Expression and distribution of a vacuolar aquaporin in young and mature leaf tissues of Brassica napus in relation to water fluxes

Recently, it has been shown that water fluxes across biological membranes o ccur not only through the lipid bilayer but also through specialized water- conducting proteins, the so called aquaporins. In the present study, we inv estigated in young and mature leaves of Brassica napus L. the expression an d localization of a vacuolar aquaporin homologous to radish gamma -tonoplas t intrinsic protein/vacuolar-membrane integral protein of 23 kDa (TIP/VM 23 ), In-situ hybridization showed that these tonoplast aquaporins are highly expressed not only in developing but also in mature leaves, which export ph otosynthates. No substantial differences could be observed between differen t tissues of young and mature leaves. However, independent of the developme ntal stage, an immunohistochemical approach revealed that the vacuolar memb rane of bundle-sheath cells contained more protein cross-reacting with anti bodies raised against radish gamma -TIP/VM 23 than the mesophyll cells. The lowest labeling was detected in phloem cells. We compared these results wi th the distribution of plasma-membrane aquaporins cross-reacting with antib odies detecting a domain conserved among members of the plasma-membrane int rinsic protein 1 (PTP1) subfamily. We observed the same picture as for the vacuolar aquaporins. Furthermore, a high density of gold particles labeling proteins of the PIP1 group could be observed in plasmalemmasomes of the va scular parenchyma. Our results indicate that gamma -TIP/VM 23 and PIP1 homo logous proteins show a similar expression pattern. Based on these results i t is tempting to speculate that bundle-sheath cells play an important role in facilitating water fluxes between the apoplastic and symplastic compartm ents in close proximity to the vascular tissue.