Modern agriculture faces a dual challenge: meeting the ever-growing global food demand while simultaneously reducing the environmental costs of chemical fertilizers. Against this backdrop, monomeric amino acids, as biostimulants and novel nitrogen fertilizer carriers, are redefining the boundaries of plant nutrition. Unlike traditional studies of amino acid mixtures, recent explorations of the functions of single amino acids have revealed a crucial fact: different amino acids play highly differentiated roles within plants, their functions extending far beyond simply providing nitrogen. This article, based on the latest research progress, systematically elucidates the unique functions of monomeric amino acids in agriculture from five dimensions.
1.Monomeric Amino Acids：As a Directly Absorbable Organic Nitrogen Source: Breaking Through the Traditional Nitrogen Nutrition Paradigm
Traditional plant nutrition has long held that crops primarily absorb inorganic ammonium and nitrate nitrogen, while organic nitrogen requires mineralization by soil microorganisms before utilization. However, research over the past two decades has completely overturned this understanding. Plant roots and leaves possess a complete amino acid absorption system, capable of directly taking up molecular-level amino acids.
Lysine represents a milestone in this regard. Greenhouse trials conducted by the USDA Agricultural Research Service showed that maize grown using L-lysine as the sole nitrogen source exhibited no significant differences in growth indicators, biomass accumulation, or amino acid profile compared to treatments using ammonium nitrate. This groundbreaking finding demonstrates that single amino acids can completely replace inorganic nitrogen fertilizers to support the entire growth cycle of field crops. More importantly, lysine carries a positive charge within the agronomic pH range (3-9), allowing it to be adsorbed by negatively charged sites in the soil, resulting in a significantly lower leaching risk compared to nitrate ions. This characteristic provides a novel approach to addressing persistent problems such as low nitrogen fertilizer utilization and nitrate pollution in groundwater.
Histidine also showed potential as an effective nitrogen source, while alanine and arginine were less effective. This interspecies difference in amino acids suggests that the application of single amino acids cannot be generalized; their molecular structure (charge properties, side chain size) directly determines soil behavior and plant utilization efficiency.
2. Stress-Resistant Physiological Protectants: From Osmotic Regulation to Oxidative Defense
One of the most prominent functions of monomeric amino acids in agriculture is enhancing crop resistance to abiotic stresses. Research in this field has progressed from phenomenological description to molecular mechanism verification.
Proline is undoubtedly the core player. Under stresses such as drought, salinity, heavy metals, and extreme temperatures, the accumulation of proline in crops such as rice, corn, and wheat can increase by 4-20 times. Genetic evidence confirms the causal relationship between this accumulation and stress resistance: rice overexpressing the proline synthesis gene OsOAT maintains high proline levels even under normal conditions and exhibits stronger drought and oxidation resistance; while gene knockout lines are highly sensitive to low temperatures and drought. Proline’s mechanism of action is dual: as an amphoteric molecule, it regulates osmotic pressure and stabilizes protein and membrane structures; simultaneously, as a direct scavenger of reactive oxygen species (ROS), its pyrrolidine ring can react with hydroxyl radicals to generate hydroxyproline derivatives.
Gamma-aminobutyric acid (GABA) and branched-chain amino acids (leucine, isoleucine, and valine) also accumulate significantly under flooding and hypoxia stress, possessing both signaling and metabolic regulatory functions. Asparagine exhibits particularly unique behavior under salt stress: it preferentially promotes the uptake of phosphorus, calcium, and potassium by maize roots while inhibiting the transport of sodium ions to leaves, thereby achieving ion homeostasis regulation.
It is noteworthy that this protective function is dose-dependent. L-methionine promotes lettuce growth at low concentrations but inhibits it at high concentrations, suggesting that excessive stimulation may damage photosynthetic structures. This necessitates precise regulation in agricultural applications.
3. Direct Drivers of Photosynthesis and Yield Formation
The contribution of monomeric amino acids to crop yield increases stems not only from nitrogen supply but also from their direct intervention in photosynthetic metabolism. A systematic review published in 2025 summarized two core pathways: promoting chlorophyll synthesis and optimizing photosystem II (PSII) electron transport efficiency.
