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.