Plant pathogens, such as the Pseudomonas syringae pv. tomato (1), can enter through leaf pores known as stomata, which control respiration and releaseof water (2). In response to infection, plants release L-malic acid (3) from their roots, a food source for the beneficial bacterium Bacillus subtilis (4). The bacteria release toxins that suppress the root’s antimicrobial defenses (5) and stave off other potentially pathogenic bacterial strains (6), allowing B. subtilis to colonize the roots. B. subtilis colonization, in turn, causes the plant to produce abscisic acid, which leads to stomatal closing (7), helping prevent further infection. Similarly, when plants are infected with the tomato blight fungus Alternaria solani (8), nearby plants can initiate their own defenses by sensing warning signals transmitted between the plants via a plant root-fungal symbiotic system, the mycorrhiza, containing the beneficial fungus Glomus mosseae (9).
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