Climate stress tolerance can be managed through increased crop inputs (water, nutrients and soil amendments) and through improved management technologies, such as conservation tillage, shading and frost prevention, each of which can help mitigate environmental stress. While these approaches can be effective, rapid and flexible, they are time-intensive and depend upon the availability and cost-effectiveness of the needed inputs.
The application of exogenous chemicals including microbial products, biofertilizers and biostimulants for the management of plant stress responses is an area of tremendous interest and unmet potential that has the added benefit of being rapid and targeted with a generally low cost of implementation. The use of biostimulants and plant hormones to achieve climate stress tolerance is however, strongly constrained by a lack of understanding of the mechanisms involved and by regulatory restrictions that constrain the use of any product that explicitly targets plant growth and development processes. This constraint applies even if the changes in plant growth and development or plant hormone levels that result from product application do not differ from those that occur naturally in well-adapted species.
While biostimulants have been gaining acceptance as a mechanism to enhance crop resilience, many biostimulant products have been observed to have variable benefits, resulting in commercial uncertainty which constrains adoption. This occurs in part because biostimulant response depends on unpredictable plant stress events and complex soil and genetic interactions. Uncertainty is exacerbated by inadequate understanding of modes of action, in part because regulatory constraints discourage producers from pursuing a full understanding of the mechanisms involved for fear of disclosing plant growth regulatory effects.
The roles of exogenous hormones in crop stress regulation
Exogenous bioactive compounds, whether naturally occurring or chemically synthesized, have been used to help alleviate plant stress. These compounds can be applied directly or induced through microbial inoculation with uptake via multiple pathways, including passive diffusion, active transport through plasma membrane transporters, endocytosis, stomata and via symbiotic mechanisms like nodulation and mycorrhiza. Table 1 illustrates how the exogenous application of these compounds, including both agonists and antagonists, enhances stress tolerance across different crop species.
| Plant hormone | Chemical nature | Compound name | Treated crop | Stress condition |
| Abscisic acid | Natural occurring | (+)-cis,trans-ABA (S-ABA) | Maize | Drought |
| Rice | Salt | |||
| Synthetic agonist | Quinabactin | Soybean, barley, maize | Drought | |
| Opabactin | Wheat | Drought | ||
| Auxin | Natural occurring | Indole-3-acetic acid (IAA) | Rice | Drought and heat |
| Synthetic antagonist | Naphthaleneacetic acid (NAA) | Chufa | Alkaline | |
| Pea | Drought | |||
| 2,4-Dichlorophenoxyacetic acid (2,4-D) | Wheat | Salt | ||
| Brassinosteroid | Natural occurring | 24-Epibrassinolide (EBL) | Wheat | Drought |
| Perennial Ryegrass | Salt | |||
| Synthetic agonist | BB16 | Strawberry | Salt, drought | |
| Cytokinin | Synthetic agonist | 6-Benzyladenine (6-BA) | Maize | Flooding |
| Winter wheat | Heat | |||
| Kinetin | Common sage | Salt | ||
| N-(2-chloro-4-pyridyl)-N′-phenylurea (CPPU) | Rice | Drought | ||
| Rice | Salt | |||
| Synthetic antagonist | PI-55 | Bulbine natalensis Baker and Rumex crispus L. | Cadmium | |
| 2-Chloro-6-(3-methoxyphenyl)aminopurine (INCYDE) | ||||
| Ethylene | Synthetic agonist | Ethephon | Mustard | Salt |
| Synthetic antagonist | Aminoethoxyvinylglycine (AVG) | Cotton | Flooding | |
| Persimmon | Cold storage | |||
| 1-Methylcyclopropene (1-MCP) | Rice | Salt | ||
| Gibberellin | Natural occurring | GA3 | Mustard | Salt |
| Wheat | Salt | |||
| Synthetic antagonist | Chlormequat chloride (CCC) | Rice | Salt | |
| Mepiquat chloride (MC) | Cotton | Salt | ||
| Flurprimidol | Red Firespike | Drought | ||
| Trinexapac-ethyl | Perennial ryegrass | Drought | ||
| Kentucky bluegrass | Drought | |||
| Paclobutrazol (PBZ) | Pomegranate | Cold | ||
| Wheat | Heat stress due to late Sow | |||
| Strawberry | Drought | |||
| Jasmonic acid | Agonist | Methyl jasmonate (MeJA) | Black locust | Salt |
