Osmotic stress decreases PIP aquaporin transcripts in barley roots but H₂O₂ is not involved in this process

Previous reports indicate that salt stress reduces the root hydraulic conductance and the expression of plasmamembrane-type aquaporins (PIPs). As a molecular mechanism for this phenomenon, the present study found evidence that the osmotic component, but probably not an ion-specific component, decreases PIP transcripts. Eight of ten PIP transcripts were reduced to less than half by 360 mM mannitol treatment for 12 h in comparison with control samples. A large decrease of HvPIP2;1 protein was also recorded. This reduction of both transcripts and proteins of HvPIP2s should be physiologically effective for preventing or reducing dehydration at an initial phase of severe salt/osmotic stress. Root cell sap osmolality increased from 278 to 372 mOsm 24 h after 360 mM mannitol treatment. These steps can secure survival and growth recovery with water reabsorption in barley. Our data also suggest that H₂O₂ seems not to be the main cause of osmotic stress-induced transcriptional down-regulation within the concentrations (20–500 μM) and time periods (24 h) examined, although H₂O₂ was previously proposed to be involved in the mechanisms of salinity/osmotic tolerance.