Biostimulants represent a shift in crop production, functioning as biological catalysts that help awaken plant’s innate potential rather than simply supplying them with nutrients. These bioactive compounds which range from humic substances and polysaccharides to consortia of endophytic bacteria and arbuscular mycorrhizal fungi, operate through epigenetic mechanisms that trigger dormant metabolic pathways, amplifying nutrient absorption, fortify cell walls against pathogens. Conventional fertilizers which only supply nutrients linearly, biostimulants orchestrate a whole physiological change, they rewrite root architecture, unlock phosphorous locked in soil.
A plant biostimulant is any substance or microorganism of established biological origin applied to plants with the aim to enhance nutrition efficiency, abiotic stress tolerance and/or crop quality traits, regardless of its nutrients content. Unlike traditional crop protection chemical inputs, such as pesticides and herbicides, plant stimulants are unique, single products that possess multiple avenues for promoting crop growth. As there is widespread depletion of soil health due to the overuse of chemicals and fertilizers, an increasing number of farmers in the country are incorporating these biologicals into their farming regimes. Increased investment and the need to improve yields per hectare are expected to drive the India biostimulants market growth. In addition, as the demand for organic foods increases, so does the demand for organic farming, which is further predicted to aid the market’s growth in the country. The India biostimulants market size was valued at USD 355.53 million in 2024. The market is projected to grow from USD 410.78 million in 2025 to USD 1,135.96 million by 2032, exhibiting a CAGR of 15.64% during the forecast period.
A key advantage of biostimulants lies in their multilevel mode of action, affecting plant performance from early developmental stages to final yield and quality. Their application has been shown to improve germination, biomass accumulation, and crop quality through coordinated effects at the molecular, biochemical, and physiological levels. At the molecular level, biostimulants can modulate signallingpathways, gene expression and protein synthesis, leading to changes in primary and secondary metabolism. These molecular adjustments translate into enhanced physiological processes, including photosynthesis, respiration, transpiration, and antioxidative defense responses. Furthermore, biostimulants modulate phytohormonal balance and improve water and nutrient uptake, transport, and utilization efficiency, thereby contributing to improved plant growth and stress resilience
Despite recent efforts to clarify the regulatory status of biostimulants, there is no legal or regulatory definition of plant biostimulants anywhere in the world, including in the European Union and in the United States. Two main groups are distinguished: microbial and non-microbial plant biostimulants.
MICROBIAL BIOSTIMULANTS
Microbiological biostimulants are formulations containing non-toxic and non-pathogenic plant growth-promoting bacteria (PGPB) and plant growth-promoting fungi (PGPF). They may include a single microbial strain or a consortium of multiple organisms and can be applied through soil, foliar spraying, or seed coating.
Bacterial biostimulants include both rhizospheric and endophytic strains, commonly from the genera Rhizobium, Acetobacter, Azospirillum, Bacillus, or Pseudomonas. Bacteria-based biostimulants enhance nutrient availability, primarily through the production of organic acids and enzymes that solubilize mineral compounds, such as phosphorus, into plant-accessible forms. The most effective strains are often selected from endophytic bacteria belonging to Stenotrophomonas spp., Delftia spp., Brevundimonas spp.,and Novosphingobium spp. based on high enzymatic and metabolic activity, ACC (1-aminocyclopropane-1-carboxylic acid) deaminase activity, and production of various metal-complexing compounds. Strains of Burkholderia sp. and Phyllobacterium sp. are particularly promising due to their abilities to fix nitrogen, produce IAA (indole-3-acetic acid)-like compounds, synthesize extracellular polymers, and form biofilms.
Plants also assimilate bioactive metabolites produced by PGPR, such as cytokinins and auxins, which stimulate cell division and elongation. Furthermore, microbial activity in soil improves its physicochemical properties, which is beneficial not only for plants but also for the broader soil biotic community. Indirect plant growth promotion effects may also arise from their antagonistic activity against plant pathogens and pests, especially through the production of antibiotics and siderophores.
Fungal biostimulants employed as plant growth-promoting agents in agriculture and horticulture primarily include arbuscular mycorrhizal fungi and species belonging to the genus Trichoderma. AMF form symbiotic associations with up to 80% of terrestrial plants. Their hyphal networks absorb and transport mineral nutrients, such as nitrogen and phosphorus, to plant roots, which are essential for both roots and root hairs. They also improve water conditions, thereby influencing nutrient uptake by plant roots. Studies also indicate that AMF modulate the expression of genes involved in nitrogen uptake and metabolism and also produce phytohormones, such as indole-3-acetic acid and gibberellins, which promote plant growth.
