Brassinosteroids are steroidal plant hormones that regulate a broad spectrum of growth and stress responses, including cell elongation, photosynthesis, and immunity. Exogenous application of 24-epibrassinolide, often abbreviated EBR, has previously shown promise in reducing disease symptoms in a variety of crops, but the metabolic mechanisms underlying this effect in kiwifruit had remained poorly characterized. Because postharvest fruit cannot rely on the inducible immune systems of growing plants in the same way, and because chemical fungicides face increasing regulatory and consumer pressure, understanding how hormone priming reshapes the fruit’s own chemical arsenal is of considerable practical interest. The new study addresses this gap by treating harvested kiwifruit with EBR, challenging the fruit with B. cinerea, and then tracking the resulting metabolic changes at 24, 48, and 72 hours after infection.

The physiological outcome was clear: EBR treatment significantly reduced lesion development compared with untreated controls. This visible protection provided the starting point for the molecular analysis. The researchers employed an integrated physiological and metabolomic approach, profiling the full complement of differentially accumulated metabolites at each of the three time points. They identified 35 differentially accumulated metabolites at 24 hours, 64 at 48 hours, and 57 at 72 hours, a pattern that itself hints at the dynamics of the response: a modest early shift, a pronounced mid-stage surge, and a sustained but slightly attenuated late phase as the fruit begins to recover metabolically.

To make sense of these hundreds of changing compounds, the team performed KEGG pathway enrichment analysis, which maps individual metabolites onto known biochemical networks. Four core pathways emerged as the backbone of the EBR-induced response: phenylpropanoid and flavonoid biosynthesis, amino acid metabolism, purine and cytokinin signaling, and cuticular and lipid metabolism. Each of these pathways contributes a distinct layer of defense, and their coordinated activation suggests that EBR primes the fruit to deploy chemical barriers, physical barriers, and signaling molecules in a structured sequence rather than as a diffuse stress reaction.

The phenylpropanoid and flavonoid pathways are among the most intensively studied branches of plant chemical defense. They generate a diverse array of secondary metabolites, including lignin precursors, antimicrobial phenolics, and antioxidant flavonoids, many of which directly inhibit fungal growth or reinforce cell walls against penetration. Their enrichment in the EBR-treated fruit indicates that the hormone steers metabolic flux toward the production of antifungal compounds, effectively arming the fruit’s chemical frontline before and during fungal attack. Because B. cinerea is a necrotroph that feeds on dead tissue and exploits oxidative damage, the antioxidant dimension of flavonoid accumulation may be particularly significant, blunting the reactive oxygen bursts that the pathogen uses to kill host cells.

Amino acid metabolism formed the second pillar of the response. Changes in free amino acid pools can influence resistance in several ways: some amino acids serve as precursors for defensive secondary metabolites, others act as signaling molecules that modulate hormone crosstalk, and still others alter the nutritional environment presented to the pathogen. A pathogen attempting to colonize fruit encounters not only preformed and induced antimicrobials but also a host whose nitrogen economy has been rearranged, potentially limiting the availability of the nutrients the fungus needs to establish infection. The study’s finding that amino acid metabolism is a core enriched pathway suggests that EBR treatment extends beyond simple toxin production and touches the fundamental resource allocation of the fruit under siege.

The third pillar, purine and cytokinin signaling, points to the communication layer of the defense response. Purine metabolism underpins energy transfer and nucleotide availability, while cytokinins are hormones classically associated with cell division but increasingly implicated in stress signaling and defense regulation. Their involvement indicates that EBR-treated fruit are not merely accumulating defensive chemicals but actively reconfiguring their signaling networks, coordinating the timing and magnitude of the response across tissues. This signaling activation was most prominent at the mid-stage of the trajectory, consistent with the 48-hour time point showing the largest number of differentially accumulated metabolites.

The fourth pillar, cuticular and lipid metabolism, addresses the physical dimension of defense. The cuticle is the waxy outer layer of the fruit surface and constitutes the first barrier any pathogen must breach. Lipid metabolism also supplies the building blocks for cuticular components and for signaling lipids involved in stress responses. By enriching this pathway, EBR treatment appears to reinforce the fruit’s physical armor at the same time that it stocks the chemical arsenal within. The authors note that this combination of chemical and physical barriers, together with signaling activation and metabolic reallocation, constitutes a potential coordinated defense response rather than a collection of isolated effects.

