Scientists decode the molecular network shaping Freesia flower pigmentation
image: Proposed model of flavonoid biosynthesis regulation in Freesia flower.
Credit: Horticulture Research
Flavonoids are crucial compounds influencing flower coloration, fertility, and stress resistance. Among them, proanthocyanidins (PAs), anthocyanins, and flavonols share intertwined biosynthetic pathways, yet their coordinated regulation remains poorly understood in flowers. This study identified four TT2-type MYB transcription factors—FhMYBPA1–4—that orchestrate PA biosynthesis in Freesia hybrida. These regulators directly activate key structural genes such as FhLAR and FhANR, functioning in concert with the bHLH protein FhTT8L and the WD40 protein FhTTG1. The findings reveal a hierarchical and feedback regulatory network balancing activators and repressors, offering new insight into how flowers synchronize pigment production and stress-related metabolism.
Proanthocyanidins (PAs) are tannin-like flavonoids that protect plants from UV light, pathogens, and herbivores while contributing to the taste of fruits, teas, and wines. Despite extensive research on seeds and fruits, the regulatory mechanisms of PA biosynthesis in flowers—especially monocots—remain largely uncharacterized. In Freesia hybrida, a model ornamental species rich in flavonoids, previous studies revealed activators like FhPAP1 for anthocyanins and FhMYBF1 for flavonols, but the regulators responsible for PAs were unknown. Due to this gap, and the intricate overlap between these biosynthetic branches, a comprehensive study of TT2-type MYB regulators was necessary to uncover how floral flavonoid pathways interact and are fine-tuned during development.
A research team from Northeast Normal University, Jilin University, and the China Tobacco Gene Research Center has uncovered the molecular mechanism driving flavonoid biosynthesis in Freesia hybrida. The study (DOI: 10.1093/hr/uhae352), published on December 12, 2024, in Horticulture Research, systematically characterized four TT2-type MYB transcription factors that control the production of proanthocyanidins. Through gene expression profiling, protein interaction assays, and transient transformation analyses, the researchers established an integrative model showing how these regulators coordinate multiple metabolic pathways that define floral color and defense in Freesia.
Using comparative transcriptomics and functional assays, the team identified four TT2-type MYBs—FhMYBPA1, FhMYBPA2, FhMYBPA3, and FhMYBPA4—that cluster phylogenetically with known PA regulators. These nuclear-localized proteins act as transcriptional activators, significantly correlating with PA accumulation in developing flowers, especially in the torus, a tissue previously overlooked for its metabolic importance. Transient overexpression of FhMYBPAs in Freesia petals induced strong PA accumulation and upregulated structural genes such as FhLAR and FhANR.
Further assays demonstrated that FhMYBPAs interact with the bHLH partner FhTT8L to enhance promoter activation of PA biosynthetic genes, while also activating FhTTG1 and MYB repressors (FhMYB27 and FhMYBx)—revealing a hierarchical and feedback regulation mechanism. Dual-luciferase and electrophoretic mobility shift assays confirmed that FhMYBPAs directly bind MYB-core motifs within promoters of target genes. Comparative analysis of multiple Freesia MYB factors uncovered both specialization and overlap in regulating anthocyanins, flavonols, and PAs, indicating that floral coloration and defense share a tightly integrated transcriptional network.
“Our research highlights the remarkable complexity of floral metabolism,” said corresponding author Prof. Yueqing Li. “We found that the TT2-type MYB regulators not only activate PA biosynthesis but also participate in a feedback system involving both activators and repressors. This ensures a balanced production of pigments and protective compounds. The discovery of FhMYBPA2 as a particularly efficient activator suggests potential for targeted metabolic engineering to modify flower color intensity or enhance plant resilience.”
This study sheds light on how flowers orchestrate multiple flavonoid pathways to create diverse colors and protective functions. The identified regulatory modules provide valuable genetic tools for breeding ornamental plants with tailored pigmentation and enhanced stress tolerance. Moreover, understanding PA regulation in Freesia offers opportunities for developing new varieties with improved antioxidant or nutritional properties. Beyond ornamentals, these insights advance the broader field of plant metabolic engineering, paving the way for manipulating flavonoid networks to improve crop quality, resilience, and potential nutraceutical applications.
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References
DOI
Original Source URL
https://doi.org/10.1093/hr/uhae352
Funding information
This work was supported by the National Natural Science Foundation of China (32100211, 32272751), the China Postdoctoral Science Foundation (2023 M740580), the Department of Science and Technology of Jilin Province (20220508112RC), Science and Technology Projects of HNTI (KY2022YC0003), and the Fundamental Research Funds for the Central Universities (2412023YQ005, 2412024QD024).
About Horticulture Research
Horticulture Research is an open access journal of Nanjing Agricultural University and ranked number one in the Horticulture category of the Journal Citation Reports ™ from Clarivate, 2023. The journal is committed to publishing original research articles, reviews, perspectives, comments, correspondence articles and letters to the editor related to all major horticultural plants and disciplines, including biotechnology, breeding, cellular and molecular biology, evolution, genetics, inter-species interactions, physiology, and the origination and domestication of crops.