Trichoderma spp. strains provide multiple benefits to plants and soil and have been widely applied in biocontrol, soil bioremediation, and industrial applications due to their production of numerous enzymes. They also exhibit plant growth-promoting activity. Studies have shown that cultivation with various Trichoderma spp. strains enhances the uptake of micro- and macro elements and induces plant resistance against phytopathogens. These fungi can also influence the expression of genes involved linked to photosynthesis and other key metabolic processes. Other fungi including Aspergillus spp., Penicillium spp., Coprinus spp., Sterigmatomyces spp., Acremonium spp., andPaecilomyces spp., play roles in mycoremediation, producing extracellular polymeric substances (EPS) and facilitating the biosorption of heavy metals such as Pb, Cd, Hg, Cu, and Co.
NON-MICROBIAL BIOSTIMULANTS
Humic Substances
Humic substances (HS), including humic and fulvic acids, are natural components of the soil organic matter derived from the decomposition of plant, animal, and microbial residues. Their application generally promotes plant growth, although the response varies with the source, plant species, environmental conditions, dose, and application method. HS improve root nutrition by increasing nutrient availability and uptake, stimulate plasma membrane H⁺-ATPases, promote cell enlargement and enhance respiration and invertase activity. They may also affect hormone-related processes and improve tolerance to environmental stress by modulating phenylpropanoid metabolism and stress-related enzymes. The meta-analysis reported an average increase of 22 ± 4% in shoot dry weight and 21 ± 6% in root dry weight after HS application.
Protein Hydrolysates and Other N-Containing Compounds
Protein hydrolysates mainly consist of amino acids and peptides formed by chemical or enzymatic hydrolysis of plant and animal materials, whereas other nitrogenous biostimulants include betaines, polyamines and non-protein amino acids. These compounds can enhance nitrogen uptake and assimilation through regulation of enzymes, genes, and root nutrient-acquisition pathways, and influence carbon and nitrogen metabolism. Proline and glycine betaine provide chelating and antioxidant effects that can enhance micronutrient acquisition and reduce the effect of environmental stresses. Protein hydrolysates also promote soil microbial activity, biomass and respiration, thus improving soil fertility. Their application has been associated with improvements in crop yield and quality, but concerns exist regarding the use of animal-derived hydrolysates in the food chain.
Seaweed Extracts and Botanicals
Seaweed extracts are a complex mixture of bioactive compounds, including laminarin, alginates, carrageenans, nutrients, sterols, betaines, and hormone-like substances. They represent a class of plant biostimulants that can be applied to the soil, hydroponic solution, or directly on the leaves, where they improve water retention and aeration, nutrient availability, and beneficial microflora activity. In plants, seaweed extracts can stimulate seed germination and early development, improve crop production, and enhance resistance to environmental stresses through antioxidant activity and hormone-like regulation of gene expression. The main mechanism of Ascophyllum nodosum, a common commercial seaweed species, appears to be the stimulation of plant hormone gene expression. Botanicals or plant extracts with biologically active compounds are another source of potential biostimulants, although their mode of action and efficacy have been less studied than that of seaweed extracts and pesticides.
Chitosan and Other Biopolymers
Chitosan is a deacetylated chitin polymer; its oligomers interact with cellular components and receptors participating in plant defence and stress response signalling cascades. Upon treatment, this biopolymer can trigger intracellular hydrogen peroxide accumulation and calcium signalling, thus modulating physiological and molecular plant responses. Chitosan has been extensively studied as a means to protect plants against fungal pathogens. Moreover, it has been shown to improve drought, salinity, and freezing tolerance as well as quality traits in several species. Chitosan can induce stomatal closure via an abscisic acid (ABA)-dependent pathway, thus helping plants withstand environmental stresses. Other elicitor biopolymers such as laminarin, a polysaccharide derived from seaweed, can also be considered for plant biostimulation and protection purposes since their signalling pathways are at least partially overlapped with those of chitosan.
