Cadmium is one of the most troubling contaminants in the global food supply, and chocolate has become one of its most recognizable carriers. For cacao farmers in Latin America, particularly those growing on volcanic soils where cadmium is naturally abundant, the metal’s presence in beans has led to rejected shipments and lost income under tightening European Union limits. A new study published in the journal Plant and Soil offers the first whole-transcriptome picture of how cacao trees respond to cadmium at the molecular level, and it reveals a surprisingly coordinated defense strategy that runs from hormone signaling in the roots all the way to the stomata on the leaf surface. The work was led by researchers at the Colombian Corporation for Agricultural Research (AGROSAVIA) and The Pennsylvania State University, who compared two cacao genotypes with dramatically different tendencies to accumulate the metal.
The team selected two genotypes from a large screening program: PA121, a high accumulator designated PA121(HA), and TSH660, a low accumulator designated TSH660(LA). Seedlings grown from open-pollinated seeds of each genotype were raised hydroponically in half-strength Hoagland’s solution at AGROSAVIA’s Palmira Research Station in Valle del Cauca, Colombia. After cadmium nitrate was added to bring the solution to 10 parts per million, the researchers sampled leaf and root tissues at zero, 24, and 48 hours for RNA sequencing, and again after 60 days for elemental analysis by inductively coupled plasma optical emission spectrometry. The results of the tissue analysis were striking. Cadmium-treated PA121 roots contained roughly 1.6 times more cadmium than TSH660 roots, and PA121 leaves accumulated approximately 6.4 times more cadmium than TSH660 leaves. Under control conditions without added cadmium, the two genotypes were statistically indistinguishable, confirming that the divergence emerged specifically under cadmium exposure.
The transcriptomic data revealed that cadmium mobilized thousands of genes. After 48 hours of exposure, the shared response across both genotypes involved 4,185 differentially expressed genes in roots but only 1,999 in leaves, underscoring that the root is the primary battleground of the cadmium response. Root expression skewed heavily toward downregulation, with 56 percent of responsive genes turned off, while leaf expression skewed toward upregulation. The most telling genotype difference appeared in the leaves: TSH660, the low accumulator, mounted only 180 differentially expressed genes in leaves at 48 hours, compared with 1,538 in PA121. The researchers interpret this as evidence of an avoidance strategy in TSH660, which keeps cadmium out of its tissues and therefore has little need for internal stress responses, whereas PA121 must activate extensive cellular machinery to cope with the cadmium it inevitably imports.
To make sense of the massive gene lists, the team used gene ontology enrichment, semantic similarity clustering, and KEGG pathway mapping. The single largest functional cluster in roots encompassed 767 genes tied to hormone biosynthesis and signaling, spanning abscisic acid, auxin, ethylene, strigolactone, and melatonin pathways. Several central enzymes of abscisic acid biosynthesis were strongly induced, including two orthologs of nine-cis-epoxycarotenoid dioxygenase, the rate-limiting step of the pathway, which were upregulated 7.5-fold and 8.6-fold. The second-largest cluster involved catabolism and detoxification, containing 31 lignin-forming peroxidase genes, of which 25 were downregulated, and 21 glutathione S-transferase genes, of which 15 were downregulated in the shared response. The broad suppression of lignin-forming peroxidases intrigued the authors, because lignified tissues are major sites of cadmium sequestration, and reduced peroxidase activity can elevate monolignols and coumarins, molecules with documented metal-chelating properties that may help mobilize or immobilize cadmium in the rhizosphere.
The study also documented a sweeping reconfiguration of metal transport. Among 82 differentially expressed genes encoding metal transporters, several stood out. TcIRT1, a ZIP-family iron transporter previously shown to import both iron and cadmium, was sharply repressed, down 48-fold in PA121 roots and nearly 23-fold in TSH660 roots. TcNRAMP5, a known contributor to cadmium uptake, was downregulated 9.6-fold specifically in TSH660 roots, while TcNRAMP6 was repressed in both genotypes. The heavy metal ATPase TcHMA3 was strongly downregulated in PA121 roots, a pattern opposite to its Arabidopsis ortholog but consistent with repression of xylem-loading transporters described in other species. Meanwhile, TSH660 uniquely upregulated a different HMA5 ortholog, TcRAN1, predicted to localize to the chloroplast membrane and linked to ethylene-coupled copper transport. These genotype-specific patterns suggest that differences in cadmium accumulation may hinge on which transporters are silenced and which are reinforced, rather than on any single master gene.
Beyond the membrane-level controls, the study pointed to an unexpected connection between cadmium stress and plant water relations. In a companion physiological experiment at Penn State, PA121 seedlings were exposed to 0, 6, or 12 parts per million cadmium under greenhouse conditions. Measurements with a LI-6400XT photosynthesis system showed that cadmium significantly reduced stomatal conductance, accounting for about 21 percent of its variation, with plants at 12 parts per million showing significantly lower conductance than controls by the second day. Critically, photosynthetic rate was unaffected, meaning the plants were conserving water without sacrificing carbon gain. The physiological data dovetailed with the hormonal measurements: 36 hours after cadmium treatment, root abscisic acid concentrations in treated seedlings averaged 1.95 nanomolar, roughly nine times the 0.21 nanomolar measured in controls, while leaf abscisic acid remained unchanged.
