Growth Stimulatory Effects and Genome-Wide Transcriptional Changes Produced by Protein Hydrolysates in Maize Seedlings

Enriching NPK Mineral Fertilizer with Plant-Stimulating Peptides Increases Soilless Tomato Production, Grower Profit, and Environmental Sustainability

The need to increase agricultural production to feed a steadily growing population may clash with the more environmentally friendly but less efficient production methods required. Therefore, it is important to try to reduce the use of chemical inputs without compromising production. In this scenario, natural biostimulants have become one of the most sought-after and researched technologies. In the present study, the results of a greenhouse experiment on hydroponic tomatoes (Solanum lycopersicum L.) are presented, which involved comparing the use of ordinary NPK fertilizer (Cerbero®) with the use of NPK fertilizers enriched with 0.5% protein hydrolysate of plant origin (Cerbero Green®) at both standard (100%) and reduced (70%) fertilization rates. The results highlight how the use of Cerbero Green® fertilizers improves the production performance of tomatoes. More specifically, they show that the use of Cerbero Green® leads to higher marketable yields, especially under reducing fertilizer use, ensuring a positive net change in profit for the grower. In addition, carbon footprint analysis has revealed that the use of Cerbero Green® reduces the environmental impact of hydroponic tomato growing practices by up to 8%. The observed higher yield of hydroponically grown tomatoes even with reduced fertilization rates underlines once again the key role of natural biostimulants in increasing both the economic and environmental sustainability of horticultural production.

Discovery of the biostimulant effect of asparagine and glutamine on plant growth in Arabidopsis thaliana

Protein hydrolysates have gained interest as plant biostimulants due to their positive effects on plant performances. They are mainly composed of amino acids, but there is no evidence of the role of individual of amino acids as biostimulants. In this study we carried out in vitro experiments to monitor the development of Arabidopsis seedlings on amino acid containing media in order to analyze the biostimulant properties of the twenty individual proteinogenic amino acids. We demonstrated that proteinogenic amino acids are not good nitrogen sources as compared to nitrate for plant growth. Biostimulant analyses were based on leaf area measurements as a proxy of plant growth. We developed the Amino Acid Use Efficiency index to quantify the biostimulating effect of individual amino acids in the presence of nitrate. This index allowed us to classify amino acids into three groups, characterized by their inhibiting, neutral, and beneficial effects regarding leaf area. Glutamine and asparagine demonstrated the most significant effects in promoting leaf area in the presence of nitrate supply. The stimulating effect was confirmed by using the L and D enantiomeric forms. Both L-glutamine and L-asparagine stimulated leaf area at low concentrations, emphasizing their biostimulating properties. Our plant growth design and AAUE index pave the way for the identification of other bioactive molecules in protein hydrolysates and for the comparison of biostimulant performances.

Amino Acids Biostimulants and Protein Hydrolysates in Agricultural Sciences

The effects of different types of biostimulants on crops include improving the visual quality of the final products, stimulating the immune systems of plants, inducing the biosynthesis of plant defensive biomolecules, removing heavy metals from contaminated soil, improving crop performance, reducing leaching, improving root development and seed germination, inducing tolerance to abiotic and biotic stressors, promoting crop establishment and increasing nutrient-use efficiency. Protein hydrolysates are mixtures of polypeptides and free amino acids resulting from enzymatic and chemical hydrolysis of agro-industrial protein by-products obtained from animal or plant origins, and they are able to alleviate environmental stress effects, improve growth, and promote crop productivity. Amino acids involve various advantages such as increased yield and yield components, increased nutrient assimilation and stress tolerance, and improved yield components and quality characteristics. They are generally achieved through chemical or enzymatic protein hydrolysis, with significant capabilities to influence the synthesis and activity of some enzymes, gene expression, and redox-homeostasis. Increased yield, yield components, and crop quality; improved and regulated oxidation-reduction process, photosynthesis, and physiological activities; decreased negative effects of toxic components; and improved anti-fungal activities of plants are just some of the more important benefits of the application of phenols and phenolic biostimulants. The aim of this manuscript is to survey the impacts of amino acids, different types of protein hydrolysates, phenols, and phenolic biostimulants on different plants by presenting case studies and successful paradigms in several horticultural and agricultural crops.

Enzyme-Based Biostimulants Influence Physiological and Biochemical Responses of Lactuca sativa L

Biostimulants (BSs) are natural materials (i.e., organic or inorganic compounds, and/or microorganisms) having beneficial effects on plant growth and productivity, and able to improve resilience/tolerance to biotic and abiotic stresses. Therefore, they represent an innovative alternative to the phyto- and agrochemicals, being environmentally friendly and a valuable tool to cope with extreme climate conditions. The objective of this study was to investigate the effects of several biomolecules (i.e., Xylanase, β-Glucosidase, Chitinase, and Tramesan), alone or in combinations, on lettuce plant growth and quality. With this aim, the influence of these biomolecules on biomass, pigment content, and antioxidant properties in treated plants were investigated. Our results showed that Xylanase and, to a lesser extent, β-Glucosidase, have potentially biostimulant activity for lettuce cultivation, positively influencing carotenoids, total polyphenols, and ascorbic acid contents; similar effects were found with respect to antioxidative properties. Furthermore, the effect of the more promising molecules (Xylanase and β-Glucosidase) was also evaluated in kiwifruit cultured cells to test their putative role as sustainable input for plant cell biofactories. The absence of phytotoxic effects of both molecules at low doses (0.1 and 0.01 µM), and the significantly enhanced cell biomass growth, indicates a positive impact on kiwifruit cells.

