Microbial Health of the Rhizosphere

Microbial Health of the Rhizosphere

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Microbial Health of the Rhizosphere
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The importance of the interactions between microorganisms, plants and the rhizosphere was realized as early as 1904 by Soil Bacteriologist and Professor of Agronomy at the Technical College of Munich, Lorenz Hiltner. “The term rhizosphere was introduced by Hiltner in 1904.” (Gobran, 2001) Hiltner “emphasized the critical role of microbial activities in the ‘rhizosphere’ in the nutrition and general health of plants.” (Curl, 1986) The term rhizosphere was used to “describe specifically the interaction between bacteria and legume roots.” (Lynch, 1990)

Today there is debate among microbiologists and plant scientists regarding the definition of rhizosphere (Curl, 1986). “Rhizo” is derived from the Greek word “rhiza,” meaing “root”. “Sphere” is “one’s field of action, influence, or existence: one’s natural surroundings.” (Lynch, 1990 “Rhizosphere is the zone where root activity significantly influences biological properties.” (Manthey, 1994)

There are three main areas of research that are done on the rhizosphere. The first one is the “influence of roots on microorganisms.” The second is “influence of microorganisms on plant growth,” and the third is “rhizosphere influence on soil-borne pathogens and plant disease.” (Curl, 1986)

When Hiltner first talked about the rhizosphere in 1904, he stated, “The nutrition of plant in general certainly depends upon the composition of the soil flora in the rhizosphere…If plants have the tendency to attract useful bacteria by their root excretions, it would not be surprising if they would also attract uninvited guests which, like the useful organisms, adapt to specific root excretions.”

This speech identified two of the main topics of rhizosphere research: “(1) the relation of the rhizosphere to plant nutrition, growth, and development, and (2) the influence of rhizosphere phenomena on pathogens and pathogenesis.” (Curl, 1986) It was realized as early as 1904 that rhizosphere microorganisms can cause disease or transmit viruses as well as benefit the plant.

There are microorganisms in the bulk soil as well as in the rhizosphere. The microorganisms in the soil include bacteria, fungi, protists, actinomycetes, and nematodes. These microorganisms are not distributed uniformly around the soil; they are congregated around nutrient sources. A nutrient source for these microorganisms is organic matter. (Curl, 1986)

In the rhizosphere there are different amounts and types of microorganisms than there are in the bulk soil due to different substrate, or “the surface on which an organism grows or is attached.” Other factors that vary from rhizosphere to bulk soil are the acidity, moisture, nutrients, electrical conductivity, and redox potential (Lynch, 1990).

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All of these factors lead to diversity of microorganism population in the rhizosphere. “The root-soil interface is a unique environment for microorganisms.” (Curl, 1986)

The environment in the rhizosphere is determined by three things: plants, soil, and organisms. The three factors related to plants that are important to rhizosphere health are nutrition, structure and rhizodeposition, or the material transfer from roots to soil. The three soil factors that affect the rhizosphere are moisture level, structure, and nutrients. The three organism factors that affect the rhizosphere are growth, interactions, and nutrient supply. (Curl, 1986)

A main predisposing determinant of rhizosphere microorganism colonization is the spermosphere’s colonization by microorganisms. The spermosphere is the area surrounding a germinating seed. (Lynch, 1990). If the germinating seed has a large microorganism population, the plant is likely to have a large microorganism population as well.

Microorganism colonization happens in two phases. First, the bacteria around the germinating seed get pushed downward as the roots extend through the soil. This would account for the lower population of microorganisms on the roots that are furthest down. Next, the microorganisms multiply and compete for survival when the population reaches the limits of the ecological niche. (Lynch, 1990)

There are four main kinds of interactions that occur when two or more organisms live in the same area. The first is commensalism, or symbiosis, which is an interaction that “promotes the growth of individuals within the populations.” Commensalism implies that one organism benefits from the presence of another organism, but the second organism is neither benefiting from or being harmed by the first organism. The second is competition, or antibiosis. The third, which rarely occurs, is neutralism, and the fourth is mutualism. (Curl, 1986)

In the rhizosphere, there are four symbiotic interactions that occur: (1) microorganisms may release nutrients from the soil that were previously unavailable to the plant, (2) “one species may synthesize a growth factor which is essential for growth and reproduction by another organism, (3) one organism may provide microclimate changes that make the area more suitable for another organism, and (4) one organism may neutralize a toxin that is making life impossible for another organism. (Curl, 1986)

Microorganisms can become competitive in the microenvironment of the rhizosphere. The most common type of competition occurs when there is higher demand than supply of nutrients for microorganisms, but it can also occur when one microorganism becomes dominant and therefore has a high competitive ability for substrate colonization. “Microorganisms which possess the necessary attributes for rapid substrate colonization also will win out in the end over competitors.”

Rhizosphere, plant, and microorganism interactions are very complex. The relationship between organisms and roots is highly variable. It can be beneficial, harmful, or neutral. “The goal in manipulating the rhizosphere must be to increase the balance of beneficial over harmful affects.” (Lynch, 1990)
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