Soil salinization and desertification due to climate change are the most relevant challenges for the agriculture of the 21st century. Soil compost amendment and plant growth promoting rhizobacteria (PGP-R) are valuable tools to mitigate salinization and desertification impacts on agricultural soils. Selection of novel halo/thermo-tolerant bacteria from the rhizosphere of glicophytes and halophytes, grown on soil compost amended and watered with 150/300 mM NaCl, was the main objective of our study. Beneficial effects on the biomass, well-being and resilience, exerted on the assayed crops (maize, tomato, sunflower and quinoa), were clearly observable when soils were amended with 20% compost despite the very high soil electric conductivity (EC). Soil compost amendment not only was able to increase crop growth and biomass, but also their resilience to the stress caused by very high soil EC (up to 20 dS m−1). Moreover, compost amendment has proved itself a valuable source of highly halo- (4.0 M NaCl)/thermo tolerant rhizobacteria (55 °C), showing typical PGP features. Among the 13 rhizobacterial isolates, molecularly and biochemically characterized, two bacterial strains showed several biochemical PGP features. The use of compost is growing all around the world reducing considerably for farmers soil fertilization costs. In fact, only in Italy its utilization has ensured, in the last years, a saving of 650 million euro for the farmers, without taking into account the environment and human health benefits. Furthermore, the isolation of halo/thermo-tolerant PGPR strains and their use will allow the recovery and cultivation of hundreds of thousands of hectares of saline and arid soils now unproductive, making agriculture more respectful of agro-ecosystems also in view of upcoming climate change.

The salinization of soil is the process of progressive accumulation of salts such as sulfates, sodium, or chlorides into the soil. The increased level of salt has significant effects on glycophyte plants, such as rice, maize, and wheat, which are staple foods for the world's population. Consequently, it is important to develop biotechnologies that improve crops and clean up the soil. Among other remediation methods, there is an environmentally friendly approach to ameliorate the cultivation of glycophyte plants in saline soil, namely, the use of microorganisms tolerant to salt with growth-promoting features. Plant growth-promoting rhizobacteria (PGPR) can improve plant growth by colonizing their roots and playing a vital role in helping plants to establish and grow in nutrient-deficient conditions. Our research aimed to test in vivo halotolerant PGPR, isolated and characterized in vitro in a previous study conducted in our laboratory, inoculating them on maize seedlings to improve their growth in the presence of sodium chloride. The bacterial inoculation was performed using the seed-coating method, and the produced effects were evaluated by morphometric analysis, quantization of ion contents (sodium, potassium), produced biomass, both for epigeal (shoot) and hypogeal (root) organs, and by measuring salt-induced oxidative damage. The results showed an increase in biomass and sodium tolerance and even a reduction of oxidative stress in seedlings pretreated with a PGPR bacterial consortium (Staphylococcus succinus + Bacillus stratosphericus) over the control. Moreover, we observed that salt reduces growth and alters root system traits of maize seedlings, while bacterial treatment improves plant growth and partially restores the root architecture system in saline stress conditions. Therefore, the PGPR seed-coating or seedling treatment could be an effective strategy to enhance sustainable agriculture in saline soils due to the protection of the plants from their inhibitory effect.

Soil salinity, due to natural phenomena or human activities, alters the water potential, which in turn affects plant growth, negatively influencing their nutrient and water uptake. Plant-growth-promoting rhizobacteria (PGPR) can be used to counteract these negative effects, especially in glycophytes. The aim of our study was to characterize physiologically, genetically, and biochemically the novel halotolerant/halophilic bacteria isolated in our previous work. We evaluated the plant-growth-promoting (PGP) features and NaCl regulation’s roles in them. In this regard, analysis based on 16S rDNA sequences confirmed that our isolates are distinct bacterial strains, probably belonging to new species, which we named Bacillus sp. M21 and M23 and Halomonas sp. QH23 and QH24. In literature, it is known that many species of Halomonas and Bacillus genera produce factors regulating plant development, such as indole-3-acetic acid (IAA), ammonium, and siderophores; and their efficiency in promoting plant growth and productivity was also demonstrated in vivo. We demonstrated that the newly isolated strains exhibit different PGP activities, highlighting how the latter are regulated by NaCl and in a strain-dependent manner. In particular, the main results showed that NaCl negatively affects the production of IAA in QH23, M21 and M23, whereas it promotes it in QH24, where it is strictly salt-dependent. Both Halomonas strains produce ammonium only in the presence of NaCl, directly correlated to its concentration. The opposite occurs in Bacillus strains, where salt reduces its production up to ten times. Overall, the results underline how halotolerance is a preferable, but not sufficient, condition for considering a PGPR potentially useful in applications aimed at improving the tolerance and productivity of crops in saline soils.