Plant tissue culture and molecular biology as tools in understanding plant development and in plant improvement Indra K. Vasil University of Florida, Gainesville, Florida, USA The ability to introduce foreign genes into plant cells and to study their transient expression as well as their stable integration into the plant genome, followed by the regeneration of fertile transgenic plants from single transformed cells, forms the basis of many exciting achievements. Together, these provide a new insight into the molecular basis of plant growth and development, and unique opportunities for engineering better and more efficient plants for the future. Current Opinion in Biotechnology 1991, 2:158-163 Introduction Tissue culture and molecular biology are two important and interacting components of plant biotechnology. Al- though the 'prehistory' of plant tissue culture began more than 230 years ago, the beginnings of modem research in this field can be attributed to the attainment of un- limited growth of cultured plant cells and organs. This was achieved independently and simultaneously by Gau- theret, Nobecourt and White in the 1930s [1]. The dis- covery of plant growth regulators, such as auxins and cy- tokinins, and their critical role in shoot and root morpho- genesis, played an important part in the rapid develop- ment of tissue culture methods [2]. These efforts culmi- nated in 1965 in the experimental demonstration of the totipotency of plant cells (first theorized by Haberlandt at the turn of the century) by the recovery of whole to- bacco plants from single cultured cells [3]. The development of recombinant DNA technology in the 1970s, and its application to plants, has for the first time provided opportunities for investigation and understand- ing of the molecular basis of plant development, and for the identification and cloning of plant genes. The successful and now almost routine use of the Ti plas- mid of Agrobacterium tumefaciens to introduce foreign genes stably into plant cells has created unparalleled op- portunities for the molecular genetic manipulation of di- cotyledonous plants. Similarly, the recent development of methods for the direct delivery of DNA into protoplasts, as well as into intact cells, has pemlitted parallel experi- ments to be carried out even with those species, such as the cereals, that are not amenable to transformation by Agrobacterium. A perusal of contemporary literature clearly demonstrates that advances both in the molecular understanding of plant development and in genetic manipulation could not have been made without the combined and innovative application of the techniques of cell and tissue culture and of molecular biology. In order to highlight these ad- vances, I have selected a limited range of examples, which admittedly show my biases and interests, from amongst the many elegant reports in the literature that have ap- peared over the last year or so. Variation induced in tissue culture Until about 1960 the dogma was widely accepted that plant cells proliferating in vitro underwent only normal mitoses and therefore gave rise to clonal populations of cytologically normal cells. It was assumed, therefore, that plants regenerated from such cells were tree to type and clonal in nature. Early cytological studies, documenting the presence of polyploidy, aneuploidy and chromoso- mal rearrangements in cultured cells [44], quickly dis- pelled this myth, and also raised concerns about the ge- netic fidelity of plants derived from cultured cells. More recent studies have established clearly that the variabil- ity observed in cultured cells can be either the mani- festation of pre-existing variability generated during cel- lular differentiation and plant development, or induced de novo during culture [7,8]. Interestingly, cells in orga- nized meristems appear to be largely immune to such changes. In addition, embryogenic cell cultures are also rather resistant to changes induced in vitro, and there is increasing evidence that stringent selection in favour Abbreviations Adh--alcohol dehydrogenase; CaMV-cauliflower mosaic virus; Sh-l--shrunken-'l. 158 © Current Biology Ltd ISSN 0958-1669