Do Plants Absorb Amino Acids?

What Amino Acids can Plants Uptake?

Plants can uptake various amino acids through their roots and their leaves. Here are some key examples:

• Glutamate: This amino acid is efficiently absorbed by plant roots and plays a pivotal role in nitrogen assimilation. It is actively transported and can also be taken up by leaves, contributing to the synthesis of other important compounds [Näsholm et al., 2009; Tegeder & Rentsch, 2010].
• Glycine: Readily absorbed by roots, glycine is vital for stress responses and nitrogen metabolism. Research suggests it can also be absorbed through leaves [Rentsch et al., 2007; Tegeder & Rentsch, 2010].
• Lysine: Efficiently absorbed through roots, lysine is essential for protein synthesis and various metabolic processes. It can also be taken up by leaves via specialized transporters [Fischer et al., 1998; Tegeder & Rentsch, 2010].
• Arginine: The uptake of this cationic amino acid is facilitated by the AAP5 transporter, enabling efficient absorption at varying soil concentrations [Santiago & Tegeder, 2016].
• Phenylalanine: Absorbed by roots, phenylalanine is involved in producing phenolic compounds and other secondary metabolites. It may also be absorbed through leaves and contribute to improved nitrogen metabolism [Tegeder & Rentsch, 2010; Näsholm et al., 2009].
• Cysteine: Efficiently absorbed by roots, cysteine is important for protein and antioxidant synthesis. It may also influence nitrogen content and overall plant health when absorbed through leaves [Ziegler et al., 2015; Näsholm et al., 2009].
• Other Amino Acids: Several other amino acids, such as asparagine and glutamine, can be transported within the plant and potentially absorbed through leaves. The specifics of uptake can depend on the plant species and environmental factors [Tegeder & Rentsch, 2010].

Organic Nitrogen Uptake through Roots

In many ecosystems, organic nitrogen, particularly amino acids, is often more abundant than inorganic nitrogen due to the breakdown of soil proteins. Plants have specialized transporters in their roots and also benefit from symbiotic relationships with mycorrhizal fungi to facilitate this uptake.

• Specialized Transporters: The gateway for amino acids into plant cells are specialized transporter proteins embedded in the plasma membrane of root cells and mycorrhizal fungi [Näsholm et al., 2009]. The transporters exhibit high specificity and affinity for various amino acids, ensuring efficient uptake even at low soil concentrations.
• Proton-Coupled Symport: The uptake of amino acids is often coupled with the inward movement of protons (H⁺). This proton gradient, generated by proton pumps, provides the energy required for the active transport of amino acids against their concentration gradient [Bush, 1993].
• Mycorrhizal Networks: Mycorrhizal fungi form symbiotic associations with plant roots, significantly expanding the plant's absorptive surface area. The fungal hyphae can access organic nitrogen sources in soil regions beyond the reach of plant roots, and then transfer the absorbed amino acids to the plant, playing a crucial role in enhancing plant N acquisition [Smith & Read, 2007]. 

Factors Influencing Root Uptake

• Soil Amino Acid Concentration: The concentration of amino acids in the soil solution directly influences uptake rates. Higher concentrations generally lead to increased uptake, although transporter saturation can occur.
• Plant Nitrogen Status: The plant's internal nitrogen status can regulate the expression and activity of amino acid transporters. Nitrogen-deficient plants tend to upregulate transporter activity to increase nitrogen acquisition.
• Competition with Microbes: Plants compete with soil microbes for available amino acids. The outcome of this competition depends on factors such as the relative abundance of plants and microbes, their respective uptake kinetics, and the soil conditions.
• Mycorrhizal Associations: Mycorrhizal fungi can significantly enhance a plant's ability to compete for organic nitrogen by increasing the absorptive surface area and accessing nitrogen sources unavailable to plant roots alone.

Organic Nitrogen Uptake through Leaves

Plants can also absorb amino acids through their leaves, potentially enhancing growth and stress tolerance. This process, called foliar absorption, occurs primarily through specialized transport proteins in the leaf cell membranes.

• Foliar Absorption: Amino acids applied to the leaf surface can penetrate the cuticle, a waxy layer that protects the leaf from water loss, and enter the leaf tissues. The efficiency of this process can be enhanced by the use of surfactants, which help reduce surface tension and improve the penetration of amino acids.
• Transport Proteins: Once inside the leaf, amino acids are taken up by specialized transport proteins located in the cell membranes. These transporters facilitate the movement of amino acids from the intercellular spaces (apoplast) into the cytoplasm of the leaf cells.

Factors Influencing Leaf Uptake

Several factors influence the uptake of amino acids through plant leaves. These factors can enhance or inhibit the efficiency of absorption, and they include:

• Leaf Structure and Condition: The physical condition of the leaf surface, including the presence of aqueous pores and the integrity of the cuticle, can affect how easily amino acids are absorbed. Leaf age and health also play a role in uptake efficiency. These factors collectively determine how effectively plants can absorb amino acids through their leaves, influencing their growth and metabolic processes.
• Temperature: Optimal temperatures significantly affect amino acid uptake. For instance, glycine uptake in pea leaves was found to be optimal at around 30°C, indicating that temperature can enhance metabolic processes related to absorption.
• Environmental Conditions: Factors such as humidity, light intensity, and overall climate can impact leaf physiology and, consequently, amino acid uptake. High humidity may facilitate absorption, while extreme temperatures or drought conditions can hinder it.
• Metabolic activity: The metabolic state of the plant plays a crucial role in amino acid uptake. Active respiration and photosynthesis are essential for energy-dependent transport mechanisms, as ATP is often required for the uptake of amino acids like glycine.

Post-Uptake Metabolism

Within the plant, uptaked amino acids undergo a series of intricate metabolic transformations.

• Rapid Metabolism: Once inside the plant cell, amino acids are rapidly metabolized. They can be directly incorporated into proteins or broken down into their constituent parts [Schmidt & Stewart, 1999; Thornton, 2001; Persson et al., 2006].
• Transamination: A key metabolic process is transamination, where the amino group from one amino acid is transferred to another molecule, creating a new amino acid. This allows the plant to synthesize the specific amino acids it needs for protein synthesis and other metabolic processes.
• Deamination: In some cases, amino acids undergo deamination, where the amino group is removed, releasing ammonia. The ammonia is then assimilated into organic compounds through the glutamine synthetase/glutamate synthase (GS/GOGAT) pathway.

Benefits of Amino Acids

The amino acids provide plants several key advantages:

• Enhanced Growth: Foliar application of amino acids has been linked to increased productivity and improved physiological characteristics in various crops, such as tomatoes and soybeans.
• Stress Tolerance: Amino acids play a role in stress responses, helping plants cope with environmental challenges.
• Nitrogen Use Efficiency: The uptake of amino acids is energetically favorable compared to inorganic nitrogen sources, as it allows plants to utilize nitrogen more efficiently without the need for additional energy expenditure for assimilation