34.21: The Apoplast and Symplast

Plant growth depends on its ability to take up water and dissolved minerals from the soil. The root system of every plant is equipped with the necessary tissues to facilitate the entry of water and solutes. The plant tissues involved in the transport of water and minerals have two major compartments - the apoplast and the symplast. The apoplast includes everything outside the plasma membrane of living cells and consists of cell walls, extracellular spaces, xylem, phloem, and tracheids. The symplast, in contrast, consists of the entire cytosol of all living plant cells and the plasmodesmata - which are the cytoplasmic channels interconnecting the cells.

There are several potential pathways for molecules to move through the plant tissues: The apoplastic, symplastic, or transmembrane pathways. The apoplastic pathway involves the movement of water and dissolved minerals along cell walls and extracellular spaces. In the symplastic route, water and solutes move along the cytosol. Once in this pathway, materials need to cross the plasma membrane when moving from cell to neighboring cell, and they do this via the plasmodesmata. Alternatively, in the transmembrane route, the dissolved minerals and water move from cell to cell by crossing the cell wall to exit one cell and enter the next. These three pathways are not mutually exclusive, and some solutes may use more than one route to varying degrees.

Another potential route is the vacuolar pathway, but this route is mostly restricted to water molecule movement. Here, water moves through the vacuoles of plant cells through osmosis. The mechanism is similar to the symplastic route, but instead of transport being limited to the cytosol, the water passes through the vacuoles. Further, vacuolar transport is facilitated by two proton pumps - ATPase and PPase - that energize the solute uptake. Vacuoles also comprise of specialized transport proteins - the aquaporins - that participate in the transport of water and solutes such as glucose and sucrose.

Plants take up water and dissolved minerals from the soil and transport these resources to distant tissues. The resources enter and move in two distinct compartments: the apoplast and the symplast. 

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

Plant cells may also excrete excess protons into the apoplast due to changes in pH. 

Additionally, chemicals that enter the apoplast in the root may trigger the secretion of molecules that bind and excrete toxins.  

The cytoplasm of plant cells forms another continuous system, called the symplast. Neighboring plant cells are connected by channels allowing water and small molecules to move from one cell to the next. 

The continuum of interconnected plant cells facilitates the transport of signal molecules, such as hormones. For example, the hormone cytokinin moves freely through the symplast to regulate cell proliferation.

The symplast is also a conduit for small RNAs, which activate defense responses against plant pathogens.

Resources usually travel on both the apoplastic and symplastic pathways within the plant. For instance, minerals that are taken up by the roots must cross a selectively permeable membrane at least once in order to leave the root and travel to other plant tissues.