Phytosterols are steroid-like triterpenes that occur in plant tissues and contribute to membrane structure as free sterols, sterol esters, steryl glycosides and acylated steryl glycosides. β-Sitosterol, campesterol and stigmasterol are prevalent plant phytosterols, with β-sitosterol described as the most abundant. Their functions and commercial uses include roles in food fortification, nutraceuticals and pharmaceuticals. The review cites a market assessment of more than $700 million in 2019 and a projected compound annual growth rate of 8.7% from 2020 to 2027, but these commercial estimates are background to the review rather than experimental outcomes.

Plant extraction and chemical processing can impose substantial demands on raw materials, energy and solvents. Solvent extraction, including the Soxhlet method and cold pressing, may require large quantities of plant biomass because phytosterol concentrations in raw materials are low, while output can vary with source and season. Chemical esterification permits structural modification but can involve complicated procedures, side reactions and low selectivity, including formation of phytosterol oxides, dienes and trienes that reduce yield and complicate purification. Enzymatic ester synthesis with lipases offers a more selective biological alternative, and the review describes it as potentially more ecologically sustainable and safer, although it does not establish a universal life-cycle advantage.

The biosynthetic pathway begins with acetyl-CoA and proceeds through the mevalonate pathway in plants, with some capacity to draw precursors from the plastidial MEP pathway. HMG-CoA reductase, abbreviated HMGR, converts HMG-CoA to mevalonate and is identified as a major rate-limiting control point. Its activity is regulated through transcription, feedback inhibition, phosphorylation, ubiquitination and endoplasmic-reticulum-associated degradation. Squalene synthase then forms squalene, squalene epoxidase produces 2,3-oxidosqualene, and oxidosqualene cyclases create cyclic sterol or triterpene skeletons. Sterol methyltransferases and sterol methyl oxidases subsequently affect the side-chain and methyl-group patterns that distinguish individual phytosterols.

Because this pathway is tightly regulated, raising the expression of one upstream gene does not necessarily produce a proportional increase in a target compound. Plant experiments reviewed in the article illustrate that complexity. Overexpressing HMGR1 in Arabidopsis increased several sterols while also accumulating upstream intermediates. Co-expression of HMGR and squalene synthase in Panax notoginseng produced approximately 1.8-fold higher β-sitosterol and 2.0-fold higher stigmasterol relative to wild-type cells. In rice, altering several mevalonate-pathway genes while reducing expression of competing MEP-pathway genes increased campesterol by 1.41- to 2.24-fold in one set of transgenic lines; stigmasterol and β-sitosterol also rose in subsets of lines, although not uniformly across all lines.

Protein engineering can increase pathway performance more directly than changing gene expression alone. A serine-to-alanine substitution at position 359 in HMG-CoA synthase 1 from Brassica juncea produced approximately tenfold greater enzyme activity than the wild-type enzyme in the reported assays. When introduced into Arabidopsis, tobacco and tomato, the mutant increased total leaf phytosterols by 20-30% relative to the wild-type overexpression construct. Removing the membrane-binding N-terminal domain from HMGR has also increased phytosterol accumulation in transgenic plants, including a 3.2-fold increase in tobacco seeds and a tenfold increase in leaves in one study, although some mutants showed abnormal floral development or reduced seed set. The review therefore identifies tissue-specific expression and developmental testing as important considerations.

Downstream enzymes can determine which phytosterol accumulates, but their effects may also alter plant growth or stress resistance. Stable overexpression of a squalene synthase in Nicotiana benthamiana produced a 160% increase in stigmasterol relative to the control while sitosterol and campesterol remained unchanged; the same engineered plants showed reduced resistance to the oomycete Phytophthora infestans. An oxidosqualene cyclase from Taraxacum coreanum reduced β-sitosterol, campesterol and stigmasterol in tobacco because it competed for the shared intermediate 2,3-oxidosqualene and supported other triterpene routes. By contrast, cotton engineered to express a sterol C-24 methyltransferase showed higher sitosterol and a changed campesterol-to-brassicasterol ratio, accompanied by shorter, thicker fibers. Such findings are species- and tissue-specific and do not establish a universal effect on every engineered crop.

