Microspore-derived Embryo Formation and Morphological Changes during the Isolated Microspore Culture of Radish (Raphanus sativus L.)

Raphanus sativus L. cv. Taebaek, a efficiently microspore-derived embryo (MDE)-forming cultivar, and 'Chungwoon', a non-MDE-forming cultivar were selected as donor plants for isolated microspore culture. Radish flower bud of 2.0 (small, S), 4.0 (medium, M), and 6.0 (large, L) ${\pm}$ 0.5 mm in length were isolated to determine the temporal relationship between flower bud size and MED yield. Anatomical observations revealed no difference in the structure of the flower buds between the two cultivars. In both cultivars, the stigmas were much longer than the floral leaf in M-sized flower buds. The MDE yields for 'Taebaek' per petri dish were 6.6 and 1.3 for M- and L-sized of flower buds, respectively, but MDE formation was not induced in the S flower buds. On the other hand, 'Chungwoon' failed to form MDEs in all flower buds. The microspore density of 'Taebaek' was 1.3 times more than that of 'Chungwoon' for M sized flower buds. Of the M-sized buds from 'Taebaek' and 'Chungwoon', 92.1 and 81.6%, respectively, were in the late uninucleate microspore stage, which is characterized by the highest frequency of MDE formation. Anatomical observations of MDE formation revealed that the microspores were able to divide to form a primordium from which cell division took place continuously in the 'Teabeak' cultivar. However, the microspores of 'Chungwoon' failed to progress beyond the primodium stage, resulting in lack of MDE formation. By contrast, after the formation of the primordium, various developmental stages of embyos from microspore were observed in the 'Taebaek' cultivar. These results can be used to determine MDE forming potentials of radish cultivars.