The rhizosphere encompasses the millimeters of soil surrounding a plant root where complex biological and ecological processes occur. This review describes recent advances in elucidating the role of root exudates in interactions between plant roots and other plants, microbes, and nematodes present in the rhizosphere. Evidence indicating that root exudates may take part in the signaling events that initiate the execution of these interactions is also presented. Various positive and negative plant-plant and plant-microbe interactions are highlighted and described from the molecular to the ecosystem scale. Furthermore, methodologies to address these interactions under laboratory conditions are presented.

The Role of Root Exudates in Rhizosphere Interactions with Plants and Other Organisms

Plant roots serve a multitude of functions in the plant including anchorage, provision of nutrients and water, and production of exudates with growth regulatory properties. The root–soil interface, or rhizosphere, is the site of greatest activity within the soil matrix. Within this matrix, roots affect soil structure, aeration and biological activity as they are the major source of organic inputs into the rhizosphere, and are also responsible for depletion of large supplies of inorganic compounds. Roots are very complicated morphologically and physiologically, and their metabolites are often released in large quantities into the soil rhizosphere from living root hairs or fibrous root systems. Root exudates containing root-specific metabolites have critical ecological impacts on soil macro and microbiota as well as on the whole plant itself. Through the exudation of a wide variety of compounds, roots impact the soil microbial community in their immediate vicinity, influence resistance to pests, support beneficial symbioses, alter the chemical and physical properties of the soil, and inhibit the growth of competing plant species. In this review, we outline recent research on root exudation and the role of allelochemicals in the rhizosphere by studying the case of three plants that have been shown to produce allelopathic root exudates: black walnut, wheat and sorghum

The role of root exudates and allelochemicals in the rhizosphere

Plant genomes contribute to the structure and function of the plant microbiome, a key determinant of plant health and productivity. High-throughput technologies are revealing interactions between these complex communities and their hosts in unprecedented detail.

The plant microbiome.

Natural products in crop protection.

The chalcone synthase superfamily of type III polyketide synthases.

The root exudates produced by sorghums contain a biologically active constituent known as sorgoleone. Seven sorghum accessions were evaluated for their exudate components. Except for johnsongrass, which yielded 14.8 mg root exudate/g fresh root wt, sorghum accessions consistently yielded approximately 2 mg root exudate/g fresh root wt. Exudates contained four to six major components, with sorgoleone being the major component (> 85%). Three-dimensional structure analysis was performed to further characterize sorgoleone's mode of action. These studies indicated that sorgoleone required about half the amount of free energy (493.8 kcal/mol) compared to plastoquinone (895.3 kcal/mol) to dock into the QB-binding site of the photosystem II complex of higher plants. Light, cryo-scanning, and transmission electron microscopy were utilized in an attempt to identify the cellular location of root exudate production. From the ultrastructure analysis, it is clear that exudate is being produced in the root hairs and bei...

Mode of Action, Localization of Production, Chemical Nature, and Activity of Sorgoleone: A Potent PSII Inhibitor in Sorghum spp. Root Exudates1

The relative phytotoxicity of sorgoleone as measured by seed germination and seedling growth of selected crop and weed species and inhibition of photosynthetic oxygen evolution in atrazine-resistan...

Phytotoxicity and Distribution of Sorgoleone in Grain Sorghum Germplasm

Effects of the alleochemical sorgoleone on photosynthetic electron transport by oxygen-evolving chloroplast thylakoids and Triton X-100-prepared Photosystem II (PSII) membranes were analyzed. The Hill activity of the thylakoids proved to be at least as sensitive to inhibition by sorgoleone as it was to DCMU [3-(3,4-dichlorophenyl)-1,1-dimethylurea], a potent herbicidal inhibitor of PSII. However, a Photosystem I (PSI) partial reaction was not affected by a 10-fold greater concentration of sorgoleone than is required for complete inhibition of Hill activity. Measurements of flash-induced chlorophyll a variable fluorescence showed that sorgoleone neither dissipated excitation energy nor diminished the amplitude of chlorophyll a variable fluorescence. However, it inhibited the decay of variable fluorescence as effectively as DCMU, which blocks the oxidation of the PSII-reduced primary electron acceptor, Q-A, by the PSII secondary electron acceptor, QB, by displacing QB from the D1 protein. Additionally, sorg...

Inhibition of a photosystem II electron transfer reaction by the natural product sorgoleone

Herbicidal Activity and Site of Action of the Natural Product Sorgoleone

Uptake, Translocation, and Metabolism of14C-MSMA in Organic Arsenical-Resistant and -Susceptible Mississippi Biotypes of Common Cocklebur (Xanthium strumariumL.)

Chandrashekhar I. Nimbal

Papers

Mode of Action, Localization of Production, Chemical Nature, and Activity of Sorgoleone: A Potent PSII Inhibitor in Sorghum spp. Root Exudates1

Phytotoxicity and Distribution of Sorgoleone in Grain Sorghum Germplasm

Inhibition of a photosystem II electron transfer reaction by the natural product sorgoleone

Herbicidal Activity and Site of Action of the Natural Product Sorgoleone

Uptake, Translocation, and Metabolism of14C-MSMA in Organic Arsenical-Resistant and -Susceptible Mississippi Biotypes of Common Cocklebur (Xanthium strumariumL.)