Plant Nutrition and Mutualistic Relationships

Mutualism Across Kingdoms and Domains

• Mutualism: Relationships between different species where each provides a substance or service that benefits the other.
• Mutualisms can occur within the same kingdom or between different kingdoms/domains.

Examples of Mutualism

Animal-Fungus

• Leaf-cutter ants:
  • Harvest leaves but don't eat them directly.
  • Cultivate a fungus on the leaves.
  • Ants eat the cultivated fungus.

Fungus-Bacterium

• Lichens (e.g., Peltigera):
  • Mutualistic association between a fungus and a photosynthetic partner (cyanobacterium).
  • Cyanobacterium provides carbohydrates.
  • Fungus provides anchorage, protection, minerals, and water.

Animal-Bacterium

• Puffer fish (fugu):
  • Contain tetrodotoxin, a nerve toxin.
  • The toxin is synthesized by mutualistic bacteria (Vibrio species).
  • Fish gains a chemical defense.
  • Bacteria live in a high-nutrient, low-competition environment.
• Preparation of fugu requires specially trained chefs to remove poisonous parts.

Plant-Bacterium

• Floating fern Azolla:
  • Provides carbohydrates for nitrogen-fixing cyanobacteria in its leaves.
  • Receives nitrogen from the cyanobacteria.

Plant-Fungus

• Mycorrhizae:
  • Mutualistic associations between roots and fungi.
  • Fungus absorbs carbohydrates from roots.
  • Fungal mycelium (hyphae network) increases the surface area for water and mineral uptake by roots.

Plant-Animal

• Plants defended by ants:
  • Plant provides carbohydrate-rich nectar from nectaries.
  • Ants protect the plant from predators and competitors.

Bacteria and Plant Nutrition

• Soil bacteria play roles in plant nutrition through:
  • Mutually beneficial chemical exchanges with plant roots.
  • Enhancing decomposition of organic materials.
  • Increasing nutrient availability.

Rhizobacteria

• Rhizobacteria: Bacteria living closely with plant roots or in the rhizosphere (soil surrounding roots).
• Form mutually beneficial associations with plant roots.
• Depend on plant-secreted nutrients (sugars, amino acids, organic acids).
• Plants may allocate up to 20% of photosynthetic production to fuel bacterial communities.

Benefits for Plants

• Antibiotics protect roots from disease.
• Absorption of toxic metals.
• Increased nutrient availability.
• Conversion of gaseous nitrogen into usable forms.
• Production of chemicals that stimulate plant growth.
• Inoculation of seeds with plant-growth-promoting rhizobacteria can increase crop yield and reduce the need for fertilizers and pesticides.

Types of Rhizobacteria

• Free-living in the rhizosphere.
• Endophytes: Live between cells within the plant.
• Unique composition of bacterial communities endophytically and in the rhizosphere depending on root secretions and microbial products.

Experiment: Variability of Bacterial Communities

• Experiment to determine the complexity and factors affecting bacterial communities in and around roots.
• Metagenomics technique used to estimate the number of bacterial "species".
  • Analyzed DNA coding for 16S ribosomal RNA subunits.
  • Sequences >97% identical were grouped into "taxonomic units" or "species".

Variables Tested

• Location (endophytic, inside/outside rhizosphere).
• Soil type (clayey/porous).
• Developmental stage of root system (old/young).

Results

• Bacterial community composition varied markedly by location and soil type.
• Example:
  • Bacteria inside roots in clayey soil and bacteria inside roots in porous soil are 34% similar.
  • Soil bacteria outside the rhizosphere in younger and older roots in porous soil are 80% similar.

Bacteria in the Nitrogen Cycle

• Nitrogen is essential for synthesizing proteins and nucleic acids.
• Nitrogen deficiency is a major limitation to plant growth.
• Plants use $NO<em>3^-$ and $NH</em>4^+$.
• Nitrogen sources:
  • Weathering of rocks.
  • Lightning (produces small amounts of $NO_3^-$, carried in rain).
  • Activity of bacteria (major source).
• Nitrogen cycle: Natural processes where nitrogen-containing substances are made available, used, and returned.

Nitrification

• Two-step process to create $NO_3^-$.
  • Oxidation of ammonia ($NH<em>3$) to nitrite ($NO</em>2^-$).
  • Oxidation of nitrite ($NO<em>2^-$) to nitrate ($NO</em>3^-$).
  • Mediated by different types of nitrifying bacteria.
• Plant enzyme reduces $NO<em>3^-$ back to $NH</em>4^+$, which is incorporated into organic compounds.
• Nitrogen is transported from roots to shoots via the xylem as $NO_3^-$ or as organic compounds.

Nitrogen Loss

• Loss occurs in anaerobic soils when denitrifying bacteria convert $NO<em>3^-$ to $N</em>2$ (diffuses into the atmosphere).

Ammonium Acquisition

• Plants can acquire nitrogen as $NH_4^+$.
  • Nitrogen-fixing bacteria convert gaseous nitrogen ($N<em>2$) to $NH</em>3$, which becomes $NH_4^+$ in soil.
  • Ammonification: Decomposers convert organic nitrogen from dead material into $NH_4^+$.

