Lecture 21 – Plasmodesmata and Plant Vacuoles

Q: What is the phragmoplast and what does it do?

A double band of microtubules that delivers Golgi-derived vesicles to the developing cell plate during cytokinesis. The vesicles contain cell wall material and fuse to form the new cell wall.

Q: Where does the phragmoplast start forming, and how does it grow?

It starts in the center between the two daughter nuclei and moves laterally outward to meet the existing cell wall.

Q: What does the pre-prophase band (PPB) leave behind after it disappears?

A "footprint" at the division site — factors in the band create interactions that guide the phragmoplast to the correct fusion point with the existing cell wall.

Q: What are the three MT arrays visible during plant cell division?

The pre-prophase band (PPB), the mitotic spindle, and the phragmoplast.

Q: What are plasmodesmata?

Cytoplasmic connections between plant cells, 50–60 nm in diameter. They are unique to plants and analogous to (but ~10× larger than) gap junctions in animal cells.

Q: What can move through plasmodesmata?

Small molecules (ions, sugars) move passively down a concentration gradient. Large molecules (selected RNAs and proteins) can also move through, but this movement is regulated by gating.

Q: How many plasmodesmata does a typical plant cell have, and how are they arranged?

Hundreds per cell; all plant cells have them. They often cluster into pit fields scattered around the cell surface.

Q: When do plasmodesmata form?

During cell division, when the ER gets trapped in the developing cell plate.

Q: What is the internal structure of a plasmodesma?

The ER traverses the channel as the desmotubule (important for regulation). The plasma membrane lines the outside and is continuous between adjacent cells. A cylinder of cytoplasm passes through between the desmotubule and the plasma membrane.

Q: What is the size exclusion limit (SEL) of plasmodesmata, and how was it determined?

About 1.0 kDa. Determined by microinjection experiments where fluorescent dextrans of different sizes (0.1, 1.0, 3.0, 5.0 kDa) were injected into cells and movement into adjacent cells was observed.

Q: What role do actin filaments play in plasmodesmata?

They help regulate movement through PD. When pharmaceuticals break down actin filaments, movement through plasmodesmata is affected.

Q: How do RNA viruses exploit plasmodesmata to spread?

They encode a movement protein (MP) that carries a peptide signal to open the PD gate, increasing the SEL selectively. The MP binds to the viral genome and carries it from cell to cell.

Q: Give an example of long-distance signalling through plasmodesmata.

A protein made in the leaf that detects daylight changes can move to the shoot tip to trigger flower production.

Q: What types of macromolecules move through PD to regulate plant development?

Many types of RNAs and proteins, including transcription factors, move selectively from cell to cell.

Q: What is the vacuole and what surrounds it?

A fluid-filled compartment surrounded by the tonoplast membrane. It drives turgor pressure via osmosis and can make up 30–90% of total cell volume.

Q: What are the four important roles of plant vacuoles?

Storage (ions, organic acids, sugars, proteins), digestion (proteases, nucleases), pH and ion homeostasis (usually ~pH 5.5), and defense (toxic compounds, enzymes).

Q: What are the two types of vacuoles found in many plant cells?

Protein storage vacuoles (often in seed cells; nutrient reserve for the developing embryo during germination) and lytic vacuoles (low pH, involved in digestion and ion homeostasis).