Impact of the arbuscular mycorrhizal symbiosis on tomato tolerance to drought stress

Microorganisms play a key role in preserving soil fertility in forest agro-ecosystems. Arbuscular mycorrhizal (AM) fungi, which form symbioses with the roots of the most important crop species, are usually considered biofertilizers. AM fungi improve plant mineral nutrient acquisition, in exchange for C compounds derived from the photosynthetic process, which can result in a positive host growth response and in an improved response to stress. Previous works, mainly performed in pots, suggest that the effects of the AM symbiosis on plant-water relationships are often subtle, transient, and probably circumstance- and symbiont-specific. Additionally, the mechanisms involved in the mitigation to water stress effects on plant growth by AM fungi are still to be elucidated. Within the framework of a project supported by CNR (AQUA), we studied the impact of the AM symbiosis on tomato (var. San Marzano nano) tolerance to water stress. The first step consisted in developing a protocol for water stress in presence/absence of an AM fungus. Two AM species have been selected to be inoculated separately, i.e. Funneliformis mosseae and Rizophagus intraradices, while non-mycorrhizal plants acted as controls. Leaf water potential (?leaf) and gas exchanges have been monitored until the plants reached a water stress state. By varying the duration of the drought, two levels of water stress were achieved, as confirmed by the measurement of leaf water potential: moderate stress (water potential approximately -0.8 MPa) and severe water stress (leaf water potential below -1.2 MPa), no stress (controls, leaf water potential at -0.3 MPa). Water Use Efficiency (WUE) has been calculated, showing a difference between the two considered AM fungi. At the end of the experiment (when plants have reached a severe stress state), AM colonization degree has been calculated, and morphometric parameters have been considered (e.g., shoot length and diameter, internodal distance, stomatal density). For the following biochemical and molecular analyses, leaves and roots from all the set of plants have been collected. The levels of ABA, proline, H2O2 have been assessed, and the analysis of CAT and SOD activity is being determined. Gene expression analyses (RT-qPCR experiments) will be performed considering genes potentially involved in water stress response (e.g., a tomato gene coding for a dehydrin and LeNCED1) and in mycorrhizal symbiosis (e.g., fungal and plant PT genes), on leaves and root samples respectively. On the basis of the results so far obtained, R. intraradices has been selected for further experiments devoted to verify the impact of the AM symbiosis on the plant responses to biotic stresses (i.e., aphids and nematodes) during drought conditions.