Root Exudates :Functions in plant-microbe interaction
- 1. Root exudates: Functions in plant-microbe interactions Debayan Nandi M.Sc student Dept. of Biotechnology Prepared by
- 2. Root exudates Root exudates, are chemicals exuded by the roots of growing plants. They are readily available to micro- organisms and are of great importance for the population of the plant root surfaces. Root exudation includes the secretion of ions, free oxygen and water, enzymes, mucilage, and a diverse array of carbon- containing primary and secondary metabolites. Definition Fig. Release of root exudates in the rhizosphere Root Root border cells
- 3. Introduction Roots are integral to a plant’s growth and survival; and secrete an enormous range of compounds into the surrounding soil Root secretes to compete with the invading root systems of neighboring plant species and with soil-borne microorganisms, including bacteria, fungi, and insects The compounds secreted are referred as root exudates and serve important roles as chemical attractants and repellants in the rhizosphere, the narrow zone of soil that surrounds the root system Studying the root system of the model plant Arabidopsis thaliana (L.) has been indispensable in advancing our understanding of the impact of agricultural practices on root development and the impact of roots
- 4. Root exudates Through the exudation of a wide variety of compounds 1. Roots may regulate the soil microbial community. 2. Encourage beneficial symbioses. 3. Change the chemical and physical properties of the soil . 4. Inhibit the growth of competing plant species . 5. 5% to 21% of all photosynthetically fixed carbon being transferred to the rhizosphere through root exudates Functions
- 5. Components of root exudates Root exudates Low molecular weight exudates High molecular weight exudates Organic acid Amino acids Sugar Phenolics Protein Polysaccharide
- 6. Fig.1. Carbon flow in plants- carbon dioxide is assimilated by plants and used for synthesis of metabolites and release root exudates; its various components to influence soil properties and residing microbiota (Musilova, Ridl et al. 2016).
- 7. Root interactions 2. Root-root Communication Allelopathy is mediated by the release of certain secondary metabolites by plant roots and plays an important role in the establishment and maintenance of terrestrial plant communities. parasitic plants often use secondary metabolites secreted from roots as chemical messengers to initiate the development of invasive organs (haustoria) required for heterotrophic growth A secondary metabolite secreted by the roots of knapweed (Centaurea maculosa) provides a classic example of root exudates exhibiting negative root-root communication in the rhizosphere.
- 8. 2. Root-Microbe Communication roots have been shown to play an important role in root microbe interactions include flavanoids present in the root exudates of legumes that activate Rhizobium meliloti genes responsible for the nodulation process (Peters et al., 1986) Erwinia spp., Pseudomonas spp., and Agrobacterium spp., possess quorum-sensing systems that control the expression of several genes required for pathogenicity Root exudates from pea (Pisum sativum) seedlings were found to contain several bioactive components that mimicked AHL signals in well-characterized bacterial reporter strains, stimulating AHL-regulated behaviors in some strains against pathogenicity
- 9. Factors affecting exudates 1. Microorganisms In rhizosphere, the roots have to compete with the microorganisms in their vicinity for water, nutrients, space that mainly includes fungi and bacteria and insects feeding on an abundant source of organic material Root exudates acts as messengers in physical and biological interaction with the other plant roots and microorganisms and also encourages beneficial symbioses inhibit growth of pathogenic microbes The rate of exudation or exudates formation is increased by the presence of microbes in the rhizosphere (Matilla et al. 2010)
- 10. 2. Soil pH and fertilizers High concentrations of anions and organic acids in root exudation lead to P deficiency and this lowers rhizospheric pH, making Mn, Fe and Zn to be more available in calcareous soils As micronutrients such as Mn, Fe and Al occur in high concentrations below pH (5.5) hence rhizospheric acidification below pH 5.5 can cause some major macronutrients to become limiting However, organic acids from root exudates are able to solubilize unavailable soil Ca, Fe and Al phosphates Rooibos tea (Aspalathus linearis L.) can actively modify their mycorhizospheric pH by extruding OH- and HCO3 - to facilitate growth in low pH soils (pH usually between 3 to 5) (Dakora and Phillips, 2002)
- 11. 3. Plant characteristics (species, age, nutrients, light) Plant age alters the rhizosphere microorganism and the stage of plant maturity controls the magnitude of rhizosphere effect and degree of response to specific microorganisms Some microbes were found to be more effective at the time of flowering than in the seedling or full maturity stage The effect of light on the production of pectin and polygalacturonase (PG) in the root exudates of Trifolium alexandrinum inoculated with an efficient strain of Rhizobium trifolii was investigated. The pectin methyl esterase PME and PG increased with an increase in the duration of light to which plants were exposed (Chhonkar, 1978).
