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DEVELOPMENT AND HORMONE ACTION

Expansins Abundant in Secondary Xylem Belong to Subgroup A of the -Expansin Gene Family^1,[w]

Department of Forest Genetics and Plant Physiology, Umea Plant Science Center, Swedish University of Agricultural Sciences, 901 83 Umea, Sweden (M.G.-M., E.J.M., H.A., J.S., B.S.); Royal Institute of Technology, Department of Biotechnology, AlbaNova University Center, SE–106 91 Stockholm, Sweden (A.W., F.S., T.T.T.); Department of Plant Biology and Forest Genetics, Genetic Center, Swedish University of Agricultural Sciences, 750 07 Uppsala, Sweden (K.B.); and Department of Biology, University of York, York YO10 5YW, United Kingdom (S.M.-M.)

Differentiation of xylem cells in dicotyledonous plants involves expansion of the radial primary cell walls and intrusive tip growth of cambial derivative cells prior to the deposition of a thick secondary wall essential for xylem function. Expansins are cell wall-residing proteins that have an ability to plasticize the cellulose-hemicellulose network of primary walls. We found expansin activity in proteins extracted from the cambial region of mature stems in a model tree species hybrid aspen (Populus tremula x Populus tremuloides Michx). We identified three -expansin genes (PttEXP1, PttEXP2, and PttEXP8) and one -expansin gene (PttEXPB1) in a cambial region expressed sequence tag library, among which PttEXP1 was most abundantly represented. Northern-blot analyses in aspen vegetative organs and tissues showed that PttEXP1 was specifically expressed in mature stems exhibiting secondary growth, where it was present in the cambium and in the radial expansion zone. By contrast, PttEXP2 was mostly expressed in developing leaves. In situ reverse transcription-PCR provided evidence for accumulation of mRNA of PttEXP1 along with ribosomal rRNA at the tips of intrusively growing xylem fibers, suggesting that PttEXP1 protein has a role in intrusive tip growth. An examination of tension wood and leaf cDNA libraries identified another expansin, PttEXP5, very similar to PttEXP1, as the major expansin in developing tension wood, while PttEXP3 was the major expansin expressed in developing leaves. Comparative analysis of expansins expressed in woody stems in aspen, Arabidopsis, and pine showed that the most abundantly expressed expansins share sequence similarities, belonging to the subfamily A of -expansins and having two conserved motifs at the beginning and end of the mature protein, RIPVG and KNFRV, respectively. This conservation suggests that these genes may share a specialized, not yet identified function.

² Present address: Institut de Recherche en Biologie Végétale (IRBV), Université de Montréal, 4101 Sherbrooke Street East, Montréal, Quebec, Canada H1X 2B2.

³ Present address: Wood Anatomy and Quality Laboratory, Department of Wood Properties, Forestry and Forest Products Research Institute, P.O. Box 16, Tsukuba Norin Kenkyu Danchi-nai, Tsukuba Science City, Ibaraki 305–8687, Japan.

^[w] The online version of this article contains Web-only data.

Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.104.039321.

^* Corresponding author; e-mail ewa.mellerowicz{at}genfys.slu.se; fax 46–90–786–5901.

Received January 17, 2004; returned for revision March 31, 2004; accepted April 3, 2004.

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