Plant take up most of the mineral nutrient from rhizosphere. Root exudate, root border cells and rhizodeposits are component of rhizosphere that affect microbial colonization, multiplication and secretion of organic bioactive compounds. Rhizodeposits released by plant root in rhizosphere include root exudate, mucilage, lysate produced and contain monopolysaccharides, organic acids, phenolic compounds, amino acid and protein (Dennis et al., 2010; Park and Ryu 2021).  Rhizosphere microorganisms release extracellular enzymes for degradation of different polymeric compound and may suppress plant pathogenic fungi (Egamberdieva et al., 2011). The root exudation profiles can shape the root microbiome of diseased plant. Plant depute beneficial rhizosphere communities by modifying the exudation pattern in response to above ground pathogens to benefit plant (Yuan et al., 2018). The study was made on the influences of root exudate changes mediated by biocontrol agent Bacillus cereus AR156 while controlling tomato bacterial wilt caused by Ralstonia solanacearum. It was observed that B cereus AR156 induce specific components in plant root exudate (Wang et al., 2019). Phytochemicals repel, inhibit or kill pathogenic microorganisms in the rhizosphere (Baetz and Martinoia 2014). The roots of nitrogen-fixing legumes exude phenolics and aldonic acids that serves as a signal to attract Rhizobiaceae bacteria. An acidic environment can pose threat to acquisition of nutrient by plant roots and threaten the survival of beneficial microorganisms and root. Rooibos tea (Aspalathus linearis L.) modify rhizosphere pH by extruding OH- and HCO3- to facilitate growth in pH 3 – 5 (Dakora and Phillips 2002).

More rhizosphere-enhanced metabolites (REMs) and rhizosphere-abated metabolites (RAMs) were detected in sand and clay substrate compared to soil substrate. Miller et al.(2019) study demonstrated that substrate influence the root exudate profiles. Exometabolites can have nutritional value and signaling functions. Few microbes can migrate from the rhizosphere into rhizoplane and from there into the root where they become endophytes. The mature cluster root secrete organic acid and also exude isoflavonoids and fungal cell wall degrading enzymes, leading to decrease in bacterial abundance (Weisskopf et al., 2006).

The exometabolites has to cross atleast one membrane to transit from the cytoplasm of root cells into rhizosphere (Joelle et al., 2017).

  1. Small hydrophilic compounds could diffuse from the root into the rhizosphere driven by the large concentration gradient. The carbon and nitrogen flow at the soil-root interface is bidirectional with carbon and nitrogen lost from root and taken up from soil simultaneously (Jones et al., 2009).
  2. Channel proteins could facilitate such diffusion
  3. Active (ATP-driven) or secondary active (proton gradient driven) transporters could shuttle compounds across membranes against a concentration gradient.
  4. Diffusion of compounds is possible only in young root tissue, which is still devoid of Casparian strips or suberized endodermis as both block apoplasmic flow in adult tissues (Joelle et al., 2017).

The root tip having apical and root cap meristem is resistant to infection caused by number of pathogens. Wen et al.(2009) report extracellular DNA (exDNA) is a component of root cap slime and that exDNA degradation during inoculation by a fungal pathogen results in loss of root tip resistance to infection.

Root derived antimicrobial compound include inodole, terpenoid, benzoxazinone, flavonoid, isoflavonoid and phenolics (Lanoue et al., 2009). Terpenes are natural products with diverse structure and biological activity. Arabidopsis thaliana mutant that lacks diterpene   rhizathalene is susceptible to opportunistic root herbivore fungus gnat (Bradysia sp.) and suffers substantial damage of peripheral tissue at the larval feeding sites (Huang and Osbourn 2019).

Maize sesquiterpenoid phytoalexin zealexin is accumulated in Fusarium graminearum infected tissue. Zealexins exhibits antifungal activity against number of plant pathogenic fungi (Huffaker et al., 2011). Triterpenoids are the most potent antifungal defense compound released by plant root. Roots of Arabidopsis thaliana produce triterpenoids such as tricyclic triterpene diol and arabidiol.  In a degradation reaction induced by Pythium irregular infection, arabidol is cleaved to homoterpene (E)-4,8-dimethyl-1,3,7 –nonatriene (DMNT) which has a role in defense against Pythium irregulare infection (Sohrabi and Ali 2017). Diterpenoid kauralexins accumulates in response to fungal attack (Schmelz et al., 2011). Soybean secretes large amount of soyasaponin (triterpenoid glycosides) as root exudates (Tsuno et al., 2018). Quinonemethide triterpenoid 22β-hydroxy-maytenin and maytenin present in root cap and near vascular cylinder of Peritassa laevigata in-vitro root suggest role in plant defense against infection by microorganisms as well as in the root exudation process. The root culture obtained from P. laevigata may accumulate secondary metabolites which are cytotoxic such as quinonemethide triterpenes(Pina et al., 2016).

Plants modify the soil properties by modulating the composition of the root exudates to ensure survival under adverse condition.


