Rhamnolipids(RLs) are glycolipid biosurfactants naturally produced by Pseudomonas and Burkholderia bacteria, well-known for surface-active properties and eco-friendliness. They reduce surface/interface tensions, inhibit pathogens, and have low toxicity, making them promising for agriculture—an industry needing greener practices by cutting synthetic inputs. This review covers their antimicrobial effects on phytopathogens (mainly fungi), ability to trigger plant defenses and protect crops, recent research on their role as plant biostimulants (improving soil health and abiotic stress tolerance), and prospects as biopesticides for sustainable agriculture.
1. Introduction
Global agriculture faces urgent challenges: boosting productivity/quality to feed a growing population amid climate change, while tackling diseases/pests that cause major yield losses. Synthetic pesticides/fertilizers, though widely used, harm human health, environment, and biodiversity. Thus, microbial biosurfactants (e.g., glycolipids, lipopeptides) are proposed as safer alternatives.
Rhamnolipids (RLs) are glycolipids—secondary metabolites of Pseudomonas (mainly pathogenic Pseudomonas aeruginosa) and some Burkholderia species—and the most studied microbial biosurfactants. They are low-molecular-weight amphiphilic compounds first isolated from P. aeruginosa fermentation, initially described as two L-rhamnoses linked to two β-hydroxydecanoic acids. Later, their structure was clarified: one/two hydrophobic alkyl chains connected via O-glycosidic bonds to one (mono-RLs) or two (di-RLs) rhamnose hydrophilic heads (linked by α−1,2-glycosidic bonds, Fig. 1).
As biosurfactants, RLs lower surface/interfacial tensions by reducing interphase repulsive forces. For producing bacteria, RLs support biofilm development, motility, antimicrobial activity, and nutrient assimilation via substrate solubilization. RL mixtures (from fermentation) contain mono-RLs and di-RLs; their composition depends on bacterial species/strain, carbon source, and culture conditions, leading to over 60 homologues with 8–16 carbon chains.
Surfactants are vital for industries (food, cosmetics, agriculture) due to surface-modifying, emulsifying, and other properties. To reduce synthetic surfactants’ impact, RLs have gained interest. They are mostly produced via P. aeruginosa fermentation (using soluble/insoluble carbon sources) with yields of 1–200 g L⁻¹. Scaling up production and cutting costs are key hurdles; solutions include genetic engineering of non-pathogenic strains and using low-cost renewable carbon sources (e.g., plant oils, agricultural wastes like sugarcane bagasse, rice straw).
RLs have a low Critical Micelle Concentration (CMC) for effective surface tension reduction and stability under extreme temperature (20–80 °C), pH (4.0–12.0), and salinity (up to 8% NaCl). They are biodegradable (aerobically fully, anaerobically partially—unlike synthetic Triton X-100) in soil, wastewater, and soil suspensions, and have low cytotoxicity, mutagenicity, and ecotoxicity.
Beyond surfactant properties, RLs have valuable biological activities for agriculture: direct antimicrobial effects on phytopathogens (via plasma membrane permeabilization/lysis, lowering resistance risk), ability to stimulate plant immunity, biostimulant potential (improving nutrient use, abiotic stress tolerance, crop quality), soil bioremediation, and soil health improvement (modifying physical structure and microbial communities). They also aid agrochemical formulation. Thus, RLs can reduce synthetic inputs, supporting sustainable agriculture. This review focuses on RLs’ roles in crop protection, biostimulation, soil health, and agrochemical formulation.
Fig. 1. Structural formulas of the main congeners produced by Pseudomonas sp. (a) Mono-rhamnolipid (b) Di-rhamnolipid. ©MolView.
2. Direct effects of Rhamnolipids against plant pathogens and pests
Biocontrol (or bioprotection) reduces phytopathogens via biological substances/organisms (called biopesticides in crop protection). Definitions vary globally: the U.S. EPA defines biopesticides as natural-material-derived, while France includes macroorganisms, microbes, chemical mediators, and natural substances. The EU lacks a clear definition, regulating biopesticides under synthetic pesticide rules, but is debating a unified framework for “Farm to Fork” and Green Deal goals. The biopesticides market is growing—projected to reach $13.9 billion by 2028 (up from $6.7 billion in 2023).
RLs inhibit diverse microorganisms: animal pathogens (fungi, bacteria, viruses) and, for phytopathogens, fungi and oomycetes (no effect on phytopathogenic bacteria, Table 1). Most RLs studied are P. aeruginosa-derived crude/purified mixtures (dominated by Rha-C10-C10 and Rha-Rha-C10-C10). Structure affects activity: di-RLs drive antifungal effects against Botrytis cinerea, Phytophthora spp., and Fusarium spp., while mono-RLs better inhibit Alternaria alternata and Cladosporium sp. No difference between mono- and di-RLs was seen against Aspergillus flavus, suggesting pathogen-dependent efficacy.
