Nitric oxide mediates humic acids-induced root development and plasma membrane H+-ATPase activation

Regulation mechanism of exogenous nitric oxide on phenanthrene uptake by ryegrass roots

Polycyclic aromatic hydrocarbons (PAHs) constitute a category of persistent organic contaminants that possess the potential to induce carcinogenic, teratogenic, and mutagenic consequences. Our previous findings have revealed that plant roots actively take up PAHs through co-transport with protons, and auxin can promote PAHs uptake by wheat roots. It remains unclear whether nitric oxide (NO), a signaling molecule involved in numerous physiological processes in plants and downstream of auxin, can affect PAHs uptake by plant roots. In our study, 50 μmol/L sodium nitroprusside (SNP) significantly enhanced phenanthrene uptake after 4 h of exposure. After the addition of SNP (50 μmol/L), the H+ flux on root surface increased, and H+-ATPase activity was activated, indicating that exogenous NO promotes phenanthrene uptake by plant roots via activating H+-ATPase. By studying the effects of 50 μmol/L cyclic guanosine monophosphate (cGMP), 5 mmol/L Ca2+, and 50 μmol/L adenosine monophosphate (AMP) on phenanthrene uptake by ryegrass roots and measuring root calcium-dependent protein kinases (CDPK) activity, we demonstrated that exogenous NO promotes phenanthrene uptake through the signaling pathway of NO, cGMP, Ca2+, CDPK, 14-3-3 protein and H+-ATPase. The results contribute significant insights into elucidating the underlying mechanisms of NO modulating PAHs absorption by plant roots, thereby offering crucial strategies for advancing food safety measures and enhancing the phytoremediation potential of soils and waters contaminated with PAHs.

Indole-3-acetic acid enhances the co-transport of proton and phenanthrene mediated by TaSAUR80-5A in wheat roots

Polycyclic aromatic hydrocarbons (PAHs) are a type of organic pollution that can accumulate in crops and hazard human health. This study used phenanthrene (PHE) as a model PAH and employed hydroponic experiments to illustrate the role of indole-3-acetic acid (IAA) in the regulation of PHE accumulation in wheat roots. At optimal concentrations, wheat roots treated with PHE + IAA showed a 46.9% increase in PHE concentration, whereas treatment with PHE + P-chlorophenoxyisobutyric acid resulted in a 38.77% reduction. Transcriptome analysis identified TaSAUR80-5A as the crucial gene for IAA-enhancing PHE uptake. IAA increases plasma membrane H+-ATPase activity, promoting active transport of PHE via the PHE/H+ cotransport mechanism. These results provide not only the theoretical basis necessary to better understand the function of IAA in PAHs uptake and transport by staple crops, but also a strategy for controlling PAHs accumulation in staple crops and enhancing phytoremediation of PAH-contaminated environments.

Investigating the carcinogenic and non-carcinogenic health hazards of heavy metal ions in Spinacia oleracea grown in agricultural soil treated with biochar and humic acid

This study addressed the bioaccumulation and human health risk among the consumption of Spinacia oleracea grown in agricultural soil treated with humic acid (189-2310 ppm) and biochars (0.00-5.10%.wt). The biochars came from two local feedstocks of rice-husk (RH) and sugar-beet-pulp (SBP) pyrolyzed at temperatures 300 and 600 °C. Total concentrations of Cu, Cd, and Ni found in both the soil and biomass/biochar exceeded global safety thresholds. The bioaccumulation levels of HMs in spinach leaves varied, with Fe reaching the highest concentration at 765.27 mg kg-1 and Cd having the lowest concentration at 3.31 mg kg-1. Overall, the concentrations of Zn, Cd, Pb, and Ni in spinach leaves exceeded the safety threshold limits, so that its consumption is not recommended. The assessment of hazard quotient (HI) for the HMs indicated potential health hazards for humans (HI > 1) from consuming the edible parts of spinach. The biochar application rates of 4.35%wt and 0.00%.wt resulted in the highest (3.69) and lowest (3.15) HI values, respectively. The cumulative carcinogenic risk (TCR) ranged from 0.0085 to 0.0119, exceeding the cancer risk threshold. Introducing 5.10%wt biomass/biochar resulted in a 36% rise in TCR compared to the control. The utilization of humic acid alongside HMs-polluted biochars results in elevated levels of HMs bioaccumulation exceeding the allowable thresholds in crops (with a maximum increase of 49% at 2000 ppm humic acid in comparison to 189 ppm). Consequently, this raised the HI by 46% and the TCR by 22%. This study demonstrated that the utilization of HMs-polluted biochars could potentially pose supplementary health hazards. Moreover, it is evident that the utilization of HMs-polluted biochars in treating metal-contaminated soil does not effectively stabilize or reduce pollution.

Humic acid improves wheat growth by modulating auxin and cytokinin biosynthesis pathways

Humic acids have been widely used for centuries to enhance plant growth and productivity. The beneficial effects of humic acids have been attributed to different functional groups and phytohormone-like compounds enclosed in macrostructure. However, the mechanisms underlying the plant growth-promoting effects of humic acids are only partially understood. We hypothesize that the bio-stimulatory effect of humic acids is mainly due to the modulation of innate pathways of auxin and cytokinin biosynthesis in treated plants. A physiological investigation along with molecular characterization was carried out to understand the mechanism of bio-stimulatory effects of humic acid. A gene expression analysis was performed for the genes involved in auxin and cytokinin biosynthesis pathways in wheat seedlings. Furthermore, Arabidopsis thaliana transgenic lines generated by fusing the auxin-responsive DR5 and cytokinin-responsive ARR5 promoter to ß-glucuronidase (GUS) reporter were used to study the GUS expression analysis in humic acid treated seedlings. This study demonstrates that humic acid treatment improved the shoot and root growth of wheat seedlings. The expression of several genes involved in auxin (Tryptophan Aminotransferase of Arabidopsis and Gretchen Hagen 3.2) and cytokinin (Lonely Guy3) biosynthesis pathways were up-regulated in humic acid-treated seedlings compared to the control. Furthermore, GUS expression analysis showed that bioactive compounds of humic acid stimulate endogenous auxin and cytokinin-like activities. This study is the first report in which using ARR5:GUS lines we demonstrate the biostimulants activity of humic acid.

Mitigation of Huanglongbing: Implications of a Biologically Enhanced Nutritional Program on Yield, Pathogen Localization, and Host Gene Expression Profiles

Huanglongbing (HLB, citrus greening disease), the most destructive disease affecting citrus production, is primarily linked to the gram-negative, insect-vectored, phloem-inhabiting α-proteobacterium 'Candidatus Liberibacter asiaticus' (CLas). With no effective treatment available, management strategies have largely focused on the use of insecticides in addition to the destruction of infected trees, which are environmentally hazardous and cost-prohibitive for growers, respectively. A major limitation to combating HLB is the inability to isolate CLas in axenic culture, which hinders in vitro studies and creates a need for robust in situ CLas detection and visualization methods. The aim of this study was to investigate the efficacy of a nutritional program-based approach for HLB treatment, and to explore the effectiveness of an enhanced immunodetection method to detect CLas-infected tissues. To achieve this, four different biologically enhanced nutritional programs (bENPs; P1, P2, P3, and P4) were tested on CLas-infected citrus trees. Structured illumination microscopy preceded by a modified immunolabeling process and transmission electron microscopy were used to show treatment-dependent reduction of CLas cells in phloem tissues. No sieve pore plugging was seen in the leaves of P2 trees. This was accompanied by an 80% annual increase in fruit number per tree and 1,503 (611 upregulated and 892 downregulated) differentially expressed genes. These included an MLRQ subunit gene, UDP-glucose transferase, and genes associated with the alpha-amino linolenic acid metabolism pathway in P2 trees. Taken together, the results highlight a major role for bENPs as a viable, sustainable, and cost effective option for HLB management.

Analysis of the molecular composition of humic substances and their effects on physiological metabolism in maize based on untargeted metabolomics

Introduction

Humic substances (HSs), components of plant biostimulants, are known to influence plant physiological processes, nutrient uptake and plant growth, thereby increasing crop yield. However, few studies have focused on the impact of HS on overall plant metabolism, and there is still debate over the connection between HS' structural characteristics and their stimulatory actions.