Baetz, U. and Martinoia, E.  2014 Root Exudates: The Hidden Part of Plant Defense. Trends Plant Sci. 19(2): 90- 98

doi: 10.1016/j.tplants.2013.11.006

Dakora, F. D. and Phillip, D. A. 2002 Root Exudates as Meditors of Mineral Acquisition in Low-Nutrient Environments.  Plant and Soil 245(1): 35 -47

doi: 10.1023/A:1020809400075

Dennis, P. G., Miller, A. J. and Hirsch, P. R. 2010 Are Root Exudates more Important than Other Sources of Rhizodeposits in Structuring Rhizosphere Bacterial Communities. FEMS Microbiol. Ecol. 72(3): 313 – 327

doi: 10.1111/j.1574-6941.2010.00860

Egamberdieva, D., Renella, G., Wirth, S., Islam, R. 2011 Enzyme Activities in the Rhizosphere of Plants. In: “Soil Enzymology”.  Shukla, G.  and Varma, A (eds.) Publisher: Springer-Verlag Berlin Heidelberg. Chapter 8,  pp: 149 – 166

doi: 10.1007/978-3-642-14225-3_8

Huffaker, A., Kaplan, F., Vaughan, M. M., Dafoe, N. J., Ni, X., Rocca, J. R., Alborn, H. T., Teal, P. E. A. and Schmelz, E. A. 2011 Novel Sesquiterpenoids Constitute a Dominant Class of Pathogen-Induced Phytoalexins in Maize. Plant Physiol. 156(4): 2082 – 2097

Huang, A. C. and Osbourn, A. 2019 Plant Terpenes that Mediates Below-Ground Interactions: Prospects for Bioengineering Terpenoids for Plant Protection. Pest Manag. Sci. 75(9): 2368 – 2377

doi: 10.1002/ps.5410

Miller, S. B., Heuberger, A. L., Broeckling, C. D. and Jahn, C. E. 2019 Non-Targeted Metabolomics Reveals Sorghum Rhizosphere-Associated Exudates are Influenced by The Belowground Interaction of Substrate and Sorghum Genotype. Int. J. Mol. Sci. 20(2): 431

doi: 10.3390/ijms20020431

Joelle, S., Enrico, M. and Trent, N. 2017 Feed Your Friends: Do Plant Exudate Shape the Root Microbiome? Trends in Plant Science 23(1): 1 – 17

doi: 10.1016/j.tplants.2017.09.003

Jones, D. L., Nguyen, C. and Finlay, R. D. 2009 Carbon Flow in the Rhizosphere: Carbon Trading at the Soil-Root Interface. Plant and Soil 321: 05 – 33

Lanoue, A., Burlat, V., Henkes, G. J., Koch, I., Schurr, U. and Rose, U. S. R. 2009 De novo Biosynthesis of Defense Root Exudates in Response to Fusarium Attack in Barley. New Phytol. 185(2): 577 – 588

Park, Y-S. and Ryu, C-M. 2021 Understanding Plant Social Networking System: Avoiding Deleterious Microbiota but Calling Beneficials. Int. J. Mol. Sci. 22(7): 3319

doi: 10.3390/ijms22073319

Pina, E. S., Silva, D. B., Teixeira, S. P., Coppede, J. S., Furlan, M., Franca, S. C., Lopes, N. P., Pereira, A. M. S. and  Lopes, A. A. 2016 Mevalonate-Derived Quinonemethide Triterpenoid from In-Vitro Roots of Peritassa laevigata and their Localization in Root Tissue by MALDI Imaging. Sci. Rep. 6: 22627

Schmelz, E. A., Kaplan, F., Huffaker, A., Dafoe, N. J., Vaughan, M. M., Ni, X., Rocca, J. R., Alborn, H. T. and Teal, P. E. 2011 Identity Regulation and Activity of Inducible Diterpenoid Phytoalexins in Maize. Proc. Natl. Acad. Sci. USA 108(13): 5455 – 5460

doi: 10.1073/pnas.1014714108

Sohrabi, R. and Ali, T. 2017 Formation and Exudation of Non-Volatile Products of the Arabidiol   Triterpenoid Degradation Pathway in Arabidopsis Roots. Plant Signaling Behavior. 12(1): e1265722

Tsuno, Y., Fujimatsu, T., Endo, K., Sugiyama, A. and Yazaki, K. 2018 Soyasaponins: A New Class of Root Exudates in Soybean (Glycine max). Plant Cell Physiol. 59(2): 366 – 375

doi: 10.1093/pcp/pcx192

Wang, N., Wang, L., Zhu, K., Hou, S., Chen, L., Mi, D., Gui, Y., Jiang, C. and Guo, J-H. 2019 Plant Root Exudates are Involved in Bacillus cereus AR156 Mediated Biocontrol Against Ralstonia solanacearum. Front Microbiol. 10: 98

doi: 10.3389/fmicb.2019.00098

Wen, F., White, G. J., Van Etten, H. D., Xiong, Z. and Hawes, M. C. 2009 Extracellular DNA is Required for Root Tip Resistance to Fungal Infection. Plant Physiol. 151(2): 820 – 829

Weisskopf, L., Abou-Mansour, E., Fromin, N., Tomasi, N., Santelia, D., Edelkott, I., Neumann, G., Aragna, M., Tabacchi, R. and Martinoia, E. 2006 White Lupin has Developed a Complex Strategy to Limit Microbial Degradation of Secreted Citrate required for Phosphate Acquisition. Plant Cell Environ. 29(5): 919 – 927 3040.2005.01473.x

Yuan, J., Zhao, J., Wen, T., Zhao, M., Li, R., Goossens, P., Huang, Q., Bai, Y., Vivanco, J. M., Kowalchuk, G. A., Berendsen, R. L. and Shen, Q. 2018 Root Exudate Drive the Soil-borne  Legacy of Aboveground Pathogen Infection. Microbiome 6(1): 156

doi.: 10.1186/s40168-018-0537-x

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