Inorganic Compounds
Beneficial inorganic elements are chemical elements that can promote plant growth or improve stress tolerance even if they are not essential nutrients for all plant species. The major beneficial elements identified are aluminium (Al), cobalt (Co), sodium (Na), selenium (Se) and silicon (Si) which act on different ways depending on plant species or environments. They can improve cell wall integrity, regulate osmotic potentials, reduce transpiration rates, act as cofactors, improve nutrient uptake, enhance antioxidant protection and involve in plant hormone signalling. Beneficial inorganic salts including silicates, phosphites, phosphates and carbonates may influence osmotic, pH and redox balances and activate stress-related enzymes. Their biostimulant roles should be differentiated from their conventional roles of fertilizers or fungicides.
BIOSTIMULANTS IMPROVE TOLERANCE TO ABIOTIC STRESS
Different microbial and non-microbial biostimulants could be used to improve plant resistance to drought, salinity, extreme temperatures, nutrient deprivation, heavy-metal toxicity, and oxidative stress. Biostimulants could be utilized to optimize the crop physiology and biochemistry in order to increase resistance and productivity through multiple mechanisms. Thus, the application of antioxidant- and amino-acid-based biostimulants increased cowpea productivity under heat and water stress, while protein hydrolysates improved maize tolerance to drought, oxygen deprivation, and Fe deficiency. Moreover, the treatment of crops with glycine betaine, melatonin, polyamines, silicon, and various beneficial microorganisms combined with plant extracts enhanced photosynthesis, water-use efficiency, mineral contents, and yield under stress conditions.
The biostimulant activity of various types of arbuscular mycorrhizal fungi (AMF), plant growth-promoting rhizobacteria (PGPR), and beneficial endophytic bacteria on crop productivity under multiple abiotic stresses was reported. AMF inoculation in combination with compost application allowed quinoa to maintain high productivity under water stress. At the same time, the treatment of crops with PGPR and various strains of nitrogen-fixing bacteria improved plants’ phosphorus nutrition and adaptation to drought and decreased osmotic potential. Moreover, several strains of PGPR and AMF could increase crop productivity under Fe-deficient conditions probably due to enhanced Fe uptake and changes in phytohormone contents. The overall biostimulatory potential of these microorganisms was also manifested in their ability to increase the efficiency of photosystems and protect plants from oxidative stress.
The review also describes the beneficial impact of biostimulants on plant productivity under heavy-metal and oxidative stresses. Thus, silicon and iron-oxide nanoparticles improved the resistance of common bean to cadmium toxicity. Similarly, liquiritoside treatment alleviated lead poisoning of Chinese cabbage, while co-composted biochar application enhanced the biometric, physiological, and biochemical parameters of Brassica napus under chromium stress. Finally, rice-bran enzyme extract treatment improved the photosynthetic apparatus of pepper under ozone stress.
Salinity stress is another important aspect, where multiple studies have shown the positive effect of biochemical and microbial biostimulants. The application of 2-keto-L-gulonic acid, 5-aminolevulinic acid, protein hydrolysates obtained from plant sources decreased the damage from salt stress, while the inoculation with Pseudomonas, Bacillus, Rhizobium, Penicillium, and other bacteria and fungi increased the photosynthetic performance, growth, germination, and resistance to stress. The efficiency was reported to depend on the crop species, biostimulant concentration, and stress levels, which should be considered when selecting a product and the dose.
CONCLUSION
Biostimulants represent a promising strategy to enhance plant growth and resilience under both abiotic and biotic stresses, contributing to sustainable agriculture by improving physiological, biochemical, and molecular responses, including antioxidant activity, osmolyte accumulation, photosynthetic efficiency, and regulation of defense-related genes. However, their effectiveness strongly depends on product composition, production method, application timing, soil conditions, and plant developmental stage.
From an applied perspective, such advances will support the development of precision biostimulant strategies, optimizing input efficiency, reducing environmental impacts, and improving crop resilience under climate change. Importantly, these innovations should be aligned with harmonized regulatory and methodological frameworks to facilitate reliable product assessment and adoption in agricultural practice.
Author
Nishant Sidnal is a postgraduate student pursuing a Master’s degree in Genetics and Plant Breeding at Anand Agricultural University, Gujarat. Driven by curiosity and a passion for research, he enjoys exploring new concepts and translating theoretical knowledge into practical applications.