Casein as protein and hydrolysate: Biostimulant or nitrogen source for Nicotiana tabacum plants grown in vitro?

In contrast to inorganic nitrogen (N) assimilation, the role of organic N forms, such as proteins and peptides, as sources of N and their impact on plant metabolism remains unclear. Simultaneously, organic biostimulants are used as priming agents to improve plant defense response. Here, we analysed the metabolic response of tobacco plants grown in vitro with casein hydrolysate or protein. As the sole source of N, casein hydrolysate enabled tobacco growth, while protein casein was used only to a limited extent. Free amino acids were detected in the roots of tobacco plants grown with protein casein but not in the plants grown with no source of N. Combining hydrolysate with inorganic N had beneficial effects on growth, root N uptake and protein content. The metabolism of casein-supplemented plants shifted to aromatic (Trp), branched-chain (Ile, Leu, Val) and basic (Arg, His, Lys) amino acids, suggesting their preferential uptake and/or alterations in their metabolic pathways. Complementarily, proteomic analysis of tobacco roots identified peptidase C1A and peptidase S10 families as potential key players in casein degradation and response to N starvation. Moreover, amidases were significantly upregulated, most likely for their role in ammonia release and impact on auxin synthesis. In phytohormonal analysis, both forms of casein influenced phenylacetic acid and cytokinin contents, suggesting a root system response to scarce N availability. In turn, metabolomics highlighted the stimulation of some plant defense mechanisms under such growth conditions, that is, the high concentrations of secondary metabolites (e.g., ferulic acid) and heat shock proteins.

Biostimulant Properties of Protein Hydrolysates: Recent Advances and Future Challenges

Over the past decade, plant biostimulants have been increasingly used in agriculture as environment-friendly tools that improve the sustainability and resilience of crop production systems under environmental stresses. Protein hydrolysates (PHs) are a main category of biostimulants produced by chemical or enzymatic hydrolysis of proteins from animal or plant sources. Mostly composed of amino acids and peptides, PHs have a beneficial effect on multiple physiological processes, including photosynthetic activity, nutrient assimilation and translocation, and also quality parameters. They also seem to have hormone-like activities. Moreover, PHs enhance tolerance to abiotic stresses, notably through the stimulation of protective processes such as cell antioxidant activity and osmotic adjustment. Knowledge on their mode of action, however, is still piecemeal. The aims of this review are as follows: (i) Giving a comprehensive overview of current findings about the hypothetical mechanisms of action of PHs; (ii) Emphasizing the knowledge gaps that deserve to be urgently addressed with a view to efficiently improve the benefits of biostimulants for different plant crops in the context of climate change.

Transcriptome, Biochemical and Phenotypic Analysis of the Effects of a Precision Engineered Biostimulant for Inducing Salinity Stress Tolerance in Tomato

Salinity stress is a major problem affecting plant growth and crop productivity. While plant biostimulants have been reported to be an effective solution to tackle salinity stress in different crops, the key genes and metabolic pathways involved in these tolerance processes remain unclear. This study focused on integrating phenotypic, physiological, biochemical and transcriptome data obtained from different tissues of Solanum lycopersicum L. plants (cv. Micro-Tom) subjected to a saline irrigation water program for 61 days (EC: 5.8 dS/m) and treated with a combination of protein hydrolysate and Ascophyllum nodosum-derived biostimulant, namely PSI-475. The biostimulant application was associated with the maintenance of higher K⁺/Na⁺ ratios in both young leaf and root tissue and the overexpression of transporter genes related to ion homeostasis (e.g., NHX4, HKT1;2). A more efficient osmotic adjustment was characterized by a significant increase in relative water content (RWC), which most likely was associated with osmolyte accumulation and upregulation of genes related to aquaporins (e.g., PIP2.1, TIP2.1). A higher content of photosynthetic pigments (+19.8% to +27.5%), increased expression of genes involved in photosynthetic efficiency and chlorophyll biosynthesis (e.g., LHC, PORC) and enhanced primary carbon and nitrogen metabolic mechanisms were observed, leading to a higher fruit yield and fruit number (47.5% and 32.5%, respectively). Overall, it can be concluded that the precision engineered PSI-475 biostimulant can provide long-term protective effects on salinity stressed tomato plants through a well-defined mode of action in different plant tissues.