Transport and storage provide another layer for metabolic engineering. The review organizes plant interventions as “push,” “block” and “pull”: increasing biosynthetic flux, suppressing competing routes and altering the storage or transport of products. Overexpression of the rice SCYL2 vesicle-associated protein increased campesterol, stigmasterol and β-sitosterol accumulation, whereas an scyl2 mutant had reduced amounts. These changes can interact with membrane biology, development and stress responses. The review emphasizes that long-term effects on growth, fertility, stress tolerance and overall fitness are rarely evaluated, limiting conclusions about agricultural deployment.

Heterologous production moves selected phytosterol pathways into hosts such as bacteria, algae and yeast. These systems allow engineers to redirect acetyl-CoA and other precursors, replace parts of a native ergosterol pathway with plant enzymes and optimize fermentation. Engineered E. coli has assembled modules for cycloartenol production, while yeast can reconstitute complete pathways when required redox partners, including NADPH-cytochrome P450 reductase, are supplied. Screening can identify more efficient enzymes: an Ajuga reptans DWARF1 enzyme converts 24-methylenecholesterol to campesterol in one step rather than through the less efficient two-step route used by orthologs such as rice DWARF1. The review also identifies a scarcity of commercial analytical standards for intermediate sterols as a practical obstacle to locating metabolic bottlenecks.

Higher titers introduce a second problem because excess free sterol can be toxic to the producing cell. Yeast may respond by esterifying sterols, creating a metabolic sink that can lower the concentration of free product. Deleting genes encoding the acyltransferases ARE1 and ARE2 can increase free sterols, but complete removal of esterification may impair growth. For 24-epi-ergosterol, a more balanced system retained ARE1 while overexpressing ARE2 and the ester-hydrolyzing enzymes YEH1 and YEH2. This “push-and-pull” design achieved 2.76 grams per liter of 24-epi-ergosterol in fed-batch fermentation. A separate campesterol strategy restored sterol homeostasis rather than simply blocking storage and reached 5.88 grams per liter in fed-batch culture. The results show that controlled storage can be compatible with high production, although they do not guarantee the same performance in every strain or product.

Systems-level engineering integrates metabolic changes with fermentation and cellular organization. One study combined modular expression of sterol genes with lipid-droplet regulation, raising campesterol from 88.3 milligrams per liter to 169.20 milligrams per liter in shake-flask cultures. Optimization of metal-ion regulation and operation in a 5-liter fed-batch bioreactor increased the titer to 1.44 grams per liter. Lipid droplets can provide soluble reservoirs for hydrophobic products, but the review frames this as a strategy for mitigating toxicity and improving total titers rather than as proof of industrial scale. Another study engineered Yarrowia lipolytica to produce a blend of pollen sterols corresponding to the profile identified as necessary for honey-bee brood development. When the engineered yeast was included in bee feed, brood production increased in the absence of natural pollen. That experiment tested a functional nutritional application in bees and does not establish a clinical benefit in humans.

Looking ahead, the review proposes combining CRISPR-based editing, new host organisms and machine-learning tools for enzyme design. Models including UniKP, EnzyACT, DeepEnzyme and TCNeKP predict aspects of enzyme activity or kinetics from protein sequences, structures and reaction conditions, potentially narrowing the search for useful mutations. Better analytical methods are also needed: researchers have used transient Nicotiana benthamiana expression to generate a library of phytosterol intermediates for gas chromatography–mass spectrometry characterization and constructed a customized analytical standard mixture for diagnosing rice mutants. The review’s broader prospect is that improved pathway models, enzyme screening, storage control and fermentation could support more efficient production. It does not establish that engineered systems have already replaced agricultural extraction or that all phytosterols are now economically accessible at industrial scale.

Subject of Research: Review of metabolic engineering strategies for plant and microbial phytosterol production

Article Title: The potential of metabolic engineering for sustainable phytosterol production

Article References: Hu, R., Di, X., Li, G., Schaller, H., Hemmerlin, A., & Liao, P. (2026). The potential of metabolic engineering for sustainable phytosterol production. Advanced Biotechnology, 4(2), Article 15. https://doi.org/10.1007/s44307-026-00108-9

Image Credits: AI Generated

DOI: 10.1007/s44307-026-00108-9

Keywords: phytosterols, metabolic engineering, campesterol, beta-sitosterol, stigmasterol, CRISPR, enzyme engineering, synthetic biology, yeast, lipid droplets, potential, metabolic