Cytology of the (1)glucan (Callose) in plasmodesmata and sieve plate pores

This chapter discusses the cytology of the (1)lucan (callose) in plasmodesmata and sieve plate pores. Plasmodesmata (Pd) are co--xial membranous channels that cross walls of adjacent plant cells, linking the cytoplasm, plasma membranes and endoplasmic reticulum (ER) of cells and allowing direct cytoplasmic cell--o--ell communication of both small molecules and macromolecules (proteins and RNA). Transport through Pd mediates many processes in plants, among them information transfer for coordination of development, movement of photosynthesis products from mature to developing and storage tissues, responses to pathogen infection and systemic gene silencing. A basic structure of simple primary Pd consists of two coaxial membrane tubes. The inner membrane along the Pd axis, termed the desmotubule, is continuous with and connects the ER of the bordering cells; the outer coaxial membrane is continuous with and connects plasma membranes of adjacent cells. Between the membranes is a sleeve that interconnects the cytoplasm of the neighboring cells. Within the cytoplasmic sleeve are particles whose identity is still unknown, but have been interpreted to be cytoskeletal proteins.