, 308, and 515 gene probes involved in carbon (C), N, phosphorus (P) and sulfur (S) cycling, respectively, were detected. Compared with the weak response of genes in the bulk soil, N addition significantly increased the abundance of genes involved in C fixation (tkta, rubisco, TIM), C degradation (amyA, ara, chitinase), methanogenesis (mcra), N2-fixation (nifH), denitrification (narG, nirK/S, nosZ), polyphosphate degradation (ppx), and sulfite reduction (dsrA, dsrB, cysJ) in the rhizosphere. The abundance of genes responsible for C-, N-, P-, and S-cycling increased with the N addition rate and was mainly regulated by variations in plant biomass and root exudates. The quantity of soil dissolved organic C played a crucial role in determining the abundance of genes related to the cycling of C, N, P, and S. Plant biomass indirectly affected gene abundance by increasing organic acids in root exudates and dissolved organic C. These findings facilitate our understanding of microbial function to elevated N levels and highlight the key role of root exudates and plant biomass in regulating microbial functions under future N-enrichment scenarios.

Aligning theoretical and empirical representations of soil carbon-to-nitrogen stoichiometry with process-based terrestrial biogeochemistry models

Soil carbon-nitrogen (C:N) stoichiometry acts as a control over decomposition and soil organic matter formation and loss, making it a key soil property for understanding ecosystem dynamics and projected ecosystems responses to global environmental change. However, the controls of soil C:N and how they respond to increasing pressures from global change agents are not fully understood. The “foundational” controls on soil C:N, namely plant and microbial C:N, have been used to predict soil C:N, but fail to accurately simulate all ecosystems and may be insufficient for predictions under global environmental change. We present an “emerging” representation of controls of soil C:N that includes plant-microbe-mineral feedbacks that have been shown to regulate soil C:N. We argue that including representation of these emerging drivers in process-based terrestrial biogeochemistry models, which include biological N fixation, mycorrhizae, priming, root exudation of organic acids, and mineralogy (including soil texture, mineral composition, and aggregation), will improve mechanistic representation of soil C:N and associated processes. Such improvements will produce models that will better simulate a variety of ecological states and predict soil C:N when global changes modify plant-microbe-mineral interactions. Here, we align our empirical understanding of controls of soil C:N with those controls represented in models, identifying contexts where emerging drivers might be particularly important to represent (e.g., priming and root exudation in nutrient-limited conditions) and areas of future work. Additionally, we show that implementing emerging drivers of soil C:N results in different simulated outcomes at steady state and in response to elevated atmospheric CO2. Our review and preliminary simulations support the need to incorporate emerging drivers of soil C:N into process-based terrestrial biogeochemistry models, allowing for both theoretical exploration of mechanisms and potentially more accurate predictions of land biogeochemical responses to global change.

Effects of Magnesium Imbalance on Root Growth and Nutrient Absorption in Different Genotypes of Vegetable Crops.

Magnesium (Mg) plays a crucial role in crop growth, but how Mg supply level affects root growth and nutrient absorption in vegetable crops with different genotypes has not been sufficiently investigated. In this study, the responses of tomato (Solanum lycopersicum L.) and cucumber (Cucumis sativus L.) crops to different levels of Mg supply were explored. Four levels of Mg treatment (i.e., 0.2, 1.0, 2.0, 3.0 mmol/L) were applied under hydroponic conditions, denoted as Mg0.2, Mg1, Mg2, and Mg3, respectively. The results showed that with increasing Mg levels, the plant biomass, root growth, and nutrient accumulation in both vegetable crops all increased until reaching their maximum values under the Mg2 treatment and then decreased. The total biomass per tomato plant of Mg2 treatment was 30.9%, 14.0%, and 14.0% higher than that of Mg0.2, Mg1, and Mg3 treatments, respectively, and greater increases were observed in cucumber plant biomass (by 54.3%, 17.4%, and 19.9%, respectively). Compared with the Mg0.2 treatment, the potassium (K) and calcium (Ca) contents in various plant parts of both crops remarkably decreased under the Mg3 treatment. This change was accompanied by prominently increased Mg contents in various plant parts and para-hydroxybenzoic acid and oxalic acid contents in root exudates. Irrespective of Mg level, plant biomass, root growth, nutrient accumulation, and root exudation of organic acids were all higher in tomato plants than in cucumber plants. Our findings indicate that excessive Mg supply promotes the roots to exude phenolic acids and hinders the plants from absorbing K and Ca in different genotypes of vegetable crops despite no effect on Mg absorption. A nutritional deficiency of Mg stimulates root exudation of organic acids and increases the types of exuded organic acids, which could facilitate plant adaption to Mg stress. In terms of root growth and nutrient absorption, tomato plants outperform cucumber plants under low and medium Mg levels, but the latter crop is more tolerant to Mg excess.

