Role of the molecular structure of humified organic matter in rice plant response to environmental lead pollution.

Humified organic matter has been shown to decrease Pb toxicity in plants. However, there are still gaps in our understanding of the mechanism by which this phenomenon occurs. In this study, we aimed to assess the ability of humic substances (HSs), humic acids (HAs), and fulvic acids (FAs) to enhance defense mechanisms in rice plants under lead (Pb)-stressed conditions. HS fractions were isolated from vermicompost using the chemical fractionation methodology established by the International Humic Substances Society. These fractions were characterized by solid-state NMR and FTIR. Chemometric analysis was used to compare humic structures and correlate them with bioactivity. Three treatments were tested to evaluate the protective effect of humic fractions on rice plants. The first experiment involved the application of humic fractions along with Pb. The second comprised pretreatment with humic fractions followed by subsequent exposure to Pb stress. The third experiment involved Pb stress and subsequent treatment with humic fractions. The root morphology and components of the antioxidative defense system were evaluated and quantified. The results showed that HS + Pb, HA + Pb, and FA + Pb treatment preserved root growth and reduced the levels of O2- and malondialdehyde (MDA) in the roots by up to 5% and 2%, respectively. Pretreatment of the plants with humic fractions promoted the maintenance of root growth and reduced the contents of O2-, H2O2, and MDA by up to 48%, 22%, and 20%, respectively. Combined application of humic fractions and Pb reduced the Pb content in plant tissues by up to 60%, while pretreatment reduced it by up to 80%. The protective capacity of humic fractions is related to the presence of peptides, lignin, and carbohydrate fragments in their molecular structures. These results suggest that products could be developed that can mitigate the adverse effects of heavy metals on agricultural crops.

Spectroscopic-chemometric modeling of 80 humic acids confirms the structural pattern identity of humified organic matter despite different formation environments.

The structure of humic substances (HSs) and the humification process are critical topics for understanding the dynamics of carbon on the planet. This study aimed to assess the structural patterns of 80 humic acid (HA) samples isolated from different soils, namely, Histosols, Ferralsols, Cambisols, Mollisols, Planosols and vermicompost, by spectroscopic characterization using solid-state 13C nuclear magnetic resonance cross-polarization/magic angle spinning combined with chemometric techniques. All 80 HAs had a similar structural pattern, regardless of their source of origin, but they had different relative quantities of organic C species. The different structural amounts of the various organic C fractions generated different properties in each of the HAs. This explains why there were similarities in the HS functions but why the intensities of these functions varied among the samples from the different soil types and environments, confirming that HSs are a group of compounds with a structural identity distinct from the molecules that give rise to them. There appears to be no single definition for the humification process; therefore, for the soils from each source of origin, a specific humification process occurs that depends on the characteristics of the local environment. Humification can be understood as a process that is similar to a chemical reaction, where the key factor that determines the formation of the products is the structural characteristics of the reactants (organic substrates deposited in the soil). The degree to which the reaction progresses is governed by the reaction conditions (chemical, physical, and biological properties of the soil). The structural patterns for HSs obtained in this study justify the existence of HSs structured as self-assembled, hydrophilic and hydrophobic domains that, under certain conditions, can undergo transformations, altering the balance of organic carbon in the environment.

Environmental implications of the organic matter structure for white-rot fungus Pleurotus eryngii growth in a tropical climate.

White-rot fungi (Pleurotus eryngii) are decomposers of lignocellulosic substrates. The relationship between the structure of humified organic matter and P. eryngii growth, is poorly understood. This study aimed to evaluate the relationship between the growth and development of white-rot fungi (P. eryngii) in two structurally different sources of humified organic matter. Fungus growth and development (mycelium diameter, fresh and dry mycelium mass, mycelium density, and biological yield) were evaluated in experiments with the application of humic substances (HS) extracted from vermicompost (VC) and peat. Both HS were characterized by CP/MAS 13C NMR spectroscopy associated with chemometrics analysis. The HS present different structural characteristics, with those extracted from VC having a predominance of functionalized C-aliphatics (carbohydrates), low hydrophobicity, and a 90% proportion of cellulose/hemicellulose carbon in the composition. HS extracted from peat have a predominance of C-aromatics (lignin fragments), higher hydrophobicity, and a proportion of lignin carbon of up to 80%. The results showed that P. eryngii growth is dependent on the C-cellulosic and C-lignin balance. HS extracted from lignin-rich peat regulates the fungus growth at initial times and sometimes inhibits the biological performance. The highly cellulosic HS from VC regulate the fungus growth at later times and its biological performance.

