D. P. S. Verma, Z. Hong, A. Delauney and J. Olson

Dept. of Mol. Genetics and Plant Biotechnology Center

Ohio State University, Columbus, OH 43210

Callose, 1,3-b glucan, is synthesized on the nascent cell plate and several other locations in the plant.  We cloned an Arabidopsis cDNA encoding a callose synthase (CalS1) catalytic subunit.  The CalS1 gene comprises 41 exons with 40 introns and is transcribed into a 6.0 Kb mRNA.  The deduced peptide, with an approximate molecular mass of 226 kD, shares sequence homology with the yeast 1,3-b-glucan synthases (FKS1p) and is distinct from plant cellulose synthases (CelS).  CalS1 contains 16 predicted transmembrane helices with an N-terminal region and a large central loop facing the cytoplasm.  CalS1 interacts with two cell plate-associated proteins, phragmoplastin (dynamin homolog) and a novel UDP-glucose transferase (UGT1) that copurifies with the CalS complex.  Following fusion with GFP, CalS1 was localized at the growing cell plate.  Expression of CalS1 transgenic tobacco cells enhanced callose synthesis on the forming cell plate.  These data suggest that plant CalS may form a complex with UDP-glucose transferase  and possibly with sucrose synthase to facilitate availability of substrate for callose synthesis.  UGT1 also interacts with Rop1, a Rho-like protein, and this interaction occurs only in its GTP-bound configuration.  The fusion protein of GFP-UGT1 was found to be associated with the forming cell plate during cytokinesis.  We propose that UGT1 may transfer UDP-glucose from sucrose synthase to callose synthase and thus participates in the synthesis of callose at the forming cell plate. There are 12 CalS-like enzymes in Arabidopsis and each may be tissue specific and or regulated under different physiological conditions.