In recent years evidence has been collected which suggests that in higher plants a part of the genome is expressed in both phases of the life cycle. However, to establish the biological significance of this phenomenon, it is important to determine what proportion of the genome is shared by the sporophytic and gametophytic generations.

The expression of genes controlling dimeric (or multimeric) enzymes can be detected by comparing the electrophoretic patterns of pollen and sporophyte from heterozygous F/S plants: three bands are expected in the sporophyte, only two in the gametophyte in the case of haplo-diploid determination; the same pattern if the enzyme is of diploid origin; different banding when different genes are expressed in pollen and in diploid tissues. The possibility that causes other than haploid transcription may be responsible for these results can be ruled out by including pollen with heterozygous gene duplication in the comparison of electrophoretic pattern. This pollen type can be obtained by using B-A translocations (Sari Gorla et al., MNL 57:94-95). This method, tested on ADH-1, allowed us to establish the haplo-diploid expression of GOT-1, while beta-GLU proved to be controlled by different genes in pollen and sporophyte.

Subsequently, PHI was studied by means of this system. It, too, revealed haplo-diploid expression for the gene: two bands for normal pollen from F/S plants, three bands, as in the sporophytic tissues, for hyperploid pollen.

Evidence of the same type of expression has been obtained with regard to GDH, which is probably an esameric enzyme.

MDH expression was studied by means of the IEF technique. In fact in maize at least five forms of this enzyme exist, which form heterodimers between allelic and non-allelic isozymes within the same subcellular location (mitochondria and cytosol). Thus the resulting electrophoretic pattern is quite complex and difficult to analyze by means of the described method. The pollen-sporophyte comparison of F1 plants derived from two inbreds showing different IEF patterns for only one band, revealed three bands in the sporophyte and two in the pollen. This is an indication that at least one MDH form is expressed in the gametophytic phase, and it should be MDH-4 (Goodman classification), since it is a soluble enzyme and the two parental lines showed different electrophoretic variants only for this isozyme.

UDPGpp was considered, also because of its possible significance in pollen biology. We screened a very large number of genotypes (124 inbreds, 8 popcorn, 22 samples from Argentine and ten from Italian populations, about twenty defective kernel mutants in different background) without finding any variant form. It seems that there is no "permissible" variability of the gene coding this enzyme, which is evidently active in both pollen and sporophyte.

M. Sari Gorla, C. Frova, G. Binelli and E. Ottaviano
 
 

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