Transport proteins and salt tolerance in plants

Evidence indicates that plant salt tolerance operates at a cellular level. Commonly proposed cellular mechanisms include ion sequestration in vacuoles or ion exclusion at plasma membranes. Plasma membrane ATPase and vacuolar ATPase and pyrophosphatase are proton pumps that provide an energy source for transport of ions across the plasma membrane and tonoplast, respectively. Membrane Na(+)/H(+) antiporters take advantage of the proton gradient formed by these pumps to exchange Na(+) for H(+) across a membrane. Therefore, activity and expression of these proton pumps and Na(+)/H(+) antiporters are investigated in numerous plant species under saline environment. In this review, information is presented on responses of tonoplast and plasma membrane ATPases and Na(+)/H(+) antiporters to salinity. Inconsistencies exist in some of the information and this may be due to differences in cultivars, experimental conditions, salt level used and plant age. Correlation between increased activity and expression of these transport proteins and adaptation to salinity is proposed, although this correlation is based on untested hypotheses. This precludes a general conclusion to be drawn concerning the involvement of membrane transport systems in plant salt tolerance. It is obvious that further extensive studies are needed in this area. (C) 2003 Elsevier Science Ireland Ltd. All rights reserved.