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BIOCHEMICAL PROCESSES AND MACROMOLECULAR STRUCTURES

Interactions between MUR10/CesA7-Dependent Secondary Cellulose Biosynthesis and Primary Cell Wall Structure^1,[OA]

John Innes Centre, Norwich NR4 7UH, United Kingdom (S.B., K.R., G.J.S.); Institute of Food Research, Norwich Research Park, Colney, Norwich NR4 7UA, United Kingdom (P.R.); Department of Biochemistry, University of Cambridge, Cambridge CB2 1QW, United Kingdom (C.J.B.); Centre for Novel Agricultural Products, Department of Biology, University of York, York YO10 5DD, United Kingdom (N.G.T.); and Max-Planck Institute for Molecular Plant Physiology, 14476 Golm, Germany (L.N., M.P.)

Primary cell walls are deposited and remodeled during cell division and expansion. Secondary cell walls are deposited in specialized cells after the expansion phase. It is presently unknown whether and how these processes are interrelated. The Arabidopsis (Arabidopsis thaliana) MUR10 gene is required for normal primary cell wall carbohydrate composition in mature leaves as well as for normal plant growth, hypocotyl strength, and fertility. The overall sugar composition of young mur10 seedlings is not significantly altered; however, the relative proportion of pectin side chains is shifted toward an increase in 1 5--arabinan relative to 1 4--galactan. mur10 seedlings display reduced fucogalactosylation of tightly cell wall-bound xyloglucan. Expression levels of genes encoding either nucleotide sugar interconversion enzymes or glycosyl transferases, known to be involved in primary and secondary cell wall biosynthesis, are generally unaffected; however, the CesA7 transcript is specifically suppressed in the mur10-1 allele. The MUR10 locus is identical with the CesA7 gene, which encodes a cellulose catalytic subunit previously thought to be specifically involved in secondary cell wall formation. The xylem vessels in young mur10 hypocotyls are collapsed and their birefringence is lost. Moreover, a fucogalactosylated xyloglucan epitope is reduced and a 1 5--arabinan epitope increased in every cell type in mur10 hypocotyls, including cells that do not deposit secondary walls. mur10 also displays altered distribution of an arabinogalactan-protein epitope previously associated with xylem differentiation and secondary wall thickening. This work indicates the existence of a mechanism that senses secondary cell wall integrity and controls biosynthesis or structural remodeling of primary cell walls and cellular differentiation.

² Present address: Universität Freiburg, Institut für Biologie III, Schaenzlestrasse 1, 79104 Freiburg, Germany.

³ Present address: University of Natural Resources and Applied Life Sciences, Department of Applied Plant Sciences and Plant Biotechnology, Institute of Plant Protection, Peter Jordanstr. 82, A–1190 Vienna, Austria.

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: Georg J. Seifert (georg.seifert{at}boku.ac.at).

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www.plantphysiol.org/cgi/doi/10.1104/pp.106.087700

^* Corresponding author; e-mail georg.seifert{at}boku.ac.at; fax 00431–47654–3359.

Received August 1, 2006; accepted October 5, 2006; published October 20, 2006.