RHIZOSPHERE: CHARACTERISTICS, MICROBIOLOGY AND IMPORTANCE - BIOLOGY - 2024

The rhizosphere is the zone of soil that surrounds a root of a plant. Both the biology and the chemistry of the soil are influenced by this root. This area is approximately 1 mm wide and does not have a defined border, it is an area influenced by compounds exuded by the root and by microorganisms that feed on the compounds.

The term rhizosphere is derived from the Greek word rhiza meaning "root" and "sphere meaning field of influence." It was the German scientist Lorenz Hiltner (1904) who first described it as "the zone of soil immediately adjacent to the roots of legumes that supports high levels of bacterial activity."

Composition of the rhizosphere

However, the definition of the rhizosphere has evolved as other physical, chemical and biological properties have been discovered. The rhizosphere is highly influenced by the roots of plants that promote intense biological and chemical activities.

Organisms that coexist in the rhizosphere exhibit a variety of interactions with each other and with plants. These interactions can affect the growth of a wide range of crops, which is why rhizospheres are very important as substitutes for chemical fertilizers and pesticides.

Characteristics of the rhizosphere

It is thin and is subdivided into three basic zones

Structurally, the rhizosphere is about 1mm wide and has no sharp edges. Despite this, three basic zones have been described in the rhizosphere:

- The endorizosphere

It consists of the root tissue and includes the endodermis and cortical layers.

- The rhizoplane

It is the surface of the root, where soil particles and microbes adhere. It is made up of the epidermis, the cortex and the layer of mucilaginous polysaccharides.

- The ectorizosphere

It is the outermost part; that is, the soil that is immediately adjacent to the root.

In some cases other important rhizospheric layers may be found, such as the mycorizosphere and rhizovain.

Different compounds are released in the rhizosphere

During the growth and development of a plant, a variety of organic compounds are produced and released through exudation, secretion, and deposition. This causes the rhizosphere to be rich in nutrients, compared to the rest of the soil.

The root exudates include amino acids, carbohydrates, sugars, vitamins, mucilages, and proteins. The exudates act as messengers that stimulate the interactions between the roots and the organisms that inhabit the soil.

Changes the pH of the soil around the roots

The rhizosphere environment generally has a lower pH, with less oxygen and higher concentrations of carbon dioxide. However, exudates can make the soil in the rhizosphere more acidic or alkaline, depending on the nutrients that the roots are taking from the soil.

For example, when a plant absorbs nitrogen into ammonium molecules, it releases hydrogen ions that will make the rhizosphere more acidic. In contrast, when a plant absorbs nitrogen into nitrate molecules, it releases hydroxyl ions that make the rhizosphere more alkaline.

Microbiology

As mentioned above, the rhizosphere is an environment with a high density of microorganisms of various species.

For a better understanding, the microorganisms of the rhizosphere can be classified into three large groups, according to the effect they cause on plants:

Beneficial microbes

This group includes organisms that promote plant growth directly - for example, by providing necessary nutrients to the plant - or indirectly, inhibiting harmful microbes through various resistance mechanisms.

In the rhizosphere there is constant competition for resources. Beneficial microbes limit the success of pathogens with several mechanisms: the production of biostatic compounds (which inhibit the growth or multiplication of microorganisms), competition for micronutrients, or by stimulating the plant's immune system.

Commensal microbes

In this category are most microbes that do not directly harm or benefit the plant or the pathogen. However, commensal microbes are likely to affect any other micro-organism to some extent, through a complex network of interactions that would have an indirect effect on the plant or pathogen.

Although there are specific microorganisms that are capable of protecting the plant (directly or indirectly) against pathogens, their effectiveness is largely influenced by the rest of the microbial community.

Thus, commensal microorganisms can compete effectively with other microorganisms, exerting an indirect effect on the plant.

Pathogenic microbes

A wide range of soil-borne pathogens can affect plant health. Before infection, these harmful microbes compete with many other microbes in the rhizosphere for nutrients and space. Nematodes and fungi are the two main groups of soil-borne plant pathogens.

In temperate climates, pathogenic fungi and nematodes are agronomically more important than pathogenic bacteria, although some bacterial genera (Pectobacterium, Ralstonia) can cause substantial economic damage to some crops.

Viruses can also infect plants through roots, but require vectors such as nematodes or fungi to enter the root tissue.

Importance

Attracts beneficial microorganisms

The high levels of moisture and nutrients in the rhizosphere attract much greater numbers of microorganisms than other parts of the soil.

Some of the compounds secreted in the rhizosphere promote the establishment and proliferation of microbial populations, much higher compared to the rest of the soil. This phenomenon is known as the rhizosphere effect.

Offers protection against pathogenic microorganisms

The root cells are under continuous attack by microorganisms, which is why they have protection mechanisms that guarantee their survival.

These mechanisms include the secretion of defense proteins and other antimicrobial chemicals. It has been determined that the exudates in the rhizosphere vary according to the growth stages of the plant.

Protects roots from desiccation

Several studies suggest that the soil in the rhizosphere is significantly more humid than the rest of the soil, which helps protect the roots from drying out.

The exudates released by the roots at night allow the expansion of the roots in the soil. When perspiration resumes with daylight, exudates begin to dry out and adhere to soil particles in the rhizosphere. As the soil dries and its hydraulic potential decreases, the exudates lose water to the soil.

References

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