Humic acids induce a eustress state via photosynthesis and nitrogen metabolism leading to a root growth improvement in rice plants.

Chemical eustressors induce a eustress state "positive stress" increasing the resistance and improve the plant growth. The potentiality of humic acids (HA) to act as a eustressor has been scarcely explored. The present study aims to evaluate how HA with different structural characteristics induce differently, a eustress state in rice plants through the regulation of photosynthesis. The photosynthetic performance index showed an initial eustress state in plant by HA application characterized by reduction in photosynthesis followed by an increase in photosynthetic efficiency. The HA as a chemical eustressor triggering changes in plant metabolism indicate that the interaction of HA with root system induces a roots growth stimulus preceded by an initial positive stress. The eustress caused by HA is differentiated and is related to its chemical-physics characteristics. The HAVC, with a predominance of CAlkyl-(O,N), CAlkyl-di-O, CAromatic-O structures and greater polarity, stimulated the accumulation of N-NO3- and of soluble sugars in the sheath, increase carbohydrates content in the root and the root emission, resulting in higher total biomass production. The HASOIL, with a predominance of CCOOH-(H,R), CAlkyl-O, CAromatic-H,R structures and greater hydrophobicity caused a decrease in N-NH4+ and N-amine. The HARN, with a predominance of CAlkyl-O, CAlkyl-H,R, and CO, characterized by average polarity, caused an increase in photosynthetic pigment and N-NH4+ content. These results are keys to understand that quality of soil organic matter is related to plant development and that HA are efficient proxies for elucidate its function in natural environments.

Fire lead to disturbance on organic carbon under sugarcane cultivation but is recovered by amendment with vinasse.

Sugarcane burning has been widely practiced in Brazil and worldwide. In the long term, this farming practice can cause soil erosion, reduction in organic carbon (OC) and consequently, changes in the structure of soil organic matter (SOM). Such changes may be difficult to reverse. This study aimed to assess the medium- and long-term effects of sugarcane burning on SOM characteristics, both in terms of quantity and structural quality and evaluate the application of vinasse as a strategy to attenuate fire-induced changes in burned soil. The experiment was conducted in a 50-year-old sugarcane field on soils classed as Cambissolo Háplico (Inceptisol). Four plots were sampled: a) burning of sugarcane for harvest for 37 years (SCB37); b) renewal of the sugarcane field and burning for harvest for 3 years (SCB3); c) renewal of the sugarcane field without burning for harvest for 3 years (SCWB), and d) renewal of the sugarcane field and burning for harvest with the application of vinasse for 3 years (SCV). Chemical and physical characterization of SOM was performed by solid-state spectroscopy (UV-vis, ATR-FTIR e 13C NMR CP/MAS) and chemometric techniques. The results showed that sugarcane burning drastically impacts SOM content and its chemical structure, however, the application of vinasse preserves and restores the soil from the fire effects. Content of soil OC, particulate OC, mineral-associated OC, humic acid, humin and light fraction OM that were affected by fire, had an increase and recovery of contents by the vinasse application. Solid state spectroscopy showed that labile structures were lost in humic acids (HA) by fire and recalcitrant structures were preserved. The application of vinasse incorporated fragments of lipids and carbohydrates in HA structure. Burning sugar cane straw affects the integrity of soil organic matter but can be restored by applying vinasse.

Carbon input and the structural quality of soil organic matter as a function of agricultural management in a tropical climate region of Brazil.

