Intercellular attachment is an essential process in the morphogenesis of multicellular organisms. A unique mutant, nolac-H18 (nonorganogenic callus with loosely attached cells), generated by T-DNA transformation using leaf-disk cultures of haploid Nicotiana plumbaginifolia, lost the ability to form tight intercellular attachments and adventitious shoots. The gene tagged with T-DNA, named NpGUT1 (glucuronyltransferase 1), was similar to the gene for the catalytic domains of animal glucuronyltransferases and was expressed predominantly in shoot and root apical meristems. The transformation of NpGUT1 complemented the nolac-H18 mutation, and the expression of antisense NpGUT1 RNA produced crumbled shoots. The mutation caused defects in the glucuronic acid of rhamnogalacturonan II of pectin, which drastically reduced the formation of borate cross-linking of rhamnogalacturonan II. NpGUT1, which encodes a unique glucuronyltransferase, is a glycosyltransferase gene identified in pectin biosynthesis and is essential for intercellular attachment in plant meristems and tissues. S patially and temporally controlled intercellular attachment and communication are indispensable for the organization of plant tissues, making them critical for normal development and morphogenesis in every multicellular organism. Plant cell walls are composed primarily of cellulose microfibrils, hemicellulose, pectic polysaccharides, and small amounts of structural proteins (1-3). Pectin is believed to be involved in intercellular attachment because it is localized mainly in the primary cell wall, middle lamella, and cell corners. Pectin consists mostly of three structurally well-characterized polysaccharides: homogalacturonans (HGs) and highly branched rhamnogalacturonans I and II (RG-I and RG-II). Compared with cellulose and hemicellulose, little is known about the synthesis and assembly of pectins. The biosynthesis of HG, RG-I, and RG-II likely requires at least 41 unique glycosyltransferases (1). The activities of several transferases involved in the biosynthesis of pectin have been identified (1). However, none of these enzymes have been purified, and their genes have never been identified.Recently, we established a system for producing mutants called nolac (nonorganogenic callus with loosely attached cells) by T-DNA transformation, which involves in vitro cultures of leaf disks of haploid Nicotiana plumbaginifolia (4). These mutants are defective in intercellular attachment, which results in the failure of organogenesis. Haploid N. plumbaginifolia plants (5) are suitable for generating and studying such mutants, because mutations have a direct effect on phenotype and because cells with embryo-lethal mutations can be maintained in tissue culture as unorganized callus, which enables us to analyze mutant cell walls. We identified 199 lines of callus with loosely attached cells from cultures of 2,970 leaf disks that had been transformed with T-DNA. Although, only 25 of these continued to grow on the medium, nolac-H18 had a growth rate that was s...

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