Methods

In this study, two different HSs (AHA, Aojia humic acid and SHA, Shandong humic acid) screened in a previous experiment were chosen for foliar spraying, and plant samples were collected on the tenth day after spraying (62 days after germination) to investigate the effects of different HSs on photosynthesis, dry matter accumulation, carbon and nitrogen metabolism and overall metabolism in maize leaf.

Results and discussion

The results showed different molecular compositions for AHA and SHA and a total of 510 small molecules with significant differences were screened using an ESI-OPLC-MS techno. AHA and SHA exerted different effects on maize growth, with the AHA inducing more effective stimulation than the SHA doing. Untargeted metabolomic analysis revealed that the phospholipid components of maize leaves treated by SHA generally increased significantly than that in the AHA and control treatments. Additionally, both HS-treated maize leaves exhibited different levels of accumulation of trans-zeatin, but SHA treatment significantly decreased the accumulation of zeatin riboside. Compared to CK treatment, AHA treatment resulted in the reorganization of four metabolic pathways: starch and sucrose metabolism, TCA cycle, stilbenes, diarylheptanes, and curcumin biosynthesis, and ABC transport, SHA treatment modified starch and sucrose metabolism and unsaturated fatty acid biosynthesis. These results demonstrate that HSs exert their function through a multifaceted mechanism of action, partially connected to their hormone-like activity but also involving hormoneindependent signaling pathways.

PHB3 regulates lateral root primordia formation via NO-mediated degradation of AUXIN/INDOLE-3-ACETIC ACID proteins

We have previously shown that Arabidopsis thaliana Prohibitin 3 (PHB3) controls auxin-stimulated lateral root (LR) formation; however, the underlying molecular mechanism is unknown. Here, we demonstrate that PHB3 regulates lateral root (LR) development mainly through influencing lateral root primordia (LRP) initiation, via affecting nitric oxide (NO) accumulation. The reduced LRP in phb3 mutant was largely rescued by treatment with a NO donor. The decreased NO accumulation in phb3 caused a lower expression of GATA TRANSCRIPTION FACTOR 23 (GATA23) and LATERAL ORGAN BOUNDARIES DOMAIN 16 (LBD16) through inhibiting the degradation of INDOLE-3-ACETIC ACID INDUCIBLE 14/28 (IAA14/28). Overexpression of either GATA23 or LBD16 in phb3 mutant background recovered the reduced density of LRP. These results indicate that PHB3 regulates LRP initiation via NO-mediated auxin signalling, by modulating the degradation of IAA14/28.

Humic Substances as Microalgal Biostimulants—Implications for Microalgal Biotechnology

Humic substances (HS) act as biostimulants for terrestrial photosynthetic organisms. Their effects on plants are related to specific HS features: pH and redox buffering activities, (pseudo)emulsifying and surfactant characteristics, capacity to bind metallic ions and to encapsulate labile hydrophobic molecules, ability to adsorb to the wall structures of cells. The specific properties of HS result from the complexity of their supramolecular structure. This structure is more dynamic in aqueous solutions/suspensions than in soil, which enhances the specific characteristics of HS. Therefore, HS effects on microalgae are more pronounced than on terrestrial plants. The reported HS effects on microalgae include increased ionic nutrient availability, improved protection against abiotic stress, including against various chemical pollutants and ionic species of potentially toxic elements, higher accumulation of value-added ingredients, and enhanced bio-flocculation. These HS effects are similar to those on terrestrial plants and could be considered microalgal biostimulant effects. Such biostimulant effects are underutilized in current microalgal biotechnology. This review presents knowledge related to interactions between microalgae and humic substances and analyzes the potential of HS to enhance the productivity and profitability of microalgal biotechnology.

The Central Role of Soil Organic Matter in Soil Fertility and Carbon Storage

The aim of the paper is to give an overview on the chemistry of soil organic carbon (SOC) affecting nutrient availability, the emission of greenhouse gases and detoxifying harmful substances in soil. Humic substances represent the stable part of SOC, accounting for between 50 and more than 80% of organically bound carbon in soil. Humic substances strongly affect the soil solution concentration of several plant nutrients and may increase P-, Fe-, and Cu- solubility, thereby increasing their plant availability. Soil organic carbon, mainly humic substances, can detoxify monomeric Al in acid soils, can strongly bind toxic heavy metals, making them unavailable to the plant roots, and may strongly bind a vast variety of harmful organic pollutants. Increasing SOC is an important goal in agriculture. The inclusion of mixtures of semi-perennial plant species and cultivars may strongly increase SOC and humic substance content in soils. To increase SOC, farmyard manure and its rotted or composted forms are superior compared to the separate application of straw and slurry to soil. The storage of carbon, mainly in organic form, in soils is very important in the context of the emission of greenhouse gases. Worldwide, soils release about 10 times more greenhouse gases compared to fossil fuel combustion. Small increments in SOC worldwide will strongly affect the concentration of atmospheric CO2. The public discussion on soil fertility and greenhouse gas emissionshas been politically controlled in a way that leaves the important and positive contribution of soil organic carbon and mainly humic substances partly misinterpreted and partly underestimated.

Alkaline extract from vermicompost reduced the stress promoted by As on maize plants and increase their phytoextraction capacity

Arsenic (As) represents an environmental risk and phytoremediation has been identified as a good technique to recover contaminated soils. Plants defense mechanisms needed to be enhanced against As stress-promoting action by biostimulants such as humic materials. This work sought to determine the effectiveness of an alkaline vermicompost extract (AEV) and in mitigating stresses promoted by As in maize plants, increasing their potential use for phytoextraction. The AEV were extracted from vermicompost and two preliminary assays in Leonard pots were carried out: the first one to define the best AEV concentration-response dose and the second to point out the toxic As concentration. The second step was to set up a 28-day long experiment with the following four treatments: control, AEV, As, As + AEV. AEV attenuated As-induced stress in maize plants. Maize dry biomass was reduced in the As treatment and rebalanced to values similar to the control in the As + HS treatment while the plants treated only with HS showed the highest biomass among the treatments. The concentrations of P, Fe, Cu, Mn and Ni, and catalase (CAT), ascorbate peroxidase (APX) and superoxide dismutase (SOD) antioxidant activity increased in the As treatment and decreased in the As + AEV treatment. The rate of photosynthesis decreased, and the internal CO₂ concentration increased with stress induced by As, where both effects were attenuated by AEV. Our results show the positive effect of the AEV in alleviating As abiotic stress on maize growth, offering new options of employment of humic substances in phytoremediation process.

Humic Acid Alleviates Fe Chlorosis in Graminaceous Plants Through Coordinated Fe-Dependent and Fe-Independent Mechanisms

Many studies have shown the close relationship between the beneficial action of soil and sedimentary humic acids on the growth of plants cultivated in calcareous soils and their ability to improve Fe plant nutrition. These results have been ascribed to the humic acid (HA) capability to improve Fe solubility and bioavailability. However, other effects more related to a humic acid action on the specific mechanisms activated in roots of plants under Fe deficiency cannot be ruled out. Although this question has been studied in dicotyledonous plants, in graminaceous plants there are no specific studies. Here we investigate the ability of a humic acid extracted from peat (HA) to improve Fe nutrition in wheat plants cultivated under Fe deficient and sufficient conditions. The results show that HA can improve the physiological status of Fe deficient wheat plants by alleviating some of the deleterious consequences of Fe deficiency on plant development and increasing the plant ability to secrete phytosiderophores to the nutrient solution. This action of HA is associated with increases in the Fe-active pool in leaves that might be related to the mobilization of the Fe complexed by HA resulting from the interaction of HA with the phytosiderophores in the nutrient solution. The Fe translocation from the root to the shoot may be favored by the action of trans-Zeatin Riboside (tZR) since the leaf concentration of this phytohormone was enhanced by HA in Fe deficient plants.