Perhaps the most conceptually important contribution of the study is its temporal analysis, which uncovered a triphasic metabolic trajectory. In the first phase, at 24 hours, the fruit exhibits early chemical defense priming, laying down the initial metabolic groundwork for resistance. In the second phase, at 48 hours, defense amplification takes hold, with signaling activation and a broad surge in differentially accumulated metabolites corresponding to the peak of the response. In the third phase, at 72 hours, the fruit enters metabolic recovery, as the defensive buildup subsides and resources are presumably reallocated back toward maintaining fruit quality. This staged architecture mirrors the priming-and-amplification logic seen in systemic immunity in growing plants and demonstrates that harvested fruit retain a remarkable capacity for organized, time-dependent metabolic defense when appropriately stimulated.

The practical implications extend in two directions. Scientifically, the study provides a comprehensive metabolomic framework for understanding EBR-mediated resistance, giving researchers a map of the pathways and candidate metabolites that can be targeted in future breeding, treatment optimization, or mechanistic studies. Agriculturally and commercially, the identification of specific defense-related metabolites offers potential biomarkers for selecting or verifying resistant fruit, and supports the development of EBR-based treatments as a component of postharvest disease management in kiwifruit, an approach that could reduce reliance on synthetic fungicides during storage. The work was financially supported by the National Natural Science Foundation of China, the Natural Science Foundation of Chongqing, and postgraduate research innovation projects at Chongqing University of Arts and Sciences, with Zhexin Li and Guohua Liu serving as corresponding authors. As the kiwifruit industry continues to grapple with postharvest losses, this study suggests that the fruit’s own metabolism, when nudged in the right direction at the right time, can be recruited as a powerful ally against one of its most persistent enemies.

Subject of Research: Metabolomic mechanisms of 24-epibrassinolide-induced resistance to gray mold in postharvest kiwifruit

Article Title: EBR-induced metabolic reprogramming contributes to kiwifruit resistance against gray mold

Article References: Luo, J., Li, L., Zhang, D., Wang, X., Zeng, T., Zhong, H., Gong, Z., Zhao, X., Peng, J., Luo, Z., Li, Z., & Liu, G. (2026). EBR-induced metabolic reprogramming contributes to kiwifruit resistance against gray mold. BMC Plant Biology. https://doi.org/10.1186/s12870-026-10140-5

Image Credits: AI Generated

DOI: 10.1186/s12870-026-10140-5

Keywords: kiwifruit, Botrytis cinerea, 24-epibrassinolide, brassinosteroids, gray mold, metabolomics, postharvest disease, plant immunity, phenylpropanoid pathway, flavonoids, disease resistance, plant hormones

Bagging is a long-standing, pesticide-free technique used across fruit orchards in East Asia. By enclosing young fruit in paper covers, farmers shield them from insect damage, sunburn, and pesticide residue, often producing a more uniform and attractive appearance at harvest. Yet the practice has a well-known side effect: bagged fruit frequently emerges paler than its sun-exposed counterparts. For most crops this trade-off is acceptable, but the underlying biology has remained murky, particularly in kiwifruit, where the pigments involved and their regulatory networks had not been systematically dissected until now.

A team of researchers at Chongqing University of Arts and Sciences, led by corresponding author Xuelian Sang, set out to close that gap. They combined three complementary layers of analysis—physiological measurements of pigment content, untargeted metabolomic profiling of the fruit skin, and transcriptomic sequencing to capture which genes were switched on or off. This integrative approach, applied to ‘Hongyang’ kiwifruit (Actinidia chinensis), allowed them to connect the visible phenotype of pale skin to specific molecular events occurring along the flavonoid and carotenoid biosynthetic pathways.

The physiological measurements delivered the first surprise. When the researchers quantified pigment levels in bagged versus unbagged fruit, the dominant effect was a pronounced reduction in carotenoid accumulation—the pigments that give kiwifruit skin its characteristic yellowish-brown tone. Anthocyanins, by contrast, played only a minor role. Even in the control fruit, anthocyanin content was strikingly low, measuring just 11 micrograms per gram of skin at 130 days after pollination. This means that in ‘Hongyang’, the story of bagging-induced color loss is overwhelmingly a carotenoid story, with anthocyanins as a supporting character rather than the protagonist.

