INDUCED MUTAGENSIS AS A BREEDING STRATEGY FOR IMPROVEMENT OF SOLANACEOUS VEGETABLES

摘要

The Solanaceae are an agriculturally significant family with a wide variety of domesticated and cultivated vegetables and fruit species such as potato, tomato, eggplant, and sweet peppers, in addition to ornamental and medicinal plants such as petunia, tobacco, and garden nightshade. Since the discovery of hybrid vigor (heterosis) in the latter half of the 19th century, most crops are now grown from F_1 hybrid seed for both crop yield and quality improvement. For the development of new varieties or improvement of specific characters in existing cultivars, plant breeders rely on the genetic resources of wild relatives and induced or spontaneous mutants. Induction of mutations is achievable through the use of chemical mutagens such as ethyl methionine sulfonate, radiation such as gamma rays, X-rays and ion beams, and genetic engineering. The process of inducing mutations and searching for relevant mutants of agronomic applicability requires patient exploration and keen observation of morphological characters during selection. In this respect we have worked relentlessly to develop novel male-sterile mutants of tomato, a well known and worldwide-consumed fruit, and African nightshade, a leafy vegetable currently underutilized because of its very low leaf yields. In hybrid seed production, induced male sterility is potentially useful for pollination control, thus reducing the costs and labor involved when manual emasculation (removal) of the anthers or male flowers is done. Through seed irradiation with gamma rays, we have induced stable male-sterile mutants in 'First' tomato, a popular Japanese breeding line with superior features. The mutants, named T-l, T-2, and T-3, are characterized by pollen degradation at the tetrad, meiocyte, and microspore stages, respectively. In addition, a temperature-dependent, photoperiod-independent male-sterile mutant, T-4, which is sterile in spring (with some residual fertility) and shows partial restoration of fertility in autumn, has been isolated. In a two-line hybrid seed production system that uses T-4 as a seed parent, the risk of contamination is minimal and the selfed-seed progeny are easily distinguishable at the seedling stage by narrow-leaf markers. Male sterility also has the potential to alter the vegetative-reproductive balance and biomass partitioning in leafy vegetables with prolific fruit and seed production, such as African nightshade. Through seed irradiation with 12~C~(5+) ion beams, we have induced a novel African nightshade mutant with abnormal floral organs. The flowers are sepaloid in mid-spring, stamenless in late spring, and indeterminate in summer; structure and fertility are partly restored, with berry and seed set, in autumn. In this review we use the gene-based ABCDE and protein-based quartet models of floral organ identity to discuss the possible molecular background of the dynamic nature of this mutant. We also discuss other mutants induced in the tomato 'First', such as parthenocarpic and short-internode mutants, and the similarities and differences of their genetic backgrounds to those of well established mutants on the basis of allelism test-crosses.