Carbon (C) stabilization and the quality of soil organic matter (SOM) in a tropical climate are key aspects regulating carbon dioxide emissions and maintaining the C cycle. Soil management influences the accumulation of C, regulating the balance between mineralization and/or the humification of SOM. This study aimed to quantify inputs of different chemical forms of C into soil and to evaluate the structural chemical characteristics of humified SOM. Four management systems were established: Forest (F), Pasture (P), Conventional tillage (T), and No-tillage (NT). Total organic carbon (TOC) and nitrogen (TN) by depth, chemical forms of organic matter input, and spectroscopic characterization of SOM in the form of humic acids (HA) were analyzed. The results obtaining by PCA-13C NMR show that the forest accumulated a high amount of C on the surface (surpassing 20 Mg ha-1), favoring the formation of aliphatic HA (CAlkyl-H,R; CAlkyl-O,N; CAlkyl-O). In the NT management that increases biomass in the soil (14 Mg ha-1), the mineralization process occurred to a greater extent, allowing HA to form with a predominance of aromatic structures (CArm-H,R and CArm-O,N). The PCA-FTIR analysis showed that the P system contributed to the formation of similar HA to those under F management. The T management system incorporated the least TOC and TN, with different HA types being formed in these soils than what was found in other managements. Thus, minimally managed and more stabilized systems in tropical climates form HA of structural and compositional similarity, regardless of the nature of C (C3 or C4). In contrast, soils subjected to agricultural uses that promote higher or lower C inputs, form HA that are structurally different from P and F. This study demonstrates the need for developing experiments for model building to elucidate the relationships among C input, management type, and the formation of humic substances.

Physiological response of Cucurbita pepo var. pepo mycorrhized by Sonoran desert native arbuscular fungi to drought and salinity stresses

ABSTRACT Plants response to symbiosis with arbuscular mycorrhizal fungi (AMF) under water stress is important to agriculture. Under abiotic stress conditions native fungi are more effective than exotics in improving plant growth and water status. Mycorrhization efficiency is related to soil fungi development and energy cost-benefit ratio. In this study, we assessed the effect on growth, water status and energy metabolism of Cucurbita pepo var. pepo when inoculated with native AMF from the Sonoran desert Mexico (mixed isolate and field consortium), and compared with an exotic species from a temperate region, under drought, low and high salinity conditions. Dry weights, leaf water content, water and osmotic potentials, construction costs, photochemistry and mycorrhization features were quantified. Under drought and low salinity conditions, the mixed isolate increased plant growth and leaf water content. Leaf water potential was increased only by the field consortium under drought conditions (0.5-0.9 MPa). Under high salinity, the field consortium increased aerial dry weight (more than 1 g) and osmotic potential (0.54 MPa), as compared to non-mycorrhized controls. Plants inoculated with native AMF, which supposedly diminish the effects of stress, exhibited low construction costs, increased photochemical capacity, and grew larger external mycelia in comparison to the exotic inoculum.

Physiological response of Cucurbita pepo var. pepo mycorrhized by Sonoran desert native arbuscular fungi to drought and salinity stresses.

Plants response to symbiosis with arbuscular mycorrhizal fungi (AMF) under water stress is important to agriculture. Under abiotic stress conditions native fungi are more effective than exotics in improving plant growth and water status. Mycorrhization efficiency is related to soil fungi development and energy cost-benefit ratio. In this study, we assessed the effect on growth, water status and energy metabolism of Cucurbita pepo var. pepo when inoculated with native AMF from the Sonoran desert Mexico (mixed isolate and field consortium), and compared with an exotic species from a temperate region, under drought, low and high salinity conditions. Dry weights, leaf water content, water and osmotic potentials, construction costs, photochemistry and mycorrhization features were quantified. Under drought and low salinity conditions, the mixed isolate increased plant growth and leaf water content. Leaf water potential was increased only by the field consortium under drought conditions (0.5-0.9MPa). Under high salinity, the field consortium increased aerial dry weight (more than 1g) and osmotic potential (0.54MPa), as compared to non-mycorrhized controls. Plants inoculated with native AMF, which supposedly diminish the effects of stress, exhibited low construction costs, increased photochemical capacity, and grew larger external mycelia in comparison to the exotic inoculum.

