Induced Gene and Chromosome Mutants

Plant scientists, including breeders, can use an arsenal of physical and chemical mutagens and appropriate selection techniques to ‘manufacture’ in their experimental plots gene and chromosome mutants to compensate for the erosion of natural sources of genetic variability. They also have the capability of generating in this type of genetic manipulation the entire array of genetic variation inherent in all loci controlling each plant trait, and thus in a relatively short time producing most, if not all, of the genetic variants that have ever occurred in the evolution of a given agricultural plant. This capability is required not only for the breeder concerned with developing new cultivars to meet the numerous and varied demands of the modern farmer, processer and consumer, but also for the geneticist, physiologist, anatomist and biochemist concerned with unravelling important plant processes and their genetic control. In short, these scientists need inexhaustible supplies of genetic variability, often never before selected in Nature or by earlier plant breeders. Numerous experiments demonstrate that induced mutants have considerably extended the genetic variability of a phenotype. An outstanding example is eceriferum (‘waxless’ plant surfaces) in barley. Spontaneous mutations produced several well known variants controlled by about six loci. Genetic analyses of over 1300 induced and the few spontaneous mutants have determined that this trait is controlled by at least 77 loci (Lundqvist 1976, and personal communication). There are numerous alleles at some of these loci. Other examples are described in this paper. The quantity and quality of artificially induced genetic variability in plants is in no small part due to the contributions of improved mutagens, mutagen treatments and selection techniques. A new potent and unique mutagen, sodium azide, is particularly successful in inducing putative point mutations. Recent experiments with barley and Salmonella have revealed that it is not azide per se but an activated metabolite that is the mutagenic agent. The metabolite has been isolated and crystallized and can now be synthesized in vitro . These findings usher in a new category of mutagens and suggest new avenues for understanding the interaction of mutagens with chromosomes and genes and for greater control of the induction of genetic variability in plants. The considerable success of varietal development through induced mutants is well documented: 465 culvitars of sexually and vegetatively reproducing crops have been released that owe some of their production advantage to an induced gene or chromosome mutant. These cultivars have led to considerable economic impact in a number of countries. In breeding research, induced mutants are indispensable for probing and elucidating the pathway and genetic control of important plant processes such as wax synthesis and deposition (von Wettstein-Knowles 1979), nitrogen assimilation (Kleinhofs et al . 1980), photorespiration and different facets of photosynthesis (Somerville & Ogren 1980; Miles et al . 1979; Simpson & von Wettstein 1980) . In the manipulation of plant genes (genetic engineering) in breeding research, it becomes increasingly necessary to pinpoint these genes on chromosomes. For this endeavour, an abundant array of induced chromosome mutants such as trisomics, telotrisomics, acrocentrics, inversions, translocations and deletions is required. This important activity can now be complemented by ever-improving chromosome banding techniques.