Plant-Microbe interaction

Plant-Microbe interaction 

Plants are non-motile but they constantly encounter both the biotic and abiotic stress. There is a constant war between the pathogenic microbes and the host plant – the outcome of which determines resistance or disease.

Abiotic factors

• Soil properties

pH, soil type, macronutrient distribution, organic matter, salinity, soil structure, and moisture content drive the microbial community formation.

• Environmental factors

Climate, light, water, ultraviolet (UV) radiation and geographic location.

Biotic factors

• Plant factors

Plant immune system, plant compartment, metabolite secretions, plant age, plant-plant interactions, root morphology, root exudates plant genotype

• Anthropogenic factors

Agricultural practices including dosage of fertilizers, pesticide sprays, cultivation practices, pollution, and several other human activities that distrub the quality of soil, air and water.

•  Plants secrete various organic compounds resulting in a nutritionally enriched environment favorable for microbial growth.
•  As a result, plants are heavily colonized with a diversity of microbes whose reservoir is primary the soil. 
•  Microbes that colonize plants are called either epiphytes (colonize plant surface) or endophytes (colonize plants interior)  
• Microbial communities influence plants in direct and indirect ways. 
• Plants and microbes can have variety of interactions including pathogenic, symbiotic and associative.

Pathogenic Relationship 

• The contact between plant and pathogenic microorganism lead to a particular chain of events in plant organism. 
•  The extracellular space between cell wall and plasma membrane acts as a first battle field between plants and pathogens. 
• Bacteria, fungi, viruses and oomycetes that colonize the living plant tissues are encased in this narrow region in the initial step of infection. 
• Therefore, the apoplastic region is believed to be an interface which mediates the first crosstalk between host and pathogen. 
•  The secreted proteins and other metabolites, derived from both host and pathogen, interact in this apoplastic region and govern the final relationship between them. 

 Types of pathogen based on effects :

1. Necrotrophy : plant cells are killed 

2. Biotrophy : plant cells remain alive 

3. Hemibiotrophy : plant cells initially alive later killed.

Symbiotic relationships 

 Symbiosis refers to relationships between organisms of different species that show an intimate association with each other. 

Symbiotic relationships provide at least one of the participating species with a nutritional advantage. 

3 types of symbiosis have been recognized depending on the nature of relationship. 

• Parasitism
• Mutualism 
• Commensalism 

Parasitism 

Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life.Parasites are therefore chemoautotrophs.

Mutualism 

• Mutualism is a biological interaction between two species where in both species benefit from each other. 
• This term was discovered by Pierre van Benden. 
• They exchange food or provide shelter or protection, but may still be able to live independent life.

Mutualism examples 
There are many different examples of mutualistic relationship : 
Plants and Microbes : eg. Rhizobium in root nodules. 
Nitrogen fixing bacteria
Plant growth promoting rhizobacteria(PGPR
Biocontrol microorganisms
Mycoparasitic fungi and protozoa.
Lichen -  Fungus and algae 
Terrestrial plants and insects : eg. Pollination
Mycorrhizae - Fungus and plant roots. There are three types of mycorrhizae

• Ectomycorrhiza e.g. Amantia spp.
• Endomychorrhizae e.g.Glomus spp.
• Ectendomycorrhizae e.g. Endogone sp.

Commensalism 

• Commensalism is the relationship between two different species in which one species benefits from the relationship and the other is neither harmed or helped. 
• Very few of such relationships exist, as it is very unlikely the two organisms can live together without them affecting each other. 
• Commensalim example - Penicillium digitatum is responsible for stimulation of the fungus Diplodia natalensis that cause fruit infection citrus.

Other relationships

Ammensalism 

Ammensalism is the ecological interaction in which an individual species harms another without obtaining benefit. This type of symbiotic relationship is common, but not considered as an important process structuring communities because they are accidental and do not benefit the species doing the harm.

Examples 

 Algal blooms can lead to the death of many species of fish and other animals, however the algae do not benefit from the deaths of these individuals. 

Black walnut trees secretes a chemical from its roots which prevents the growth of neighboring trees.  Elephants stepping on ants or leveling brush does not benefit the elephant but harms the ants and brush.

Synergism 

The interaction or cooperation of two or more organizations, substances or other agents to produce a combined effect greater than the sum of their separate effects. 

Example : E.coli, which obtains nutrients from food materials ingested by host produces vitamins which are used by the host. 

The synergistic action of Lactobacillus bulgaricus and Streptococcus thermophillus in the production of yoghurt.

Inquilinism

This is a type of relationship in where one organism uses the other as a permanent housing or place to abode 

e.g. phanerogamic plants (Cuscuta, Loranthus) grow on trees 

Metabiosis

This is a relationship in which one organism creates or prepares a suitable environment for the other.

e.g. apple tissue infected with Glomerella cingulata supported the growth of bacterial food borne pathogens (E coli and Listeria monocytogenes) Presence of high population of Salmonella in bacterial soft rot of fresh fruits and vegetables