The review, led by Eliakira Kisetu Nassary of Sokoine University of Agriculture in Tanzania together with colleagues, followed the PRISMA-ScR framework, the recognised reporting standard for scoping reviews. The team searched Scopus, Web of Science, PubMed, ScienceDirect and Google Scholar for publications from January 2010 through February 2026, screening 286 records and ultimately including 95 peer-reviewed studies in the evidence synthesis. Rather than simply summarising the literature, the authors assigned each study to a specific segment of the citral biosynthetic pathway and scored the depth of experimental validation using a purpose-built tool they call the Evidence Maturity Index, or EMI. The index ranks evidence hierarchically: transcriptomic prediction scores lowest at 1.0, integrated transcriptome-metabolome studies score 1.5, recombinant enzyme characterisation scores 2.0, and in planta functional validation, the strongest form of proof, scores 2.5.

The biosynthetic story begins in the chloroplast. The plastidial methylerythritol phosphate, or MEP, pathway supplies the isoprenoid building blocks that are assembled into geranyl diphosphate, the immediate precursor of all monoterpenes. Transcriptome assemblies and comparative expression analyses have repeatedly flagged genes encoding 1-deoxy-D-xylulose-5-phosphate synthase (DXS), its reductoisomerase partner DXR, and geranyl diphosphate synthase (GPPS) as central players in precursor production. In the review’s evidence map, the MEP pathway was the single most heavily studied segment, accounting for 24 of the 95 included studies, with GPPS-related work adding another 16. But here lies the review’s central warning: sheer volume of publications did not translate into mechanistic certainty, because most of these investigations relied on gene expression patterns rather than direct measurements of enzyme activity or metabolic flux.

The best-resolved step in the entire pathway turns out to be the conversion of geranyl diphosphate into geraniol. Despite being represented by only 13 studies, geraniol synthase achieved one of the highest validation scores in the EMI analysis. Recent work combining recombinant enzyme assays, subcellular localisation and functional analyses revealed something unexpected: a dual-localised geraniol synthase operating in both plastids and cytosol, working alongside a previously uncharacterised cytosolic geranyl pyrophosphatase. This cooperation between cellular compartments refines the older picture of monoterpenes being built exclusively inside chloroplasts and demonstrates that citral formation depends on metabolic exchange across cellular boundaries. Comparable compartmental coordination has since been documented in camphor trees, suggesting the pattern may be widespread among aromatic plants.

The final oxidation step, in which geraniol is converted into the aldehydes geranial and neral, has historically been the murkiest part of the pathway. For years, alcohol dehydrogenases were implicated purely on the basis of transcript abundance and correlations with citral content. That changed when recent functional studies in lemongrass deployed recombinant enzyme assays, virus-induced gene silencing and transient overexpression to demonstrate a direct enzymatic contribution to citral accumulation. The review notes that these findings require an update to previous classifications that treated ADH-mediated oxidation as merely predictive. Even so, comparative enzyme kinetics among ADH isoforms, their substrate preferences and their relative contributions to geranial versus neral formation remain unexplored, leaving the terminal chemistry only partially resolved.

The weakest link, according to the EMI, is transcriptional regulation. Only nine studies examined regulatory factors, and nearly all relied on co-expression networks and candidate gene prediction. Transcriptomic analyses have associated terpene accumulation with transcription factors of the AP2/ERF and MYB families, but direct evidence of promoter binding or transcriptional activation in Cymbopogon is essentially absent. A compelling comparator exists: in the laurel family, a MYB44 transcription factor was experimentally shown to regulate alcohol dehydrogenase expression, providing a mechanistic framework that lemongrass researchers could replicate using chromatin immunoprecipitation, yeast one-hybrid assays and promoter-reporter systems. Until such experiments are performed in Cymbopogon itself, the regulatory hierarchy controlling citral production remains an inference rather than a demonstrated fact.

Beyond the pathway itself, the review highlights how environmental conditions, agronomic practices and post-harvest handling shape citral accumulation in ways that are still poorly connected to molecular mechanisms. Drought studies have revealed significant genotype-by-environment interactions for essential oil yield, and transcriptomic evidence suggests water stress alters expression of upstream isoprenoid genes, including GPPS-related components. Harvest interval, cutting frequency and developmental stage all modify oil yield and composition, likely through changes in glandular structure, precursor availability and enzymatic activity. Crucially, the authors urge a clear separation between biosynthetic regulation and post-harvest chemistry: drying conditions, storage duration and distillation procedures can shift the geranial-to-neral ratio through volatilisation, oxidation or chemical conversion, meaning that two samples of the same cultivar may differ chemically without any difference in genetic capacity.

