New Maize Protoplast System Accelerates Discovery of Defensive Terpene Genes

For decades, maize has served as a flagship model for understanding how plants defend themselves against hungry insects. When caterpillars chew through maize leaves, the plant responds by releasing a cocktail of volatile terpenoids—fragile, airborne molecules that summon parasitic wasps to attack the herbivores, prime neighboring tissues for future assaults, and in some cases directly poison the attackers themselves. Yet despite the ecological importance of these compounds, researchers studying the genes behind maize terpene biosynthesis have long lacked a fast, reliable tool for probing their function. A new study published in Crop Health changes that, describing a maize protoplast transfection system that allows scientists to test terpene synthase genes and their regulators in days rather than the months or years demanded by conventional genetic transformation.

The research, led by Jinfeng Qi and Jianqiang Wu of the Kunming Institute of Botany at the Chinese Academy of Sciences, addresses a stubborn bottleneck in plant functional genomics. Terpene synthases, or TPSs, are the enzymes that convert linear prenyl diphosphate substrates into the enormous diversity of terpenoid structures found across the plant kingdom. The traditional approach to determining what a given TPS produces involves expressing the gene in bacteria such as Escherichia coli and analyzing the products. But bacterial systems come with well-known liabilities: they cannot perform the eukaryotic post-translational modifications that may be essential for enzyme activity, high-level expression often drives misfolded proteins into insoluble inclusion bodies, and the prokaryotic cellular environment lacks the chaperones and folding machinery needed to keep some plant enzymes soluble and active. Generating stable transgenic maize lines to bypass these problems is expensive and slow, given the crop’s recalcitrance to transformation.

Protoplasts—plant cells stripped of their walls by enzymatic digestion—offer an attractive alternative. Exogenous DNA can be delivered into protoplasts by polyethylene glycol-mediated transfection, driving transient expression of genes of interest within 24 to 48 hours. Maize protoplasts have already proven useful for promoter analysis, protein interaction studies, and investigating nonvolatile defensive metabolites such as benzoxazinoids. But nobody had shown whether they could support the study of volatile terpene biosynthesis, which poses a distinct analytical challenge: the products evaporate, making detection and quantification far more delicate than measuring water-soluble compounds. The new work demonstrates that, with the right combination of growth conditions, culture parameters, and extraction chemistry, maize protoplasts can indeed become miniature terpene factories whose output is readily captured and measured.

The first obstacle the team faced was a genuine biological paradox. Protocols for isolating stable maize protoplasts typically call for etiolated seedlings grown in complete darkness, because the large central vacuoles characteristic of skotomorphogenic cells maintain high turgor pressure and help the naked cells survive the mechanical stresses of centrifugation and pipetting. However, light is a critical environmental cue for terpenoid metabolism: it drives plastid development and the accumulation of farnesyl diphosphate, the C15 precursor of sesquiterpenes. When the researchers compared seedlings grown in darkness with those given one, two, three, or eight days of dim light at 4.25 micromoles per square meter per second, they found that the partially greened seedlings accumulated dramatically more precursor and product. After methyl jasmonate treatment, the greenish seedlings released 2.7-fold more (E)-α-bergamotene and 3.8-fold more (E)-β-farnesene than etiolated controls, and farnesyl diphosphate levels rose up to 5.9-fold with eight days of light exposure. The compromise—eight days of dim light—balanced cellular robustness with metabolic competence, resolving what the authors call the light paradox.

Leaf anatomy added a second layer of optimization. The maize leaf is a developmental gradient running from the division-active basal meristem to the fully mature tip. Protoplasts isolated from the base and middle of the second leaf showed excellent integrity and transfection efficiencies above 90 percent with an enhanced green fluorescent protein reporter, but a substantial fraction of protoplasts from the leaf tips ruptured after transfection. The likely explanation lies in cellular architecture: in mature tip cells, the central vacuole has fragmented into smaller vacuoles and the cytoplasm is packed with chloroplasts, undermining osmotic stability and mechanical strength. The base and middle regions of greenish seedling leaves were therefore selected for all subsequent experiments, underscoring how developmental stage alone can make or break a protoplast protocol.