| Peach | Cold storage | |||
| Salicylic acid | Natural occurring | 2-Hydroxybenzoic acid | Maize | Drought |
| Mustard | Salt | |||
| Acetylsalicylic acid (ASA) | Maize | Cold | ||
| Strigolactone | Agonist | GR24 | Cucumber | Salt |
| Barley | Cadmium | |||
| Maize | Drought | |||
| Apple | Saline-alkali | |||
| Sweet orange | Cold storage | |||
| Cotton | Salt |
Naturally occurring auxins, including indole-3-acetic acid (IAA), 4-chloroindole-3-acetic acid, indole-3-butyric acid (IBA) and phenylacetic acid (PAA), have been utilized to manipulate plant stress responses examples of which included the exogenous application of IAA improved grain yield in rice under drought and heat stress by enhancing pollen viability and spikelet fertility and enhanced protective mechanisms and resilience in vegetative growth under salinity stress of rapeseed seeds soaked in IBA. While synthetic and natural auxins are primarily used for growth promotion or inhibition, auxin also plays a crucial role in regulating crop stress responses. For example, the application of naphthaleneacetic acid (NAA) induces early IAA-dependent accumulation of H2O2, enhancing antioxidant capacity and improving drought stress tolerance in soybean seedlings.
Abscisic acid occurs as the (+)-cis,trans-isomer (S-ABA), a chiral structure that is challenging to synthesize and is commercially produced through the fermentation of phytopathogenic fungi. The application of S-ABA has been shown to enhance stress tolerance in various crops. In salt-treated rice, S-ABA improved photosynthesis by reducing Na+ accumulation in leaves and increasing the activity of antioxidant enzymes. Similar stress-alleviating effects have been observed in maize and apple under drought conditions. Additionally, synthetic receptor agonists of abscisic acid, such as pyrabactin analogs (e.g. quinabactin and opabactin), have been developed to selectively control stomatal conductance, offering potential applications for reducing water loss during drought stress.
Brassinosteroids include c. 70 polyhydroxylated sterol derivatives. Despite significant efforts to chemically synthesize brassinosteroids, only two brassinosteroid analogs, EBL and BB16, are widely used in agriculture. Foliar applications of both EBL and BB16 have been shown to enhance fruit production and improve fruit quality in strawberry plants subjected to salt and water stress and have shown stress protective effects on rice and lettuce.
Natural cytokinins are N6-substituted adenine derivatives, including compounds such as N,N′-diphenylurea, N6-[(3-methylbut-2-en-1-yl)amino]purine (iP), zeatin, Z-9-riboside and meta-Topolin, all of which are isolated from plant materials. While many synthetic 6-benzyladenine (6-BA)-containing compounds with CK-like bioactivities have been reported, only a few are commercially available due to challenges in achieving efficient synthesis. Exogenous application of 6-BA has been shown to alleviate waterlogging-induced damage in maize by activating the ROS scavenging system and promoting plant growth. Additionally, two novel cytokine derivatives, PI-55 and INCYDE, have been developed to block cytokine signaling and inhibit cytokine degradation by targeting cytokine oxidase/dehydrogenase. These treatments prolong cytokinin activity in plants and positively affect shoot and root growth, as well as fresh weight, in medicinal plant seedlings grown in the presence of cadmium.
Ethylene applications, inhibitors or antagonists are commonly used to promote rapid and uniform ripening in fruits but also have effects on plant stress responses. Mustard plants treated with ethephon, an ethylene agonist, under salt stress showed a significant reduction in oxidative damage and an enhancement in photosynthetic nitrogen use efficiency. Aminoethoxyvinylglycine (AVG) and 1-methylcyclopropene (1-MCP) are two effective ethylene antagonists that inhibit ethylene biosynthesis and signaling pathways respectively. Treatments with AVG and 1-MCP prevented weight loss and reduced the decline in soluble solids content during cold storage of persimmon fruit by minimizing the incidence and severity of peel blackening and softening. AVG and 1-MCP also function as stress protectors during the growth period of crops such as rice, cotton and apple.
