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Background: Traditional agriculture is on the front line of climate change, being most impacted by the increase in the intensity and frequency of extreme events, such as floods, drought and rising temperatures. Local ecological knowledge is a recognized keystone of successfully managed socioecological systems, but loss of soil fertility, water scarcity, incidence of diseases and decreased production due to climate change are linked to the greater vulnerability experienced by traditional farmers. Plant biostimulants are natural products used to stimulate nutrient uptake and efficiency by crops, increase tolerance to abiotic/biotic stress and improve quality without negative impacts on the environment if obtained from renewed sources. Humic substances are some of the most used plant biostimulants in agriculture and play a central role in plant adaptation. Materials and methods: We reviewed and discussed a sample set of papers (n = 52) about humic substances to mitigate abiotic stress in crops using data basis from Web of Science (Clarivate Analytics), Scopus—IBM (International Business Machines Corporation), and Scielo (Scientific Electronic Library Online). Results: The predominance of authors in the global south is notable, but it is not a coincidence, since this is where the effects of climate change will have the greatest impact. The mechanisms involved in the stress mitigation involve the activation of signaling factors, gene response induction, the accumulation of osmoprotective and anti-oxidant compounds, the induction of antioxidative metabolism, ion homeostasis, membrane transport and adjustment of hormonal balance. The intriguing question is: how can a complex mixture of molecules affect so many distinct effects on plants responsible for plant adaptation? Conclusions: The complexity of humic substances challenges our knowledge method, but supramolecular chemistry may provide answers that enable us to broaden our understanding of the plant defense mechanisms modulated by these substances. [ABSTRACT FROM AUTHOR]

Background: Huanglongbing (HLB) is a devastating citrus disease. Before callose deposition, the bacterial infection causes oxidative stress, starting cell damage. Humic substances are among the most efficient antioxidants found in nature. Furthermore, it is also previously reported that humic substances can induce a phenylpropanoid metabolism contributing to the production of antimicrobial compounds. It has been noted by technicians and growers in the Brazil's main citrus belt that orchards continuously treated with humic substances can live well with the symptoms of HLB. Methods: We treated two young citrus orchards (Citrus sinensis cultivars Baía and Pera) with soluble humic substances (HS) isolated from peat either combined or not with a plant growth-promoting bacteria consortium. The activity of key enzymes related to the response against biotic stresses, such as peroxidase (POX), β-1,3-glucanase (PR) and phenylalanine ammonia-lyase (PAL), as well as the differential transcription level of target genes linked to plant stress response by qRT-PCR was monitored for six months. Results: No differences were obtained between sole humic substances and their application combined with plant growth-promoting bacteria, indicating that the microbial consortium had no modulatory effect on HS-treated plants. The treatments promoted the activities of POX, PAL and β-1,3-glucanase, which remained significantly higher concerning the control throughout the evaluation period. In addition, treatments positively regulated the transcription levels of CsPR-7, CsPR-3 and CsPR-11 genes. The experimental data were qualitatively similar to those found in commercial orange orchards treated continuously for 12 and 6 years with humic substances, where the activity and transcription levels were also more significant than for untreated plants. Conclusion: Our results indicate that humic substances can trigger immune-mediated responses in plants and they can be used as a natural chemical priming agent to mitigate disease symptoms and contribute to more resilient citrus cultivation. [ABSTRACT FROM AUTHOR]

The use of humic substances in agriculture as a biostimulant emerged as one of the promising methods to promote sustainable production. Different molecular, biochemical, and physiological processes are triggered, resulting in nutrient efficiency use and protection against abiotic stress. Understanding plant changes promoted by humic substances is essential for innovative and tailored biostimulation technologies. Cell metabolites are the final target of the response chain, and the metabolomic approach can be helpful in unveiling pathways related to plant response. This study aimed to evaluate a global metabolic alteration of rice leaves induced by humic acids (HA) applied in a hydroponics system. Using 1H NMR and GC-TOF/MS analysis, we observed a significant decrease in all main metabolites classes in leaves treated with HA, including lipids, organic acids, amino acids, and carbohydrates. Metabolites in higher concentrations in HA-treated plants are candidates as markers of HA bioactivity, including amino acids, intermediates of tricarboxylic acid cycle, and lipids, and aromatic compounds related to plant-stress response. [ABSTRACT FROM AUTHOR]

Background: The bacterial-spot disease caused by different Xanthomonas species is one of the major tomato diseases that reduce crop production and quality. Pesticides indiscriminate usage has resulted in an increase in resistant bacterial strains as well as contamination of farmers, consumers and the environment. Plant growth-promoting bacteria and humic acids can act as elicitors of plant defence mechanism causing extensive transcriptional and metabolic reprogramming which, in turn, produce a range of plant chemical defences. The purpose of this study was to study how humic acids and plant growth-promoting bacteria, when applied to the substrate, affected the severity of bacterial spot symptoms in tomato leaves. Materials and methods: One-month-old Micro-Tom tomato (Solanum lycopersicum L.) were transferred to 3 L pots filled with a sterile mixture of sand and vermiculite (2:1, v:v) and treated or not (control) with 250 mL of 4.5 mmol C. L−1 of humic acids, Herbaspirillum seropedicae (108 CFU. mL−1) and the combination of humic acids plus H. seropedicae. One day after substrate treatment, the leaves were inoculated (or not) with X. euvesicatoria (Xe). The area below the disease progression curve based on severity scores and the number of symptomatic leaflets was used to assess phytopathogen virulence. The concentration of oxalic, citric and succinic acids in leaf extracts were determined using HPLC analysis. Results: Sole or combined H. seropedicae (BAC) and humic acids (HA) application promoted shoot and root growth related to control when plants were challenged with Xe pathogen. For plants inoculated with Xe, more significant plant-growth promotion results were obtained for HA + BAC treatment. The first visible symptoms were observed 16 days after inoculation with 2 × 104 CFU. g−1 of Xe cells in leaves of control plants. HA and BAC applied alone or combined reduced disease severity. Only plants treated with HA were able to reduce disease incidence (number of the leaflets with symptoms). Organic acids, such as oxalic, citric and succinic acids, rose in Xe-inoculated leaves. The reduced amount of organic acids in diseased leaves treated with HA + BAC may be linked to a decrease in disease progression. Conclusion: Humic acids and H. seropedicae increased growth by modulating the content of organic acids in leaf tissue, attenuating the symptoms of the bacterial spot disease. [ABSTRACT FROM AUTHOR]