CO2 concentration and water availability alter the organic acid composition of root exudates in native Australian species

PurposeRoot exudation of organic acids (OAs) facilitates plant P uptake from soil, playing a key role in rhizosphere nutrient availability. However, OA exudation responses to CO2 concentrations and water availability remain largely untested.MethodsWe examined the effects of CO2 and water on OA exudates in three Australian woodland species: Eucalyptus tereticornis, Hakea sericea and Microlaena stipoides. Seedlings were grown in a glasshouse in low P soil, exposed to CO2 (400 ppm [aCO2] or 540 ppm [eCO2]) and water treatments (100% water holding capacity [high-watered] or 25–50% water holding capacity [low-watered]). After six weeks, we collected OAs from rhizosphere soil (OArhizo) and trap solutions in which washed roots were immersed (OAexuded).ResultsFor E. tereticornis, the treatments changed OArhizo composition, driven by increased malic acid in plants exposed to eCO2 and increased oxalic acid in low-watered plants. For H. sericea, low-watered plants had higher OAexuded per plant (+ 116%) and lower OArhizo per unit root mass (–77%) associated with larger root mass but fewer cluster roots. For M. stipoides, eCO2 increased OAexuded per plant (+ 107%) and per unit root mass (+ 160%), while low-watered plants had higher citric and lower malic acids for OArhizo and OAexuded: changes in OA amounts and composition driven by malic acid were positively associated with soil P availability under eCO2.ConclusionWe conclude that eCO2 and altered water availability shifted OAs in root exudates, modifying plant–soil interactions and the associated carbon and nutrient economy.

The involvement of organic acids in soil fertility, plant health and environment sustainability.

Increasing demand for safe food by an ever-growing human population emphasizes the urgency for increasing crop yields and reducing the losses caused by abiotic and biotic stresses; a partial solution to this problem is to develop a better understanding of plant-microbe interactions. Plant roots continuously release a wide range of compounds including organic acids in root exudates. These root exudates stimulate growth of specific microbial communities in the rhizosphere, which affect complex biological and physico-chemical interactions occurring between plant roots and the surrounding soil environment. In addition, organic acids are also released by different microbes and during decomposition of organic matter and plant residues in the soil. Interestingly, the available organic acids in the rhizosphere play crucial roles in various physio-chemical processes including the chemoattraction of microbes (both beneficial and pathogenic), mineralization and solubilization of complex minerals (P, K and Zn), biocontrol of phytopathogens, induction of systemic resistance, biogas formation, mitigation of abiotic stresses and, detoxification of metals and residual pesticides. Thus, organic acids play a significant role in the sustainable management of the soil ecosystem and in environmental sustainability. This review discusses the role of organic acids in the stimulation or enrichment of specific root-associated microbial communities and their effect on plant-microbe interactions at the root surface. In addition, the potential for root microbiome modification to enhance nutrient cycling and nutrient acquisition, and in amelioration of environmental stresses for increasing food production is discussed.

