There are three primary effects that biologicals have on plants: communication between the plant and the microbe, changes in plant gene expression and physiological changes.
One thing we’ve learned is that different microbial organisms communicate differently. For example, there’s a plant root growing through the soil: When it needs nitrogen, it releases a chemical signal into the soil, and if Bradyrhizobium is present, it will migrate to the plant. Once in the rhizosphere, the Bradyrhizobium releases a signal, causing the plant to curl a root hair around the Bradyrhizobium. This is the beginning of the nodulation process to make nitrogen.
This is just one of many communication mechanisms between plants and microbials. In the case of Trichoderma, it emits signals to plant roots, which then trigger changes in gene expression in the plant. The gene expression relies on signaling cascades, meaning one change has a compound effect, triggering other key systems in the plant.
An example of how a plant might respond at the molecular level is changing its biochemical pathways. As the Trichoderma colonizes the roots, it sends a signal to the plant, telling it to upregulate whole biochemical pathways. One such pathway is the Reactive Oxygen Cycling pathway, which is what plants use to purge free radicals, or toxins, from their cells. If the plant can’t eliminate the toxins, DNA and protein can become damaged leading to decreased photosynthesis and yield.
Reactive oxygen species also cause damage in human systems; however, we can take vitamins and consume high antioxidant foods, such as dark chocolate and red wine, to rid us of these toxins. The difference is that plants can’t take vitamins and instead use their own biochemistry to convert these free radicals to a harmless state.
By upregulating the biochemical pathway, as in the example above, the plant cells are better able to mitigate stress, such has drought, high heat, disease and so on, thus enabling the plant to perform at its highest potential even in the presence of stress. While the effect of biologicals on plant performance appears simple, the colonization, signaling, and plant responses suggest a synergy that is elegant in its complexity.