Glycine significantly increased chlorophyll a content in lettuce leaves within a concentration range of 250-500 mg/L, and its derivative, glycine betaine, also upregulated ascorbate peroxidase activity. Glutamic acid (Glu), as a precursor to γ-aminobutyric acid (GABA) and proline, significantly improved the photosynthetic rate of hawthorn leaves by optimizing the PSII electron transport chain; isotope labeling confirmed that exogenous glutamate could enter the leaf metabolic pool in its intact molecular form.
Field data provided strong support. In maize trials in Guangdong and Dongguan, root irrigation with 12% amino acid water-soluble fertilizer increased maize stalk diameter by 28.6%-46.4% and root indicators by 7.8%-57.6%, with significant synergistic effects when combined with insecticides. A three-year field trial in southern Spain showed that application of amino acid biostimulants alone increased durum wheat yield by 29.5% and grain protein content by 4.2%. In a rice demonstration project in Yongtai, Fujian, Taiwanese amino acid liquid fertilizer achieved yield increases of 6.7%-20% or more in different treatment areas. The core mechanism lies in promoting increased tillering and heading rate.
4. Quality Formation and Secondary Metabolic Regulation
The role of monomeric amino acids in improving the quality of agricultural products is receiving increasing attention, especially aromatic amino acids, which influence sensory quality and nutrient density through secondary metabolic pathways.
Phenylalanine is a prime example in this field. It upregulates the synthesis of phenols, stilbenes, anthocyanins, and other secondary products by activating key genes in the phenylpropanoid metabolic pathway, VvPAL and VvCHS. When applied to grapes, this process is mediated by abscisic acid, significantly improving wine quality. Tryptophan works synergistically with phenylalanine to increase the content of lupin alkaloids and phenolic substances, simultaneously promoting plant height, seed yield, and crude protein accumulation.
Alanine exhibits a unique aroma-improving function in apples. Studies have found that foliar application of alanine can significantly increase the activity of alcohol dehydrogenase and alcohol acyltransferase in Fuji apples, thereby increasing the total amount of ester aroma compounds, with better effects than leucine, isoleucine, and valine.
Basic amino acids (lysine, histidine) are often applied in the form of zinc chelates to improve onion bulb size and pyruvate content by altering nitrogen metabolism pathways. A research review by the Jiangsu Academy of Agricultural Sciences further confirms that amino acids participate in the signal regulation network of fruit quality formation, acting not only as nutrient carriers but also as physiological activators.
5. Multifunctional Roles in the Soil-Microbe-Plant Interaction Interface
The role of monomeric amino acids in the rhizosphere micro-ecosystem is being re-evaluated. First, amino acids can be directly utilized by soil microorganisms as carbon and nitrogen sources, regulating microbial community structure and metabolic activity. Second, some amino acids can activate plant systemic immune responses. Research from the Jiangsu Academy of Agricultural Sciences indicates that amino acids participate in the expression regulation of plant-pathogen interaction-related genes, and both the MAPK signaling pathway and plant hormone transduction pathway are induced by amino acids.
The synergistic effect of amino acids and pesticides is of particular interest. The combination of 12% amino acids and 1% bromocyanamide not only promotes corn growth but also increases the pesticide’s efficacy against fall armyworm by 6.5%-31.2%, with the efficacy still exceeding 50% 20 days after application. This “pesticide and fertilizer co-source” strategy provides a feasible solution for reducing pesticide use and simplifying operations.
In summary, single amino acids in agriculture are far more than simple “nutritional supplements”; they are multifunctional agricultural inputs that integrate nitrogen supply, stress signaling, photosynthesis promotion, quality improvement factors, and pesticide synergists. Proline’s osmotic protection, lysine’s low-leaching nitrogen source, phenylalanine’s secondary metabolism regulation, and glycine’s photosynthesis promotion—each amino acid carries unique molecular identity information. Modern agriculture is shifting from “what crops are fed and what crops eat” to “what crops need and what they need,” and the differentiated and precise application of single amino acids is a microcosm of this paradigm shift.