Responses to aluminum and cadmium of a RNAi sorghum line with decreased levels of phosphoenolpyruvate carboxylase 3 (PPC3)

Phosphoenolpyruvate carboxylase (PEPC) is an enzyme family with central roles in carbon and nitrogen metabolisms. In order to obtain knowledge about the function of specific PEPC isoenzymes, we have characterized RNAi sorghum lines with decreased level of PPC3 (Ppc3 lines). PPC3 is the main root PEPC, and participates in responses to salinity and ammonium stress. This prompted us to study the responses of Ppc3 lines to Al and Cd stress. Both Al and Cd treatments decreased germination rate, growth rate, and shoot and root biomass production. These detrimental effects were higher in Ppc3 lines than in Wt. Root PPC3 expression and PEPC activity was increased by Al and Cd in Wt, but not in Ppc3 lines. The treatments also increased PPC2 and PPCK gene expression, and PEPCk activity, but these changes did not substitute the lack of PPC3. The production and secretion of exudates by plant roots play an important role in plant tolerance to Al and Cd toxicity. Al (2 days of treatment) caused a 40-fold increase of the citrate content of root exudates, without differences between Wt and Ppc3 lines. On the contrary, Cd (7 days of treatment) caused a 400-fold increase of citrate in Wt exudates, but only a 30-fold increase in Ppc3 lines. These results indicate that although PPC3 is not necessary for early production of organic acids in root exudates, it is crucial for maintaining high level of synthesis and accumulation of citrate, and that PPC3 is the main PEPC isoenzyme responsible for this response to heavy metal stress.

Rhizobacteria Mitigate the Negative Effect of Aluminum on Pea Growth by Immobilizing the Toxicant and Modulating Root Exudation.

High soil acidity is one of the main unfavorable soil factors that inhibit the growth and mineral nutrition of plants. This is largely due to the toxicity of aluminum (Al), the mobility of which increases significantly in acidic soils. Symbiotic microorganisms have a wide range of beneficial properties for plants, protecting them against abiotic stress factors. This report describes the mechanisms of positive effects of plant growth-promoting rhizobacteria Pseudomonas fluorescens SPB2137 on four pea (Pisum sativum L.) genotypes grown in hydroponics and treated with 80 µM AlCl3. In batch culture, the bacteria produced auxins, possessed 1-aminocyclopropane-1-carboxylate (ACC) deaminase activity, alkalized the medium and immobilized Al, forming biofilm-like structures and insoluble phosphates. Inoculation with Ps. fluorescens SPB2137 increased root and/or shoot biomass of Al-treated plants. The bacteria alkalized the nutrient solution and transferred Al from the solution to the residue, which contained phosphorus that was exuded by roots. As a result, the Al concentration in roots decreased, while the amount of precipitated Al correlated negatively with its concentration in the solution, positively with the solution pH and negatively with Al concentration in roots and shoots. Treatment with Al induced root exudation of organic acids, amino acids and sugars. The bacteria modulated root exudation via utilization and/or stimulation processes. The effects of Al and bacteria on plants varied depending on pea genotype, but all the effects had a positive direction and the variability was mostly quantitative. Thus, Ps. fluorescens SPB2137 improved the Al tolerance of pea due to immobilization and exclusion of toxicants from the root zone.

Novel Insights into the Impact of Nano-Biochar on Composition and Structural Transformation of Mineral/Nano-Biochar Heteroaggregates in the Presence of Root Exudates.

Multiple lines of existing evidence indicate that natural organic matter (NOM) could protect poorly crystalline Fe(III) (oxyhydr)oxides from Fe(II)-catalyzed mineral transformation. Conversely, we find that nano-sized biochar (nano-BC), a pyrogenic form of NOM, promotes the phase transformation of ferrihydrite (Fh) in nano-BC/Fh heteroaggregates in the presence of aqueous Fe(II) and rice root exudates. The nano-BC/Fh heteroaggregates are composed of a core-shell like structure where the inner-layered nano-BC is more compacted and plays the dominant role in accelerating the phase transformation of Fh relative to that in the outer sphere. The extent of phase transformation is more regulated by the reversible redox reactions between quinone and hydroquinone in nano-BC than the electron transfer via its condensed aromatic structures. Furthermore, the reductive organic acids in root exudates contribute to the mineral transformation of nano-BC/Fh associations by donating electrons to Fe(III) through nano-BC. Our results suggest that heteroaggregates between nano-BC and Fe minerals are subjected to partial dissociation during their co-transport, and the stably attached nano-BC is favorable to the phase transformation of poorly crystalline Fe minerals (e.g., Fh), which might have profound implications on biogeochemical cycles of carbon and Fe in the prevailing redox environments.

