In the columbine genus Aquilegia, an early-diverging eudicot model, the showy petaloid sepals and the broad multi-whorled stamen domain are governed by floral organ-identity genes in ways that depart from the canonical Arabidopsis ABC model: B-class MADS-box genes promote anthocyanin-based petaloidy of the first-whorl sepals and, unexpectedly, are required to maintain expression of the C-class gene AqAG1AqAG1 in the outer stamen whorls. Sharma and Kramer (2017) left four linked questions open, and none of them has been resolved in the subsequent literature: (1) which loci specify fundamental sepal identity and the petaloid papillate epidermis (AGL6 homologs are untested candidates in Aquilegia); (2) whether Aquilegia B-class genes act solely by activating anthocyanin-pathway genes or also repress photosynthesis-related loci such as BANQUO homologs; (3) what mechanistic basis — spatial reach of canonical upstream activators (WUSCHEL/LEAFY), cis-regulatory architecture at AqAG1AqAG1, or evolutionarily drifted regulatory redundancy — underlies the B→C dependency; and (4) why the requirement for B-class function in maintaining the C-domain boundary differs between Aquilegia species and among ranunculid taxa. Later work has enriched the comparative context (Delphinium, Nigella, Thalictrum) but has not tested these questions in Aquilegia or explained the B→C connection.

Background

Floral organ identity in flowering plants is described by the ABC(E) model: sepal identity requires A-class function, petal identity A+B, stamen identity B+C, and carpel identity C, with E-class SEPALLATA genes participating in all whorls. The A-, B-, and C-class functions are encoded by MADS-box transcription factors — B class by APETALA3 (AP3) and PISTILLATA (PI) homologs, and C class by AGAMOUS (AG) homologs, which also terminate the floral meristem. In the reference plant Arabidopsis thaliana, AG expression is activated by LEAFY (LFY, a floral meristem identity transcription factor) together with WUSCHEL (WUS, a homeodomain protein produced in the meristem center whose protein moves non-cell-autonomously outward), and B-class genes are not required to maintain C-class expression (Lohmann et al. 2001). Petaloidy denotes the acquisition of petal-like, pollinator-attracting features (color, epidermal cell shape) by organs other than petals.

Aquilegia (columbine, Ranunculaceae) belongs to the Ranunculales, the sister lineage to all other eudicots, and is an established evo-devo model. Its flowers have five petaloid sepals, five spurred nectariferous petals, many whorls of stamens forming a broad stamen domain, two whorls of staminodes, and carpels. Gene duplications in this lineage produced three AP3 paralogs (AqAP3AqAP3-1/-2/-3), a single PI homolog (AqPIAqPI), and two AG homologs. Functional studies by virus-induced gene silencing (VIGS, a transient knockdown method using a viral vector) showed that AqAP3AqAP3-3 specifies petal identity and that B-class genes were recruited to novel functions such as staminode identity (Kramer et al. 2007; Zhang et al. 2013).

