In our studies on the effect of aspartate derived amino acids on the growth of excised mature embryos of normal lines of maize, we have observed that threonine alone either at 1 mM or 2.5 mM in the medium caused slight inhibition of root growth especially when such growth was determined by length. Reduced root growth by a threonine effect was observed previously by Green and Donovan (Crop Sci. 20: 358, 1980) and our results confirm this report. The threonine effect on root growth was not observed in fl1-a seedlings, and this fact was the basis of our hypothesis suggesting that the differences between the behavior of mutant and normal embryos under LT inhibition could be located at the level of homoserine dehydrogenase enzyme (Rapela, 1980, submitted to Experientia).
The slight growth inhibitory effect of threonine was observed by us in roots of several seedlings of inbred lines and hybrids of maize and also in the BP inbred line (see this MNL, above). Since this line has less sensitivity to LT inhibition, it was a good material for studying the effect of such amino acids because BP embryos grow and develop in these inhibitory mediums. Confirming Green and Donovan's report, the LT inhibitory effect was much more marked in the root zone than in the shoot zone. These differences could be explained if root and shoot zones differ in the rate of translocation of the amino acids or differ in the sensitivity of the regulator enzymes to their specific inhibitors. The first point remains to be studied, but the second one is very interesting to focus.
Two enzymes are implicated in the flow regulation of aspartate derived amino acids: aspartokinase and homoserine dehydrogenase. Between both, homoserine has unusual properties and characteristics and allows some physiological and genetic speculations about its regulation. Thus, based on the findings of Bryan's group (Biochim. Biophys. Acta 171:205, 1969; Plant Physiol. 55:991, 1975 and 55:999, 1975) about the changes in the regulation of homoserine dehydrogenase during growth of maize, we attempt to give an extension of these ideas applied to the behavior of a maize seedling growing in an inhibitory LT medium. This extension is based on features of the spatial variation of sensitivity of the homoserine dehydrogenase to threonine inhibition in roots, as well as spatial and temporal variation in shoots. Since lysine alone did not have visible inhibitory effects on root and shoot growths, this extension can be applied to the conjunct LT inhibition:
As we can see in the figure, the root zone would be much more affected than the shoot zone by LT inhibition. The maximum inhibition is located in the apical root zone, and the lowest in the basal root zone. Thus, the lengthening of roots would be strikingly inhibited while the weight increase of roots would be less inhibited.
The cumulative LT inhibition is certainly the external cause of the lack of embryo growth. But we suggest that threonine alone is responsible for the unlike root-shoot behavior under LT inhibition, despite the aforementioned translocating problems. The unlike root-shoot behavior also appears in fl1-a seedlings (see Table 1 in this MNL, above), and we think that it occurs the same way. This fact does not invalidate the supposition of differences in homoserine dehydrogenase, since such enzyme probably differs more in the high-low sensitivity than in the whole sensitivity-insensitivity to the threonine effect.
The use of slight threonine root inhibition for isolating possible overproducer lysine-threonine-methionine individuals is very attractive but very difficult to put into practice. The degree of difference between root inhibition and no inhibition is low. So, this method could never replace the LT selection method, but it would be very important for isolating individuals with homoserine dehydrogenase less sensitive to threonine.
Miguel Angel Rapela
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