Use of Ore-Derived Humic Acids With Diverse Chemistries to Elucidate Structure-Activity Relationships (SAR) of Humic Acids in Plant Phenotypic Expression

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Nitric oxide and hydrogen sulfide: an indispensable combination for plant functioning

Nitric oxide (NO) and hydrogen sulfide (H₂S) are gasotransmitters, which are involved in almost all plant physiological and stress-related processes. With its antioxidant regulatory properties, NO on its own ameliorates plant stress, while H₂S, a foul-smelling gas, has differential effects. Recent studies have shown that these signaling molecules are involved in intertwined pathway networks. This is due to the contrasting effects of NO and H₂S depending on cell type, subcellular compartment, and redox status, as well as the flux and dosage of NO and H₂S in different plant species and cellular contexts. Here, we provide a comprehensive review of the complex networks of these molecules, with particular emphasis on root development, stomatal movement, and plant cell death.

Genetic variation in the promoter of an R2R3-MYB transcription factor determines fruit malate content in apple (Malus domestica Borkh.)

Deciphering the mechanism of malate accumulation in apple (Malus domestica Borkh.) fruits can help to improve their flavor quality and enhance their benefits for human health. Here, we analyzed malate content as a quantitative trait that is determined mainly by genetic effects. In a previous study, we identified an R2R3-MYB transcription factor named MdMYB44 that was a candidate gene in qtl08.1 (quantitative trait locus mapped to chromosome 8) of fruit malate content. In the present study, we established that MdMYB44 negatively regulates fruit malate accumulation by repressing the promoter activity of the malate-associated genes Ma1 (Al-Activated Malate Transporter 9), Ma10 (P-type ATPase 10), MdVHA-A3 (V-type ATPase A3), and MdVHA-D2 (V-type ATPase D2). Two single-nucleotide polymorphisms (SNPs) in the MdMYB44 promoter, SNP A/G and SNP T/-, were experimentally shown to associate with fruit malate content. The TATA-box in the MdMYB44 promoter in the presence of SNP A enhances the basal activity of the MdMYB44 promoter. The binding of a basic-helix-loop-helix transcription factor MdbHLH49 to the MdMYB44 promoter was enhanced by the presence of SNP T, leading to increased MdMYB44 transcript levels and reduced malate accumulation. Furthermore, MdbHLH49 interacts with MdMYB44 and enhances MdMYB44 activity. The two SNPs could be used in combination to select for sour or non-sour apples, providing a valuable tool for the selection of fruit acidity by the apple breeding industry. This research is important for understanding the complex molecular mechanisms of fruit malate accumulation and accelerating the development of germplasm innovation in apple species and cultivars.

Bioactivity of Humic Acids Extracted From Shale Ore: Molecular Characterization and Structure-Activity Relationship With Tomato Plant Yield Under Nutritional Stress

The increasing demands for biostimulants in the agricultural market over the last years have posed the problem of regulating this product category by requiring the industry to make available the information about efficacy and safety, including the explanation of mode of action and the definition of bioactive constituents. In the present study, we tested the biostimulant proprieties of a sedimentary shale ore-extracted humic acid (HA) on Micro Tom tomato plants under increasing nutritional stress and investigated the correlation with the chemical features of HA by means of ultra-high resolution FT-ICR MS, FT-ATR, and ¹³C-NMR. Humic acid application proved effective in alleviating the nutritional stress by improving nutrient use efficiency, with results comparable to the control treatment supplied with higher NPK nutrition. Increased yield (up to +19%) and fruit quality (in the range +10-24%), higher ascorbic acid content and a better root growth were the main parameters affected by HA application. Molecular-level characterization identified the possible chemical drivers of bioactivity, and included flavonoids, quinones, and alkaloids among the most represented molecules, some of which exhibiting antioxidant, pro-oxidant, and antimicrobial activity. The redox effect was discussed as a determinant of the delicate homeostasis balance, capable of triggering plant defense response and eventually inducing a protective priming effect on the plants.

Ethylene and Nitric Oxide Involvement in the Regulation of Fe and P Deficiency Responses in Dicotyledonous Plants

Iron (Fe) and phosphorus (P) are two essential elements for plant growth. Both elements are abundant in soils but with poor availability for plants, which favor their acquisition by developing morphological and physiological responses in their roots. Although the regulation of the genes related to these responses is not totally known, ethylene (ET) and nitric oxide (NO) have been involved in the activation of both Fe-related and P-related genes. The common involvement of ET and NO suggests that they must act in conjunction with other specific signals, more closely related to each deficiency. Among the specific signals involved in the regulation of Fe- or P-related genes have been proposed Fe-peptides (or Fe ion itself) and microRNAs, like miR399 (P), moving through the phloem. These Fe- or P-related phloem signals could interact with ET/NO and confer specificity to the responses to each deficiency, avoiding the induction of the specific responses when ET/NO increase due to other nutrient deficiencies or stresses. Besides the specificity conferred by these signals, ET itself could confer specificity to the responses to Fe- or P-deficiency by acting through different signaling pathways in each case. Given the above considerations, there are preliminary results suggesting that ET could regulate different nutrient responses by acting both in conjunction with other signals and through different signaling pathways. Because of the close relationship among these two elements, a better knowledge of the physiological and molecular basis of their interaction is necessary to improve their nutrition and to avoid the problems associated with their misuse. As examples of this interaction, it is known that Fe chlorosis can be induced, under certain circumstances, by a P over- fertilization. On the other hand, Fe oxides can have a role in the immobilization of P in soils. Qualitative and quantitative assessment of the dynamic of known Fe- and P-related genes expression, selected ad hoc and involved in each of these deficiencies, would allow us to get a profound knowledge of the processes that regulate the responses to both deficiencies. The better knowledge of the regulation by ET of the responses to these deficiencies is necessary to properly understand the interactions between Fe and P. This will allow the obtention of more efficient varieties in the absorption of P and Fe, and the use of more rational management techniques for P and Fe fertilization. This will contribute to minimize the environmental impacts caused by the use of P and Fe fertilizers (Fe chelates) in agriculture and to adjust the costs for farmers, due to the high prices and/or scarcity of Fe and P fertilizers. This review aims to summarize the latest advances in the knowledge about Fe and P deficiency responses, analyzing the similarities and differences among them and considering the interactions among their main regulators, including some hormones (ethylene) and signaling substances (NO and GSNO) as well as other P- and Fe-related signals.

Chemical Structure and Biological Activity of Humic Substances Define Their Role as Plant Growth Promoters

Humic substances (HS) are dominant components of soil organic matter and are recognized as natural, effective growth promoters to be used in sustainable agriculture. In recent years, many efforts have been made to get insights on the relationship between HS chemical structure and their biological activity in plants using combinatory approaches. Relevant results highlight the existence of key functional groups in HS that might trigger positive local and systemic physiological responses via a complex network of hormone-like signaling pathways. The biological activity of HS finely relies on their dosage, origin, molecular size, degree of hydrophobicity and aromaticity, and spatial distribution of hydrophilic and hydrophobic domains. The molecular size of HS also impacts their mode of action in plants, as low molecular size HS can enter the root cells and directly elicit intracellular signals, while high molecular size HS bind to external cell receptors to induce molecular responses. Main targets of HS in plants are nutrient transporters, plasma membrane H⁺-ATPases, hormone routes, genes/enzymes involved in nitrogen assimilation, cell division, and development. This review aims to give a detailed survey of the mechanisms associated to the growth regulatory functions of HS in view of their use in sustainable technologies.

A conserved, buried cysteine near the P-site is accessible to cysteine modifications and increases ROS stability in the P-type plasma membrane H+-ATPase

Sulfur-containing amino acid residues function in antioxidative responses, which can be induced by the reactive oxygen species generated by excessive copper and hydrogen peroxide. In all Na+/K+, Ca2+, and H+ pumping P-type ATPases, a cysteine residue is present two residues upstream of the essential aspartate residue, which is obligatorily phosphorylated in each catalytic cycle. Despite its conservation, the function of this cysteine residue was hitherto unknown. In this study, we analyzed the function of the corresponding cysteine residue (Cys-327) in the autoinhibited plasma membrane H+-ATPase isoform 2 (AHA2) from Arabidopsis thaliana by mutagenesis and heterologous expression in a yeast host. Enzyme kinetics of alanine, serine, and leucine substitutions were identical with those of the wild-type pump but the sensitivity of the mutant pumps was increased towards copper and hydrogen peroxide. Peptide identification and sequencing by mass spectrometry demonstrated that Cys-327 was prone to oxidation. These data suggest that Cys-327 functions as a protective residue in the plasma membrane H+-ATPase, and possibly in other P-type ATPases as well.

