Plant Molecular Biology 34: 163–168, 1997.
1997 Kluwer Academic Publishers. Printed in Belgium.
Expression in anthers of two genes encoding Brassica oleracea
transmembrane channel proteins
e K. Ruiter
, Gerben J. van Eldik, Marinus M.A. van Herpen, Jan A.M. Schrauwen
George J. Wullems
Department of Experimental Botany, University of Nijmegen, Toernooiveld 1, 6525 ED Nijmegen, Netherlands
author for correspondence);
Present address: Department of Brassica and Oilseeds Research, John Innes
Centre, Norwich Research Park, Colney, Norwich NR4 7UH, UK
Received 24 June 1996; accepted in revised form 31 January 1997
Key words: Brassica oleracea, transmembrane channel proteins, ...
Screening of an anther cDNA expression library resulted in the isolation of two almost identical cDNA clones,
termed mipA andmipB, showing homology with sequencesencoding transmembrane channel proteins fromthe MIP
family. Both clones were expressed in several tissues, but not in pollen. MipA was preferentially expressed in the
surrounding sporophytic tissues of stamens. Anthers subjected to drought were induced to accumulate even more
mip transcripts, which was entirely due to higher mipA gene expression. On basis of isolation procedures, sequence
homology and drought inducibility of mipA we conclude that the encoded proteins probably are constituents of the
pollen coat and are aquaporins.
During early pollen development, a ﬂow of water
occurs from the tapetum to the growing pollen grains.
This water is supplied via the vascularsystem in the ﬁl-
ament, and, within the anther, transported towards the
various tissues. At shedding, pollen is strongly dehyd-
rated, which may be seen as a condition essential to
survive water stress during the period of pollen transfer
. Water is lost from the grains, but also from the
locular space, prior to shedding, at the end of pollen
development . Moreover, speciﬁc water relocation
in endothecial and epidermal cells is a prerequisite to
anther dehiscence [4, 18, 29]. After landing of the pol-
len grains on the stigma, pollen hydrates by absorption
of stigmatic water, and then germinates [10, 27]. Thus,
transport of water is an essential element for successful
anther and pollen development, anther dehiscence and
The nucleotidesequence data reported will appear in the EMBL,
GenBank and DDBJ Nucleotide Sequence Databases under the
accession number X95639 (mipA) and X95640 (mipB).
Membranes are barriers to water ﬂow , which
can be overcome by aquaporin-like proteins that
form water-selective transmembrane channels [2, 32].
Aquaporinsoccur both in the plasma membrane[7, 26]
and the tonoplast . These proteins are assumed to
be involved in symplastic and transcellular transport of
water. Aquaporins are members of the MIP family of
passive transmembrane transporters, a family named
after the major integral membrane protein (MIP) of
the bovine lens ﬁbre cell membrane . These MIPs
are speciﬁc to a single type of membrane, such as
the plasma membrane (PIP), the tonoplast (TIP) or the
peribacteroid membrane of root nodules [7, 12, 19, 22,
25, 26]. The occurrence of individual MIP proteins is
often restricted to single-cell types. Expression of the
corresponding genes in tissue-speciﬁc, as is described
for genes encoding
-TIPs in seeds, proteins located
in the tonoplast of protein storage vacuoles [19, 20, 21,
22], and for the root-speciﬁc mip genes [34, 42, 44].
In Brassica oleracea, the pollen grains are surroun-
ded by a lipidic pollen coat, also containing proteins.
Screening of an anther cDNA expression library with
GR: 201001894, Pips nr. 134105 BIO2KAP
plan3591.tex; 14/04/1997; 7:24; v.7; p.1