Key Takeaways & Executive Findings
- •• • In the conventional (WT) cultivar, adaxial corolla epidermal cells exhibited a spatially sequential expansion pattern: basal cells expanded first, followed by middle cells, with adaxial expansion rates exceeding abaxial rates. In the Xianglei (XL) cultivar, adaxial cell expansion rates were significantly reduced (p < 0.01), and the degree of cell expansion was negligible. This cellular arrest directly accounts for the non-opening phenotype, which extends the bud stage and alters the harvest window for medicinal material, potentially affecting biomass accumulation and secondary metabolite profiles. • • Transcript profiling by qRT-PCR revealed that LmXTH6, LmXTH7, and LmXTH33 were highly expressed in WT during pre-anthesis and anthesis, but their expression was significantly suppressed in XL (p < 0.05). LmEXP7 peaked before corolla opening in WT, whereas its expression remained low in XL at the same stage. These data establish that the down-regulation of cell wall loosening enzymes (XTHs and EXP) is a primary molecular lesion underlying the non-opening trait, providing candidate targets for marker-assisted selection. • • Ruthenium red staining quantified by Image J showed that the overall degree of pectin de-methylesterification in XL corolla cell walls was higher than in WT, consistent with the elevated expression of LmPME28 and LmPME45 (p < 0.05). Increased de-methylesterification can promote calcium-mediated pectin cross-linking, enhancing cell wall rigidity. This biochemical shift likely acts in concert with reduced XTH/EXP activity to lock the corolla in a closed conformation, offering a mechanistic explanation for the prolonged bud stage. • • STRING-based protein-protein interaction network prediction identified potential interactions among LmXTHs, LmEXP7, and LmPMEs, suggesting a coordinated regulatory module. The inverse expression patterns—low XTH/EXP and high PME—indicate that the Xianglei phenotype arises from a coordinated cell wall remodeling program rather than a single gene defect. This multi-gene signature can be exploited for breeding programs aimed at manipulating flower opening time and improving mechanical harvestability or ornamental value.
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Abstract
Dried flower buds or newly opened flowers of Lonicera macranthoides are used as Shanyinhua (Lonicerae Flos) for medicinal purposes. The Xianglei cultivar, developed from a natural mutant, exhibits a distinctive phenotype characterized by non-opening corolla and prolonged bud stage. This study aimed to elucidate the cellular and molecular mechanisms underlying this unique corolla phenotype. The flower of conventional cultivar (WT) and the Xianglei cultivar (XL) of L. macranthoides were used as materials. Corolla adaxial epidermal cells and longitudinal sections were observed using light microscopy. The expression patterns of xyloglucan endotransglucosylase/hydrolase genes (LmXTH6, LmXTH7, LmXTH33), expansin gene (LmEXP7), and pectin methylesterase genes (LmPME28, LmPME45) were analyzed by qRT-PCR. Protein-protein interaction networks were predicted using the STRING database. Ruthenium red staining coupled with Image J quantitative analysis was performed to assess the degree of pectin de-methylesterification in corolla cell walls. In the WT, the adaxial cells of corolla displayed a spatially sequential expansion, with the basal cells expanding first, followed by the middle cells. Moreover, the expansion rate of the adaxial cells is higher than that of the abaxial cells. In contrast, the expansion rate of adaxial cells in XL was significantly reduced, and the degree of cell expansion was not obvious. At the molecular level, LmXTH6, LmXTH7, and LmXTH33 were highly expressed in WT during the pre-anthesis and anthesis stages, but their expression was significantly inhibited in XL. LmEXP7 expression peaked before corolla opening in WT, while its expression was lower in XL during the same period. Ruthenium red staining results indicated that the overall degree of de-methylesterification in XL was higher than that in WT, consistent with the expression trends of LmPME28 and LmPME45. In conclusion, the non-opening corolla of the Xianglei cultivar is closely related to the inhibition of corolla cell expansion. The low expression of XTHs and EXP collectively leads to a decrease in the expansion capacity of Xianglei corolla cells, while the up-regulation of PMEs increases pectin de-methylesterification in the cell wall, potentially enhancing cell wall rigidity. The synergy of XTH/EXP-PME may be an important reason for the non-opening corolla of Xianglei, providing experimental evidence for the study of the regulatory mechanism of the excellent phenotype of Xianglei L. macranthoides and offering new insights into the cell wall regulatory mechanisms of plant floral organ morphogenesis.
