Metabolism of the Cytokinins in Mosses

Abstract

One of the most pronounced effects of cytokinin is the induction of buds on the protonema of mosses (Hahn and Bopp, 1968; Szweykowska, 1974; Nehlsen, 1978). Most of the moss species treated with cytokinin between 10 nM and 100 µM show a similar reaction. Certain protonemal filaments which are target cells for bud formation quickly change their morphogenetic behavior and form buds instead of filamentous side branches. In some mosses a short side branch which has already been formed can also change its shape under the influence of cytokinin. The primary steps of this reaction can be seen as early as 6 to 10 h after application as a modification of the tip region of the cells and shortly later in an increase of the RNA content in these cells. These reactions of the moss protonema are very specific, because only N-6-substituded adenine derivatives — true cytokinins — are active (Hahn and Bopp, 1968), whereas other substances like substituted ureas, which have a strong effect in other test systems (Takahashi et al., 1978), do not induce buds in the mosses. Also the ribosides of benzyladenine and other cytokinins are much less active than the purine bases themselves when they are applied from outside (Whitacker and Kende, 1974; Spiess, 1975). But this lack of an effect seems to be more a consequence of a restricted uptake than of an inefficiency of the substance. It can be shown that the application of ribose-1-phosphate together with the kinetin enhances the bud formation by about 100%, whereas ribose-5-phosphate does not change the number of buds induced by kinetin (Fig. 1). From this one may conclude that for the effect of cytokinin in the cells the ribosides and not the bases are necessary, but no further experiments or arguments are available at the moment to support this assumption.