The significance of the silk attachment region in the expression of certain R alleles
--B. M. Prasanna and K. R. Sarkar

The pigmentation patterns of R-st, R-mb and R-nj are attractive from a developmental viewpoint, as the anthocyanins are distributed specifically in the outermost differentiated aleurone layer of the endosperm. We analyzed the onset and progression of anthocyanin pigmentation in R-st, R-mb, R-nj:Illinois, standard R-r, R-sc (ex. R-mb) and R-mb/R-nj in the summer seasons at New Delhi from 1989 to 1992. All these alleles were in the same stock background.

The pattern alleles exhibited diversity in the onset and mode of pigment progression. Pigments could be visually detected on the 11th day after pollination (DAP) in the aleurones of R-st, R-mb, standard R-r and R-sc (ex. R-mb), while R-nj recorded a conspicuous delay (18th DAP). R-st and R-mb pattern formation reflected the systematic (clonal) development of the aleurone. In R-st, the colored spots mostly showed groups of pigmented cells in squares (4 or 16 cells) or rectangles (2, 8 or 32 cells), confirming the observations by Coe (in Maize Breeding and Genetics, ed. D. B. Walden, pp. 447-459, 1978) that divisions during clonal development of aleurone occur in alternating perpendicular planes. The colored spots in R-mb showed a large number of uniformly dark cells with clearly defined borders. The R-nj allele revealed a very unique mode of pigmentation. We observed that in R-nj:Illinois, pigmentation started from the tip of the immature kernel, but only from a specific site, the silk attachment region. Anthocyanin pigments diffused gradually from this site in a typical sun ray-like manner towards the periphery of the crown. Similarly, in both standard R-r and R-sc, pigmentation occurred first in the cells surrounding the silk attachment region, but progressed in a wave-like manner.

The differences in the pattern formation of R-mb and R-nj, representing clonal and aclonal systems, could be clearly visualized in the R-mb/R-nj heterozygotes. When R-mb was used as a female parent in crosses with R-nj, kernels with full action of both the alleles, that is, a typical Navajo pattern along with marbled sectors in the aleurone, could be obtained. In kernels where both R-mb and R-nj expressed, the marbled spots appeared first on the 11th DAP and intensified by the 18th DAP, while the Navajo coloration just started from the silk attachment region on the 18th DAP.

Our study indicated that the Navajo pattern might not be solely due to the delayed onset, as was evident by the clearcut differences in the manner of pigment expression in R-nj and other R alleles. The important questions are - How are only the cells around the silk attachment region endowed with pigment-producing potential in the Navajo aleurone, although all the aleurone cells have the same genetic constitution? Why does diffusion-based pigmentation occur only in R-nj and not in standard R-r? It would be tempting to predict that for the pigmentation to start only from the silk attachment region, a signalling source might be responsible. However, when the R gene is under the control of a transposable element (as in R-st or R-mb), the response might be lacking, leading to the appearance of pigmented clones anywhere on the kernel. At present, we know little about how the activity of such signalling sources is controlled and the molecular nature of the 'regulators' in R-nj, R-st and R-mb. Wilson (in Maize Breeding and Genetics, ed. D. B. Walden, pp. 405-419, 1978) noted that important metabolic events like starch synthesis, protein body formation and transport start from the silk attachment region of the kernel. It is apparent that this site has some significance in the anthocyanin pigmentation of certain R alleles, as in other important cellular processes in the developing endosperm. 

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