Root Nodules and Nitrogen Fixation
Lateral Root Development and Nodulation
Lateral Root Development (Recap)
Lateral roots emerge from the root, regulated by:
Auxin: A diurnal oscillating pulse initiates priming just behind the root apical meristem.
Environmental Signals: Nitrogen and phosphorus levels interact with auxin signaling to modify root architecture based on the plant's needs.
Adaptations to Nutrient Limitation
Nitrogen Limitation: Plants exhibit increased lateral root branching at specific sites.
Phosphorus Limitation: Some plants develop proteoid roots, which are clusters of lateral roots that release organic acids and chelate metals to enhance phosphate absorption.
Plants evolve specific root system adaptations based on their environmental origins to efficiently acquire nutrients with limited resources.
Root Nodules and Nitrogen Fixation
A specific adaptation evolved approximately 100 million years ago in certain plants, involving the formation of root nodules.
These nodules facilitate a symbiotic relationship with soil bacteria (Rhizobia) that fix nitrogen.
Most crop plants lack this symbiosis, necessitating the use of nitrogen fertilizers, which have significant environmental consequences.
Understanding this symbiosis could reduce the need for nitrogen fertilizers, a major expense for farmers.
Nitrogen-fixing bacteria convert atmospheric nitrogen into ammonia within the nodules, providing a direct nitrogen source for the plant.
Microorganism-Driven Development
Plant and animal development are often influenced by microorganisms due to their long-term co-evolution.
Microorganisms can act as pathogens or provide mutual benefits.
Many microorganisms have evolved to exploit developmental mechanisms in plants and animals.
Nodulation is a well-studied example of microorganism-driven developmental modification.
Rhizobia induce the formation of nodules, a new organ, through specific signals.
Rhizobia vs. Agrobacterium
Rhizobium: Forms beneficial nodules that fix nitrogen.
Agrobacterium: A closely related bacterium that causes crown gall tumors, which are harmful to the plant.
Agrobacterium induces tumors by inserting DNA into the plant genome, a mechanism used in plant genetic transformation.
Some Agrobacterium species have been reclassified as Rhizobia, highlighting their close phylogenetic relationship.
Tumor formation in Agrobacterium involves auxin-induced cell division.
Other Examples of Development Regulation
Insect-induced galls: Insects lay eggs in galls that take resources from the plant.
Nematode-induced root galls: Nematodes infect roots and disrupt plant development, targeting auxin and cytokinin pathways.
Nodule Structure
Nodules have a structured internal organization.
Vascular tissue extends from the root into the nodule to supply it with photosynthetic products. These include mainly organic acids that are transported into the cells colonized by the bacteria, because the plant usually has enough carbon.
Rhizobia colonize cells intracellularly, a rare instance of plants tolerating intracellular symbionts.
Infection threads facilitate the infection of new nodule regions as the nodule grows.
Nodule Function
Plants supply carbon to the bacteria, while bacteria convert atmospheric nitrogen into ammonia.
Ammonia is assimilated into amino acids, serving as a nitrogen source for the plant.
Evolutionary Context
Arbuscular mycorrhizal fungi (AM) are considered a likely precursor to root development, representing an ancient symbiosis.
The symbiosis with nitrogen-fixing Rhizobia is a more recent development, around 100 million years ago.
A predisposition event occurred in a clade of plants (including Fabales, Fagales, Cucurbitales, and Rosales), leading to nodulation capabilities.
Actinorhizal plants in this clade form similar structures in symbiosis with actinomycetes bacteria.
The current hypothesis suggests that the predisposition evolved once, with subsequent loss of symbiosis in many species.
Climate and Nodulation
High atmospheric CO_2 concentrations (around 1,500-2,000 ppm) during the expansion of legumes may have favored nodulation.
Plants in high CO_2 environments often become nitrogen-limited.
Nodulation provided a competitive advantage by supplying an alternative nitrogen source, allowing plants to capitalize on high CO_2 levels. Approximately 50% of leaf protein is rubisco, that requires nitrogen to properly function.
Decreasing CO_2 levels may have contributed to the loss of symbiosis in some legumes.
The symbiosis has a cost, requiring plants to supply photosynthetic products to the bacteria.
Stringent regulation of symbiosis is essential to avoid excessive carbon investment when nitrogen is not needed.
Facultative Symbiosis
The symbiosis is facultative; neither the plant nor the bacteria necessarily requires it.
Bacteria differentiate into specialized forms within nodules and often die with the plant.
Plants release chemical signals to attract bacteria and initiate symbiosis when nitrogen is scarce.
High nitrogen levels reduce the production of these signals.
Infection Process
Bacteria colonize young, elongating root hairs.
Root hairs curl around the bacteria.
Bacteria digest the root hair cell wall to form an infection thread.
The infection thread, an inward growth of the plasma membrane, carries bacteria into the root cortex.
Cell divisions are initiated in the cortex.
Bacteria are released intracellularly into cells via endocytosis, surrounded by a plant-derived membrane.
The plant controls nutrient transport across this membrane, regulating bacterial function.
Nodule Development
Nodule development is similar to lateral root development.
Bacterial signals (Nod factors) trigger the first cell divisions, involving the pericycle.
Cortical cells primarily divide to form the nodule.
The nodule differentiates, forming vascular tissue connected to the root to supply carbon and transport fixed nitrogen (usually in the form of amino acids).
Infection and nodule development are coordinated and regulated by the plant but initiated by bacterial signals.
Nod Factors
Rhizobia use Nod factors, signaling molecules with a short N-acetylglucosamine backbone (similar to chitin).
The backbone is modified with various side groups and a lipid tail, providing chemical specificity.
Different Rhizobium species produce different Nod factor signals, tailored to specific legume species.
Nod factors are perceived by specific receptors on plant roots, initiating symbiosis.
Genes for Nod factor synthesis are regulated by the NodD transcription factor, activated by flavonoids from the plant.
Nod factors are necessary for nodule formation and infection thread development.
Purified Nod factors can induce nodule formation.
Mutant Studies
Researchers have identified mutants defective in nodulation to understand the process.
Mutants were screened for defects such as no nodules, infection without nodules, or nodules without infection.
Many mutants also show defects in symbiosis with mycorrhizal fungi.
Shared Signaling Pathways
Mycorrhizal fungi also infect the root and initiate cortical cell divisions.
Plants release signals (flavonoids for Rhizobia, strigolactones for mycorrhizae) to trigger factor synthesis.
Strigolactones also function in shoot branching.
Very similar signalling pathways were activated in the root in response to Rhizobia and micorrhizal fungi, and genetic evidence showed it.
Mycorrhizal factors (MYC factors) have been identified, sharing a similar structure with Nod factors. They bind different receptors but activate similar genes.
The pathway involves calcium spiking responses, but branches to trigger nodulation or mycorrhizae formation.
The mycorrhizal pathway likely predates and was hijacked by Rhizobia.
Evolution of Nodulation
If mycorrhizae are so widespread, why is nodulation so specific?
Certain key genes are missing in most land plants, preventing them from forming symbiosis with rhizobia.
Developmental Similarities
Lateral root and nodule development share similarities, including initiation of cell divisions, primordium formation, and differentiation into an organ.