Mycorrhizal symbiosis promotes the nutrient content accumulation and affects the root exudates in maize

BackgroundArbuscular mycorrhizal fungi (AMF) are a group of important symbiotic microorganisms found in ecosystems. Maize is the second most produced food crop globally. To investigate the mechanisms by which mycorrhizal symbiosis improves maize yields, the effects of mycorrhizal symbiosis on root vigor, nutrient accumulation in various tissues, and root exudates were investigated. We propose the following hypothesis: The secretion of organic acids in root exudates has antagonistic or synergistic effects, which are related to the rhizosphere environment. AMF symbiosis will enhance this effect.ResultRhizophagus aggreatus, Claroideoglomus etunicatum, and Funneliformis mosseae were used to inoculate maize plants separately; meanwhile, maize was inoculated with the above three fungi together for another processing. The plant tissues were sampled at five growth stages: V12 (twelve-leaf), VT (Tassel), R1 (Silking), R2 (Blister), and R4 (Dough stage). The root vigor, and nutrient content in different maize organs and organic acids in root exudates were determined in these stages. The results show that mycorrhizal symbiosis significantly improved the root vigor of maize, especially for plants inoculated with F. mosseae. AMF symbiosis significantly increased N, P, and K accumulation. Mixed inoculation with arbuscular mycorrhizal fungi significantly promoted the accumulation of N and K in maize. P accumulation was significantly promoted by C. etunicatum inoculation. Mycorrhizal symbiosis reduced the levels of protocatechuic, vanillic, citric, and ferulic acid in maize root exudates and increased the levels of p-hydroxybenzoic and caffeic acid. Except for syringic, chlorogenic and succinic acid, the levels of other organic acids in root exudates were higher in plants inoculated with F. mosseae than in other treatments.ConclusionThis study demonstrates that mycorrhizal symbiosis improves root vigor and promotes nutrient accumulation at various sites; in addition, mycorrhizal symbiosis affects the content of organic acids in root exudates.

The enhanced mechanisms of Hansschlegelia zhihuaiae S113 degrading bensulfuron-methyl in maize rhizosphere by three organic acids in root exudates

The residues of bensulfuron-methyl (BSM), a sulfonylurea herbicide, in soil have caused serious damage to the rotation of susceptible crops. Many studies have reported that the removal of BSM in soil was achieved by adding degrading bacteria. However, the mechanisms used by bacteria to degrade BSM in the crop rhizosphere remain unclear. In this study, a BSM-degrading bacterium, Hansschlegelia zhihuaiae S113, was applied to investigate the enhancement of effects mediated by organic acids during the bioremediation of BSM-contaminated maize rhizosphere soil. Organic acids, such as L-malic acid, tartaric acid, and fumaric acid, identified in maize root exudates, significantly stimulated the expression of cheA, which encoded the histidine kinase in strain S113 and contributed to the chemotactic response. This process accelerated the accumulation of strain S113 around the maize roots and promoted the colonization process on maize roots. The growth of strain S113 was significantly increased by L-malic acid but not tartaric acid or fumaric acid. After the S113 suspension was root-irrigated to BSM-contaminated soil, the density of strain S113 colonizing root surfaces and in rhizosphere soil reached 1.1 × 104 cells/g for roots and 4.9 × 104 cells/g in dry soil at 15 d, leading to 80.9% BSM degradation efficiency. The treatment with the addition of a mixture of S113 and L-malic acid completely degraded BSM in rhizosphere soil due to the strong attraction and growth promotion of strain S113 by L-malic acid, with a higher efficiency than that with the extra addition of fumaric acid (89.7%) or tartaric acid (87.0%). This paper revealed the enhancement effects of organic acids identified in root exudates for the in situ bioremediation of BSM-contaminated rhizosphere soil.

