New Insights on Plant Salt Tolerance Mechanisms and Their Potential Use for Breeding

Abstract

Soil salinization is a major threat to agriculture in arid and semi-arid regions, where water scarcity and inadequate drainage of irrigated lands severely reduce crop yield. Salt accumulation inhibits plant growth and reduces the ability to uptake water and nutrients, leading to osmotic or water-deficit stress. Salt is also causing injury of the young photosynthetic leaves and acceleration of their senescence, as the Na⁺ cation is toxic when accumulating in cell cytosol resulting in ionic imbalance and toxicity of transpiring leaves. To cope with salt stress, plants have evolved mainly two types of tolerance mechanisms based on either limiting the entry of salt by the roots, or controlling its concentration and distribution. Understanding the overall control of Na⁺ accumulation and functional studies of genes involved in transport processes, will provide a new opportunity to improve the salinity tolerance of plants relevant to food security in arid regions. A better understanding of these tolerance mechanisms can be used to breed crops with improved yield performance under salinity stress. Moreover, associations of cultures with nitrogen-fixing bacteria and arbuscular mycorrhizal fungi could serve as an alternative and sustainable strategy to increase crop yields in salt-affected fields.

Keywords: beneficial soil microorganisms; detoxification pathways; engineering of plant salinity tolerance; salinity; tolerance mechanisms; transport of sodium.

FIGURE 1

Schematic overview of sodium uptake into roots and transport mechanisms into leaves. Na⁺ enters root cells and cross the plasma membrane via NSCC, CNGC, members of the HKT gene subfamily 1, and apoplastic pathways. To cope with salt stress, Na⁺ is sensed by the hyperosmotic and ionic sensors leading to activated Ca²⁺, ROS, and hormone signaling pathways. CDPKs, CBLs, CIPKs, MAPK become active and transduce signal downstream gene transcription in the nucleus. This signaling pathways result in activation of detoxification mechanisms, including the plasma membrane Na⁺/H⁺ antiporter (SOS1), HKT, and the tonoplast Na⁺, K⁺/H⁺ exchanger (NHX). SOS1 extrudes Na⁺ from the cortex cells at the root–soil interface, while at the xylem parenchyma cells; it loads Na⁺ into xylem sap. The HKT1 protein mediates the reverse flux and unloads Na⁺ from the xylem vessels to prevent overaccumulation in photosynthetic tissues. Other candidates for loading Na⁺ to the xylem are the outward-rectifying K⁺ channels KORC and NORC. At the tonoplast membrane, NSCCs include the slow vacuolar (SV) and fast vacuolar (FV) conductances, whereas the vacuolar K⁺ (VK) channel is selective for K⁺, while TPC1 is perfectly leaky for Na⁺ (Maathuis and Amtmann, 1999; Shabala and Cuin, 2007; Kronzucker and Britto, 2011). To maintain low concentration of Na⁺ in leaves, it is either retranslocated with HKT gene through the phloem to lower leaves and down to the roots, or detoxified by sequestration into the vacuole with NHX proteins. NSCCs, nonselective cation channels; HUK, HKT, high potassium affinity transporter; ROS, reactive oxygen species; CDPKs, calcium-dependent protein kinases; CBLs, calcineurin B-like proteins; CIPKs, CBL-interacting protein kinases; MAPK, mitogen-activated protein kinase; KOR, outward-rectifying K⁺channels; NORC, nonselective outward-rectifying channels.