Loss of deeply conserved C-class floral homeotic gene function and C- and E-class protein interaction in a double-flowered ranunculid mutant

Abstract

In the model plant Arabidopsis thaliana, a core eudicot, the floral homeotic C-class gene AGAMOUS (AG) has a dual role specifying reproductive organ identity and floral meristem determinacy. We conduct a functional analysis of the putative AG ortholog ThtAG1 from the ranunculid Thalictrum thalictroides, a representative of the sister lineage to all other eudicots. Down-regulation of ThtAG1 by virus-induced gene silencing resulted in homeotic conversion of stamens and carpels into sepaloid organs and loss of flower determinacy. Moreover, flowers exhibiting strong silencing of ThtAG1 phenocopied the double-flower ornamental cultivar T. thalictroides 'Double White.' Molecular analysis of 'Double White' ThtAG1 alleles revealed the insertion of a retrotransposon causing either nonsense-mediated decay of transcripts or alternative splicing that results in mutant proteins with K-domain deletions. Biochemical analysis demonstrated that the mutation abolishes protein-protein interactions with the putative E-class protein ThtSEP3. C-and E-class protein heterodimerization is predicted by the floral quartet model, but evidence for the functional importance of this interaction is scarce outside the core eudicots. Our findings therefore corroborate the importance and conservation of the interactions between C-and E-class proteins. This study provides a functional description of a full C-class mutant in a noncore ("basal") eudicot, an ornamental double flower, affecting both organ identity and meristem determinacy. Using complementary forward and reverse genetic approaches, this study demonstrates deep conservation of the dual C-class gene function and of the interactions between C-and E-class proteins predicted by the floral quartet model. floral organ identity genes | MADS-box genes | solo long terminal repeats | RNA silencing C urrent understanding of floral patterning has emerged primarily from studies in the core eudicot model plants Arabidopsis thaliana and Antirrhinum majus. In these species, the genetic ABCE model predicts how combinatorial expression of four classes of transcription factors specifies organ identity in the floral meristem (1-4). According to the latest Arabidopsis model, which incorporates the role of the E-class proteins, once flowering has initiated, A-and E-class proteins specify sepals; A-, B-, and E-class proteins specify petals; B-, C-, and E-class proteins specify stamens; and C-and E-class proteins specify carpels and terminate floral meristem development (2, 5, 6). The underlying biochemical mechanism for specifying organ identity has been described by the floral quartet model, which predicts that correct transcription of organ-specific genetic programs requires the formation of hetero-multimeric complexes between these four interacting classes of transcription factors (5,(7)(8)(9). Mutations affecting the class-A, -B, -C, and -E functions are homeotic, resulting in the replacement of one organ type by another. Loss of expression of the Arabidopsis C-class gene AGAMOUS (AG) results in conversi...

Ranunculacean flower terata: Records, a classification, and some clues about floral developmental genetics and evolution

“…In Thalictrum thalictroides, the cultivar "Double White" is sterile, all floral organs being replaced with white petaloid sepals. In agreement with the ABC model, the expression of the C function ThtAG1 gene is reduced, associated with structural defects in the genomic region, which entail the production of abnormal mRNAs and truncated proteins unable to interact with proteins encoded by the putative E-function gene ThtSEP3 (Galimba et al, 2012). In Nigella damascena, a widespread floral mutant coexists with the normal type in gardens and flowery meadows.…”

“…Petaloid stamens were also observed in Actaea spicata (tribe Cimicifugeae, subfamily Ranunculoideae), in Anemonella thalictroides (formerly Thalictrum anemonoides) (Penzig, 1890), in Caltha palustris (tribe Caltheae, subfamily Ranunculoideae) in which sexual organs can be completely replaced by petals in horticultural forms named double marsh marigolds (Smith, 1928;Wijnands, 1993). In Thalictrum thalictroides, all organs can be replaced by petaloid sepals, and such variants are used in horticulture (Galimba et al, 2012). Flowers in which the involucral bracts become proliferous to the exclusion of all the other parts of the flower have been found in some of the cultivated double varieties of Nigella damascena (Masters, 1869).In Xanthorhiza simplicissima (formerly X. apiifolia, subfamily Coptidoideae), flowers can abort or become unisexual flowers, with reduction in stamen and carpel number (Penzig, 1890).…”

Mapping and allelic sequencing of a long sterile lemma trait in rice

“…Then, FBP7 and FBP11 genes, which regulated the identity and development of an ovule in petunia, were cloned in 1995 (Angenent et al 1995;Colombo et al 1995) and designated to an additional class D MADS box gene. Furthermore, the SEP1/2/3 genes were found as necessary for the development of petals, stamens, and carpels in Arabidopsis and were identified as a new class gene (E-class gene) of the floral quartet model (Pelaz et al 2000;Honma and Goto 2001;Galimba et al 2012). The findings of D-class and E-class enriched the ABC model and made the classical one extending to the ABCDE model.…”

Thermal Control Suitable for Increasing Petals in <i>Eustoma grandiflorum</i> (Raf.) Shinn.

“…Loss of expression of the Arabidopsis C-class gene AGAMOUS (AG) results in the conversion of stamens and carpels to petals and sepals, respectively and indeterminacy of the floral meristem, leading to double flowers with excess petals. The same mechanism as that of Arabidopsis can be adopted in some floricultural plants such as Ipomea nil, Gentiana scabra, and Thalictrum thalictroides (Galimba et al, 2012;Nakatsuka et al, 2015;Nitasaka, 2003). In the case of the double-flowered Eustoma, several theories have been postulated about the genetic mechanism.…”