Sensitivity of cell hydraulic conductivity to mercury is coincident with symplasmic isolation and expression of plasmalemma aquaporin genes in growing maize roots

Root elongation occurs as individual cells along the growing zone increase in volume. This increase is caused by water entering the cell either by moving across the cell membrane from the apoplast via aquaporins, or entering through plasmodesmata that symplastically connect cells to each other or with the sieve element. In this investigation we used mercury, a known inhibitor of aquaporin water channels, to manipulate the water permeability of growing maize root cells. 20 muM HgCl2 was found to reduce root elongation by around 75% and this reduction in growth was greatest in the older growing cells, with little effect on the younger cells near the root tip. Cell hydraulic conductivity (Lp) of cells close to the root tip (at 3 mm) remained unaffected by mercury treatment in contrast to older growing and non-growing cells where Lp was greatly reduced. Using reverse transcription-polymerase chain reaction analysis, younger root regions were shown to express higher levels of two plasmalemma intrinsic protein genes than older root regions further away from the root tip. However, a gene encoding a tonoplast aquaporin was expressed at similar levels in both regions of the growing zone. The fluorescent tracer, carboxyfluorescein, demonstrated symplastic connection between the phloem and root cortical cells at 3 mm but not at 5 or 20 mm. The data are consistent with a decrease in symplastic continuity along the growing zone and highlight a change in the principal pathway of water uptake during the development of the growing root cell.