Putative Mutator-induced a2 mutants with colored scutellum

From our 1979 a2 bm bt purple Mutator plot, we isolated 3 stable pale purple a2 mutant kernels (a2-Mus1, a2-Mus2, and a2-Mus3). We extracted the homozygous a2-Mus mutants, crossed these mutants to purple Mutator lines, and selfed the F1 to see whether somatic mutability could be induced in the a2-Mus alleles. We did not recover somatic mutability, but we did observe the segregation of pale purple kernels with full purple scutella. We checked the pedigree, and found that the purple Mutator lines which the mutants were crossed into carried the R-scm2 -allele. In our 1987 crossing block, we planted pale purple kernels a2-Mus aleurone phenotype) with purple scutella from segregating selfed ears, and selfed and outcrossed the resulting plants to a2 testers a2 bt). For all 3 a2-Mus mutants, selfed ears were recovered that were homozygous a2-Mus, and either homozygous or segregating (3:1 purple:colorless) for scutellum color (two ears from our a2-Mus3 selfs segregated for both the a2 mutant and scutellum color, and were presumably the result of heterofertilization events, but other selfed ears of this mutant were homozygous a2-Mus.) For a2-Mus2 and a2-Mus3, the outcrosses to a2 testers produced ears with all pale purple kernels, and either all or half of the kernels had purple scutella. No crosses of a2-Musl to a2 testers set seed.

These data suggest that the A2 locus may have separate components responsible for color in the aleurone and in the scutellum. Chen et al. (Genetics 116:469) have described an organ-specific Mutator-induced Adh1 mutant that has a Mu insert in the promoter region of the Adh1 gene. This appears to be an example of a gene that has a compound regulatory region, with sequences in the promoter that are required for the expression of the gene in certain tissues. Perhaps A2 is regulated in an analogous manner.

Alternatively, the A2 coding region could contain tissuespecific components for scutellum and aleurone color. A Mu insert in one of these components could disrupt transcription or tissue-specific splicing of RNA transcripts in one tissue, but not the other. Molecular analysis of the a2-Mus mutants could provide valuable insight into the regulation and expression of the A2 gene, and perhaps other genes, in maize.

We are moving our a2-Mus mutants into a B Pl background to observe whether purple plant color is expressed in homozygous a2-Mus plants. If the a2-Mus mutants behave as do previously reported a2 mutants, these homozygous a2-Mus plants should be brown, with some tissue deterioration. If the putative scutellum color component also allows the expression of plant color, then purple plants will be observed.

Not all Mutator-induced a2 mutants show scutellum color in the presence of R-scm2 as do the 3 a2 -Mus mutants described above. The only other Mutator-induced a2 mutant we have in an R-scm2 background, a2-Mum3 (a mutable allele), has colorless scutellum. Furthermore, it is possible that the 3 a2-Mus alleles described above are the result of the same mutational event, since the purple Mutator parent was used as a male in the isolation plot in which the 3 a2-Mus mutants were isolated, and we have no way of telling whether these mutants arose independently of each other, or whether they arose from a tassel sector. Thus it is possible that a2 mutations that express scutellum color are a very rare event. We would be happy to share our a2-Mus mutants with anyone wishing to work with them.

Philip S. Stinard and Donald S. Robertson


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