Nearly 140 different gibberellins have been identified in higher plants and fungi. Commercially, gibberellins are typically obtained through the fermentation of phytopathogenic fungi, with GA3 being the most widely produced form, followed by GA4 and GA7. The application of GA3 has been shown to improve grain yield in salt-stressed wheat, with the effect being particularly pronounced in salt-sensitive cultivars. Several gibberellin biosynthesis inhibitors act as growth retardants by reducing longitudinal shoot growth, resulting in a more compact plant architecture that lowers the risk of lodging and improves tolerance to various abiotic stresses. Strawberries treated with the gibberellin inhibitor Paclobutrazol (PBZ) under drought conditions exhibited enhanced enzymatic and non-enzymatic antioxidant activities, increased relative water content and improved photosynthetic rate, resulting in higher fruit yield.
Among jasmonates, (+)-7-iso-JA-L-isoleucine (JA-Ile), a stereoisomer conjugated with the amino acid isoleucine, has been recognized for its bioactivity in plants upon exogenous application. The oxidative damage caused by salt stress in black locust was alleviated by the exogenous application of Methyl jasmonate (MeJA), which significantly enhanced the activities of antioxidant enzymes. Crops such as beans, cauliflower and okra have improved resistance to various stresses through the application of MeJA.
Natural salicylic acid (SA) is primarily produced by plants, and its analogs are widely used as pharmaceuticals due to their anti-inflammatory properties. SA or acetylsalicylic acid (ASA) treatments, applied through seed soaking or root treatment, significantly enhanced the tolerance of maize seedlings and young plants to chilling stress. The stress-tolerant properties of SA have been observed in crops like rice, tomato and sunflower.
Biostimulants and their role in abiotic stress tolerance
Biostimulants have emerged as promising products to enhance crop resilience against abiotic stresses. According to the International Standards Organization definition, biostimulants are ‘product(s) that contain substance(s), microorganism(s), or mixtures thereof, that, when applied to seeds, plants, the rhizosphere, soil, or other growth media, act to support a plant’s natural nutrition processes independently of the biostimulant’s nutrient content’. Biostimulants can contain various natural compounds of microbial or non-microbial origin, as well as beneficial microorganisms, including bacteria, fungi and yeasts. The most widely studied biostimulants include those derived from seaweed, humic substances, protein hydrolysates and living microbes. A recent meta-analysis assessed the effectiveness of various biostimulants on crop yield improvement and estimated an increase of c. 18%. Table 2 summarizes some examples of different biostimulants and their role in plant stress mitigation. Here, we highlight some of the studies that provide experimental evidence for a direct mechanistic effect and discuss the possible mode of action of the different compounds.
| Biostimulant category | Natural source | Active ingredient | Treated crop | Stress condition |
| Seaweed extracts | Ascophyllum nodosum | IAA, GA, CK, phenols, biopolymers, sugars | Tomato | Drought |
| Sargassum spp. | IAA, phenols, biopolymers, sugars | Tomato | Salt | |
| Sargassum wightii | IAA, GA, CK, phenols, biopolymers, sugars | Okra | Salt | |
| Garlic extract | Allium sativum | IAA, GA, ABA, Kinetin, ascorbic acid, sugars | Broad bean | Drought |
| Carrot extract | Daucus carota | IAA, GA, ABA, CK, ascorbic acid, sugars | ||
| Moringa extracts | Moringa oleifera | IAA, GB, CK, ABA, sugars, phenols, ascorbic acid | Common bean | Salt, heat |
| Protein hydrolysate | Sugar cane molasses and yeast extract (Saccharomyces cerevisiae) | Glycine betaine, peptides, amino acids | Tomato | Drought |
| Pumpkin seeds | Amino acids, peptides | Common bean | Salt | |
| Humic acid | Leonardite | Humic acid | Finger millet | Salt |
| Non-specified | Humic acid | Rice | Salt | |
| Organic matter | Humic acid | Maize | Drought | |
| PGPB | Azotobacter vinellandii SRI Az3 | IAA, GA | Rice | Drought |
| Bacillus amyloliquefaciens RWL-1 | ABA | Rice | Salt | |
| Bacillus cereus SA1 | IAA, GA | Tomato | Drought | |
