Induced mutations - A new paradigm in plant breeding

The use of ionizing radiation, such as X-rays, gamma rays and neutrons and chemical mutagens for inducing variation, is well established. Induced muta tions have been used to improve major crops such as wheat, rice, barley, co tton, peanuts, and beans, which are seed propagated. Since the establishmen t of the Joint FAO/IAEA Division of the Nuclear Techniques in Agriculture, more than 1800 cultivars obtained either as direct mutants or derived from their crosses have been released worldwide in 50 countries. In vegetatively propagated plants, many of mutants were derived from irradiating rooted st em cuttings, detached leaves, and dormant plants. According to the FAO/IAEA database, of the 465 mutants released among the vegetatively propagated pl ants, most are in the floricultural plants and a few in fruit trees. These include chrysanthemum, Alstroemeria, dahlia, bougainvillea, rose, Achimenes , begonia, carnation, Streptocarpus, and azalea. The irradiation of in vitr o cultured date palm, apple, potato, sweet potato and pineapple now provide s a means to treat large populations which would not have been possible bef ore. Irradiation of micropropagated plants, axillary and adventitious buds, apical meristems, regenerative callus cultures, anthers and microspores, a nd somatic embryos provides a miniaturized version of trees and seeds in th e Petri dish instead of the field. During the last decade, the use of radio -actively labeled probes in recombinant DNA research for cloning and mappin g plant genes and transgenesis, particularly for RFLP, microsatellite based DNA fingerprinting, has become a routine procedure. Many homeotic mutants that change floral development have been isolated in Arabidopsis, Petunia, Antirrhinum and Lycopersicon. Mutants of Arabidopsis are being used to anal yze genes, which determine response to auxins, cytokinins, gibberellin, abs cisic acid and ethylene in plant growth, floral development and senescence, fruit formation and ripening. These mutants are facilitating the isolation , identification and cloning of the genes, which would ultimately help in d esigning crops with improved yield, increased stress tolerance, longer shel f-life and reduced agronomic inputs. The identification and analysis of mut ants by using molecular techniques of DNA fingerprinting and mapping with P CR based markers, such as RAPDs, AFLP and STMS, and mutant tagging shall br ing a new dimension in gene technology. Already, mutations can be linked to changes in DNA sequences for some plant traits and to establish molecular maps in structural and functional genomics of crop plants. These in turn wo uld lead to a rapid enhancement of crop yields and quality.