Response of the proline requiring mutants to D-proline

The proline requiring mutants (pro) of Zea mays, identifiable on the basis of their abnormal endosperm morphology, are seedling-lethal monogenic recessive mutants. Their growth is resumed if L-proline is added to the medium; other amino acids as well as proline precursors do not restore mutant growth (Gavazzi et al., Theor. Appl. Gen. 46:339-345, 1975 and Racchi et al., Plant Sci. Lett. 13:357-364, 1978). All mutants so far isolated appear functionally allelic when crossed inter se.

Little is known about the metabolism of "non protein" amino acids in plants (Robinson, Life Science 19:1097-1102, 1976). D-alanine is the D-isomer most frequently found in higher plants, followed by D-tryptophan and a few others, while the presence and transformation of D-proline (as far as I know) has not yet been reported.

Here results are presented on the growth of pro mutant embryos in presence of D-proline. A partial phenotypic repair is observed (Fig. 1) for all the pro alleles, except for pro1-1121, on media enriched with 2mM D-proline, while at a higher concentration (4mM D-proline) there is a complete repair. Non-mutant control siblings are not affected in their growth by the D-proline addition to the minimal media. This observation is in contrast to the inhibitory effect previously reported (Robinson, 1976) for other D-amino acids in higher plants. D-isomers of nearly half the protein amino acids, when fed to plants, are converted to N-malonyl derivatives, to the corresponding alpha-ketoacids and to the L-isomer. Two different routes could be postulated for the conversion to the L-isomer: a direct racemization or a specific oxidation followed by a transamination of the resulting alpha-ketoacid. The only report about a possible direct racemization is that of Miura and Mills (Plant Physiol. 47:483-487, 1971) on D-tryptophan in cell cultures of tobacco.

The observation that pro mutants are repaired when fed with D-proline could be explained by assuming the presence of a proline racemase which converts D-proline directly to the L-isomer. The other possible route, through the corresponding alpha-ketoacid, is incompatible with the observations. In this respect proline requiring mutants might be of value in elucidating this area of amino acid metabolism.

Figure 1.

Chiara Tonelli

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