Which Traits of Humic Substances Are Investigated to Improve Their Agronomical Value?

Humic substances (HSs) are chromogenic organic assemblies that are widespread in the environment, including soils, oceans, rivers, and coal-related resources. HSs are known to directly and indirectly stimulate plants based on their versatile organic structures. Their beneficial activities have led to the rapid market growth of agronomical HSs. However, there are still several technical issues and concerns to be addressed to advance sustainable agronomical practices for HSs and allow growers to use HSs reliably. First, it is necessary to elucidate the evident structure (component)–function relationship of HSs. Specifically, the core structural features of HSs corresponding to crop species, treatment method (i.e., soil, foliar, or immersion applications), and soil type-dependent plant stimulatory actions as well as specific plant responses (e.g., root genesis and stress resistance) should be detailed to identify practical crop treatment methodologies. These trials must then be accompanied by means to upgrade crop marketability to help the growers. Second, structural differences of HSs depending on extraction sources should be compared to develop quality control and assurance measures for agronomical uses of HSs. In particular, coal-related HSs obtainable in bulk amounts for large farmland applications should be structurally and functionally distinguishable from other natural HSs. The diversity of organic structures and components in coal-based HSs must thus be examined thoroughly to provide practical information to growers. Overall, there is a consensus amongst researchers that HSs have the potential to enhance soil quality and crop productivity, but appropriate research directions should be explored for growers’ needs and farmland applications.

Coated Diammonium Phosphate Combined With Humic Acid Improves Soil Phosphorus Availability and Photosynthesis and the Yield of Maize

Controlled release phosphorus (P) fertilizers and humic acid (HA) applications are two effective and significant techniques or measures for preventing P loss and enhancing maize development. However, the underlying physiological mechanism of how the controlled release P fertilizers combined with HA affect the maize production and P-use efficiency (PUE) remains unknown. The effects of applying coated diammonium phosphate (CDAP) and HA together on soil nutrient supply intensity, soil phosphatase activity, photosynthesis, endogenous hormone contents, and yield of maize, as well as PUE, were examined in this study. In a pot experiment, two types of P fertilizers-CDAP and diammonium phosphate (DAP)- as well as two HA application rates (0 and 45 kg ha^-1) and two P levels (60 and 75 kg P₂O₅ ha^-1) were utilized. Results showed that the key elements that influence the growth and yield of the maize were the availability of P content in soil, plant photosynthesis, and hormone levels. The combination of CDAP and HA had a greater impact on yield and PUE over the course of 2 years than either DAP alone or DAP combined with HA. Besides, using CDAP in combination with HA increased the yield and PUE by 4.2 and 8.4%, respectively, as compared to the application of CDAP alone at 75 kg P₂O₅ ha^-1. From the twelve-leaf to milk stages, the available P content in the soil was increased by an average of 38.6% with the combination of CDAP and HA compared to the application of CDAP alone at 75 kg P₂O₅ ha^-1. In addition, the application of CDAP combined with HA boosted the activities of ATP synthase, as well as the content of cytokinin (CTK), and hence improved the maize photosynthetic rate (Pn). When compared to the application of CDAP alone or DAP combined with HA, the Pn of CDAP + HA treatments was enhanced by 17.9-35.1% at the same P rate. In conclusion, as an environmentally friendly fertilizer, the combined application of CDAP and HA improved the intensity of the soil nutrient supply, regulated photosynthetic capabilities, and increased the yield and PUE, which is important for agricultural production, P resource conservation, and environmental protection.

Identification of nitric oxide (NO)-responsive genes under hypoxia in tomato (Solanum lycopersicum L.) root

Flooding periods, as one probable consequence of climate change, will lead more frequently to plant hypoxic stress. Hypoxia sensing and signaling in the root, as the first organ encountering low oxygen, is therefore crucial for plant survival under flooding. Nitric oxide has been shown to be one of the main players involved in hypoxia signaling through the regulation of ERFVII transcription factors stability. Using SNP as NO donor, we investigated the NO-responsive genes, which showed a significant response to hypoxia. We identified 395 genes being differentially regulated under both hypoxia and SNP-treatment. Among them, 251 genes showed up- or down-regulation under both conditions which were used for further biological analysis. Functional classification of these genes showed that they belong to different biological categories such as primary carbon and nitrogen metabolism (e.g. glycolysis, fermentation, protein and amino acid metabolism), nutrient and metabolites transport, redox homeostasis, hormone metabolism, regulation of transcription as well as response to biotic and abiotic stresses. Our data shed light on the NO-mediated gene expression modulation under hypoxia and provides potential targets playing a role in hypoxia tolerance. These genes are interesting candidates for further investigating their role in hypoxia signaling and survival.

Crosstalk among hydrogen sulfide (H2S), nitric oxide (NO) and carbon monoxide (CO) in root-system development and its rhizosphere interactions: A gaseous interactome

Root development in higher plants is achieved by a precise intercellular communication which determines cell fate in the primary embryonic meristem where the gasotransmitters H₂S, NO and CO participate dynamically. Furthermore, the rhizosphere interaction of these molecules with microbial and soil metabolism also affects root development. NO regulates root growth and architecture in association with several other biomolecules like auxin indole-3-acetic acid (IAA), ethylene, jasmonic acid (JA), strigolactones, alkamides and melatonin. The CO-mediated signal transduction pathway in roots is closely linked to the NO-mediated signal cascades. Interestingly, H₂S acts also as an upstream component in IAA and NO-mediated crosstalk during root development. Heme oxygenase (HO) 1 generates CO and functions as a downstream component in H₂S-mediated adventitious rooting and H₂S-CO crosstalk. Likewise, reactive oxygen species (ROS), H₂S and NO crosstalk are important components in the regulation of root architecture. Deciphering these interactions will be a potential biotechnological tool which could provide benefits in crop management in soils, especially under adverse environmental conditions. This review aims to provide a comprehensive update of the complex networks of these gasotransmitters during the development of roots.

Bioactivity of Size-Fractionated and Unfractionated Humic Substances From Two Forest Soils and Comparative Effects on N and S Metabolism, Nutrition, and Root Anatomy of Allium sativum L

Humic substances (HS) are powerful natural plant biostimulants. However, there is still a lack of knowledge about the relationship between their structure and bioactivity in plants. We extracted HS (THE1-2) from two forest soils covered with Pinus mugo (1) or Pinus sylvestris (2). The extracts were subjected to weak acid treatment to produce size-fractionated HS (high molecular size, HMS1-2; low molecular size, LMS1-2). HS were characterized for total acidity, functional groups, element and auxin (IAA) contents, and hormone-like activity. HS concentrations ranging from 0 to 5 mg C L-1 were applied to garlic (Allium sativum L.) plantlets in hydroponics to ascertain differences between unfractionated and size-fractionated HS in the capacity to promote mineral nutrition, root growth and cell differentiation, activity of enzymes related to plant development (invertase, peroxidase, and esterase), and N (nitrate reductase, glutamine synthetase) and S (O-acetylserine sulphydrylase) assimilation into amino acids. A positive linear dose-response relationship was determined for all HS in the range 0-1 mg C L-1, while higher HS doses were less effective or ineffective in promoting physiological-biochemical attributes of garlic. Bioactivity was higher for size-fractionated HS according to the trend LMS1-2>HMS1-2>THE1-2, with LMS2 and HMS2 being overall more bioactive than LMS1 and HMS1, respectively. LMS1-2 contained more N, oxygenated functional groups and IAA compared to THE1-2 and HMS1-2. Also, they exhibited higher hormone-like activities. Such chemical properties likely accounted for the greater biostimulant action of LMS1-2. Beside plant growth, nutrition and N metabolism, HS stimulated S assimilation by promoting the enrichment of garlic plantlets with the S amino acid alliin, which has recognized beneficial properties in human health. Concluding, this study endorses that i) treating THE with a weak acid produced sized-fractionated HS with higher bioactivity and differing in properties, perhaps because of novel molecular arrangements of HS components that better interacted with garlic roots; ii) LMS from forest soils covered with P. mugo or P. sylvestris were the most bioactive; iii) the cover vegetation affected HS bioactivity iv); HS stimulated N and S metabolism with relevant benefits to crop nutritional quality.

