Jong Hyun Ham

Many farmers support advanced technologies to reduce inputs that result in greater sustainability and profitability on their farms and protect the inputs they do use. One area of interest is improving the efficacy of seed treatments with active chemical or biological materials, which could be a more cost-effective way to deliver crop protection inputs.

The global seed treatment market is envisioned to grow from $6.4 billion in 2020 to $11.3 billion by 2025, according to the MarketsandMarkets research service. This indicates great potential for the seed treatment industry and the demand for more innovative and effective products. The study of seed treatments is critical to ensuring that products are both economically and environmentally beneficial.

Beneficial microorganisms, including fungi and bacteria, are considered excellent for seed treatments because they would be less harmful to the environment than chemical materials. Moreover, beneficial microorganisms would provide long-term positive impacts on plant growth if they can successfully colonize host plants and persist after the initial stages of development.

There have been numerous studies reporting the effects of seed treated with beneficial microorganisms on various crops, including soybean. The mechanisms underlying the beneficial effects on plants include:

The increase of available plant nutrients through metabolic activities such as nitrogen fixation, phosphate solubilization and production of siderophores, which help transport iron across cell membranes.

Production of plant growth hormones, such as indole-3-acetic acid (IAA or auxin).

Suppression of diseases through antagonistic activities against pathogens or by inducing plant defense systems.

In addition to growth promotion and disease suppression, protection from abiotic stresses, such as drought, flooding, nutrient deficiencies or abnormally high or low temperatures, is another beneficial mechanism that some plant-growth promoting rhizobacteria provide to host plants. Beneficial bacteria in soybeans and other legume plants have been studied with the diverse aspects of growth-promoting activity mentioned above. Particularly, rhizobia, which are symbiotic nitrogen-fixing bacteria that cause the formation of nodules on legumes, are an important group of beneficial bacteria for soybeans and other legume crops. Some strains of rhizobia demonstrate multiple beneficial roles, including synthesis of indole-3-acetic acid, production of siderophores and suppression of plant pathogens. Bacterial endophytes of soybeans — bacterial organisms that live within soybeans — have also been reported to exert positive effects on growth, nodulation and disease suppression.

With the support of the Louisiana Soybean and Grains Research and Promotion Board, we have screened and characterized soybean rhizobacteria from Louisiana soybean fields based on their various beneficial activities for the growth and health of soybean plants to develop biological agents for soybean seed treatment. The first screening process was performed focusing on antifungal activity against a fungal soybean pathogen, Rhizoctonia solani (Figure 1). Interestingly, three of the bacterial isolates showing strong antifungal activities against the fungal pathogen also enhanced the growth and root development of soybeans in greenhouse trials (Figure 2). Although none of the three strains showed growth-promoting activity in field trials, we observed a substantial reduction of purple seed stain caused by Cercospora kikuchii with one of the three strains tested and the commercial seed-treating product, indicating that the bacterial strain is a good candidate of biological agent that can be used for seed treatment.

Furthermore, we extended the screening of rhizospheric bacteria based on other beneficial activities that were previously mentioned, including IAA production, nitrogen fixation, phosphate solubilization and siderophore production. Several strains of these beneficial bacteria showed significant growth-promoting activity when used to treat soybean seeds. Most biostimulants and biofertilizers are formulated with a single promising biological agent, which could result in a significant reduction of activity when the environment is not favorable. To avoid this type of risk, as well as to increase beneficial activity through additive or synergistic effects, we have formulated and tested mixtures of multiple bacterial agents with different types of biological activities beneficial to soybean growth and health. Strikingly, certain combinations of multiple bacterial strains caused a substantial increase of soybean growth and yield, indicating additive or synergistic effects of simultaneous seed treatment with multiple beneficial bacteria (Figures 3 and 4).

In greenhouse tests to compare with conventional fertilization, seed treatments with those bacterial mixtures were more effective than the application of a commercial fertilizer in the pot soil. In a preliminary field trial at the LSU AgCenter Central Research Station in Baton Rouge, they also produced soybean plants showing smaller damages of foliar diseases and insect pests compared to the control group, which had no seed treatment. Each bacterial mixture tested is composed of nine to 23 bacterial strains. With new funding from the United Soybean Board, we are conducting field trials with these bacterial mixtures in three locations of Louisiana (Baton Rouge, Alexandria and Bossier City) and optimizing formulation conditions through testing various protective materials and subsets of each bacterial mixture (Figure 5).

Beneficial microorganisms that facilitate the growth of plants have long been studied as an ideal biological tool to improve crop production in environmentally friendly ways. We are working to develop an innovative seed treatment technology with bacterial agents that originated from Louisiana soils through designing bacterial mixtures based on biological data of their components. This study will greatly contribute to the sustainable production of soybeans in Louisiana through the reduction of crop management costs for fertilization and pesticide application and an increase in productivity and the improvement of soil health and food safety.

Jong Hyun Ham is a professor in the Department of Plant Pathology and Crop Physiology.

(This article appears in the summer 2021 issue of Louisiana Agriculture.)

Figure 1. A soybean-associated beneficial bacterium showing antifungal activity against the fungal pathogen Rhizoctonia solani. The large inhibition zone surrounding the bacterium indicates its antagonistic activity against the fungal pathogen grown on the culture medium. Photo by Rosalie Calderon

Figure 2. Soybean plants grown from the seeds without any treatment (left), with a commercial seed treatment product (center), and a selected bacterial agent (right). This picture shows the more vigorous growth of soybean plants grown from the seeds treated with a selected beneficial bacterium. Photo by Rosalie Calderon

Figure 4. Soybean pods harvested from one plant grown in the greenhouse. Imposed seed treatments for this result are 1) no treatment check (T1), 2) a commercial seed treatment product (T2), 3) the bacterial mixture #1 (T3), 4) the bacterial mixture #2 (T4), and 5) the bacterial mixture #3 (T5). This picture shows the yield increase from the seed treatments with bacterial mixtures of beneficial bacteria. Photo by Rosalie Calderon

Figure 5. Rosalie Calderon, a Ph.D. candidate of the Department of Plant Pathology and Crop Physiology, is testing beneficial bacteria for their growth-promoting activity on soybean plants. Photo by Johnson Leonard