PLANT - MICROBE INTERACTION

PLANT - MICROBE INTERACTION

• The interactions of plants with microbes both in the soil and above ground shoot are of great importance for the growth and productivity of plants in agricultural and natural ecosystems.
• Many microbe-plant interactions do no harm to the plant, whereas the microbe gains some advantage. Such relationships, in which one partner benefits but the other is neither hurt nor helped, is called Commensalism.
• Many other important interactions are beneficial to both the microorganism and the plant (i.e., are mutualistic).
• Microbe-plant interactions can be broadly divided into two classes:

a)     Epiphytes - Microbes that live on the surface of plants are called Epiphytes.

b)     Endophytes - Microbes that colonize internal plant tissues are called Endophytes.

 

PHYLLOSPHERE MICROORGANISMS

• The Phyllosphere is a term used in microbiology to refer to the total above-ground portions of plants as habitat for microorganisms. 
• The Phyllosphere region can be further sub-divided into

a)     Caulosphere – Stems

b)     Phylloplane – Leaves

c)     Anthosphere – Flowers

d)     Carposphere – Fruits

 

Microorganisms in Phyllosphere region

• Phyllosphere is home to a diverse assortment of microbes including bacteria, filamentous fungi, yeasts, and photosynthetic and heterotrophic protists.
• Proteobacteria, Pseudomonas syringae and Erwinia, and Pantoea spp. are most important bacterial species present in Phyllosphere.
• Another abundant bacterial genus, Sphingomonas, produces pigments that function like sunscreen so it can survive the high levels of UV irradiation occurring on these plant surfaces.

Beneficial activities of Phyllosphere Microorganisms

• Plant protection

ü  Antibiosis

ü  Competetion

ü  Anti-pests

• Plant growth

ü  Plant growth promoting hormones

• Ecosystem service

ü  Removal of Environmental pollutants.

 

Harmful activities of Phyllosphere Microorganisms

• Plant pathogens

ü  Blight

ü  Rust

ü  Spot

ü  Wilt 

• Human pathogens

ü  Enteric bacteria

ü  Fungal spores

 

RHIZOSPHERE MICROORGANISMS

• The Rhizosphere is the narrow region of soil that is directly influenced by root secretions, and associated soil microorganisms known as the root microbiome.
• In 1904 the German Agronomist and Plant Physiologist Lorenz Hiltner first coined the term "Rhizosphere" to describe the plant-root interface, a word originating in part from the Greek word "rhiza", meaning root.
• Hiltner described the Rhizosphere as the area around a plant root that is inhabited by a unique population of microorganisms influenced by the chemicals released from plant roots.
• The rhizosphere represents one of the most diverse habitats on the planet and is central to ecosystem functioning. Infinite dynamic interactions between root exudates, microbial activity, genetic exchange, nutrient transformation, and gradient diffusion are most likely the factors shaping this below-ground world.
• In the years since, the rhizosphere definition has been refined to include three zones.

a)    Endorhizosphere - The Endorhizosphere includes portions of the cortex and endodermis in which microbes and cations can occupy the "free space" between cells (apoplastic space).

b)   Rhizoplane - The Rhizoplane is the medial zone directly adjacent to the root including the root epidermis and mucilage.

c)  Ectorhizosphere - The outermost zone is the Ectorhizosphere which extends from the rhizoplane out into the bulk soil. 

Microorganisms in Rhizosphere region

• Bacteria belonging to the genera Azospirillum sp., Azotobacter sp., Azoarcus sp., Arthrobacter sp., Klebsiella sp., Burkholderia sp., Citrobacter sp., Pseudomonas sp., Bacillus sp., Paenibacillus sp., Herbaspirillum sp., Erwinia sp., Flavobacterium sp., Serratia sp., Xanthomonas sp., Rhizobium sp., Alcaligenes faecalis, Enterobacter cloacae, Gluconacetobacter diazotrophicus and Bradyrhizobium japonicum have been shown to produce plant growth promoting substances.

Beneficial activities of Rhizosphere Microorganisms

• Plant growth promoting rhizobacteria are free - living, soil - borne bacteria, which enhance the growth of the plant either directly or indirectly.

a)     The direct mechanisms involve

ü  N₂ fixation

ü  Phytohormones production (Auxins and Gibberellins)

ü  Siderophores production

ü  Ammonia production

ü  Solubilization of Phosphorous

ü  Lowering of Ethylene concentration (ACC [Aminocyclopropane-1-carboxylate] deaminase activity).

b)     Some bacteria support plant growth indirectly by

ü  Production of Antagonistic substances (Antibiotics and Lytic enzymes)

ü  Hydrogen cyanide (HCN) production

ü  Competition

ü  Exopolysaccharide (EPS) production

ü  Induced Systemic Resistance (ISR) towards plant pathogens.

Harmful activities of Rhizosphere Microorganisms

• Plant pathogens

ü  Rot

ü  Wilt 

• Human pathogens

ü  Enteric bacteria