Interactions between MUR10/CesA7 Dependent Secondary Cellulose Biosynthesis and Primary Cell Wall Structure

Sonia Bosca San Jose , Christopher J. Barton , Neil G. Taylor , Peter Ryden , Lutz Neumetzler , Markus Pauly , Keith Roberts , and Georg J. Seifert ^*

John Innes Centre, Norwich Research Park, Colney Norwich, NR4 7UH, UK
Dept Biochemistry, University of Cambridge, Tennis Court Rd, Cambridge CB2 1QW, UK
Centre for Novel Agricultural Products, Department of Biology, University of York, York, YO10 5DD, UK
Institute of Food Research, Norwich Research Park, Colney, Norwich, NR4 7UA, UK
Max- Planck Institute for Molecular Plant Physiology, Am Mühlenberg 1, 14476 Golm, Germany

^* Corresponding author; email: georg.seifert{at}boku.ac.at.

Primary cell walls are deposited and remodelled during celldivision and expansion. Secondary cell walls are deposited inspecialized cells after the expansion phase. It is presentlyunknown if and how these processes are interrelated. The Arabidopsisthaliana MUR10 gene is required for normal primary cell wallcarbohydrate composition in mature leaves as well as for normalplant growth, hypocotyl strength and fertility. The overallsugar composition of young mur10 seedlings is not significantlyaltered, however the relative proportion of pectin side chainsis shifted towards an increase in 1 $->$ 5- ${alpha}$ -arabinan relative to 1 $->$ 4- ${beta}$ -galactan.mur10 seedlings display reduced fucogalactosylation of tightlycell wall bound xyloglucan. Expression levels of genes encodingeither nucleotide sugar interconversion enzymes or glycosyltransferases, known to be involved in primary and secondarycell wall biosynthesis are generally unaffected, however theCesA7 transcript is specifically suppressed in the mur10-1 allele.The MUR10 locus is identical with the CesA7 gene, which encodesa cellulose catalytic subunit previously thought to be specificallyinvolved in secondary cell wall formation. The xylem vesselsin young mur10 hypocotyls are collapsed and their birefringenceis lost. Moreover, a fucogalactosylated xyloglucan epitope isreduced and 1 $->$ 5- ${alpha}$ -arabinan epitope increased in every cell typein mur10 hypocotyls, including cells that do not deposit secondarywalls. mur10 also displays an altered distribution of an arabinogalactan-protein(AGP) epitope previously associated with xylem differentiationand secondary wall thickening. This work indicates the existenceof a mechanism that senses secondary cell wall integrity andcontrols biosynthesis or structural remodelling of primary cellwalls and cellular differentiation.