Effects of Cytokinin

The cytokinins seem most likely to play a direct role in the regulation of cell division in vivo since they promote cell division in cultured plant tissues. This, plus reports that kinetin stimulates division in the protozoan, Paramecium cau-datum (Guttman and Back 1958), and in yeast (Barea et al. 1974), indicate a fairly widely occurring cytokinin-sensitive control site. 

The effect of cytokinin on division is dependent upon the state of differentiation of the treated cells. In nondividing differentiated cells, for example, cytokinin usually triggers cell proliferation, while in rapidly dividing meristematic cells it retards division (Van t Hof 1968). This indicates that sites controlling division change during differentiation, or at least that the hormonal sensitivity of these sites changes. 

Cytokinin deprivation of tobacco or sycamore cells in suspension culture temporarily induces partial synchronization of division beginning about 18 h after re-addition of the hormone (Jouanneau 1971, Peaud-Lenoel and Jouan-neau 1971, Roberts and Northcote 1970). This indicates that cytokinins may influence a specific regulatory point within the cell cycle. 

Perhaps the most likely point of cytokinin control of the cell cycle is in G2 or the entry into mitosis from G2. Transfer of cultured soybean cells to cytokinin-free medium leads to cessation of division but not to DNA synthesis, so the amount of DNA per cell approximately doubles (Short et al. 1974). This would not be expected if the cells became blocked at some point preceding S (i.e., at Gi). 

Cytokinins are reported to affect both the and G2 phases of the cell cycle of root meristems, causing a reduction in G (MacLeod 1968) and a lengthening of G2 (MacLeod 1968, Van t Hof 1968). This indicates that even though cytokinins may act most directly on a particular step of the cell cycle 

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Bates, L.M. Hall, A.E. (1981). Stomatal closure with soil water depletion not associated with changes in bulk leaf water status. Oecologia, 50, 62-5. Blackman, P.G. Davies, W.J. (1983). The effects of cytokinins and ABA on... 

We have developed recently a couple of classes of anticytokinins which antagonizes the effect of cytokinins 47,48). The anticytokinin activity of N4-substituted 4-amino-... 

Addition of large amounts of these growth hormones tend to suppress ethylene production. However, after ethylene production starts and accelerates, only cytokinin consistently suppresses ethylene production. This suggests a special antagonism between cytokinins and ethylene production. This antagonism is consistent with the well known retardation effect of cytokinins on loss of chlorophyll and protein in aging leaves (19). 

Elliott, D.C. (1983) The pathway of betalain biosynthesis effect of cytokinin on enzymic oxidation and hydroxylation of tyrosine in Amaranthus tricolor seedlings. 

Fig. (17). Effects of cytokinins and influence of light on shoot formation on intemodal segments The segments were cultured on WP solid medium containing cytokinin at 25 °C for 8 weeks whether in 16 hr/day light (80 gEm S" ) or in the dark. Bars represent standard deviation of the means, n=6.

There are also other data, mostly from older literature, on auxin-mediated stimulation of cytoplasmic streaming in epidermal cells of various roots [99-101] and in Tradescantia stamen hair cells [102]. Some of this work deserves to be followed up in the light of current interest in the actomyosin cytoskeleton. Regrettably, no data seem to exist on the effects of cytokinins on cytoplasmic streaming. 

Schaeffer and Sharpe (1971) have reported an effect of cytokinin on the metabolism of [mer/iy/- C]methionine. There is an increase in lipid labeling, but there is also a marked decrease in the conversion of methyl groups to CO2 in the presence of cytokinin. Phosphatidylcholine was isolated from the lipid fraction and found to be more labeled in the cytokinin-treated samples. However, PC was by no means the only lipid labeled, and the position of label was not determined. 

Anticytokinins agents that partially negate the physiological effects of cytokinins, e.g. the synthetic kinetin inhibitor, 6-methylpurine. 

It follows that the preferential incorporation of [ P]Pi into PI and PA, evidence for the operation of a PI cycle, is confirmed. However, the difference between kinetin and water-treated cells has completely disappeared. The capricious variability of callus tissue has done its deadly experimental work for us and broken the correlation. In the Popperian sense we have made real scientific progress because we have now shown that if there was an effect of kinetin on Pl-turnover, then it was gratuitous (like the effect of cytokinin on membrane potential in this system, or on Ca -transport in the two end regions of soybean hypocotyls), and not necessary for cell division. 

The mode of action of cytokinin in the control of cell proliferation in cotyledonary callus of soybean remains unknown. Bevan and Northcote [47] showed that the stimulation by kinetin of the increase in numbers of polyribosomes on transfer of these cells to fresh medium was yet another gratuitous effect of cytokinin, not necessary for cell division. They also showed that treatment with cytokinin, as compared with water, led to no detectable change in the relative levels of any mRNAs, and had no detectable effect on bulk protein synthesis or on the synthesis of any particular polypeptides. These results have always pointed to the cytoplasm, rather than the nucleus, as the site of the cytokinin signal-translating machinery. It seems that on cytokinin-starvation, cells can arrest at a number of positions in the mitotic cycle. This suggests that there are a number of different processes in the cell cycle requiring the presence of cytokinin [48]. This may be the reason for the unusual stability of cytokinin-dependence, a feature characteristic of the differentiated state which these cells would otherwise be expected to lose. If there are a large number of cytokinin-surmountable hurdles in the process of cell proliferation for cultured soybean cells, the mode of action , when we find it, is unlikely to be simple. 

The unexpected result of this work is that heat induced increases in the ipt mRNA (Figs. 2 and 3), but did not produce effective alterations in plant development. Preliminary analysis of cytokinin levels indicates that even without heat induction, the transgenic plants contain cytokinin levels 8-fold over that of control plants, while cytokinin levels in heat-induced plants is 60-170-fold over that found in control plants (Morgan, unpubl.). One explanation for these results is that the cytokinin levels after heat shock may be beyond the ability of the plant to respond. The supraoptimal cytokinin levels after hs may negate the developmental effects of cytokinins that were found at control temperatures. Our current work is focusing more closely on defining the optimal response times and processes. 

Fig.3. Effect of cytokinin treatment on yield parameters of soybean plants [9]...

In Vitro Effects of Cytokinin on RNA-Polymerase Associated Protein Kinase... 

Khanam N, Khoo C, Khan AG (2000) Effects of cytokinin/auxin combinations on organogenesis, shoot regeneration and tropane alkaloid production in Duboisla myoporoides. Plant Cell Tissue Organ Cult 62(2) 125-133. doi 10.1023/A 1026568712409... 

In apical meristems of Vicia faba roots, lAA increases the duration of S with no change in the initial rate of DNA synthesis (Webster 1967). This indicates that the effects of lAA on S, as with the previously-mentioned effects of cytokinins on S, are not by an overall modification of DNA synthesis. This is supported by the data of Nagl (1972) showing that total suppression of mitosis by lAA in onion root tips occurs with no inhibition of DNA replication. 

Cell division in higher plants requires both cytokinin and auxin. While the cytokinins exhibit the most dramatic effects on cell division they are ineffective in the absence of auxin. The effects of cytokinin and auxin on cell division vary with concentration and with the state of differentiation of the cells. Nondividing differentiated cells are induced to divide when exposed to cytokinin and auxin, while division in rapidly dividing meristematic cells is more likely to be retarded in their presence. This indicates that the control sites for cell cycling may be altered during differentiation, perhaps by a shift in sensitivity to auxins and cytokinins. 

The most common effects of cytokinin, ethylene, and abscisic acid on stem and coleoptile cell elongation are inhibitory. They inhibit both auxin-enhanced... 

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