Both R and Sn control the distribution of anthocyanins in plant and seed tissues. However, they differ in regard to their map position (Sn lying 2cM distal to R) as well as in their tissue-specific expression. In fact while some tissues such as anthers and roots represent a common domain of expression of both R and Sn, others like scutellar node, mesocotyl, leaf base, midrib, seed glumes and pericarp are exclusive of Sn expression.
Several Sn geographic alleles have been studied. One of them, symbolized Sn:bol3, differs from the others in its potential for pigment accumulation in the seedling tissues (at least five times greater than in others) and in its light independent pigmentation of seed integuments. Sn:bol3 is unstable giving rise to a series of derivatives with a lower level of pigmentation.
The molecular cloning of the Sn gene is of particular interest in order to answer specific questions concerning its structure and the regulation of its expression. Since the product of the gene is unknown, an indirect approach to cloning Sn is necessary. Therefore the approach we adopted is based on the idea of a molecular similarity between Sn and R, cloned by S. Dellaporta. A 600 bp HincII - BGlII fragment (pR-nj:1) isolated from the R-nj allele crosshybridizes to Sn:bol3. SstI-digested DNA from plants homozygous for r Sn:bol3 was analyzed by Southern blot. pR-nj: 1 DNA hybridizes to three SstI fragments of Sn:bol3. This pattern is similar to the hybridization pattern observed with R-r in which molecular analysis discloses the presence of three components. If a similar structure exists for Sn it is possible that these two genes are related in the sense that both derived from a common progenitor containing a triplicate structure. This finding should open the way to an analysis of the basis of the tissue specificity of Sn, its response to light, and the molecular events leading to Sn:bol3 instability.
C. Tonelli, G. Consonni, G. Gavazzi and S. Dellaporta1
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