Abstract: Plasmodesmata are supracellular organelles connecting two adjacent plant cells, providing a pathway of direct intercellular communication. It is generally accepted that the size exclusion limit (SEL) of plasmodesmata is about 800- 1000 Da. There are many evidences indicating that plasmodesmata SEL varies in different tissues under different physiological conditions. Under certain circumstances plasmodesmata SEL could be very high and macromolecules could traffic through plasmodesmata. Interaction with plasmodesmata, virus movement protein (MP) could modify SEL of plasmodesmata and enable virus transfer from cell to cell. In maize mutant knl, the protein KN1 might be signal molecule which results in the formation of tumors in epidermis and other tissues. P-protein could move from companian cell to sieve tube through plasmodesmata. The high plasmodesmal SEL in some tissues and the change of plasmodesmal SEL during development might play an important role in the regulation of development. The mechanisms of intercellular trafficking of macromolecules via plasmodesmata are discussed.