Plants need different types of macro and micronutrients to support their growth and development. While some of these nutrients are absorbed directly from the soil, many are made available through the activities of beneficial microbes living harmoniously with plant roots. The rhizosphere, which encompasses the thin layer of soil surrounding and influenced by plant roots, is home to a diverse microbial community.
Certain bacteria in the rhizosphere play key roles in plant nutrition by facilitating the breakdown or transformation of organic and inorganic compounds into plant-available forms. Understanding and harnessing these plant-microbe interactions could improve agricultural sustainability and crop yields.
The Rhizosphere Microbiome
The rhizosphere is surrounded by an abundance and diversity of microorganisms, supported by the nutrients released from plant roots. Root exudates like sugars, organic acids, and amino acids attract and stimulate the growth of microbial populations. Bacteria dominate the rhizosphere microbial community, with proteobacteria, actinobacteria, firmicutes, and bacteroidetes among the most prevalent phyla. The composition of the rhizosphere microbiome is influenced by many factors, including plant species, soil type, and environmental conditions. Plants also selectively recruit beneficial microbes to the rhizosphere to meet their nutritional needs.
The rhizosphere has a bacterial population that is estimated to be 10 to 100 times greater than in regular soil. These bacteria are crucial because they help in various nutrient cycles that affect the nutrition and health of plants. They do important tasks like fixing nitrogen, breaking down phosphate, producing siderophores, and signaling with plant hormones. Manipulating the rhizosphere microbiome through plant breeding or probiotic applications is an area of increasing research interest.
Nitrogen is an essential macronutrient for plants, required for biosynthesis of amino acids, proteins, and nucleic acids. However, diatomic nitrogen (N2) from the atmosphere is not directly usable by plants. Specialized bacteria can convert N2 into ammonia (NH3) through the process of nitrogen fixation. Common rhizosphere inhabitants like Rhizobium, Bradyrhizobium, Azorhizobium, Allorhizobium, and Azospirillum can form symbiotic relationships with legumes and fix nitrogen. The bacteria colonize root nodules, receiving nutrients from the plant while providing fixed nitrogen from N2 in the air.
It is estimated that 50-70 million metric tons of nitrogen are fixed through these legume-rhizobia symbioses worldwide each year. Other free-living bacteria like Azotobacter and Clostridium contribute smaller amounts of fixed nitrogen in the rhizosphere. Inoculation with nitrogen fixing bacteria is known to benefit crop growth by increasing nitrogen availability. Exploiting plant-microbe nitrogen fixation systems could reduce reliance on synthetic nitrogen fertilizers.
Phosphorus is another macronutrient essential for plants as a component of key molecules like nucleic acids, phospholipids, and ATP. While total phosphorus may be abundant in soils, it is often bound up in organic matter or insoluble mineral complexes, limiting its availability to plants. Many rhizosphere bacteria can solubilize otherwise unavailable phosphorus through processes like acidification, chelation, and exchange reactions.
Common phosphate-solubilizing bacteria include Pseudomonas, Bacillus, Rhizobium, Burkholderia, Enterobacter, and Erwinia species. They release organic acids like gluconic and citric acids that acidify the rhizosphere, driving dissolution of insoluble phosphates like tricalcium phosphate. Phosphate-solubilizing bacteria also exude metabolites like siderophores that can chelate cations like Ca2+, Fe3+, and Al3+ that immobilize phosphate in the soil. Inoculating seeds with these bacteria, especially for crops like wheat, rice, and maize grown in soils with abundant insoluble phosphates, can enhance phosphate nutrition and increase yields.
Unlocking Micronutrients for Plant Uptake
While nitrogen, phosphorus, and potassium are considered macronutrients needed in relatively large amounts, plants also require various micronutrients like iron, zinc, manganese, copper, boron, molybdenum, and chlorine. These micronutrients serve as essential cofactors in plant enzymes and proteins. However, like macronutrients, micronutrients are often bound in forms not directly accessible for plant absorption and utilization. Beneficial rhizosphere bacteria can transform unavailable micronutrients into plant-available forms through various mechanisms.
For example, iron is abundant in soils but primarily present in insoluble Fe3+ forms. Bacteria like Pseudomonas and Bacillus secrete siderophores, which are small molecules with a high affinity for Fe3+. Siderophores solubilize otherwise inaccessible iron by chelating it, then the iron-siderophore complex is taken up by the bacteria and provided to plants in a usable form. Similarly, some bacteria release organic acids that solubilize mineral-occluded forms of micronutrients like zinc and manganese. Inoculating soils with assemblages of plant growth-promoting rhizobacteria could enhance micronutrient availability in agricultural soils where low micronutrient mobility otherwise limits crop growth and yields.
Plant growth and development are heavily influenced by phytohormones like auxins, cytokinins, gibberellins, and ethylene. Some rhizosphere bacteria can produce or modulate these phytohormones in ways that stimulate plant growth. For example, Pseudomonas and Azospirillum species synthesize auxins like indole-3-acetic acid (IAA) that promote root proliferation, increasing root surface area for nutrient absorption.
Bacteria may also metabolize plant-produced ethylene, a stress hormone that inhibits growth. By modulating phytohormones, beneficial rhizosphere bacteria support increased nutrient uptake as well as enhanced plant growth, development, and stress resilience. Applying auxin- and cytokinin-producing bacteria could reduce reliance on exogenous hormone applications in agriculture.
Harnessing beneficial plant-microbe associations has promising applications for enhancing agricultural sustainability and food security. Commercial products containing mixes of plant growth-promoting rhizobacteria (PGPR) are already available. Further research can help identify effective PGPR strains for specific crops and soils using culture-based or metagenomic approaches.
Inoculating seeds or soils with custom PGPR consortia could reduce fertilizer requirements while increasing yields. PGPR also hold potential for success in marginal soils. Integrating beneficial rhizosphere microbiomes with appropriate crop rotations and reduced tillage practices may enhance soil health while reducing agriculture’s environmental footprint.
However, important challenges remain. Plant-microbe associations are complex, and introducing non-native strains risks unintended ecological impacts. Effects of PGPR may be context-dependent based on environment, plant genotypes, and native soil microbiota. Better understanding these interactions will inform practical PGPR applications. Policy and regulatory hurdles must also be addressed.
Rhizosphere bacteria play critical roles in soil nutrient transformations that influence plant nutrition and agricultural productivity. Nitrogen fixation, phosphate solubilization, siderophore production, and phytohormone modulation are key mechanisms that enhance availability of nutrients like nitrogen, phosphorus, and many other mictonutrients. Characterizing and harnessing beneficial plant-microbe associations could reduce reliance on fertilizers and improve agricultural sustainability. However, ecological complexities warrant further research to translate these fundamental processes into effective management practices. Continued exploration of the relationships between plants and their root microbiomes will open promising doors for food security and environmental stewardship.