VICTORIA, BRITISH COLUMBIA, CANADA

University of Victoria
 
 

The R locus - regulator and regulated

--E. Derek Styles

Phenotypically at least, the R locus controls the concentration, tissue specificity, and the pattern of distribution within a tissue of anthocyanins and related 3-hydroxy flavonoids. This is demonstrated most conclusively when different R alleles are compared against a common genetic background. In this sense the R locus acts as a regulatory locus, and whether or not the locus is eventually assigned an enzymatic role, this regulatory function will still need to be explained. Comparisons against common genetic backgrounds have built-in limitations, however, because a specific genetic background may lack factors that allow certain allelic differences to be expressed. Obviously the genetic backgrounds against which different R alleles are compared need to carry the so-called complementary factors known to be required for r locus expression. Factors such as C2; C (if aleurone color is being studied); A1, A2, Bz1, Bz2 (if anthocyanins are used as a phenotype) are usually considered essential to study R locus expression. Some factors, such as Pr, probably modify the substrate for the R locus, others (A1, Bz1) modify the product. Factors that enhance the expression of R controlled pigments (in, Pl, a3) are not so regularly incorporated in such comparison studies, though in theory at least, they should be required for maximum expression of the R locus potential, and, as with Pl and the 'cherry' alleles, they may also be essential in uncovering R allele differences.

We can report on a specific example of how the expression of a R allele is affected by both known and unknown factors in the genetic background. As with many of our studies that have turned out to be interesting, we did not plan on doing a detailed study on this particular R allele, and much of what we can report at this time comes from a retrospective study of our records. The R allele in question was carried in a 'demonstration' ear kept mostly because it had mosaic pericarp. Beyond this the ear has no pedigree. We were interested in the R allele because in the backcross generation to r-g it appeared to determine a very strong silk pigmentation. We initially classified the allele as 'R-g' because it determined green anthers and colored aleurone. It also conditioned coleoptile and seedling root pigment, and thus grouped with Stadler's 'Group D' R alleles. We have not backcrossed this allele into an inbred line such as W22, but rather we have tried to identify the factors that modify (regulate?) its expression independently of other genetic background effects.

Although initially characterized as R-g this R allele consistently determines red anthers with Pl. With a3 and Pl the anthers range into purple, and a3 plants show the instability typically of Group D R alleles in the presence of a3, i.e., tissues that can range from green with small purple sectors to uniform diffuse purple. We have recently been trying to characterize a factor tentatively named 'Pth', that with Group D R alleles, a3, and Pl determines a strong purple cob pith pigment. The R allele from the 'demonstration' ear does not require a3 to express the pith pigment with Pth and Pl, but when a3 is present together with Pth and Pl, the whole cob is purple, pith, pedicels, pericarp - the works! The one characteristic that we were originally interested in, the silk pigment, shows variability, presumably due to genetic background differences, but so far we have not been able to isolate specific factors that will assure a good silk color expression.

In summary, we have isolated an R allele that with pl is a typical Group D R-g allele. As with other R alleles of this group, with a3 the plant phenotypes can range from green with small purple sectors to uniform diffuse purple. With Pl both A3 and a3 plants have red anthers, and a3 Pl plants have the potential to produce strong purple anthers. With a3, Pl, and a factor we have tentatively termed Pth, all parts of the plant can be purple. If the R locus is a 'regulatory' locus, clearly it, or at least some allelic forms of it, can also be regulated. The allele described above has the potential to determine pigment in all parts of the plant. Yet in a W22 background, with pl and A3, it would have been characterized as 'R-g', with the inherent assumption that it determined (regulated?) the characteristic tissue specificities associated with R-g alleles, namely, colored aleurone but green anthers and plant. It is worth noting that Group D R-g alleles are paramutable, i.e., if maintained heterozygous with a paramutagenic allele such as R-st, the aleurone pigmenting potential can be markedly and progressively reduced to the point that even some R R R endosperms may be colorless or near colorless. Thus can the biter become bitten, the regulator become regulated!


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