Integrated transcriptome and metabolome analyses shed light on the defense mechanisms in tomato plants after (E)-2-hexenal fumigation

Tomato is a widely cultivated fruit and vegetable and is valued for its flavor, colour, and nutritional value. C₆-aldehydes, such as (E)-2-hexenal, not only have antibacterial and antifungal properties but also function as signaling molecules that control the defense mechanisms of plants, including tomatoes. In this study, we used liquid chromatography-mass spectrometry (LC-MS) and RNA sequencing techniques to generate metabolome and transcriptome datasets that elucidate the molecular mechanisms regulating defense responses in tomato leaves exposed to (E)-2-hexenal. A total of 28.27 Gb of clean data were sequenced and assembled into 23,720 unigenes. In addition, a non-targeted metabolomics approach detected 739 metabolites. There were 233 significant differentially expressed genes (DEGs) (158 up-regulated, 75 down-regulated) and 154 differentially expressed metabolites (DEMs) (86 up-regulated, 69 down-regulated). Most nucleotides and amino acids (L-Phenylalanine, L-Asparagine, L-Histidine, L-Arginine, and L-Tyrosine) and their derivatives were enriched. The analyses revealed that mitogen-activated protein kinase (MPK), pathogenesis-related protein (PR), and endochitinase (CHIB) were primarily responsible for the adaptation of plant defense responses. Therefore, the extensive upregulation of these genes may be associated with the increased plant defense response. These findings help us comprehend the defense response of plants to (E)-2-hexenal and improve the resistance of horticultural plants.

Chemical Characterization of a Collagen-Derived Protein Hydrolysate and Biostimulant Activity Assessment of Its Peptidic Components

Protein hydrolysates (PHs) are plant biostimulants consisting of oligopeptides and free amino acids exploited in agriculture to increase crop productivity. This work aimed to fractionate a commercial collagen-derived protein hydrolysate (CDPH) according to the molecular mass of the peptides and evaluate the bioactivity of different components. First, the CDPH was dialyzed and/or filtrated and analyzed on maize, showing that smaller compounds were particularly active in stimulating lateral root growth. The CDPH was then fractionated through fast protein liquid chromatography and tested on in vitro grown tomatoes proving that all the fractions were bioactive. Furthermore, these fractions were characterized by liquid chromatography-electrospray ionization-tandem mass spectrometry revealing a consensus sequence shared among the identified peptides. Based on this sequence, a synthetic peptide was produced. We assessed its structural similarity with the CDPH, the collagen, and polyproline type II helix by comparing the respective circular dichroism spectra and for the first time, we proved that a signature peptide was as bioactive as the whole CDPH.

Deciphering the Potential Role of Symbiotic Plant Microbiome and Amino Acid Application on Growth Performance of Chickpea Under Field Conditions

The unprecedented rise in the human population has increased pressure on agriculture production. To enhance the production of crops, farmers mainly rely on the use of chemical fertilizers and pesticides, which have, undoubtedly, increased the production rate but at the cost of losing sustainability of the environment in the form of genetic erosion of indigenous varieties of crops and loss of fertile land. Therefore, farming practices need to upgrade toward the use of biological agents to maintain the sustainability of agriculture and the environment. In this context, using microbial inoculants and amino acids may present a more effective, safer, economical, and sustainable alternative means of realizing higher productivity of crops. Therefore, field experiments were performed on chickpea for two succeeding years using Rhizobium and L-methionine (at three levels, i.e., 5, 10, and 15 mg L-1) separately and in combinations. The results show that the application of Rhizobium and all the three levels of L-methionine increased the growth and yield of chickpea. There was a higher response to a lower dose of L-methionine, i.e., 5 mg L-1. It has been found that maximum grain yield (39.96 and 34.5% in the first and second years, respectively) of chickpea was obtained with the combined use of Rhizobium and L-methionine (5 mg L-1). This treatment was also the most effective in enhancing nodule number (91.6 and 58.19%), leghemoglobin (161.1 and 131.3%), and protein content (45.2 and 45%) of plants in both years. Likewise, photosynthetic pigments and seed chemical composition were significantly improved by Rhizobium inoculation. However, these effects were prominent when Rhizobium inoculation was accompanied by L-methionine. In conclusion, utilizing the potential of combined use of L-methionine and microbial inoculant could be a better approach for developing sustainable agriculture production.

Harnessing Synergistic Biostimulatory Processes: A Plausible Approach for Enhanced Crop Growth and Resilience in Organic Farming

Simple Summary

Demand for organically grown crops has risen globally due to its healthier and safer food products. From a sustainability perspective, organic farming offers an eco-friendly cultivation system that minimizes agrochemicals and producing food with little or no environmental footprint. However, organic agriculture’s biggest drawback is the generally lower and variable yield in contrast to conventional farming. Compatible with organic farming, the selective use of biostimulants can close the apparent yield gap between organic and conventional cultivation systems. A biostimulant is defined as natural microorganisms (bacteria, fungi) or biologically active substances that are able to improve plant growth and yield through several processes. Biostimulants are derived from a range of natural resources including organic materials (composts, seaweeds), manures (earthworms, fish, insects) and extracts derived from microbes, plant, insect or animal origin. The current trend is indicative that a mixture of biostimulants is generally delivering better growth, yield and quality rather than applying biostimulant individually. When used correctly, biostimulants are known to help plants cope with stressful situations like drought, salinity, extreme temperatures and even certain diseases. More research is needed to understand the different biostimulants, key components, and also to adjust the formulations to improve their reliability in the field.

