Anatomy of angiosperms: Wall ingrowths and transfer cells

1. Wall ingrowths and transfer cells
The formation of wall ingrowths increases plasma membrane surface
areas of transfer cells involved in membrane transport of nutrients in
plants. Construction of these ingrowths provides intriguing and
diverse examples of localized wall deposition. Flange wall ingrowths
resemble secondary wall thickenings of tracheary elements in
morphology and probable mechanisms of deposition.
By contrast, reticulate wall ingrowths, deposited as discrete papillate
projections, branch and fuse to create a fenestrated wall labyrinth
representing a novel form of localized wall deposition.
Papillate wall ingrowths are initiated as patches of disorganized
cellulosic material and are compositionally similar to primary walls,
except for a surrounding layer of callose and enhanced levels of
arabinogalactan proteins at the ingrowth/membrane interface.
How this unusual form of localized wall deposition is constructed is
unknown but may involve constraining cellulose-synthesizing rosette
complexes at their growing tips.
Wall ingrowths are modified primary walls

2. • Transfer Cells (Fig. 2.1):
• These are specialized parenchyma cells characterized by
wall ingrowths. The ingrowths are formed by the deposition
of wall materials on the inner side of primary wall. The
plasma membrane of the cell gets invaginated along with
the outline of wall ingrowths and thus the surface area of
plasma membrane is increased by many folds.
• These cells are concerned with absorption and secretion
where the invaginated plasma membrane facilitates the
process. They occur in association with xylem and phloem,
glandular hair, hydathodes, nectaries, salt glands, synergids
etc.

3. • Transfer cells are plant cells with secondary wall ingrowths.
These cells are ubiquitous, occurring in all plant taxonomic
groups and in algae and fungi. Transfer cells form from
differentiated cells across developmental windows and in
response to stress.
• They are considered to play a central role in nutrient
distribution by facilitating high rates of transport at
bottlenecks for apo-/symplasmic solute exchange.
• These properties are conferred by their unique structural
features—an invaginated secondary wall ensheathed by an
amplified area of plasma membrane enriched in a suite of
solute transporters.
• Phloem parenchyma cells, called transfer cells and border
parenchyma cells, are located near the finest branches and
terminations of sieve tubes in leaf veinlets, where they also
function in the transport of foods.

4. • A study of the fine structure of minor veins of
mature leaves of 975 species and 242 families
of Angiosperms shows that transfer cells are
widespread amongst herbaceous
Dicotyledons, are much rarer in woody
Dicotyledons, and are virtually absent from
the Monocotyledons.

5. • Four types of transfer cell are recognized in minor
veins, all possessing irregular ingrowths of wall
material protruding into their protoplasts, and all being
regarded as modified parenchyma of the minor vein.
• Two types occur in phloem. One (the A-cell), with
ingrowths distributed right round its periphery, is
associated specifically with the sieve elements.
• The other (the B-cell) occurs more generally
throughout the phloem and has zones of wall
ingrowths oriented towards sieve elements and their
associated companion cells or A-cells.
• Two other types (C- and D-cells) occur in xylem
parenchyma and bundle sheath respectively, and have
ingrowths only on walls in contact with or in close
proximity to vessels or tracheids. Each species has a
characteristic combination of types of transfer cell.

8. Transfer cell