The Coffee ecosystem is a self-sustaining stable ecosystem because of the Agroforestry model. Multiple crops grow harmoniously with a heterogeneous tree population. Coffee soils are rich in organic matter because of the enormous amounts of leaf litter due to the constant shedding of leaves from trees, shrubs, and the coffee bush. This provides the base for the proliferation of microbes, most of which are beneficial to coffee and multiple crops associated with the coffee plantation. A well-structured and regulated community of microorganisms is always associated with the coffee plant.
This article singles out the role of Plant growth promoting Rhizobacteria (PGPR ) and how this interaction could pave way for a new wave of farming in eco-friendly coffee. Our current understanding has demonstrated two main aspects. First, we have very little research data about the role of PGPR in the coffee ecosystem. Second, isolation and characterization of PGPR will hold the promise of improving coffee and multiple cop yields.
Plant growth-promoting rhizobacteria (PGPR) are naturally occurring soil bacteria that aggressively colonize plant roots and benefit plants by providing growth promotion. Rhizobacteria are often referred to as plant growth-promoting rhizobacteria or PGPRs.
The term PGPR was coined by Joe Kloepper in the late 1970s and was defined by Kloepper and Schroth, as ”the soil bacteria that colonize the roots of plants by following inoculation onto the seed and that enhance plant growth. PGPRs are reported to influence growth, yield, and nutrient uptake.
A review of the literature clearly defines the rhizosphere, as the zone of soil surrounding a plant root where the biology and chemistry of the soil are influenced by the root. This zone is about 1 mm wide but has no distinct edge. Rather, it is an area of intense biological and chemical activity influenced by compounds exuded by the root, and by microorganisms feeding on the compounds. While the term ‘rhizobacteria’ implies a group of rhizosphere bacteria competent in colonizing the root environment. About 2–5% of the rhizosphere bacteria are PGPR.
Scientific Literature also clearly elucidates the role of PGPR in increasing the availability of nutrients. PGPRs increase the availability of nutrients through the solubilization of unavailable forms of nutrients and by the production of siderophores which aids in the facilitating of nutrient transport. Phosphorus, a limiting nutrient for plant growth, can be plentiful in soil but is most commonly found in insoluble forms. Organic acids and phosphatases released by rhizobacteria found in plant rhizospheres facilitate the conversion of insoluble forms of phosphorus to plant-available forms such as H2PO4−. The well-known mechanism for growth promotion is through producing various plant growth hormones that include Gibberellin and Indole-3-acetic acid (IAA). Production of siderophores that chelate iron and make it available to the plant root.
PGPR that synthesizes auxins, gibberellins, and kinetins or that interferes with plant ethylene synthesis has been identified.
PGPR has the potential to curb the adverse effects of various stresses such as salinity, drought, heavy metals, floods, and other stresses on plants by inducing the production of antioxidant enzymes such as catalase, peroxidase, and superoxide dismutase.
Acinetobacter, Agrobacterium, Arthobacter, Azotobacter, Azospirillum, Burkholderia, Pseudomonas putida, Azospirillum fluorescens, and Azospirillum lipoferum and notable nitrogen-fixing bacteria associated with legumes includes Allorhizobium, Azorhizobium, Bradyrhizobium, and Rhizobium, Serratia, Thiobacillus, Pseudomonads, and Bacilli.
Broad composition of PGPR
Amino acids, monosaccharides, organic acids, carbohydrates, sugars, vitamins, mucilage, and proteins.
Advantages of PGPR
Defend the rhizosphere and root against pathogenic microorganisms.
Increasing nitrogen fixation
Help in promoting free-living nitrogen-fixing bacteria
Attract and repel particular microbe species and populations.
Increase the supply of other nutrients, such as phosphorus, iron, and other macronutrients and micronutrients.
Produce plant hormones that enhance other beneficial bacteria or fungi.
Change the chemical properties of the soil around the roots.
Keep the soil around the roots moist.
Encourage plant growth.
Stabilise soil aggregates.
Inhibit the growth of competing plant species
Relationship of PGPR with plants
PGPRs have different relationships with different species of host plants. The two major classes of relationships are rhizospheric and endophytic.
Rhizospheric relationships consist of the PGPRs that colonize the surface of the root, or superficial intercellular spaces of the host plant, often forming root nodules.
Endophytic relationships involve the PGPRs residing and growing within the host plant in the apoplastic space.
Rhizobacteria are also able to control plant diseases that are caused by other bacteria and fungi. The disease is suppressed through induced systemic resistance and through the production of antifungal metabolites. Pseudomonas biocontrol strains have been genetically modified to improve plant growth and improve the disease resistance of agricultural crops.
PGPR functions in three different ways.
Synthesizing particular compounds for the plants
Facilitating the uptake of certain nutrients from the soil
Preventing plant diseases
Future Area of Research
Isolation and characterization of PGPR microbes from different coffee Agro Climatic regions.
Host specificity and compatibility. The growth-promoting ability of some bacteria may be highly specific to certain plant species, cultivar,s and genotypes.
Development of PGPR into commercial biofertilizers and biopesticides. Which are region-specific (Altitude, High Rainfall, Soil type. Tree diversity)
Private-Public Partnerships for Increased Knowledge and Improved Training
Every step in the process from microbe isolation to licensing is laborious, expensive, and requires time. Collaboration between industrial, academic, and government research should become an important part of the product development process.
Recently PGPR has attracted the attention of agriculturists as soil inoculums to improve plant growth and yield. PGPR has the potential to contribute to the development of sustainable agricultural systems. It will not only help build up a vibrant plant-soil microbial relationship but will also enhance the ecological integrity of the coffee ecosystem. This in turn will reduce the dependency on chemical fertilizers and build healthy soils.
Anand T Pereira and Geeta N Pereira. 2009. Shade Grown Ecofriendly Indian Coffee. Volume-1.
Anand Titus Pereira & Gowda. T.K.S. 1991. Occurrence and distribution of hydrogen-dependent chemolithotrophic nitrogen-fixing bacteria in the endo rhizosphere of wetland rice varieties grown under different Agro-climatic Regions of Karnataka. (Eds. Dutta. S. K. and Charles Sloger. U.S.A.) In Biological Nitrogen Fixation Associated with Rice production. Oxford and I.B.H. Publishing. Co. Pvt. Ltd. India.
Subba Rao. N. S. 1998. Soil Microorganisms And Plant Growth. Oxford and IBH Publishing Co.
Bopanna, P.T. 2011.The Romance of Indian Coffee. Prism Books ltd.