Anatomy of Angio: Plasmodesmata

1. As mentioned previously, the protoplasts of adjacent plant
cells are interconnected by narrow strands of cytoplasm
called plasmodesmata (singular: plasmodesma), which
provide potential pathways for the passage of substances
from cell to cell (van Bel and van Kesteren, 1999; Haywood et
al., 2002).
Although such structures have long been visible with the light
microscope (Fig. 4.19)—they were first described by Tangl in
1879—it was not until they could be observed with an
electron microscope that their nature as cytoplasmic strands
was confi rmed.

2. Plasmodesmata are the structural and functional analogs of the gap
junctions found between animal cells.
At the gap junctions the plasma membranes of adjacent cells are
associated in placques having narrow channels called “connexons,”
through which the protoplasts of the two cells communicate. The
presence of a cell wall precludes direct contact between the plasma
membranes of adjacent plant cells; consequently the plant body
essentially is partitioned into two compartments, the symplast (or
symplasm) and the apoplast (or apoplasm).
The symplast is composed of the plasma membrane-bound
protoplasts and their interconnections, the plasmodesmata; the
apoplast consists of the cell wall continuum and intercellular spaces.
Accordingly the movement of substances from cell to cell by means of
plasmodesmata is called symplastic transport (symplasmic transport),
and the movement of substances in the cell wall continuum is called
apoplastic transport (apoplasmic transport).

4. • Plasmodesmata Contain Two Types of Membranes: Plasma Membrane and
Desmotubule A plasmodesma is a plasma membrane-lined channel typically
traversed by a tubular strand of tightly constricted endoplasmic reticulum
called a desmotubule.
• In most plasmodesmata the desmotubule does not resemble the adjoining
endoplasmic reticulum. It is much smaller in diameter and contains a central,
rod-like structure. Considerable controversy has centered on the
interpretation of the central rod. Most investigators believe it represents the
merger of the inner leaflets, or inner portions, of the bilayers of the
endoplasmic reticulum forming the desmotubule. If this interpretation is
correct, the desmotubule lacks a lumen, or opening, and the major pathway
through which substances move from cell to cell via plasmodesmata is the
region between the desmotubule and the plasma membrane. This region,
called the cytoplasmic sleeve, is subdivided into 2.5 nanometer diameter
microchannels by globular particles embedded in both the plasma membrane
and desmotubule and interconnected by spoke-like extensions.

5. Plasmodesmata May Be Classified as Primary or
Secondary According to Their Origin Many
plasmodesmata are formed during cytokinesis as
strands of tubular endoplasmic reticulum become
entrapped within the developing cell plate.
The plasmodesmata formed during cytokinesis are
called primary plasmodesmata. Plasmodesmata can
also be formed de novo across existing cell walls.
These postcytokinetically formed plasmodesmata
are referred to as secondary plasmodesmata, and
their formation is essential to establish
communication between ontogenetically unrelated
cells