Abstract

Demand for organically grown food crops is rising substantially annually owing to their contributions to human health. However, organic farm production is still generally lower compared to conventional farming. Nutrient availability, content consistency, uptake, assimilation, and crop responses to various stresses were reported as critical yield-limiting factors in many organic farming systems. In recent years, plant biostimulants (BSs) have gained much interest from researchers and growers, and with the objective of integrating these products to enhance nutrient use efficiency (NUE), crop performance, and delivering better stress resilience in organic-related farming. This review gave an overview of direct and indirect mechanisms of microbial and non-microbial BSs in enhancing plant nutrient uptake, physiological status, productivity, resilience to various stressors, and soil-microbe-plant interactions. BSs offer a promising, innovative and sustainable strategy to supplement and replace agrochemicals in the near future. With greater mechanistic clarity, designing purposeful combinations of microbial and non-microbial BSs that would interact synergistically and deliver desired outcomes in terms of acceptable yield and high-quality products sustainably will be pivotal. Understanding these mechanisms will improve the next generation of novel and well-characterized BSs, combining microbial and non-microbial BSs strategically with specific desired synergistic bio-stimulatory action, to deliver enhanced plant growth, yield, quality, and resilience consistently in organic-related cultivation.

Divergence in Corn Mycorrhizal Colonization Patterns Due to Organic Treatment

Excessive application of chemical fertilizers and other agrochemicals can cause large imbalances in soils and agricultural ecosystems. In this context, mycorrhizae represent a viable solution to mitigate these negative effects. Arbuscular mycorrhizae are vital symbionts due to the multiple benefits they bring to both crops and the entire agroecosystem. The main purpose of this study was to observe whether differentiated fertilization has an influence on mycorrhizal colonization patterns in corn. Observed frequencies and intensities of colonization varied widely between phenophases and treatments, with 20% variation for frequency and 14% for intensity, which implies the constant development of both partners during the vegetation period. Arbuscules and vesicles were present in all development stages, but the overall mean was lower than 4% for arbuscules and 1% for vesicles in the analyzed root fragments. Intensity was highly correlated with frequency of colonization compared with arbuscules, where the coefficient was 0.54, and vesicles, with a coefficient of 0.16. Both PCA and NMDS provided good graphical solutions, with a high resolution due to explained variance and good spatial position of vectors. The use of mycorrhizal maps permits the full exploration of colonization patterns and fungal strategy, and the assessment of mycorrhizae-free areas. For the untreated variant, the strategy was oriented toward a longitudinal colonization followed by an irregular development of hyphae with multiple non-colonized areas. Treatment acts to stimulate the appearance of mycorrhizal spots, which further develop radially.

The potential use of Chlamydomonas reinhardtii and Chlorella sorokiniana as biostimulants on maize plants

Nitrogen deficiency and drought stress are among the major stresses faced by plants with negative consequence on crop production. The use of plant biostimulants is a very promising application in agriculture to improve crop yield, but especially to prevent the effect of abiotic stresses. Algae-derived biostimulants represent an efficient tool to stimulate the root development: while macroalgae have already been widely adopted as a source of biostimulants to improve plants growth and resilience, far less information is available for microalgae. The objective of this work is to investigate the stimulant ability on maize roots of two green algae species, Chlamydomonas reinhardtii and Chlorella sorokiniana, being respectively the model organism for Chlorophyta and one of the most promising species for microalgae cultivation at industrial scale. The results obtained demonstrate that both C. reinhardtii and C. sorokiniana cells promoted the development of maize root system compared to the untreated negative control. C. sorokiniana specifically increased the number of secondary roots, while improved micro-nutrients accumulation on roots and shoots was measured in the case of C. reinhardtii treated plants. When these microalgae-derived biostimulants were applied on plants grown in stress conditions as nitrogen deficiency, improved development of the root system was measured in the case of plants treated with C. sorokiniana biomass. Microalgae cultivation for biostimulant production can thus be considered as a bio-based process providing solutions for improving plant resilience toward stress conditions.

Recent Advances in the Molecular Effects of Biostimulants in Plants: An Overview

As the world develops and population increases, so too does the demand for higher agricultural output with lower resources. Plant biostimulants appear to be one of the more prominent sustainable solutions, given their natural origin and their potential to substitute conventional methods in agriculture. Classified based on their source rather than constitution, biostimulants such as humic substances (HS), protein hydrolysates (PHs), seaweed extracts (SWE) and microorganisms have a proven potential in improving plant growth, increasing crop production and quality, as well as ameliorating stress effects. However, the multi-molecular nature and varying composition of commercially available biostimulants presents challenges when attempting to elucidate their underlying mechanisms. While most research has focused on the broad effects of biostimulants in crops, recent studies at the molecular level have started to unravel the pathways triggered by certain products at the cellular and gene level. Understanding the molecular influences involved could lead to further refinement of these treatments. This review comprises the most recent findings regarding the use of biostimulants in plants, with particular focus on reports of their molecular influence.

