Performances of mycorrhizal tomatoes under water and nutrient stress conditions

Tomato (Solanum lycopersicum) has been one of the first non-legume models to be used in Arbuscular Mycorrhiza (AM) studies, since it responds to mycorrhizal colonization (1) and native microbiota (2) providing a useful tool to investigate metabolic processes and gene expression regulation in plant-microbe interaction. Tomato avoids the overlapping signal transduction pathways, since it does not establish the Rhizobium - AM tripartite symbiosis that occurs in legumes (3), thus permitting to investigate additional clues concerning the non-common symbiotic signaling pathway. Our work is developed in the frame of the TOMRES European Project, whose purpose is to enhance resilience to combined water and nutrient stress in tomato, as a model crop, in the context of global climate change evoking a scenario of frequent drought events and with the aim to reduce the environmental impact of agricultural activities. Our working hypothesis is that mycorrhization is a beneficial driver for tolerating adverse environmental conditions. The aim of the poster is to illustrate the performances of the tomato reference genotype M82 when grown in a miniature industrial culture system, i.e. tomatoes are grown in little volume plastic-pots (100 ml), using a quartz sand-vermiculite substrate with and without a monospecific inoculum of Funneliformis mosseae, an AM fungus which is widespread in nature. The well-watered and water stress conditions, with about 100% and 40% soil water content, respectively, were identified in a preliminary experiment measuring the stem water potential by using a Sholander pressure chamber. Seedlings were maintained for 16 h light at 24°C and 8 h dark at 21°C. The low nutrient solution was a modified Long-Ashton solution containing 3.2 μM phosphate to favor mycorrhization. After 70 days from the start of the experiment, at the sampling time biometric parameters and SPAD index were measured. The mycorrhizal status was also monitored with morphological observations. The results indicate that under nutrient deprivation and in a small-substrate volume the growth of M82 is in general stunted and that water stress reduces root and shoot biomass, regardless of the AM fungus presence. Even though plants in both well-watered and water stress conditions show a good level of mycorrhization, the AM symbiosis leads to a growth reduction: non mycorrhizal plants show higher biomass in both watering conditions compared to mycorrhizal plants. However, the AM symbiosis seems to improve the nutritional status of water- stressed plants over time: after 22 days from the start of the experiment the SPAD index was lower in mycorrhizal plants, but after 70 days the values were reversed. In conclusion, these results suggest that the beneficial effects usually provided by AM fungi to tomato are more limited when the M82 cultivar is maintained in small-plastic pots, as the ones used in the industrial cultivation, but some parameters, as the general nutritional status, seem to be positively affected. 1) Fiorilli et al. (2009) New Phytologist 184, 975–987 2) Chialva et al. (2018) New Phytologist https://doi. org/10.1111/nph.15014 3) Barker et al. (1998) The Plant Journal 15, 791–797