## 1 Introduction  
Bacteria living in the rhizosphere can be classified based on their degree of intimacy with the root: bacteria colonizing the root surface are defined as rhizosphere, while others, such as endophytes, enter inside plant tissues and are often found in roots or even in shoots [1]. According to one of the first definitions, endophytes are bacteria or fungi that are able to live in the plant endosphere during all or part of their life cycle without inducing any damage to the plant [1]. Later, [PERSON] et al. [2] suggested to classify endophytes as any bacterium that can be isolated from surface-sterilized plant tissue that does not harm the plant. More recently, [PERSON] et al. [3] suggested that endophytes are bacteria, archaea, fungi, and protists able to colonize a plant's interior, regardless of their interaction with the plant. Except for already established seed-endophytes, the most common mode of entry of endophytic bacteria into plant tissues is through primary and lateral root cracks, and diverse tissue wounds that release plant metabolites which attract bacterial cells [4]. Less frequently, bacterial endophytes enter plants through stomata (particularly on leaves and young stems), lenticels, germinating radicles and emerging lateral roots or root hair cells [4]. The soil is a sink of rhizosphere bacteria that are able to efficiently colonize root surfaces and penetrate inside plant tissues [4]. Although many of the genes involved in endophytic behavior have not yet been identified, the occurrence of genetic differences between bacteria living in the rhizosphere and those able to colonize the interior of plant organs has been speculated [5]. Living inside the plant offers the possibility of establishing more efficient plant-bacterial communication [6; 7]. Moreover, endophytic bacteria are less exposed to variations in soil properties such as pH, water availability, and competition with the external microbiota [4]. From the plant interior, bacterial endophytes can improve plant growth and health by several mechanisms including the solubilization of phosphate [8], synthesis of phytohormones [9; 10], nitrogen fixation [11; 12; 13], siderophore production [14; 15], and suppression of phytopathogenic microorganisms [16; 17]. In addition, bacterial endophytes can increase plant tolerance to stress through the reduction of ethylene, which is synthesized by a plant exposed to either biotic stresses (phytopathogenic fungi, bacteria and nematodes) or abiotic stresses (higher concentration of heavy metals, organic pollution, flooding, drought and salinization). This occurs through the action of the enzyme 1-aminocyclopropane-1-carboxylate (ACC) deaminase, which cleaves ACC (the immediate precursor of ethylene) to alpha-ketobutyrate and ammonia. As a direct consequence, the amount of ethylene in the host plant is reduced [18; 19]. Soil salinization is one of the most common environmental factors limiting the yield and health of crop plants [20]. Nearly 831 Mha of land worldwide suffers from an excess of salt [20], while salinity related to human activities affects about 76 Mha of land worldwide (Food and Agriculture Organization of the United Nations- FAO 2015) [20]. In Europe, Mediterranean countries are more affected by salinization, which is often coupled with soil alkalization; in Italy, soil salinization accounts for 3.2 Mha [20]. Plants respond to drought and salinity in a similar way, which is related to water stress. High soil salinity leads to osmotic stress, Na\({}^{+}\) and Cl\({}^{-}\) toxicity, ethylene production, plasmolysis, nutrient imbalance, the production of reactive oxygen species (ROS), and interference with the photosynthetic process. These adverse effects are induced in plants growing in these conditions, and both growth and yield are impaired. As a consequence, seed germination is inhibited, seedling growth and vigor are reduced, and flowering and fruit yield are decreased.