PGPR microorganisms are those bacteria that colonise the root and its area of influence (rhizospheric soil) and are extremely important in agriculture as they colonise plants using various mechanisms. These mechanisms are either intracellular (iPGPR), where they are incorporated within the root cells, generating nodules that are visible to the naked eye, or extracellular (ePGPR), existing in the rhizosphere, in the rhizoplane or in the spaces between the cells of the root cortex (Ahemad & Kibret, 2013). During this colonisation process, the bacteria are attracted by chemotaxis based on compounds present in the root exudates, such as amino acids and sugars, providing a rich source of energy and nutrients that allow the bacteria to adhere to the root surface (Correa, 2008). On the other hand, microorganisms are capable of adapting to any carbon source, i.e. they can assimilate any organic compound and use it as a nutrient to obtain sufficient energy for their development. This metabolic versatility makes them perfect candidates for potential degradation of organic pollutants present in soils and water. Azotobacter species are known to tolerate up to 5% concentration of pesticides and also degrade heavy metals. A. chroococcum and A. vinelandii demonstrated their biodegradation efficacy against many commonly used pesticides such as endosulfan, chlorpyrifos, pendimethalin, phorate, glyphosate and carbendazim (Dubey et al., 2005, Meena et al., 2020).
Seed treatment is one of the most efficient, economical and environmentally friendly methods of crop protection, as a small amount of active ingredient can control problems that threaten the crop from the time of sowing. In addition, seed treatment provides protection during the critical stages of germination and crop establishment. The combined use of seed treatment together with the addition of PGPRs would improve crop establishment, as the PGPRs would bring more vigour to the seed, decreasing the possible damaging effect of the pesticide.
In trials on seeds treated with a conventional fungicide, it has been shown that the use of microorganisms applied in irrigation advances the germination of these seeds by 14.95%, and also increases the foliar and root development of plants.
The bars represent mean values of the germination percentage of the seeds subjected to the different treatments. Statistical differences between treatments are shown with asterisks (T-student, * p<0.05, **p<0.01,***p<0.001). ST: seeds treated with chemical fungicide.
The bars represent mean values of aerial, root and total length of the seeds subjected to the different treatments. Statistical differences between treatments are shown with asterisks (T-student, * p<0.05, **p<0.01,***p<0.001). ST: seeds treated with chemical fungicide.
On the other hand, the use of synthetic fungicides in crop plans provides the plant with protection against diseases and pests, but may produce detrimental effects on the plant microbiome. The use of PGPR-based products in conventional crop strategies would improve plant vigour and health by restoring the microbiome.
Trials conducted on lettuce where a conventional fungicide was applied at levels where chemical stress occurred in the crop showed that the application of VIBACTER® reduced stress symptoms by 26.6% and also improved rooting, biomass and the physiological state of the crop.
The bars indicate mean values of phytotoxicity of the plants where the different treatments were applied. Analysis of variance (p<0.05) and subsequent Duncan’s multiple range test. Different letters for each treatment and evaluation indicate significant differences. 0: no symptoms, 1: mild symptoms, 2: severe symptoms.
Rhizobacteria play a key role in biological and conventional approaches to agriculture, as they have the ability to detoxify heavy metals, degrade pollutants and reduce chemical stress. This contributes to improving the physiological state of crops, which in turn leads to higher yields.
REFERENCES
- Ahemad, M., & Kibret, M. (2013). Recent trends in microbial biosorption of heavy metals: a review. Biochemistry and Molecular Biology, 1(1), 19-26.
- Franco Correa, M. (2008). Evaluación de caracteres PGPR en actinomicetos e interacciones de estas rizobacterias con hongos formadores de micorrizas.
- Dubey, S. C., Singh, V., Priyanka, K., Upadhyay, B. K., & Singh, B. (2015). Combined application of fungal and bacterial bio-agents, together with fungicide and Mesorhizobium for integrated management of Fusarium wilt of chickpea. BioControl, 60(3), 413-424.
- Mahmood, A., Turgay, O. C., Farooq, M., & Hayat, R. (2016). Seed biopriming with plant growth promoting rhizobacteria: a review. FEMS microbiology ecology, 92(8), fiw112.
Meena, R. S., Kumar, S., Datta, R., Lal, R., Vijayakumar, V., Brtnicky, M.,… & Marfo, T. D. (2020). Impact of agrochemicals on soil microbiota and management: A review. Land, 9(2), 34.
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