Lecture 25: Competition in the Rhizosphere

Soil surrounding the roots

the surface of the roots

the interstitial space between cells of the roots

Soil is the environment for interaction between: 1. Pathogens a. Harmful to plant 2. The plant(s) 3. Biocontrol PGPRs a. Symbiotes: integrates, sometimes irreversible relationships b. Commensals: independent colonisers; don’t really add or take away anything

• Amino acids • Organic acids • Putrescine • Sugars • Vitamins etc Plants secrete 20% of the fixed carbon they produce into the soil. This allows for a high biomass in the rhizosphere, causing bacteria to chemotax to the roots. Plants do this to feed the bacteria

• attach and form a microcolony • Colony enlarges, forming a exopolysaccharide biofilm around it

• Various forms of attachment and motility systems in different bacteria • EPS (exopolysaccharides) and fimbriae a. Allow bacteria to attach to surfaces • Flagella - allow chemotaxis • Type 4 pili - allow bacteria to move around on the rhizoplane

Through enzyme secretory pathways • Type 2: secrete toxins to kill fungi, insects and other bacteria • Type 3: effectors

• Chitinases a. Prod. by certain Pseudomonas spp. b. Hydrolyse fungal cell (by hydrolysing chitin) walls - biocontrol • Pectate lyase a. Digest plant material (e.g., potatoes) b. allows bacteria to live off decaying material • AHL Lactonases a. Degrade QS molecules (what bacteria to know their position and surrounding bacterial population b. Interferes with Quorum sensing of other bacteria - changing their behaviour • Bacteriocins a. Diverse, narrow-spectrum proteinaceous toxins b. Allows bacteria to kill each other to fight for rhizosphere space c. There is often immunity genes against these

Type 3 secretory effectors Affects the plant/fungal cells; e.g., by toying with their immune system

• Erwinia amylovora - cause fire blight • Xanthomonas campestris - cause black rot/leaf spot a. excessive secretion of type 2 enzymes • Steptomyces scabies (common scab) a. plant immune system trying to fight off bacteria

Biocontrol PGPR bacteria • Suppress pathogenic plant bacteria • Contribute to: a. biofertilisation b. root growth stimulation c. Plant stress control d. Pathogen antagonism

• Biofertilisation a. Plants need various things. E.g., nitrogen, phosphorus, etc. which they may not be very good at acquiring themselves b. Rhizobium - ‘symbiotic’ nitrogen fixation c. Azospirillum, Azotobacter - free-living N2 fixers (nitrogen bio-fertilisers) d. Phosphate solubilisation - inorganic phosphate to organic acids • Root growth stimulation • Plant stress control • Pathogen antagonism

• Hormones • Auxins - plant growth hormones, promote growth and development a. Indole-3-acetic acid (IAA) • Bigger plant = more exudate!!!! • Azotobacter, Pseudomonas sp. a. Encode genes for production and degradation of IAA b. also produce other plant growth factors - “Gibberellins” and “Cytokinins”

• Bacillus, Enterobacter, certain Pseudomonas sp a. Prod enzymes for synthesis of “2-3- butanediol acetoin” (plant growth promoting metabolites) b. (“Volatile”) • Pyrroloquinoline quinone (PQQ) a. Antioxidant b. Antifungal activity c. ISR induction d. (“Cofactor”)

“Aminocyclopropane-1-carboxylic acid” (ACC) ethylene precursor • Stress -> ethylene production • = Inhibition of root elongation, accelerated abscission, aging and senescence • ACC deaminase turns the ACC into NH3 and alpha-ketobutyrate • ACC deaminase protects against: a. Polyaromatic hydrocarbons b. Phytopathogenic bacteria c. Heavy metals (e.g., Ca2+, Ni2+) d. Drought e. Salt

• Siderophores a. Pyoverdin b. Pyochelin • Phenazines a. Phenazine-1-carboxylate b. Phenazin-1-carboxamide c. Pyocyanin • Antibiotics a. Hydrogen cyanide b. 2-4-DAPG c. Pyoluteorin d. Pyrrolnitrin • Cyclic lipopeptides a. Viscosinamide

• Produce by many bacteria • Iron/other metal-binding small (500- 1000Da) molecules • Facilitate metal ion scavenging • Once secreted, bind to a metal, and then is re-up-taken by a different bacteria • E.g., Pyoverdin: a. Fe3+ scavenger b. Can inhibit pathogen growth via iron limitation • E.g., Pyochelin a. Weak Fe3+, strong Cu2+ & Zn2+ chelator b. Can function as antifungal antibiotic

• Function as powerful surfactants • Antibiotic activity due to membrane solubilisation • Allow bacteria to move across plant surface • Without these genes (when KO) they dont move

E.g., Pyocyanin & Phenazine-1-carboxamide • Inhibit electron transport in pathogenic species • Catalyse formation of hydroxyl radicals in combination with ferripyochein - damage lipids and other molecules • Contribute to oxygen shuttling in microoxic environments • May contribute to iron mobilization in soil

Raises percentage of culturable soil bacteria from <1% to ~50% • Bacteria can get in but not out and then be cultured on soil

> 30,000 different species of bacteria Even the same species can have HUGE diversity between them Do different things previously discussed

Symbiosis of plants with AMF occur in more than 90% of plant species For the plant: • improved phosphate uptake • improved nitrogen uptake For the fungus: 1. Carbon - hexose sugars