Statistical method for testing the neutral mutation hypothesis by DNA polymorphism.

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

http://ucce.ucdavis.edu/files/datastore/234-17.pdf

Preharvest and postharvest factors influencing vitamin C content of horticultural crops.

Microbial communities play a pivotal role in the functioning of plants by influencing their physiology and development. While many members of the rhizosphere microbiome are beneficial to plant growth, also plant pathogenic microorganisms colonize the rhizosphere striving to break through the protective microbial shield and to overcome the innate plant defense mechanisms in order to cause disease. A third group of microorganisms that can be found in the rhizosphere are the true and opportunistic human pathogenic bacteria, which can be carried on or in plant tissue and may cause disease when introduced into debilitated humans. Although the importance of the rhizosphere microbiome for plant growth has been widely recognized, for the vast majority of rhizosphere microorganisms no knowledge exists. To enhance plant growth and health, it is essential to know which microorganism is present in the rhizosphere microbiome and what they are doing. Here, we review the main functions of rhizosphere microorganisms and how they impact on health and disease. We discuss the mechanisms involved in the multitrophic interactions and chemical dialogues that occur in the rhizosphere. Finally, we highlight several strategies to redirect or reshape the rhizosphere microbiome in favor of microorganisms that are beneficial to plant growth and health.

The rhizosphere microbiome: significance of plant beneficial, plant pathogenic, and human pathogenic microorganisms

Biological effects of resveratrol.

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

Biorational alternatives are gaining increased attention for weed control because of concerns related to pesticide usage and dwindling numbers of labeled products, particularly for minor-use crops. Allelopathy offers potential for biorational weed control through the production and release of allelochemics from leaves, flowers, seeds, stems, and roots of living or decomposing plant materials. Under appropriate conditions, allelochemics may be released in quantities suppressive to developing weed seedlings. Allelochemics often exhibit selectivity, similar to synthetic herbicides. Two main approaches have been investigated for allelopathic weed suppression. One is use of living rotational crops or mulches that interfere with the growth of surrounding weeds [e.g., tall red fescue, Festuca arundinacea Schreb.; creeping red fescue, F. rubra L. subsp. commutata; asparagus, Asparagus officinalis L. var. altilis); sorghum, Sorghum bicolor (L.) Moench; alfalfa, Medicago saliva L.; black mustard, Brassica nigra (L.) Koch; and oat, Avena saliva L.]. Attempts to select germplasm with enhanced suppressive ability have been limited. The second is use of cover crop residues or living mulches to suppress weed growth for variable lengths of time (e.g., winter rye, Secale cereale L.; winter wheat, Triticum aestivum L.; and sorghum). Cover crop residues may selectively provide weed suppression through their physical presence on the soil surface and by release of allelochemics or microbially altered allelochemics. The ability to understand the physiological basis for allelopathy in a crop plant may allow the weed scientist or ecologist to work closely with molecular biologists or traditional plant breeders to selectively enhance the traits responsible for weed suppression.

Utilization of Allelopathy for Weed Management in Agroecosystems

In contrast to plant-animal interactions, the conceptual framework regarding the impact of secondary metabolites in mediating plant-plant interference is currently less well defined. Here, we address hypotheses about the role of chemically-mediated plant-plant interference (i.e., allelopathy) as a driver of Mediterranean forest dynamics. Growth and defense abilities of a pioneer (Pinus halepensis) and a late-successional (Quercus pubescens) Mediterranean forest species were evaluated under three different plant interference conditions: (i) allelopathy simulated by application of aqueous needle extracts of Pinus, (ii) resource competition created by the physical presence of a neighboring species (Pinus or Quercus), and (iii) a combination of both allelopathy and competition. After 24 months of experimentation in simulated field conditions, Quercus was more affected by plant interference treatments than was Pinus, and a hierarchical response to biotic interference (allelopathy < competition < allelopathy + competition) was observed in terms of relative impact on growth and plant defense. Both species modulated their respective metabolic profiles according to plant interference treatment and thus their inherent chemical defense status, resulting in a physiological trade-off between plant growth and production of defense metabolites. For Quercus, an increase in secondary metabolite production and a decrease in plant growth were observed in all treatments. In contrast, this trade-off in Pinus was only observed in competition and allelopathy + competition treatments. Although Pinus and Quercus expressed differential responses when subjected to a single interference condition, either allelopathy or competition, species responses were similar or positively correlated when strong interference conditions (allelopathy + competition) were imposed.

/pdf/the-impact-of-competition-and-allelopathy-on-the-trade-off-4cxdv66qma.pdf

The Impact of Competition and Allelopathy on the Trade-Off between Plant Defense and Growth in Two Contrasting Tree Species.

Allelopathy can be defined as an important mechanism of plant interference mediated by the addition of plant-produced secondary products to the soil rhizosphere. Allelochemicals are present in all types of plants and tissues and are released into the soil rhizosphere by a variety of mechanisms, including decomposition of residues, volatilization, and root exudation. Allelochemical structures and modes of action are diverse and may offer potential for the development of future herbicides. We have focused our review on a variety of weed and crop species that establish some form of potent allelopathic interference, either with other crops or weeds, in agricultural settings, in the managed landscape, or in naturalized settings. Recent research suggests that allelopathic properties can render one species more invasive to native species and thus potentially detrimental to both agricultural and naturalized settings. In contrast, allelopathic crops offer strong potential for the development of cultivars that are ...

Weed and Crop Allelopathy

Flavonoids are biologically active low molecular weight secondary metabolites that are produced by plants, with over 10,000 structural variants now reported. Due to their physical and biochemical properties, they interact with many diverse targets in subcellular locations to elicit various activities in microbes, plants, and animals. In plants, flavonoids play important roles in transport of auxin, root and shoot development, pollination, modulation of reactive oxygen species, and signalling of symbiotic bacteria in the legume Rhizobium symbiosis. In addition, they possess antibacterial, antifungal, antiviral, and anticancer activities. In the plant, flavonoids are transported within and between plant tissues and cells, and are specifically released into the rhizosphere by roots where they are involved in plant/plant interactions or allelopathy. Released by root exudation or tissue degradation over time, both aglycones and glycosides of flavonoids are found in soil solutions and root exudates. Although the relative role of flavonoids in allelopathic interference has been less well-characterized than that of some secondary metabolites, we present classic examples of their involvement in autotoxicity and allelopathy. We also describe their activity and fate in the soil rhizosphere in selected examples involving pasture legumes, cereal crops, and ferns. Potential research directions for further elucidation of the specific role of flavonoids in soil rhizosphere interactions are considered.

https://www.springer.com/cda/content/document/cda_downloaddocument/Flavonoids.pdf?SGWID=0-0-45-1382603-0

Leslie A. Weston

Papers

The role of root exudates and allelochemicals in the rhizosphere

Utilization of Allelopathy for Weed Management in Agroecosystems

The Impact of Competition and Allelopathy on the Trade-Off between Plant Defense and Growth in Two Contrasting Tree Species.

Weed and Crop Allelopathy

Flavonoids: Their Structure, Biosynthesis and Role in the Rhizosphere, Including Allelopathy