Crude RL extracts (cell-free broth) often have stronger effects than purified congeners—lower concentrations reach IC₅₀. In vitro inhibition assays use RL concentrations of 1–3000 μg mL⁻¹, with Minimal Inhibitory Concentrations (MIC) rarely exceeding 200 μg mL⁻¹, showing potent fungicidal activity. RLs target diverse fungal/oomycete families (e.g., Aspergillaceae, Peronosporaceae), with well-studied pathogens including Alternaria sp., Botrytis sp., Fusarium sp., Phytophthora sp., and Pythium sp.
RL effects on fungi/oomycetes include: mycelial growth inhibition (e.g., A. alternata, Fusarium verticillioides), spore germination delay/inhibition (e.g., Cercospora kikuchii, Magnaporthe grisea), and zoospore lysis/motility inhibition (e.g., Phytophthora capsici, Pythium aphanidermatum). RLs disrupt zoosporic pathogen plasma membranes; fungal sensitivity may relate to membrane lipid composition (e.g., ergosterol content makes Sclerotinia sclerotiorum more sensitive than B. cinerea).
For pests, di-RLs kill green peach aphids (Myzus persicae)—100% mortality at 100 μg mL⁻¹ via cuticular membrane disruption.
Table 1. Direct effects of rhamnolipids (RLs) against pests.
Bioinspired synthetic RLs (made via green chemistry) also inhibit phytopathogens: synthetic mono-RLs stop B. cinerea growth in tomato, and three synthetic RLs (especially C12 fatty acid tail) inhibit Zymoseptoria tritici (natural RLs have no effect). Like natural RLs, synthetic RL nanoemulsions do not inhibit Pseudomonas syringae DC3000 pv tomato.
In planta tests (Table 2) confirm RLs protect crops (cucumber, tomato, maize, etc.) from fungal diseases (e.g., Alternaria early blight, Botrytis gray mold). Preventive RL application (foliar spraying, soil addition, seed treatment) works best: e.g., RL soil application reduces cucumber damping-off by 1/3; pea seed/root RL treatment (25 μg mL⁻¹) fully suppresses Fusarium oxysporum wilt. Curative treatments also work—RL spraying on tomato reduces Alternaria solani infection, matching chemical fungicide efficacy. For aphids, RL supernatant (100 μg mL⁻¹) causes 100% mortality, similar to commercial insecticide.
RLs synergize with other compounds: combining RLs with yeast (Rhodotorula glutinis) or laurel essential oil enhances A. alternata control in cherry tomatoes; RLs + phenazines better reduce Pythium splendens damping-off (RLs solubilize phenazines for better fungal access).
Table 2. Plant protection tests with rhamnolipids (RLs) against pests.
3. Indirect effects by plant defense mechanisms triggering
RLs stimulate plant defenses, similar to their role in human/animal immunity. They likely trigger Microbe-Associated Molecular Patterns (MAMP)-triggered immunity: plants recognize MAMPs via surface Pattern-Recognition Receptors (PRRs), activating signaling cascades for basal immunity. No RL-specific plant receptor is known, but plasma membrane recognition is hypothesized.
RLs induce early defense responses: Reactive Oxygen Species (ROS) accumulation, hypersensitive response (HR)-like effects (grapevine, rapeseed), stomatal closure (rapeseed), and Ca²⁺ influx/phosphorylation cascades (grapevine). Late responses include modified defense gene expression, hormonal pathway induction (salicylic acid, jasmonic acid, ethylene), and callose deposition (grapevine, rapeseed, Arabidopsis thaliana). In cherry tomatoes, RLs boost antioxidant enzyme activity; proteomics show RLs activate systemic defenses (dependent on application method: foliar vs. root).
Synthetic RLs also promote plant defenses: bioinspired RLs enhance antioxidant enzyme activity and weakly induce defense genes/metabolites (some linked to abiotic stress in wheat); synthetic RL bolaforms and nanoemulsions induce hormone-related defense genes in A. thaliana.
RLs protect A. thaliana from non-RL-sensitive pathogens (e.g., P. syringae DC3000 pv tomato) via defense stimulation—even at 0.2 mg mL⁻¹. Pure RLs (recombinant Pseudomonas putida) reduce Heterodera schachtii (nematode) infection in A. thaliana (no direct nematode inhibition) by boosting H₂O₂ production (no hormonal gene induction). Di-RL efficacy depends on acyl chain length (C10-C12/C10-C12:1 most effective at 2 ppm), but high doses reduce plant growth (mono-RLs at 8.3 ppm also harm Arabidopsis).
RLs protect plants via dual action (direct fungal inhibition + defense stimulation) for RL-sensitive fungi/oomycetes, and only defense stimulation for non-sensitive pathogens (bacteria, nematodes). However, field trial data on RL efficacy is lacking.
