The induction of somatic embryogenesis and subsequent plant regeneration in maize can be achieved with many inbred lines, although a number of elite inbreds show poor response in vitro. By manipulation of the callus induction medium, it was found that the embryogenesis induction response is largely a physiological phenomenon; a change in the concentration or molecular configuration of the plant growth regulator (PGR) dramatically altered the response of each inbred examined.
Immature embryos from the inbreds B73 and MS71, reciprocal crosses between B73 and MS71 (Fl), and self-pollinations of the hybrids derived from these crosses (F2) were isolated when they had attained a length of 1.2-1.8 mm. All culture induction media contained N6 salts, 9% sucrose, and 0. 1 to 100 µM of 2-, 3-, or 4-chlorophenoxyacetic acid (CPA). Cultures were scored for embryo formation after three weeks. Treatments which were scored with a " ± " (Table 1) produced embryoids at frequencies occasionally approaching those scored with a " + " (optimum induction frequency and growth of embryogenic tissue), but either the growth of the tissue was slow (at concentrations above the optimum) or the concentration of the PGR was high enough only to induce embryoid-like structures in a few isolated cases (at concentrations below the optimum). Treatments scored with a "-" indicate either germination of the immature embryos (concentrations below the optimum) or tissue necrosis with little or no detectable growth (concentrations above the optimum).
Each form of CPA elicited a similar concentration-dependent response. At low concentrations, the scutellum swelled slightly but did not form embryoids. Embryoids formed from the scutellum when the concentration was increased, and a further increase in concentration resulted in toxicity. While each inbred showed a similar overall response pattern, the relative sensitivity of each to different concentrations of the three forms of CPA differed between genotypes. This was particularly evident in the comparison of B73 (a BSSS derivative) and MS71 (an A619 derivative). B73 showed a greater sensitivity than MS71 in response to each concentration of each form of CPA.
Reciprocal crosses between these two inbreds sometimes showed a similar response pattern to that of the maternal parent of the cross, as was especially evident in the case of MS71 x B73. Comparison of the F2 generation embryos showed essentially no difference in sensitivity to the PGRs. In addition, the response tended to be uniform for all embryos in each petri dish; there was no evident Mendelian segregation as would have been expected for nuclear genes (Table 2). The case where partial response was seen (at concentrations just above or below the optimum concentration) appeared to be physiological in nature--swelling of the scutellum with no embryoid formation at lower concentrations and toxicity at higher levels. In these instances, any induction of embryogenesis was sporadic and occurred at a low frequency.
These results indicate that the induction in vitro of somatic embryogenesis in maize is greatly influenced by the concentration and molecular structure of a given PGR and, under the appropriate conditions, can be achieved at relatively high frequencies independent of the genetic background of the explant tissue. Despite the similarity of the response of F1 hybrids to that of the maternal parent of each hybrid in many cases, it is unclear whether the inheritance of culture response is controlled by cytoplasmic genes. Further, the lack of well-defined segregation ratios in the F2 generation indicates a complex mode of inheritance.
K. Close and L. Ludeman
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