Cytokinin Biosynthesis in Higher Plants

Abstract

Since cytokinin nucleotides have been detected within different tRNA species there has been continued discussion about the biosynthesis of the free cytokinins (Fig. 1). While a group of authors prefers a de novo synthesis from adenine monomers (Chen et al., 1976; Chen and Petschow, 1978; Chen and Melitz, 1979; Burrows, 1978; Stuchbury et al., 1979; Taya et al., 1978), there are others who sees the pathway via the hydrolysis of polynucleotides (Holtz and Klambt, 1975, 1978; Klemen and Klämbt, 1974; Helbach and Klämbt, in press; Maass and Klämbt in press. 1980), to be more likely. The first group used slime molds or callus tissues from cytokinin autonomous tobacco or crown gall, which were supplied with radioactive adenine. Their research was restricted to investigation into the possibility for a de novo synthesis by detection of radioactive cytokinins. Chen and Melitz (1979) reported a crude enzyme system being able to synthesize cytokinins directly from 5’AMP and IPP. Taya and coworkers found that 5’AMP is a direct precursor for IPAde in Dictyostelium discoideum. IPAde is an intermediate derivative in the biosynthesis of discadenine, which is involved in spore germination. However, the tRNA and its turnover was not taken into consideration in any of these publications. The second group published results on the turnover rates of tRNA in bacteria and roots of higher plants and compared the content of “bound” cytokinins to the free ones. Moreover, Holtz and Klämbt (1978) isolated the Δ²isopentenyldiphosphate: tRNA Δ²-isopentenyltransferase, which isopentenylated tRNA, oligo N, MS2, poly A, and also oligo A-(Ap)_3–7A- but not ApApA, ApA or other monomeric adenine derivatives. These data, of course, are not conclusive because the expected isopentenylation of adenine or its derivatives could be catalyzed by another enzyme.

Part of the doctoral thesis, Bomm 1980