This combination of findings allowed the researchers to construct an integrated model of cadmium defense. In their hypothesis, cadmium perception in the root, likely initiated by reactive oxygen species and calcium signaling as documented in other plants, triggers abscisic acid biosynthesis in root tissue. The hormone signal travels to close stomata, reducing transpiration-driven mass flow through the xylem and thereby slowing the passive delivery of dissolved cadmium to the root surface. In parallel, the plant suppresses the expression of cadmium-permeable transporters such as IRT1 and NRAMP5, cutting off active import routes. Additional transcriptional changes point to adjustments in nitrogen, phosphorus, and sulfur transport, enhanced chelation through coumarin-like compounds, and a shift in carbohydrate metabolism that includes upregulation of raffinose biosynthesis genes and cuticle-related eceriferum genes, consistent with lowered leaf water potential and reduced water loss.
The parallels with drought response were not coincidental. When the team compared their cadmium transcriptome against a previously published cacao drought-response dataset from the cacao gene atlas, they found correlated changes in abscisic acid-related genes, including late embryogenesis abundant proteins and PYL receptors, as well as overlapping patterns in auxin and ethylene signaling gene families. This overlap suggests that breeding or engineering for improved water-use efficiency could carry collateral benefits for cadmium exclusion, a finding with immediate practical relevance for breeding programs in Colombia, Ecuador, and Peru, where both drought and cadmium constrain cacao production. The authors note, however, that the two stresses are not identical; cadmium altered nine auxin signaling genes compared with six under drought, and only some showed matching regulation, indicating distinct as well as shared pathways.
The study is not without limitations, which the authors acknowledge candidly. Hormone quantification was performed only on PA121 seedlings due to regulatory restrictions on moving cacao material between the United States and Colombia, so the abscisic acid response of the low-accumulator genotype remains unverified. Transporter regulation was assessed at the transcript level only, and the authors emphasize that functional validation through mutants, transporter assays, or gene editing will be essential before candidate genes can be deployed in marker-assisted selection. Still, the identification of specific, reproducibly regulated genes such as TcIRT1, TcNRAMP5, TcNRAMP6, and multiple HMA family members provides a concrete molecular toolbox for breeders seeking low-cadmium varieties that maintain normal growth on contaminated soils, the ideal agronomic phenotype of tolerance combined with exclusion.
As the first published whole-transcriptome analysis of cadmium response in Theobroma cacao, the study elevates chocolate’s source crop into the growing roster of plant species with detailed heavy-metal response maps. It also reframes cadmium accumulation not as passive leakage but as an active, hormonally orchestrated process that plants attempt to regulate, with different genotypes achieving very different degrees of success. The sequencing data, statistical workflows, and gas-exchange datasets have been deposited in public repositories including GenBank, Zenodo, and GitHub, making the resource freely available to the international cacao research community. Whether the candidate genes identified here will translate into chocolate with safer cadmium levels will depend on the next phase of functional studies, but the roadmap now exists, and it points toward a future in which the genetics of cadmium exclusion can be bred deliberately rather than discovered by accident.
Subject of Research: Molecular and physiological mechanisms of cadmium response and accumulation in seedlings of two Theobroma cacao genotypes, combining RNA-seq transcriptomics, hormone quantification, and gas-exchange measurements.
Subject of Research: Agriculture
Article Title: Molecular and physiological mechanisms of the cadmium response in seedlings of two Theobroma cacao L. genotypes
Article References: Delgadillo-Durán, P., Menéndez-Burns, F. M., Rodríguez-Medina, C., Montenegro, A. C., Istvan, A., Guiltinan, M. J., Yockteng, R., & Maximova, S. N. (2026). Molecular and physiological mechanisms of the cadmium response in seedlings of two Theobroma cacao L. genotypes. Plant and Soil. https://doi.org/10.1007/s11104-026-08980-z
Image Credits: AI Generated
DOI: 10.1007/s11104-026-08980-z
Keywords: cadmium, Theobroma cacao, abscisic acid, RNAseq, gas exchange, heavy metals, NRAMP, IRT1, HMA transporters, stomatal conductance, chocolate, genotype variation
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Alan Morgan. (September 8, 2026). How two cacao genotypes respond to cadmium at molecular and physiological levels. Scienmag. https://scienmag.com/how-two-cacao-genotypes-respond-to-cadmium-at-molecular-and-physiological-levels/
Alan Morgan. “How two cacao genotypes respond to cadmium at molecular and physiological levels.” Scienmag, 8 September 2026, https://scienmag.com/how-two-cacao-genotypes-respond-to-cadmium-at-molecular-and-physiological-levels/. Accessed 8 September 2026.
Alan Morgan. “How two cacao genotypes respond to cadmium at molecular and physiological levels.” Scienmag. September 8, 2026. https://scienmag.com/how-two-cacao-genotypes-respond-to-cadmium-at-molecular-and-physiological-levels/
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