| Bacillus strains | IAA | Rice | Drought | |
| Ensifer meliloti RD64 | IAA | Alfalfa | Drought | |
| Enterobacter cloacae | ACC deaminase | Wheat | Salt, heavy metal | |
| Leclercia adecarboxylata MO1 | IAA | Tomato | Salt | |
| Pseudomonas azotoformans | ET | Tomato | Salt | |
| Pseudomonas fluorescens G20-18 | CK | Tomato | Drought | |
| Pseudomonas putida H-2-3 | GA | Soybean | Drought | |
| Pseudomonas sp. UW4 | ET | Tomato | Salt | |
| Streptomyces sp., Pseudomonas sp. | Polysaccharide | Wheat | Salt | |
| AMF | Rhizophagus irregularis | IAA, CK, GA, ET | Black locust | Drought |
| Funneliformis mosseae | Biopolymers, chelating compounds | Maize | Heat | |
| Endophytic fungi | Paecilomyces formosus LHL10, Penicillium funiculosum LHL06 | Chelating compound | Soybean | Drought, heavy metal |
| Paecilomyces formosus LHL10 | GA, IAA | Cucumber | Salt | |
| Trichoderma spp. | GA, ABA, SA, IAA, CK | Wheat | Drought |
Nonmicrobial biostimulants
Seaweed-derived biostimulants, like Seaweed Extracts (SE), contain a rich array of bioactive compounds, including phytohormones like auxins, CKs and GAs, vitamins, amino acids and polysaccharides, which contribute to their beneficial effects. Tomato treated with Seaweed Extracts improved fruit yield under salinity conditions. The effect was partially attributed to the presence of phytohormones in the extract. The plant hormones auxin, GAs and CKs have been detected in Seaweed Extracts and can persist in the final applied extract, although in variable concentrations. For example, GAs and CKs have been observed to range from 0.3 to 4.7 μg g–1 and from 0.06 to 4.6 μg g–1 of seaweed dry weight, respectively, while auxin has been identified with concentrations between 0.01 and 12 μg g–1. Seaweed Extract has shown the capability to alter the endogenous levels of plant hormones. Seaweed Extracts from Kappaphycus alvarezii significantly increased endogenous ABA and CKs concentrations in durum wheat in both non-stressed and drought conditions. The observed increase of maize tolerance to drought stress upon Seaweed Extract application was attributed to both the increase of polyamines and to the stimulation of IAA and GA endogenous production.
Many Seaweed Extract biostimulants enhance stress resistance by boosting the plant’s antioxidant defenses, by triggering various physiological and biochemical pathways, such as the synthesis of osmoprotectants and thus reducing oxidative stress caused by abiotic factors. The stress-protective effect of Seaweed Extract against salt stress was associated with the presence of phenolic compounds that can both act as ROS scavengers and chelate toxic ions. Antioxidant and stress-protective effects have also been attributed to the presence of non-structural carbohydrates and biopolymers. Algal polysaccharides such as ulvans, alginates and fucans can act as elicitors by activating stress-related pathways in plants, thus resulting in hormone-like effects. In-soil application of algal polysaccharides improved salt stress tolerance of wheat, as a function of the molecular weight and the sulfate content of the polymers, and induced an antioxidant response by modulating Na+ uptake and mobilization within the crop and by regulating the expression of Na+ transporters. Finally, the presence of polyamines in Seaweed Extract can have osmoregulatory effects and protect crops in water-deficient conditions.
Plant extracts (PE) are usually concentrated liquids or powders extracted from various plant species. Some PE may contain natural plant growth regulators, such as auxins, CKs and GAs, which are an inevitable consequence of their plant origin. PE can also induce hormone production. The application of PE on common flax increased the endogenous levels of GA3 and IAA both at the shooting and rooting stages. Other bioactive compounds present in PE, such as sugars, can up-regulate growth-related genes while downregulating stress-related ones, and thus alleviate salt stress in rice. Extracts obtained from Moringa oleifera protected common bean from salinity and heat stress. PE also contains a variety of other bioactive compounds, including alkaloids, flavonoids, phenols, terpenoids and essential oils, which contribute to their biological properties.
