A MITE insertion abolishes the AP3-3 self-maintenance regulatory loop in apetalous flowers of Nigella damascena


 MADS-box transcription factors are important regulators of floral organ identity through their binding to specific motifs, termed CArG, in the promoter of their target genes. Petal initiation and development depend on class A and B genes, but MADS-box genes of the AP3 clade are key regulators of this process. In the early diverging eudicot Nigella damascena, an apetalous [T] morph is characterized by the lack of expression of the NdAP3-3 gene, the latter expression being petal-specific in the wild-type [P] morph. All [T] morph plants are homozygous for an NdAP3-3 allele with a MITE insertion in the second intron of the gene. Here, we investigate to which extent the MITE insertion impairs the NdAP3-3 gene regulation. We found that expression of NdAP3-3 is initiated in the [T] morph, but the MITE insertion prevents its positive self-maintenance by affecting the correct splicing of the mRNA. We also found specific CArG features in the promoter of the NdAP3-3 genes with a petal-specific expression. However, they are not sufficient to drive an expression only in petals of transgenic Arabidopsis, highlighting the existence of Nigella specific cis/trans-acting factors in regulating AP3 paralogs.