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.
Meaning of Apoplast

Significance of 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.

Apo Plastic Transport

  • 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.

Apo Plastic Colonization

  • 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.

Symplast

  • 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.

Tonoplast

  • 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.

Vacuolar Pathway

  • 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.

FAQs

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.