Flower Development and Perianth Identity Candidate Genes in the Basal Angiosperm Aristolochia fimbriata (Piperales: Aristolochiaceae)

Abstract

Aristolochia fimbriata (Aristolochiaceae: Piperales) exhibits highly synorganized flowers with a single convoluted structure forming a petaloid perianth that surrounds the gynostemium, putatively formed by the congenital fusion between stamens and the upper portion of the carpels. Here we present the flower development and morphology of A. fimbriata, together with the expression of the key regulatory genes that participate in flower development, particularly those likely controlling perianth identity. A. fimbriata is a member of the magnoliids, and thus gene expression detected for all ABCE MADS-box genes in this taxon, can also help to elucidate patterns of gene expression prior the independent duplications of these genes in eudicots and monocots. Using both floral development and anatomy in combination with the isolation of MADS-box gene homologs, gene phylogenetic analyses and expression studies (both by reverse transcription PCR and in situ hybridization), we present hypotheses on floral organ identity genes involved in the formation of this bizarre flower. We found that most MADS-box genes were expressed in vegetative and reproductive tissues with the exception of AfimSEP2, AfimAGL6, and AfimSTK transcripts that are only found in flowers and capsules but are not detected in leaves. Two genes show ubiquitous expression; AfimFUL that is found in all floral organs at all developmental stages as well as in leaves and capsules, and AfimAG that has low expression in leaves and is found in all floral organs at all stages with a considerable reduction of expression in the limb of anthetic flowers. Our results indicate that expression of AfimFUL is indicative of pleiotropic roles and not of a perianth identity specific function. On the other hand, expression of B-class genes, AfimAP3 and AfimPI, suggests their conserved role in stamen identity and corroborates that the perianth is sepal and not petal-derived. Our data also postulates an AGL6 ortholog as a candidate gene for sepal identity in the Aristolochiaceae and provides testable hypothesis for a modified ABCE model in synorganized magnoliid flowers.

Floral MADS‐box protein interactions in the early diverging angiosperm Aristolochia fimbriata Cham. (Aristolochiaceae: Piperales)

“…For example, in Antirrhinum majus (Plantaginaceae), Medicago truncatula (Leguminosae), Eschscholzia californica (Papaveraceae), and Aquilegia coerulea (Ranunculaceae), APETALA1/FRUIT-FULL homologs play roles in floral meristem alone, sometimes sepal identity, branching, leaf morphogenesis, and even fruit development (Benlloch et al, 2006;Huijser et al, 1992; Pabón-Mora et al, 2012; Pabón-Mora, Sharma, Holappa, Kramer, & Litt, 2013). However, in the early divergent angiosperm Aristolochia fimbriata (Aristolochiaceae), the orchids Phalaenopsis, Aphrodite, and Oncidium Gower Ramsey, and in the non-core eudicot Nigella damascena (Ranunculaceae), AGL6 orthologs are expressed specifically in the perianth (Hsu et al, 2015;Pabón-Mora, Suárez-Baron, Ambrose, & González, 2015;Su et al, 2013), and control the identity of sepals and petals (Wang et al, 2015), which has positioned them into the model as the non-core eudicot putative A-function genes. Redundancy between AGL6 and SEP genes in Petunia hybrida and Oryza sativa has been shown to occur during meristem determinacy and in petal and stamen identity (Ohmori et al, 2009;Rijpkema, Zethof, Gerats, & Vandenbussche, 2009).…”

“…Aristolochia species are unique within Piperales in that they exhibit a monosymetric perianth formed by three petaloid sepals, fused to form a convoluted and tubular structure differentiated in three regions: the utricle, the tube, and the limb (González & Stevenson, 2000b;Pabón-Mora et al, 2015). Aristolochia species are unique within Piperales in that they exhibit a monosymetric perianth formed by three petaloid sepals, fused to form a convoluted and tubular structure differentiated in three regions: the utricle, the tube, and the limb (González & Stevenson, 2000b;Pabón-Mora et al, 2015).…”

“…Aristolochia species are unique within Piperales in that they exhibit a monosymetric perianth formed by three petaloid sepals, fused to form a convoluted and tubular structure differentiated in three regions: the utricle, the tube, and the limb (González & Stevenson, 2000b;Pabón-Mora et al, 2015). Also, Aristolochia species possess a gynostemium, which is a 3, 5, 6 (less often 9, 10, or 12)-lobed crown-like structure found inside the utricle, above the inferior ovary, formed by the congenital fusion of the anthers and the commissural, bifid stigmatic lobes (González & Stevenson, 2000a, 2000bPabón-Mora et al, 2015;Suárez-Baron, Pérez-Mesa, Ambrose, González, & Pabón-Mora, 2016). Also, Aristolochia species possess a gynostemium, which is a 3, 5, 6 (less often 9, 10, or 12)-lobed crown-like structure found inside the utricle, above the inferior ovary, formed by the congenital fusion of the anthers and the commissural, bifid stigmatic lobes (González & Stevenson, 2000a, 2000bPabón-Mora et al, 2015;Suárez-Baron, Pérez-Mesa, Ambrose, González, & Pabón-Mora, 2016).…”

“…Redundancy between AGL6 and SEP genes in Petunia hybrida and Oryza sativa has been shown to occur during meristem determinacy and in petal and stamen identity (Ohmori et al, 2009;Rijpkema, Zethof, Gerats, & Vandenbussche, 2009). However, in the early divergent angiosperm Aristolochia fimbriata (Aristolochiaceae), the orchids Phalaenopsis, Aphrodite, and Oncidium Gower Ramsey, and in the non-core eudicot Nigella damascena (Ranunculaceae), AGL6 orthologs are expressed specifically in the perianth (Hsu et al, 2015;Pabón-Mora, Suárez-Baron, Ambrose, & González, 2015;Su et al, 2013), and control the identity of sepals and petals (Wang et al, 2015), which has positioned them into the model as the non-core eudicot putative A-function genes.…”

“…We have recently studied flower development as well as the spatio-temporal expression patterns of the floral identity MADS-box genes in the Magnoliid Aristolochia fimbriata (Aristolochiaceae: Piperales). Aristolochia species are unique within Piperales in that they exhibit a monosymetric perianth formed by three petaloid sepals, fused to form a convoluted and tubular structure differentiated in three regions: the utricle, the tube, and the limb (González & Stevenson, 2000b;Pabón-Mora et al, 2015). The perianth works as a specialized trapping system for pollinators, ensuring cross-pollination Oelschlägel, Gorb, Wanke, & Neinhuis, 2009;Oelschlägel et al, 2015).…”