# Plant-Growth-Promoting Bacteria Mitigating Soil Salinity Stress in Plants
## 1 Introduction  
Global food needs are growing significantly, in line with population growth, raising a number of nutrition issues as well as the need for new approaches and actions. Climate change raises a number of questions for the coming decades and furthers uncertainty on how to deal with these questions and how to provide food. Rising temperatures, declining freshwater supplies, and the increasing frequency of extreme events such as floods, fires, and drought, require an integrated approach, joint action, and indepth research to overcome and adapt. As one of the most important factors, soil salinity occurs all over the world, causing diverse effects on soil biota and soil properties. Salinity is a major problem in agriculture, reaching about 20% of cultivated lands and more than 30% of the irrigated ones [1]. Crops have a very diverse response to soil salinity, which is basically related to their ability to survive and to produce a satisfactory amount of yield while growing in salinity conditions. Soluble salts aggravate soil fertility, causing diverse negative effects on plant health [2]. Soil electrical conductivity (EC) concentrations of up to 0.7 dS.m\({}^{-1}\) do not lead to damage in crop plants, but when it is more than 2.5-3 dS.m\({}^{-1}\), the yield decreases dramatically [3].  
Soil microbes possess the ability to modify the rhizosphere and thus can ameliorate its deleterious effects on plant growth and development under stressful conditions. The field method is the most acceptable method to justify the ability of a population or consortium to improve plant development in real conditions. Nowadays, the introduction of selected microbes in soil and, especially, in soil rhizosphere is often found as an approach for field application [4, 5]. It is clear that rhizosphere bacteria interact with the plant roots, exchanging different molecules. Plant roots excrete exudates and signal molecules until 40% of their carbon is fixed photosynthetically; thus, plants select or modify the microbial consortium in the rhizosphere. Exudates are very diverse, and their type depends on thespecific plant species and ecosystem, stage of development, or soil biota. [6]. Previous studies have demonstrated that PGPB alleviated the negative effects of salinity on the growth of tomato, pepper, canola, sunflower, and cotton [4; 7; 8].  
Problems related to the cultivation of agricultural crops, expressed as stressors, have led to a number of scientific developments. Publications related to this area in one of the major abstract and citation databases worldwide, such as Scopus, have shown increasing research work done during the last few years. Those devoted to general stress have reached more than 3000 documents in the last decade, and publications specifically targeting salinity and the use of PGP microorganisms are also growing, albeit at a slower pace, reaching about 32% of the total (Figure 1a). At the same time, the distribution of the type of documents shows an increase in reviews, books, and book chapters dealing with soil salinity and the use of PGP microorganisms (Figure 1b).  
Despite the gradually increasing number of review papers dealing with the role of PGPB in the attenuation of salinity stress to plants [5; 9; 10; 11; 12; 13; 14; 15; 16; 17], the need for these kinds of documents has increased due to the rapid development of knowledge in the field. Most of the papers discuss the damage produced by salinity to plants and the general mechanisms that describe the bacteria as plant-growth promoters. There are not many documents summarizing the exact mechanisms by which PGPB alleviate salinity stress.  
This review focuses on the recent scientific findings on the role of PGPB in alleviating plant growth and development under soil salinity.