Germination cytokinins

The accumulated assay results obtained with the 4-alkyl-2-methylpyrrolo[2,3-d]pyrimidines are of special interest. Can-pounds 25 and 26 exhibited significant cytokinin activity in all assays utilized. On the other hand, compound 27 was a potent antagonist in the tobacco bioassay but had little activity in the other two assay systems, while 29 acted as an anticytokinin in the tobacco bioassay and a cytokinin in the lettuce seed germination assay, but was without activity in the Amaranthus test system. 

Cytokinins also delay senescence and hence can be used to prolong the shelf life of fresh vegetable and cut flowers and to promote seed germination. Cytokinins appear to be involved with transfer RNA (t-RNA) and, therefore, they possibly play a role in regulating the incorporation of specific amino acids into proteins. 

In addition to its spore germination inhibitor activity, it shows phytohormone (cytokinine) activity. Its derivative without the 3-methylbut-2-enyl residue, 3-(L-3-amino-3-carboxypropyl)-adenine 2, shows the same regulatory function as discadenine. ... 

Cytokinins act mainly through cell cycle regulation [15]. They stimulate cell division, prevent abscission, prevent rooting, enhance germination, and prevent senescence. Kinetin was firstly described in 1955 [16]. Benzyladenin was discovered by Strong in 1958 [17]. Both compounds are still the most commonly used cytokinins in plant micropropagation [7]. In addition. Strong [17] described thidia-zuron, which is in use mainly to induce senescence in cotton. 

Fig. 2. Somatic embryogenesis in Hinoki cypress. A Collected open-pollinated cones. B Proliferation of induced embiyogenic tissue on medium containing auxin and cytokinin. C Embryogenic cells. D-F Different developmental maturation stages of somatic embryos. G Production of somatic embryos. H Germination of somatic embryos. I Plantlets growing in vitro. J Acclimatized plants derived from somatic embryos. K-L Somatic plants growing out in the field. Bars 1mm (C-F), 1cm (A-B, G-J), Im (K-L)...

Although roots, particularly the root apices, are a major site of cytokinin biosynthesis in plants, there is evidence now that other meristematic tissues and organs including the cambium, developing buds, seeds and fruits and the embryonic axis of germinating seed have the ability to synthesize cytokinins under optimal growth conditions [6, 14]. Recently, stem and leaves have also been shown to be additional sites of cytokinin production [6]. The view that root-produced cytokinins move in the xylem to the shoot to participate in the control of development and senescence is widely accepted [15]. The major question that remains to be clarified is under what conditions the observed cytokinin activity in other plant parts is derived solely from the roots, and when, and to what degree it is derived by synthesis in situ. 

In this paper we have examined [ H]-MVA incorporation into cytokinins in crown gall tissues. In addition, results are described which provide evidence of [ C]-adenine incorporation into cytokinins by germinating seeds and young leaves under normal physiological conditions. 

Cytokinin Biosynthesis by Lupin Seeds During Early Stages of Germination... 

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. 

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