Siedlecka A, Wiklund S, Péronne M-A, Micheli F, Lesniewska J, Sethson I, Edlund U, Richard L, Sundberg B, Mellerowicz EJ
Pectin methyl esterase inhibits intrusive and symplastic cell growth in developing wood cells of Populus
Plant Physiology: 2008 146:554-565

Abstract
Wood cells, unlike most other cells in plants, grow by a uniquecombination of intrusive and symplastic growth. Fibers growin diameter by diffuse symplastic growth, but they elongatesolely by intrusive apical growth penetrating the pectin-richmiddle lamella that cements neighboring cells together. In contrast,vessel elements grow in diameter by a combination of intrusiveand symplastic growth. We demonstrate that an abundant pectinmethyl esterase (PME; EC 3.1.1.11) from wood-forming tissuesof hybrid aspen (Populus tremula x tremuloides) acts as a negativeregulator of both symplastic and intrusive growth of developingwood cells. When PttPME1 expression was up- and down-regulatedin transgenic aspen trees, the PME activity in wood-formingtissues was correspondingly altered. PME removes methyl estergroups from homogalacturonan (HG) and transgenic trees had modifiedHG methylesterification patterns, as demonstrated by two-dimensionalnuclear magnetic resonance and immunostaining using PAM1 andLM7 antibodies. In situ distributions of PAM1 and LM7 epitopesrevealed changes in pectin methylesterification in transgenictrees that were specifically localized in expanding wood cells.The results show that en block deesterification of HG by PttPME1inhibits both symplastic growth and intrusive growth. PttPME1is therefore involved in mechanisms determining fiber widthand length in the wood of aspen trees.

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