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.

Traditional agriculture represents the most-extensive food-producing segment in the world. However, these agroecosystems are widely and closely associated with rural poverty, reflecting the dualism between the subsistence and the commodity-producing sector in the peripheric countries. Therefore, socially adapted technologies may be a reliable and helpful methodology to enhance subsistence crop production. Humic substances are natural organic biostimulants extractable as water suspensions from renewable sources such as agricultural biomass and farming residues. These easy-to-handle extracts may be mixed with plant-growth-promoting bacteria (PGPB) and used as biostimulants within a low-cost technological application in the circular economy strategy. Few investigations have been focused on the use of biostimulant practices on marginal or subsistence crops. Cassava (Manihot esculenta Crantz) and okra (Abelmoschus esculentus) are two essential foods for poor communities of rural territories in tropical and subtropical countries. The aim of this study was to evaluate the effect of the foliar application of a humic/PGPB mixed biostimulant on cassava and okra crops grown in an agricultural soil with very low natural fertility. In pot trials, the applied biostimulant improved the plant development with a 200% increase of the root weight in cassava, while the preservation of active diazotrophic bacteria was improved by 10- and 100-times in cassava and okra in the mixed formulation with humic acid. In real field systems, the plant treatment increased the yield of cassava and okra by 70% and 50%, respectively thereby allowing a simultaneous nitrogen savings with the best yield performance obtained at the lower N fertilization rate. The use of biostimulants can play a role in the transition process, helping the food security and the autonomy of impoverished farmers. Combining the elements of traditional knowledge and modern science is essential to create innovative technologies enabling the sustainable management of agroecosystems.

Peats are an important reserve of humified carbon in terrestrial ecosystems. The interest in the use of humic substances as plant growth promoters is continuously increasing. The objective of this study was to evaluate the bioactivity of alkaline soluble humic substances (HS), humic (HA) and fulvic acids (FA) isolated from peats with different decomposition stages of organic matter (sapric, fibric and hemic) in the Serra do Espinhaço Meridional, state of Minas Gerais. Dose-response curves were established for the number of lateral roots growing from the main plant axis of tomato seedlings. The bioactivity of HA was greatest (highest response in lateral roots at lowest concentration) while FA did not intensify root growth. Both HS and HA stimulated root hair formation. At low concentrations, HS and HA induced root hair formation near the root cap, a typical hormonal imbalance effect in plants. Transgenic tomato with reporter gene DR5::GUS allowed the observation that the auxin-related signalling pathway was involved in root growth promotion by HA.

Intensive agriculture maintains high crop yields through chemical inputs, which are well known for their adverse effects on environmental quality and human health. Innovative technologies are required to reduce the risk generated by the extensive and harmful use of pesticides. The plant biostimulants made from humic substances isolated from recyclable biomass offer an alternative approach to address the need for replacing conventional agrochemicals without compromising the crop yield. The stimulatory effects of humic substances are commonly associated with plant hormones, particularly auxins. However, jasmonic acid (JA) is crucial metabolite in mediating the defence responses and governing plant growth and development. This work aimed to evaluate the changes in the biosynthesis and signalling pathway of JA in tomato seedlings treated with humic acids (HA) isolated from vermicompost. We use the tomato model system cultivar Micro-Tom (MT) harbouring a reporter gene fused to a synthetic promoter that responds to jasmonic acid (JERE::GUS). The transcript levels of genes involved in JA generation and activity were also determined using qRT-PCR. The application of HA promoted plant growth and altered the JA status, as revealed by both GUS and qRT-PCR assays. Both JA enzymatic synthesis (LOX, OPR3) and JA signalling genes (JAZ and JAR) were found in higher transcription levels in plants treated with HA. In addition, ethylene (ETR4) and auxin (ARF6) signalling components were positively modulated by HA, revealing a hormonal cross-talk. Our results prove that the plant defence system linked to JA can be emulated by HA application without growth inhibition.

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.

Plant biostimulants (PBs) have been considered the new wave for ecological intensification and sustainability, but are they sustainable? They increase nutrient use efficiency and reduce the impact of abiotic stress in plants. However, commercially available PBs based on humic substances are obtained using non-renewable sources of organic matter. At the same time, the microbial inoculants include a discussion of the properties of microorganisms and formulation design, as well as standards of purity and process control. Farmers depend on biological inputs like others to generate additional income for agribusiness. We produced a composite PB using humic substances isolated from vermicompost with KOH 5% and microbial consortia of plant growth-promoting bacteria (H. seropedicae, G. diazotrophicus, Bacillus spp.) grown in a simple medium with molasses and fishmeal as sources of C and N, respectively, in a homemade reactor at 37 °C for 36 h. The on-farm PB was applied directly in a passion fruit trial, and plant health and yield were monitored. The plants treated with the PB showed decreased visual symptoms of pests and diseases concurrent to higher activities of the enzymes used to monitor the induction of the plant resistance system (1,3-β glucanase, peroxidase, and phenylalanine ammonia-lyase). Plants treated with the PB yielded more than 50% more passion fruit than the control in soil with natural low fertility, fertilized with vermicompost. It is possible to produce PBs directly on the farm, leveraging locally available resources and simple technologies to sustainably enhance plant health and productivity.