Metabolomic profiling sharpened the picture further. The team detected 33 differential metabolites between bagged and control fruit skins—15 that increased and 18 that decreased. Crucially, every one of the differential metabolites assigned to the flavonoid biosynthesis pathway, designated map00941 in the Kyoto Encyclopedia of Genes and Genomes, was present at significantly lower levels in the bagged fruit. This wholesale suppression of flavonoid pathway output suggested that bagging was not merely tweaking a single enzymatic step but dampening the entire metabolic pipeline that feeds pigment production.

To understand how that suppression was orchestrated, the researchers turned to their transcriptomic data. The decrease in anthocyanin accumulation in bagged fruit was accompanied by the downregulation of several structural genes that encode the enzymes of anthocyanin biosynthesis, including CHS (chalcone synthase), CYP73A (a cytochrome P450 enzyme), DFR (dihydroflavonol 4-reductase), and F3H (flavanone 3-hydroxylase). These enzymes sit at critical junctions of the pathway: CHS catalyzes the first committed step toward all flavonoids, while DFR channels intermediates toward the anthocyanin branch. When these genes fall silent, the flux of precursors into colored compounds slows to a trickle. The team also observed differential expression of MBW transcription factors—the protein complexes composed of MYB, bHLH, and WD40 subunits that act as master switches controlling anthocyanin gene expression—indicating that bagging exerts its influence at the regulatory level as well as the enzymatic one.

The metabolite data added an intriguing wrinkle. Among the 33 differential metabolites, one compound moved in the opposite direction from the anthocyanins: neohesperidin, a flavanone glycoside, increased in bagged fruit, while four other metabolites—butin, afzelechin, eriodictyol, and gallocatechin—all decreased alongside the anthocyanin decline. The authors are careful to frame these patterns as correlative rather than causative, but the inverse relationship between neohesperidin accumulation and anthocyanin reduction hints at a possible regulatory link. One speculative mechanism is metabolic competition: if pathway intermediates are diverted toward neohesperidin synthesis, fewer substrates remain available for the anthocyanin branch. Alternatively, neohesperidin accumulation could be a downstream consequence of the same transcriptional reprogramming. Distinguishing between these possibilities will require targeted experiments, but the correlation provides a concrete lead for future functional studies.

The carotenoid side of the story proved equally revealing. Bagging markedly altered the expression of two key genes in the carotenoid biosynthesis pathway. The gene crtZ, which encodes beta-carotene hydroxylase—an enzyme that converts beta-carotene into zeaxanthin—was downregulated by a staggering 95 percent. Meanwhile, LCY-epsilon, which encodes lycopene epsilon-cyclase, an enzyme that channels carotenoid precursors into the alpha-carotene and lutein branch, was upregulated 2.16-fold. The near-total shutdown of crtZ is particularly striking, as it effectively blocks the hydroxylation steps needed to produce the oxygenated xanthophylls that contribute significantly to skin color. Together, these transcriptional shifts plausibly account for the observed drop in total carotenoid content, though the authors note the association remains to be validated experimentally.

What makes this study methodologically notable is its integrative design. Rather than relying on a single omics layer, the researchers cross-referenced physiology, metabolomics, and transcriptomics to build a coherent causal chain: bagging alters light exposure, which reshapes transcription factor activity, which suppresses structural genes, which depletes pigment precursors, which lightens the skin. Each link in that chain is supported by data, even if the full regulatory logic—from photoreceptor signaling to MBW complex assembly—remains to be worked out. The approach also surfaced candidate genes and metabolites that breeders and growers can now target, whether through optimized bagging schedules, selective breeding for pigment stability, or post-harvest treatments designed to restore coloration.

