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B and C class MADS-box genes and the developmental genetics of maize flower development


The ABC model of flower development describes how a flower is patterned and the genes necessary for floral organ identity. However, it is not clear that the ABC model can be generally applied to the flowering plants, as it was based solely on genetic studies from the core eudicot species Arabidopsis and Antirrhinum. This dissertation describes an examination of maize orthologs of B and C class genes, and compares their function with B and C class genes of Arabidopsis to understand the degree to which the ABC model is conserved. B class genes from maize were found to rescue Arabidopsis B class mutants, and the maize B class proteins were shown to bind DNA as an obligate heterodimer as has been demonstrated in Arabidopsis. These findings indicate conservation in biochemical function of the maize and Arabidopsis B class proteins. Furthermore, these findings support the conclusion that the lodicule, a grass specific organ of uncertain homology, represents a modified petal. A comparative expression approach was used to further verify the relationship of lodicules to the organs of non-grass flowers. B class genes were shown to be expressed in a whorl of foliar organs outside the stamens in Streptochaeta, a basal grass that diverged before the evolution of lodicules, and in the petals of the outgroup species Joinvillea and Chondropetalum strongly supporting the interpretation that lodicules are modified petals, and further supporting conservation of B class function between Arabidopsis and maize. Zag1 and Zmm2 are duplicate pair of C class genes from maize that are hypothesized to have partitioned the C class function of establishing stamen and carpel identity. Rescue of the Arabidopsis C class mutant ag with the two maize genes confirms that their protein products have subfunctionalized, with ZAG1 better able to promote carpel identity, and ZMM2 better able to promote stamen identity. A more recent duplicate of Zmm2 was isolated, Zmm23, as were mutant alleles of zmm2 and zmm23. While the zmm2 zmm23 double mutant had no phenotype, the zag1 zmm2 zmm23 showed a considerable enhancement of the previously described zag1 phenotype substantiating a C class function for Zmm2 and Zmm23

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