The Apoplast and Symplast

Plants take up water and dissolved minerals from the soil and move them across root tissues before they enter the vascular system for transport to distant tissues.

To move these resources efficiently, plants use two main pathways: the apoplast and the symplast.

The apoplast is a continuous system of extracellular spaces, including cell walls and the spaces between cells. Water, gases, and minerals move freely and passively through the apoplast without crossing plasma membranes.

Plant cells also pump protons into the apoplast, creating a gradient that helps drive the cotransport of nutrients into the cell.

The apoplast can also act as a filtering zone where cell walls and secreted molecules bind some harmful substances and limit their movement into plant cells.

The cytoplasm of plant cells forms another continuous system, called the symplast. Neighboring plant cells connect through channels called plasmodesmata, which allow water and small molecules to move between cells without crossing plasma membranes, similar to the movement through the apoplast.

This network of connected cells helps transport signaling molecules. For example, the hormone cytokinin moves through the symplast to regulate cell proliferation.

The symplast also transports small RNAs that help trigger defense responses against plant pathogens.

Resources often move through both apoplastic and symplastic pathways within the plant. For example, in roots, the Casparian strip blocks apoplastic flow, so these substances must cross the plasma membrane before entering the vascular tissue.