The practical implications extend beyond kiwifruit. Bagging is used on apples, pears, grapes, mangoes, and many other crops worldwide, and the molecular principles uncovered here—light-dependent carotenoid accumulation, transcriptional control of flavonoid flux, and the interplay between competing branches of the phenylpropanoid pathway—are broadly conserved across plant species. For the ‘Hongyang’ industry specifically, the study suggests that growers who want the protective benefits of bagging without sacrificing skin color may need to time bag removal carefully, allowing a window of light exposure before harvest to rebuild carotenoid reserves. It also provides a reference framework for optimizing bagging practices to improve both the appearance and quality of kiwifruit, turning what was once an empirical art into a science guided by genes and metabolites. As consumer demand for visually flawless, residue-free fruit continues to grow, studies like this one demonstrate how genomics can illuminate the quiet chemistry unfolding beneath a paper bag.

Subject of Research: Molecular mechanisms by which bagging treatment inhibits skin coloration in 'Hongyang' kiwifruit through carotenoid and flavonoid pathway regulation

Article Title: Integrative transcriptomic and flavonoid metabolomic analysis reveals how bagging inhibits skin coloration in ‘Hongyang’ kiwifruit (Actinidia chinensis)

Article References: Jue, D., Liu, Y., Liu, Y., Zhang, Z., Ren, C., Chen, T., Zhang, Q., & Sang, X. (2026). Integrative transcriptomic and flavonoid metabolomic analysis reveals how bagging inhibits skin coloration in ‘Hongyang’ kiwifruit (Actinidia chinensis). BMC Genomics. https://doi.org/10.1186/s12864-026-13413-3

Image Credits: AI Generated

DOI: 10.1186/s12864-026-13413-3

Keywords: kiwifruit, bagging treatment, skin coloration, carotenoid biosynthesis, anthocyanin, flavonoid metabolism, transcriptomics, metabolomics, Actinidia chinensis, gene expression, MBW transcription factors, fruit quality

Kiwifruit is what botanists call a climacteric fruit, meaning it continues to ripen after harvest by producing its own ethylene, a gaseous plant hormone that orchestrates softening, sweetening, and aroma development. Global production of the fruit has surpassed four million tons annually since 2017, reaching roughly 4.4 million tons by 2024, with the green-fleshed Hayward cultivar dominating commercial markets thanks to its characteristic flavor. Because kiwifruit is typically picked while still firm and physiologically mature but far from ripe, its flavor, sweetness, and aroma remain underdeveloped at harvest and must be coaxed along with ethylene treatment before it reaches consumers.

The research team, led by Inhwan Kim and Eunyoung Park of Chung-Ang University and Seoul National University along with colleagues at the Korea Food Research Institute, set out to answer a question that has lingered in postharvest science: how exactly do ethylene and 1-methylcyclopropene alter not just the quantity of aroma compounds in kiwifruit, but their actual sensory impact? Previous studies had catalogued volatile changes during ripening, but raw concentrations can be misleading, because odor thresholds vary enormously between compounds. A chemical present in large amounts may barely register to the human nose, while a trace compound can dominate the aroma experience.

To capture the full picture, the researchers subjected Hayward kiwifruit imported from New Zealand to three treatments in sealed chambers at 20 degrees Celsius: an ethylene exposure of 1000 microliters per liter for 22 hours, a 1-methylcyclopropene treatment at 1.4 microliters per liter, and an untreated control. Calcium hydroxide was included in each chamber to absorb carbon dioxide and keep conditions consistent. Fruit from each group was then analyzed at days zero, three, five, seven, and ten of storage, yielding a time-resolved portrait of aroma chemistry under each regime.

The analytical workhorse of the study was headspace solid-phase microextraction coupled with gas chromatography-mass spectrometry, a technique that allows volatile compounds released from the fruit pulp to be captured on an absorbent fiber and then separated and identified with high precision. Using isotopically labeled internal standards, the team quantified 27 distinct volatile compounds: nine aldehydes, seven esters, four alcohols, five terpenes, one ketone, and one furanone. Ten of these, including hexanal, hex-2-enal, nonanal, benzaldehyde, methyl hexanoate, and the pinenes, were confirmed against authentic reference standards, while the rest were identified through mass spectral libraries and retention indices.

The headline numbers are striking. Over ten days of storage, total volatile content in untreated control fruit climbed from 761 to 6335 micrograms per kilogram, an 8.32-fold increase, while ethylene-treated fruit surged even further, from 927 to 8771 micrograms per kilogram, a 9.46-fold rise. In stark contrast, fruit treated with 1-methylcyclopropene, which jams the ethylene receptors on cell membranes and prevents the ripening signal from being received, saw its volatile content fall to just 71.5 percent of its starting level by day ten. The fruit had essentially been frozen in an immature aromatic state.

