Andrés Calderín García

Humic acids of vermicompost as an ecological pathway to increase resistance of rice seedlings to water stress

This paper discussed the potential role of humic acids (HA) in preventing oxidative stress in rice plants submitted to water stress. The rice seedlings (Oryza sativa L. cv. IACUB-30) was grown in nutrient solution and HA were extracted from vermicompost and analysed using Fourier-transform infrared (FTIR) spectroscopic and chemical methods. Changes in plant anatomy and morphology before and after stress initiation were employed as biomarkers of plant response. The growth rate over time, water content, dry-mass content, leaf carbohydrates, protein content and amino-acid content were among the parameters evaluated. Peroxidases (POX) activity, proline content, H2O2 content and membrane permeability were also studied. The results show that HA induced POX activity leading to a reduction in H2O2 content and greater conservation of membrane permeability. These results indicate that the HA play an important role as ecological and safety alternative to prevent oxidative stress in plant caused by drought stress. Our findings provide novel evidence for the protective action of HA against oxidative stress caused by water deficits.

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.

Property enhancement of soy protein isolate-based films by introducing POSS.

To enhance the mechanical and water-resistant properties of soy protein isolate (SPI) based films, hydrophobic TriSilanolPhenyl polyhedral oligomeric silsesquioxanes (POSS) was incorporated to modify the SPI films. POSS has three SiOH groups in a molecular, which is employed to cross-link SPI with the help of 3-glycidoxypropyltrimethoxysilane (GPTMS). POSS is a structure of eight phenol groups, playing a critical role in improving the physical and mechanical properties. The X-ray diffraction (XRD) and attenuated total reflectance (ATR) Fourier transform infrared spectroscopy (FT-IR) were used to characterize the films. The degree of reaction of SiOH groups in the POSS was estimated to be 53.0% according with the absorbance of ATR FT-IR spectra. Although the elongation at break was reduced by 52.6%, the tensile modulus, tensile strength and 10% offset yield strength were significantly increased by 86.6%, 34.0% and 56.8%, respectively, due to the cross-linking reactions among SPI, GPTMS and POSS. The results of water-resistant tests showed that the 24-hour water absorption was dramatically reduced by 54.7%.

Humic Substances and Plant Defense Metabolism

Humic substances (HS) affect most plant metabolic processes. Regardless of their source, HS help regulate enzymatic systems related to primary, secondary, and defense metabolisms in response to environmental stress. Morphologically, the HS–plant interaction results in increased root length and the emanation of lateral roots. These morphological changes occur in response to complex regulatory and stress response processes activated by the application of HS and similar chemical fractions. Given that the roots are the main plant organs that interact with HS, HS–root interaction mechanisms are one of the most important topics in HS–plant research. Specifically, there is a known biochemical relationship between humic compounds and major plant metabolic processes. New findings about the modes of metabolite action in plants have increased our understanding of how HS help to optimize plant metabolism. Advanced technologies, such as large-scale and spectroscopy, have also increased our understanding of the modes of action of HS. The application of techniques such as amplified fragment length polymorphism (AFLP) and microarray analysis in study of HS-treated plants has demonstrated that approximately 6.1–9 % of differentially expressed genes correspond to metabolic pathways that are associated with defense mechanisms in response to stimuli. These results suggest that HS induce plant adaptive responses to environmental stress. In this study, we discuss how HS contribute to improved plant performance through complex metabolic mechanisms. We apply new findings about the modes of action of metabolites related to antioxidant mechanisms to understand HS modes of action and examine HS effects in plants by using spectroscopic techniques to study root interactions. We also propose a framework for investigating the use of HS in agriculture to improve the growth of food plants grown in high-stress environments.

Hybrid boron nitride-natural fiber composites for enhanced thermal conductivity

Thermal conductivity was dramatically increased after adding natural fiber into hexagonal boron nitride (hBN)/epoxy composites. Although natural fiber does not show high-thermal conductivity itself, this study found that the synergy of natural fiber with hBN could significantly improve thermal conductivity, compared with that solely using hBN. A design of mixtures approach using constant fibers with increasing volume fractions of hBN was examined and compared. The thermal conductivity of the composite containing 43.6% hBN, 26.3% kenaf fiber and 30.1% epoxy reached 6.418 W m−1 K−1, which was 72.3% higher than that (3.600 W m−1 K−1) of the 69.0% hBN and 31.0% epoxy composite. Using the scanning electron microscope (SEM) and micro computed tomography (micro-CT), it was observed that the hBN powders were well distributed and ordered on the fiber surfaces enhancing the ceramic filler’s interconnection, which may be the reason for the increase in thermal conductivity. Additionally, the results from mechanical and dynamic mechanical tests showed that performances dramatically improved after adding kenaf fibers into the hBN/epoxy composite, potentially benefiting the composite’s use as an engineered material.

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.

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.

Effects of Humic Materials on Plant Metabolism and Agricultural Productivity

Humic materials, such as compost or vermicompost, have a high potential for increasing plant production, even in unfavorable environmental conditions. The use of these materials involves two types of action: direct (on plant metabolism) and indirect (increased soil fertility). The materials are composed of mineral elements, plant hormones, and amino acids that create beneficial conditions for crop yield and anti-stress adaptation. These products increase soil mineral content, reduce compaction, increase water retention and aggregate stability, and provide an adequate niche in the soil for microbial development. At the same time, humic materials stimulate root growth, which allows for a greater coverage of plant nutrition and greater activity of biotic and abiotic anti-stress enzyme systems. This stimulus translates into improved plant conditions and the optimization of metabolic processes, which leads to greater agricultural yield. The high potential of humic materials is based on their structural richness, which makes them an ecological and sustainable alternative for plant improvement. In this chapter, we will discuss improvement technologies.

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.

Humic acid differentially improves nitrate kinetics under low‐ and high‐affinity systems and alters the expression of plasma membrane H+‐ATPases and nitrate transporters in rice

Humic acids (HAs) have a major effect on nutrient uptake, metabolism, growth and development in plants. Here, we evaluated the effect of HA pretreatment applied with a nutrient solution on the uptake kinetics of nitrate nitrogen (N-NO3−) and the metabolism of nitrogen (N) in rice under conditions of high and low NO3− supply. In addition, the kinetic parameters of NO3− uptake, N metabolites, and nitrate transporters (NRTs) and the plasma membrane (PM) H+-ATPase gene expression were examined. The plants were grown in a growth chamber with modified Hoagland and Arnon solution until 21 days after germination (DAG), and they were then transferred to a solution without N for 48 h and then to another solution without N and with and without the addition of HAs for another 48 h. After this period of N deprivation, the plants received new nutrient solutions containing 0.2 and 2.0 mM N-NO3−. Treatment of rice plants with HA promoted the induction of the genes OsNRT2.1-2.2/OsNAR2.1 and some isoforms PM H+-ATPase in roots. The application of HAs differentially modified the parameters of the uptake kinetics of NO3− under both concentrations. When grown with 0.2 mM NO3−, the plants pretreated with HA had lower Km and Cmin values as well as a higher Vmax/Km ratio. When grown with 2 mM NO3−, the plants pretreated with HA had a higher Vmax value, a greater root and shoot mass, and a lower root/shoot ratio. The N fractions were also altered by pretreatment with HA, and a greater accumulation of NO3− and N-amino was observed in the roots and shoots, respectively, of plants pretreated with HA. The results suggest that pretreatment with HA modifies root morphology and gene expression of PM H+-ATPases and NO3− transporters, resulting in a greater efficiency of NO3− acquisition by high- and low-affinity systems.