Dark septate endophyte decreases stress on rice plants

Abstract Abiotic stress is one of the major limiting factors for plant development and productivity, which makes it important to identify microorganisms capable of increasing plant tolerance to stress. Dark septate endophytes can be symbionts of plants. In the present study, we evaluated the ability of dark septate endophytes isolates to reduce the effects of water stress in the rice varieties Nipponbare and Piauí. The experiments were performed under gnotobiotic conditions, and the water stress was induced with PEG. Four dark septate endophytes were isolated from the roots of wild rice (Oryza glumaepatula) collected from the Brazilian Amazon. Plant height as well as shoot and root fresh and dry matter were measured. Leaf protein concentrations and antioxidant enzyme activity were also estimated. The dark septate endophytes were grown in vitro in Petri dishes containing culture medium; they exhibited different levels of tolerance to salinity and water stress. The two rice varieties tested responded differently to inoculation with dark septate endophytes. Endophytes promoted rice plant growth both in the presence and in the absence of a water deficit. Decreased oxidative stress in plants in response to inoculation was observed in nearly all inoculated treatments, as indicated by the decrease in antioxidant enzyme activity. Dark septate endophytes fungi were shown to increase the tolerance of rice plants to stress caused by water deficiency.

Nitrogen-fixing bacteria and arbuscular mycorrhizal fungi in Piptadenia gonoacantha ( Mart. ) Macbr

Abstract The family Leguminosae comprises approximately 20,000 species that mostly form symbioses with arbuscular mycorrhizal fungi (AMF) and nitrogen-fixing bacteria (NFB). This study is aimed at investigating and confirming the dependence on nodulation and biological nitrogen fixation in the specie Piptadenia gonoacantha (Mart.) Macbr., which belongs to the Piptadenia group. Two consecutive experiments were performed in a greenhouse. The experiments were fully randomized with six replicates and a factorial scheme. For the treatments, the two AMF species and three NFB strains were combined to nodulate P. gonoacantha in addition to the control treatments. The results indicate this species’ capacity for nodulation without the AMF; however, the AMF + NFB combinations yielded a considerable gain in P. gonoacantha shoot weight compared with the treatments that only included inoculating with bacteria or AMF. The results also confirm that the treatment effects among the AMF + NFB combinations produced different shoot dry weight/root dry weight ratios. We conclude that AMF is not necessary for nodulation and that this dependence improves species development because plant growth increases upon co-inoculation.

Production of native arbuscular mycorrhizal fungi inoculum under different environmental conditions

Abstract In order to obtain an arbuscular mycorrhizal fungi (AMF) native inoculum from Sierra de Moa and determine the most appropriate conditions for its big scale production, four light and temperature combinations were tested in three plant species (Calophyllum antillanum, Talipariti elatum and Paspalum notatum). Growth and development parameters, as well as the mycorrhizal functioning of the seedlings were evaluated. The natural light treatment under high temperatures (L-H) was the most suitable for the growth and development of the three plant species, showing the highest total biomass values, mainly of root, and a positive root-shoot ratio balance. This treatment also promoted higher values of root mycorrhizal colonization, external mycelium and AMF spore density. A total of 38 AMF species were identified among the plants and environmental conditions tested. Archaeospora sp.1, Glomus sp.5, Glomus brohultii and G. glomerulatum were observed in all the treatments. The L-H condition can be recommended for native inoculum production, as it promotes a better expression of the AM symbiosis and an elevated production of mycorrhizal propagules.

Dark septate endophyte decreases stress on rice plants.

Abiotic stress is one of the major limiting factors for plant development and productivity, which makes it important to identify microorganisms capable of increasing plant tolerance to stress. Dark septate endophytes can be symbionts of plants. In the present study, we evaluated the ability of dark septate endophytes isolates to reduce the effects of water stress in the rice varieties Nipponbare and Piauí. The experiments were performed under gnotobiotic conditions, and the water stress was induced with PEG. Four dark septate endophytes were isolated from the roots of wild rice (Oryza glumaepatula) collected from the Brazilian Amazon. Plant height as well as shoot and root fresh and dry matter were measured. Leaf protein concentrations and antioxidant enzyme activity were also estimated. The dark septate endophytes were grown in vitro in Petri dishes containing culture medium; they exhibited different levels of tolerance to salinity and water stress. The two rice varieties tested responded differently to inoculation with dark septate endophytes. Endophytes promoted rice plant growth both in the presence and in the absence of a water deficit. Decreased oxidative stress in plants in response to inoculation was observed in nearly all inoculated treatments, as indicated by the decrease in antioxidant enzyme activity. Dark septate endophytes fungi were shown to increase the tolerance of rice plants to stress caused by water deficiency.