Dissolution and Aggregation of Metal Oxide Nanoparticles in Root Exudates and Soil Leachate: Implications for Nanoagrochemical Application.

Knowledge of dissolution, aggregation, and stability of nanoagrochemicals in root exudates (RE) and soil leachate will contribute to improving delivery mechanisms, transport in plants, and bioavailability. We characterized aggregation, stability, and dissolution of four nanoparticles (NPs) in soybean RE and soil leachate: nano-CeO2, nano-Mn3O4, nano-Cu(OH)2, and nano-MoO3. Aggregation differed considerably in different media. In RE, nano-Cu(OH)2, and nano-MoO3 increased their aggregate size for 5 days; their mean sizes increased from 518 ± 43 nm to 938 ± 32 nm, and from 372 ± 14 nm to 690 ± 65 nm, respectively. Conversely, nano-CeO2 and nano-Mn3O4 disaggregated in RE with time, decreasing from 289 ± 5 nm to 129 ± 10 nm, and from 761 ± 58 nm to 143 ± 18 nm, respectively. Organic acids in RE and soil leachate can be adsorbed onto particle surfaces, influencing aggregation. Charge of the four NPs was negative in contact with RE and soil leachate, due to organic matter present in RE and soil leachate. Dissolution in RE after 6 days was 38%, 1.2%, 0.5%, and <0.1% of the elemental content of MoO3, Cu(OH)2, Mn3O4, and CeO2 NPs. Thus, the bioavailability and efficiency of delivery of the NPs or their active ingredients will be substantially modified soon after they are in contact with RE or soil leachate.

Mass spectrometry-based quantification and spatial localization of small organic acid exudates in plant roots under phosphorus deficiency and aluminum toxicity.

Low-molecular-weight organic acid (OA) extrusion by plant roots is critical for plant nutrition, tolerance to cations toxicity, and plant-microbe interactions. Therefore, methodologies for the rapid and precise quantification of OAs are necessary to be incorporated in the analysis of roots and their exudates. The spatial location of root exudates is also important to understand the molecular mechanisms directing OA production and release into the rhizosphere. Here, we report the development of two complementary methodologies for OA determination, which were employed to evaluate the effect of inorganic ortho-phosphate (Pi) deficiency and aluminum toxicity on OA excretion by Arabidopsis roots. OA exudation by roots is considered a core response to different types of abiotic stress and for the interaction of roots with soil microbes, and for decades has been a target trait to produce plant varieties with increased capacities of Pi uptake and Al tolerance. Using targeted ultra-performance liquid chromatography coupled with high-resolution tandem mass spectrometry (UPLC-HRMS/MS), we achieved the quantification of six OAs in root exudates at sub-micromolar detection limits with an analysis time of less than 5min per sample. We also employed targeted (MS/MS) matrix-assisted laser desorption/ionization (MALDI) mass spectrometry imaging (MSI) to detect the spatial location of citric and malic acid with high specificity in roots and exudates. Using these methods, we studied OA exudation in response to Al toxicity and Pi deficiency in Arabidopsis seedlings overexpressing genes involved in OA excretion. Finally, we show the transferability of the MALDI-MSI method by analyzing OA excretion in Marchantia polymorpha gemmalings subjected to Pi deficiency.

