Roots: Contribution to the Rhizosphere

Abstract

Diverse compounds exuded by different parts of the root system create a unique environment known as the rhizosphere. Interactions among microorganisms found within the rhizosphere and communication between these microorganisms and plant roots play an important role in maintaining plant health. In 2004, the discovery of deoxyribonucleic acid (DNA)‐based neutrophil extracellular traps (NETs) have transformed our understanding of mammalian immune responses. Subsequent studies revealed that plant roots express a similar mechanism in the rhizosphere, where root border cells secrete extracellular DNA together with antibiotic amino acids, enzymes and other proteins, to trap pathogens and prevent their invasion of the root system. As in animals, the defense response is impaired if extracellular proteins and/or extracellular DNA are degraded by protease and/or by DNase at the time of inoculation with fungal or bacterial pathogens. Defining species‐specific mechanisms of plant extracellular trapping may yield new avenues for engineering the rhizosphere to improve crop production.

Key Concepts

  • Extracellular DNA (exDNA) in the rhizosphere has long been observed, and was presumed to result from cell death.
  • Recent studies have revealed that exDNA is a key component of an extracellular matrix involved in trapping and immobilisation of pathogens.
  • The specificity of exDNA‐based trapping may play a critical role in microbial population structure in the rhizosphere.
  • The discovery that DNA‐based extracellular trapping operates in animals and plants suggests that this previously unrecognised process is an ancient underpinning of eukaryotic defence.
  • Efforts to engineer the rhizosphere may be facilitated by a better understanding of how plants control the environment surrounding their roots.

Keywords: mucilage; border cells; root cap; sloughed root cap cells; microbial ecology; root exudates

Figure 1. Functional categories of root exudates.
Figure 2. Sources and effects of root exudates. (a) Regions of the root known to release exudates. (b) Host‐specific interactions of microorganisms with border cells: 1, release of border cells (arrow) into the environment; 2, induction of zoosphere germination by border cells; 3, binding of bacteria to border cells; 4, colonisation of border cells by soil‐borne fungus; 5, attraction of pathogenic nematode to border cells.
Figure 3. Use of rhizosphere interactions for improvement of plant health and the environment.
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Curlango‐Rivera, Gilberto, Gunawardena, Uvini, Wen, Fushi, Zhao, Xiaowen, Xiong, Zhongguo, and Hawes, Martha C(Jan 2016) Roots: Contribution to the Rhizosphere. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0002335.pub3]