Dynamics of aquaporins and water relations during hypocotyl elongation in Ricinus communis L. seedlings.

The rate of water flow across biological membranes can be modulated by aquaporins which are expressed in many cells and tissues. The biological functions of these water channels in cellular processes have often been anticipated from the expression pattern, although the participation in the underlying process is not known in many cases. Ten putative aquaporin transcripts were identified in castor bean (Ricinus communis L.) seedlings and the water channel activity of three selected genes was analysed by heterologous expression in Xenopus oocytes, as well as the spatial and temporal expression by in situ hybridization/immunolocalization along the hypocotyl's axis. Water relations parameters were studied in elongating and non-elongating tissues using the cell pressure probe technique. These results indicate that (i) the amount of the RcPIP2-1 aquaporin correlated best with the elongation activity of the etiolated hypocotyl and (ii) the hydraulic conductivity of cortex cells is significantly higher in the elongating region of the hypocotyl compared with the non-elongating, mature region.