A 2019 whole-bud transcriptome comparison of single- and double-flower Lagerstroemia speciosa (giant crape myrtle) proposed a hormone- and transcription-factor-mediated regulatory network for stamen petaloidy — the homeotic conversion of stamens into petal-like organs that produces the prized double-flower phenotype — centered on the floral homeotic genes MADS16, AP2, SOC1, and AGAMOUS (AG). The proposal, however, was purely correlative. Later work (2020–2026) has refined the candidate list (suppression of C-class AG homologs, AP2/miR172-related regulation), delivered a chromosome-level genome, finer anatomical staging, and heterologous overexpression phenotypes in Arabidopsis and tobacco, but no endogenous functional test of any petaloidy candidate has been performed in Lagerstroemia itself. The open problem is to establish, by endogenous functional experimentation in Lagerstroemia, which regulators causally drive the stamen-to-petal conversion, whether their differential expression is cause or consequence, and what direct regulatory interactions connect them.

Background

Lagerstroemia speciosa (giant crape myrtle or banaba; family Lythraceae) is a summer-flowering woody ornamental tree of South and Southeast Asia. Like many ornamentals, it gains horticultural value from double flowers, in which extra petal-like organs replace reproductive ones. In L. speciosa the double phenotype arises by stamen petaloidy: a homeotic transformation (conversion of one organ type into another) in which stamens develop as petal-like structures, producing showier blossoms but male sterility. Floral organ identity is classically described by the ABC(E) model, in which overlapping combinations of homeotic transcription-factor genes specify sepals (A function), petals (A+B), stamens (B+C), and carpels (C): the MADS-box transcription factors AGAMOUS (AG, C function) and APETALA3/PISTILLATA (B function) specify stamen identity, while APETALA2 (AP2, A function) and microRNA miR172 antagonize AG in the outer whorls; SOC1-class and other MADS-box genes connect floral induction to organ development. Loss or suppression of C-class activity, or ectopic A/B-class activity, in the third whorl is a recurring route to petaloid stamens across ornamentals.

The source study (Hu et al. 2019, on a natural double-flower variant of L. speciosa in which all stamens are petaloid) contrasted transcriptomes of single- and double-flower buds at three stamen developmental stages defined by bud diameter and axial-section anatomy. Because no reference genome existed, transcripts were assembled de novo into 73,536 unigenes (consensus transcript sequences) and 30,714 differentially expressed genes (DEGs) were identified, with 22 validated by quantitative RT-PCR (qRT-PCR). Weighted gene co-expression network analysis (WGCNA, a method that clusters genes by correlated expression across samples) identified a co-expression module whose expression stably distinguished the two phenotypes, with four floral homeotic genes as hubs — MADS16, AP2/AIL5, SOC1, and AG — co-expressed with hormone-signaling and transcription-factor genes (109 hormone-pathway DEGs across auxin, gibberellin, jasmonic acid, and other pathways). The authors themselves flagged the limits: whole flower buds were sequenced, so organ-level attribution was impossible; co-expression identifies candidates but not function; and spatial–temporal verification and applicability to other double-flower materials were untested.

Since then the same and adjacent groups have substantially built out the resource and candidate layer: a chromosome-level L. speciosa genome (Wan et al. 2024), genome-wide MADS-box analyses in L. indica and L. speciosa (Qiao et al. 2024; Wan et al. 2025), a stamen-focused comparative transcriptome of an independent double variety ('Zijuan', Zhang et al. 2025) showing C-class AGAMOUS homolog (AG1 and MADS3) suppression and activated auxin/gibberellin/jasmonate responses, with WGCNA linking AG and MADS3 to the RNA-processing gene SMD1B, and heterologous overexpression of L. speciosa MADS-box genes (LsAG2, LsDEF1) and of LsAP2/ERF10 in Arabidopsis and tobacco respectively (Yang et al. 2023; Wan et al. 2026). Virus-induced gene silencing (VIGS, a transient knockdown method using recombinant viral vectors) has been demonstrated in the genus (Li et al. 2020; Yang et al. 2024). In a closely related woody ornamental, rose, the RhAGL24–RhARF18–RhAG module regulating stamen petaloidy was dissected with VIGS plus promoter-binding assays (Liu et al. 2022). What no study has done is functionally test any petaloidy candidate inside Lagerstroemia.

Problem Statement

Establish the causal regulatory mechanism of stamen petaloidy in Lagerstroemia speciosa. Specifically: determine, by endogenous loss- and/or gain-of-function perturbation in Lagerstroemia, which of the candidate regulators identified by the comparative transcriptome studies — the C-class AGAMOUS homologs (AG, AG1, MADS3), the MADS16, AP2-family (including LsAP2/ERF genes), and SOC1 hub genes, and the associated hormone-signaling genes (auxin, gibberellin, jasmonic acid pathways) — are necessary and/or sufficient for the homeotic conversion of stamens into petal-like organs in the double-flower phenotype; whether the phenotype-associated expression differences (notably C-class suppression and AP2 downregulation) are cause or consequence of the conversion; and which direct regulatory interactions (for example miR172–AP2–AG, AG/MADS3–SMD1B, or transcription factor–hormone gene links) mediate the network. The mechanism should account for the natural single- versus double-flower difference at the organ level (which organs express and require the identified regulators, at which developmental stages), thereby converting the existing correlative network into a validated explanation of how transcript-level differences produce the stamen-to-petal developmental transition.

References

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