The Myb-domain proteins P and C1 control the accumulation of the phlobaphene pigments in the pericarp and cob glumes, and of the anthocyanin pigments in the aleurone, respectively. Recent studies have shown that P and C1 bind to identical sites in the promoter of the a1 flavonoid biosynthetic gene. The a1 promoter has a modular structure with proximal haPBS (high affinity P binding sites) and more distal laPBS (low affinity P binding sites) (Sainz et al, Plant Cell 9:611-625, 1997). The a1-mum2 allele harbors a Mu1 insertion in between these two PBS, in an element that has been found to be conserved in the promoters of other flavonoid biosynthetic genes (Tuerck, J.A. and Fromm, M.E., Plant Cell 6:1655-1663, 1994; Lesnick, M.L. and Chandler, V.L., Plant Physiol. 117:437-445, 1998). In the presence of Mutator activity, a1-mum2 alleles provide densely spotted aleurones on a colorless background (Fig. 1). To investigate the pattern of pericarp variegation provided by the excision of Mutator, a1-mum2 lines were crossed to P-rr stocks. In the absence of a1 gene function, P-rr pericarps accumulate a characteristic brown pigment. Seeds derived from plants of the genotype P-rr/P-wr a1-mum2/a1 were investigated after self pollination or after crossing to a1/a1 pollen (26A:4-5 x 13-8) for aleurone spotting and pericarp variegation. While aleurone spotting is evident in most kernels (Fig. 1A, B), the expected red pericarp variegated sectors are rare, usually very fine and are present at a much lower frequency than the spots in the aleurone (Fig. 1A, B). The high spotting frequency in the aleurone, and the presence of a large number of spotted aleurones in each ear are indicative of the presence of Mutator activity during ear development. The low frequency at which red revertant somatic sectors appear in the pericarp could indicate either that Mutator does not transpose with high frequency in the pericarp, or that the cis-acting regulatory elements in which Mu1 is inserted in the a1-mum2 allele are much more important for the regulation of a1 by P than by C1. Understanding the reason for the low frequency of variegation in the pericarp may expose a property of Mutator unexpected from previous studies, or reveal a fundamental difference in the way the P and C1 Myb-domain transcription factors control the expression of a gene common to the two main branches of maize flavonoid biosynthesis.
1. Pericarp and aleurone pigmentation in the a1-mum2
allele. A) Kernels derived from selfing P-rr/P-wr a1-mum2/a1
plants (26A:4-6@) were selected to illustrate the variety of red revertant
sectors found in the pericarp (arrows indicate the width of sectors). B)
A sector of an ear derived from the cross P-rr/P-wr a1-mum2/a1 x
a1/a1 (26A:4-5 x 13-8) shows the low frequency of red variegated
pericarps. Arrows indicate two such sectors.
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