Salt stress enhances proline utilization in the apical region of barley roots.

Accumulation of an osmoprotectant, proline, is enhanced in response to salinity in plants. Here, by immunohistochemical analysis, we demonstrated that proline transporter (HvProT) was highly expressed in the apical region of barley roots under salt stress. Free proline was accumulated more in the basal region than in the apical region of barley roots under salt stress, although expression level of HvProT was higher in the apical region. On the other hand, salt stress increased proline and hydroxyproline contents in the cell wall fraction of the root apical region, suggesting increment of proline utilization. Expression of the genes encoding cell wall proteins (proline rich protein and extensin) and cellulose synthase was induced in barley roots by salt stress. These findings indicated that free proline transported by HvProT presumably behaved as a component of cell wall synthesis in the apical region of barley roots under salt stress.