Two maize lines, floury-a 79-7660 and BP 79-7511 (see MNL 55:53 and 54), were screened for resistance to lysine-threonine 5 mM inhibition by evaluating seedlings derived from whole kernels. With the aim of testing the new hypothesis of Phillips et al. (Crop Sci. 21:601-607) on our mutant and normal lines selected by the growth of their embryo cultures in lysine-threonine medium, the following procedure was used. -Thirty kernels were selected from the middle part of the ear and surface-sterilized for 20 min in 2.5% sodium hypochlorite 1:8 v/v. Murashige-Skoog medium (without changes), containing 0.2% sucrose and 0.8% agar, and either 0 (control) or 5 mM lysine-threonine was autoclaved and poured into test tubes (10 ml per tube). Fifteen kernels of each strain were tested in the inhibitory medium (one in each tube) and the rest were used as control. The incubation period was 6 days, at 28 C in the dark. The experiment was repeated twice.
As shown in Table 1, we have tested 8 parameters. Among them, the primary root length (treated/control) is the most representative of the extent of the inhibition. Seedling growth was inhibited to a different degree in both lines. However, either the mutant floury-a or the normal BP line could be considered resistant, because the root length in lysine-threonine medium exceeded 50% of the root length of control, according to the classification of Phillips et al.
Surprisingly, the normal BP line showed the higher degree of resistance. Such a result is somewhat contradictory with our previous determinations of the floury-a and BP excised mature embryos in treated 2.5 mM/control test, in which the floury-a embryos showed a higher degree of resistance compared with BP ones.
It is interesting to note, however, that contrary to the results obtained by Phillips et al., the resistance of the floury-a and BP line was not only expressed when seedlings were derived from whole kernels but also in seedlings derived from excised embryos.
The Phillips hypothesis of the high methionine/lysine or methionine/(lysine + threonine) ratios could explain our experimental results with the floury-a and BP seedlings derived from whole kernels. However, such hypothesis does not explain the behavior of such seedlings derived from excised embryos.
We have previously suggested (MNL 55:54) that the free amino acid pool could be affecting the lysine-threonine response. Such hypothesis has also been proposed by Phillips et al. (Crop Sci. 21:601-607). Either the floury-a or the BP line has a higher free amino acid pool. Such a pool could affect the lysine-threonine response either on the seedlings derived from whole kernels or on the seedlings derived from excised embryos. However, the finding that the endosperm tissue was not required to provide resistance suggests an alternative explanation. That is, probably the floury-a gene or a gene closely linked with it could affect some regulatory enzyme of the aspartate family of amino acids and produce desensitization to the lysine-threonine effect (MNL 55:55-57). Further tests will be performed in this way.
Miguel Angel Rapela
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