Growth promotion of tomato plants through bacteria derived from spent substrate of edible mushrooms

Tomatoes are one of the most widely grown vegetables on the planet, feeding millions and forming the backbone of countless diets. To keep up with the world's hunger, farmers must constantly find ways to grow more food without harming the land. For decades, the standard solution has been chemical fertilizers, but these can leach into groundwater and damage soil health over time. Nature, however, offers a quieter alternative: tiny bacteria that live in the soil and help plants grow. These microscopic helpers, often called plant growth-promoting bacteria, do not just sit idle; they produce natural compounds that unlock nutrients, fight off harmful microbes, and even stimulate the plant's own growth hormones. The challenge for scientists is finding the right strains of these bacteria and figuring out the best way to deliver them to crops.

A team of researchers in Brazil turned to an unlikely source to find these helpers: the leftover material from mushroom farms. When edible mushrooms are harvested, they leave behind a spent substrate—a mixture of husks, seeds, wood, and other agricultural by-products that has already been partially broken down by the fungi. This rich, organic waste is teeming with microbial life. The researchers collected twenty different bacterial strains from this spent mushroom substrate and put them through a series of tests to see which ones were the most capable of helping a tomato plant. They looked for specific biological activities, such as the ability to produce enzymes that break down tough plant fibers, substances that grab iron from the soil, and natural growth hormones that encourage roots to spread.

The screening process revealed that every single bacterial strain they tested had some useful ability, but three stood out as particularly promising. One strain, named BUD 5, was a powerhouse that produced a growth hormone called indole-3-acetic acid, along with enzymes that break down cellulose and compounds that secure iron. Another, BUD 7, also produced the growth hormone and enzymes that break down cellulose. A third, BUD 19, was a strong producer of iron-grabbing compounds and also produced cellulose-breaking enzymes. To see if these bacteria could actually make a difference in a living plant, the team moved their work from the lab bench to a greenhouse. They grew tomato plants in pots and applied the bacteria in three different ways: soaking the seeds before planting, spraying the leaves, or doing both. They monitored the plants over two growing seasons, measuring everything from how tall they grew to the size and weight of their root systems.

The results showed that the bacteria did indeed help the tomatoes, though the effects varied depending on the year and how the bacteria were applied. In the first year, the combination of treating the seeds and spraying the leaves with the BUD 5 strain produced the most dramatic results. These plants developed root systems that were nearly forty percent larger in volume than the untreated control plants. The leaves of plants treated with BUD 5 also showed a significant boost in fresh weight, growing almost forty-five percent heavier than the control group. The bacteria seemed to work by encouraging the plants to build more root mass, which in turn allowed them to pull more water and nutrients from the soil, fueling the growth of the leaves and stems above ground.

The second year of the experiment confirmed that these bacteria were reliable, even if the specific winners changed slightly. While the control plants grew just as tall as the treated ones in terms of height, the bacteria-treated plants often developed more robust root systems and accumulated more dry matter in their roots and shoots. For instance, treating seeds with the BUD 7 strain led to a root system that was over twenty percent larger than the control. The researchers also measured how efficiently the plants used light for photosynthesis, the process by which plants turn sunlight into energy. The bacteria-treated plants maintained healthy photosynthetic machinery, with some treatments showing a marked increase in the efficiency of electron transport, a key step in converting light energy into chemical energy. This suggests that the bacteria helped the plants use sunlight more effectively, providing the extra energy needed to support the larger root and shoot growth.

What makes this discovery particularly interesting is the source of the bacteria. By using the spent substrate from mushroom cultivation, the researchers tapped into a reservoir of microorganisms that are already adapted to breaking down complex organic matter. These bacteria are naturally equipped with the tools to degrade tough plant fibers and recycle nutrients, traits that proved beneficial when transferred to tomato plants. The study demonstrates that these waste products from mushroom farms are not just trash to be discarded, but a valuable resource for finding new biological tools for agriculture. The three selected strains, BUD 5, BUD 7, and BUD 19, showed that they could consistently promote growth, improve root development, and enhance the plant's ability to capture energy from the sun.

The study did not find that the bacteria worked the same way every single time; the results shifted between the two years, likely due to natural variations in environmental conditions. However, the consistent trend was that the bacteria helped the plants grow more vigorously, particularly in their root systems. This is a crucial finding because a stronger root system means a plant is better equipped to handle stress and access nutrients. The researchers concluded that these bacterial isolates have real potential to be developed into bio-inputs, offering a sustainable way to boost tomato production without relying on chemical fertilizers. The work highlights a simple but powerful idea: by looking at the waste of one agricultural process, we can find the keys to improving another.