Kinetics of N⁶-(Δ²-Isopentenyl)Adenosine Degradation in Tobacco Cells

Abstract

Uptake and degradation of the cytokinin, N-6A2-isopentenyl) adenosine, were studied in tobacco cells grown as cell suspensions. Degradation occurs by cleavage of the isopentenyl chain which gives adenyllc products.Rate of N'(A2-isopentenyl)l8_l4Cladenosine degradation increases severalfold after a 3-to 4-hour delay when ceUs have been exposed to a cytokini.Consequently, only rates of N-_A2-isopentenyl)adenosine degradation measured during the first 3 hours of incubation with 18-4Cl-N-6(A2-isopentenyl)adenosine are representative of the intrinsic in vivo cytokinin degradatve lctivity of tobacco cells. Within these limits, it appears that cytokinin degradative activity is high in cytokinin-autonomous tobacco cells, as indicated by the half life of the supplied N6(A2 isopntenyl adenosine (about 3 hours) when it is supplied at the physiological concentration of 0.2 micromolar. This cytokiin degradative activity appears to be under the control of cytokinins themselves because N6-(A2-isopentenyl)adenosine degradative activity is increased several-fold folowing a 3-to 4-hour delay after these cells have been exposed to a cytokinin.Studies of cytokinin metabolism in plant cells, tissues and organs have established that adenine and/or adenosine and/or adenylic nucleotides are catabolic products of zeatin (12), BA (3), i6Ade' or i6Ado (8, 10, 11). These observations indicate that cytokinin degradation occurs by side-chain cleavage in a manner specific for plants because in animal systems i6Ado is shown to be converted to inosine (5, 6).Presumably, irreversible inactivation of cytokinins by degradation plays an important role in the expression of their biologic activities in plants, but to our knowledge, no quantitative study of cytokinin degradation in plants has been reported. Paces et al. (11) detected an enzymic activity in crude extracts of tobacco tissues which catalyzed conversion of i6Ado to adenosine. Whitty and Hall (14) partially purified such an enzymic activity from corn kernels which they termed cytokinin oxidase. This enzymic activity removed the A2 -isopentenyl chain as the aldehyde from either N6-(A2-isopentenyl)-substituted adenine or adenosine (1). It is not known, however, if degradation of cytokinin in vivo occurs specifically at the level of the base, riboside or ribotide(s). Occurrence of cytokinin activity in cytokinin-autonomous plant tissues grown in vitro has been demonstrated (2, 4, 13). Cytokinin-autonomous 'Abbreviations: i6Ade: N6-(W'-isopentenyl)adenine; i6Ado: N6-(A'-isopentenyl)adenosine; i6AMP, i6ADP, i6ATP: N6-(A2-isopentenyl)adenosine-5'-mono-, di-, and triphosphate; Ado: adenosine; Ade: adenine. cells apparently produce enough cytokinins to meet their own requirements for cell division, either by increased cytokinin biosynthesis or by reduced cytokinin degradation, as compared to cytokinin-requiring cells. In a previous paper (10), we reported that i6Ado is rapidly degraded in cytokinin-requiring tobacco cells. To evaluate the second hypothesis, we further studied the degrada...

Genotypic Variation in Cytokinin Oxidase from Phaseolus Callus Cultures

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Genome-wide expression profiling of ARABIDOPSIS RESPONSE REGULATOR 7(ARR7) overexpression in cytokinin response

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Regulation of cytokinin content in plant cells

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