Apoplast

The apoplast is the part of a plant’s tissue outside the plasma membrane, consisting of the cell walls, the middle lamella, and the extracellular spaces including the lumina of xylem vessels. It forms a continuous network through which water and solutes can move, mainly by diffusion and bulk flow, without crossing plasma membranes. The apoplast is distinct from the symplast, the cytoplasmic continuum connected by plasmodesmata between cells.

In roots, the apoplast plays a key role in radial transport from the soil toward the vascular cylinder. However, the endodermis contains a Casparian strip, a band of suberin and lignin that blocks apoplastic flow at this boundary. This forces substances to cross the plasma membranes of endodermal cells to enter the stele, providing a checkpoint for selective uptake into the vascular system.

In leaves and other tissues, the apoplastic route transports water and minerals from the xylem to the photosynthetic tissues through cell walls and intercellular spaces. Its contribution to overall transport is influenced by cell-wall properties and additional barriers that regulate permeability.

The apoplast is also relevant to plant defense and physiology. It is a site where rapid exchange with the external environment occurs and where extracellular defense compounds can accumulate. Pathogens may interact with the apoplast, and plants can deploy immune responses in this space.

Understanding the apoplast helps explain plant water relations, nutrient uptake, and extracellular defense mechanisms, alongside the complementary symplastic pathway.