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J. Biol. Chem., Vol. 281, Issue 19, 13708-13716, May 12, 2006

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Depletion of UDP-D-apiose/UDP-D-xylose Synthases Results in Rhamnogalacturonan-II Deficiency, Cell Wall Thickening, and Cell Death in Higher Plants^*

Joon-Woo Ahn¹, Rajeev Verma, Moonil Kim^¶, Jae-Yong Lee^||, Yu-Kyung Kim, Jae-Wook Bang^**, Wolf-Dieter Reiter², and Hyun-Sook Pai^||3

From the Laboratory of Plant Genomics, Korea Research Institute of Bioscience and Biotechnology, Taejon 305-333, Korea, Department of Molecular and Cell Biology, University of Connecticut, Storrs, Connecticut 06269-3125, ^¶BioNanotechnology Center, Korea Research Institute of Bioscience and Biotechnology, Taejon 305-333, Korea, ^||Department of Biology, Yonsei University, Seoul 120-749, Korea, and ^**Department of Biology, Chungnam National University, Taejon 305-764, Korea

D-Apiose serves as the binding site for borate cross-linking of rhamnogalacturonan II (RG-II) in the plant cell wall, and biosynthesis of D-apiose involves UDP-D-apiose/UDP-D-xylose synthase catalyzing the conversion of UDP-D-glucuronate to a mixture of UDP-D-apiose and UDP-D-xylose. In this study we have analyzed the cellular effects of depletion of UDP-D-apiose/UDP-D-xylose synthases in plants by using virus-induced gene silencing (VIGS) of NbAXS1 in Nicotiana benthamiana. The recombinant NbAXS1 protein exhibited UDP-D-apiose/UDP-D-xylose synthase activity in vitro. The NbAXS1 gene was expressed in all major plant organs, and an NbAXS1-green fluorescent protein fusion protein was mostly localized in the cytosol. VIGS of NbAXS1 resulted in growth arrest and leaf yellowing. Microscopic studies of the leaf cells of the NbAXS1 VIGS lines revealed cell death symptoms including cell lysis and disintegration of cellular organelles and compartments. The cell death was accompanied by excessive formation of reactive oxygen species and by induction of various protease genes. Furthermore, abnormal wall structure of the affected cells was evident including excessive cell wall thickening and wall gaps. The mutant cell walls contained significantly reduced levels of D-apiose as well as 2-O-methyl-L-fucose and 2-O-methyl-D-xylose, which serve as markers for the RG-II side chains B and A, respectively. These results suggest that VIGS of NbAXS1 caused a severe deficiency in the major side chains of RG-II and that the growth defect and cell death was likely caused by structural alterations in RG-II due to a D-apiose deficiency.

Received for publication, November 18, 2005 , and in revised form, March 8, 2006.

The nucleotide sequence(s) reported in this paper has been submitted to the GenBank^TM/EBI Data Bank with accession number(s) AY391715 [GenBank] .

^* This research was supported in part by grants from the Plant Diversity Research Center of the 21st Century Frontier Research Program, the Molecular and Cellular BioDiscovery Program, and the Plant Signaling Network Research Center (at Korea University) of the Science Research Center Program (to H.-S. Pai), all of which were funded by the Ministry of Science and Technology of Korea. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

The on-line version of this article (available at http://www.jbc.org) contains supplemental Figs. 1-3.

¹ Present address: Dept. of Chemistry and Biomolecular Sciences, Macquarie University, NSW 2109, Australia.

² Funded by United States Dept. of Energy Grant DE-FG02-95ER20203. To whom correspondence may be addressed. Tel.: 860-486-5733; Fax: 860-486-4331; E-mail: wdreiter{at}uconn.edu.

³ To whom correspondence may be addressed. Tel.: 82-2-2123-5653; Fax: 82-2-312-5657; E-mail: hspai{at}yonsei.ac.kr.

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