How it Works: Bacillus Subtilis OKB105

After deciding to use seed coating with Bacillus subtilis, the next step was to choose a specific strain. After extensive research and searching for a supplier that could provide the exact strain we needed(shocking, nobody wants to sell bacteria to high schoolers), we ultimately selected Bacillus subtilis OKB105.This specific strain produces spermidine, a compound made up of two or more amino groups, which plays two important roles in plant growth. The first being promoting the production of Expansin, a protein which makes cell walls less rigid. Increased fluidity allows for an increase in turgor pressure, enabling cells to expand and grow. Once a cell reaches a certain size, it hits a threshold, which signals cell division.

The second benefit of OKB105 is its ability to reduce ethylene production. Ethylene is a hormone associated with plant stress, fruit ripening, and aging. High ethylene levels prevent root elongation, while lower levels promote an increase in vegetation. In my previous research, I noticed that most experiments only measured plant growth effects within the first 20 to 30 days. During that period, there was a significant increase in vegetation compared to control plants. This was a strong indicator that ethylene production was being suppressed. However, I was immediately worried that if the bacteria continuously lowered ethylene levels, the plant might prioritize vegetative growth and flower production over fruit ripening, and this would not be captured in the studies that I had researched.

To understand this further, I researched a study that specifically examined the spermidine production of Bacillus subtilis OKB105. The study also measured ethylene levels after applying OKB105 to the roots of tobacco plants using a root drenching method. Results showed that after one day, ethylene production dropped by 50.65%, and after three days, it decreased by 20.61%. While there were diminishing effects over time, it’s important to consider the inoculation method. Root drenching provides quick colonization, but because the bacteria do not grow along with the roots, environmental factors can significantly reduce bacterial presence over time.

The method we are using, seed coating, differs in that it delays bacterial activation until germination begins. However, because the bacteria are present from the moment the seed starts rooting, they have greater resistance to environmental factors. This leads to prolonged colonization and sustained effects, reducing the risk of rapid declines in ethylene suppression after initial application.

Returning to the ethylene levels in our trials, measurements after one day showed ethylene content around 0.5 µg/g, which is ideal for tomato plants. This balance ensures both strong vegetative growth and adequate flowering, without preventing fruit ripening. This is how our chosen bacteria, Bacillus subtilis OKB105, supports plant growth by optimizing spermidine and ethylene levels.