Insights on the impact of arbuscular mycorrhizal symbiosis on tomato tolerance to water stress

Arbuscular mycorrhizal (AM) fungi, which form symbioses with the roots of the most important crop species, are usually considered bio-fertilizers, whose exploitation could represent a promising avenue for the development in the future of a more sustainable next-generation agriculture. The best understood function in symbiosis is an improvement in plant mineral nutrient acquisition, as exchange for C compounds derived from the photosynthetic process: this can enhance host growth and tolerance to environmental stresses, e.g., water stress (WS). However, physiological and molecular mechanisms occurring in AM-colonized plants and directly involved in the mitigation of water stress effects need to be further investigated. The main goal of this work is to verify the potential impact of the AM symbiosis on the plant response to WS. To this aim, the effect of two AM fungi (Funneliformis mosseae and Rhizophagus intraradices) on tomato under WS condition was studied. A combined approach, involving eco-physiological, morphometric, biochemical and molecular analyses, has been used to highlight the mechanisms involved in plant response to water stress during AM symbiosis. Gene expression analyses focused on a set of target genes putatively involved in the plant response to drought and, in parallel, we considered the expression changes induced by the imposed stress on a group of fungal genes playing a key role in water-transport process. Taken together, results show that AM symbiosis positively affects the tolerance to water stress in tomato with a different plant response depending on the AM fungi species involved.