Intercropping improves heavy metal phytoremediation efficiency through changing properties of rhizosphere soil in bamboo plantation

Moso bamboo is considered a potential species for heavy metal (HM) phytoremediation; however, the effect of intercropping on rhizosphere and phytoextraction remains to be elucidated. We comparatively investigated rhizobacteria, soil properties, and phytoextraction efficiency of monoculture and intercropping of Moso bamboo and Sedum plumbizincicola in Cu/Zn/Cd-contaminated soil. Compared with monocultures, intercropping increased the bacterial α-diversity indices (Shannon, Chao1) and the number of biomarkers. Intercropping reduced the contents of soil organic matter (SOM), available nutrients, and Cd and Cu in rhizosphere soils, and reduced the Cd and Zn contents in tissues of sedum. By contrast, Cd and Zn contents in tissues of bamboo increased, and the increase of organic acid in root exudates from intercropping could facilitate the HM absorption. The total amount of Cu, Zn, and Cd removed from the soil in intercropping system was 1.2, 1.9, and 1.8 times than those in monoculture bamboo, respectively. The abundances of Proteobacteria, Acidobacteria, Verrucomicrobia and Actinobacteria were higher in intercropping, playing an important role in soil nutrient cycles and HM remediation. These bacterial communities were closely correlated (P < 0.01) with SOM, available nitrogen, available phosphorus, and HMs. The results suggested this intercropping pattern can increase HM removal efficiency from polluted soils.

Root morphological, Cd accumulation and tolerance characteristics of 2 Dianthus caryophyllus cultivars under Cd stress

For studying the physiological response of two different Dianthus caryophyllus cultivars on Cd stress, pot experiment was carried out to measure proline and glutathione in leaves, five types of organic acids in root exudates (oxalic acid, malic acid, acetic acid, tartaric acid, citric acid), soluble sugars and free amino acids, root length, root surface area, root volume, root projected area and Cadmium content in soil, plant roots and aboveground. According to the effects of cadmium stress on the physiological and biochemical characteristics of two Dianthus caryophyllus, the results showed that: the growth of two cultivars are affected, “Master” and “Xiao Yan” are manifested as plant height, leaf, flower buds and biomass decreased, but the “Master” by the stronger inhibitory effect. The root length, total root surface area, total root projected area, and average root diameter of the “Master” increased under cadmium stress, but the root volume decreased. However, the root length, total root surface area, root volume, and root projected area of the “Xiao Yan” under cadmium stress decreased, while the average root diameter increased. The glutathione in the leaves of the two cultivars decreased, the proline content of the leaves of the “Xiao Yan” increased, while that of the “Master” decreased. In the root exudates, the free amino acid content of the two cultivars are reduced, and the secretion of organic acids is also inhibited (except for the citric acid secreted in the “Master”), while the soluble sugar content in the root exudates is expressed as “Xiao Yan” increased, and the “Master” decreased. According to the physiological response to the two cultivars under cadmium stress, the “Xiao Yan” is more suitable for soil restoration in the mining area of Lanping area.

The arbuscular mycorrhizal mycelium from barley differentially influences various defense parameters in the non-host sugar beet under co-cultivation.

The interactions between arbuscular mycorrhizal fungi (AMF) and non-host species are poorly studied. Particularly scarce is information on members of the Amaranthaceae/Chenopodiaceae family. Sugar beet (Beta vulgaris) plants were co-cultivated with a host species (Hordeum vulgare) in the presence (+AMF) or absence of Rhizophagus intraradices to explore the hypothesis that the presence of an active, pre-established AMF mycelium induces defense responses in the non-host species. Biomass of sugar beet did not respond to the +AMF treatment, while its root exudation of organic acids and phenolic acids was drastically decreased upon co-cultivation with +AMF barley. The most conspicuous effect was observed on a wide range of potential defense parameters being differentially influenced by the +AMF treatment in this non-host species. Antioxidant defense enzymes were activated and the level of endogenous jasmonic acid was elevated accompanied by nitric oxide accumulation and lignin deposition in the roots after long-term +AMF treatment. In contrast, significant reductions in the levels of endogenous salicylic acid and tissue concentration and exudation of phenolic acids indicated that AM fungus hyphae in the substrate did not induce a hypersensitive-type response in the sugar beet roots and downregulated certain chemical defenses. Our results imply that the fitness of this non-host species is not reduced when grown in the presence of an AMF mycelium because of balanced defense costs. Further studies should address the question of whether or not such modulation of defense pattern influences the pest resistance of sugar beet plants under field conditions.

