Interactions of P-pr with other P alleles
--O. Prem Das and Joachim Messing

Since P-pr is a hypermethylated derivative of P-rr, it was of interest to determine the phenotype of heterozygotes between the two. Therefore, P-pr/P-ww plants of the family #6601 (Fig. 1, preceding note) were crossed reciprocally to plants homozygous for P-rr-4026 (the parent of P-pr) in the W22 inbred background. Molecular analysis was used to distinguish P-rr/P-pr from P-rr/P-ww progeny plants in a small-scale greenhouse planting. All P-rr/P-ww plants showed full red pigmentation in pericarp, as expected from the dominant P-rr allele. Surprisingly, P-pr/P-rr plants showed variable patterned pigmentation in pericarp. Larger plantings of families from reciprocal crosses of families 6601, 6671, 6672 and 6673 (Fig. 1, preceding note) to P-rr-4026 gave similar results. When P-ww homozygotes from these families were used in such crosses, all progeny displayed uniform red pericarp color. Therefore, effects caused by the genetic background or by the P-ww allele were unlikely. In contrast, when plants carrying P-pr were crossed to P-rr, pericarp pigmentation was highly variable, ranging from fully red to color only at the silk attachment point. Striking patterns, and sectors with differing patterns, both of which are typical of P-pr, were also seen. Qualitatively, pigmentation was heavier than in P-pr/P-ww plants, and in P-pr/P-wr plants from crosses to W22.

These observations suggest that P-pr suppresses the normal expression of P-rr. The degree of suppression varies between progeny, and in sectors of an ear. In addition to P-rr, P-pr also suppresses pericarp pigmentation conditioned by three other P alleles, P-cw (white-cap pericarp, colorless cob), P-or (orange pericarp, red cob) and P-ow. For all three, suppression was variable as with P-rr, and control crosses to P-ww siblings showed no reduction or variation in pigmentation. However, crosses to inbred lines carrying the P-wr allele (W22, Mo17, B73 and BSSS53) show the typical variation in pericarp pigmentation expected from P-pr, and no reduction or variation in cob pigmentation. These properties of P-pr may resemble the phenomenon termed co-suppression observed for transgenes in tobacco and petunia (Matzke et al., EMBO J. 8:643, 1989; Napoli et al., Plant Cell 2:279, 1990; and van der Krol et al., Plant Cell 2:291, 1991). As in co-suppression, P-pr may suppress itself (note the lower pigmentation in P-pr/P-pr vs. P-pr/P-ww, preceding note), and related genes. Of particular interest is the possibility of a heritable change in P-rr as a result of interaction with P-pr. This can be determined either by changes in phenotype or by changes in DNA methylation in outcrossed progeny from a heterozygote. 


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