【Division of Innovative Research】Takuro SHINANO

Rhizosphere?

Though plant roots are in the soil, the limited area apart from only 100 fÊm from root surface is the so-called "Rhizosphere". The rhizosphere is under the effect of both plant roots and soil, and this area has been recognized as a very important area in the meaning of biological, physiological, and chemical prospects. Though, because the rhisophere is inside "Soil", the heterogeny makes the research on this area difficult. Plant roots secrete several kinds of organic acids, enzymes to the rhizosphere, and these organic compounds and enzymes affect on the soil property to change its solubility, etc. This process surely makes the rhizosphere more suitable for not only plant growth but also for microbe growth. It is said that only 1% of total microbes in the soil are culturable and most of them are still unknown. It is sure that some of them might be pathogenic to plants, but we are also looking for useful microbes from the rhizosphere.

 

Isolated nitrogen fixing bacteria from the rhizosphere of a Larch plant in Siberia, Russia. The culture does not contain any nitrogenous compounds, while very active proliferation in a nitrogen limiting medium was observed.

 

A plant also actively changes its rhizosphere by secreting organic acids and/or enzymes (such as asic phosphatase). The photo shows a proteoid root developed under phosphorus deficient conditions, which has a high activity of citrate and acid phosphatase secretion. Microbes living in the rhizosphere are also affected by the plant.

 

It is considered that plants and microbes are mutually interacted through the rhizosphere. To absorb nutrients from a soil solution, a plant develops a large root system in the soil. However, it is well known that a large part of minerals in the soil are not readily usable by plants. Recent evidence supports the idea that the associative cooperation between plants and microbes could enhance the availability of soil characteristics more suitable for plant growth. Thus, by applying microbes directly or indirectly into agricultural technology, it is expected to improve and stabilize productivity in the future. As agriculture is recently in a very severe condition in regard to food safety, conservation of the environment, and conservation of the ecosystem, the study on Rhizospheres will be one approach to get over these difficulties.