The practical implications reach into breeding and biotechnology. Current cultivar improvement relies heavily on phenotypic evaluation, oil yield and chemical profiling, with molecular markers directly tied to citral biosynthesis still scarce. The review argues that genome-wide association studies and quantitative trait locus mapping across diverse Cymbopogon germplasm, combined with multi-environment phenotyping, could identify genomic regions controlling citral concentration, geranial-neral ratios and environmental stability. Emerging technologies could accelerate this transition: long-read sequencing promises better genome annotation and resolution of terpene gene families, single-cell transcriptomics could reveal which specialised cells actually manufacture citral, spatial metabolomics can map where intermediates accumulate within tissues, and artificial intelligence-assisted multi-omics integration may predict pathway behaviour under contrasting production environments.

The overarching lesson of the evidence map is that research volume and mechanistic resolution are not the same thing. Pathway segments drowning in transcriptomic datasets scored lower on the Evidence Maturity Index than segments supported by fewer but functionally decisive experiments, a pattern the authors say should redirect future effort toward enzyme kinetics, metabolic flux analysis, targeted gene perturbation using RNA interference or CRISPR/Cas editing, and integrated studies that simultaneously measure gene expression, protein activity, metabolite flux and final oil composition. As global demand for naturally derived bioactive compounds pushes lemongrass cultivation into new regions and production systems, the gap between knowing which genes are present and knowing which genes control the chemistry becomes the decisive bottleneck. Closing it, the review concludes, will determine whether citral-rich lemongrass cultivars of the future are selected by trial and error or engineered by design.

Subject of Research: Molecular pathways and validation status of citral biosynthesis in Cymbopogon species

Article Title: Evidence mapping of molecular pathways regulating citral biosynthesis and validation status in Cymbopogon spp.

Article References: Nassary, E. K., Magubika, A. J., Mhagama, E. A., Salimu, A. I., Kilasi, N. L., Tryphone, G. M., & Shitindi, M. J. (2026). Evidence mapping of molecular pathways regulating citral biosynthesis and validation status in Cymbopogon spp.. Discover Plants, 3(1), Article 378. https://doi.org/10.1007/s44372-026-00852-1

Image Credits: AI Generated

DOI: 10.1007/s44372-026-00852-1

Keywords: citral, Cymbopogon, lemongrass, monoterpene biosynthesis, MEP pathway, geraniol synthase, alcohol dehydrogenase, evidence mapping, transcriptomics, metabolomics, essential oil, plant breeding

The research, carried out by Zhuo Yan, Wenqian Ren, Xingfu Shang, Xiujing Zou and Xueguang Wang at Shanghai University, describes a series of Al-P(x)-O solid acid catalysts prepared by a one-pot, PEG-20000-assisted synthesis. Rather than depositing an acid phase onto a preformed support, the team co-assembles aluminum and phosphorus precursors in a single vessel with the polymer additive, then calcines the resulting material. By simply varying the phosphorus-to-aluminum molar ratio, the authors tune the fundamental nature of the acid sites on the catalyst surface, and that single synthetic knob turns out to govern the entire catalytic behavior in the acetalization of citral with 1,2-propanediol.

The mechanistic heart of the paper lies in how the P/Al ratio reshapes the acid-type distribution. Comprehensive characterization using X-ray diffraction, Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy, Brunauer-Emmett-Teller surface area analysis, scanning electron microscopy, ammonia temperature-programmed desorption and pyridine-adsorbed infrared spectroscopy shows that materials low in phosphorus behave essentially as Lewis acids, while increasing phosphorus content introduces and then enriches Brønsted acid sites. At an intermediate composition the catalyst becomes a genuinely bifunctional Lewis-Brønsted system, in which the two acid types operate synergistically on the same surface. This deliberate transformation from purely Lewis acidity to a balanced dual-acid architecture is presented by the authors as a general design strategy for high-performance solid acid catalysts, not merely a recipe for one reaction.

The payoff is striking. The optimized composition, designated Al-P(1.20)-O for its phosphorus-to-aluminum molar ratio of 1.20, converts 64.6 percent of citral under the reaction conditions while directing 97.2 percent of the converted material to the desired citral propylene glycol acetal, the fragrance compound registered as CAS 10444-50-5. In a business where even small losses of a precious aldehyde to polymerization, oxidation or isomerization cut directly into margins, a selectivity approaching unity means that nearly every molecule of converted citral emerges as sellable product. The acetal itself is prized because it preserves the citrus olfactory profile of the parent aldehyde while dramatically improving chemical stability in formulated products.

