Zeatin biosynthesis from N-(Delta-isopentenyl)adenine in Actinidia and other woody plants.
Tissue cultures of Actinidia kolomikta can be maintained as callus through repeated passages on a nutrient medium devoid of cytokinin but containing inorganic nutrients, sucrose, and other basal organics plus auxin. Under these conditions, actively growing callus contained 2 and 0.5 nmol of the cytokinins zeatin [io(6)Ade; 6-(4-hydroxy-3-methylbut-2-enylamino)purine] and N(6)-(Delta(2)-isopentenyl)adenine (i(6)Ade), respectively, per gram (fresh weight). When tissues were transferred from cytokininless medium to 30 muM i(6)Ade, endogenous io(6)Ade increased linearly to 160 nmol/g (fresh weight) during 8 hr, and i(6)Ade increased to 5 nmol/g (fresh weight) in 2 hr and then declined. The apparent K(m) for i(6)Ade in A. kolomikta and Actinidia chinensis x Actinidia arguta callus and in tissue slices of A. arguta stems was 12 muM. In addition, the reaction(s) converting i(6)Ade to io(6)Ade was O(2)-requiring and specific for i(6)Ade versus N(6)-(Delta(2)-isopentenyl)adenosine (i(6)A). When A. kolomikta callus was fed 30 muM i(6)A, io(6)Ade increased and reached a concentration corresponding to 6 nmol/g (fresh weight) in 8 hr. Ribosylzeatin (io(6)A) did not increase. Under N(2) during i(6)A feeds, i(6)A accumulated rather than being metabolized to i(6)Ade, suggesting that i(6)A normally may be metabolized via i(6)AMP and i(6)Ade to io(6)Ade. A survey of 30 species of woody plants in 20 families of dicotyledonous angiosperms indicated that the ability to accumulate io(6)Ade (>/=10 nmol/g) in 24-hr feeds with 30 muM i(6)Ade was restricted to certain systematic groups-e.g., order Ericales, families Oleaceae and Rubiaceae. This suggests that plants may differ in their pathways for io(6)Ade biosynthesis and that cytokinin biochemistry has potential as a taxonomic character above the species and genus levels.
Arnold Arboretum, Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, MA 02138.
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