How do plants ‘use’ the Bacillus subtilis applied to the soil to help protect the plant canopy from attack by diseases?

Scientists have measured a peculiar interaction between a plant under attack from a disease in the canopy. Root colonization by Bacillus subtilis increases (below right). The increased level of Bacillus subtilis on the root system sends signals to the plants own chemical pathways that produce abscisic acid and salicylic acid which are involved in controlling the closure of stomata. When stomata close they reduce the entry points for the disease into the plant.