Download What is a Plant Cell? Continued

844
The Plant Cell
LETTER TO THE EDITOR
What is a Plant Cell? Continued
Are animal and plant cells so different
that there should be different definitions for the limits of their cells? A “no”
answer is certainly reasonable, but
Andrew Staehelin (What is a Plant
Cell? A Response, 1991, Plant Cell 3,
553) should have acknowledged that
many prominent plant biologists have
answered “yes.”
What an animal biologist or a microbiologist refers to as a “cell,” the
unit bounded by the plasma membrane, a traditional plant biologist
would cal1 the “protoplast.” Many plant
biologists reserve the term “cell” for
the protoplast and the cell wall. No
less authorities on plant cells than
Katherine Esau and Ray Evert explicitly define the term this way (K. Esau,
1953, Plant Anatomy, New York: John
Wiley & Sons, pp. 13 and 33; K. Esau,
1977, Anatomy of Seed Plants, New
York: John Wiley & Sons, pp. 17, 43,
and 504; P. Raven, R. Evert, and S.
Eichhorn, 1986, Biology of Plants, New
York: Worth Publishers, pp. 16 and
726). Thus, Helen Stafford was in good
company when she asked for the rationale for calling the wall extracellular
(What is a Plant Cell? 1991, Plant Cell
3, 331).
Many eminent plant cell biologists,
such as Keith Roberts and Andrew
Staehelin, instead share the terminology of animal cell biologists. For example, the definitive text Molecular
Biology of the Cell (B. Alberts et al.,
1989, New York: Garland Publishing),
of which Keith Roberts is a coauthor,
includes the statement that “the plant
cell wall is an elaborate extracellular
matrix that encloses each cell in a
plant” (p. 1137).
1s the issue just semantics or are
there genuine differences between
animal and plant cells that justify different terminology? There is strong
logic for providing a unifying definition
of a cell for all kingdoms, in which the
limit of the cell is the plasma membrane. Obviously, the plant cell wall is
in large part analogous to the extracellular matrix of animal cells, and plant
cell biologists should be included in
symposia on, and funding for, the
extracellular matrix.
If the logic of this terminology is
followed, then the use of the term
“protoplast” should be modified. Most
plant biologists refer to the units isolated from tissues by using cell wall
digestion enzymes as “protoplasts.”
Once a wall regenerates, a “cell” suspension culture is maintained. These
usages imply that the term “protoplast”
has two internally consistent meanings,
i.e., (1) the product of enzymatic digestion and (2) the part of a plant cell
within and including the plasma membrane. If the unifying definition of a
“cell” were to become widely accepted,
only the first usage of the term “protoplast” would survive.
Strictly speaking, according to the
logic of the unifying definition of a cell,
a protoplast and a cell are identical
entities, and even the remaining usage of the term “protoplast” could be
dropped. Yet this term is retained in
the text Molecular Biology of the Cell,
which defines a protoplast as a “wallless cell” (p. 1143). The modifier “wallless” emphasizes the abnormal nature
of this state. The implication is that a
plant cell has a wall but the wall is not
part of the cell. Thus, the retention of
the term “protoplast” in this text indicates that in plants the term “cell”
necessarily implies the presence of a
wall even though a wall is not included
in the definition.
About the only time that a higher
plant cell lacks an extracellular matrix
and becomes wall-less is when the
wall is removed enzymatically. In contrast, many animal cells lack an extracellular matrix, and the term “protoplast” is nonexistent in the animal literature. In almost all plant cell types,
the middle lamella defines the outer
limit of the wall so that individual plant
“cells” can be recognized. In animal
tissues, it is usually more difficult to
assign territoriality to the extracellular
matrix; a more inclusive definition of
an animal cell might embarrass those
asked to point one out.
The traditional inclusion of the cell
wall in the definition of the plant cell
emphasizes that higher plant cells
essentially always have walls and that
the limits of those walls are assignable as units. Reasonable arguments
exist for changing that definition, and
it is healthy that the meanings of biological terms evolve. But it is important to understand that, at present, one
term is being used differently by members of the same community and that
there are meaningful rationales for
each definition.
Fred Sack
Department of Plant Biology
Ohio State University
Columbus, OH 43210-1293
What is a Plant Cell? Continued.
F. Sack
Plant Cell 1991;3;844
DOI 10.1105/tpc.3.9.844
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