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Elm embryos: early opportunity for ease of induction
A key step in plant genetic manipulation is regeneration of plants from the genetically transformed cells. In some instances this may be straightforward, but many species present a real challenge. Corredoira and colleagues at the Galician Agrobiological Research Institute (pp. 637-644) discuss the possibility of genetic manipulation of elm (Ulmus glabra and U. minor), in relation to the transfer of resistance to Dutch elm disease. The loss to this disease of much of the European elm population is well documented. However, despite much intensive research effort, it remains a major practical problem. Therefore the authors have sought to develop a method for growth of an elm tissue culture with the potential for induction of somatic embryos. As starting material they used normal zygotic embryos from which callus cultures were established. Induction of somatic embryogenesis was then attempted by manipulation of the hormone content of the medium. Their data show that there is a short period during the development of the zygotic embryo in which it is possible to establish embryogenic cultures. This occurs at the 'cotyledonary stage', after the embryos have developed bilateral symmetry, but before the deposition of storage reserves. These results illustrate a feature widely seen in plant tissue culture: the developmental/physiological status of the tissue used to initiate a culture can influence the subsequent potential of that culture. In this instance, it may be significant that the phase of reserve deposition is, in many dicots, associated with a peak of ABA content in the embryo. Possibly this induces changes that depress the embryogenic potential of the callus culture. So, selection of embryos at an appropriate stage is the key to success, but, as the authors point out, this is only the beginning. Subsequent performance of the embryos must be optimized before growth of trees from genetically modified callus becomes a real possibility.
Professor J. A. BryantUniversity of Exeter, UK
j.a.bryant{at}exter.ac.uk
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