Abstract
While soil salinity is a global problem, how salt enters plant root cells from the soil solution remains underexplored. Non‐selective cation channels (NSCCs) are suggested to be the major pathway for the entry of sodium ions (Na+), yet their genetic constituents remain unknown. Yeast PQ loop (PQL) proteins were previously proposed to encode NSCCs, but the role of PQLs in plants is unknown. The hypothesis tested in this research is that PQL proteins constitute NSCCs mediating some of the Na+ influx into the root, contributing to ion accumulation and the inhibition of growth in saline conditions. We identified plant PQL homologues, and studied the role of one clade of PQL genes in Arabidopsis and barley. Using heterologous expression of AtPQL1a and HvPQL1 in HEK293 cells allowed us to resolve sizable inwardly directed currents permeable to monovalent cations such as Na+, K+, or Li+ upon membrane hyperpolarization. We observed that GFP‐tagged PQL proteins localized to intracellular membrane structures, both when transiently over‐expressed in tobacco leaf epidermis and in stable Arabidopsis transformants. Expression of AtPQL1a, AtPQL1b, and AtPQL1c was increased by salt stress in the shoot tissue compared to non‐stressed plants. Mutant lines with altered expression of AtPQL1a, AtPQL1b, and AtPQL1c developed larger rosettes in saline conditions, while altered levels of AtPQL1a severely reduced development of lateral roots in all conditions. This study provides the first step toward understanding the function of PQL proteins in plants and the role of NSCC in salinity tolerance.
Highlights
Soil salinization is a global issue that limits agricultural productivity, with more than 800 million hectares of the world's arable land being classified as salt-affected (FAO, 2008)
We observed that AtPQL1a and HvPQL1 are involved in the transport of monovalent cations, such as Na+, K+, and Li+ (Figures 2-3)
The PQL1 permeability was inhibited by high external Ca2+ and pH acidification (Figure 3c,d), which is typical for previously described Ca2+-sensitive voltage-insensitive non-selective cation channels (vi-Non-selective cation channels (NSCCs)) reported for root-derived protoplasts from several plant species (Buschmann et al, 2000; Demidchik & Tester, 2002; Spalding et al, 1992; Tyerman et al, 1997)
Summary
Soil salinization is a global issue that limits agricultural productivity, with more than 800 million hectares of the world's arable land being classified as salt-affected (FAO, 2008). The addition of extracellular Ca2+ alleviates the toxic effects of salt stress on plants by reducing Na+ influx (reviewed by Tester & Davenport, 2003), suggesting that Ca2+-sensitive pathways play an important role in Na+ influx. The transport activities of ScPQL1 and ScPQL2 are similar to NSC1 activity (Bihler et al, 1998, 2002), making them a putative candidate for NSCC In this manuscript we identified plant homologues of ScPQL1 and ScPQL2 and characterized the role of clade 1 PQLs in Arabidopsis thaliana and barley under control and salt stress conditions. Our findings provide a first step toward understanding plant PQL function, indicating that clade 1 PQLs show similar transport properties to other Ca2+-sensitive viNSCCs, and play an important role in lateral root development
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