Rhizosphere

Bringhurst et al., 2001

Cardon and Gage, 2006

Hegde and Fletcher, 1996

Gregory, 2006

Kupier et al., 2002

Marschner, 1995

Pinton et al., 2007

Villacieros et al., 2003

The rhizosphere is the area of soil around plant roots that has enhanced microbial growth and activity compared to bulk soil. This microhabitat is in constant flux where plant and microbial derived compounds enter the rhizosphere. Interactions between plants and microorganisms may be mediated by these compounds. Nutrient and water concentrations and pH are also in flux here because of uptake and redox reactions. In all, the rhizosphere is defined by the presence of a plant root and the community that develops here is very diverse with dynamic environmental constrainsts.

Colonization

Roots are colonized by many types of soil microorganisms. Different organisms can have different niches in the rhizosphere. For example, Pseudomonas fluorescens can colonize the entire root system when it alone is inoculated on alfalfa roots. But when it is inoculated with Sinorhizobium meliloti the extent of colonization differs. Psuedomonas fluorescens is displaced by Sinorhizobium meliloti on the root hairs. Some rhizosphere organisms actually attach to the root surface (the rhizoplane). There are several steps in the colonization of this niche, including: movement to the root surface or chemotaxis, adsoprtion, anchoring, and often specific microbial gene expression follows (such as nodulation or virulence genes).

Colonization of a root by two different bacteria. One is a red fluorescent strain and the other is a green fluorescent strain. Both strains colonize the emerging lateral root which appears yellow. From Bringhurst et al., 2001.
A green fluorescent strain is colonizing root hairs shown in red. From Bringhurst et al., 2001.

Exudation

Roots produce many types of compounds that are released into the rhizosphere. Several classes of organic compounds have been identified, including: sugars and polysaccharides, amino acids, nucleotides, organic acids, fatty acids, sterols, growth factors, hormones, enzymes, flavonoids, and others. These compounds serve many functions and they pose a significant carbon cost for the plant. Root released compounds and exudates are believed to strongly influence the composition of the microbial community in the rhizopshere. Also, the rhizosphere microbial composition can change over time. This may be associated with abiotic factors such water availabilty, or biotic factors such as root age and exudation pattern.

Pattern of root exudation (near root tip) and microorganism colonization (more colonization near mature root zones). From Marschner, 1995.
This figure shows the change in root exudates flushed from pots or washed off mulberry roots (Morus spp. - Moraceae) over time. From Hegde and Fletcher, 1996.
Table showing the composition of exudates in Barmultra grass (Lolium multiflorum - Poaceae) during development from seed to root of an established plant. From Kuiper et al., 2002

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