In a multicellular organism, cells must communicate to work together in a coordinated manner. One way that cells communicate is through direct contact with other cells. The points of contact that connect adjacent cells are called intercellular junctions.
Intercellular junctions are a feature of fungal, plant, and animal cells. However, different types of junctions are found in different kinds of cells. Intercellular junctions found in animal cells include tight junctions, gap junctions, and desmosomes. The junctions connecting plant cells are called plasmodesmata (singular = plasmodesma). While they are functionally similar to animal gap junctions, they differ structurally.
Plasmodesmata are passageways that connect adjacent plant cells. Just as two rooms connected by a doorway share a wall, two plant cells connected by a plasmodesma share a cell wall.
The plasmodesma “doorway” creates a continuous network of cytoplasm—like air flowing between rooms. It is through this cytoplasmic network—called the symplast—that most nutrients and molecules are transferred among plant cells.
A single plant cell has thousands of plasmodesmata perforating its cell wall, making a giant communication network across the entire plant. The number and structure of plasmodesmata vary across cells and change in individual cells. Most water and nutrients that move through a plant are transported by vascular tissue—xylem and phloem. However, plasmodesmata also transport these materials among cells and ultimately throughout the plant.
Plasmodesmata are versatile and continuously alter their permeability. In addition to water and small molecules, they can transport specific macromolecules, such as receptor-like protein kinases, signaling molecules, transcription factors, and RNA-protein complexes.
As cells grow, the density of their plasmodesmata decreases unless they produce secondary plasmodesmata. Certain parasitic plants develop secondary plasmodesmata that connect them to hosts, allowing them to extract nutrients.