Evaluation of the Potential Use of a Collagen-Based Protein Hydrolysate as a Plant Multi-Stress Protectant

Protein hydrolysates (PHs) are a class of plant biostimulants used in the agricultural practice to improve crop performance. In this study, we have assessed the capacity of a commercial PH derived from bovine collagen to mitigate drought, hypoxic, and Fe deficiency stress in Zea mays. As for the drought and hypoxic stresses, hydroponically grown plants treated with the PH exhibited an increased growth and absorption area of the roots compared with those treated with inorganic nitrogen. In the case of Fe deficiency, plants supplied with the PH mixed with FeCl₃ showed a faster recovery from deficiency compared to plants supplied with FeCl₃ alone or with FeEDTA, resulting in higher SPAD values, a greater concentration of Fe in the leaves and modulation in the expression of genes related to Fe. Moreover, through the analysis of circular dichroism spectra, we assessed that the PH interacts with Fe in a dose-dependent manner. Various hypothesis about the mechanisms of action of the collagen-based PH as stress protectant particularly in Fe-deficiency, are discussed.

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.

Quantitative Proteomics of Maize Roots Treated with a Protein Hydrolysate: A Comparative Study with Transcriptomics Highlights the Molecular Mechanisms Responsive to Biostimulants

Protein hydrolysate (PH)-based biostimulants offer a cost-effective and sustainable approach for the regulation of physiological processes in plants to stimulate growth and improve stress tolerance. Understanding the mode of action of PHs is challenging, but it is indispensable to improve existing candidates and to develop novel molecules with enhanced stimulatory effects. Hence, the present study aimed to understand the proteome level responses in the B73 maize roots treated with APR, a PH biostimulant, at two increasing concentrations and to compare and integrate it with the transcriptomic data obtained previously under identical experimental conditions. Results indicate that APR induced dose-dependent global changes in the transcriptome and proteome of maize roots. APR treatment altered the expression and abundance of several genes and proteins related to redox homeostasis, stress response, glycolysis, tricarboxylic acid cycle, pentose phosphate pathway, and other metabolic pathways of carbohydrates, amino acids, and lipids. Further, metabolic processes of phytohormone, secondary metabolites, especially phenylpropanoids, flavonoids, and terpenoids and transport, and cytoskeletal reorganization associated mechanisms were stimulated. Our results suggest that APR treatment altered the redox homeostasis and thus triggered an oxidative signal. This could be one of the key regulators of the cascade of downstream events involving multiple signaling, hormonal, and metabolic pathways, resulting in an altered physiological and metabolic state which consequently could lead to improved growth and stress adaptation observed in biostimulant-treated plants.

An Enzymatically Hydrolyzed Animal Protein-Based Biostimulant (Pepton) Increases Salicylic Acid and Promotes Growth of Tomato Roots Under Temperature and Nutrient Stress

Biostimulants may be particularly interesting for application in agricultural and horticultural crops since they can exert a growth-promoting effect on roots. This may be important for promoting longitudinal and lateral root growth and therefore increasing belowground vegetative growth, which may in turn lead to improved aboveground vegetative growth and increased yields. Here, we examined the effects and mechanism of action of an enzymatically hydrolyzed animal protein-based biostimulant (Pepton) on the root growth of tomato plants, with an emphasis on its possible role on chorismate-derived hormones (auxin, salicylic acid, and melatonin). Tomato plants growing in hydroponic systems were exposed to either nutrient stress conditions (experiment 1) or suboptimal temperatures (experiment 2) in a greenhouse, and the concentration of auxin, salicylic acid, and melatonin in roots were measured just prior and after the application of the biostimulant. Results showed that the application of Pepton exerted a growth-promoting effect on roots in plants growing under suboptimal conditions, which might be associated with enhanced salicylic acid levels in roots. The extent of effects of this enzymatically hydrolyzed animal protein-based biostimulant might strongly depend on the growth conditions and stage of root system development. It is concluded that an enzymatically hydrolyzed animal protein-based biostimulant (Pepton) may exert a positive effect enhancing primary and lateral root growth of tomato plants growing under suboptimal conditions, by stimulating the biosynthesis of specific hormonal pathways, such as salicylic acid under stress.

