Plant Molecular Biology 46: 289–299, 2001.
© 2001 Kluwer Academic Publishers. Printed in the Netherlands.
Expression pattern of transcripts encoding water channel-like proteins in
Norway spruce (Picea abies)
and Inger Hakman
Department of Botany, Stockholm University, 10691 Stockholm, Sweden
Institute of Plant Genetics, Slovak
Academy of Sciences, Akademicka 2, P.O. Box 39A, 950 07 Nitra, Slovak Republic;
present address: Department
of Biology and Environmental Science, Kalmar University, 93182 Kalmar, Sweden (
Author for correspondence;
Received 29 December 2000; accepted in revised form 20 February 2001
Key words: aquaporin, conifer, in situ hybridizations root, tonoplast intrinsinc protein (TIP), vascular tissue
With the aim of identifying cells and tissues with high expression of aquaporins (water channels) or homologous
genes in Norway spruce (Picea abies), we report the expression patterns of such transcripts in seedlings, in roots of
various ages, and in needles. In situ hybridization experiments with a conserved area of a tonoplast intrinsic protein
(TIP) gene from P. abies gave high expression signals in differentiating vascular tissues and in the columella cells of
the seedling root cap. High-staining signals were also seen in guard cells and in the bundle sheath cells of needles.
Moreover, a slightly increased staining signal was seen in cells forming lateral roots as well as in adventitious
roots formed from hypocotyl cuttings. By using PCR-based procedures we also identiﬁed a full-length aquaporin-
like cDNA (mipr) from roots of two-week old seedlings. Sequence homology analysis of the gene suggests that
it belongs to the TIP subgroup within the large MIP (major intrinsic protein) family. A phylogenetic analysis of
the plant MIP family, including both plasmamembrane (PIP) and tonoplast intrinsic protein (TIP) from Picea,
suggests that MIP subgroups evolved already 330 million years ago, as this is the dating of conifer and angiosperm
Aquaporins, water channel proteins that are mem-
bers of the large MIP (major intrinsic protein) family
of transmembrane channel proteins, have now been
identiﬁed in a number of plants (for recent reviews
see, for example, Maurel 1997; Maurel, et al., 1997;
Schäffner, 1998; Chrispeels et al., 1999). These pro-
teins were named aquaporins (AQP) because of their
high efﬁciency and selectivity to conduct water in the
direction of an osmotic gradient (Agre et al., 1993).
A number of processes that require transport of wa-
ter across cellular membranes have been suggested
to involve AQPs, from single-cell expansion and os-
moregulation to long-distance transport of both water
and assimilates (e.g. Maurel, 1997). The discovery of
AQPs in plants has directed new attention to the dy-
namics of water transport in plants, particularly under
water-limiting conditions (Mullet and Whitsitt, 1996).
Proteins of the MIP family are found in animals,
plants, yeast and bacteria (e.g. Park and Saier, 1996).
In plants, by sequence homology analyses, MIP pro-
teins form three groups (Yamada et al., 1995; Park and
Saier, 1996; Weig et al., 1997). One group contains
the TIPs (tonoplast intrinsic proteins) and another one
the PIPs (plasma membrane intrinsic proteins) that
have been localized to the tonoplast and to the plasma
membrane, respectively. The third group comprises
NOD26 found in the peribacteroid membrane of root
nodules. Functional aquaporins have been identiﬁed in
all three groups.
Large gene families that display differences in their
tissue and developmental expression patterns encode
plant MIPs and in Arabidopsis thaliana,over20dif-
ferent genes have been identiﬁed (Weig et al., 1997).