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DEVELOPMENT AND HORMONE ACTION

Pectin Methyl Esterase Inhibits Intrusive and Symplastic Cell Growth in Developing Wood Cells of Populus^1,2,[W],[OA]

Anna Siedlecka, Susanne Wiklund, Marie-Amélie Péronne, Fabienne Micheli³, Joanna Leniewska⁴, Ingmar Sethson, Ulf Edlund, Luc Richard, Björn Sundberg^* and Ewa J. Mellerowicz

Department of Forest Genetics and Plant Physiology, Umeå Plant Science Centre, SE 901 83 Umeå, Sweden (A.S., M.-A.P., J.L., B.S., E.J.M.); Organic Chemistry, Umeå University, SE 901 87 Umeå, Sweden (S.W., I.S., U.E.); and Laboratoire de Physiologie Cellulaire et Moléculaire des Plantes, Université Pierre et Marie Curie, CNRS-FRE 2846, F94200 Ivry sur Seine, France (F.M., L.R.)

Wood cells, unlike most other cells in plants, grow by a unique combination of intrusive and symplastic growth. Fibers grow in diameter by diffuse symplastic growth, but they elongate solely by intrusive apical growth penetrating the pectin-rich middle lamella that cements neighboring cells together. In contrast, vessel elements grow in diameter by a combination of intrusive and symplastic growth. We demonstrate that an abundant pectin methyl esterase (PME; EC 3.1.1.11) from wood-forming tissues of hybrid aspen (Populus tremula x tremuloides) acts as a negative regulator of both symplastic and intrusive growth of developing wood cells. When PttPME1 expression was up- and down-regulated in transgenic aspen trees, the PME activity in wood-forming tissues was correspondingly altered. PME removes methyl ester groups from homogalacturonan (HG) and transgenic trees had modified HG methylesterification patterns, as demonstrated by two-dimensional nuclear magnetic resonance and immunostaining using PAM1 and LM7 antibodies. In situ distributions of PAM1 and LM7 epitopes revealed changes in pectin methylesterification in transgenic trees that were specifically localized in expanding wood cells. The results show that en block deesterification of HG by PttPME1 inhibits both symplastic growth and intrusive growth. PttPME1 is therefore involved in mechanisms determining fiber width and length in the wood of aspen trees.

² This article is dedicated to the memory of Anna Siedlecka, who died tragically in spring 2004.

³ Present address: CIRAD-CP, UMR PIA, F34398 Montpellier, France.

⁴ Present address: Department of Botany, University of Biaystok, wierkowa 20 b, 15–950 Biaystok, Poland.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Björn Sundberg (bjorn.sundberg{at}genfys.slu.se).

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

^[OA] Open Access article can be viewed online without a subscription.

www.plantphysiol.org/cgi/doi/10.1104/pp.107.111963

^* Corresponding author; e-mail bjorn.sundberg{at}genfys.slu.se.

Received October 30, 2007; accepted November 24, 2007; published December 7, 2007.

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