Fulvic acids (FA), humic acids (HA) and humates are highly recalcitrant organic substances derived from the decomposition of plant and animal matter. Foliar application of HA improved drought resistance of wheat by increasing its antioxidant response. The stress-protective effects of HA and humates were attributed to a hormone-like activity leading to the activation of stress-dependent pathways. HA can help regulate osmotic pressure within plant cells, aiding in the maintenance of turgor pressure and overall cellular function during water stress and improving grain yield by 16% in two different maize genotypes exposed to drought stress. These HA-induced stress-protective effects were observed along with shifts in endogenous hormonal levels, including an increase of IAA and a decrease of ABA concentrations. The growth-stimulating effect of HA on wheat was also related to the modulation of endogenous hormonal pathways, as gene analysis revealed the HA-induced up-regulation of genes involved in the biosynthesis of auxin and CKs.
Protein hydrolysates (PH) are derived from the enzymatic or chemical hydrolysis of proteins from various sources, including animal byproducts, plant biomass and microbial fermentations of various carbon compounds. PH are usually rich in amino acids and peptides, which act as biostimulants through both soil and foliar application. PH stimulate root and shoot development and promote overall plant vigor, which is crucial for coping with abiotic stressors like drought and salinity. In some cases, the presence of tryptophan – an auxin precursor – in PH has been attributed to the stimulation of auxin-like responses resulting in the promotion of seed germination and plant growth of pea (Pisum sativum L.). Root growth promotion by PH can enhance the plant’s ability to access water and nutrients, improving resilience against drought and nutrient deficiencies. The PH treatment of water-stressed tomato plants was associated with increased GA1, GA3 and IAA endogenous levels and decreased ABA concentrations. Metabolic analyses suggested that PH of various origins alter both phytohormone profiles and fatty acid metabolism of the treated plants.
PH can influence plant stress response by mechanisms not obviously related to hormone metabolism. PH improve nutrient solubility and availability, facilitating the uptake of essential nutrients by plants during stress conditions. Seed treatment with PH-protected tomato plants from heat and drought-induced damage by preserving yield and quality traits. These protective effects were observed along with an increase in antioxidant content within the plant. The application of PH to maize seedlings in a hydroponic system improved the plant’s tolerance to salt, nutrient deficiency and hypoxia stress conditions. The effects were traced back to a PH-related modulation of nitrate transporters and ROS gene expression. PH function can also vary depending on the source of hydrolysates. For example, 2 out of 11 PH tested as seed primers resulted in salt stress-alleviating properties. By contrast, different types of PH had similar but crop-dependent salt stress alleviating effects.
Microbial biostimulants
Interest in the use of plant microbial inoculation to enhance crop stress tolerance is founded on the observation that the imposition of abiotic stress often results in functional changes to the plant microbiome that can enhance plant stress tolerance, and from the observation that stress-tolerant species are often associated with specific microbial partners critical for the tolerance of those species to the stress. Microbiome shifts under plant stress vary with the specific abiotic stress conditions, with some ‘core species’ remaining preserved, indicating a strict relationship between the plant and its associated bacteria. Drought conditions enriched Actinobacteria over other Bacteroidetes and Proteobacteria in bulk soil, due to the higher resistance of Actinobacteria to drought conditions. The inoculation of stress-tolerant microbes to the plant has also been shown to increase abiotic stress tolerance. Microbial biostimulants, including both bacteria and fungi, have the potential to synthesize IAA, as predicted from the analysis of 7282 prokaryotic genomes and empirical evidence and the beneficial effect of microbial biostimulants has often been attributed to the production of phytohormones. It has been widely hypothesized that treating plants with stress-tolerant-plant-beneficial microbes can help restore stress-induced microbiome imbalances and contribute to stress alleviation.
