Sucrose synthase isozyme SUS1 in the maize root cap is preferentially localized in the endopolyploid outer cells Aleš Kladnik, Barbara Vilhar, Prem S. Chourey, and Marina Dermastia Abstract: The structure of the maize (Zea mays L.) root cap was studied to quantitatively evaluate the relationship among the size of the cells, their endopolyploidy level, and the abundance of the sucrose synthase isozyme SUS1. Me- dian longitudinal root cap sections were analysed using immunolocalization, quantitative DNA staining, and image cytometry. Both the immunolocalization signal for the SUS1 protein and the endopolyploidy level increased from ca- lyptrogen towards the root cap periphery and were thus the highest in the outer cells. These cells had a nuclear DNA content of mostly 8C or higher and the largest volumes of all root cap cells. The high amount of SUS1 protein in the outer, endopolyploid cells suggests an association between endoreduplication and the abundance of this enzyme. The outer cells are involved in mucilage production; hence, there is a possibility that sucrose synthase provides monosaccharide precursors for mucilage synthesis. Key words: nuclear DNA amount, endoreduplication, immunolocalization, image cytometry, Zea mays L. Résumé : Les auteurs ont étudié la structure de la coiffe racinaire chez le maïs (Zea mays L.), afin d’évaluer quantita- tivement la relation entre la dimension des cellules, leur niveau d’endoploïdie et l’abondance de l’isozyme SUS1 de la sucrose synthase. Ils ont analysé des sections longitudinales médianes à l’aide de l’immunolocalisation, de la coloration quantitative de l’ADN, et de l’imagerie cytométrique. Le signal d’immunolocalisation de la protéine SUS1 aussi bien que le niveau de ploïdie, augmentent des cellules calyptrogènes en allant vers la périphérie de la coiffe, et sont ainsi les plus élevés dans les cellules externes. Ces cellules possèdent un contenu en ADN nucléique généralement de 8C ou plus, ainsi que les plus gros volumes de toutes les cellules de la coiffe. La grande quantité de protéines SUS1 dans les cellules externes endoploïdes suggèrent une association entre l’endoreduplication et l’abondance de cet enzyme. Les cellules externes sont impliquées dans la production du mucilage et par conséquent les auteurs discute la possibilité que la sucrose synthase produise les monosaccharides précurseurs à la synthèse du mucilage. Mots clés : quantité d’ADM nucléique, endoreduplication, immunolocalisation, imagerie cytométrique, Zea mays L. [Traduit par la Rédaction] Kladnik et al. 103 Introduction The root cap has several important physiological func- tions. It protects the root apical meristem from physical damage and pathogen attack, senses gravity, aids in the pas- sage of the root in soil by decreasing the frictional resistance through cell sloughing and mucilage secretion, modifies the conditions in the rhizosphere, and regulates root growth as a response to the received signals (Barlow 1975; Bengough and McKenzie 1997; Hawes et al. 2000). Cells of maize (Zea mays L.) root caps originate from the calyptrogen, the distal part of the root apical meristem. Calyptrogen cells divide to form additional layers of cells, which then increase in size during the course of differentia- tion (Kiesselbach 1949). The nondividing nuclei of the dif- ferentiated cells outside the meristem zone still exhibit DNA synthesis due to endoreduplication cycles (Barlow 1976). Consequently, the endopolyploidy level of the nuclei in- creases as the cells advance through the root cap, and the outermost cells of the columella contain nuclei with DNA content up to 16C (Barlow 1977). Endopolyploid cells have an increased growth potential and metabolic activity, but they lose the cell division capacity (Barlow 1978; Middleton and Gahan 1979; Kondorosi et al. 2000; Edgar and Orr- Weaver 2001). Because endopolyploidy occurs in tissues with high metabolic activity, it is believed to provide a mo- lecular basis for increased levels of gene expression (Joubès and Chevalier 2000). The endopolyploid cells in the outer part of the maize root cap contain hypertrophied Golgi bodies, which form large polysaccharide-filled secretory vesicles involved in accumu- lation and secretion of mucilage (Morre and Jones 1967). Secreted polysaccharides are characteristically rich in fu- cose, with glucose, galactose, arabinose, xylose, and man- nose as other major monomers (Harris and Northcote 1970; Can. J. Bot. 82: 96–103 (2004) doi: 10.1139/B03-143 © 2004 NRC Canada 96 Received 26 May 2003. Published on the NRC Research Press Web site at http://canjbot.nrc.ca on 10 February 2004. A. Kladnik, B. Vilhar, and M. Dermastia. 1 Department of Biology, Biotechnical Faculty, University of Ljubljana, Vecna pot 111, SI-1001 Ljubljana, Slovenia. P.S. Chourey. Department of Plant Pathology, University of Florida, Gainesville, FL 32601, U.S.A., and USDA, Agricultural Research Service, Gainesville, FL 32611-0680, U.S.A. 1 Corresponding author (e-mail: marina.dermastia@uni-lj.si).