Biology Reference
In-Depth Information
ELECTIVE CELL DEATH IN PLANTS
Plants also show elective cell death during their (life-long) development. Elective cell
death is seen as a response to pathogenic infection
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and also as a means to eliminate embry-
onic structures that are not necessary to the future development of the plant. Elective cell
death in plants is sometimes referred to as 'sensecence', although that term can create confu-
sion with quite different processes in animals.
)
It also refers to tissue scale phenomena that
may not map well on to deaths of individual cells.
An example of a temporary structure that has to be cleared away is the endosperm, a trip-
loid food-storing organ. The endosperm of the castor bean Ricinus communis stores oil and
proteins and surrounds the cotyledons of the growing embryo. During development, these
resources are transferred to the cotyledons and, once the stores in the endosperm are
exhausted, the endosperm undergoes elective cell death, apparently by the release of
massive quantities of endopeptidases from specific storage organs.
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Another example of
elective cell death in early development is the inner integument of rapeseed plant Brassica
rapa. The integuments cover the ovule of a flower, and turn into the tough outer coat of
the seed. The thick inner integument remains as protection for the seed but, shortly after
embryonic growth begins, the inner integument destroys itself by elective cell death and
this avoids obstructing embryonic development.
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The process of development in plants is unending, and remodelling takes place
throughout life. This means that complete organs such as leaves and shoots can be eliminated
by programmed cell death in response to various triggers, including prolonged darkness.
The process is local, and one shaded leaf of a plant can be degenerating while others, better
lit, thrive, and it is active, requiring changes of expression in many genes.
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At the time of writing, very little is known about the internal mechanisms that control elec-
tive cell death in plants. Attempts to identify processes in plants that are typical of elective
cell death in animals are also meeting with success. For example, dying broccoli cells
show severing of DNA very similar to that seen in animal apoptosis and the cells express
homologues of regulators of animal apoptosis such as bax.
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Genomic and proteomic tech-
niques are now identifying groups of proteins that are regularly associated with elective
cell death, and these may offer a promising direction for future research.
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DEATH FOR LIFE
That the default state of an animal cell (and perhaps a plant cell) should be to die rather
than to proliferate and to thrive often strikes students, brought up on the Darwin
e
Wallace
theory of evolution, as paradoxical. It is important to note, though, that for a metazoan
animal or higher plant, cell proliferation is not the same thing as organism reproduction.
A somatic cell may reproduce in a limited way by proliferating, but none of the daughter cells
that it founds will outlive the body of which it forms a part. Only the germ cells, each of
which carries half of the genome carried by the somatic cells, can give rise to the next
)
And since the word derives from the Latin senex meaning 'old man', the animal world seems to have
a more just claim on the term than do plants.
