Ecology - Mutualistic and Other Interactions Between Bacteria and Plants

Bacteria-Plant Interactions

• Commensal: Bacteria benefit without harming the plant.
  • Occurs in the plant rhizosphere and phyllosphere.
• Parasitic: Bacteria cause disease in the plant.
  • Includes bacterial plant pathogens and tumor-forming bacteria like Agrobacterium.
• Mutualistic: Both bacteria and plant benefit (symbiosis).
  • E.g., $N_2$-fixing symbiosis.

Plant Rhizosphere/Phyllosphere

• Rhizosphere: Area around the roots.
  • Bacteria benefit from root exudates (rhizodeposits) such as sugars, amino acids, and organic acids.
• Phyllosphere: Aerial surfaces of the plant, mainly leaves.

Bacterial Adaptation to the Phyllosphere

• Efflux pumps: toxin secretion
• Antibiotics: inhibit other bacteria
• Biosurfactants: increase wettability of plant surface; enhance leaching of substrates
• Nutrient uptake & metabolic adaptations: utilisation of excreted plants metabolites for growth
• EPS -extracellular polymeric substances; help maintain hydrated layer surrounding the bacteria; protect from desiccation, assist in bacterial aggregation
• Flagellum: motility; invasion
• Quorum sensing: communication with other members of same species; epiphytic fitness
• Auxins: plant hormones to stimulate plant metabolite production
• Protective proteins/pigments: e.g. protection against UV, reactive $O_2$ species

Selected Plant Pathogens

• Xylella fastidiosa: infects grapevine and citrus fruits, transmitted by leafhoppers (endophyte).
• Xanthomonas campestris: infects Brassicaceae, enters via wounds (endophyte).
• Pantoea stewartii: infects maize, vectored by corn flea beetles (endophyte).
• Burkholderia cepacia: infects onion, colonizes seed in soil (epiphyte).

Agrobacterium tumefaciens

• Causes crown gall disease.
• Involves horizontal gene transfer.
• Ti (tumor-inducing) plasmid: contains genes for auxin/cytokinin production, opine synthesis, and vir genes (for T-DNA transfer).

Dinitrogen Fixing Bacteria and Archaea

• Convert atmospheric nitrogen ($N<em>2$) to ammonia ($NH</em>3$).

Types:

• Free-living (aerobes): Klebsiella, Azotobacter.
• Free-living (anaerobes): Clostridium, Desulfovibrio.
• Symbiotic: Rhizobium, Mesorhizobium.
• Some Cyanobacteria (Anabaena, Nostoc) can be free-living or symbiotic.

N2 Fixation – Production of Ammonia

• Chemical: 700-900 K (ca. 500oC), 100-900 atm
• Biological: 300 K (27oC), 1 atm

Nitrogenase

$8H^+ + 8e^- + N<em>2 + 16 MgATP \longrightarrow 2NH</em>3 + H_2 + 16 MgADP + 16 Pi$

N2 Fixation – Nodule Forming Bacteria

• Rhizobial: Rhizobium, Mesorhizobium, Sinorhizobium (Alphaproteobacteria), Parasponia (Cannabaceae).
• Actinorhizal: Frankia (Actinobacteria), Betulaceae (e.g., Alnus) and Rosaceae (e.g., Purshia).

Root Invasion by Dinitrogen Fixing Rhizobia

• Bacterial protein secretion into plant cells - via type III and/or type IV secretion systems
• Bacterial nod and vir genes
• Expression of plant genes (e.g. SYM)

Heterocysts in Cyanobacteria

• Contain nitrogenase.
• Obtain carbon from neighboring vegetative cells.
• Supply fixed N as amino acids.

Life Cycle of Heterocyst Forming Anabaena (Cyanobacteria)

• Heterocysts: form in response to N deprivation
• Akinetes: resting structures, germinate under favourable growth conditions
• Hormogonia: motile filaments, dispersal forms; e.g. to establish new symbioses with plants (e.g. Azolla or water fern)

Azolla-Anabaena Symbiosis

• Azolla: provides cavities filled with $N_2$ and additional carbon from photosynthesis
• Used in rice paddies as biofertilizer