Equally important for any prospective industrial user is the catalyst’s endurance. In continuous flow experiments the optimized Al-P(1.20)-O catalyst maintained its performance over 300 hours on stream without measurable deactivation, a duration that distinguishes it from many homogeneous acid catalysts and from solid acids that leach or coke rapidly. Thermogravimetric analysis confirmed excellent thermal stability of the material, and the authors demonstrated that the catalyst can be regenerated after use, restoring its activity for further service. For a fragrance industry moving toward greener, solvent-minimized continuous processes, a heterogeneous catalyst that survives weeks of operation and can be regenerated rather than discarded represents a meaningful advance over liquid mineral acids and soluble Lewis acids that must be neutralized and washed out of the product stream.

The one-pot synthesis itself carries practical weight. Traditional routes to acidic aluminum phosphates often involve multi-step impregnation or hydrothermal treatments that are difficult to scale. The PEG-20000-assisted method reported here uses the high-molecular-weight polymer as a structure-directing and dispersing agent, allowing the aluminum and phosphate species to assemble homogeneously into a crystalline framework during a single thermal treatment. Because the acid-site population is set by the feed composition rather than by a post-synthetic modification whose reproducibility is hard to guarantee, scale-up becomes far more predictable. The authors argue that this synthetic simplicity, combined with the tunability of the acid chemistry, highlights the industrial potential of crystalline aluminum phosphates in fragrance synthesis.

The chemistry underlying acetalization explains why the Lewis-Brønsted balance matters. Converting citral with 1,2-propanediol proceeds through initial activation of the aldehyde carbonyl, nucleophilic attack by one hydroxyl group of the diol, dehydration to a hemiacetal and subsequent ring closure to the five-membered cyclic acetal with loss of water. Lewis acid sites polarize the carbonyl effectively but can also promote side reactions on citral’s sensitive conjugated diene system, whereas moderate Brønsted acidity efficiently catalyzes the dehydration and cyclization steps. The study’s results indicate that when both site types are present in the right proportion, each handles the step it does best, accelerating the overall reaction while suppressing the pathways that destroy the fragile terpene aldehyde.

Earlier attempts at this transformation underscore the difficulty. Aluminum sulfate, copper methanesulfonate, stannic chloride-doped polyaniline and lithium tetrafluoroborate have all been explored as acetalization catalysts, along with heteropolyacids supported on molecular sieves and acidic ion-exchange resins. Each approach suffers from some combination of corrosion, catalyst separation difficulties, leaching, limited lifetime or insufficient selectivity. The aluminum phosphate system, being a robust crystalline inorganic solid with no soluble components, sidesteps most of these problems while matching or exceeding the performance of the alternatives in the vapor-phase process described.

Beyond citral, the implications of the work extend across the broader field of acid catalysis on mixed oxides. Aluminum phosphates and alumina-aluminum phosphate hybrids are being studied for applications ranging from biodiesel production and glycerol valorization to propane combustion and the synthesis of glycol ethers, and the demonstration that a simple stoichiometric parameter can switch the dominant acid mechanism offers a transferable principle for catalyst designers. The Shanghai team frames the P/Al-ratio-controlled Lewis-to-Brønsted transformation as a key strategy for tailoring solid acids, a principle that could guide the development of catalysts for acetalizations, esterifications, etherifications and condensations throughout green chemistry.

For now, the citral propylene glycol acetal stands as the flagship demonstration. As demand grows for natural fragrance ingredients that survive formulation, storage and shipping, chemistry that extends their shelf life without introducing harmful residues becomes ever more valuable, and safety assessments of the acetal by the fragrance industry reflect that interest. With a simple, scalable synthesis, tunable surface chemistry, 300-hour stability and selectivity above 97 percent, the aluminum phosphate catalysts reported in this study move a long-standing laboratory reaction considerably closer to the industrial reactor, and they suggest that the humble aluminum phosphate family may hold far more catalytic potential than its modest reputation implies.

Subject of Research: One-pot synthesis of tunable aluminum phosphate solid acid catalysts for citral acetalization with 1,2-propanediol

Article Title: One-Pot Synthesis of Aluminum Phosphate Catalysts for Acetalization of Citral with 1,2-Propanediol

Article References: Yan, Z., Ren, W., Shang, X., zou, X., & Wang, X. (2026). One-Pot Synthesis of Aluminum Phosphate Catalysts for Acetalization of Citral with 1,2-Propanediol. Catalysis Letters, 156(10), Article 272. https://doi.org/10.1007/s10562-026-05507-0

Image Credits: AI Generated

DOI: 10.1007/s10562-026-05507-0

Keywords: aluminum phosphate, citral, acetalization, 1,2-propanediol, solid acid catalyst, Brønsted acidity, Lewis acidity, fragrance chemistry, catalyst stability, one-pot synthesis, green catalysis, flavor and fragrance industry