Introduction

The rhizosphere is defined as the region of soil surrounding plant roots which is under the influence of the root. This region is centered around the root, and is best defined by the biotic response to the influence of the root (Figure 1). Practically, this region is measured using biological indicators such as microbial density, enzymatic activity, or mapping root-derived chemical gradients. Thus, the spatial limits of the rhizosphere are determined by the soil biotic community under the direct or indirect influence of plant roots. The composition and dynamics of this biotic community is dependent on plant species, root architecture, plant carbon allocation, soil physical and chemical properties, microbial population diversity, among a host of other factors.

The plant root system, though comprising approximately half of a given plant's biomass, is relatively poorly studied compared to aboveground tissues. Consequently, examination of the ecological interactions in the rhizosphere lags behind aerial studies. This is primarily due to the technical challenges of working in a complex soil matrix. More problematic still is that much of the rhizosphere community is microbial, and only a fraction of microbe species are amenable to laboratory culture (see below). Though root biology and ecology are more challenging than aboveground studies, the biological and ecological importance of the root system and surrounding rhizosphere has prompted many detailed studies of root biology and rhizosphere interactions.

Roots are highly branched organs which aid the plant in uptake of water and nutrients from the soil. This branched nature results in a vast surface area available for colonization by soil organisms. Due to the challenges associated with quantifying root characteristics in a natural soil matrix, estimates of root surface area vary by orders of magnitude. One study reported that one square meter of soil in temperate grassland ecosystems contains an estimated 80 m2 of root surface area. Another study reported that a single 1-month-old rye plant can generate 620 km of root length, with over a billion root hairs, and over 600 m2 of total surface area. Regardless of the precise values (which vary by plant species, soil type, nutrient status, etc.), this incredible surface area generates an abundant and heterogeneous matrix for soil biota to thrive.

The rhizosphere contains a complex food web with the plant as the primary source of carbon. Aerial plant parts

Figure 1 Schematic representation of a plant root and surrounding rhizosphere. Though drawn here as a discrete boundary, the precise delineation separating bulk and rhizosphere soil is typically vague. Mucilage is secreted by plant roots and typically is composed of polymers such as polysaccharides and polypeptides. Biofilm is a dense microbial layer immediately adhering to the root surface. Shading is approximately proportional to nutrient density in the soil, with a decreasing gradient from the root surface toward the bulk soil.

Figure 1 Schematic representation of a plant root and surrounding rhizosphere. Though drawn here as a discrete boundary, the precise delineation separating bulk and rhizosphere soil is typically vague. Mucilage is secreted by plant roots and typically is composed of polymers such as polysaccharides and polypeptides. Biofilm is a dense microbial layer immediately adhering to the root surface. Shading is approximately proportional to nutrient density in the soil, with a decreasing gradient from the root surface toward the bulk soil.

convert carbon dioxide to carbohydrates through the process of photosynthesis. Fixed carbon is transferred through the plant vasculature to the root system, generally in the form of carbohydrates, amino acids, and other primary plant metabolites. These compounds serve as carbon and nitrogen substrates to support root system growth and this growth subsequently impacts the rhizosphere by modulating interactions with rhizosphere organisms through the secretion of organic compounds into the soil (root exudation), regulation of border cell release to the soil, and the alteration of the physical properties of individual root cells, the root system as a whole, and the physical properties of the soil. Plant-derived contributions to the rhizosphere subsequently influence the physiology, behavior, fitness, and interactions of the organisms inhabiting that area. These interactions can be positive, negative, or neutral to plant fitness, and each specific contribution to the rhizosphere may induce a specific change in community activity or structure.

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