Interaction between Humic Substances and Plant Hormones for Phosphorous Acquisition

Phosphorus (P) deficiency is a major constraint in highly weathered tropical soils. Although phosphorous rock reserves may last for several hundred years, there exists an urgent need to research efficient P management for sustainable agriculture. Plant hormones play an important role in regulating plant growth, development, and reproduction. Humic substances (HS) are not only considered an essential component of soil organic carbon (SOC), but also well known as a biostimulant which can perform phytohormone-like activities to induce nutrient uptake. This review paper presents an overview of the scientific outputs in the relationship between HS and plant hormones. Special attention will be paid to the interaction between HS and plant hormones for nutrient uptake under P-deficient conditions.

Discriminating the Short-Term Action of Root and Foliar Application of Humic Acids on Plant Growth: Emerging Role of Jasmonic Acid

Humic substances (HS, fulvic and humic acids) are widely used as fertilizers or plant growth stimulants, although their mechanism of action still remains partially unknown. Humic substances may be applied either directly to the soil or as foliar sprays. Despite both kind of application are commonly used in agricultural practices, most of the studies regarding the elicited response in plants induced by HS are based on the root-application of these substances. The present work aimed at discriminating between the mechanisms of action of foliar application versus root application of a sedimentary humic acid (SHA) on plant development. For this purpose, six markers related to plant phenotype, plant morphology, hormonal balance and root-plasma membrane H⁺-ATPase were selected. Both application strategies improved the shoot and root growth. Foliar applied- and root applied-SHA shared the capacity to increase the concentration of indole-3-acetic acid in roots and cytokinins in shoots. However, foliar application did not lead to short-term increases in either abscisic acid root-concentration or root-plasma membrane H⁺-ATPase activity which are, however, two crucial effects triggered by SHA root-application. Both application modes increased the root concentrations of jasmonic acid and jasmonoyl-isoleucine. These hormonal changes caused by foliar application could be a stress-related symptom and connected to the loss of leaves trichomes and the diminution of chloroplasts size seen by scanning electron microscopy. These results support the hypothesis that the beneficial effects of SHA applied to roots or leaves may result from plant adaptation to a mild transient stress caused by SHA application.

An Update on Nitric Oxide Production and Role Under Phosphorus Scarcity in Plants

Phosphate (P) is characterized by its low availability and restricted mobility in soils, and also by a high redistribution capacity inside plants. In order to maintain P homeostasis in nutrient restricted conditions, plants have developed mechanisms which enable P acquisition from the soil solution, and an efficient reutilization of P already present in plant cells. Nitric oxide (NO) is a bioactive molecule with a plethora of functions in plants. Its endogenous synthesis depends on internal and environmental factors, and is closely tied with nitrogen (N) metabolism. Furthermore, there is evidence demonstrating that N supply affects P homeostasis and that P deficiency impacts on N assimilation. This review will provide an overview on how NO levels in planta are affected by P deficiency, the interrelationship with N metabolism, and a summary of the current understanding about the influence of this reactive N species over the processes triggered by P starvation, which could modify P use efficiency.

Molecular Composition of Size-Fractionated Fulvic Acid-Like Substances Extracted from Spent Cooking Liquor and Its Relationship with Biological Activity

The treatment of spent cooking liquor is critical for clean production of pulp and paper industry. There is a compelling need to develop a cost-effective and green technology for reuse of organic matter in spent cooking liquor to mitigate the negative impacts on the environment. The objective of this study is to examine the chemical structure of fulvic acid-like substances extracted from spent cooking liquor (PFA) and their relationship with bioactivity in plant growth. Compared with the benchmark Pahokee peat fulvic acid (PPFA), PFA has less aromatic structure, but higher content of lignin, carbohydrates, and amino acid. After fractionation, protein/amino proportion decreased with increasing molecular weight, but the aromaticity increased. Under salt stress, rice seedling growth was promoted by PFA with low molecular weight (<5 kDa), but inhibited by fraction with high molecular weight (>10 kDa). Principal component analysis suggested that promoted growth was more related with chemical structure (O- and N-alkyl moieties) than with molecular weight. This study provided the theoretical basis for development of an innovative green technology of sustainable reuse of spent cooking liquor in agriculture.

Root ABA and H+-ATPase are key players in the root and shoot growth-promoting action of humic acids

Although the ability of humic (HA) and fulvic acids (FA) to improve plant growth has been demonstrated, knowledge about the mechanisms responsible for the direct effects of HA and FA on the promotion of plant growth is scarce and fragmentary. Our study investigated the causal role of both root PM H+-ATPase activity and ABA in the SHA-promoting action on both root and shoot growth. The involvement of these processes in the regulation of shoot cytokinin concentration and activity was also studied. Our aim was to integrate such plant responses for providing new insights  to the current model on the mode of action of HA for promoting root and shoot growth. Experiments employing specific inhibitors and using Cucumis sativus L. plants show that both the root PM H+-ATPase activity and root ABA play a crucial role in the root growth-promoting action of SHA. With regard to the HA-promoting effects on shoot growth, two pathways of events triggered by the interaction of SHA with plant roots are essential for the increase in root PM H+-ATPase activity-which also mediates an increase in cytokinin concentration and action in the shoot-and the ABA-mediated increase in hydraulic conductivity (Lpr).

Novel and conserved functions of S-nitrosoglutathione reductase in tomato

Nitric oxide (NO) is emerging as a key signalling molecule in plants. The chief mechanism for the transfer of NO bioactivity is thought to be S-nitrosylation, the addition of an NO moiety to a protein cysteine thiol to form an S-nitrosothiol (SNO). The enzyme S-nitrosoglutathione reductase (GSNOR) indirectly controls the total levels of cellular S-nitrosylation, by depleting S-nitrosoglutathione (GSNO), the major cellular NO donor. Here we show that depletion of GSNOR function impacts tomato (Solanum lycopersicum. L) fruit development. Thus, reduction of GSNOR expression through RNAi modulated both fruit formation and yield, establishing a novel function for GSNOR. Further, depletion of S. lycopersicum GSNOR (SlGSNOR) additionally impacted a number of other developmental processes, including seed development, which also has not been previously linked with GSNOR activity. In contrast to Arabidopsis, depletion of GSNOR function did not influence root development. Further, reduction of GSNOR transcript abundance compromised plant immunity. Surprisingly, this was in contrast to previous data in Arabidopsis that reported that reducing Arabidopsis thaliana GSNOR (AtGSNOR) expression by antisense technology increased disease resistance. We also show that increased SlGSNOR expression enhanced pathogen protection, uncovering a potential strategy to enhance disease resistance in crop plants. Collectively, our findings reveal, at the genetic level, that some but not all GSNOR activities are conserved outside the Arabidopsis reference system. Thus, manipulating the extent of GSNOR expression may control important agricultural traits in tomato and possibly other crop plants.

Nitric oxide and plant mineral nutrition: current knowledge

Plants under conditions of essential mineral deficiency trigger signaling mechanisms that involve common components. Among these components, nitric oxide (NO) has been identified as a key participant in responses to changes in nutrient availability. Usually, nutrient imbalances affect the levels of NO in specific plant tissues, via modification of its rate of synthesis or degradation. Changes in the level of NO affect plant morphology and/or trigger responses associated with nutrient homeostasis, mediated by its interaction with reactive oxygen species, phytohormones, and through post-translational modification of proteins. NO-related events constitute an exciting field of research to understand how plants adapt and respond to conditions of nutrient shortage. This review summarizes the current knowledge on NO as a component of the multiple processes related to plant performance under conditions of deficiency in mineral nutrients, focusing on macronutrients such as nitrogen, phosphate, potassium, and magnesium, as well as micronutrients such as iron and zinc.

