Cytokinins from germinating seeds

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. 

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. 

---