Polymethoxyflavones (PMFs) accumulate mainly in the colored outer peel, or flavedo, of citrus fruit and are associated with anticancer, anti-inflammatory, metabolic-protective, and neuroprotective activities reported in experimental studies. Their multiple methoxy groups improve lipophilicity and help shape bioactivity, but installing those groups requires specialized O-methyltransferases (OMTs). Many known citrus OMTs recognize only narrow substrate sets or act poorly on accessible, early-stage intermediates. Apigenin has therefore served as a common proposed entry point for engineered production, although obtaining it from plants can be costly and inefficient. Because of these challenges, deeper investigation is needed into enzymes and precursor routes that can support efficient, controllable PMF biosynthesis.
On 6 April 2026, researchers from Zhejiang University, working with the Institute of Fruit Tree Research at the Quzhou Academy of Agriculture and Forestry Science, reported (DOI: 10.1093/hr/uhag128) the findings in Horticulture Research, volume 13, issue 8. The team combined developmental profiling of ‘Bingtangcheng’ sweet orange (Citrus sinensis) peel with gene-expression analysis, in-plant validation, enzyme assays, kinetic measurements, mutagenesis, and molecular modeling. Their results establish CsOMT5 as a positive regulator of citrus PMF accumulation and identify CsOMT5 as the first reported plant enzyme to methylate naringenin selectively at the C5 position. The work focuses on a previously uncharacterized citrus methylation step.
The researchers first used high-performance liquid chromatography (HPLC) to track sinensetin, nobiletin, 5,6,7,4′-tetramethoxyflavone, heptamethoxyflavone, and tangeretin across seven fruit-development stages. Total PMFs in the flavedo peaked at about 3.6 milligrams per gram of fresh weight 60 days after flowering, while the white inner peel contained only trace amounts. Transcriptome comparisons and reverse transcription quantitative polymerase chain reaction (RT-qPCR) linked this pattern to strong, flavedo-specific expression of CsOMT5. Expression was high in sweet orange but negligible in the mandarins, tangelo, and pomelo examined. Increasing CsOMT5 expression roughly fourfold raised total PMFs by 1.17-fold; virus-induced gene silencing (VIGS) reduced gene expression to about 63% of the control level and lowered total PMFs by 28.72%. In cell-free assays, CsOMT5 used S-adenosyl-L-methionine (SAM) as the methyl donor and modified flavanones, flavones, dihydroflavonols, flavonols, and caffeic acid at several molecular positions. Most importantly, it converted naringenin into 5-methylnaringenin with C5 selectivity. The enzyme bound naringenin with a Michaelis constant of 5.19 micromolar. Mutational analysis identified residues controlling this activity: N14S, I256V, and G305L increased activity about fivefold, whereas T18M, I120N, and G305F eliminated detectable activity.
In discussing the significance, the authors said the study moves PMF pathway design closer to an accessible upstream precursor rather than relying only on later flavone intermediates. They said CsOMT5 offers both catalytic breadth and unusually precise C5 methylation, giving synthetic biologists a useful component that could work alongside other OMTs to assemble specific PMFs. The findings do not yet constitute an industrial production process, they added, but they provide three essential building blocks for one: a scalable starting substrate, a functionally validated enzyme, and residue-level targets for improving catalytic performance.
The discovery could support microbial or plant-cell factories that convert naringenin into selected PMFs through coordinated enzyme pathways, potentially reducing reliance on citrus-peel extraction and seasonal raw materials. The activity-enhancing mutations also provide a starting point for protein engineering, while the loss-of-function residues may help researchers screen for related C5-selective enzymes. In horticulture, understanding CsOMT5 regulation may eventually aid efforts to adjust PMF composition in citrus fruit. Substantial work remains before commercial use, including completing downstream pathway steps, balancing metabolic flux, testing engineered hosts, improving yield and selectivity, confirming long-term process stability, and evaluating purification costs under realistic production conditions. Any health-related application will also require product-specific efficacy, safety, and regulatory assessment.
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References
DOI
10.1093/hr/uhag128
Original Source URL
https://doi.org/10.1093/hr/uhag128
Funding information
This research was supported by the National Natural Science Foundation of China (U25A20688), the Hangzhou Joint Fund of the Zhejiang Provincial Natural Science Foundation of China under Grant No. LHZSD24C150001, the National Key R&D Program of China (2023YFD2300604), and the Fundamental Research Funds for the Zhejiang Provincial Universities (226-2024-00211).
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.