Download Cell Division Inhibitors Clyde L. Elmore University of

Cell Division Inhibitors
Clyde L. Elmore
University of California, Davis
There are at least three major functions for chemicals that
are applied to soil or to a plant to occur before they are a
herbicide. The sum of these functions is termed the mode of action.
First the chemical has to be at a site so that it can be taken up
into the plant. This is the uptake phase. Uptake is through the
root hair and root, shoot or leaves. Secondly it has to be moved
within a plant to a site, where activity occurs. This process is
called translocation or being systemic. The last process is
critical because the chemical must have some affect, at a certain
site (s) on a critical function of the plant. This location is
called the site of action, and the affect is the mechanism of
action. All of these functions must take place before the chemical
is active as an herbicide.
The materials that are considered cell division disruptors or
mitotic poisons can be found in more than one chemical family. The
older herbicides DCPA
(Dacthal),
carbanilates
(propham and
chlorpropham), bensulide (Betasan and Prefar) and pronamide (Kerb)
are all examples of older compounds that have some effect on cell
division. Also the family of substituted dinitroaniline herbicides
(trifluralin,
benefin,
oryzalin,
pendimethalin,
isopropalin,
profluralin, ethalfluralin, nitralin, dinitramine, and prodiamine}
all affect cell division. More recently the pyridine herbicides
(dithiopyr and thiazopyr) also are cell division inhibitors.
Application of these herbicides are to the soil and uptake is
primarily in the young shoot and root in the meristematic areas.
These herbicides do not inhibit seed germination, but affect young
seedlings and especially new roots and shoots. Broadleaf shoots are
more susceptible than grass shoots. Roots of both plant types are
susceptible. There must also be some uptake into young roots from
root hairs. The herbicide moves to the endodermis cells in the root
and there affects new lateral root formation. The affect that is
most often observed on roots of plants is a enlarged, globose root
tip and the roots are almost devoid of lateral roots. These affects
are rate dependant and also somewhat dependant on the metabolic
rate of the herbicide; for example, pendimethalin seems to be
metabolized faster than oryzalin thus damage is greater in oryzalin
treated roots than pendimethalin. Oryzalin seems to be a more
active herbicide, yet the rapid metabolism of pendimethalin just
decreases the activity faster than oryzalin. Shoots or axillary
buds in broad leaves are affected by the stunting of the new shoot,
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or stopping the axillary bud from developing.
These herbicides are not translocated long distances in the
plant. There seems to be some cell to cell movement. Thus, the
materials must be applied to soil and either mixed into the soil
mechanically to place the chemical close to the seedling or it much
be leached in sightly with rainfall or irrigation water. If the
herbicide is not at the site of rooting of the new germinating
seed, then the seedling may grow through or below the herbicide
affording tolerance. This also is true if a large grass seedling
such as wild oat germinates below the herbicide. No control will
result. This is a way of achieving positional selectivity with
annual crops. Tree or ornamental shrub roots will grow well below
the herbicide and not be injured.
The action of these herbicides is centered in the cell cycle
and cell division in particular. In a rapidly growing plant, cells
are dividing rapidly, elongating, and dividing again. The cell
cycle of a growing cell is the period between the formation of the
cell by division of its mother cell and the time when the cell
itself divides to form two daughter cells. The primary seats of
division are at the root and shoot tips (meristems) . During the
cells cycle there are phases called Gl, S, G2 and mitosis (Figure
1). The Gl phase is considered the pre-DNA synthesis phase, the s
phase is when DNA synthesis occurs and the G2 phase is considered
the pre-mitosis phase.
During the mitosis phase there are further distinct occurances
(Figure 2) . There is an interphase stage before mitosis, which is
the whole of the cell cycle, except mitosis. It is during this
stage that proteins are made to prepare for mitosis. Microtubules
are protein strands that are involved in cell division and the
formation of new cell walls. Microtubules are present around the
nucleus at the interphase stage in what is called the preprophase
band. This area is where the future cell wall will be formed.
During prophase, the second phase of mitosis, the chromosomes are
formed. During metaphase, the next phase, the chromosomes align at
the center plane of the cell and microtubules form a spindle shape
to a pole at the opposite edges of the cell, where the daughter
cells will be formed. At anaphase the chromosome pulls apart with
the microtubules apparent between the two halves of the cell. At
telophase the cell plate is formed between the nuclei, and the two
new cells are formed with the new cell wall intact between the
cells. The new cells then stay in interphase until the mitosis
process starts again to form new cells. Thus, microtubules, formed
from tubulin, are formed (polymerized) and depolymerized (reduced
to tubulin) several times during the cycle. They have different
functions and are found at different places in the cell. During the
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formation of the new cell plate in the telophase stage of mitosis,
the microtubules help "guide' vesicles containing new cell material
to the center plane to make the new wall. Most cell di vision
inhibitors act upon the tubulin to stop their polymerization and
thus interrupt nuclear material movement, or they affect the
microtubules in the process of forming the new cell wall.
All dinitroaniline herbicides affect the polymerization of
tubulin into microtubules. Pronamide (Kerb) also disrupts mitosis.
It causes root tip swelling and rapid affects on mitosis. Instead
of the absence of microtubules, there is a shortening of the
microtubule, which does not allow normal division to occur. With
DCPA, microtubules were disrupted, but the main effect seems to be
the lack of the cell wall forming properly between the new cells,
thus forming binucleate or multinucleate cells, more similar to
propham or chlorpropham.
Cell division and microtubule function is common to most plant
and animal cells. The proteins involved in the processes are
apparently different enough between fungi, mammals, and green
plants for selectivity to occur between organisms.
Dithiopyr and thiazopyr disrupts cell division by
inhibiting mitosis in the late prometaphase and causing multipolar
mitosis. Thiazopyr does not bind to tubulin but instead may bind to
another microtubule-associated protein. The microtubules are
shortened to the point that the spindle fibers, normally formed to
separate chromosomes to the polls of the cell before the cell wall
is formed to form daughter cells, do not allow normal division.
Cortical microtubules, which normally prevent isodiametric cell
expansion, are also essentially absent resulting in club-shaped
root tips.
All of these herbicides are selective on certain crops and
have been extremely useful fro weed management in many agricultural
crops, turfgrasses and ornamentals.
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Figure 1.
Plant Cell Cycle
Mitosis (M)
(
Pre Prophase
(G2)
Pre DNA synthesis
(GI)
DNA synthesis )
(S)
Interphase
Figure 2.
MITOSIS
/
metaphase (2N)
prophase (2N)
Interphase
1
\
(IN)
(IN)
anaphase (2N)
telaphase
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