Tomato Root Growth Inhibition by Salinity and Cadmium Is Mediated By S-Nitrosative Modifications of ROS Metabolic Enzymes Controlled by S-Nitrosoglutathione Reductase

S-nitrosoglutathione reductase (GSNOR) exerts crucial roles in the homeostasis of nitric oxide (NO) and reactive nitrogen species (RNS) in plant cells through indirect control of S-nitrosation, an important protein post-translational modification in signaling pathways of NO. Using cultivated and wild tomato species, we studied GSNOR function in interactions of key enzymes of reactive oxygen species (ROS) metabolism with RNS mediated by protein S-nitrosation during tomato root growth and responses to salinity and cadmium. Application of a GSNOR inhibitor N6022 increased both NO and S-nitrosothiol levels and stimulated root growth in both genotypes. Moreover, N6022 treatment, as well as S-nitrosoglutathione (GSNO) application, caused intensive S-nitrosation of important enzymes of ROS metabolism, NADPH oxidase (NADPHox) and ascorbate peroxidase (APX). Under abiotic stress, activities of APX and NADPHox were modulated by S-nitrosation. Increased production of H2O2 and subsequent oxidative stress were observed in wild Solanumhabrochaites, together with increased GSNOR activity and reduced S-nitrosothiols. An opposite effect occurred in cultivated S. lycopersicum, where reduced GSNOR activity and intensive S-nitrosation resulted in reduced ROS levels by abiotic stress. These data suggest stress-triggered disruption of ROS homeostasis, mediated by modulation of RNS and S-nitrosation of NADPHox and APX, underlies tomato root growth inhibition by salinity and cadmium stress.

Nitrosative stress-mediated inhibition of OsDHODH1 gene expression suggests roots growth reduction in rice (Oryza sativa L.)

This study monitored the transcriptional response of OsDHODH1 under nitrosative stress conditions relative to the transcripts accumulations for the core mitochondrial cytochrome c oxidase1 (CcOX1) subunit, nuclear CcOX subunits 5b and 5c, two rice nitrate reductases (OsNIA1 and OsNIA2), and nitric oxide excess 1 (OsNOE1) genes. Our findings reveal that short-term exposure of rice seedlings to 1 mM SNP (Nitric oxide donor) applied exogenously for 1 h resulted in significant down-regulation of OsDHODH1 expression in all rice cultivars. In addition, the transcriptional patterns for the CcOX subunits, which are known to have a high affinity for nitric oxide, showed that the core catalytic subunit (OsCcOX1) and the nuclear subunit (OsCcOX5b) were up-regulated, while the nuclear subunit (OsCcOX5c) gene expression was suppressed. OsGSNOR1 expression was enhanced or decreased concomitant with a decrease or increase in SNO accumulation, particularly at the basal level. Moreover, high OsNIA1 expression was consistent with impaired root development, whereas low transcript accumulation matched a balanced root-growth pattern. This suggests that OsNIA1 expression would prevail over OsNIA2 expression under nitrosative stress response in rice. The level of malondialdehyde (MDA) content increased with the increase in SNP concentration, translating enhanced oxidative damage to the cell. We also observed increased catalase activity in response to 5 mM SNP suggesting that potential cross-talk exist between nitrosative and oxidative stress. These results collectively suggest a possible role of OsDHODH1 and OsCcOX5b role in plant root growth during nitrosative stress responses.

Humic Substances Contribute to Plant Iron Nutrition Acting as Chelators and Biostimulants

Improvement of plant iron nutrition as a consequence of metal complexation by humic substances (HS) extracted from different sources has been widely reported. The presence of humified fractions of the organic matter in soil sediments and solutions would contribute, depending on the solubility and the molecular size of HS, to build up a reservoir of Fe available for plants which exude metal ligands and to provide Fe-HS complexes directly usable by plant Fe uptake mechanisms. It has also been shown that HS can promote the physiological mechanisms involved in Fe acquisition acting at the transcriptional and post-transcriptional level. Furthermore, the distribution and allocation of Fe within the plant could be modified when plants were supplied with water soluble Fe-HS complexes as compared with other natural or synthetic chelates. These effects are in line with previous observations showing that treatments with HS were able to induce changes in root morphology and modulate plant membrane activities related to nutrient acquisition, pathways of primary and secondary metabolism, hormonal and reactive oxygen balance. The multifaceted action of HS indicates that soluble Fe-HS complexes, either naturally present in the soil or exogenously supplied to the plants, can promote Fe acquisition in a complex way by providing a readily available iron form in the rhizosphere and by directly affecting plant physiology. Furthermore, the possibility to use Fe-HS of different sources, size and solubility may be considered as an environmental-friendly tool for Fe fertilization of crops.

Protein Profiling of Arabidopsis Roots Treated With Humic Substances: Insights Into the Metabolic and Interactome Networks

Background and Aim: Humic substances (HSs) influence the chemical and physical properties of the soil, and are also known to affect plant physiology and nutrient uptake. This study aimed to elucidate plant metabolic pathways and physiological processes influenced by HS activity. Methods: Arabidopsis roots were treated with HS for 8 h. Quantitative mass spectrometry-based proteomics analysis of root proteins was performed using the iTRAQ (Isobaric Tag for Relative and Absolute Quantification) technique. Out of 902 protein families identified and quantified for HS treated vs. untreated roots, 92 proteins had different relative content. Bioinformatic tools such as STRING, KEGG, IIS and Cytoscape were used to interpret the biological function, pathway analysis and visualization of network amongst the identified proteins. Results: From this analysis it was possible to evaluate that all of the identified proteins were functionally classified into several categories, mainly redox homeostasis, response to inorganic substances, energy metabolism, protein synthesis, cell trafficking, and division. Conclusion: In the present study an overview of the metabolic pathways most modified by HS biological activity is provided. Activation of enzymes of the glycolytic pathway and up regulation of ribosomal protein indicated a stimulation in energy metabolism and protein synthesis. Regulation of the enzymes involved in redox homeostasis suggest a pivotal role of reactive oxygen species in the signaling and modulation of HS-induced responses.

Comparative Proteomic Analysis during the Involvement of Nitric Oxide in Hydrogen Gas-Improved Postharvest Freshness in Cut Lilies

Our previous studies suggested that both hydrogen gas (H₂) and nitric oxide (NO) could enhance the postharvest freshness of cut flowers. However, the crosstalk of H₂ and NO during that process is unknown. Here, cut lilies (Lilium "Manissa") were used to investigate the relationship between H₂ and NO and to identify differentially accumulated proteins during postharvest freshness. The results revealed that 1% hydrogen-rich water (HRW) and 150 μM sodium nitroprusside (SNP) significantly extended the vase life and quality, while NO inhibitors suppressed the positive effects of HRW. Proteomics analysis found 50 differentially accumulated proteins in lilies leaves which were classified into seven functional categories. Among them, ATP synthase CF1 alpha subunit (chloroplast) (AtpA) was up-regulated by HRW and down-regulated by NO inhibitor. The expression level of LlatpA gene was consistent with the result of proteomics analysis. The positive effect of HRW and SNP on ATP synthase activity was inhibited by NO inhibitor. Meanwhile, the physiological-level analysis of chlorophyll fluorescence and photosynthetic parameters also agreed with the expression of AtpA regulated by HRW and SNP. Altogether, our results suggested that NO might be involved in H₂-improved freshness of cut lilies, and AtpA protein may play important roles during that process.

Recycling of wastes from fish beneficiation by composting: chemical characteristics of the compost and efficiency of their humic acids in stimulating the growth of lettuce

Waste from the beneficiation of fish was composted with crushed grass aiming to characterize their chemical composition and investigate the possibility of the use of the final compost as source of humic acids (HA) able to stimulate the growth of lettuce. Compost presented pH value, C/N ratio, and electrical conductivity that allow its use as an organic fertilizer. The element content was present in the following order of abundance in the compost: P > Ca > N > Mg > K > Fe > Zn > Mn > Mo > Cu, and the humus composition was similar to that observed in others kind of organic residues composted. The high content of oxygen pointed out a high level of oxidation of HA, in line with the predominance of phenolic acidity in the functional groups. The 13C-NMR spectra showed marked resonances due to the presence of lipids and other materials resistant to degradation as methoxy substituent and N-alkyl groups. A concentration of 20 mg L-1 HA increased significantly both dry and wet root matter in lettuce but the CO2 assimilation, stomatal conductance, and number of lateral roots of the plants were not affected. However, increases of 64% in the water-use efficiency was observed due to the HA addition, probably related to the root morphology alteration which resulted in 1.6-fold increase of lateral root average length and due to the higher H+ extrusion activity. Reuse of residues from the fish beneficiation activity by composting may represent a safe tool to increase the value of recycled organic residues and generate HA with potential use as plant growth stimulants.

