Plant, soil and microbe respond to allelochemicals. Allelochemicals present in soil are exposed to physiochemical and biological processes which may be detoxified or become more toxic or serve as a carbon skeleton to produce new toxins by soil microorganisms (Blum et al.,1999; Jilani et al., 2008). Transformation of allelochemicals by soil microbes may result into compounds with modified biological properties. The allelochemical activity is influenced by organic matter, reactive mineral surfaces, ion exchange capacity, inorganic ions (Inderjit 2001) and the bioavailability of allelochemicals in soil depends on adsorption, leaching and degradations by abiotic and biotic factors (Bhowmik 2018).
Allelochemicals are exuded into the rhizosphere from roots or is released through volatilization, leaching or released upon degradation of plant residue in soil. Therefore, rhizosphere has concentrated distribution of allelochemicals (Liu et al., 2018). The root-soil interface is the site of activity and has impact on soil microbial community (Bertin et al., 2003). The soil microorganisms produce secondary metabolite from the metabolic activity or by decomposition of organic residue. Bacterium Bacillus subtilis strain B-916 induce disease resistance in rice (Ren et al., 2006). Allelopathy influences surrounding plants. Phenolics and their derivatives, terpenoids and alkaloids are the main categories of plant allelochemicals which are released as root exudate, volatiles from leaves and through decomposition products (Qu and Wang 2008, Li et al., 2010). Alleochemicals from decomposed straw suppress weed as well as can reduce the incidence of pests and diseases (Cheng and Cheng 2015). Moreover, straw mulch improves the soil organic matter content and increase soil fertility. Terpenoid (E)-nerolidol is a volatile signal that elicit defense response against fungal pathogen Colletotrichum fructicola and piercing herbivore Empoasca onukii providing resistance to tea plants against the fungal pathogen and piercing herbivore (Chen et al., 2020). Differences in root exudates among populations of invasive plant may have an impact on plant performance. Flavonoid in the root exudate of the invasive plant enhanced arbuscular mycorrhizal fungal colonization (Tian et al., 2021). Flaveria bidentis (L.) disrupts soil microbial community to its benefit. Bacillus is plant growth promoting rhizobacteria in the rhizosphere of F. bidentis. Bacilli can enhance the plant nutrient uptake and prevent colonization of the root surface by plant pathogenic fungi (Chen et al., 2021; Sun et al., 2022). Organic acid, esters, diterpenes and cyclohexanone are potent allelochemicals in root exudate and can inhibit fungal pathogen (Kong et al., 2002; Li et al., 2013a; Li et al., 2013b).
The root exudate has a role in communication between plants and microbes in the rhizosphere. The plant genotype influence soil microbial community structure and the difference in the rhizosphere microbial community may contribute to the difference in resistance to the fungal pathogen Fusarium oxysporum f. sp. niveum (An et al., 2011). The rice root exudate has antifungal properties. By using watermelon/aerobic rice intercropping system the wilt disease caused by fungal pathogen F. oxysporum f. sp. niveum in watermelon can be controlled (Hao et al., 2010). Chinese chive plants intercropped with tomato suppressed the occurrence of bacterial wilt of tomato caused by Pseudomonas solanacearum via allelopathic effect (Yu 1999). The allelopathic effect of root exudate provide resistance to cucumber cultivars against Fusarium wilt (Wu et al., 2006). The type and quantity of allelochemical in root exudates can be one of the factor for the difference in eggplant resistance to Verticillium dahliae (Zhou et al., 2011). The isothiocyanate produced by Brassica sp. inhibit soil borne fungal pathogens of potato namely Colletotrichum coccodes, Rhizoctonia solani and Helminthosporium solani (Taylor et al., 2014). The root exudate from banana plant resistant to Panama wilt, inhibited spore germination of fungal pathogen Fusarium oxysporum f. cubense, whereas the root exudate from banana variety susceptible to Panama wilt did not (Buxton 1962). The composition of root exudate from wilt- susceptible and wilt-resistant plants were different and the difference in spore germination response are part of wilt resistance mechanism in the rhizosphere of banana (Buxton 1962; Li et al., 2013b). Allelochemicals exuded by plant root or are present in soil, may interact synergistically, which may provide an alternative for environmentally friendly compounds with structural diversity (phytotoxicity and nutrient availability).
