Plant-growth-promoting rhizobacteria to improve crop growth in saline soils: a review

Saline soils are a major issue for agriculture because salt turns agronomically useful lands into unproductive areas. The United Nations Environment Program estimates that approximately 20 % of agricultural land and 50 % of cropland in the world is salt-stressed. Soil salinisation is reducing the area that can be used for agriculture by 1-2 % every year, hitting hardest in the arid and semi-arid regions. Salinity decreases the yield of many crops because salt inhibits plant photosynthesis, protein synthesis and lipid metabolism. Plant-growth-promoting rhizobacteria (PGPR), beneficial bacteria that live in the plant root zone named the rhizosphere, is one of the solutions to solve this issue. Indeed rhizobacteria counteract osmotic stress and help plant growth. This article reviews the benefits of plant-growth-promoting rhizobacteria for plants growing in saline soils. The major points are (1) plants treated with rhizobacteria have better root and shoot growth, nutrient uptake, hydration, chlorophyll content, and resistance to diseases; (2) stress tolerance can be explained by nutrient mobilisation and biocontrol of phytopathogens in the rhizosphere and by production of phytohormones and 1-aminocyclopropane-1-carboxylate deaminase; (3) rhizobacteria favour the circulation of plant nutrients in the rhizosphere; (4) rhizobacteria favour osmolyte accumulation in plants; (5) plants inoculated with rhizobacteria have higher K+ ion concentration and, in turn, a higher K+/Na+ ratio that favour salinity tolerance; and (6) rhizobacteria induce plant synthesis of antioxidative enzymes that degrade reactive oxygen species generated upon salt shock.