Production of native arbuscular mycorrhizal fungi inoculum under different environmental conditions.

In order to obtain an arbuscular mycorrhizal fungi (AMF) native inoculum from Sierra de Moa and determine the most appropriate conditions for its big scale production, four light and temperature combinations were tested in three plant species (Calophyllum antillanum, Talipariti elatum and Paspalum notatum). Growth and development parameters, as well as the mycorrhizal functioning of the seedlings were evaluated. The natural light treatment under high temperatures (L-H) was the most suitable for the growth and development of the three plant species, showing the highest total biomass values, mainly of root, and a positive root-shoot ratio balance. This treatment also promoted higher values of root mycorrhizal colonization, external mycelium and AMF spore density. A total of 38 AMF species were identified among the plants and environmental conditions tested. Archaeospora sp.1, Glomus sp.5, Glomus brohultii and G. glomerulatum were observed in all the treatments. The L-H condition can be recommended for native inoculum production, as it promotes a better expression of the AM symbiosis and an elevated production of mycorrhizal propagules.

Nitrogen-fixing bacteria and arbuscular mycorrhizal fungi in Piptadenia gonoacantha (Mart.) Macbr.

The family Leguminosae comprises approximately 20,000 species that mostly form symbioses with arbuscular mycorrhizal fungi (AMF) and nitrogen-fixing bacteria (NFB). This study is aimed at investigating and confirming the dependence on nodulation and biological nitrogen fixation in the specie Piptadenia gonoacantha (Mart.) Macbr., which belongs to the Piptadenia group. Two consecutive experiments were performed in a greenhouse. The experiments were fully randomized with six replicates and a factorial scheme. For the treatments, the two AMF species and three NFB strains were combined to nodulate P. gonoacantha in addition to the control treatments. The results indicate this species' capacity for nodulation without the AMF; however, the AMF+NFB combinations yielded a considerable gain in P. gonoacantha shoot weight compared with the treatments that only included inoculating with bacteria or AMF. The results also confirm that the treatment effects among the AMF+NFB combinations produced different shoot dry weight/root dry weight ratios. We conclude that AMF is not necessary for nodulation and that this dependence improves species development because plant growth increases upon co-inoculation.

Involvement of Hormone- and ROS-Signaling Pathways in the Beneficial Action of Humic Substances on Plants Growing under Normal and Stressing Conditions.

The importance of soil humus in soil fertility has been well established many years ago. However, the knowledge about the whole mechanisms by which humic molecules in the rhizosphere improve plant growth remains partial and rather fragmentary. In this review we discuss the relationships between two main signaling pathway families that are affected by humic substances within the plant: one directly related to hormonal action and the other related to reactive oxygen species (ROS). In this sense, our aims are to try the integration of all these events in a more comprehensive model and underline some points in the model that remain unclear and deserve further research.

Assessment of the Use of Natural Materials for the Remediation of Cadmium Soil Contamination.

Rice plants accumulate cadmium (Cd2+) within the grain, increasing the danger of human exposure. Natural materials have been used in soil remediation, but few studies have examined the risks (based on the bioavailability of these metals to plants) of using these materials, so the practice remains controversial. In the present study, we evaluated the effectiveness of biochar produced from sugarcane bagasse, vermicompost (VC), vermicompost solid residue (VCR) and humin for remediation of Cd2+-contaminated soils. We characterized the interactions between these materials and Cd2+ and evaluated their capacity to alter Cd2+ availability to rice plants. Our results show that under the conditions in this study, biochar and humin were not effective for soil remediation. Although biochar had high Cd2+ retention, it was associated with high Cd2+ bioavailability and increased Cd2+ accumulation in rice plants. VC and VCR had high Cd2+ retention capacity as well as low Cd2+ availability to plants. These characteristics were especially notable for VCR, which was most effective for soil remediation. The results of our study demonstrate that in the tested materials, the bioavailability of Cd2+ to plants is related to their structural characteristics, which in turn determine their retention of Cd2+.

