A sustainable microbial approach to enhancing tomato salt tolerance: Functional role of halophilic plant-associated bacteria in growth promotion and fruit quality improvement

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 plant growth-promoting bacteria (PGPB) represents a sustainable strategy to mitigate the detrimental effects of salinity stress and to enhance plant performance under adverse environmental conditions. This PhD project was divided into two main parts. The first investigated the effects of the halotolerant strain PVr_9 on tomato plants grown under in vitro, greenhouse, and open-field conditions. Inoculation with PVr_9 led to increased root growth and shoot biomass, higher proline accumulation, and enhanced APX activity, overall improving the physiological status of plants subjected to salt stress. These effects were associated with the upregulation of the salt tolerance genes SOS1 and NHX1, the induction of PAP, and the downregulation of cysteine synthase. Moreover, PVr_9 induced the expression of defence-related proteins even in the absence of stress, suggesting a priming effect against phytopathogens. In field-grown plants, PVr_9 also improved the nutraceutical quality of tomato fruits, increasing the levels of carotenoids, lycopene, β-carotene, and L-ascorbic acid, while reducing contaminant accumulation. The second part of the study focused on endophytic bacteria isolated from halophytic plants, selected for their natural adaptation to saline environments. After in vitro screening and compatibility tests, three strains, Curtobacterium sp., Sphingomonas sp., and Pseudomonas sp., were assembled into a microbial consortium and applied to industrial tomato plants (cv. Heinz 1301) through both root and foliar inoculation. Root treatment with the consortium, in plants exposed to 200 mM NaCl, resulted in a significant increase in biomass, chlorophyll content, proline, and total soluble sugars, as well as enhanced activity of the antioxidant enzymes APX and CAT. Gene expression analyses revealed the upregulation of SOD1, SOD3, P5CS2, OAT, and P5CR, indicating the activation of antioxidant and osmoprotective pathways. The absence of major variations in Na⁺ and K⁺ content suggests that the observed tolerance was mainly due to improved redox balance and osmolyte metabolism rather than direct regulation of ion homeostasis. Overall, this work highlights the potential of halotolerant plant growth promoting bacteria, used either as single strains or in compatible microbial consortia, as sustainable biotechnological tools to improve tomato resilience and productivity in saline environments. Root inoculation proved to be the most effective strategy, supporting the development of microbial-based solutions applicable to agriculture in salt-affected soils within the framework of current and future climate challenges.