Rhizosphere effect

1. Rhizosphere effect
BY:
N. MATHAN
BP211508
II MSC APPLIED MICROBIOLOGY
SACRED HERAT COLLEGE (AUTONOMOUS)
TIRUPATTUR

2.  What is Rhizosphere?
 The rhizosphere is the narrow region of soil around the plant root that is influenced by several
factors like the root exudates and the associated soil microorganisms.
 The rhizosphere is considered the most active region of soil as it receives the nutrients from the
nutrients, in addition to the microorganisms that are present around the root.
 It is a dynamic environment fluctuating with the stages of root growth and senescence.
 Rhizosphere as a region was defined more than a century ago by Lorenz Hiltner as a soil
compartment influenced by plant roots.
 The rhizosphere is an important part of soil microbiology which is responsible for various
metabolic processes occurring in the soil like cycling of nutrients and uptake of carbon.
 The roots of crop plants create an interface between the plant and the soil environment, thus
establishing an enormous reservoir of the microbial community.
 The area of rhizosphere usually extends a few millimeters from the root surface where the roots
release various compounds like root exudates, mucilage, and sloughed-off root cells that
support higher microbial populations and activities than in bulk soil.

3.  What is the Rhizosphere effect?
 The rhizosphere effect is the influence of plant roots on the development of soil microorganisms as a
result of the physical and chemical alteration of soil and the release of root secretions and exudates
within the rhizosphere.
 The rhizospheric effect is observed on the basis of the microbial biomass of the rhizosphere when
compared to the biomass of the bulk soil.
 The rhizosphere effect on soil microbial population can be measured by comparing the population
density [colonies forming units (CFU)] between the rhizosphere soil (R) and the bulk soil (S), for which
the “R/ S ratio” is employed.
 The rhizosphere effect is higher for bacteria > fungi > actinomycetes > protozoa.
 The microorganism diversity is higher near to the rhizoplane, which then decreases with an increase in
distance from the rhizoplane.
 The interaction between plant nutrients in soil and plant exudates modifies the microclimate of the
rhizosphere.
 The rhizosphere effect is a result of the interaction between the plant root and the microbial
community of the region, where both factors influence each other.
 In the rhizosphere, microbial activity influences the plant root, and the plant root secretions influence
the microbial biomass.

4. Microorganisms found in Rhizosphere (Rhizosphere microbiome)
 The microbial population in the rhizosphere consists of different groups of microorganisms like bacteria, fungi, parasites,
viruses, and algae.
 The microbial population in the rhizosphere is known as the rhizosphere microbiome and the microbial population in
such an area much higher than the bulk soil.
 In the rhizosphere, there is a microbial population distinct from the rest of the soil.
 Bacteria in the rhizosphere are larger and have higher proportions of Gram-negative and denitrifying bacteria than those
in the bulk soil.
 Rhizosphere fungal populations, abundant in both pathogenic and mycorrhizal species, can be 10 to 20 times higher
than those in the non-rhizosphere.
 Protozoa and other microfauna also thrive in the rhizosphere because that is where food is most plentiful.
 The type and population of microorganisms in the rhizosphere are highly influenced by the type of plant grown on the
soil.
 Microbes in the bulk soil often experience long periods of nutrient deprivation; they have different survival strategies in
dealing with starvation and stress.
 The rhizosphere bacterial community is recruited from the main reservoir of microorganisms present in the soil.
 Next to the recruitment of specific soil microbes into the rhizosphere microbiome, plant roots also influence specific
functions of the microbiome.
 Some of the examples of microorganisms found in the rhizosphere region include Bacillus, Arthrobacter, Pseudomonas,
Agrobacterium, Alcaligenes, Clostridium, Flavobacterium, Corynebacterium, Micrococcus, Xanthomonas, Amanita,
Tricholoma, Torrendia, Descomyces, Thelephora, Verticillium, Phytophthora, Rhizoctonia, Micromonospora,

6.  Plant Growth Promoting Rhizobacteria (PGPR) are a group of bacteria that enhances plant growth via
various plant growth-promoting substances as well as biofertilizers.
 PGPR act as biofertilizers are efficient soil microbes for sustainable agriculture and hold great promise in
the improvement of agriculture yields.
 These bacteria enhance crop growth and can help in the sustainability of the safe environment and crop
productivity.
 Some common examples of PGPR genera exhibiting plant growth-promoting activity are
Pseudomonas, Azospirillum, Erwinia, Mycobacterium, Azotobacter, Bacillus, Burkholderia, Enterobacter,
Rhizobium, Mesorhizobium, Flavobacterium, etc.
 PGPR have different roles in promoting soil health as well as crop health. These bacteria function as
biofertilizers, biocontrol agents, and biological fungicides.
 The function of PGPR as biofertilizers is by virtue of the production of growth-stimulating
phytohormones such as indole-3-acetic acid (IAA), gibberellic acid (GA3 ), zeatin, ethylene, and abscisic
acid.
 Besides, these bacteria also promote plant nutrition as bacteria like Rhizobium act as phosphate
solubilizing bacteria increase the availability of accumulated phosphate, increase the efficiency of
biological nitrogen fixation and render availability of iron and zinc through the production of plant
growth-promoting substances.
 PGPR also protects plants against pathogens by direct antagonistic interactions between the biocontrol
agent and the pathogen, as well as by induction of host resistance.

7.  Factors influencing their growth and activities
 Microbial growth and activities in soil depend on resources (carbon and other mineral
nutrients) available and the physiochemical conditions of their habitat.
 A. Nutrients
 The majorities of soil microorganisms are presumably chemoorganoheterotrophic and use
organic compounds as carbon and energy sources.
 The humic fraction of soil organic matter provides the stable microbial nutrient base for
microorganisms; however, humic substances have incredibly complex structures and are
resistant to decomposition.
 In addition, humus is associated with mineral particles and forms organo-clay complexes.
Therefore, humus only serves as a slow-release source of carbon and energy for oligotrophic
microorganisms.
 In the rhizospheric area, other forms of nutrients are released by the plant in the form of
root secretions and root exudates.

8. B. Physiochemical factors of soil
 The physical factors of soil like soil moisture, soil atmosphere, and soil temperature also affect
the growth and activities of microorganisms in the soil and the rhizosphere region.
 Soil water affects not only the moisture available to organisms, but also the soil aeration
status, soil temperature, and a variety of soil chemical reactions.
 Soil temperature affects the rates of physical, chemical, and biological processes in the soil.
 Soil pH influences microorganisms indirectly by changing the chemical forms, solubility, and
availability of chemical compounds.
 C. Interactions
 Different soil organisms frequently interact with each other; the interactions can be positive,
negative, or neutral.
 Positive interactions (commensalism, synergism, and mutualism) enhance the abilities of
populations to survive within a particular habitat; negative interactions (competition,
amensalism, and predation/ parasitism) limit the population growth.
 Interactions like symbiosis and mycorrhiza favor the growth of organisms and their activities in
in the soil.

9. THANK YOU