Detecting the volatile products required its own series of refinements. The team compared direct overnight headspace adsorption with solid-phase microextraction against a strategy in which protoplasts are first lysed and extracted with an organic solvent. Extraction with n-hexane or n-pentane vastly outperformed direct adsorption, and n-pentane was chosen for its lower toxicity and greater volatility. The final workflow concentrates the extract, heats it to 65 degrees Celsius to drive volatiles into the headspace, and captures them on a polydimethylsiloxane solid-phase microextraction fiber before gas chromatographic analysis on a Shimadzu GC-2014 system. Culture conditions mattered as well: a nutrient-rich medium supplemented with sucrose, glucose, and Murashige and Skoog salts supported higher terpene accumulation than the standard W5 solution, sealed containers and darkness outperformed light and open vessels, and larger culture volumes helped—in 50-milliliter headspace tubes, (E)-β-farnesene levels ran 60 percent higher than in 2-milliliter tubes.

Promoter choice and cofactor supply further sharpened the system’s sensitivity. When the maize gene ZmTPS10 was expressed from the maize ubiquitin promoter, protoplasts accumulated more (E)-α-bergamotene and (E)-β-farnesene than when the cauliflower mosaic virus 35S promoter drove the same gene. Spraying the mother plants with methyl jasmonate eight hours before protoplast isolation—an approach the authors describe as in planta priming—boosted terpene accumulation in subsequently transfected protoplasts more than tenfold compared with untreated controls, exploiting the intact plant’s metabolic machinery to enrich precursors rather than adding expensive, hydrolysis-prone farnesyl diphosphate directly to the medium. Because all terpene synthases carry a conserved DDxxD domain that binds magnesium, the researchers also tested whether adding 10 millimolar magnesium ions to the culture medium would help. It did: bergamotene rose 2.7-fold and farnesene 2.0-fold, confirming that cofactor availability limits ZmTPS10 activity inside the transfected cells.

To show that the platform extends beyond single-enzyme biochemistry, the team used it to interrogate transcriptional regulation. A previous study had identified ZmMYC2a, a basic helix-loop-helix transcription factor in the jasmonate signaling pathway, as a positive regulator of (E)-α-bergamotene and (E)-β-farnesene biosynthesis. Overexpressing ZmMYC2a in maize protoplasts significantly elevated both volatiles relative to a green fluorescent protein control, demonstrating that the system can resolve regulatory layers as well as catalytic function. The authors argue that the high efficiency of multi-plasmid co-transfection opens the door to reconstructing entire defense signaling cascades—receptor to kinase to transcription factor to structural gene—inside a single batch of cells. Because maize protoplasts provide a homologous cellular environment with the correct cofactors, membranes, and post-translational modification machinery, the resulting data should carry higher physiological fidelity than results from heterologous bacterial or yeast systems.

The implications reach well beyond one crop. Graminaceous staples such as wheat and sorghum share maize’s stubborn resistance to genetic transformation, and a rapid protoplast-based assay could serve as an efficient shortcut for functional gene studies across these species, with potential extension to other non-model plants through optimization of osmotic regulators, enzyme cocktails, and ionic conditions. Ecologically, the stakes are considerable: maize terpene volatiles recruit parasitoid wasps against fall armyworm and other pests above ground, attract entomopathogenic nematodes against the western corn rootworm below ground, and even directly damage the peritrophic matrix of lepidopteran midguts. By shrinking the timeline for assigning function to the roughly 30 TPS genes in the maize genome—about half of which still lack confirmed products—this system promises to accelerate the discovery of defensive chemistry that could inform breeding programs and crop synthetic biology for years to come.

Subject of Research: A maize protoplast transfection system for analyzing the biosynthesis and regulation of volatile terpenoid defense compounds

Article Title: A maize protoplast transfection system for studying the biosynthesis of volatile terpenoids

Article References: Qi, J., Li, M., Hu, Z., Li, R., Li, J., Zhang, M., Ma, C., & Wu, J. (2026). A maize protoplast transfection system for studying the biosynthesis of volatile terpenoids. Crop Health, 4(1), Article 13. https://doi.org/10.1007/s44297-026-00076-5

Image Credits: AI Generated

DOI: 10.1007/s44297-026-00076-5

Keywords: maize, protoplast transfection, terpene synthases, volatile terpenoids, ZmTPS10, methyl jasmonate, plant defense, ZmMYC2a, solid-phase microextraction, secondary metabolism, crop biotechnology, protoplast