Theor Appl Genet (1987) 75:217-221 9 Springer-Verlag 1987 Isolation of an amylose-free starch mutant of the potato (Solanum tuberosum L.) J. H. M. Hovenkamp-Hermelink 1, E. Jacobsen 1 ,, A. S. Ponstein 2, R. G. F. Visser 1, G. H. Vos-Scheperkeuter 2, E. W. Bijmolt 1, j. N. de Vries 1, B. Witholt 2 and W. J. Feenstra 1 1 Department of Genetics, University of Groningen, Kerklaan 30, NL-9751 NN Haren, The Netherlands 2 Department of Biochemistry, University of Groningen, Nijenborgh 16, NL-9747 AG Groningen, The Netherlands Received June 3, 1987; Accepted July 17, 1987 Communicated by G. Wenzel Summary. An amylose-free potato mutant was isolated after screening 12,000 minitubers. These minitubers had been induced on stem segments of adventitious shoots, which had been regenerated on leaf explants of a mono- ploid potato clone after R6ntgen-irradiation. The mu- tant character is also expressed in subterranean tubers and in microspores. Starch granules from the mutant showed a strongly reduced activity of the granule bound starch synthase and loss of the major 60 kd protein from the starch granules. Key words: Solanum tuberosum - Mutant - Starch composition - Granule-bound starch synthase - amy- lose-free Introduction Two types of starch occur in plants: assimilatory starch is formed in the chloroplasts in green tissues as the re- sult of photosynthesis. At night this starch is broken down into sugars, which are transported to other or- gans. Reserve starch is synthesized from the transported sugar-molecules. It accumulates in amyloplasts in spe- cific storage organs, such as endosperm or perisperm in seeds, and in vegetative organs such as potato tubers (Shannon and Garwood 1984). Normally, the major constituents of starch are amy- lose and amylopectin, which have different properties with respect to water solubility and iodine binding ca- pacity. The amylose content in most reserve starches is approximately 20%-25% (Shannon and Garwood 1984). However, in many plant species mutants are * To whom correspondence should be addressed known with altered contents of amylose and amy- lopectin. In maize (Zea rnays L.) mutants exist contain- ing starch almost without amylose (waxy mutants). Such mutants are also known in other plant species such as rice, barley, sorghum and amaranth (Amaranthus hy- pochondriacus L.). Mutants with an increased amylose content were found in maize, e.g., dull and amylose ex- tender (Okuno and Sakaguchi 1984; Shannon and Gar- wood 1984). In all these cases starch in endosperm or perisperm was involved. To date, no mutant plant has been found in which the reserve starch in a vegetative organ, such as the potato tuber, is affected. From the work of de Nettancourt and Dijstra (1969) we know that mutation leading to amylose-free starch can occur in a Solanum species. These authors observed waxy-like microspores in developing buds of S. verrucosum. The spontaneous mutation frequency was extremely low: among 2.5 million microspores analysed, two showed the characteristics of amylose-free starch. Hovenkamp- Hermelink et al. (1987 a) found occasional cells in mini- tubers of a monoploid clone of S. tuberosum showing reddish-brown starch granules when exposed to an I2- KI solution. This indicated the presence of amylose-free starch in these cells. In order to obtain a solid plant mu- tant we carried out an experiment in which potato tubers, obtained on X-rayed plant material, were screened for an altered starch composition. Waxy alleles in maize and other plant species proved to be recessive to the wildtype allele. Therefore, in the search for a potato mutant we used a monoploid clone (2n=x=12) as starting material. Adventitious shoots formed on leaf explants of potato can be readily used for the isolation of mutants. According to Broertjes and van Harten (1985) such shoots arise mainly from single cells, since the large majority of the mutant plants which are obtained do not show chimerism. Large scale