Evolution and function of MADS‐box genes involved in orchid floral development

Orchids are known for their beauty and complexity of flower and ecological strategies. The evolution in orchid floral morphology, structure, and physiological properties has held the fascination of botanists for centuries, from Darwin through to the present. In floral studies, MADS‐box genes contributing to the now famous ABCDE model of floral organ identity control have dominated conceptual thinking. The sophisticated orchid floral organization offers an opportunity to discover new variant genes and different levels of complexity to the ABCDE model. Recently, several remarkable research reports on orchid MADS‐box genes, especially B‐class MADS‐box genes, have revealed the evolutionary track and important functions on orchid floral development. Diversification and fixation of both paleoAP3 gene sequences and expression profiles might be explained by subfunctionalization and even neofunctionalization. Knowledge about MADS‐box genes encoding ABCDE functions in orchids will give insights into the highly evolved floral morphogenetic networks of orchids.     

The ABCs of flower development: mutational analysis of AP1/FUL‐like genes in rice provides evidence for a homeotic (A)‐function in grasses

The well‐known ABC model describes the combinatorial interaction of homeotic genes in specifying floral organ identities. While the B‐ and C‐functions are highly conserved throughout flowering plants and even in gymnosperms, the A‐function, which specifies the identity of perianth organs (sepals and petals in eudicots), remains controversial. One reason for this is that in most plants that have been investigated thus far, with Arabidopsis being a remarkable exception, one does not find recessive mutants in which the identity of both types of perianth organs is affected. Here we report a comprehensive mutational analysis of all four members of the AP1/FUL‐like subfamily of MADS‐box genes in rice (Oryza sativa). We demonstrate that OsMADS14 and OsMADS15, in addition to their function of specifying meristem identity, are also required to specify palea and lodicule identities. Because these two grass‐specific organs are very likely homologous to sepals and petals of eudicots, respectively, we conclude that there is a floral homeotic (A)‐function in rice as defined previously. Together with other recent findings, our data suggest that AP1/FUL‐like genes were independently recruited to fulfil the (A)‐function in grasses and some eudicots, even though other scenarios cannot be excluded and are discussed.     

Functional specialization of duplicated AP3‐like genes in Medicago truncatula

The B–class of MADS box genes has been studied in a wide range of plant species, but has remained largely uncharacterized in legumes. Here we investigate the evolutionary fate of the duplicated AP3‐like genes of a legume species. To obtain insight into the extent to which B‐class MADS box gene functions are conserved or have diversified in legumes, we isolated and characterized the two members of the AP3 lineage in Medicago truncatula: MtNMH7 and MtTM6 (euAP3 and paleoAP3 genes, respectively). A non‐overlapping and complementary expression pattern of both genes was observed in petals and stamens. MtTM6 was expressed predominantly in the outer cell layers of both floral organs, and MtNMH7 in the inner cell layers of petals and stamens. Functional analyses by reverse genetics approaches (RNAi and Tnt1 mutagenesis) showed that the contribution of MtNMH7 to petal identity is more important than that of MtTM6, whereas MtTM6 plays a more important role in stamen identity than its paralog MtNMH7. Our results suggest that the M. truncatula AP3‐like genes have undergone a functional specialization process associated with complete partitioning of gene expression patterns of the ancestral gene lineage. We provide information regarding the similarities and differences in petal and stamen development among core eudicots.     

CURVED CHIMERIC PALEA 1 encoding an EMF1‐like protein maintains epigenetic repression of OsMADS58 in rice palea development

Floral organ specification is controlled by various MADS‐box genes in both dicots and monocots, whose expression is often subjected to both genetic and epigenetic regulation in Arabidopsis thaliana. However, little information is known about the role of epigenetic modification of MADS‐box genes during rice flower development. Here, we report the characterization of a rice gene, CURVED CHIMERIC PALEA 1 (CCP1) that functions in palea development. Mutation in CCP1 resulted in abnormal palea with ectopic stigmatic tissues and other pleiotropic phenotypes. We found that OsMADS58, a C‐class gene responsible for carpel morphogenesis, was ectopically expressed in the ccp1 palea, indicating that the ccp1 palea was misspecified and partially acquired carpel‐like identity. Constitutive expression of OsMADS58 in the wild‐type rice plants caused morphological abnormality of palea similar to that of ccp1, whereas OsMADS58 knockdown by RNAi in ccp1 could rescue the abnormal phenotype of mutant palea, suggesting that the repression of OsMADS58 expression by CCP1 is critical for palea development. Map‐based cloning revealed that CCP1 encodes a putative plant‐specific EMBRYONIC FLOWER1 (EMF1)‐like protein. Chromatin immunoprecipitation assay showed that the level of the H3K27me3 at the OsMADS58 locus was greatly reduced in ccp1 compared with that in the wild‐type. Taken together, our results show that CCP1 plays an important role in palea development through maintaining H3K27me3‐mediated epigenetic silence of the carpel identity‐specifying gene OsMADS58, shedding light on the epigenetic mechanism in floral organ development.