Researchers found a much faster way to screen soil bacteria as potential biostimulants and bio-pesticides. Prof. Ian Dubery (left) and Dr. Msizi Mhlongo (right) from the University of Johannesburg identified 10 times more signal compounds from the bacteria, compared to most recent studies. Rhizobacteria can help protect crops from abiotic and biotic stresses, by boosting plant growth and plant self-defense. Farmers apply the bacteria as seed coatings or inoculants. "Biologicals" can reduce the need for chemical fertilizers and pesticides. Credit: Therese van Wyk, University of Johannesburg
Certain soil bacteria can serve as allies to plants by producing volatile organic compounds (VOCs) that act as natural and environmentally friendly fertilizers, pesticides or aids to boost the plant immune system. Rather than conducting lengthy and intensive greenhouse and field studies to identify potential crop-boosting biologics, analysts can use chemical screening methods to more quickly identify specific beneficial compounds that bacteria can produce. A team of researchers from the University of Johannesburg have developed a method based on gas chromatography (GC) and high resolution mass spectrometry (MS) that identified 10-20 times more VOCs produced by soil bacteria than other current methods.
The researchers grew four strains of promising rhizobacteria – Pseudomonas koreensis, Pseudomonas fluorescens, Lysinibacillus sphaericus and Paenibacillus alvei – and analyzed the VOCs produced by these strains using solid-phase micro-extraction (SPME) GC-MS. The sensitivity and high scanning rate of the high-resolution time-of-flight (TOF) MS method, combined with multivariate data analysis, enabled the team to distinguish compounds with very close molecular masses and better separate out molecular peaks, said lead author Msizi Mhlongo. As a result, a total of 121 different VOCs produced by the four strains could be differentiated, including three forms of salicylic acid – methyl salicylate, isoamyl salicylate and n-hexyl salicylate. These compounds can help protect plants from disease by triggering Induced Systemic Resistance (ISR) and Systemic Acquired Resistance (SAR), processes that strengthen the plants’ immune system through chemical signaling.
The analytical approach developed by the researchers can help better identify rhizobacteria that are beneficial for plant growth and protection, and could also help gain crop-specific insights for selecting biostimulants and biofertilizers. High resolution analyses of the VOCs produced by rhizobacteria can also identify biomarkers for SAR and ISR induction, and other beneficial qualities, by obtaining comprehensive metabolomic profiles for different strains. Faster screening for these biomarkers could then be conducted prior to greenhouse and field trials in order to rule out non-starters and select the most promising strains for further study. This research was published in the journal Metabolites.
“As more research is done, biostimulant approaches become more reliable,” Mhlongo said. “Plant growth promoting rhizobacteria (PGPR) are crop specific. What works well for tomatoes, wheat, and maize (corn) might not work for spinach. This kind of research helps us know which plant-specific bacteria will promote growth and induced resistance.”
Future work will include investigating the volume, concentration and consistency of VOC production by the studied rhizobacteria strains to further evaluate their potential biologic tools in agriculture.