Characterization of gamma-irradiated seeds of a wild Namibian marama bean (Tylosema esculentum) with microsatellite markers

Legumes have been used as food by indigenous people of Africa yet their full potential as food sources has not been completely realized. With modern technologies such as marker-assisted selection and mutational breeding, there is scope for rapid improvement of germplasm in these underutilized species. One approach is to induce genetic mutations through irradiation of plant tissues with neutrons or gamma rays generating molecular changes. The irradiation leaves residual radiation in the exposed tissues creating offspring with new features. Genetic mutations in plants can improve their performance, germination, quality, size and disease resistance. The effects of radiation applied to marama bean (Tylosema esculentum) were explored. The aim of this study was to screen individuals of T. esculentum derived from seeds irradiated with gammarays using microsatellite markers. There were two irradiation treatments -50 and 100 gamma ray irradiations that were compared to non-irradiated seeds as controls. For each treatment, seeds were imbibed and germinated. Seed emergence rates and percentages were ascertained. Each set of seeds was genotyped using a SSR marker - MARA039, located in a sub-unit of a polygalacturonase – an enzyme that mainly regulates seed germination. In regard to emergence, significant differences between the irradiated seeds and non-irradiated counterparts were noted. Irradiated seeds germinated faster and in adosage-dependent manner compared to non-irradiated counterparts (p>0.05). Genotyping revealed that the total number of AGA repeats in the microsatellite region shifted from its original five in the controls to between four and seven in the irradiated seeds. Furthermore, there were several point mutations in the irradiated seeds as compared to the controls. The study findings, suggest that irradiation may induce beneficial mutational changes such as faster germination rate of the marama bean, considering that the mutated microsatellite repeat region is located in gene encoding the enzyme regulating seed germination in plants. Key words: Marama bean, Tylosema esculentum, mutational breeding, irradiation, seed cycle, SSR.