Tonoplast Na～+/H～+ Antiporter ATPase And PPase In Populus Euphratica And Its Relation To Salt Tolerance

Populus euphratica. is famous for its high tolerance against the adverse condition,especially drought and salinity,and mainly distributed in the edge of the desert in Northwest China where this kind of poplar is the only one arboreal species that can form forest.The Populus euphratica calli were cultivated to study its salt tolerance ,which was compared with Populus tremula.The calli of P.tremula. grows more quickly than that of P. euphratica normally. Different levels of salinity (NaCl and KCl) caused serious inhibition of cells growth in P. tremula,but as to P. euphratica,the inhibition was mild. In the presence of 50mmol NaCl, growth of P.euphratica was accelerated.KCl is more deleterious to P.euphratica,but NaCl is more deleterious to P. tremula.Under salt stress, the P.euphratica cells were found through X-ray microanalysis to accumulate much more Na~+ in the vacuole than in the cytoplasm, This mechanism conferred the high salt tolerance to P.euphratica by moderating ion toxicity in the cytoplasm and maintaining the osmotic potential of the cells. But this mechanism was absent from the P.tremula,which may be a major reason for its salt sensitivity.Through differential centrifugation and sucrose density gradient centrifugation, tonoplast vesicles were isolated from P.euphratica and P.tremula calli broken by blender. After establishing pH gradient by V-ATPase, Na~+ could dissipate the pH gradient, which indicates there is Na~+/H~+ antiporter in the tonoplast vesicles from P.euphratica(Km=20.91mm)and P.tremula calli(Km=11.4mM).Na~+-dependent H~+ efflux showed a high selectivity for Na~+ over K~+, Amiloride could inhibit the antiport activity, MIA, which is the analog of amitoride,could inhibit the antiport activity,too.The antiporter of P.euphratica is more sensitive to MIA. Maybe there is some difference between their protein structures.Under the NaCl and KCl stress, the hydrolyzing activity of the V-ATPase, proton pumping activity of the V-ATPase and V-PPase,antiport activity of the Na~+/H~+ antiporter of P. tremula were decreased. Sodium gluconate and ABA also have the same inhibitions.Comparatively, P.euphratica is more resistive, which suggests that these enzymes associate with salt tolerance.