Root colonization by Pseudomonas chlororaphis primes tomato (Lycopersicum esculentum) plants for enhanced tolerance to water stress

Previous research demonstrated that Pseudomonas chlororaphis subsp. aureofaciens strain M71, a plant growth promoting bacterium (PGPB), exerts beneficial effects on plant metabolism and primes tolerance mechanisms against biotic stresses in tomatoes. We designed an experiment to assess whether root colonization with P. chlororaphis is also able to improve tolerance to water stress in tomatoes. Our results show that inoculation with P. chlororaphis stimulates the antioxidant activity of well-watered tomatoes while maintaining a steady-state level of reactive oxygen species (ROS), increases the expression of genes encoding for the biosynthesis of leaf terpenes, stimulates the production of both the phytohormones ABA and IAA, in turn affecting plant shape (number of leaves) and height (length of internodes), without altering photosynthesis. Upon exposure to mild water stress conditions, an improved antioxidant activity in tomatoes 'primed' by P. chlororaphis inoculation limited the accumulation of reactive oxygen species (ROS) in leaves and thus enhanced tolerance, also through increase of the (osmolyte) proline content. Moreover, P. chlororaphis inoculation further enhanced the ABA level in leaves of water-stressed tomatoes allowing a more efficient modulation of stomatal closure that resulted in an improved water use efficiency (WUE) and biomass accumulation.