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Protoplast to plant: problems and pointers
As noted before in these pages, plant breeders often turn to wild relatives of crop species for 'new' genetic traits and then seek methods of introgressing the relevant genes into the crop. One such method is protoplast fusion which bypasses barriers to sexual hybridization and generates a somatic hybrid that possesses the genomes of both parental species. It is hoped that the somatic hybrid behaves as a stable tetraploid and thus inherits the desirable traits from both parents. There have indeed been some successes with somatic hybridization but there have also been many failures. One stage at which the process may break down is the regeneration of plants from the fused protoplasts. This is the problem faced by a Polish group (Tylicki and colleagues (pp. 269-278) working with Solanum lycopersicoides. This is a potential source of several useful traits but somatic hybridization with other Solanum species has proved very difficult because of frequent failures at the regeneration stage. Indeed, S. lycopersicoides itself does not regenerate well and so the authors have undertaken a very detailed analysis of the structure and behaviour of protoplasts. There are many interesting features in this paper but perhaps the most significant is the finding that an apparently homogeneous source of cells (a culture of root primordia) can give rise to a heterogeneous population of protoplasts. Four types were distinguished: mononuclear, polynuclear (arising by protoplast fusion during isolation), anuclear and 'homogeneous' (which, from their ultrastructural appearance were probably undergoing cell death). Of these, only the mononuclear were able to establish cultures from which plants could be regenerated. This careful study has thus suggested that the heterogeneity of the original protoplast population is a possible bottleneck in regeneration. It is to be hoped that the authors can go on to obtain regeneration of somatic hybrids containing the S. lycopersicoides genome.
Professor J. A. BryantUniversity of Exeter, UK
j.a.bryant{at}exeter.ac.uk
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