Salt stress mitigation and improvement in fruit nutritional characteristics of tomato plants: New opportunities from the exploitation of a halotorelant Agrobacterium strain

Soil salinity is considered one of the most limiting factors in large agricultural areas, and tomato (Solanum lycopersicum L.) is highly sensitive to this abiotic stress, which exacerbates under drought. The use of halotolerant PGPR is a promising strategy to enhance tomato tolerance and productivity in saline soils. The present study tested the capability of the recently discovered bacterial strain PVr_9 to increase tolerance to salinity in tomato plants in vitro, greenhouse and open field conditions. When inoculated with PVr_9, in vitro seedlings showed a significant increase in primary root length, number of secondary roots and fresh weight both in absence of stress and when exposed to 150 mM NaCl. In plants exposed to salt treatment, PVr_9 upregulated the salt tolerance genes SOS1 and NHX1, and the enzyme prolyl aminopeptidase involved in proline metabolism. Proteins associated with resistance against pathogens were also upregulated by PVr_9 in absence of salt stress. In greenhouse, PVr_9-inoculated tomato plants treated for 1-week with 150 mM NaCl showed a significant increase in shoot fresh biomass, chlorophyll, proline content, and APX activity. Tomato fruits from PVr_9 inoculated plants, exposed in the field to salt stress during the flowering stage, showed higher levels of carotenoids, lycopene, and l-ascorbic acid, as well as an increased antioxidant capacity compared to fruits of uninoculated plants. These results indicate PVr_9 as a potential biostimulant in tomato for better tackling soil salinization in a context of climate change and expansion of coastal salinity, while there is large scope for the improvement of the nutritional characteristics of tomato fruits.