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Published on December 7, 2007; 10.1104/pp.107.111963

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Received October 30, 2007
Accepted November 24, 2007

Pectin methyl esterase inhibits intrusive and symplastic cell growth in developing wood cells of Populus

Anna Siedlecka , Susanne Wiklund , Marie-Amelie Peronne , Fabienne Micheli , Joanna Lesniewska , Ingmar Sethson , Ulf Edlund , Luc Richard , Bjorn Sundberg *, and Ewa J. Mellerowicz

Umea Plant Science Centre, Department of Forest Genetics and Plant Physiology, SE 901 83 Umea, Sweden; Organic Chemistry, Umea University, SE 901 87 Umea, Sweden; Laboratoire de Physiologie Cellulaire et Moleculaire des Plantes, Universite Pierre et Marie Curie-CNRS FRE 2846, 3, rue Galilee, F94200 Ivry sur Seine, France

* Corresponding author; email: Bjorn.Sundberg{at}genfys.slu.se.

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 L. x tremuloides Michx.) acts as a negative regulator of both symplastic and intrusive growth of developing wood cells. When PttPME1expression 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, and the transgenic trees had modified homogalacturonan methylesterification patterns, as demonstrated by two-dimensional NMR and immunostaining using PAM1 and LM7 antibodies. The in situ distributions of PAM1 and LM7 epitopes revealed changes in pectin methylesterification in the transgenic trees that were specifically localized in expanding wood cells. The results show that en-block de-esterification of homogalacturonan 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.






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