Zinc treatment of hydroponically grown barley plants causes a reduction in root and cell hydraulic conductivity and isoform-dependent decrease in aquaporin gene expression

The cellular and molecular basis of a reduction in root water uptake in plants exposed to heavy metals such as zinc (Zn) is poorly studied. The aim of the present study on hydroponically grown barley (Hordeum vulgare) was to test whether any reduction in root hydraulic conductivity (Lp) in response to Zn treatment is accompanied by a reduction in cell Lp and gene expression level of aquaporin (AQP) isoforms. Plants were grown in the presence of 0.25M, (control), 0.1 and 1mM Zn in the root medium and analysed when they were 16-18days old. Root and cell Lp was determined through exudation and cell pressure probe analyses, respectively, and gene expression of five candidate AQPs was analysed [real time quantitative polymerase chain reaction (PCR)]. Zinc treatments caused significant reductions (25-83%) in transpiration rate, root and shoot fresh weight, surface area and stomatal conductance. Zinc concentrations in tissues increased more than 100-fold. Root Lp decreased by 24% (0.1mM Zn) and 58% (1mM Zn), while cell Lp decreased by 45 and 71%, respectively. Gene expression of AQPs decreased by 14-80%; decreases were statistically significant for HvPIP1;3, HvPIP2;4 and HvPIP2;5. Turgor in root cortex cells was not reduced by Zn treatments. It is concluded that reductions in plant water flow in response to Zn treatment are facilitated through decreases in root (Lp) and shoot (stomata) hydraulics. The decrease in root Lp is facilitated through reductions in cell Lp and AQP gene expression and may also reflect increased suberization in the endodermis.