ÁCIDOS HÚMICOS/FÚLVICOS E NITROGÊNIO NA PRODUTIVIDADE DA CULTURA DO MILHO

O milho possui grande importância para a economia mundial, neste contexto cada vez mais busca-se alternativas para aumentar o rendimento de grãos. O objetivo do estudo foi avaliar os componentes morfológicos e a produtividade de grãos do milho cultivado em diferentes tratamentos com utilização isolada e/ou associada de nitrogênio e ácidos húmicos/fúlvicos. O estudo foi conduzido na Universidade Tecnológica Federal do Paraná, Campus de Dois Vizinhos, PR, durante a safra 2017/2018. Como fonte de nitrogênio foi utilizado uréia e de ácidos húmicos/fúlvicos um produto comercial (bioestimulante). Utilizou-se delineamento de blocos ao acaso com sete tratamentos e quatro repetições, sendo: 1: aplicação de ½ dose do bioestimulante em V4 + aplicação de ½ dose em V8; 2: aplicação de dose recomenda do bioestimulante em V4; 3: aplicação de dose recomenda do bioestimulante em V8; 4: nitrogênio; 5: nitrogênio + aplicação de dose recomenda do bioestimulante em V8; 6: nitrogênio + aplicação de dose recomenda do bioestimulante em V4; 7: sem nitrogênio e sem bioestimulante (testemunha). Os ácidos foram aplicados via foliar e o nitrogênio (N) (170 Kg ha-1) em cobertura em V4. Foram avaliados no final do ciclo da cultura as características morfológicas de planta e os componentes de rendimento. Os dados foram submetidos análise de variância e quando constado significância, aplicou-se teste de Scott-Knott a 5% de probabilidade. A utilização de ácidos húmicos/fúlvicos não interferiu nas características morfológicas das plantas, tampouco sobre os componentes de rendimento do milho e produtividade do milho, seja utilizado isoladamente ou associado com N. O uso de N via uréia em cobertura proporciona elevação do número de grãos por fileira e grãos por espiga, massa de mil grãos, produtividade por espiga e área.

Biostimulant Potential of Humic Acids Extracted From an Amendment Obtained via Combination of Olive Mill Wastewaters (OMW) and a Pre-treated Organic Material Derived From Municipal Solid Waste (MSW)

Olive mill wastewaters (OMW) contain significant levels of phenolic compounds with antimicrobial/phytotoxic activity and high amounts of undecomposed organic matter that may exert negative effects on soil biology. Among OMW detoxification techniques, those focusing on oxidative degradation of phenolic compounds are relevant. The composting (bio-oxidation) process in particular, exploits exothermic oxidation reactions by microorganisms to transform the organic matrix of OMW into an amendment biologically stable and feasible to use in agriculture. This process consists of an active phase during which organic compounds are rapidly decomposed, and a curing phase characterized by a slow breakdown of the remaining materials with the formation of humic substances (HS) as by-products. In this study, bio-oxidation of OMW was performed using a pre-treated organic material derived from municipal solid waste (MSW). The obtained amendment (OMWF) was stable and in accordance with the legislative parameters of mixed organic amendments. HS were then extracted from OMWF and MSW (control amendment, Amd-C), and differences in structural properties of their humic acid (HA) fraction were highlighted via spectroscopy (Fourier Transform Infrared) and Dynamic Light Scattering. To assay a potential use of HA as biostimulants for crops, 12-day old Zea Mays L. plants were supplied with HA at 0.5 mg and 1 mg C L-1 for 2 days. HA from both amendments increased plant growth, but HA from OMWF was more effective at both dosages (plus 35-37%). Also, HA from OMWF enhanced both nitrogen assimilation and glycolysis by increasing the activity of nitrate reductase (∼1.8-1.9 fold), phosphoglucose isomerase (PGI) (∼1.8-2 fold) and pyruvate kinase (PK) (∼1.5-1.8 fold), while HA from Amd-C targeted glycolysis preferentially. HA from OMWF, however, significantly stimulated plant nutrition only at lower dosage, perhaps because certain undetermined compounds from detoxified OMW and incorporated in HA altered the root membrane permeability, thus preventing the increase of nutrient uptake. Conversely, HA from Amd-C increased nutrient accumulation in maize at both dosages. In conclusion, our results indicate that the amendment obtained via OMW composting using MSW had a reduced pollution load in terms of phenolic compounds, and HA extracted from OMWF could be used as valuable biostimulants during maize cultivation.

Alleviation of iron toxicity in Schinus terebinthifolius Raddi (Anacardiaceae) by humic substances

One of the industrial pillars of Espírito Santo state, South East of Brazil, is iron-mining products processing. This activity brings to a high level of coastal pollution due to deposition of iron particulate on fragile ecosystems as mangroves and restinga. Schinus therebinthifolius (aroeira) is a widespread restinga species. This work tested iron toxicity alleviation by vermicompost humic substances (HS) added to aroeira seedlings in hydroponic conditions. Catalase, peroxidase, and ascorbate peroxidase are antioxidant enzymes that work as reactive oxygen species (ROS) scavengers: they increase their activity as an answer to ROS concentration rise that is the consequence of metal accumulation or humic substance stimulation. S. terebinthifolius seedlings treated with HS and Fe augmented their antioxidant enzyme activities significantly less than seedlings treated separately with HS and Fe; their significantly lower Fe accumulation and the slight increase of root and leaf area confirm the biostimulating effect of HS and their role in blocking Fe excess outside the roots. The use of HS can be useful for the recovery of areas contaminated by heavy metals.

Key acclimation responses to phosphorus deficiency in maize plants are influenced by exogenous nitric oxide

Improving phosphorus (P) acquisition and utilization in crops is of great importance in order to achieve a good plant nutritional state and maximize biomass production while minimizing the addition of fertilizers, and the concomitant risk of eutrophication. This study explores to which extent key processes involved in P-acquisition, and other acclimation mechanisms to low P supply in maize (Zea mays L.) plants, are affected by the addition of a nitric oxide (NO) donor (S-nitrosoglutathione, GSNO). Plants grown in a complete culture solution were exposed to four treatments performed by the combination of two P levels (0 and 0.5 mM), and two GSNO levels (0 and 0.1 mM), and responses to P-deprivation were then studied. Major plant responses related to P-deprivation were affected by the presence of the NO donor. In roots, the activity of acid phosphatases was significantly increased in P-depleted plants simultaneously exposed to GSNO. Acidification of the culture solution also increased in plants that had been grown in the presence of the NO donor. Furthermore, the potential capability displayed by roots of P-deprived plants for P-uptake, was higher in the plants that had been treated with GSNO. These results indicate that exogenous NO addition affects fundamental acclimation responses of maize plants to P scarcity, particularly and positively those that help plants to sustain P-acquisition under low P availability.

Spermine modulates fungal morphogenesis and activates plasma membrane H+-ATPase during yeast to hyphae transition

Polyamines play a regulatory role in eukaryotic cell growth and morphogenesis. Despite many molecular advances, the underlying mechanism of action remains unclear. Here, we investigate a mechanism by which spermine affects the morphogenesis of a dimorphic fungal model of emerging relevance in plant interactions, Yarrowia lipolytica, through the recruitment of a phytohormone-like pathway involving activation of the plasma membrane P-type H⁺-ATPase. Morphological transition was followed microscopically, and the H⁺-ATPase activity was analyzed in isolated membrane vesicles. Proton flux and acidification were directly probed at living cell surfaces by a non-invasive selective ion electrode technique. Spermine and indol-3-acetic acid (IAA) induced the yeast-hypha transition, influencing the colony architecture. Spermine induced H⁺-ATPase activity and H⁺ efflux in living cells correlating with yeast-hypha dynamics. Pharmacological inhibition of spermine and IAA pathways prevented the physio-morphological responses, and indicated that spermine could act upstream of the IAA pathway. This study provides the first compelling evidence on the fungal morphogenesis and colony development as modulated by a spermine-induced acid growth mechanism analogous to that previously postulated for the multicellular growth regulation of plants.

Summary: This study presents a new mechanistic model for the integrative role of the polyamine spermine and hormone auxin in the signaling of yeast-to-hypha transition, filling an important gap in fungal morphogenesis.

