Different responses of tonoplast proton pumps in cucumber roots to cadmium and copper

Cadmium (Cd) and copper (Cu) effects on the two tonoplast proton pumps were compared in cucumber roots Different alterations of vacuolar H+ transporting ATPase (V-ATPase) (EC 3 63 14) and vacuolar H+ transporting pyrophosphatase (V-PPase) (EC 3 6 1 1) activities under heavy metal stress were investigated ATP-dependent proton transport and ATP hydrolysis increased after exposure of seedlings to Cu whereas both decreased in plants stressed with Cd PP1 hydrolysis was relatively insensitive to both heavy metals However cadmium but not copper clearly inhibited PP1-driven H+ transport Changes in enzyme activities were not due to the metal action on the expression of CsVHA A CsVHA-c and CsVP genes encoding V-ATPase subunit A and c and V-PPase respectively in cucumber roots Moreover immunoblot analysis using specific antibodies against V-ATPase holoenzyme phosphoserine and phosphothreonine suggested that the phosphorylation at Set residue in regulatory subunit B of cucumber V-ATPase was not regulated by metals Oxidative alterations of membrane lipids were measured as malondialdehyde (MDA) content Cu ions in contrast to Cd visibly enhanced the lipid peroxidation in the root tonoplast fractions Because ATP and PP1 are absolutely required by V-ATPase and V-PPase respectively for proton transport their contents were determined in the control roots and roots treated with cadmium and copper Both ATP and pyrophosphate amounts decreased under heavy metal stress (C) 2010 Elsevier GmbH All rights reserved