Rhizosphere Interactions

Introduction

The rizosphere refers to the region of soil near plant roots. Compared to the rest of soil, this area is relatively luxurious- nutrients are more plentiful and bacteria abound. Sylvia et al compare the rizosphere to an oasis.

Soil Environment Associated with Plants

Rhizoplane

The rizoplane refers to the environment in immediate physical contact with the roots. Microbes that live in the rizoplane are closer to the actual roots than the microbes in the risosphere. The functional definition is everything remaining after the roots have been shaken vigorously in water. There are more microbes (as counted by CFU) in the rizoplane than in the more loosely assoicated rizosphere. Those microbes who are directly in contact with the roots tend to be found where the integrety of the root is broken. Perhaps because of this, they also tend to be found on older rather than younger roots. The distinction between bacteria which live in the rizoplane and those who live inside the root is made by naming the latter "endophytes"

Rhizosphere

There are several subjective definitions of “rhizosphere” one is the zone of influence of plant roots- that may vary for the specific influence being tracked and the specific environment. A more general, functional definition is “the dirt that clings to roots after gentle shaking in water”. In general the rizosphere is a metabolically busier, faster moving, more competitive environment than the surrounding soil.

Physical Environment

water potential

The plant roots which the rhizosphere is associated with can effect the physical environment of the rhizosphere. As as plants transpire water with more force during the day than during the night, they change the soil water potential immediately near their roots and so the rhizosphere undergoes fluctuations that the bulk soil avoids.

texture

Plant roots compact the soil on the short term as they grow, but once they die and decay, can actually leave soil more porous

ph

several factors can lower the pH in the rhizosphere. Respiration leads to carbon dioxide (and eventually to bicarbonate/carbonic acid) generation. In addition to respiration of the roots themselves, the rhizosphere is very rich in carbon results in other organisms from prokaryotes to fungi to small animals living and respiring in the rhizosphere more than in the bulk soil.

Plant Exudates

Exudates

Exudates are surpluss sugars, amino actid, or aeromatics that diffuse out of the cells.

Secretions

Secretions are metabolic byproducts released from roots.

Mucilages

Mucilagens are cells sloughed off the root cap as the root grows. As the root grows, it pushes agains soil particulate matter creating an abrasive force that removes mucilages from the epidermal part of the root. These cells include cellulose, pectin, starch, and lignin.

Mucigel

Mucigels are organisms and soil organic matter promoting connectivity between plant roots and the surrounding soil. During dry spells, mucigels are responsible for allowing plants to continue to uptake water and nutrients.

References

• Sylvia, D., Fuhrmann,J., Hartel, P., Zuberer, D. 2005. Principles and Applications of Soil Microbiology. Pearson Education Inc. New Jersey.

Edited by students of Kate Scow