Class c n n n n mutants in whorls 1 and 2: sepals (green) become carpels (blue) and petals (orange) become stamens (red). Class B mutations cause transformation of whorls 2 and 3: petals become sepals and stamens become carpels. In class C mutations, whorls 3 and 4 are transformed: stamens become petals and carpels become sepals. [See D. Wiegel and E. M. Meyerowitz, 1994, Cell 78:203.]

cloned these genes and assessed the expression patterns of their mRNAs in the four whorls in wild-type Arabidopsis plants and in loss-of-function mutants (Figure 15-28a, b). Consistent with the ABC model, A genes are expressed in whorls 1 and 2, B genes in whorls 2 and 3, and C genes in whorls 3 and 4. Furthermore, in class A mutants, class C genes are also expressed in organ primordia of whorls 1 and 2; similarly, in class C mutants, class A genes are also expressed in whorls 3 and 4. These findings are consistent with the homeotic transformations observed in these mutants.

To test whether these patterns of expression are functionally important, scientists produced transgenic Arabid-opsis plants in which floral organ-identity genes were expressed in inappropriate whorls. For instance, the introduction of a transgene carrying class B genes linked to an A-

class promoter leads to the ubiquitous expression of class B genes in all whorls (Figure 15-28c). In such transgenics, whorl 1, now under the control of class A and B genes, develops into petals instead of sepals; likewise, whorl 4, under the control of both class B and class C genes, gives rise to stamens instead of carpels. These results support the functional importance of the ABC model for specifying floral identity.

Sequencing of floral organ-identity genes has revealed that many encode proteins belonging to the MADS family of transcription factors, which form homo- and hetero-dimers. Thus floral-organ identity may be specified by a combinatorial mechanism in which differences in the activities of different homo- and heterodimeric forms of various A, B, and C proteins regulate the expression of subordinate downstream genes necessary for the formation of the differ-

(b) Loss of function W1 W2 W3 W4

(b) Loss of function W1 W2 W3 W4

ca st st ca

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