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 nonelongating, mature region.

The transport of assimilates from source to sink tissues is mediated by the phloem. Along the vascular system the phloem changes its physiological function from loading phloem to transport and unloading phloem. Sucrose carrier proteins have been identified in the transport phloem, but it is unclear whether the physiological role of these transporters is phloem unloading of sucrose or retrieval of apoplasmic sucrose back into the sieve element/companion cell complex. Here, we describe the dynamic expression of the Ricinus communis sucrose carrier RcSCR1 in the hypocotyl at different sink strengths. Our results indicate that phloem unloading in castor bean is not catalysed by the phloem loader RcSCR1. However, this sucrose carrier represents the molecular basis of the sucrose retrieval mechanism along the transport phloem, which is dynamically adjusted to the sink strength. As a consequence, we assume that other release carrier(s) exist in sink tissues, such as the hypocotyl, in R. communis.

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