1. Introduction
Lonicera macranthoides, a source of the traditional Chinese medicine Shanyinhua (Lonicerae Flos), is widely cultivated for its dried flower buds. The Xianglei cultivar, derived from a natural mutant, exhibits a distinctive non-opening corolla and a prolonged bud stage. This phenotype is agronomically significant because it extends the harvest window and may influence the accumulation of bioactive compounds. However, the cellular and molecular basis of this trait has remained undefined, impeding targeted breeding and quality control.
Previous studies on flower opening have largely focused on petal expansion in model species, implicating cell wall remodeling enzymes such as xyloglucan endotransglucosylase/hydrolases (XTHs), expansins (EXPs), and pectin methylesterases (PMEs). Yet, the specific roles of these genes in the non-opening phenotype of L. macranthoides have not been systematically investigated. This study integrates histological observation, qRT-PCR expression profiling, protein interaction prediction, and quantitative pectin de-methylesterification assays to dissect the cellular and molecular determinants of corolla non-opening. The findings reveal that suppressed cell expansion, driven by low XTH/EXP expression and elevated PME-mediated pectin de-methylesterification, underlies the Xianglei phenotype, providing a mechanistic framework for understanding floral organ morphogenesis and informing breeding strategies.
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AO Zhijie, FU Xuesen, ZHOU Simin, QU Meiling, ZHU Jiayuan, YUAN Linxiang, ZHOU Ribao, LIU Xiangdan (2026). Correlation Research of Corolla Non-Opening Phenotype with Cell Morphology and XTH/EXP-PME Expression Characteristics in Lonicera macranthoides. Chinese Traditional and Herbal Drugs. https://doi.org/10.7501/j.issn.0253-2670.2026.16.20261622
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Frequently Asked Questions
What is the primary cellular mechanism responsible for the non-opening corolla in the Xianglei cultivar?
The non-opening phenotype is primarily due to a significant reduction in adaxial corolla epidermal cell expansion. In WT, adaxial cells expand sequentially from base to middle, with adaxial expansion exceeding abaxial. In XL, adaxial cell expansion rates are drastically reduced (p < 0.01), and cells fail to elongate, resulting in a closed corolla. This cellular arrest is driven by low expression of cell wall loosening genes (LmXTH6, LmXTH7, LmXTH33, LmEXP7) and elevated expression of pectin methylesterases (LmPME28, LmPME45), which increases cell wall rigidity.
How does the expression of XTH and EXP genes differ between WT and XL during flower opening?
In WT, LmXTH6, LmXTH7, and LmXTH33 are highly expressed during pre-anthesis and anthesis, while LmEXP7 peaks before corolla opening. In XL, these genes are significantly down-regulated (p < 0.05) at the same developmental stages. This suppression reduces xyloglucan endotransglycosylation and expansin-mediated wall loosening, impairing the cell wall extensibility required for corolla expansion.
What is the role of pectin methylesterases in the Xianglei phenotype?
LmPME28 and LmPME45 are up-regulated in XL, leading to increased pectin de-methylesterification in corolla cell walls, as confirmed by ruthenium red staining and Image J quantification. De-methylesterified pectin can form calcium cross-links, enhancing cell wall rigidity. This biochemical change counteracts cell expansion and contributes to the non-opening phenotype.
Can the XTH/EXP-PME expression signature be used as a molecular marker for breeding?
Yes. The inverse expression pattern—low XTH/EXP and high PME—is a consistent molecular signature of the Xianglei phenotype. These genes can serve as candidate markers for marker-assisted selection to predict corolla opening behavior, enabling breeders to select for desired flower morphology, harvest timing, and potentially improved medicinal quality.
What are the industrial implications of the prolonged bud stage in Xianglei?
The prolonged bud stage extends the harvest window, reducing labor pressure and allowing flexible scheduling. However, the non-opening corolla may affect the accumulation of secondary metabolites and biomass. Understanding the molecular basis allows for targeted manipulation of flowering time, potentially optimizing both yield and quality of Shanyinhua. Additionally, the closed corolla may offer protection against environmental stresses, but this requires further agronomic evaluation.
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