Unstable pigmenting factors

A few years ago Dr. Charles Burnham sent us some seed from plants that were sectoring for a dark orange pigment in the husk, cob, and pericarp. In a covering letter he happened to mention also that his a3 stocks were 'quite variable.'

The orange pigment seems similar if not identical to the P-controlled cob and pericarp pigments that we call 'phlobaphenes.' The pigment can be produced in most plant parts, including sheaths, tassels, glumes, etc., but plants that are heavily pigmented are definitely unhappy. Some families derived from crosses segregate albino, dark purple, and orange-purple seedlings that die at an early stage. At least one family segregates for P-WR, P-WW and also the phlobaphene factor, so it is probably independent of the P locus, although P locus activity may be required for its expression. The homozygote may be lethal, at least in some stocks. The best and most uniform expression is in plants heterozygous for the phlobaphene factor, and also heterozygous Sm/sm. These plants are uniformly orange when mature, with orange anthers, tassels, stems, ligules, leaves and silks.

We obtained our a3 seed from Dr. Ed Coe, and like Dr. Burnham, we have found 'variability.' 'Instability' would definitely be a better word to describe the changes we are recording. The variability seems not to derive directly from the a3 locus, however, after several years of testing and retesting, the following patterns of change seem to be emerging:

The seeds that Dr. Coe provided us were from self-pollinated a3 R-g B-b/K55 A3 r-g b plants. Some 40 different a3 r-g lines have been obtained from these seeds, and for four generations we have never recorded any unexpected plant types, and either from crosses to other stocks, or from self-pollinations. In contrast, lines derived from a3 R-g/- plants not infrequently show anthocyanin sectors of several types, the most common being in the leaf blades. We now have 12 lines, of independent origin in the sense of not being closely related, that have a finely sectored, almost diffuse leaf blade pigment, the factor responsible being inherited together with the R-g allele. The leaf blade pigment does not require a3 or B-b for its expression.

Another type of sectoring occurs in R-g/- Pl/- plants. The pericarps show occasional 'cherry'-type sectors, and in one such ear the sector included a complete kernel. The plant grown from this one seed was uniformly purple in all tissues except anthers and silks. Progeny from this plant have ranged from uniform purple, to purple with fine green sectors, to green with fine purple sectors. Again the factor responsible for the pigmentation is inherited with the R-g allele.

A third type of change is associated with the anthers. Anthers become pigmented in some R-g/- plants. This is a progressive type of change, starting sometimes as a pale 'sun-red' type of pigment and sometimes as fine longitudinal sectors on the anthers only. Seed from these plants give rise to plants with a range of anther colors, from green to full red. The full red anthered plants can in turn have progeny with anthers that range from full red to a deep mahogany, and if a plant color factor is present, the plant pigment ranges into the mahogany also.

One a3 R-g/r-g line, derived from self-pollinating an a3 R-g/W22 a r-g plant, gave a very unusual spectrum of phenotypes. Some plants were uniformly purple, some were green, and some were initially green, and remained green in the lower stem and leaves, but became progressively more purple in the upper parts of the plant so that the tassel and anthers were fully purple. On further analysis we found that the R-g allele had associated with it a leaf blade factor, and that the factor responsible for the progressively appearing pigment was inherited with the r allele. As the pigment changes include the anthers, this r allele is now effectively an r-r allele. We have grown three successive generations of plants derived from this family, and the following pattern of change seems to be fairly predictable: Green-anthered 'r-g' plants when selfed give rise to plants with a range of anther colors from green to pale to red, and the associated plant pigments range similarly from green to purple. Some plants change developmentally from green to purple. Progeny from this latter class are all purple. Progeny from purple plants range from purple to dark mahogany with some plants changing developmentally from purple to mahogany. Mahogany plants have only mahogany progeny. These pigments seem not to require either a3 or B-b, but Pl will enhance the pigments and will turn the mahogany to a bronze-necrotic. It seems that we are recording a type of change that can be registered both somatically and germinally. Dare we mention the word 'paramutation?'

E. D. Styles


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