A decade of progress in rhizoengineering to exploit plant microbiome for salt stress amelioration

Abstract

The increasing salinization of soils and resulting degradation of irrigated lands have directly affected 2.6 billion hectares of dryland agriculture worldwide. This phenomenon has led to significant qualitative and quantitative losses in crop production. The absorption and accumulation of ions adversely affect plants by disrupting photosynthetic machinery, damaging tissues, disturbing the ionic balance of cells, and inducing oxidative stress. Rhizobacteria-induced salinity tolerance is a promising tool in crop plants that works by modulating the plant metabolism. Among rhizobacteria, halotolerant plant growth promoting rhizobacteria (PGPR) stand out as particularly significant because they can extend salinity tolerance in crop plants through various mechanisms, including secondary metabolite production, osmolyte accumulation, and modulation of plant metabolism via certain localized and systemic defense functions. Furthermore, the volatile organic compounds produced by PGPR play a vital role in salinity amelioration by regulating root ions uptake, promoting osmolyte related genes expression, reducing the level of oxidative stress markers such as electrolyte leakage, and maintaining endogenous hormonal levels. These novel salt-ameliorating mechanisms and their ability to improve plant fitness and enhance tolerance to salinized soils highlight halotolerant PGPR as eco-friendly and cost-effective tools for salt stress tolerance. This review focuses on elucidating the novel mechanisms used by halotolerant PGPR, their production of secondary metabolites under salinity stress, their application as bioinoculants for crop plants in salinized soils and the development of novel bioformulations for the bioremediation of agricultural soils facing salt stress-related challenges.