Combining Molecular Weight Fractionation and Metabolomics to Elucidate the Bioactivity of Vegetal Protein Hydrolysates in Tomato Plants

The comprehension of the bioactive fractions involved in the biostimulant activity of plant derived protein hydrolysates (PH) is a complex task, but it can also lead to significant improvements in the production of more effective plant biostimulants. The aim of this work is to shed light onto the bioactivity of different PH dialysis fractions (PH1 < 0.5-1 kDa; PH2 > 0.5-1 kDa; PH3 < 8-10 kDa; PH4 > 8-10 kDa) of a commercial PH-based biostimulant through a combined in vivo bioassay and metabolomics approach. A first tomato rooting bioassay investigated the auxin-like activity of PH and its fractions, each of them at three nitrogen levels (3, 30, and 300 mg L^-1 of N) in comparison with a negative control (water) and a positive control (indole-3-butyric acid, IBA). Thereafter, a second experiment was carried out where metabolomics was applied to elucidate the biochemical changes imposed by the PH and its best performing fraction (both at 300 mg L^-1 of N) in comparison to water and IBA. Overall, both the PH and its fractions increased the root length of tomato cuttings, compared to negative control. Moreover, the highest root length was obtained in the treatment PH1 following foliar application. Metabolomics allowed highlighting a response to PH1 that involved changes at phytohormones and secondary metabolite level. Notably, such metabolic reprogramming supported the effect on rooting of tomato cuttings, being shared with the response induced by the positive control IBA. Taken together, the outcome of in vivo assays and metabolomics indicate an auxin-like activity of the selected PH1 fraction.

Borage extracts affect wild rocket quality and influence nitrate and carbon metabolism

Market is increasingly demanding vegetables with high quality and nutraceutical characteristics. It was demonstrated that leafy vegetables can get benefit from biostimulants, for the reduction of nitrate concentration and the increment of antioxidants, with potential benefit for human health. The research purpose was to investigate on the role of a novel plant-based biostimulant in affecting nitrogen and carbon metabolism in wild rocket (Diplotaxis tenuifolia L.). Foliar spray treatments were performed with extracts obtained from borage (Borago officinalis L.) leaves and flowers. To evaluate the treatments effect, in vivo determinations (chlorophyll a fluorescence and chlorophyll content) were performed. At harvest, nitrate concentration, sucrose, total sugars, chlorophyll, and carotenoids levels were measured in leaves. In order to characterize the mechanism of action also at molecular level, a set of genes encoding for some of the key enzymes implicated in nitrate and carbon metabolism was selected and their expression was measured by qRT-PCR. Interesting results concerned the increment of sucrose, coherent with a high value of Fv/Fm, in addition to a significant reduction of nitrate and ABA than control, and an enhanced NR in vivo activity. Also, genes expression was influenced by extracts, with a more pronounced effect on N related genes.

Transcriptional Regulation in Rocket Leaves as Affected by Salinity

Salinity is one of the major abiotic stress causing yield losses and decreasing product quality. The beneficial effects of biostimulant products to enhance plant tolerance to abiotic stresses have been reported in several crops, but their mode of action is poorly understood. This work aims to better understand the effect of salt stress on wild rocket treated with a borage extract. The expression of some of the transcription factors (TFs) typically involved in salt stress response was studied within a 24 h period. Physiological parameters such as chlorophyll, chlorophyll a fluorescence, carotenoids, phenols, and anthocyanin were analyzed. Results obtained showed that salt stress induced a general increase in the expression levels of almost all TFs studied, whereas the treatment with the plant-base extract only induced an increase at specific time points. Moreover, the approach adopted allowed indagating the change in gene expression during time. Different pathways such as sugars metabolism, cuticular wax biosynthesis, and brassinosteroids signaling took part in plant responses.

Nitrogen Starvation Differentially Influences Transcriptional and Uptake Rate Profiles in Roots of Two Maize Inbred Lines with Different NUE

Nitrogen use efficiency (NUE) of crops is estimated to be less than 50%, with a strong impact on environment and economy. Genotype-dependent ability to cope with N shortage has been only partially explored in maize and, in this context, the comparison of molecular responses of lines with different NUE is of particular interest in order to dissect the key elements underlying NUE. Changes in root transcriptome and NH₄⁺/NO₃^- uptake rates during growth (after 1 and 4 days) without N were studied in high (Lo5) and low (T250) NUE maize inbred lines. Results suggests that only a small set of transcripts were commonly modulated in both lines in response to N starvation. However, in both lines, transcripts linked to anthocyanin biosynthesis and lateral root formation were positively affected. On the contrary, those involved in root elongation were downregulated. The main differences between the two lines reside in the ability to modulate the transcripts involved in the transport, distribution and assimilation of mineral nutrients. With regard to N mineral forms, only the Lo5 line responded to N starvation by increasing the NH₄⁺ fluxes as supported by the upregulation of a transcript putatively involved in its transport.

Exogenous Application of Amino Acids Improves the Growth and Yield of Lettuce by Enhancing Photosynthetic Assimilation and Nutrient Availability

As natural plant growth stimulators, amino acids are widely used to improve the yield and quality of crops. Several studies have illustrated the effects of different amino acids on lettuce plant parts. However, the effects of applying single amino acids on root growth remain elusive. The objective of this study was to evaluate the effect of root application of L-methionine on the growth of lettuce. In this study, two successive experiments on butterhead lettuce were conducted under hydroponic conditions. Three amino acids, L-methionine (20 mg/L), L-glycine (210 mg/L), and L-tryptophan (220 mg/L), were applied separately. L-methionine significantly increased the growth performance by 23.60%, whereas growth using L-tryptophan and L-glycine decreased by 98.78% and 27.45%, respectively. Considering the results of the first experiment, a second experiment was established with different concentrations of L-methionine (2200 mg/L, 220 mg/L, 22 mg/L, 2.2 mg/L, 0.2 mg/L, and 0.02 mg/L). The plants were allowed to grow for four weeks. Leaf width, plant area, leaf area, chlorophyll contents, etc., were evaluated. The results show that plant growth significantly improved by applying L-methionine at the lowest concentrations of 0.2 mg/L and 0.02 mg/L, which can, therefore, improve hydroponic production of lettuce and, accordingly, human nutrition.