Biological effects of root exudates from resistant and susceptible banana varieties on Fusaiurm oxysporum f. sp. cubense and Bacillus subtilis

Root exudates of banana resistant variety ('Nantianhuang') and susceptible variety ('Guijiao No. 6') to Fusarium wilt were collected in vitro by bathing root system to examine the biological effects of root exudates from banana varieties on Fusaiurm oxysporum f. sp. cubense and Bacillus subtilis. We explored the effects of root exudates of different banana varieties on the abundance of soil microorganisms and the growth of F. oxysporum f. sp. cubense and B. subtilis. The results showed that root exudates from resistant variety could significantly reduce the abundance of soil fungi and inhibit the spore germination of F. oxysporum f. sp. cubense. Root exudates from susceptible variety could significantly stimulate mycelia growth and spores germination, whereas root exudates from the tested banana varieties could significantly increase the growth and biofilm formation of B. subtilis. By dealing with the root exudates of resistant and susceptible varieties, the growth rate of mycelia were 11.28 and 12.28 mm·d-1, and the germination rate of spores were 34.6% and 79.5%, respectively. After culturing for 12 h, the growth rates of B. subtilis (OD600) were 1.27 and 1.14, and the biofilm formation (OD570) were 1.11 and 1.30 after static culturing 72 h, respectively. There were significant differences between the values of resistant and susceptible varieties. The colonization amount of B. subtilis in the rhizosphere of susceptible variety was significantly higher than that of resistant variety. The contents of free amino acids and organic acid in root exudates of the resistant variety were higher than that of susceptible variety. The content ratio of acetic acid and proline in the root exudates of resistant variety were 3.7 times and 2.4 times of that of susceptible variety. In conclusion, root exudates of banana resistant variety could inhibit the growth of F. oxysporum f. sp. cubense. Root exudates from susceptible variety could promote the growth of F. oxysporum f. sp. cubense,while that from the tested banana varieties could all significantly enhance growth, biofilm formation and colonization ability of B. subtilis.

Root exudation of organic acids as affected by plant growth-promoting rhizobacteria Bacillus subtilis RR4 in rice

ABSTRACTBacillus subtilis possesses plant growth-promoting traits and enhances the level of essential metabolites in plants for improved growth. Included among the metabolites are the primary metabolite malic acid (MA) and the secondary metabolite salicylic acid (SA). The impact of plant growth-promoting rhizobacteria (PGPR) on root exudation/secretion of metabolites (like MA and SA) remains unexplored, however. In this study, we aim to analyze the impact of B. subtilis RR4 on the root exudation pattern of MA and SA in rice (Oryza sativa L.). For this, rice plants were treated with the PGPR B. subtilis RR4 in hydroponics and the root exudates were collected at different periods to analyze the changes in the levels of MA and SA through high-performance liquid chromatography (HPLC). Analysis of HPLC chromatograms showed that progressive colonization by RR4 enhanced root exudation of MA 3-fold and of SA 7-fold at 96 h post-inoculation (hpi), as compared to the control. This study, thus, highlights the potential of B. subtilis RR4 of inducing/enhancing root exudation of MA and SA, which are stress alleviators, plant growth-promoting agents, and a source of carbon for microbes. With this knowledge, bioformulations, comprising the PGPRs and specific metabolites as PGPR stimulants, can be developed.

