[Enzyme formation in rye seedlings during the change from heterotrophic to autotrophic growth]

Abstract

1. The formation of enzymes of the oxidative and reductive pentose phosphate cycle was followed in developing rye seedlings. Ribulose diphosphate carboxylase and transketolase are absent in dry seeds; ribose phosphate isomerase is found only in trace amounts. In light as well as in the dark transketolase and ribose phosphate isomerase increase immediately after the onset of germination, whereas ribulose diphosphate carboxylase appears later. Remarkably the latter enzyme is also formed in completely dark-grown seedlings. At this stage isolated thylakoids are already present in the proplastids. The formation of these enzymes of the Calvincycle is promoted by light. 2. The formation of ribulose diphosphate carboxylase and transketolase is strongly suppressed by low temperature or chloramphenicol. Chloramphenicol also inhibits the growth of proplastids and prevents the formation of thylakoids but not of prolamellar bodies. 3. Glucose-6-phosphate dehydrogenase, phosphogluconate dehydrogenase, and shikimate dehydrogenase are already present in the dry embryo. At the beginning of seedling development there is a rapid increase in glucose-6-phosphate dehydrogenase. In the shoots the rate of this increase levels off at the moment when ribulose diphosphate carboxylase appears. This decrease in the rate of formation of glucose-6-phosphate dehydrogenase is not due to a lack of nutrients, but must be caused by a specific regulation. 4. When the formation of ribulose diphosphate carboxylase is prevented by low temperature or chloramphenicol, the increase of glucose-6-phosphate dehydrogenase continues at a constant rate. It is concluded that the formation of glucose-6-phosphate dehydrogenase is suppressed by factors which appear when enzymes of the Calvin-cycle are formed. This effect of low temperature is not directly connected with vernalisation phenomena. 5. The results suggest that the oxidative pentose phosphate cycle plays a special role in heterotrophic growth.