Zeatin biosynthesis from N 6 -(Δ 2 -isopentenyl)adenine in Actinidia and other woody plants

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Abstract
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 [io6Ade; 6-(4-hydroxy-3-methylbut-2-enylamino)purine] and N6-(.DELTA.2-isopentenyl)adenine(i6Ade), respectively, per gram (fresh weight). When tissues were transferred from cytokininless medium to 30 .mu.M i6Ade, endogenous io6Ade increased linearly to 160 nmol/g (fresh weight) during 8 hr, and i6Ade increased to 5 nmol/g (fresh weight) in 2 hr and then declined. The apparent Km for i6Ade in A. kolomikta and A. chinensis .times. A. arguta callus and in tissues slices of A. arguta stems was 12 .mu.M. In addition, the reaction(s) converting i6Ade to io6Ade was O2-requiring and specific for i6Ade versus N6-(.DELTA.2-isopentenyl)adenosine (i6A). When A. kolomikta callus was fed 30 .mu.M i6A, io6Ade increased and reached a concentration corresponding to 6 nmol/g (fresh weight) in 8 hr. Ribosylzeatin (io6A) did not increase. Under N2 during i6A feeds, i6A accumulated rather than being metabolized to i6Ade, suggesting that i6A normally may be metabolized via i6AMP and i6Ade to io6Ade. A survey of 30 species of woody plants in 20 families of dicotyledonous angiosperms indicated that the ability to accumulate io6Ade (.gtoreq. 10 nmol/g) in 24 hr feeds with 30 .mu.M i6Ade was restricted to certain systematic groups-e.g., order Ericales, families Oleaceae and Rubiaceae. This suggests that plants may differ in their pathways for io6Ade biosynthesis and that cytokinin biochemistry has potential as a taxonomic character above the species and genus levels.