# Plant-Growth-Promoting Bacteria Mitigating Soil Salinity Stress in Plants
## 2 Soil Salinity-Induced Harmful Effects on Plants  
Fertile agricultural soil is able to offer the plant all the necessary environmental and nutritional conditions for its growth and successful development, having, as a result, sustained and consistent yield of high quality. In contrast, in saline soils, the concentration of soluble salts increases to certain levels that deteriorate plant growth through diverse adverse effects. The definition was given in the middle of the 20 th century by [PERSON] [18], and it says that the saline soils are those that have an electrical conductivity higher than 4 dS.m\({}^{-1}\) at 25 \({}^{\circ}\)C. The presence of different kinds of soluble salt ions in the soils (anions: chlorides, sulfates, nitrates; cations: sodium, potassium, magnesium, calcium) is called salinization [19]. The salinization occurs in two ways: _naturally_ or _primary_, when the soils are supplied by salts as an innate characteristic, including salt lakes or scolds formation, seawater influence, and the release of soluble salts of rocks and minerals, and _artificially_ or _secondary_, by anthropogenic activities such as chemical contamination, inappropriate crop rotations, excessive irrigation, flooding, and inadequate  
Figure 1: Publications featuring the use of plant-growth-promoting (PGP) microorganisms: (**a**) in stressful conditions and, particularly, salinity for the last 10 years; (**b**) distribution of publications by type of document, including, in particular, salinity. Source: Scopus; search parameters: “stress” AND ”plant growth-promoting”; “salinity” OR “NaCl” AND ”plant growth-promoting”. The search was performed in August 2020.  
drainage [20]. Soil salt concentration also depends on the soil type: there is low accumulation capacity in sandy soils and high affinity in clay loamy soil because sodium ions bind to negatively charged clay particles. Generally, saline stress is considered one of the most severe stresses affecting living and nonliving environments, aggravated by climate change as salinity and water are very closely linked. It can be exacerbated as a consequence of drought, especially in arid and semiarid areas, representing a quite important and widespread problem [9; 21].  
The effect of soil salinity depends on plant sensitivity, salt kind and concentration, agronomic practices, and the existence of additional stressors. Crops such as the horticultural (tomatoes, potatoes, lettuce, spinach) and cereals (rice, maize, wheat, legumes) are known to be sensitive to salinity [22; 23; 24]. They suffer from soil salinity stress, resulting in the reduction of yield of up to 50-70% in rice (_Oryza sativa_ L.), chickpea (_Cicer arietinum_ L.), barley (_Hordeum vulgar_ L.), maize (_Zea mays_ L.), soybean (_Glycine max_ L.), and wheat (_Triticum aestivum_ L.), although deleterious effects are recorded even at the beginning of seed germination [25; 26; 27; 28]. Plants exposed to elevated salt concentrations are shorter, with smaller leaves and pale color. The response of plants to salinity is a reduction in growth and yield as a consequence of osmotic adjustment and salt-specific response. The first is related to the reduction in the ability of the plant to absorb water, while the second is connected to changes in enzyme activity and metabolism, in general, and hormonal imbalance, including membrane transport of nutrients and water [29; 30]. Salinity stress to plants is associated with water shortage and related to a decrease in photosynthesis, oxidative and osmotic stresses, ion toxicity, and nutrient balance; thus, the concentration of some ions increase and they became harmful [31]. The K\({}^{+}\)/Na\({}^{+}\) ratio is very important for plants, and salt stress causes disturbances in the balance between these ions, reducing this proportion and decreasing nutrient availability [4; 10]. In addition to the above effects, the stress can lead to the generation of reactive oxidative species (ROS), leading to DNA damage and protein metabolism [32].