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.

Spectroscopic-Chemical Fingerprint and Biostimulant Activity of a Protein-Based Product in Solid Form

A solid biostimulant (AA309) obtained through thermobaric hydrolysis applied on trimmings and shavings of bovine hides tanned with wet-blue technology was chemically characterized, and its effects in maize (Zea mays L.) were evaluated. AA309 contained 13.60% total nitrogen (N), mainly in organic forms (13.40%), and several amino acids, especially lysine, phenylalanine, glycine, aspartate, and isoleucine. AA309 was further analyzed using Fourier Transform Infrared (FT-IR) spectroscopy, which revealed the presence of amide I and amide II bands, indicative of peptide structures. When supplied to maize plants for 15 days at two N dosages (2.1 or 4.2 mg/kg), AA309 induced positive physiological responses, likely because of its content in amino acids functioning as signaling molecules. The low dosage was the most effective in improving leaf (+24%) and root (+98%) dry weight, photosynthetic activity (+70%), and accumulation of N (+80%), proteins (+65–75%) and antioxidants (+60%). Spectroscopic analyses (Solid-state Cross-Polarization Magic Angle Spinning Carbon-13 Nuclear Magnetic Resonance, CP/MAS ¹³C–NMR, and High resolution-magic angle spinning nuclear magnetic resonance, HR-MAS NMR) on plant tissues revealed the increase in proteins, lignin structures and cutin in AA309-treated plants compared to untreated plants. Our results indicate that AA309 could be used as a valuable biostimulant in agriculture.

Seed and Foliar Application of Amino Acids Improve Variables of Nitrogen Metabolism and Productivity in Soybean Crop

The application of amino acids in crops has been a common practice in recent years, although most of the time they are associated with products based on algae extracts or on fermented animal or vegetable wastes. However, little is known about the isolated effect of amino acids on the development of crops. Therefore, the objective of this research was to evaluate the effect of the application of isolated amino acids on the in some steps of the soybean nitrogen metabolism and on productivity. Experiments were carried out in a greenhouse and in the field with the application of the amino acids glutamate (Glu), phenylalanine (Phe), cysteine (Cys) and glycine (Gly) and as a set (Glu+Phe+Cys+Gly), as seed treatment (ST), as foliar application (FA) and both (ST+FA), at the V4 growth stage. Evaluations consisted of nitrate reductase and urease activities, nitrate, ureide, total amino acids and total nitrogen content in leaves, and productivity. The application of Glu to leaves, Cys as ST and a mixture of Glu+Cys+Phe+Gly as ST+FA in the greenhouse experiment increased the total amino acids content. In the field experiment all treatments increased the amino acid content in leaves. At the V6 stage in the field experiment, all modes of Gly application, Glu as ST and FA, Cys and Phe as ST+FA and Glu+Cys+Phe+Gly as FA increased the nitrate content in leaves. In the greenhouse, application of Cys and Phe as ST increased the production of soybean plants by at least 21%. The isolated application of Cys, Phe, Gly, Glu and the set of these amino acids as ST increased the productivity of soybean plants in the field experiment by at least 22%.

Comment on mRNA-Sequencing Analysis Reveals Transcriptional Changes in Root of Maize Seedlings Treated with Two Increasing Concentrations of a New Biostimulant

Overpopulation is already a reality, and the need for alternative technologies to meet a continuously increasing food demand has been much discussed around the world. In addition, soil decreasing fertility and desertification are obstacles that we will need to be overcome to increase crop productivity with a much-reduced dependence upon inorganic fertilizers. In this context, protein hydrolysates has emerged as an important strategy to reduce the use of inorganic fertilizers, whose applications as biostimulants for plant growth have shown very promising results.

Effects of an Animal-Derived Biostimulant on the Growth and Physiological Parameters of Potted Snapdragon (Antirrhinum majus L.)

