Update on recessive brown seed mutant (brn)

In 1984 (MGNL58:18), a putative Mu-induced, seedling lethal, recessive brown aleurone mutant was described. The scutella of mutant kernels are also brown. This past summer, crosses were made to characterize this mutant further and to locate it to chromosome arm.

A series of B-A translocations was crossed onto outcross progeny of plants heterozygous for brn, and it was found that TB-3Sb uncovers the brown kernel locus. The kernels with hypoploid endosperm were quite small or defective. Since crosses of TB-3Sb onto sibling plants not segregating for brn did not produce small, defective kernels, and since the brown kernels of selfed brn heterozygotes are completely filled, the presence of defective brown kernels in TB-3Sb crosses may indicate a deleterious hemizygous effect of brn in the endosperm. The scutella of these defective kernels were also brown, suggesting that the pigmentation in the scutellum of hemizygous brn-endospermed kernels might be the result of diffusion of the brown pigment from the aleurone and/or endosperm into the scutellum. From the same TB-3Sb crosses, some yellow germless or nearly germless kernels were observed. The latter seeds had very defective brown embryos. These kernels may be those that are hemizygous for brn in the embryo, again suggesting a possible deleterious hemizygous effect. It would also appear that at least some synthesis of brown pigment in the embryo is possible. Very few germless or defective kernels were observed on the selfed second ears from these same plants.

The results from the TB-3Sb cross suggest that the brown pigment may be produced in all of the endosperm tissue. In brn seeds the aleurone is dark brown, however, the endosperm is also brown but not as darkly pigmented as the aleurone. It could be that the aleurone is the source of all pigment production, which diffuses through the endosperm to the embryo. It is also possible, however, that all endosperm cells produce the pigment, and that the different intensity of pigmentation in the aleurone and endosperm is due to the difference in cellular morphology (e.g., aleurone with small cells capable of being intensely pigmented vs. the large endosperm cells gorged with starch granules). If all the endosperm cells are capable of synthesizing this pigment, this mutant probably should be renamed brown endosperm.

Crosses to determine linkage to genes in the short arm of chromosome 3 will be made this winter and next summer.

Yellow kernels from a selfed ear segregating for brn were grown in 1983, and the plants were selfed and outcrossed to purple aleurone stocks. The progeny plants of crosses with known heterozygote brn plants were again selfed. These selfed ears segregated purple mottled, yellow, and brown kernels. Some of the purple mottled seeds were also brown. This result would seem to indicate that the brn gene is not involved in the anthocyanin biosynthetic pathway.

Crosses of brn to a y stock will be made this winter in order to explore the relationship of brn to the biosynthesis of carotenoid pigments.

We would be happy to supply seeds to anyone interested in studying the biochemistry of this mutant, whether it be with regard to the nature of the pigment involved or the possibility that it is an auxotrophic mutant.

Philip S. Stinard


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