Characterization of cytokinin action on enzyme formation during the development of the photosynthetic apparatus in rye seedlings

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Abstract
The action of cytokinin on the formation of photosynthetic enzymes in rye seedlings, which was described earlier, is further characterized by studying the interactions of kinetin with specific inhibitors of protein synthesis and with auxins. The combined action of cytokinin and auxin simultaneously affects enzymes of the oxidative pentose phosphate cycle which show a definite relationship to the appearance of the photosynthetic apparatus. The formation of the photosynthetic enzyme carboxydismutase (EC 4.1.1.39) in rye seedlings can be almost completely prevented by chloramphenicol, but is also strongly inhibited by cycloheximide. These effects of chloramphenicol and cycloheximide cannot be overcome by the application of kinetin. Plastid growth as well as the formation of photosynthetic enzymes (carboxydismutase; NADP-dependent glyceraldehydephosphate dehydrogenase, EC 1.2.1.9) are selectively curtailed in dark- or light-grown rye seedlings by treatment with relatively high concentrations of auxins like 3-indoleacetic acid, α-naphthalene-acetic acid (NAA) or 2,4-dichlorophenoxyacetic acid. The inhibition of plastid growth and enzyme formation can be overcome to an increasing extent by applying increasing amounts of kinetin to young seedlings simultaneously with the NAA. Conversely, the promoting effect of kinetin on the formation of photosynthetic enzymes can be lowered by simultaneous application of NAA. In older seedlings the effect of NAA is no longer reversed by kinetin. An interpretation of the particularly high cytokinin requirement for the synthesis of photosynthetic enzymes in the seedlings is proposed. The appearance of the photosynthetic enzymes in the dark and in the light is accompanied by a decline in the rate of accumulation of key enzymes of the oxidative pentose phosphate cycle, especially glucose-6-phosphate dehydrogenase (EC 1.1.1.49). This effect is not due to a simple insufficiency of nutrients. Moreover, applications of glucose-6-phosphate or 6-phosphogluconate did not suggest an inducing effect of these substrates on their corresponding enzymes. No further increase of glucose-6-phosphate dehydrogenase can be achieved by application of either kinetin or NAA alone. But the formation of this enzyme continues at a high rate and reaches nearly twice the maximal activity of the controls when high concentrations of kinetin and NAA are applied simultaneously. It is concluded that a specific effect of the auxin and the diminished competition of the plastids for the promoting effect of kinetin contribute to the increased formation of glucose-6-phosphate dehydrogenase. The role of cytokinins in the control of the described enzymes is discussed.