The experiments described here were conducted with a greenhouse F1 generation of the interspecific tri-hybrid Zea mays x Zea perennis x Zea diploperennis, obtained by Maria del Carmen Molina in our Institute (MNL 58:114-115, 1984).
Immature seeds were removed and were sterilized with 2% sodium hypochlorite and washed in sterile distilled water. Immature embryos that varied in size from 1.0 to 2.0 mm long were excised and placed scutellar-side-up on a solid agar medium. Calli were initiated on the same three media (A-B-C) described earlier (Rapela, MNL 58:106-108, 1984). The cultures were incubated in the dark at 27 C.
The responses of the hybrid embryos were similar to the responses of Zea mays embryos. In Medium A, the scutellum of cultured immature embryos produced an opaque, white to pale yellow, soft and friable callus. Structures resembling the organized scutellum of the original explant were not observed in these calli. In Medium B and Medium C, the scutellum produced an opaque, white to pale yellow and compact callus. Organized structures resembling the early stages of zygotic embryos were observed after 20 days in culture. Calli formation were obtained at more or less similar rates in the three media used (Table 1).
Root regeneration, but no shoots, was obtained after the transfer of calli from Medium A to asparagine-minus Medium A without 2,4-D in light. Root and shoot regeneration were obtained after the transfer of calli from Medium B and Medium C to 2,4-D free MS medium with 2% sucrose in light.
It is important to point out that, as with maize immature embryos, embryogenic callus was obtained both in Medium B and in Medium C. In spite of the strong differences between media, only the osmotic components (sucrose and proline) seem to have importance for somatic embryo formation. These facts lead us to suppose that it is possible to develop a very simple synthetic medium to obtain high rates of somatic embryogenesis in maize, as in related species.
Table 1. Frequency (in %) of callus formation. 1 = % of immature embryos forming soft and friable callus. 2 = % of immature embryos forming compact callus. 3 = % of compact callus forming somatic embryos.
Miguel Angel Rapela
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