Root-Shoot Signaling crosstalk involved in the shoot growth promoting action of rhizospheric humic acids.

Numerous studies have shown the ability of humic substances to improve plant development. This action is normally reflected in an enhancement of crop yields and quality. However, the mechanisms responsible for this action of humic substances remain rather unknown. Our studies have shown that the shoot promoting action of sedimentary humic acids is dependent of its ability to increase root hydraulic conductivity through signaling pathways related to ABA, which in turn is affected in roots by humic acids in an IAA-NO dependent way. Furthermore, these studies also indicate that the primary action of humic acids in roots might also be physical, resulting from a transient mild stress caused by humic acids associated with a fouling-cleaning cycle of wall cell pores. Finally the role of alternative signal molecules, such as ROS, and corresponding signaling pathways are also discussed and modeled in the context of the above-mentioned framework.

Structure-Property-Function Relationship in Humic Substances to Explain the Biological Activity in Plants.

Knowledge of the structure-property-function relationship of humic substances (HSs) is key for understanding their role in soil. Despite progress, studies on this topic are still under discussion. We analyzed 37 humic fractions with respect to their isotopic composition, structural characteristics, and properties responsible for stimulating plant root parameters. We showed that regardless of the source of origin of the carbon (C3 or C4), soil-extracted HSs and humic acids (HAs) are structurally similar to each other. The more labile and functionalized HS fraction is responsible for root emission, whereas the more recalcitrant and less functionalized HA fraction is related to root growth. Labile structures promote root stimulation at lower concentrations, while recalcitrant structures require higher concentrations to promote a similar stimulus. These findings show that lability and recalcitrance, which are derived properties of humic fractions, are related to the type and intensity of their bioactivity. In summary, the comparison of humic fractions allowed a better understanding of the relationship between the source of origin of plant carbon and the structure, properties, and type and intensity of the bioactivity of HSs in plants. In this study, scientific concepts are unified and the basis for the agronomic use of HSs is established.

Vermicompost humic acids modulate the accumulation and metabolism of ROS in rice plants.

This work aims to determine the reactive oxygen species (ROS) accumulation, gene expression, anti-oxidant enzyme activity, and derived effects on membrane lipid peroxidation and certain stress markers (proline and malondialdehyde-MDA) in the roots of unstressed and PEG-stressed rice plants associated with vermicompost humic acid (VCHA) application. The results show that the application of VCHA to the roots of unstressed rice plants caused a slight but significant increase in root ROS accumulation and the gene expression and activity of the major anti-oxidant enzymes (superoxide dismutase and peroxidase). This action did not have negative effects on root development, and an increase in both root growth and root proliferation occurred. However, the root proline and MDA concentrations and the root permeability results indicate the development of a type of mild stress associated with VCHA application. When VCHA was applied to PEG-stressed plants, a clear alleviation of the inhibition in root development linked to PEG-mediated osmotic stress was observed. This was associated with a reduction in root ROS production and anti-oxidant enzymatic activity caused by osmotic stress. This alleviation of stress caused by VCHA was also reflected as a reduction in the PEG-mediated concentration of MDA in the root as well as root permeability. In summary, the beneficial action of VCHA on the root development of unstressed or PEG-stressed rice plants clearly involves the modulation of ROS accumulation in roots.

Extracting DNA of nematodes communities from Argentine Pampas agricultural soils.

We examined four strategies (Tris/EDTA, sodium dodecyl sulfate, Chelex 100 resin and cetyltrimethylammonium bromide -CTAB-) for extracting nucleic acid (DNA) from communities of nematodes. Nematodes were isolated from an agricultural area under different management of long-term crop rotation experiment from Argentina during three seasons. After DNA extraction, Polymerase Chain Reaction-amplifications were performed and considered as indicators of successful DNA extraction. The CTAB combined with proteinase K and phenol-chloroform-isoamyl alcohol was the unique successful method because positive amplifications were obtained by using both eukaryotic and nematode specific primers. This work could contribute to biodiversity studies of nematodes on agroecosystems.