Mutagenesis and the Selection of Resistant Mutants

The prospect of utilizing plant tissue cultures as a means of generating and identifying novel genetic variants is one that has sparked the interest of researchers for many years. It has resulted in large numbers of research papers describing the selection and characterization of mutants, and several exhaustive reviews (e.g., 1 ,2 ), including a treatise giving detailed coverage of the whole field (3 ). By far the largest number of articles relate to the selection of resistant mutants, which are the most straightforward category to select in vitro, and which are the focus of the present chapter. One thing that is immediately apparent on superficially scanning the literature on these mutants is a high level of repetition. This is because the concept is simple: one challenges a large population of cells with a selective agent, and identifies “survivors” on the basis of their capacity to divide to form cell masses, or organized structures. Thus, the procedures used to select mutants resistant to a range of different antimetabolites have a certain uniformity. By the same token, selection for the same type of mutant (e.g., salt tolerant) in a range of species, involves a broadly similar strategy. On closer scrutiny of the same literature, however, it becomes apparent that far from there being a simple formula to obtain “results” for selection of every phenotype in every species, a great deal of refinement, sometimes major, sometimes subtle, has been necessary in each case. This is owing to the diversity of detail, both in the tissue-culture procedures available for each crop and in the cellular consequences of exposure to different antimetabolites.