Expression and distribution of a vacuolar aquaporin in young and mature leaf tissues of Brassica napus in relation to water fluxes |
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Authors: | Nathalie Frangne Masayoshi Maeshima Anton R Schäffner Therese Mandel Enrico Martinoia Jean-Louis Bonnemain |
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Institution: | (1) Institut de Botanique, Laboratoire de Physiologie Végétale, Université de Neuchatel, 2007 Neuchatel, Switzerland, CH;(2) Laboratoire de Physiologie et Biochimie Végétales (ESA CNRS 6161), Université de Poitiers, 86000 Poitiers, France, FR;(3) Laboratory of Biochemistry, School of Agriculture, Nagoya University, Nagoya 464-01, Japan, JP;(4) Institute of Biochemical Plant Pathology, GSF Research Center for Environment and Health, 85764 Neuherberg-München, Germany, DE;(5) Institute of Plant Physiology, University of Berne, Altenbergrain 21, 3013 Berne, Switzerland, CH |
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Abstract: | Recently, it has been shown that water fluxes across biological membranes occur not only through the lipid bilayer but also
through specialized water-conducting proteins, the so called aquaporins. In the present study, we investigated in young and
mature leaves of Brassica napus L. the expression and localization of a vacuolar aquaporin homologous to radish γ-tonoplast 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 photosynthates. No substantial differences could be observed
between different tissues of young and mature leaves. However, independent of the developmental stage, an immunohistochemical
approach revealed that the vacuolar membrane of bundle-sheath cells contained more protein cross-reacting with antibodies
raised against radish γ-TIP/VM 23 than the mesophyll cells. The lowest labeling was detected in phloem cells. We compared
these results with the distribution of plasma-membrane aquaporins cross-reacting with antibodies detecting a domain conserved
among members of the plasma-membrane intrinsic protein 1 (PIP1) 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 vascular parenchyma. Our results indicate that γ-TIP/VM 23 and PIP1 homologous proteins show a similar expression pattern.
Based on these results it is tempting to speculate that bundle-sheath cells play an important role in facilitating water fluxes
between the apoplastic and symplastic compartments in close proximity to the vascular tissue.
Received: 23 December 1999 / Accepted: 3 June 2000 |
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Keywords: | : Aquaporin localization Brassica (aquaporin) Plasma membrane Vacuolar membrane Water flux |
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