Characterization of a novel glycinebetaine/proline transporter gene expressed in the mestome sheath and lateral root cap cells in barley

The accumulation of glycinebetaine (GB) is one of the adaptive strategies to adverse salt stress conditions. Although it has been demonstrated that barley plants accumulate GB in response to salt stress and various studies focused on GB synthesis were performed, its transport mechanism is still unclear. In this study, we identified a novel gene, HvProT2, encoding Hordeum vulgare GB/proline transporter from barley plants. Heterologous expression in yeast (Saccharomyces cerevisiae) mutant demonstrated that the affinity of HvProT2 was highest for GB, intermediate for proline and lowest for γ-aminobutyric acid. Transient expression of fusions of HvProT2 and green fluorescent protein in onion epidermal cells revealed that HvProT2 is localized at the plasma membrane. Relative quantification of mRNA level of HvProT2 using semi-quantitative reverse transcription-polymerase chain reaction analysis showed that HvProT2 is constitutively expressed in both leaves and roots, and the expression level was higher in old leaves than young leaves and roots. Moreover, we found that HvProT2 was expressed in the mestome sheath and lateral root cap cells. We discussed the possible involvement of HvProT2 for salt stress tolerance.