Insights Into Microbially Induced Salt Tolerance and Endurance Mechanisms (STEM) in Plants.

Manoj Kaushal

doi:10.3389/fmicb.2020.01518

Abstract

Salt stress threatens the achievement of sustainable global food security goals by inducing secondary stresses, such as osmotic, ionic, and oxidative stress, that are detrimental to plant growth and productivity. Various studies have reported the beneficial roles of microbes in ameliorating salt stress in plants. This review emphasizes salt tolerance and endurance mechanisms (STEM) in microbially inoculated (MI) plants that ensure plant growth and survival. Well-established STEM have been documented in MI plants and include conglomeration of osmolytes, antioxidant barricading, recuperating nutritional status, and ionic homeostasis. This is achieved via involvement of P solubilization, siderophore production, nitrogen fixation, selective ion absorption, volatile organic compound production, exopolysaccharide production, modifications to plant physiological processes (photosynthesis, transpiration, and stomatal conductance), and molecular alterations to alter various biochemical and physiological processes. Salt tolerance and endurance mechanism in MI plants ensures plant growth by improving nutrient uptake and maintaining ionic homeostasis, promoting superior water use efficiency and osmoprotection, enhancing photosynthetic efficiency, preserving cell ultrastructure, and reinforcing antioxidant metabolism. Molecular research in MI plants under salt stress conditions has found variations in the expression profiles of genes such as HKT1, NHX, and SOS1 (ion transporters), PIPs and TIPs (aquaporins), RBCS, RBCL (RuBisCo subunits), Lipoxygenase2 [jasmonic acid (JA) signaling], ABA (abscisic acid)-responsive gene, and APX, CAT, and POD (involved in antioxidant defense). Proteomic analysis in arbuscular mycorrhizal fungi-inoculated plants revealed upregulated expression of signal transduction proteins, including Ca2+ transporter ATPase, calcium-dependent protein kinase, calmodulin, and energy-related proteins (NADH dehydrogenase, iron-sulfur protein NADH dehydrogenase, cytochrome C oxidase, and ATP synthase). Future research should focus on the role of stress hormones, such as JA, salicylic acid, and brassinosteroids, in salt-stressed MI plants and how MI affects the cell wall, secondary metabolism, and signal transduction in host plants.

Highlights

Salinity or salt stress is a major threat to agricultural productivity and global food security
Stem, and leaf biomass was observed in AI seedlings of Elaeagnus angustifolia L. during salt stress, which was attributed to the increased activities of superoxide dismutase (SOD), CAT, and ascorbate peroxidase (APX) in the leaves relative to those of NI plants (Chang et al, 2018)
Future studies should investigate the role of phytohormonal crosstalk (e.g., BR, JA, and strigolactones) in microbially inoculated (MI) plants to understand their role in eliciting stress signals during salt stress

Summary

Introduction

Salinity or salt stress is a major threat to agricultural productivity and global food security. A significant increase in the expression levels of genes related to the antioxidant response, such as Cu-Zn SOD, CAT, APX, and GR, was reported during salinity alkalinity stress and was further enhanced by AMF inoculation, thereby enabling host watermelon plants to cope with the stress (Ye et al, 2019). Stem, and leaf biomass was observed in AI seedlings of Elaeagnus angustifolia L. during salt stress, which was attributed to the increased activities of SOD, CAT, and APX in the leaves relative to those of NI plants (Chang et al, 2018).

Results

Conclusion