Abscission () is the shedding of various parts of an organism, such as a plant dropping a leaf, fruit, flower, or seed. In zoology, abscission is the intentional shedding of a body part, such as the shedding of a claw, husk, or the autotomy of a tail to evade a predator. In mycology, it is the liberation of a fungal spore. In cell biology, abscission refers to the separation of two daughter cells at the completion of cytokinesis.
In plants
Function
A plant will abscise a part either to discard a member that is no longer necessary, such as a leaf during
autumn, or a flower following
fertilisation, or for the purposes of
reproduction. Most
deciduous plants drop their leaves by abscission before
winter, whereas
evergreen plants continuously abscise their leaves. Another form of abscission is fruit drop, when a plant abscises fruit while still immature in order to conserve resources needed to bring the remaining fruit to maturity. If a leaf is damaged, a plant may also abscise it to conserve
water or
photosynthetic efficiency, depending on the 'costs' to the plant as a whole. The abscission layer is a greenish-greyish color.
Abscission can also occur in premature leaves as a means of plant defense. Premature leaf abscission has been shown to occur in response to infestation by gall aphids. By abscising leaves that have been made host to aphid galls, plants have been shown to massively diminish the pest population, as 98% of aphids in abscised galls died. The abscission is selective, and the chance of dropping leaves increases as the number of galls increases. A leaf with three or more galls was four times more likely to abscise than a leaf with one, and 20 times as likely to be dropped as a leaf without any galls.[Williams, A.G., & T.G. Whitham (1986). Premature leaf abscission: an induced plant defense against gall aphids. Ecology, 67(6), 1619-1627.]
Process
Abscission occurs in a series of three events: 1) resorption, 2) protective layer formation, and 3) detachment.
[Addicott, F.T. 1982. Abscission. University of California Press, London, England.] Steps 2 and 3 may occur in either order depending on the species.
Resorption
Resorption involves degrading
chlorophyll to extract the majority of its nutrients.
[Keskitalo, J., G. Bergguist, P. Gardestrom, and S. Jansson. 2005. A Cellular Timetable of Autumn Senescence. Plant Phys. 139 : 1635-1648.] Nitrogen is found in chlorophyll and is often a limiting nutrient for plants, which need large quantities to form
amino acids,
nucleic acids,
proteins, and certain
plant hormones.
[Hopkins, W.G. and N.P.A. Huner. 2009. Introduction to Plant Physiology. Fourth edition. Wiley & Sons, Hoboken, NJ.] Once nitrogen and other nutrients have been extracted from chlorophyll, the nutrients will travel to other tissues of the plant.
Resorption is what causes leaves in the
autumn to change colors.
in the leaves are slower to degrade than chlorophyll, so autumn leaves appear yellow and orange.
Protective layer formation
Cells under the abscission zone divide and form a layer of cork cells.
[Kozlowski, T.T. 1973. Shedding of Plant Parts. Academic Press, New York, NY.] Situated on both sides of the abscission zone are layers of
parenchyma cells, which produce and inject
suberin and
lignin under the abscission zone into the new layer of cork cells.
Suberin and lignin create a durable and waterproof layer for the plant once the organ is detached.
Detachment
This step can occur in a variety of ways depending on the species but always occurs at the abscission zone.
[Solomon, E.P., L.R. Berg., and D.W. Martin. 2011. Biology. Ninth edition, Brooks/Cole, Belmont, CA.] Detachment can occur when layers of parenchyma cells secrete cell wall
enzymes to self-digest the
middle lamella, which holds the cell walls together at the abscission zone.
This causes the cells of the abscission zone to break apart and the leaf or other plant part to fall off.
Another way detachment occurs is through
imbibition of water.
The plant cells at the abscission zone will take in a large amount of water, swell, and eventually burst, making the organ fall off.
Once detached, the protective layer of cork will be exposed.
Mechanisms
Structural
In
deciduous trees, an abscission zone, also called a separation zone, is formed at the base of the petiole. It is composed of a top layer that has cells with weak walls, and a bottom layer that expands in the autumn, breaking the weak walls of the cells in the top layer. This allows the leaf to be shed.
Lack of chlorophyll as a trigger
The reduction of
chlorophyll production in leaves due to decreased sunlight in the autumn explains why some leaves turn yellow. However, the yellow color can attract
, so some trees turn the leaves red instead by injecting a bright pigment.
[
]
The loss of chlorophyll may also contribute to the abscission process.
Chemical
A variety of reactive oxygen species (ROS) are generated by plants during times of stress (
Biotic component and
abiotic), including
UV light, cool temperatures, excessive light, pathogens, parasites, and high
salinity. The presence and continuous production of these ROS causes disruption in the
homeostasis of the cellular components, leading to
metabolic dysfunction and expression of cell wall-degrading enzymes (WDEs).
[Sakamoto, M., I. Munemura, R. Tomita, & K. Kobayashi (2008). Reactive oxygen species in leaf abscission signaling. Plant Signal Behavior, 3(11), 1014-1015.]
Hormonal
While researchers originally believed
abscisic acid to be the hormone that stimulates abscission (for which the hormone was named), it was later proved that it does not play a primary role. In fact,
auxin, a plant hormone, and
ethylene have been implicated as prominent regulators of abscission signaling. The two compounds work in a
synergistic fashion: As the auxin levels decrease, the flux of auxin to the abscission zone is reduced. Exhaustion of auxin makes the abscission zone sensitive to ethylene. When the plant is then exposed to ethylene, gene expression of cell wall-degrading enzymes such as cellulase and polygalacturonase are activated. However, this is not to say that ethylene directly activates WDE gene expression, because the elements responsible for detecting ethylene have not been found in the gene's promoter region.
Dwindling auxin levels have also been implicated in autumn-leaf color change.
In animals
See also
-
Marcescence, the retention of normally shed plant parts
External links