Bacteria and Nitrogen Fixation

• Atmosphere: 79% nitrogen ($N_2$), but plants can't use it directly due to the triple bond.
• Nitrogen fixation: Reduction of $N<em>2$ to $NH</em>3$.
• All nitrogen-fixing organisms are bacteria.
  • Some are free-living in the soil.
  • Others live in the rhizosphere (e.g., Rhizobium).
• Rhizobium forms associations with legume roots.

Nitrogen Fixation Equation

• $N<em>2 + 8e^- + 8H^+ + 16 ATP \rightarrow 2 NH</em>3 + H<em>2 + 16 ADP + 16 P</em>i$
• Nitrogenase: Enzyme complex that drive the reaction.
• Requires 16 ATP molecules for every 2 $NH_3$ molecules.
• Requires a rich carbohydrate supply.

Rhizobium-Legume Mutualism

• Involves changes in root structure.
• Nodules: Swellings on legume roots, where plant cells are infected by Rhizobium.
• Bacteroids: Form of Rhizobium bacteria inside nodules.

Benefits of Legume-Rhizobium Relationships

• Generate more usable nitrogen than industrial fertilizers.
• Nitrogen fixation by Rhizobium requires an anaerobic environment, facilitated by:
  • Location of bacteroids inside living cells.
  • Woody external layers of root nodules.
• Leghemoglobin: An oxygen buffer, reducing free oxygen concentration (similar to hemoglobin).
  • Provides anaerobic conditions for nitrogen fixation.
  • Regulates oxygen supply for cellular respiration.
• Mutualistic relationship: Bacteria supply fixed nitrogen; plant provides carbohydrates.
• Most ammonium produced is used to make amino acids which are transported to the shoot through the xylem.
• Each legume species is associated with a specific strain of Rhizobium bacteria and each partner responds to chemical signals from the other by expressing certain genes whose products contribute to nodule formation.

Nitrogen Fixation and Agriculture

• Underlies most types of crop rotation.
• Nonlegume (e.g., maize) planted one year, followed by a legume to restore nitrogen.
• Seeds are exposed to specific Rhizobium strain before sowing.
• Legume crop is often plowed under as "green manure."
• Rice benefits indirectly from mutualistic nitrogen fixation due to Azolla and cyanobacteria.
  • Rice farmers culture Azolla, which hosts nitrogen-fixing cyanobacteria.
  • Decomposition of Azolla increases rice paddy fertility.
  • Ducks eat Azolla, adding manure.

Fungi and Plant Nutrition

• Soil fungi form mutualistic relationships with roots.
• Mycorrhizae ("fungus roots"): Intimate associations of roots and fungi.
  • Host plant provides sugar.
  • Fungus increases surface area for water uptake and supplies phosphorus and minerals.
  • Fungi secrete growth factors that stimulate roots to grow and branch and antibiotics that help protect the plant from soil pathogens.

Types of Mycorrhizae

Ectomycorrhizae

• Dense sheath (mantle) of mycelia over the root surface.
• Hyphae extend into the soil and root cortex.
• Do not penetrate root cells, form a network in the apoplast (extracellular space).
• Roots are thicker, shorter, and more branched; typically lack root hairs.
• Found in about 10% of plant families, mostly woody plants.

Arbuscular Mycorrhizae (Endomycorrhizae)

• Do not ensheath the root; embedded within it.
• Hyphae grow toward the root, penetrate between epidermal cells, and enter the cortex.
• Hyphae digest patches of cell walls but don't pierce the plasma membrane.
• Arbuscules: Branching hyphae that form structures are important sites of nutrient transfer.
• Vesicles: Oval structures within hyphae, possibly for food storage.
• Far more common than ectomycorrhizae; found in over 85% of plant species, including most crops.

Agricultural and Ecological Importance of Mycorrhizae

• Good crop yields depend on mycorrhizae formation.
• Treating seeds with spores of mycorrhizal fungi can help seedlings form mycorrhizae, facilitating recovery of damaged natural ecosystems and improving crop yield.
• Arbuscular mycorrhizae exhibit little host specificity.
  • Mycorrhizal networks may benefit some plant species more than others.

Unusual Nutritional Adaptations in Plants

• Almost all plants have mutualistic relationships with soil fungi, bacteria, or both.
• Some plants have adaptations for exploiting other organisms.

Epiphytes

• Grow on other plants but produce their own nutrients.
• Absorb water and minerals from rain through leaves.
• Examples: staghorn ferns, bromeliads, orchids.

Parasitic Plants

• Absorb water, minerals, and sometimes products of photosynthesis from living hosts.
• Haustoria: Nutrient-absorbing projections that tap into the host plant.
• Some lack chlorophyll (e.g., dodder), others are photosynthetic (e.g., mistletoe).
• Some absorb nutrients from hyphae of mycorrhizae associated with other plants (e.g., Indian pipe).

Carnivorous Plants

• Photosynthetic but supplement mineral diet by capturing insects and small animals.
• Live in nutrient-poor soils.
• Examples:
  • Pitcher plants: Modified leaves form water-filled funnels.
  • Sundews: Exude sticky fluid from tentacle-like glands.
  • Venus flytraps: Close quickly when prey triggers hairs.