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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, 138 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 139 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. 140 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 141