Tolerance of tomato plants to water stress is improved by the root endophyte Fusarium solani FsK

The endophyte Fusarium solani strain FsK may colonize the roots of tomato plants and protect them against fungal pathogens. Here we show that the fungus can alleviate the strong negative effects of water stress on the development and growth of tomato seedlings that were co-cultivated with FsK under in vitro conditions. In a succeeding pot experiment, the presence of FsK ameliorated physiological stress in plants grown in peat under water deficit irrigation conditions: plants practically restored levels of net photosynthesis rate (PN), maximal potential yield of PSII (Fv/Fm) and relative water content (% RWC) and displayed a 4-fold increase in stomatal conductivity (gs) and a 7-fold decrease in proline levels compared to mock-inoculated plants. These changes indicate a shift towards an anisohydric type response to water stress, aided by mechanisms maintaining efficient water absorption by roots and high-water conductance in planta. Effects on the accumulation of reactive oxygen species and relevant antioxidant enzymes were less dramatic. Our data indicate that the endophyte may reduce stomatal closure response to water stress, and potentially improve water acquisition by roots and/or water conductance in planta, thus preventing reduction in photosynthesis rates and minimizing water loss; this, results in increased tolerance of tomato plants to water deprivation, conferred by the endophyte.