312 Z. Hong · D.P.S. Verma
zyme which may include homomeric or heteromeric subunits in a cell specific
manner. Of the 12 callose synthase genes inArabidopsis, CalS5, CalS11 and
CalS12 are known to be involved in callose wall formation during pollen de-
velopment (Dong et al. 2005; Nishikawa et al. 2005; Enns et al. 2005). CalS12
is responsible for pathogenesis-related callose deposition (Østergaard et al.
2002; Jacobs et al. 2003; Nishimura et al. 2003).
When purified in a sucrose gradient, CalS1 exists in a complex with large
molecular mass (Hong et al. 2001b). The CalS complex may include a UDP-
glucose transferase (UGT1), a Rho-like small GTPase (Rop1), phragmoplas-
tin, sucrose synthase (SuSy), and annexin (Verma and Hong 2001). SuSy
generates UDP-glucose which can be transferred via UGT1 to the active site
of CalS forming a substrate channel (Fig. 4) (see also, Amor et al. 1995; Verma
2001). CalS has also been shown to interact with a membrane protein hav-
Fig. 4Cell plate-specific callose synthase complex.A,BTobacco BY-2 cells expressing GFP-
taggedArabidopsiscallose synthase 1 (CalS1) under the control of the 35S promoter.
In non-dividing cells (A), CalS1 is present in punctates in the cytoplasm. In cytokinetic
cells (B), CalS1 is targeted to the cell plate.C,DMore callose is deposited in the cell plate
of the transgenic tobacco BY-2 cells overexpressing CalS1 (D)thanincontrolcells(C).
These cells were stained with DAPI for nuclear DNA and aniline blue for callose, and
photographed in a fluorescent microscope with a UV filter set.EProposed model of the
callose synthase complex at the cell plate.ArabidopsisCalS1 contains 16 predicted trans-
membrane domains and utilizes UDP-glucose transferase (UGT1) for substrate binding.
UGT1 interacts with phragmoplastin (Phr) and Rho-like GTPase (Rop). Sucrose synthase
(SuSy) and annexin (ANN) may also be part of the CalS complex (Andrawis et al. 1993;
Shin and Brown 1999)