- 12. Mechanism of root exudation Fig.2. Mechanism of root exudation of compounds through the plant cell membrane PM = Plasma Membrane; TMD = Transmembrane Domain; NBD = Nucleotide Binding Domain (Bertin, Yang et al. 2003)
- 13. Rhizosphere microbes influence plant root exudation Studies reported that the colonization of arbuscular mycorrhizal fungi influences plant root exudation, e.g., increasing secretions of nitrogen, plant phenolics, gibberellins and reducing secretions of sugars, potassium ions, and phosphorus A. thaliana was found to produce distinct root exudation profiles when cultured with Pseudomonas putida KT2440 (Matilla et al. 2010). soil bacterium Achromobacter piechaudi ARV8 that has ACC deaminase activity was able to increase tomato and pepper seedling biomass (Mayak et al. 2004)
- 14. Secondary metabolites and hormones plants produce and release numerous secondary metabolites and hormones into the rhizosphere, many of which play a role in plant– microbe interactions. benzoxazinoids, found in the root exudates of maize, attract plant- beneficial rhizobacteria Pseudomonas species contain chemotaxis sensory proteins for amino acids that aid in their colonization of tomato roots presence of PGPR B. Subtilis invokes abscisic acid and salicylic acid signaling pathways in A. thaliana, resulting in the closure of stomata and the restriction of pathogen entry (Kumar et al. 2012)
- 15. Plant-microbe interaction mediated by root exudates Plant –microbe interaction Positive plant-microbe interactions Negative plant-microbe interactions Plant growth promoting bacteria (PGPB) Root nodulation Mycorrhizal associations Antimicrobial effects
- 16. Fig.2. Representation of the complex interactions mediated by root exudates that take place in the rhizosphere between plant roots and other organisms (Walker, Bais et al. 2003).
- 17. Fig. Schematic representation of possible rhizospheric interactions mediated by root exudates. (Bais et al. 2006)
- 18. Alteration in the characteristics of soil by root exudation A large range of organic and inorganic substances are secreted by roots into the soil, that leads to changes in its biochemical and physical properties (Rougier 1981). It has been speculated that as the soil dries, exudates begin to lose water to soil, its viscosity increases and hence the resistance to movement of soil particles in contact with exudates will increase. (McCully and Boyer 1997). Plants have also been found more hydrated in the early morning hours compared with the midday samplings (McCully and Boyer 1997).
- 19. This approach to visualizes root development on the observed interface to access the 3D structure of a root system Methods for the study of interaction between root and associated microorganisms Tracing of exuded compounds, including their uptake by rhizosphere microflora, can be studied through fluorescence-based methods. In the past, biosensors have been developed for studying carbon flow through plant exudates (Porteous, Killham et al. 2000), during nodulation, and for tracing bacterial quorum sensing and root colonization.
- 20. Object of study Method Root growth and morphology Observation windows + imaging system Transparent culture media, e.g., PhytagelTM or NafionTM Computed Tomography (CT) Magnetic Resonance Imaging (MRI) Neutron radiography Plant–microbe interaction (tracing of exuded compounds, carbon flow in exudates) Biosensors Fluorescence in situ hybridization Metagenomics Table.1. An overview of methods for studying roots and root exudates and their interaction with microbe in the rhizosphere. (Musilova, Ridl et al. 2016).
- 21. Fig. Differentiation of border root cells alongwith the secretion of root exudates (Musilova, Ridl et al. 2016)
- 22. Conclusion Various technological advancements and research over the past few years facilitated a better understanding of root exudates and such advancements can be applied in agriculture to achieve a good yield and increase in productivity by mediating defensive responses against various soil-borne pathogens and can be applied for developing better methods to deal with invading crops, heavy metals and various toxic compounds. Future challenges mainly includes the absolute characterization of different chemical compounds of root exudates that exhibits resistance to diseases and facilitate more beneficial association with plants and microorganisms present in the rhizosphere.
- 23. References Jones DL Darrah PR (1994). Role of root derived organic acids in the mobilization of nutrients from the rhizosphere. Plant Soil. 166:247-257. Bais HP, Prithiviraj B, Jha AK, Ausubel FM, Vivanco JM (2005) Mediation of pathogen resistance by exudation of antimicrobials from roots. Nature 434:217–221. Chet I, Ordentlich A, Shapira R, Oppenheim A (1990) Mecahnisms of biocontrol of soil-borne plant pathogens by rhizobacteria. Plant Soil 129:85–92 Doornbos RF, Van Loon LC, Bakker PAHM (2009) Beneficial Pseudomonas spp. have altered root colonization on Arabidopsis thaliana mutants affected in the expression of induced systemic resistance. IOBC/WPRS Bull 43:307–310
- 24. Musilova, L., et al. (2016). "Effects of secondary plant metabolites on microbial populations: Changes in community structure and metabolic activity in contaminated environments." International journal of molecular sciences 17(8): 1205. Porteous, F., et al. (2000). "Use of a lux-marked rhizobacterium as a biosensor to assess changes in rhizosphere C flow due to pollutant stress." Chemosphere 41(10): 1549-1554. Rougier, M. (1981). Secretory activity of the root cap. Plant Carbohydrates II, Springer: 542-574.