Weathering and soil formation in hot, dry environments mediated by plant–microbe interactions

Bioweathering in arid lands is a complex set of processes comprising a wide variety of organisms, all contributing to soil formation. Weathering starts with outcrop fragmentation by physical forces, later thermal stress and salts produce propagation of cracks that allow colonization by lithobiontic communities. Growth and development of primary colonizers produce pools of C and N available for further establishment of non-vascular plants when moisture is available. Furthermore, plants capable of living in crevices establish interactions with microbial communities and together optimize rock resources (organic or inorganic), enhance nutrient cycling, and accelerate soil development. Cacti and succulents are frequent rock colonizers in hot deserts. These plants exhibit numerous adaptations that enable them to survive in deserts including CAM biochemistry, physiological adaptations, and interactions with their associated microbiome. The associated microbiomes include plant growth-promoting microorganisms that increase essential nutrient supply (N and P) to the plants. We propose a conceptual model of weathering where microbial associates induce higher root exudation of organic acids in succulents. This model has to be experimentally tested; however, it involves several challenges, such as: (a) the difficulty of collecting exudates from the field or emulating experimental conditions similar to nature, and (b) selecting appropriate temporal scales to detect measurable changes since most cacti exhibit remarkably slow growth rates. Therefore, innovative approaches are in order.

Co-ordination of root salinity and shoot zinc level with rhizosphere organic acid secretion in maize.

Zinc deficiency and salinity stress often limits crop production in many calcareous soils. It has been suggested that root organic acids are a response of plants to both conditions. The aim of this hydroponic experiment was to investigate the effect of salinity on organic acids (malic, citric and acetic acids) release by roots in maize plant, under Zinc deficiency conditions. Treatments consisted of three zinc levels (0, 0.5 and 1 μM) and three salinity levels (0, 30 and 60 mM NaCl and CaCl2 in 1:1 ratio). Results showed that mean shoot and root dry matter yields significantly increased with increasing zinc concentration in nutrient solution, but increase in salinity levels significantly decreased mean shoot and root dry matter yields. Salinity had higher detrimental effect on maize shoot dry matter yield compared to those of zinc-deficiency treatments. By increasing zinc concentration and salinity levels in nutrient solution, the rate of malic, citric and acetic acids exudation decreased. The highest rate of malic, citric and acetic acids exudation was 6.61, 1.06 and 0.66 mg g RDW−1 respectively, that observed to the lowest level of zinc (Zinc0) and salinity (0 mM). The highest rate of zinc transport occurred at 60 mM salinity level which had the lowest rate of organic acids exudation. The rate of malic acid was significantly higher than citric acid (4 times higher) and acetic acic (15 times higher). In conclusion, it seems that zinc concentration in maize shoot control the release of root exudation of organic acids. However, it is not clear that the elevated organic acids release under saline condition is due to the lower zinc transport from root to shoot or higher roots membrane permeability or a combination of both mechanisms.

Physiological and Root Exudation Response of Maize Seedlings to TiO2 and SiO2 Nanoparticles Exposure

How the NPs effect the growth and physiological response like the release of organic acids along the root exudates is largely unknown yet. In this study, the effects of titanium dioxide (TiO2) and silicon dioxide (SiO2) nanoparticles (NPs) treatments (1000 mg/L) on maize seedlings for 6 days were examined. Plant biomass, pigment, malondialdehyde (MDA), reactive oxygen species (ROS) production, and contents of organic acids in root exudates were analyzed. SiO2 NPs significantly reduced (p 0.05) in chlorophyll a content compared to the CK, while chlorophyll b increased by 28.9% (p < 0.05), and chlorophyll a/b ratio and content of carotenoid decreased by 16.8% and 54.7% (p < 0.05), respectively. MDA content significantly diminished in roots and leaves under SiO2 NPs. However, O2·ˉ production increased in roots by 17.2% and 23.8% (p < 0.05), respectively, under TiO2 and SiO2 NPs treatment. The pH of root exudates was declined by 17.4% and 14.2% (p < 0.05) respectively under both NPs treatment. Organic acid contents under TiO2 NPs significantly heightened (p < 0.05) by 60.7%, 31.2%, and 50.5% for citric, lactic, and fumaric acid, respectively, while formic and oxalic acid decreased by 27.8% and 26.4% respectively compared to the CK. In SiO2 NPs case, oxalic acid increased by 41.1% (p < 0.05), while malic and citric acid decreased by 62.6% and 45.7% respectively compared to the CK. In conclusion, both NPs treatments showed alternative impacts on maize seedlings.