Nitrogen is vital for plant development. Though nitrogen gas (N2) comprises roughly 78% of the atmosphere, plants can’t assimilate it in this state. Specialized nitrogen-fixing bacteria can convert atmospheric nitrogen into a form readily absorbed by plant roots. These bacteria form mutualistic bonds with legumes and certain non-leguminous plants. This article delves into the intricacies of symbiotic nitrogen fixation and the bacteria responsible for it.
Introduction to Symbiotic Nitrogen Fixation
Symbiotic nitrogen fixation is the process where specific bacteria transform nitrogen gas into ammonia, making it accessible for their host plants. This supplies the plants with a usable form of nitrogen. In exchange, the plants offer nutrients to these bacteria, creating a mutually beneficial relationship between the two organisms.
There are two main types of symbiotic nitrogen fixing bacteria
- Rhizobia – live in root nodules on legumes like peas, beans, and soybeans.
- Frankia – live in root nodules formed on non-legume plants like alder and bayberry trees.
Key features of the symbiosis between nitrogen fixing bacteria and host plants
- Bacteria cannot fix nitrogen unless they are inside root nodules of the host plant.
- Plants supply carbohydrates and other sources of energy to bacteria.
- In return, the bacteria convert atmospheric nitrogen into ammonia which is usable by the plant.
- This provides the plant with needed nitrogen without having to absorb it from the soil.
- The symbiosis improves nitrogen availability in the soil, which also benefits surrounding plants.
Rhizobia are a group of soil bacteria that form symbiotic relationships with legumes. There are several different species within the rhizobia genus including.
- Rhizobium leguminosarum – nodulates peas, beans, clover
- Bradyrhizobium japonicum – nodulates soybeans
- Sinorhizobium meliloti – nodulates alfalfa
When rhizobia encounter a legume root, they recognize chemical signals from the plant. This triggers the bacteria to multiply and stimulate nodule formation by the plant. Nodule formation involves the plant curling its root hairs around the bacteria, which then enter root cells and trigger division and growth resulting in the development of a nodule.
Within the nodule, the rhizobia differentiate into a form called bacteroids. These can convert nitrogen gas into ammonia via the enzyme nitrogenase. The plant provides the bacteria with carbohydrates, energy sources, proteins, and minerals while receiving usable nitrogen in return.
These are filamentous, nitrogen fixing bacteria that form symbiotic relationships with non-legume plants called actinorhizal plants. They form root nodules with over 200 species of woody shrubs and trees, including:
The association between Frankia and actinorhizal plants provides usable nitrogen for the plant while the plant supplies the bacteria with carbohydrates and energy sources.
As with rhizobia, Frankia bacteria recognize host-specific signal compounds which prompt nodule formation. Frankia penetrate root hair cells and become enveloped in plant cell walls, forming clusters called vesicles. Within vesicles, nitrogen fixation occurs, providing the plant with ammonia.
Methods of Inoculation
In order for plants to form symbiotic relationships with nitrogen fixing bacteria, the bacteria must be present in the soil. If bacteria are absent, steps must be taken to introduce them via a process called inoculation. There are several methods of inoculating plants with rhizobia or Frankia.
- Soil inoculation – applying liquid cultures or moistened powders containing the bacteria directly to the soil. This allows the bacteria to come into contact with and infect the plant roots.
- Seed inoculation – coating seeds with a sticking agent and bacterial powder or liquid before planting. Provides early inoculation as the bacteria are present when seeds germinate.
- Foliar application – spraying a solution containing the bacteria onto plant leaves. Bacteria enter through stomata and migrate down to roots.
- Pellet inoculation – mixing peat, clay, and bacteria into small pellets which are placed in the planting hole along with seeds. Releases bacteria slowly over time.
Proper inoculant strains must be matched with the correct host species and environmental conditions. Commercial inoculants are available which contain optimized rhizobial or Frankia strains for particular legume/actinorhizal crops and soils.
Factors Affecting Nodule Formation and Nitrogen Fixation
While most legumes and actinorhizal plants can form effective symbioses with nitrogen fixing bacteria, the level of nodulation and efficiency of nitrogen fixation depends on several factors.
- Bacterial strain – some strains are more effective at nitrogen fixation than others. Using optimized strains improves the process.
- Soil nitrogen – high nitrogen levels inhibit nodulation as the plant has less need for bacterial nitrogen. Lower soil nitrogen promotes more abundant nodulation.
- Soil pH – each bacterial species thrives in specific pH ranges. Extremely acidic or alkaline soils inhibit the symbiosis.
- Soil temperature – each bacteria has an optimal temperature range. Temperatures outside this range reduce bacterial growth and nitrogen fixation.
- Soil nutrients – proper levels of elements like phosphorus, sulfur, potassium, calcium, iron, and molybdenum are needed for bacterial growth and functioning. Deficiencies impair the process.
- Soil oxygen – nitrogen fixation requires oxygen. Waterlogging reduces available oxygen and negatively affects nodulation.
- Pesticides/contaminants – certain pesticides, heavy metals, salts, or other contaminants can inhibit nitrogen fixation at high concentrations.
Optimizing these factors leads to more effective plant-bacteria symbioses and improved nitrogen fixation.
Genetic Engineering of Symbiotic Nitrogen Fixation
While tremendous progress has been made in understanding the symbiotic relationship between plants and nitrogen fixing bacteria, scientists are also exploring how to engineer this system to further improve agricultural productivity. Two major approaches are being researched.
Engineering rhizobia and Frankia strains
- Selecting or genetically modifying the most efficient nitrogen fixing strains.
- Modifying strains to be more heat or drought tolerant.
- Engineering the bacteria to fix nitrogen more rapidly or under wider environmental conditions.
Transferring nitrogen fixing genes to cereals
- Identifying the main genes involved in the nitrogen fixation process.
- Isolating these genes from rhizobia/Frankia and transferring them to cereals like rice, wheat, and corn.
- This could allow cereals to fix their own nitrogen, reducing fertilizer usage.
- Significant progress made in identifying key genes, but major challenges remain in functionally expressing them in plants.
While promising, there are potential risks involved with genetically engineering symbiotic systems, including.
- Modified bacteria escaping into the environment and impacting ecosystems.
- Transferring genes to weed species, making them more competitive.
- Unknown impacts on food safety.
More research is needed to realize the potential benefits of engineering nitrogen fixation, while managing any potential downsides. Going forward, the symbiosis between plants and nitrogen fixing bacteria will continue to play a crucial role in natural ecosystems and agricultural production.
Importance of Symbiotic Nitrogen Fixation
The symbiotic relationship between nitrogen fixing bacteria and host plants provides many important benefits.
- Supplies usable nitrogen to plants which enables growth, without the need for added fertilizers.
- Helps make barren lands arable by promoting plant growth and improving nitrogen levels in the soil over time.
- Reduces the need for synthetic nitrogen fertilizers which require significant energy to produce and can pollute waterways.
- Plays a major role in natural nitrogen cycling processes. Estimated that symbiotic nitrogen fixation accounts for 65% of nitrogen fixed on land.
- Stimulates the growth of surrounding non-legume plants in grasslands by increasing nitrogen availability in the soil.
In summary, the evolution of symbiotic nitrogen fixation was hugely impactful, allowing new growth in nitrogen-poor soils. The bacteria-plant mutualism improves soil quality for all vegetation and reduces dependence on synthetic fertilizers. Further research aims to transfer nitrogen fixing abilities to cereals like rice and wheat to reduce fertilizer needs.