To assess the effect a new animal-derived biostimulant on the growth, root morphology, nitrogen content, leaf gas exchange of greenhouse potted snapdragon, three treatments were compared: (a) three doses of biostimulant (D): 0 (D₀ or control), 0.1 (D_0.1), and 0.2 g L^-1 (D_0.2); (b) two biostimulant application methods (M): foliar spray and root drenching; (c) two F₁Antirrhinum majus L. hybrids (CV): "Yellow floral showers" and "Red sonnet." The treatments were arranged in a randomized complete-block design with four replicates, with a total of 48 experimental units. Plant height (+11%), number of shoots (+20%), total shoot length (+10%), number of leaves (+33%), total leaf area (+29%), and number of flowers (+59%) and total aboveground dry weight (+13%) were significantly increased by the biostimulant application compared to the control, regardless of the dose. The lowest dose resulted in the best effect on the ground plant dry weight (+38%) and, in order to the root system, on total length (+55%), average diameter (+36%), volume (+66%), tips (+49%), crossings (+88%), forks (+68%), projected (+62%), and total surface area (+28%). Compared to the control, plants treated with the biostimulant significantly enhanced leaf (+16%) and root (+8%) nitrogen content, photosynthetic rate (+52%), transpiration rate (+55%), and stomatal conductance (+81%), although there were no changes in dark-adapted chlorophyll fluorescence. Differences in the application method were not evident in the aboveground morphological traits, except in the plant shoot number (root drenching: +10%). The foliar spray compared to root drenching had a significant effect only on flower dry weight (3.8 vs. 3.0 g plant^-1). On the other hand, root drenching had a positive effect on ground dry weight (2.7 vs. 2.3 g plant^-1), root morphology, leaf-N and root-N content (+3%), transpiration rate (+21%), stomatal conductance (+40%), concentration of CO₂ in intracellular spaces (+11%), as well as on the efficiency of Photosystem II (+11%). A higher pot quality was obtained in "Red sonnet" compared to "Yellow floral shower." Based on our findings, applying the biostimulant to potted snapdragon at the lowest dose, as part of a fertilizing regime, improves the crop quality in an agro-environmental sustainable way.

Protein Hydrolysate Stimulates Growth in Tomato Coupled With N-Dependent Gene Expression Involved in N Assimilation

Plant-derived protein hydrolysates (PHs) have received increased attention in the last decade because of their potential to improve yield, nutritional quality as well as tolerance to abiotic stressors. The current study investigated the effects and the molecular mechanisms of a legume-derived PH under optimal and sub-optimal nitrogen (N) concentrations (112 and 7 mg L^-1, respectively) in tomato (Solanum lycopersicum L.). Growth and mineral composition of tomato plants treated with PHs by foliar spray or substrate drench were compared to untreated plants. In addition, the expression was determined of genes encoding ammonium and nitrate transporters and seven enzymes involved in N metabolism: nitrate reductase (NR), nitrite reductase (NiR), glutamine synthetase 1 (GS1), glutamine synthetase 2 (GS2), ferredoxin-dependent glutamate synthase (GLT), NADH-dependent glutamate synthase (GLS), and glutamate dehydrogenase (GDH). The root and total plant dry weight, SPAD index and leaf nitrogen content were higher by 21, 17, 7, and 6%, respectively, in plants treated by a substrate drench in comparison to untreated tomato plants, whereas foliar application of PH gave intermediate values. PH-treated plants grown with lower N availability showed reduced expression of NR and NiR as well as of nitrate and ammonium transporter transcripts in both leaf and root tissues in comparison with untreated plants; this was especially pronounced after application of PH by substrate drench. Conversely, the transcript level of an amino acid transporter gene was up-regulated in comparison with untreated plants. At high N regime, the transcript levels of the ammonium and amino acid transporters and also NR, NiR, and GLT were significantly up-regulated in root after PH foliar and substrate drench applications compared with untreated plants. An up-regulation was also observed for GS1, GS2, and GDH transcripts in leaf after substrate drench. These results highlighted the potential benefits of using legume PH in vegetable production systems to increase growth and N-nutritional status of plants.

mRNA-Sequencing Analysis Reveals Transcriptional Changes in Root of Maize Seedlings Treated with Two Increasing Concentrations of a New Biostimulant

Biostimulants are a wide range of natural or synthetic products containing substances and/or microorganisms that can stimulate plant processes to improve nutrient uptake, nutrient efficiency, tolerance to abiotic stress, and crop quality ( http://www.biostimulants.eu/ , accessed September 27, 2017). The use of biostimulants is proposed as an advanced solution to face the demand for sustainable agriculture by ensuring optimal crop performances and better resilience to environment changes. The proposed approach is to predict and characterize the function of natural compounds as biostimulants. In this research, plant growth assessments and transcriptomic approaches are combined to investigate and understand the specific mode(s) of action of APR, a new product provided by the ILSA group (Arzignano, Vicenza). Maize seedlings (B73) were kept in a climatic chamber and grown in a solid medium to test the effects of two different combinations of the protein hydrolysate APR (A₁ and A_1/2). Data on root growth evidenced a significant enhancement of the dry weight of both roots and root/shoot ratio in response to APR. Transcriptomic profiles of lateral roots of maize seedlings treated with two increasing concentrations of APR were studied by mRNA-sequencing analysis (RNA-seq). Pairwise comparisons of the RNA-seq data identified a total of 1006 differentially expressed genes between treated and control plants. The two APR concentrations were demonstrated to affect the expression of genes involved in both common and specific pathways. On the basis of the putative function of the isolated differentially expressed genes, APR has been proposed to enhance plant response to adverse environmental conditions.

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.