Plant Cell, Tissue and Organ Culture 53: 197–204, 1998. © 1998 Kluwer Academic Publishers. Printed in the Netherlands. 197 Comparison of tuber and shoot formation from in vitro cultured potato explants Dick Vreugdenhil 1,∗ , Yvonne Boogaard 1 , Richard G.F. Visser 2 & Steef M. de Bruijn 1 1 Laboratory of Plant Physiology and 2 Laboratory of Plant Breeding, Graduate School of Experimental Plant Sci- ences, Wageningen Agricultural University, Arboretumlaan 4, 6703 BD Wageningen, The Netherlands (*requests for offprints) Received 2 September 1997; accepted in revised form 26 May 1998 Key words: Solanaceae, Solanum tuberosum, sugar metabolism, tuberization Abstract The development of axillary buds of potato (Solanum tuberosum L.) plants, cultured in vitro, was analyzed. Depending on the composition of the culture medium, the buds developed into either tubers (medium with 8% sucrose), shoots (1% sucrose), or stolons (8% sucrose and 0.5 μM gibberellin). Endogenous sugar and starch levels, and key-enzymes involved in the conversion of sucrose to starch were determined at different stages of development. Moreover, the spatial distribution of sugar levels and enzyme activities were determined within the developing structures. Glucose and fructose decreased upon tuber formation, most noticeably in the swelling parts, where also starch accumulated. The activities of sucrose synthase, fructokinase and ADP-glucose pyrophospho- rylase were highest under tuber-inducing conditions, the increase being confined to the tubers, and absent in the subtending stolons. It is concluded that changes in the measured parameters, observed under tuberizing conditions, are specifically related to the formation of the tuber, and are confined to the swelling part only. Abbreviations: GA – gibberellin; AGPase – ADP-glucose pyrophosphorylase; susy – sucrose synthase; FK – fructokinase Introduction The detailed analysis of the regulation of tuber for- mation in potato is hampered by the lack of syn- chronicity of tuberization, rendering it impossible to harvest and analyze well-defined stages of develop- ment (Vreugdenhil and Struik, 1989). To overcome this problem, Hendriks et al. (1991) and Visser et al. (1994) developed an in vitro system in which tuber formation was highly predictable and synchronous. Using this method, changes in carbohydrate levels and in the activities of several enzymes were analyzed (Appeldoorn et al., 1997; Visser et al., 1994). Al- though changes observed in vitro were similar to gross changes described in vivo (Ross et al., 1994), the use of in vitro cuttings implies the risk of unwanted and unexpected artefacts, e.g., due to stress because of cut- ting and transfer to culture medium, or due to culture conditions. Therefore, we decided to compare the in vitro for- mation of tubers from axillary buds, with the growth of shoots from similar buds. Formation of a shoot rather than a tuber can be induced under similar conditions, except for change of a single parameter. Two possibilities were analyzed: − addition of GA to the medium, in the presence of high sucrose (high sucrose being one of the induc- ing factors in the tuberizing medium). Gibberellins are known to prevent tuber formation, even when other conditions are tuber-inducing (Koda and Okazawa, 1983; Vreugdenhil and Struik, 1989); − lowering the level of sucrose in the medium (Gar- ner and Blake, 1989; Okazawa, 1967). To monitor the development of axillary buds we examined the morphology of the developing structures as