Metabolism of zeatin riboside in a hormone autonomous genetic tumour line of tobacco

Abstract

The metabolic fate of externally applied [3H]-zeatin riboside ([9R]Z) was studied in a cultured genetic tumour line of Nicotiana glauca (Grah.) × N. langsdorffii (Weinm.), which grows on auxin and cytokinin free medium. Metabolism by 3.5-week-old tissues showed enhanced stability of supplied [9R]Z; unmetabolized [9R]Z accounted for 48.7 and 37.5% of extracted radioactivity following 8 and 24 h incubation, respectively; tissues of different ages (1–10 weeks following subculture) also indicated high cytokinin stability following 8 h incubation (unmetabolized [9R]Z accounted for 32.5–53.0% of extracted radioactivity). All analyses were performed by thin layer chromatography (TLC) and the results subsequently confirmed by high performance liquid chromatography (HPLC). Side-chain cleavage and modification of the purine ring were the major forms of metabolism; metabolites with an intact cytokinin moiety included zeatin (Z), [9R]Z nucleotides and glucosyl derivatives. Detailed analysis of metabolites carried out in the experiments using 3.5-week-old tissues indicated that both dihydro-derivatives as well as cis isomers of Z and [9R]Z were not formed. Adenine, adenosine and its nucleotide(s) were the main degradative metabolites; in 3.5-week-old tissues these metabolites accounted for about 5.9 and 7.8% (of 3H extracted) following 8 and 24 h incubation, respectively. In tissues of different ages (1–10 weeks following subculture), these metabolites accounted for about 7.6–22.9% of the extracted 3H. Some metabolites (zeatin, adenine and adenosine) were also detected in the staled incubation media. The observed high [9R]Z stability in this tissue may reflect low levels of cytokinin oxidase activity and/or some form of compartmentation.