Humic Substances: Determining Potential Molecular Regulatory Processes in Plants

Humic substances (HSs) have considerable effects on soil fertility and crop productivity owing to their unique physiochemical and biochemical properties, and play a vital role in establishing biotic and abiotic interactions within the plant rhizosphere. A comprehensive understanding of the mode of action and tissue distribution of HS is, however, required, as this knowledge could be useful for devising advanced rhizospheric management practices. These substances trigger various molecular processes in plant cells, and can strengthen the plant's tolerance to various kinds of abiotic stresses. HS manifest their effects in cells through genetic, post-transcriptional, and post-translational modifications of signaling entities that trigger different molecular, biochemical, and physiological processes. Understanding of such fundamental mechanisms will provide a better perspective for defining the cues and signaling crosstalk of HS that mediate various metabolic and hormonal networks operating in plant systems. Various regulatory activities and distribution strategies of HS have been discussed in this review.

Biostimulants in Plant Science: A Global Perspective

This review presents a comprehensive and systematic study of the field of plant biostimulants and considers the fundamental and innovative principles underlying this technology. The elucidation of the biological basis of biostimulant function is a prerequisite for the development of science-based biostimulant industry and sound regulations governing these compounds. The task of defining the biological basis of biostimulants as a class of compounds, however, is made more complex by the diverse sources of biostimulants present in the market, which include bacteria, fungi, seaweeds, higher plants, animals and humate-containing raw materials, and the wide diversity of industrial processes utilized in their preparation. To distinguish biostimulants from the existing legislative product categories we propose the following definition of a biostimulant as "a formulated product of biological origin that improves plant productivity as a consequence of the novel or emergent properties of the complex of constituents, and not as a sole consequence of the presence of known essential plant nutrients, plant growth regulators, or plant protective compounds." The definition provided here is important as it emphasizes the principle that biological function can be positively modulated through application of molecules, or mixtures of molecules, for which an explicit mode of action has not been defined. Given the difficulty in determining a "mode of action" for a biostimulant, and recognizing the need for the market in biostimulants to attain legitimacy, we suggest that the focus of biostimulant research and validation should be upon proof of efficacy and safety and the determination of a broad mechanism of action, without a requirement for the determination of a specific mode of action. While there is a clear commercial imperative to rationalize biostimulants as a discrete class of products, there is also a compelling biological case for the science-based development of, and experimentation with biostimulants in the expectation that this may lead to the identification of novel biological molecules and phenomenon, pathways and processes, that would not have been discovered if the category of biostimulants did not exist, or was not considered legitimate.

Biostimulants in Plant Science: A Global Perspective

This review presents a comprehensive and systematic study of the field of plant biostimulants and considers the fundamental and innovative principles underlying this technology. The elucidation of the biological basis of biostimulant function is a prerequisite for the development of science-based biostimulant industry and sound regulations governing these compounds. The task of defining the biological basis of biostimulants as a class of compounds, however, is made more complex by the diverse sources of biostimulants present in the market, which include bacteria, fungi, seaweeds, higher plants, animals and humate-containing raw materials, and the wide diversity of industrial processes utilized in their preparation. To distinguish biostimulants from the existing legislative product categories we propose the following definition of a biostimulant as "a formulated product of biological origin that improves plant productivity as a consequence of the novel or emergent properties of the complex of constituents, and not as a sole consequence of the presence of known essential plant nutrients, plant growth regulators, or plant protective compounds." The definition provided here is important as it emphasizes the principle that biological function can be positively modulated through application of molecules, or mixtures of molecules, for which an explicit mode of action has not been defined. Given the difficulty in determining a "mode of action" for a biostimulant, and recognizing the need for the market in biostimulants to attain legitimacy, we suggest that the focus of biostimulant research and validation should be upon proof of efficacy and safety and the determination of a broad mechanism of action, without a requirement for the determination of a specific mode of action. While there is a clear commercial imperative to rationalize biostimulants as a discrete class of products, there is also a compelling biological case for the science-based development of, and experimentation with biostimulants in the expectation that this may lead to the identification of novel biological molecules and phenomenon, pathways and processes, that would not have been discovered if the category of biostimulants did not exist, or was not considered legitimate.

Foliar Nutrition, Biostimulants and Prime-Like Dynamics in Fruit Tree Physiology: New Insights on an Old Topic

Despite the fact that the usage of foliar nutrients has long history, many aspects of fertilization through leaves are still unknown. Herein, we review the current knowledge regarding the canopy fertilization putting special emphasis on Fe nutrition and briefly provide insights into the nanofertilizer technology of the foliar feeding of fruit crops. In addition, this paper discusses the main aspects of the foliar application of biostimulants regarding crucial factors of fruit cropping systems, such as fruit yield/size, tolerance to environmental stresses, and nutrient availability. Also, we specifically discuss the role of hydrogen peroxide (H₂O₂) and nitric oxide (NO) as priming molecules and their possible cross-talk with biostimulants in fruit tree physiology. Finally, a view of the key issues for future fundamental and applied research in the topic is put forward.

Transcriptome-Wide Identification of Differentially Expressed Genes in Solanum lycopersicon L. in Response to an Alfalfa-Protein Hydrolysate Using Microarrays

An alfalfa-based protein hydrolysate (EM) has been tested in tomato (Solanum lycopersicon L.) plants at two different concentrations (0.1 and 1 mL L^-1) to get insight on its efficacy as biostimulant in this species and to unravel possible metabolic targets and molecular mechanisms that may shed light on its mode of action. EM was efficient in promoting the fresh biomass and content in chlorophyll and soluble sugars of tomato plants, especially when it was applied at the concentration of 1 mL L^-1. This effect on plant productivity was likely related to the EM-dependent up-regulation of genes identified via microarray and involved in primary carbon and nitrogen metabolism, photosynthesis, nutrient uptake and developmental processes. EM also up-regulated a number of genes implied in the secondary metabolism that leads to the synthesis of compounds (phenols and terpenes) functioning in plant development and interaction with the environment. Concomitantly, phenol content was enhanced in EM-treated plants. Several new genes have been identified in tomato as potential targets of EM action, like those involved in detoxification processes from reactive oxygen species and xenobiotic (particularly glutathione/ascorbate cycle-related and ABC transporters), and defense against abiotic and biotic stress. The model hypothesized is that elicitors present in the EM formulation like auxins, phenolics, and amino acids, may trigger a signal transduction pathway via modulation of the intracellular levels of the hormones ethylene, jasmonic acid and abscissic acid, which then further prompt the activation of a cascade events requiring the presence and activity of many kinases and transcription factors to activate stress-related genes. The genes identified suggest these kinases and transcription factors as players involved in a complex crosstalk between biotic and abiotic stress signaling pathways. We conclude that EM acts as a biostimulant in tomato due to its capacity to stimulate plant productivity and up-regulate stress-related responses. Its use in agricultural practices may reduce the need of inorganic fertilizers and pesticides, thereby reducing the environmental impact of productive agriculture.

Investigating organic molecules responsible of auxin-like activity of humic acid fraction extracted from vermicompost

This work studied the auxin-like activity of humic acids (HA) obtained from vermicomposts produced using leather wastes plus cattle dung at different maturation stages (fresh, stable and mature). Bioassays were performed by testing HA concentrations in the range of 100-6000mgcarbonL(-1). (13)C CPMAS-NMR and GC-MS instrumental methods were used to assess the effect of biological processes and starting organic mixtures on HA composition. Not all HAs showed IAA-like activity and in general, IAA-like activity increased with the length of the vermicomposting process. The presence of leather wastes was not necessary to produce the auxin-like activity of HA, since HA extracted from a mix of cattle manure and sawdust, where no leather waste was added, showed IAA-like activity as well. CPMAS (13)CNMR revealed that HAs were similar independently of the mix used and that the humification process involved the increasing concentration of pre-existing alkali soluble fractions in the biomass. GC/MS allowed the identification of the molecules involved in IAA-like effects: carboxylic acids and amino acids. The concentration of active molecules, rather than their simple presence in HA, determined the bio-stimulating effect, and a good linear regression between auxin-like activity and active stimulating molecules concentration was found (R(2)=-0.85; p<0.01, n=6).

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.