The in-vitro inoculation of rice seedlings with the rhizobacterium Bacillus altitudinis resulted in a phenotypic modification of root architecture that was attributed to a change in IAA endogenous levels within the root and to the genetic modulation of auxin-responsive genes involved in root formation. A similar increase in endogenous IAA was observed upon the inoculation of wheat with rhizobacteria, including Dietzia natronolimnaea, Arthrobacter protophormiae and Bacillus subtilis, resulting in an increased tolerance against both drought and salinity. A bacterial consortium composed of Staphylococcus epidermidis CK9 strain and Bacillus australimaris CK11 inoculated on Arabian balsam tree (Commiphora gileadensis), improved tolerance to both salinity and drought stress and decreased endogenous levels of ABA and JA, while stimulating SA accumulation. Further investigations suggested that microbial biostimulants can influence the expression of the TaCTR1 gene, involved in plant response to various stress types. The inoculation of Bacillus casamancensis MKS-6 and Bacillus sp. In this case, the presence of bacterial phytohormones has been shown to also counteract the growth-inhibition effect of abiotic stress. In durum wheat exposed to drought and salinity stress in a greenhouse experiment, plants treated with a microbial consortium were more tolerant compared to untreated plants. Results suggested that microbial biostimulant inoculation also improved the grain quality of wheat, enhancing protein, sugars and lipid content under stress.
Fungal biostimulants have also been effective in plant stress mitigation. Seed pretreatment with the fungal strain Trichoderma lixii improved plant and root development and osmolyte accumulation of maize exposed to salt stress. It was proposed that Trichoderma can both trigger various stress defense responses in plants and stimulate plant rooting and growth parameters via the production of IAA, as well as adsorb and chelate toxic ions via the production of siderophores, including excess Na + as the possible consequence of altered ion mobility.
Summary and the implications of regulatory constraints on the development and use of biostimulants, microbials and plant hormones to enhance climate resilience
The increased occurrence and severity of extreme weather events is among the greatest threats to agricultural productivity globally. Extreme weather events not only reduce crop productivity by compromising crop photosynthesis, metabolism and growth, they also disrupt normal agronomic practices and hence compromise farming efficiency and profitability. Mitigating unpredictable and highly localized extreme weather events will require the development of technologies that can be rapidly and locally implemented to enhance the resilience of the crop to the impending stress. These ‘rapid response’ technologies will supplement longer-term breeding and cropping system strategies that aim to enhance the natural resilience of the crop and the cropping system. Biostimulants represent powerful tools to achieve this goal.
Plant tolerance to climate stress is largely mediated through plant sensing mechanisms and the regulation of plant hormone pathways. Plant breeding, agronomic inputs (nutrients and water), plant hormones and biostimulants have all been used to improve plant stress tolerance often mediated through interactions with natural plant hormone networks. Progress in the utilization of biostimulants, microbials and plant hormones to address the challenges of climate stress is constrained by regulatory frameworks that in many parts of the world classify all products that contain or explicitly modulate plant growth regulator pathways as pesticides. The classification of a product as a pesticide is associated with very strict safety requirements that lead to a substantial financial burden on product registration and commercial use; therefore, limiting the development and use of such products in agriculture.
The term ‘plant regulator’ is further defined as: ‘any substance or mixture of substances intended, through physiological action, for accelerating or retarding the rate of growth or rate of maturation or altering the behavior of plants.’ It is specifically stated in Environmental Protection Agency (EPA) rulings that all known plant hormones are considered pesticides and that products that contain known plant hormones or that explicitly claim to act through the modification of these pathways would be deemed pesticidal.
Conclusions and future prospectives
Biostimulants, plant hormones and microbial inoculants have tremendous potential as tools that are more flexible and rapidly implementable alternatives to breeding. Coupling the application of biostimulants or plant hormones with weather prediction and just-in-time precision application has the potential to reduce the negative effects of abiotic stress on crop production while offering a highly tailored solution to local challenges.
Dora Biostimulant products are designed based on natural active ingredients to support plants when they need specific physiological responses. Biological stimulants are developed through a lot of research and innovation, aiming to bring maximum vitality, yield, and quality to crops.
If you are interested in biostimulants, especially Plant Growth Regulators (PGRs), seaweed extracts or humic acids, you can contact Dora team.