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Notes: In using an efficient and synchronised in vitro tuberisation system the transition of axillary buds from stolons to tubers in Solanum tuberosum L. cv. Bintje was followed. After 5 or 6 days on tuber-inducing medium all axillary buds had formed tubers which increased in size until the experiment was ended at day 10. Concomitantly with the visible appearance of tubers the fresh weight of the axillary buds increased as well as their starch content. Soluble sugar content, notably glucose, increased until tuberisation occurred and dropped after that. In the daily sampled explants gene expression was studied at several levels. RNA was isolated from the different explants during the whole tuberisation experiment and northern blots were probed with cDNAs encoding genes involved in starch- and patatin-biosynthesis. It was shown that in the very early stages of development hardly any transcript could be detected. Only one day before visible swelling occurred were clear signals obtained for all the genes investigated. Although it was evident that coordinate expression of starch biosynthetic genes did occur, it was not in a similar fashion for all the genes. Sucrose synthase and ADPG-pyrophosphorylase B were expressed in an identical fashion which was different from ADPG-pyrophosphorylase S, granule-bound starch synthase and branching enzyme. The RNA levels of these three latter genes reached a maximum at day 5, remaining constant until the experiment was finished. The transcript levels of sucrose synthase and ADPG-pyrophosphorylase B reached their highest level at day 5 after which they dropped to a lower level at day 10. Patatin gene expression was clearly different from that of the starch biosynthetic genes: it steadily increased from day 4 until the end of the experiment. Enzyme activities of sucrose synthase. ADPG-pyrophosphorylase and branching enzyme confirmed the RNA expression data and showed that ADPG-pyrophosphorylase enzyme activity reached a maximum at day 4 after which it dropped. The other two enzyme activities could be detected at or one day after tuberisation occurred and increased until the experiment was ended.