Effects of Other Growth Regulators

There are few specific studies of the effects of gibberellins, ethylene, abscisic acid, and other growth regulators on the cell cycle. While these substances have effects on division in some instances, evidence connecting these effects to specific steps of the cell cycle is generally lacking. 

Gibberellins. Although the gibberellins inhibit cell division in some instances (Setterfield 1963, Kaufman et al. 1969) more often they are stimulatory (Boeken and van Oostveldt 1977, Kohler 1977, Shininger 1975) especially in subapical tissue (Wardlaw 1965, Liu and Loy 1976). GA usually has less effect on mitotic activity in the shoot apical meristem itself (see review by Loy 1977), an indication that it may be more effective in initiating cell division than in accelerating division in cells already mitotically active. In instances where GA acts to shorten the cell cycle in the shoot apical meristem, it increases the proportion of cells in S and M, an indication that GA may act at the points of entry into S and M (Lyndon 1976). 

It is difficult to relate the effects of GA on division to a direct role of GA in the control of the cell cycle or even to prove that the enhancement of cell division by GA is not an indirect effect arising from enhancement of cell elongation (Jones 1973). GA increases the mitotic index in meristematic cells of the leaf and root of barley and in both cases a simultaneous increase in DNA synthesis occurs (Svarinskaya and Gavrilova 1976). GA may have some selective effect on the cell cycle in the barley root in this case since it reduces the duration of G, G2, and M. GA decreases the cell cycle time in the subapical meristem of dwarf waterm elon seedlings (Liu and Loy 1976) where it decreases time spent in S. The primary effect of GA in apical buds of Rud- 

In suspension cultures of soybean cells, Constabel et al. (1977) succeeded in enhancing growth and partially synchronizing cell division by periodically flushing with 3% ethylene. In lettuce pith tissue cultures, ethylene at high con- 

Abscisic Acid. ABA is a potent inhibitor of cell division in the developing roots of Lemna minor (Newton 1977). This effect, plus an apparent direct inhibitory effect on cell expansion in Lemna roots, contributes to the overall inhibitory action of ABA on root elongation in this aquatic plant. 

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