Apoplast in Plants Structure and Transport Role

The term apoplast was coined in 1930 by Munch in order to separate the living symplast from the dead apoplast. The apoplast is the space outside the plasma membrane within which material can disperse freely. It is broken up by the Casparian strip in roots, by air spaces between plant cells and by the plant cuticle. The apoplast route facilitates the transportation of water and solutes across a tissue or organ. This process is called apoplastic transport.

Symplast

The symplast is the inner side of the plasma membrane in which the water and low-molecular-weight solutes can freely diffuse. Symplast cells have more than one nucleus. The water enters the cytoplasm of the cell all the way through the plasma membrane; hence, the symplastic pathway should cross cell membranes. Since the symplastic pathway crosses the cell membrane, it is also called the transmembrane pathway. The movement of water in the symplastic pathway is assisted by cytoplasmic streaming.

The apoplast is important for all the plant’s interaction with the environment. The main carbon source, i.e. the carbon dioxide, wants to be solubilised in the apoplast before it imparts through the plasma membrane into the cytoplasm of the cell and is used by the chloroplasts during photosynthesis.

In poor nitrate soil, acidification of the apoplast increases cell wall extensibility and root growth rate. An apoplast is also a place for cell-to-cell communication. 

Water Transport in Apoplast

The water enters the plant all the way through the hair on the root, which transports it up and around the plant and solutes are moved around by the xylem and the phloem, using the root, stem, and plant.

Root

Water enters the root in the course of root hairs and then one of three paths (apoplast, symplast, and vacuolar to the xylem vessel.

Root Hair to Xylem

From the root hair, water again moves by osmosis down an absorption gradient toward the xylem and can take one of the three ways-apoplast, symplast of the vacuolar.

The apoplast way is where the water takes a route going from cell wall to cell wall, not entering the cytoplasm at any point. The simplest pathway is where water moves between the cytoplasm and vacuoles of adjacent cells. The apoplast pathway can only take water a certain way, near the xylem of the Casparian strip forms a tightly packed barrier to water in the cell walls and water must shift into the cytoplasm to continue. 

This gives the plant control over the ions that penetrate into its xylem vessels since water must cross a plasma membrane.

The apoplast has recently become apparent that it plays a major role in a diverse range of processes, including plant-microbe, intercellular signalling, and both water and nutrient transport. The apoplast constitutes all chambers away from plasmalemma- the interfibrillar and internal space of the cell wall and the xylem, as well as its gas and water-filled intercellular space spreading to the rhizoplane and cuticle of the outer plant surface.

The physical and chemical properties of cell walls control plant mineral nutrition, as nutrients do not simply pass through the apoplast to the plasmalemma but can also be absorbed or fixed to cell wall components. Here, the current progress understanding of the significance of the apoplast in plant mineral nutrition is reviewed. 

The contribution of the root apoplast to short-distance transport and nutrient uptakes is examined particularly in relation to sodium toxicity and aluminium tolerance. This extends to long-distance transport and the role of the apoplast as a habitat for microorganisms. In the leaf, the apoplast might have benefits over the vacuole as a site for short-term nutrient storage space and solute exchange with the atmosphere. 

Apoplastic Movement Altered at the Endodermis

The endodermis is the central, innermost layer of the cortex in some land plants. It is made of compact living cells surrounded by an outer ring endodermal cells that are impregnated with hydrophobic substances, i.e. Casparian strip to restrict the apoplastic flow of water to the inner side. 

The cells of the endodermis have their main cell walls thickened on the four side’s radial and sloping with suberin, the water-impermeable waxy substance which in young endodermal cells are deposited in the Casparian strips. The strips vary in width but are typically smaller than the cell wall on which they are deposited. For example, in smokestack (brick cylinder), if the endodermis is likened to the smokestack with the bricks representing individual cells, the Casparian strip is analogous to the mortar between the bricks. 

Apoplast and Symplast

The root hair cells absorb water from the soil by osmosis. The water that is absorbed is transported to the xylem to the root through the root cortex. Transportation occurs by osmosis. The apoplast is the route the water moves through the cell walls and intercellular space of the root cortex. In the symplastic route, the water moves through the protoplasts of the root cortex.

The apoplast route is the fully permeable route in which the water movement occurs in passive diffusion. Whereas the symplast is a selectively permeable route in which the water movement occurs by osmosis. The endodermis prevents the water and any solutes dissolved in water from passing through this layer via the apoplast pathway. Water can also pass through the endodermis by crossing the membrane of endodermal cells twice. Water moving in and out of the xylem, which is a part of apoplast, can thereby be regulated since it must enter the symplast in the endodermis.

The Difference Between Apoplast and Symplast

Similarities Between Apoplast and Symplast

• Apoplast and symplast are two ways in which the water moves from root hair cells to the xylem.

• Both the apoplast and symplast occur in the root cortex.

• Both the apoplast and symplast carry water and nutrients towards the xylem.

Pathways For Root Absorption Through Apoplast

The apoplastic pathway provides a way towards the vascular cell through free spaces and cell walls of the epidermis and cortex. An additional apoplastic route that allows direct access to the xylem and phloem is along the margins of the secondary roots. The secondary root is developed from the pericycle, a cell layer just inside the endodermis. The endodermis is characterised by the Casparian strip. Apoplast was previously defined as the whole thing but the symplast, consisting of cell walls and spaces between cells in which water and solutes can move freely.