But raw abundance tells only half the story, so the researchers calculated relative odor activity values, or rOAVs, which compare each compound’s concentration to its known odor threshold and express it relative to the most potent odorant in the sample. Compounds with rOAV values of one or higher are considered critical to the characteristic aroma, those between 0.1 and one contribute to overall flavor, and those below 0.1 are merely potential aroma players. This sensory-weighted lens revealed a clear narrative arc: early in storage, the aroma of all three groups was dominated by aldehydes, particularly hexanal and hex-2-enal, the C6 compounds responsible for the fresh, green, grassy notes characteristic of freshly cut kiwifruit.

As storage progressed, a dramatic shift occurred in the control and ethylene-treated fruit. Esters, the class of compounds responsible for fruity and sweet aromas, began to accumulate explosively. Methyl isobutyrate in stored control fruit reached 1067 micrograms per kilogram, a staggering 97-fold increase over fresh fruit, and in ethylene-treated samples it climbed 370.9-fold. Ethyl benzoate, undetectable in fresh ethylene-treated fruit, reached 2401 micrograms per kilogram after storage and ultimately displayed the highest odor activity of any compound identified by day ten in the ethylene group. Methyl butyrate, methyl isobutyrate, and ethyl benzoate all crossed the critical rOAV threshold of one by day seven in controls and by day five in ethylene-treated fruit, marking the point at which the kiwifruit’s aroma pivots from green to genuinely fruity.

Interestingly, the study punctured a long-standing assumption. Ethyl acetate is routinely monitored as the signature ester of fruit ripening, and its concentration did indeed rise during storage. But its odor threshold is exceptionally high at 6200 micrograms per kilogram, so its rOAV remained below 0.01 throughout the experiment, meaning it contributes almost nothing to the aroma humans actually perceive. The real aromatic powerhouses, the authors argue, are the short-chain esters like methyl isobutyrate and the benzoates like ethyl benzoate, which should be considered the true indicators of fruity aroma development in ripe kiwifruit. The biochemistry behind this shift involves the lipoxygenase pathway, which generates aldehyde precursors, and the enzyme alcohol acyltransferase, which esterifies acyl-CoA molecules with alcohols to form esters; prior work has shown that genes encoding these enzymes are positively correlated with ester accumulation and are suppressed by 1-methylcyclopropene treatment.

The practical implications are considerable. Principal component analysis showed that 1-methylcyclopropene-treated fruit retained an early-stage aroma profile throughout the entire storage period, clustering with fresh samples rather than progressing toward the ripe, ester-rich profile of the other groups. This confirms that blocking ethylene perception preserves shelf life and firmness but comes at the cost of aroma development, a trade-off that distributors must weigh depending on their timeline. Conversely, ethylene treatment accelerates the arrival of full fruity aroma, making it a tool for preparing ready-to-eat fruit on demand. By combining quantitative volatile profiling with odor activity analysis, the Korean team has provided the industry with a sensory-meaningful framework for deciding exactly when and how to intervene, transforming what was once an art of guesswork into a science of precision flavor control during the long journey from vine to table.

Subject of Research: Effects of ethylene and 1-methylcyclopropene on aroma volatile profiles of Hayward kiwifruit during postharvest storage

Article Title: Ethylene and 1-methylcyclopropene modulate aroma volatile profiles in kiwifruit (Actinidia spp.) during postharvest storage

Article References: Kim, I., Park, E., Lee, H., Ahn, D., Choi, J. H., Park, K.-J., Lim, J.-H., & Lee, J. (2026). Ethylene and 1-methylcyclopropene modulate aroma volatile profiles in kiwifruit (Actinidia spp.) during postharvest storage. Food Science and Biotechnology. https://doi.org/10.1007/s10068-026-02248-z

Image Credits: AI Generated

DOI: 10.1007/s10068-026-02248-z

Keywords: kiwifruit, ethylene, 1-methylcyclopropene, aroma volatiles, postharvest storage, esters, aldehydes, odor activity value, climacteric fruit, HS-SPME-GC-MS, food science, fruit ripening