Endophytic Colonization of Beauveria bassiana Enhances Drought Stress Tolerance in Tomato via “Water Spender” Pathway

Drought stress is one of the most important climate-related factors affecting crop production. Tomatoes (<i>Solanum lycopersicum</i> L.) are economically important crops which are highly sensitive to drought. The entomopathogenic fungus <i>Beauveria bassiana</i>, a widely used biological insecticide, can form symbiotic relationships with plants via endophytic colonization, increasing plant biomass and the ability to resist biotic stress. Under simulated drought stress conditions, the biomass of tomato seedlings such as plant height, root length, stem diameter, fresh weight, and relative water content, as well as the density and size of stomata in tomato leaves were significantly increased after <i>B. bassiana</i> colonization via root irrigation (<i>p</i> < 0.05). Meanwhile, the physicochemical properties associated with drought resistance such as peroxidase activity and proline content increased significantly (<i>p</i> < 0.05), while malondialdehyde reduced significantly (<i>p</i> < 0.05), and the expression levels of key genes related to stomatal development and drought tolerance pathways increased significantly (<i>p</i> < 0.05). These results indicate that the colonization of <i>B. bassiana</i> enhances the water absorption capacity of tomato seedlings and the rate of transpiration significantly and increases drought tolerance in tomato via the "water spender" pathway, which provides a new strategy for improving crop resistance to drought stress.