References:
An, M., Zhou, X., Wu, F., Ma, Y. and Yang, P. 2011 Rhizosphere Soil Microorganism Populations and Community Structures of Different Watermelon Cultivars with Differing Resistance to Fusarium oxysporum f. sp. niveum. Can. J Microbiol. 57(5): 355 – 365
doi: 10.1139/w11-015
Bertin, C., Yang, X. and Weston, L. A. 2003 The Role of Root Exudates and Allelochemicals in the Rhizosphere. Plant Soil 256: 67 – 83
doi: 10.1023/A:1026290508166
Bhowmik, P. C. 2018 Importance of Allelopathy in Agriculture: Bioavailability and Functions of Allelochemicals in Soil Environment. Indian Journal of Weed Science 50(3): 209 – 217
doi: 10.5958/0974-8164.2018.00050.3
Blum, U., Shafer, S. and Lehman, M. E. 1999 Evidence for Inhibitory Allelopathic Interactions Involving Phenolic Acids in Field Soils: Concepts vs an Experimental Model. Critical Reviews in Plant Sciences 18(5): 673 – 693
doi.org/10.1080/07352689991309441
Buxton, E. W. 1962 Root Exudates from Banana and their Relationship to Strains of the Fusarium Causing Panama Wilt. Annals of Applied Biology 50(2): 269 – 282
doi.org/ 10.1111/j.1744-7348.1962.tb06009.x
Chen, X., Li, Q., Wang, L., Meng, Y., Jiao, S., Yin, J., Xu, H., Zhang, F. 2021 Nitrogen Uptake not Transfer of Carbon and Nitrogen by CMN Explains the Effect of AMF on the Competitive Interactions between Flaveria bidentis and Native Species. Front. Ecol. Evol. 9: 625519
doi: 10.3389/fevo.2021.625519
Chen, S., Zhang, L., Cai, X., Li, X., Bian, L., Luo, Z., Li, Z., Chen, Z. and Xin, Z. 2020 (E)-Nerolidol is a Volatile Signal that Induces Defenses against Insects and Pathogens in Tea Plants. Hort. Res. 7(1): 52
doi: 10.1038/s41438-020-0275-7
Cheng, F. and Cheng, Z. 2015 Research Progress on the Use of Plant Allelopathy in Agriculture and the Physiological and Ecological Mechanisms of Allelopathy. Front. Plant Sci. 6: 1020
doi: 10.3389/fpls.2015.01020
Hao, W-Y., Ren, L., Ran, W. and Shen, Q-R. 2010 Allelopathic Effects of Root Exudates from Watermelon and Rice Plants on Fusarium oxysporum f. sp. niveum. Plant and Soil 336(1): 485 – 497
doi: 10.1007/s11104-010-0505-0
Inderjit. 2001 Environmental effects on Allelochemical Activity. 93(1): 79 – 84
doi.org/10.2134/agronj2001.93179x
Jilani, G., Mahmood, S., Chaudhry, A. N., Hassan, I. and Akram, M. 2008 Allelochemicals: Sources, Toxicity and Microbial Transformation in Soil- A Review. Annals of Microbiology 58(3): 351 – 357
doi: 10.1007/BF03175528
Kong, C., Xu, X., Hu, F., Chen, X., Ling, B. and Tan, Z. 2002 Using Specific Secondary Metabolites as Markers to Evaluate Allelopathic Potentials of Rice Varieties and Individual Plants. Chinese Science Bulletin 47(10): 839 – 843
doi.org/10.1360/02tb9189
Li, X-G., Wei, Q., Liu, B., Alam, M-S., Wang, X-X., Shen, W. and Han, Z-M. 2013a Root Exudates of Transgenic Cotton and their Effects on Fusarium oxysporum. Front. Biosci. (Landmark Ed) 18(2): 725 – 733
doi: 10.2741/4134
Li, X-G., Zhang, T-L., Wang, X-X., Hua, K., Zhao, L., Han, Z-M. 2013b The Composition of Root Exudates from Two Different Resistant Peanut Cultivars and their Effects on the Growth of Soil-Borne Pathogen. Int. J. Biol. Sci. 9(2): 164 – 173
doi: 10.7150/ijbs.5579
Li, Z-H., Wang, Q., Ruan, X., Pan, C-D and Jiang, D-A. 2010 Phenolics and Plant Allelopathy. Molecules 15(12): 8933 – 8952
doi: 10.3390/molecules15128933
Liu, J., Yan, Z., Li, X., Jin, H., Yang, X., Xie, M., Su, A. and Qin, B. 2018 Characterization of Allelochemicals from the Rhizosphere Soil of Pinellia ternate (Thnub.) and their Inhibition Activity on Protective Enzymes. Applied Soil Ecology 125: 301 – 306
doi.org/10.1016/j.apsoil.2018.01.001
Qu, X. H. and Wang, J. G. 2008 Effect of Amendments with Different Phenolic acids on Soil Microbial Biomass Activity and Community Diversity. Applied Soil Ecology 39(2): 172 – 179
doi.org/10.1016/j.apsoil.2007.12.007
Ren, H., Song, T., Wu, T. Q., Sun, L., Liu, Y. X., Yang, F., Chen, Z. Y. and Dong, H. 2006 Effects of a Biocontrol Bacterium on Growth and Defence of Transgenic Rice Plants Expressing a Bacterial Type-III Effector. Annals of Microbiology 56(4): 281 – 287
Sun, C., Li, Q., Han, L., Chen, X. and Zhang, F. 2022 The Effects of Allelochemicals from Root Exudates of Flaveria bidentis on Two Bacillus species. Front. Plant Sci. 13: 1001208
doi: 10.3389/fpls.2022.1001208
Taylor, F. I., Kenyon, D. M. and Rosser, S. 2014 Isothiocyanates Inhibit Fungal Pathogens of Potato in In Vitro Assays. Plant and Soil 382(1-2): 281 – 289
doi: 10.1007/s11104-014-2157-y
Tian, B., Pei, Y., Huang, W., Ding, J. and Siemann, E. 2021 Increasing Flavonoid Concentrations in Root Exudates Enhance Associations between Arbuscular Mycorrhizal Fungi and an Invasive Plant. The ISME Journal 15(7): 1919 – 1930
doi.org/10.1038/s41396-021-00894-1
Wu, F., Han, X. and Wang, X. Z. 2006 Allelopathic Effect of Root Exudates of Cucumber Cultivars on Fusarium oxysporum. Allelopathy Journal 18(1): 163 – 172
Yu, J. Q. 1999 Allelopathic Suppression of Pseudomonas solanacearum Infection of Tomato (Lycopersicon esculentum) in a Tomato-Chinese Chive (Allium tuberosum) Intercropping System. J. Chem. Ecol. 25: 2409 – 2417
doi:10.1023/A:1020809805081
Zhou, B. L., Chen, Z. X., Du, L., Xie, Y. H., Zhang, Q. and Ye, X. L. 2011 Allelopathy of Root Exudates from Different Resistant Eggplants to Verticillium dahliae and the Identification of Allelochemicals. African J. Biotechnol. 10(42): 8284 – 8290
doi: 10.5897/AJB10.2300
BIOLOGICAL PROCESSES- A NATURE'S GIFT 