Cytokinin synthesis

It became clear, however, that in the whole plant the major site of cytokinin synthesis was in the roots from whence it was transported to the rest of the plant via the xylem transpiration stream. Perhaps kinins were a kind of rhizocaline ... 

In our laboratory we have been using M2 seed families and mutagenized haploid protoplasts of Nicotianaplumbaginifolia in a combined approach to search both for mutants resistant to toxic concentrations of auxins and cytokinins, and for temperature-sensitive auxin auxotrophs. The hope has been that variation isolated using these rather unspecific selection procedures would include mutations in hormone uptake, biosynthesis, catabolism and reception. The isolation methods, particularly for the cloning of auxotrophs, are laborious, and during the last five years we have slowly assembled a small collection of mutants which we feel will allow us to test several current theories about auxin and cytokinin synthesis and mode of action. This paper briefly reviews our work on the mutants and their characterization. 

A critical element in understanding the physiological role of cytokinins is the knowledge of how plants control their cytokinin levels. Therefore, we need to know the pathway(s) and site(s) of cytokinin synthesis, the metabolic fate of cytokinins, and the biochemical mechanisms controlling their biosynthesis and degradation. 

To further define the site of cytokinin synthesis in germinating seeds, embryonic axes (0.4 g) and cotyledons (2 g) were carefully excised from partially imbibed (2 h) lupin seeds, and incubated in Petri dishes with [U- C]-adenine in HEPES buffer (embryos 11 /xCi in 0.6 ml buffer cotyledons 15 juCi in 2 ml buffer pH 7) on a shaker (80 rpm 4 h, 22°C, dark). Following 4-h exposure to [ " Cj-adenine these were washed with buffer and further incubated for 6 h. The cotyledons and embryos were then extracted and purified as before. The results of 2D-TLC analyses are shown in Table 2. incorporation into (diH) [9R]Z by embryos was confirmed by further analyses. However, similar analyses indicated a lack of incorporation into (diH) [9R]Z by isolated cotyledons. 

These results of [ Cj-adenine incubation studies directly demonstrate cytokinin synthesis by germinating seeds, and further indicate that possibly only the embryonic axes have the capacity to synthesize cytokinins, which are then translocated to the cotyledons, apparently accumulating therein to evoke physiological response. This is reflected in experiments with the intact seed where [ Cj incorporation into cytokinins (per g fw) was considerably higher in embryos than in the cotyledons (Table 2), while incorporation per organ is greater for the cotyledons [18]. Translocation experiments carried out with selective application of pH]-(diH) [9R]Z to embryos or cotyledons also indicate a polar movement of cytokinins from the embryonic axes to the cotyledons. The incorporation of [ C]-adenine into (diH) [9R]Z is particularly interesting because (diH)Z-type cytokinins predominate in lupin seed. 

Since GAs as diterpenes share many intermediates in the biosynthetic steps leading to other terpenoids, eg, cytokinins, ABA, sterols, and carotenoids, inhibitors of the mevalonate (MVA) pathway of terpene synthesis also inhibit GA synthesis (57). Biosynthesis of GAs progresses in three stages, ie, formation of / Akaurene from MVA, oxidation of /-kaurene to GA 2" hyde, and further oxidation of the GA22-aldehyde to form the different GAs more than 70 different GAs have been identified. 

Occurrence, chemistry, synthesis and cytokinin activity of l -methyl-rran.s-zeatin and its analogs (glycosylated adenine derivatives) 97H(46)659. 

It is possible that, over these longer periods, protein synthesis is affected. Itai Vaadia (1965, 1971) showed that during water stress cytokinin activity in the root exduate and translocation of cytokinin were reduced. Ben-Zioni, Itai Vaadia (1967) showed that the lower rate of protein synthesis of stressed leaf tissue could be raised by treatment of leaf material with kinetin. We need more information on levels of various proteins after naturally occurring stress. 

In etiolated seedlings of Arabidopsis, low doses of cytokinin (0.5-10 mmol stimulate ethylene synthesis. It has been shown that cytokinin decreases the rapid turnover of the wild-type ACS5. ° The cytokinin probably acts both by blocking the interaction of the C-terminus with Etol and by stabilizing ACS5 isozyme in an additional till now unknown manner. 

These findings started a tremendous wave of research interest (a fashion) into the potential role of cytokinins in nucleic acid synthesis (and hence the control of protein synthesis) in plants. Looking back on those years of the early 1960s, the... 

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