Halo-tolerant plant growth promoting rhizobacteria for improving productivity and remediation of saline soils

Abstract

Background: The collective impact of climate change and soil salinity is continuously increasing the degraded lands across the globe, bringing agricultural productivity and food security under stress. The high concentration of salts in saline soils impose osmotic, ionic, oxidative and water stress in plants. Biological solutions can be the most reliable and sustainable approach to ensure food security and limit the use of agro-chemicals.

Aim of review: Halo-tolerant plant growth promoting rhizobacteria (HT-PGPR) are emerging as efficient biological tools to mitigate the toxic effects of high salt concentrations and improve the growth of plants, simultaneously remediating the degraded saline soils. The review explains the role of HT-PGPR in mitigating the salinity stress in plants through diverse mechanisms and concurrently leading to improvement of soil quality.

Key scientific concepts of review: HT-PGPR are involved in alleviating the salinity stress in plants through a number of mechanisms evoking multipronged physiological, biochemical and molecular responses. These include changes in expression of defense-related proteins, exopolysaccharides synthesis, activation of antioxidant machinery, accumulation of osmolytes, maintaining the Na⁺ kinetics and improving the levels of phytohormones and nutrient uptake in plants. The modification of signaling by HT-PGPR inoculation under stress conditions elicits induced systemic resistance in plants which further prepares them against salinity stress. The role of microbial-mechanisms in remediating the saline soil through structural and compositional improvements is also important. Development of novel bioinoculants for saline soils based on the concepts presented in the review can be a sustainable approach in improving productivity of affected agro-ecosystems and simultaneously remediating them.

Keywords: Exopolysaccharides; Plant growth promoting rhizobacteria; Remediation; Salinity; Sustainable agriculture.

Graphical abstract

Fig. 1

Metabolic and genetic properties of HT-PGPR involved in salt tolerance in plants. Metabolic and genetic properties of the HT-PGPR have a direct role in the amelioration of salt-stress in plants. They can regulate the expression of ion transporters/channels such as high-affinity K⁺ transporter (HKT1), Arabidopsis K⁺ Transporter 1(AKT1), Sodium Hydrogen Exchanger 2 (NHX2), weakly voltage‐dependent nonselective cation channel (NSCC), and plasma membrane intrinsic proteins (PIPs) that collectively take part in ion homeostasis and osmatic balance in plants. All of these channels/transporters can mediate Na⁺ and K⁺ influx into plant cells and help a suitable K+: Na+ ratio in the cytoplasm which prevents cellular damage and nutrient deficiency. The presence of HT-PGPR can also modulate the Salt Overly Sensitive (SOS1) pathways. Compatible osmolytes produced by HT-PGPR can be uptaken by plant cells to reduce the osmotic potential and stabilize proteins and cellular structures from salt stress. Volatile organic compounds (VOCs) produced by HT-PGPR can also trigger the induction of HKT1 in shoots and reduction of HKT1 in roots that limit Na⁺ entry into roots and facilitating shoot-to-root Na+ recirculation. Apart from these, non-enzymatic anti-oxidants produced by HT-PGPR can control the formation of ROS in plant cells. Excretion of exopolysaccharides (EPS) facilitate binding of Na+ in roots cells and prevents their translocation to leaves thus acting as a physical barrier around the roots.

Fig. 2

Benefits of biofilm formation by EPS producing PGPR in improving the nutrient status and structure of saline soil suitable for plant and microbial associations.

Fig. 3

Application of HT-PGPR showing rejuvenation of saline soil by different mechanisms for better productivity.

Fig. 4

HT-PGPR and their metabolites for development and application of novel bioformulation for saline soil.