Rhizospheric microorganisms and effects, PGPR and Mycorrhiza

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 term `rhizosphere` was coined by Lorenz Hiltner in 1904 to describe the soil compartment that is affected by plant roots. The prefix `rhiza-` comes from the Greek word for `root`.

The rhizosphere is considered the most active region of soil as it receives the nutrients from the plant, in addition to the microorganisms that are present around the root. It is a dynamic environment that fluctuates with the stages of root growth and senescence. The rhizosphere is an important part of soil microbiology, which is responsible for various metabolic processes occurring in the soil, such as 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.

The rhizosphere is a complex system that involves interactions between the plant, the soil, and the microorganisms. These interactions can be positive, negative, or neutral, depending on the type and function of the organisms involved. Some of the benefits of rhizosphere microorganisms for plants include:

  • Enhancing plant growth and development by producing phytohormones, solubilizing minerals, fixing nitrogen, and increasing water uptake.
  • Protecting plants against pathogens by producing antibiotics, antagonizing pathogens, or inducing systemic resistance.
  • Modifying soil structure and chemistry by decomposing organic matter, altering pH, and increasing aggregation.

Some of the drawbacks of rhizosphere microorganisms for plants include:

  • Competing with plants for nutrients, water, and space.
  • Causing diseases or infections by invading plant tissues or producing toxins.
  • Interfering with plant signaling or metabolism by producing allelochemicals or degrading plant compounds.

The rhizosphere is a key component of sustainable agriculture and environmental management, as it affects plant productivity, soil quality, and ecosystem functioning. Understanding the rhizosphere processes and their regulation by plant and microbial factors is essential for developing strategies to enhance crop yield and resilience, as well as to conserve soil resources and biodiversity.