Altered expression of barley proline transporter causes different growth responses in Arabidopsis

A compatible solute, proline is accumulated in various kinds of plants and microorganisms under environmental stresses. The function of proline is thought to be an osmotic regulator under water stress, and its transport into cells is mediated by a proline transporter. Here, we report the effects of expressing the barley proline transporter (HvProT) under the control of either the CaMV35S promoter (35Sp) or a root cap promoter (RCp), on Arabidopsis growth. In Arabidopsis, transformed HvProT functions in the plasma membrane, like other amino acid transporters. Reduction in biomass production was observed in aerial parts of 35Sp-HvProT plants, and it was accompanied with decreased proline accumulation in leaves. Impaired growth of 35Sp-HvProT plants was restored by exogenously adding L: -proline. These results suggested that growth reduction was caused by a deficiency of endogenous proline. In 35Sp-HvProT plants, the amount of proline dehydrogenase (PDH) transcript was increased compared to wild type (WT) plants, with a consequent enhancement of the activity of PDH. On the other hand, the transgenic RCp-HvProT plants accumulated 2- to 3-fold more proline in the root tip region compared to WT, and root elongation was enhanced at the same time. Thus, different physiological responses were caused by the altered location in accumulation of proline using two different promoters for heterologous expression of HvProT. These results indicate the importance of proline distribution at the tissue level during vegetative development.