4. Rhamnolipids as potential plant biostimulation agents
Under EU regulations, plant biostimulants are natural compounds that improve nutrient use efficiency, abiotic stress tolerance, crop quality, or soil/rhizosphere nutrient availability. The biostimulants market is growing—projected to reach $7.6 billion by 2029 (up from $4.3 billion in 2024). Microbial biostimulants face limitations (environmental sensitivity, short shelf life), so microbial metabolites like RLs are emerging alternatives (Table 3).
RLs promote seed germination and early growth: P. aeruginosa-derived RLs boost lettuce germination at 0.75–1 g L⁻¹, corn/sunflower at 0.25 g L⁻¹, and wheat at 0.5 g L⁻¹. Partially purified RLs (0.25 g L⁻¹) stimulate okra, lettuce, and onion germination but reduce wheat/barley germination (yet protect seeds from fungi). Soybean seed soaking in RLs (0.5–1 g L⁻¹) improves lateral root development; 2 g L⁻¹ enhances imbibition. Diluted P. aeruginosa culture (0.01 g L⁻¹ RL biocomplex) boosts sunflower ion absorption and growth.
Field studies confirm RL benefits: RLs increase sunflower yield and seed lipid/protein content; RL-glycolipid mixtures (with NPK fertilizers) cut nitrogen use without reducing maize yield and boost nitrogen uptake. RLs enhance trace nutrient uptake: zinc-RL complexes improve rapeseed zinc absorption (hydroponics), and RLs increase wheat/durum zinc content (calcareous soil).
RLs mitigate abiotic stress:
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- High temperature: RLs + small peptides (tea plant foliar spraying) improve photosynthesis, antioxidant activity, and phyllosphere beneficial microbes.
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- Salinity: RLs + choline chloride (tomato, salinized soil) reduce Na⁺/K⁺ ratio, boost peroxidase activity/proline content, and enhance salt tolerance; RL drip irrigation (cotton, saline soil) lowers soil electrical conductivity, increases organic matter, and reshapes rhizosphere microbes.
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- Heavy metals: RLs + humic acid (rice, cadmium-contaminated fields) reduce grain cadmium and increase biomass/yield (RLs trap cadmium in leaves).
RLs improve soil health: they enhance nutrient cycling (cotton rhizosphere), boost microbial enzyme activity (soil nitrogen/organic matter cycling), and stabilize soil aggregates (saline-alkali soil) to support microecological functions (carbon/nitrogen metabolism).
Table 3. Biostimulation assays with rhamnolipids (RLs) on plants.
5. Other potential applications of Rhamnolipidsin agriculture and prospects
5.1 Agrochemical formulation
Synthetic surfactants are used as agrochemical adjuvants; RLs (anionic biosurfactants) are natural alternatives. They enhance leaf wetting/penetration (hydrophobic wax surfaces) and have good solubilizing/emulsifying properties (influenced by RL homolog ratio and pH). Their biodegradability (soil microbial mineralization) is a key advantage.
RLs also improve biocontrol/biostimulant formulations: adding RLs to Plant Growth-Promoting Rhizobacteria (PGPR) solutions boosts efficacy—e.g., Pseudomonas putida + RLs improve Brassica juncea growth; Pseudomonas guariconensis + RLs promote sunflower growth and control charcoal rot.
5.2 Soil bioremediation
RLs aid remediation of polluted agricultural soils: they solubilize/emulsify hydrophobic hydrocarbons (e.g., PAHs) and chelate heavy metals, enhancing bioavailability for microbial degradation. Many studies explore RLs for bioremediation, though reviews note limitations (e.g., balancing metal mobility for essential vs. toxic metals).
5.3 Considerations and challenges
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- Ecosystem impacts: RL-driven changes to soil microbial communities raise questions about long-term effects.
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- Concentration optimization: Low concentrations suffice for defense stimulation, but higher doses are needed for bioremediation/field disease control.
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- Biodegradation timing: Ensure RLs act before degradation.
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- Metal mobility risk: RLs enhance toxic metal (Pb, Cd) mobility, requiring caution when used for essential metal bioavailability.
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- Phytotoxicity: High RL doses (＞1 g L⁻¹) or specific structures harm some plants.
6. Conclusion
Rhamnolipids are promising biosurfactants for sustainable agriculture, with multiple roles: direct phytopathogen inhibition, plant defense stimulation, biostimulation (growth, stress tolerance), soil health improvement, and agrochemical formulation. However, gaps remain: unclear mechanisms of action, need for optimized application methods, long-term ecosystem impact data, and lack of field trial results. Scaling up RL production and improving cost-effectiveness are also critical. As sustainable farming demand grows, integrating RLs into agricultural strategies offers a path to eco-friendly crop production.
Fig. 2. Potential applications of rhamnolipids for agriculture.
For further information on rhamnolipids (e.g., technical specs, application case studies) or to request samples for agricultural R&D, please contact Dora Agri.