Sharma and Kramer (2017) silenced AqPIAqPI in Aquilegia coerulea 'Origami' and found two surprises. First, AqPIAqPI promotes anthocyanin pigmentation of the petaloid sepals — a non-homeotic recruitment of the B-class program into the first whorl — but does not control sepal morphology or identity: the loci determining fundamental sepal identity, including the papillate petaloid epidermal cells, remain unknown. In another ranunculid, Nigella damascena, an AGL6 homolog (a MADS-box gene of the APETALA1/AGL6 lineage) promotes sepal identity (Wang et al. 2016), making Aquilegia AGL6 homologs plausible but untested candidates. Second, AqPIAqPI knockdown caused outer stamens to transform into sepals or petals in conjunction with reduced outer expression of the C-class gene AqAG1AqAG1, implying that B-class activity is needed to maintain the outer boundary of the C domain. Parallel observations exist in other Ranunculales: the seirena mutant of Eschscholzia californica, a loss-of-function allele of a PI/GLO homolog, transforms outer stamens into sepals via reduced EscAG1 expression (Lange et al. 2013), and in N. damascena loss of function of the AP3 homolog NgsAP3NgsAP3-3 transforms outer stamens into sepals in conjunction with contraction of the NgsAG1NgsAG1 expression domain (Gonçalves et al. 2013; Wang et al. 2016); yet in Papaver B-class silencing does not produce this phenotype (Drea et al. 2007), and earlier VIGS work in the Old World Aquilegia vulgaris had not shown it either (Kramer et al. 2007). The mechanistic basis of this B→C regulatory connection is unknown: it could reflect developmental system drift (evolutionary change in the underlying regulatory interactions producing a conserved phenotype; True and Haag 2001) that increased the importance of B-class-containing MADS complexes in AG self-maintenance; or spatial limitations on WUS diffusion across the broad stamen domain of these taxa, which would make AG activation in outer stamens dependent on additional factors such as B-class genes. Related open issues are whether the Aquilegia B-class genes act only through the anthocyanin pathway or also repress photosynthesis-related loci (BANQUO1/2 homologs, which Arabidopsis AP3/PI repress to make petals white; tests in Aquilegia were inconclusive), and whether the inter-specific differences reflect VIGS artifact or genuine evolutionary divergence. Subsequent functional genomics in Aquilegia and related genera has characterized B-class target genes and AGL6, CYCLOIDEA, and AG functions in Delphinium and Nigella (Jiang et al. 2020; Zhao et al. 2023), providing comparative context but not resolving these questions.

Problem Statement

The problem is to determine the genetic and regulatory architecture by which the Aquilegia flower specifies its petaloid sepals and maintains the outer boundary of the C-function domain, at the generality raised by Sharma and Kramer (2017). Concretely, the problem comprises four linked questions:

  1. Sepal identity loci. Which genes specify fundamental sepal identity in Aquilegia, including the production of petaloid-associated papillate epidermal cells? In particular, do AGL6 homologs — which promote sepal identity in Nigella damascena and sepal and petal identity in Delphinium — perform this function in Aquilegia, or do other loci?

  2. Scope of B-class action in the sepals. Do the Aquilegia B-class genes (AqPIAqPI, AqAP3AqAP3 paralogs) act solely by activating anthocyanin-pathway genes in the sepals, or do they also repress photosynthesis-related loci such as BANQUO1/2 homologs, as Arabidopsis AP3/PI do in petals?

  3. Mechanistic basis of the B→C dependency. Why does outer stamen expression of the C-class gene AqAG1AqAG1 depend on B-class activity? Candidate explanations to be distinguished experimentally include: changed regulatory redundancy from developmental system drift (increased importance of AP3/PI-containing MADS complexes in AG activation/self-maintenance); altered cis-regulatory architecture at AqAG1AqAG1; and spatial limitations of canonical upstream activators (WUSCHEL diffusion or LEAFY reach) across a broad stamen domain, leaving outer-whorl AG activation to B-class-dependent inputs.

  4. Inter-specific variability. Does the requirement for B-class function in maintaining the C-domain boundary genuinely differ between Aquilegia species (e.g., A. vulgaris vs A. coerulea) and among ranunculid taxa (Eschscholzia vs Papaver), and if so, does the difference reflect timing, strength, or spatial pattern of gene silencing artifacts, or real evolutionary divergence in the underlying regulatory networks?

These questions share one scientific objective: to explain how the ranunculid floral organ-identity network is wired — and how that wiring evolved — such that pollinator function was transferred to the sepals without homeotic change and C-domain integrity became B-class-dependent. The four questions are posed jointly, with Aquilegia as the minimum scope and the B→C mechanism to be explained at the generality of the Ranunculales; a scope narrower than the stated comparisons — a single species, gene, or developmental stage — does not reach the target.

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