Apoplast: Meaning of Apoplast, Significance of Apoplast, Apo Plastic Transport (For CBSE, ICSE, IAS, NET, NRA 2022)

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Meaning of Apoplast

The space outside the plasma membrane within which material can diffuse freely is known as the apoplast.
The term apoplast was coined in 1930 by Münch (A German plant physiologist who proposed the Pressure Flow Hypothesis in 1930) to separate the “living” symplast from the “dead” apoplast.
Apoplast is formed by the continuum of cell walls of adjacent cells as well as the extracellular spaces.
A tissue level compartment comparable to the symplast.
The facilitation of water and solutes across a tissue or organ occurs through apo plastic route and the process is known as apo plastic transport.
The main carbon source (carbon dioxide) needs to be solubilized in the apoplast.
- Before it diffuses through the plasma membrane into the cell՚s cytoplasm (symplast) .
- Used by the chloroplasts during photosynthesis.
Ions diffuse into the apoplast of the epidermis before diffusing into the symplast in the roots.
All gaseous molecules emitted and received by plants such as plant hormones and other pheromones must pass the apoplast.

It is crucial for all the plant՚s interaction with its environment.
It also plays an important role in resistance to aluminum toxicity and resistance.
It also provides resistance to chemicals.
The apoplast provides the rich environment for microorganism՚s endophytes.
- Arises the abiotic resistance of plants.
Exclusion of aluminum ions in the apoplast prevent toxic levels.
- Inhibit shoot growth, reducing crop yields.

For water transport in plants, the apo plastic pathway is one of the two main pathways other one is the symplastic pathway.
During apo plastic transport, water, and minerals flow in an upward direction via the apoplast to the xylem in the root.
Transport velocity is higher in the apoplast than the symplast.
Through a combination of import from the xylem, absorption by cells, and export by the phloem, the concentration of solutes transported in aboveground organs.
The apo plastic pathway is also involved in passive exclusion.
Not all the ions enter through the roots making it to the xylem.
The plasma membrane excludes the ions of the endodermal cells.

Black rot is the common apo plastic disease appear in plants without restricted habitat or climate.
- Caused by the gram-negative bacteria Xanthomonas campestris.
Apoplast is a popular biotic interface and a reservoir for microbes.
When pathogens enter the plants from leaves, the first place they come across is the apo plastic space.
The major component of the volatiles is phytotoxic in rhizobacteria.
- It is identified as 2-phenylethanol.
- 2-phenylethanol can influence the regulation of WRKY18.
- 2-phyenlethanol modulates the sensitivity of ABA through WRKY18 and WRKY40.
The microbial colonization can be beneficial to establish a symbiotic relationship with the host.
The endophytic and phyllosphere microbes can indirectly promote plant growth and protect the plant from other pathogens.
- By inducing salicylic acid (SA) and jasmonic acid (JA) signaling pathways.

The inner side of the plasma membrane in which water and low-molecular-weight solutes can freely diffuse is known as the symplast.
The symplastic transport was first realized by Eduard Tangl in 1879.
Hanstein (A German botanist who was a native of Potsdam) coined the term symplast in 1880.
There is presence of more than two nucleus in symplast.
The direct flow of small molecules such as sugars, amino acids, and ions between cells is allowed by the plasmodesmata.
Molecules like transcription factors and plant viruses which larger can also be transported through with the help of actin structures.

A membrane which bounds the chief vacuole of a plant cell is called Tonoplast.
It surrounds the central or large vacuole of the plant.
By bringing in protons, the tonoplast must work to keep the vacuole acidic.
- This allows the vacuole՚s enzymes to break down food matter.
The potassium is pumped into and out of the vacuole by the tonoplast.

This is the movement of water molecules in plant cells via the vacuoles located in the cytoplasm of the cell.
The water molecules encounter high resistance.
- Little flow usually occurs making this pathway insignificant.
Water moves by osmosis across the vacuoles of the cells of the root system.
The water moves down a concentration gradient from the soil solution to the xylem.

Q 1. What is an apoplast?

Answer:

The space outside the plasma membrane within which material can diffuse freely is known as the apoplast.

Q 2. Is apoplast active or passive absorption?

Answer:

Apoplast is the passive absorption that takes place through the apoplast of the root which in turn comprises cell wall and intercellular spaces.

Q 3. Define Tonoplast.

Answer:

A membrane which bounds the chief vacuole of a plant cell is called Tonoplast.
It surrounds the central or large vacuole of the plant.
By bringing in protons, the tonoplast must work to keep the vacuole acidic.
- This allows the vacuole՚s enzymes to break down food matter.
The potassium is pumped into and out of the vacuole by the tonoplast.

Q 4. Define Vacuolar pathway.

Answer:

This is the movement of water molecules in plant cells via the vacuoles located in the cytoplasm of the cell.
The water molecules encounter high resistance.
- Little flow usually occurs making this pathway insignificant.
Water moves by osmosis across the vacuoles of the cells of the root system.
The water moves down a concentration gradient from the soil solution to the xylem.