A specialized group of soil bacteria known as Plant Growth Promoting Bacteria (PGPB) plays a crucial role in enhancing plant growth and overall health. PGPB typically resides in the soil zones around plant roots, commonly referred to as the rhizosphere or within the plant roots, known as the inner sphere. These beneficial bacteria stabilize nutrients, prevent diseases, and significantly improve plant vitality.
PGPB serves as a primary ingredient in live microbial mixtures applied by farmers in crop fields, often termed biofertilizers. The development of PGPB mixtures is pivotal for sustainable crop management, as biofertilizers are regarded as a more eco-friendly alternative to conventional chemical fertilizers.
A team of Italian researchers investigated how three different PGPB mixtures impacted natural microbial populations in the rhizosphere and endosphere of two sunflower varieties. Their objective was to evaluate whether the PGPB inoculant would exert a lasting influence on the microbial community of sunflowers, while also examining any significant differences between the microbial communities of natural and genetically modified sunflower strains.
Initially, researchers identified bacterial strains that promote plant growth by producing beneficial acids such as indole lactic acid, which enhances resistance to heavy metals, aids in mineral dissolution, and facilitates nutrient release. They cultured 40 distinct bacterial types sourced from bee guts, pollen, wheat rhizospheres, and fruit trees, assessing their acid production. From these trials, they formulated three PGPB mixtures containing six types of bacteria, including Bacillus stocks, 3-in-1 Lactobacillus family stocks, and 2-in-1 Paenibacillus sp. strain.
To evaluate the PGPB mixtures’ effectiveness on crops, the team conducted a two-year field experiment in northern Italy during 2023 and 2024. This study involved 24 fields, including 12 with genetically modified hybrid varieties and 12 plots of naturally grown, open-pollinated sunflowers. The researchers applied the three PGPB mixtures to three plots each, resulting in nine microbe-treated plots per sunflower variety and three control plots devoid of microbes. The PGPB mixture was administered at four different points during the growing season through the irrigation water, while the control plots received microorganism-free irrigation water.
Upon flowering, the researchers harvested the sunflowers and sterilized the roots using saline, effectively isolating the soil microbes in the rhizosphere from those in the endosphere. They then extracted DNA from the samples for analysis of specific genetic regions to identify the microorganisms present using 16S rRNA gene sequencing.
After reviewing the data, the researchers found notable differences in microbial communities between the 2023 and 2024 field experiments, likely attributable to variations in temperature and rainfall. Therefore, they conducted separate analyses for each growing season to accurately gauge the PGPB treatment’s effectiveness. Their findings indicated that the microbial community of the inoculated sunflowers differed significantly from that of the control group, with hybrid sunflowers demonstrating more pronounced alterations in both rhizosphere and endosphere microbial communities compared to open-pollinated varieties, suggesting a stronger response to inoculation.
The research team identified several microbial taxa as “therapeutic indicators,” indicating their abundance varied significantly between treated hybrid sunflowers and controls. The endosphere of treated hybrids showed decreased levels of Pseudonocardiaceae and Nocardiaceae, while levels of Blastocatellaceae and Flavobacteriaceae increased compared to controls. Similarly, the rhizosphere of treated hybrids contained fewer Pseudomonadaceae and Bacillusidae, while exhibiting higher levels of Gemmataceae and Vicinamibacteriaceae. The researchers noted that these microorganisms were part of the sunflowers’ native microbiome, existing in the soil prior to PGPB application.
Furthermore, the research team compared control plots to check for inherent microbial differences between the two sunflower varieties, finding no significant discrepancies in microbial phylum richness. In fact, both varieties’ rhizosphere microbial communities closely mirrored one another, with Bacillus, Pseudomonas, and Actinobacteria comprising approximately 31%, 23%, and 16% of the hybrid sunflowers’ rhizosphere, while accounting for 29%, 25%, and 16% of the open-pollinated variety’s rhizosphere, respectively.
Finally, the researchers assessed whether rhizosphere and endosphere microorganisms were similar across sunflower varieties, discovering that populations of specific microbial families, such as Streptomycetes and Burkholderiaceae, experienced parallel increases and decreases in both the endosphere and rhizosphere. This suggests a possible direct transfer of microorganisms between these layers or that sunflowers may actively select for distinct microbial types.
In conclusion, the research team determined that the PGPB mixture significantly altered the rhizosphere and endosphere of sunflowers by enriching specific beneficial microorganisms. They proposed that scientists could eventually design custom microbial biofertilizers to enhance crop resilience against drought and disease or to improve yield. They emphasized the need for continued exploration into biofertilizers and microorganisms’ roles in soil ecosystems.
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Source: sciworthy.com
