Embryo growth inhibition by certain combinations of aspartate derived amino acids has been reported for several cereal species (C. E. Green and R. L. Phillips, Crop Sci. 14:827, 1974). Thus, the embryo growth of normal maize is severely inhibited in a medium containing lysine plus threonine (LT) at 1 mM concentration (Green and Donovan, Crop Sci. 20:358, 1980). Embryos and callus of mutant lines of maize, such as o2 and su1, are an exception to the rule (Ciampi and DaSilva, Ciencia e Cultura, 29:701, 1977, and Sysoev et al., Fiz. Biok. Kult. Rast. 11:318, 1979). In addition to such mutants, we present here evidence of the lack of inhibition by LT on excised mature embryos of the fl1-a high quality protein maize.
Simultaneous addition to the culture medium of LT 1 mM is not enough to block the normal growth and development of excised mature fl1-a embryos. Increasing the concentration of amino acids to 2.5 mM resulted in growth inhibition which averaged 50% reduction in relation to the control (Table 1). The maximum inhibition observed was in the root length either at 1 mM or 2.5 mM of LT in the medium. Neither amino acids alone nor other combinations caused embryo growth inhibition.
Table 1. LT growth inhibition: % of inhibition in respect to the control
(without amino acids)
|Primary root length||35.69||87.13|
|Seedling fresh weight||17.69||47.71|
|Roots fresh weight||10.00||72.78|
|Shoots fresh weight||9.52||57.14|
|Seedling dry weight||6.67||40.00|
|Roots dry weight||9.50||50.00|
|Shoots dry weight||8.34||34.00|
The growth inhibition observed in the fl1-a embryos is strongly different from the normal-embryo one. Thus, we suppose that either (1) an internal metabolic alteration of the embryo due to an increasing content of free amino acids and/or methionine, not due to a direct effect of the fl1-a gene, or (2) a desensitization of the aspartokinase enzyme to its specific modulator lysine due to a direct effect of the fl1-a gene, could explain the differences.
The accumulated evidence up to now about the way of action of the high quality protein maize mutants gives more weight to the first hypothesis than the second.
Although the finding of an inbred normal maize line with less sensitivity to LT inhibition (see this MNL, below) shows that the presence of floury or opaque mutant genes in the background is not strictly necessary for obtaining partial desensitization to the LT inhibition, new proofs from our laboratory show that changes in the sensitivity of homoserine dehydrogenase enzyme to its specific inhibitor, threonine, could play a more important role than the previously suspected differences between normal and mutant behavior (Rapela, submitted to Experientia).
Nevertheless, whatever be the explanation, the lack of whole inhibition on the growth and development of fl1-a embryos may be converted as a useful tool for isolating ones growing on these selective culture mediums.
Miguel Angel Rapela
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