Nucleotide Metabolism during Pine Pollen Germination

Abstract

AbstractThe metabolism of purine‐ and pyrimidine nucleotides in pine pollen (Pinus mugo) grown in suspension cultures have been examined.In the ungerminated dehydrated pollen, the presence of ATP has been demonstrated. Incubation of the pollen in a germination medium leads to an exhaustion of the ATP pool, which is restored with the onset of oxygen uptake.By labelling pollen cultures with 32P‐orthophosphate, it has been possible to quantitate the nucleotide components of the pollen, and thereby to measure changes in the nucleotide pattern at various growth stages. The most marked changes occur during the initial phase of tube growth when a large increase in the ribonucleoside triphosphate and the sugar nucleotide pools is observed. The contents of ATP and UDP‐glucose are further increased if starch synthesis is initiated by the addition of sucrose to the culture medium.In order to determine whether nucleotides in pine pollen are synthesized from de novo pathways or via reutilization pathways, from breakdown products of nucleic acids, pollen was incubated with 14C‐labelled precursors of both the de novo and the reutilization pathways. Incorporation experiments established de novo synthesis of ATP and GTP from glycine, and de novo synthesis of CTP and UTP from orotic acid. The operation of pathways for the utilization of exogenous nucleosides was also demonstrated. While uridine, cytidine and adenosine are incorporated into nucleoside triphosphate to a great extent, only minor incorporation of inosine and guanosine is observed. These reutilization pathways might be of importance for the synthesis of nucleotides during tube growth in situ.Addition of inhibitors of glycolysis and oxidative phosphorylation drastically reduces the level of ribonucleoside triphosphates, indicating a rapid turnover of the nucleotide pool.