Could plant scientists breed heavy metal contamination out of cacao?

UNIVERSITY PARK, Pa. — Cadmium is a naturally occurring, toxic heavy metal found in many soils across Latin America and the Caribbean, where cacao trees are widely cultivated. It makes its way into cocoa seeds, called beans, that are roasted and made into chocolate, according to the Food and Agriculture Organization of the United Nations (FAO) — but some cacao varieties take up less of the toxic element than others. An international team of researchers, including Penn State scientists, found genetic differences in how two varieties managed cadmium uptake and facilitated the metal’s movement throughout the plant.

The findings — published this week (Sept. 8) in Plant and Soil — may be an early step toward breeding cacao that accumulates less cadmium, according to the collaborators from Penn State and Corporación Colombiana de Investigación Agropecuaria–AGROSAVIA (the Colombian Agricultural Research Corporation).

While trace amounts of cadmium in chocolate don’t present a serious risk to public health, long-term exposure to too much cadmium can lead to bone fragility as well as kidney and lung damage, explained one of the study’s two senior authors, Siela Maximova, research professor of plant biotechnology in Penn State's College of Agricultural Sciences.

“One of the key challenges in cacao farming is managing cadmium levels in the beans, especially in regions where the soil naturally contains cadmium,” she said. “If the soil doesn't have cadmium, the plants won't absorb it, and the beans won’t contain it. But in areas with contaminated soil, the only solution may be to plant cacao varieties that do not accumulate cadmium. This requires identifying and breeding naturally resistant varieties or using biotechnology to develop new ones, aiming to reduce cadmium uptake while maintaining high yield, disease resistance and good flavor.”

In the study, the team compared two cacao genotypes — distinct genetic varieties of the cacao tree — shown in previous research to take up different amounts of cadmium. They studied PA121, a low-cadmium-accumulating genotype originally collected from wild populations in the Marañon River basin in Peru, and the TSH660 genotype developed in Trinidad and Tobago as part of a breeding program to create disease-resistant, high-yielding trees that turned out to be high accumulators of cadmium.

The researchers grew cacao seedlings in a nutrient solution and exposed them to either no cadmium or low levels of cadmium. They tracked changes in gene activity over time and measured cadmium accumulation in the plants. The team also evaluated plant health by monitoring photosynthesis, water use and levels of a hormone involved in stress responses.

Although both varieties absorbed cadmium, they appeared to regulate the metal differently, with the largest differences seen in genes linked to detoxification, stress response, hormone signaling and cadmium movement within the plant. The low-accumulating variety showed gene activity patterns consistent with limiting cadmium uptake and managing stress more effectively, whereas the high-accumulating variety showed expression patterns that may facilitate greater cadmium movement through the plant. According to the researchers, these findings suggest that genetic differences influence how cacao plants handle cadmium, affecting the amount that reaches aboveground tissues and potentially the developing beans.

Maximova noted that the team measured gene expression soon after the cacao plants were exposed to cadmium but waited two months before measuring cadmium accumulation.

“We wanted to know if the plant’s immediate molecular response to cadmium helps explain why, months later, one genotype contains more cadmium than another,” she said. “This study shows that cadmium accumulation isn't determined solely by how much cadmium is present in the soil — it also depends on how the plant controls metal transport and its physiological response to cadmium.”

Maximova said that future research should focus on identifying candidate genes for genetic engineering or breeding new low-cadmium varieties of cacao.

“By identifying the genes and pathways involved, we can provide breeders with new tools to develop cacao varieties that accumulate less cadmium while preserving the traits growers and consumers value,” Maximova said. “This work could help farmers in naturally cadmium-rich regions remain competitive and contribute to a sustainable supply of high-quality cocoa in the future.”

For farmers in regions where cadmium occurs naturally in the soil, she added, these varieties may offer a practical way to meet international standards and maintain access to export markets.

In addition to Maximova, study collaborators from Penn State include co-first author Francisco Menéndez-Burns, who earned a doctoral degree in plant biology from Penn State; Mark Guiltinan, Franklin Styer Professor of Horticultural Botany and professor of plant molecular biology in the Department of Plant Science; and Albert Istvan, professor of bioinformatics in the Department of Biochemistry and Molecular Biology. Collaborators from Corporación Colombiana de Investigación Agropecuaria-AGROSAVIA include co-first author Paola Delgadillo-Durán, co-senior author Roxana Yockteng, Carlos Rodríguez-Medina and Andrea Montenegro.

The Penn State researchers’ contributions to this work were funded by the Endowed Program in the Molecular Biology of Cacao at Penn State and U.S. Department of Agriculture Hatch appropriations under project numbers PEN05003 and PEN4879 and accession numbers 7007428 and 7005892, and by the Penn State plant biology program and the Department of Plant Science. More funding information is available in the paper. This content is solely the responsibility of the authors and does not necessarily represent the views of the funders.