Analysis of the physiological mechanism underlying TDZ's high cytokinin activity

In 1982, Mok et al. abroad compared the cytokinin activity of 6 thiadiazolylurea derivatives and another three pyridylurea substances using the hypocotyl callus growth assay of Phaseolus lunatus (butter bean), and found that TDZ was the strongest one. Using soybean callus, radish cotyledons and tobacco differentiation material as test materials respectively, TDZ physiological activity tests were carried out and compared with the purine cytokinins BA, ZT, KT, etc., commonly used in tissue culture. The results showed that TDZ has very high cytokinin activity, and plant growth and cell division are very sensitive to low concentrations (10-9 to 10-5 mol/L) of TDZ; in terms of concentration level, in the radish bioassay, to produce the same effect, the concentration of TDZ is 10-2 times that of BA; in the soybean callus experiment, the concentration of TDZ is 10-4 times that of ZT. These results are consistent with the report of the activity assays carried out as early as 1978 when these aromatic urea derivatives were synthesized. Regarding the physiological mechanism of TDZ, TDZ promotes the conversion of nucleotides into nucleosides. TDZ causes changes in purine metabolism in soybean callus, accelerating cytokinin synthesis, thereby enabling these calli to continue growing on a medium without additional exogenous cytokinin; in the study, these calli were called hormone-autonomous calli. This phenomenon has also been observed in other plants. Some also proposed that TDZ benefits callus growth, bud differentiation and propagation by promoting the synthesis or accumulation of intracellular purine cytokinins. In tissue culture studies, Mok et al. also proposed that because TDZ has a benzene ring and carries another ring structure, it produces a strong cytokinin-like effect. However, at present the molecular mechanism of TDZ's highly active cytokinin effect is still not fully clear, and questions such as how it regulates gene expression all require further study. Because TDZ has a strong cytokinin effect, it shows an obvious effect of promoting axillary bud proliferation. Inducing axillary bud proliferation is an important pathway for rapid asexual propagation. It can both maintain the genetic characteristics of the species and, in a short period, propagate in large numbers to achieve factory-scale seedling production. This is especially important for those precious, rare and conventionally difficult-to-propagate plants. Some woody plants have poor regeneration ability, and the purine cytokinins used previously were difficult to use to initiate large-scale axillary bud proliferation; after TDZ appeared, ideal results were obtained in its application to the rapid propagation of some difficult-to-regenerate fruit trees, flowers and trees, opening a new situation for woody plant tissue work. In tissue culture, the combined use of a high concentration of cytokinin with a low concentration of auxin, or the combined use of two or more hormones, can achieve the effect of promoting bud proliferation. The use of TDZ is also like this; in shoot tip culture of black locust, European mountain ash and small-leaved linden, when an appropriate amount of BA, IBA or NAA is added to the medium supplemented with TDZ, the bud proliferation rate can be increased and the shoot tips can be elongated. Likewise, when TDZ is added to a medium containing BA, it also increases the bud proliferation rate of plants such as maple and muscadine grape. The combined use of multiple cytokinins is better than using them alone; this is an interesting question. This indicates that there are differences in the uptake and recognition of different hormones by cells, or differences in the mechanisms of action of various hormones; they activate different receptors, and the regulation of gene expression is related to this. The mechanism of combined hormone use still requires further study. (Jiangchengzi)