Genetics researchers target cadmium reduction in cacao

The 28th annual International Conference on the Status of Plant and Animal Genome Research took place Jan. 11-15, in San Diego, and the Jan. 12 cacao genomics workshop included a presentation on the attempt to reduce the cadmium content of the bean used for chocolate.

Jim Dunwell of the University of Reading in the United Kingdom addressed the situation including potential solutions in “Molecular Genetic Approaches to Reducing Cadmium Accumulation in Cocoa.” Dunwell said that the European Union recently implemented a cadmium limit which took effect in January 2019.

“During the processing of chocolate, we’re trying to get rid of cadmium,” Dunwell said.

Chocolate is one of the major products with high cadmium levels, and rice grown in China also has high levels of cadmium.

“It’s something which is not good for anybody,” Dunwell said. “It accumulates in your body over your life.”

The source of cadmium could be natural such as volcanic soils, and the use of phosphate fertilizers, mining activity or industrial pollution can also create cadmium in the soil.

“There are hot spots geographically where cacao grown in those areas produce beans that exceed the limit,” Dunwell said.

The four nations with the greatest cacao production are in western Africa, where the cadmium levels in the soil are the lowest. Although those four nations account for approximately 72% of the world’s chocolate production, some of the highest-quality chocolate is from South American cacao and many of those areas have cadmium levels which exceed the European Union limit. The initial maximum levels allow for a higher cadmium concentration in darker chocolate but no more than 0.00008%; the lowest maximum level is 0.00001% and the limit for cocoa powder is 0.00006%.

“Africa and Asia the range is not too bad, but if you look at the Latin American, South American region the rate is much more,” Dunwell said. “It’s a threat to the livelihoods of particularly sensitive areas of challenged parts of South America.”

Efforts have been made to track the source of cadmium in those areas.

“Some of them are natural geological sources,” Dunwell said.

Other sources are due to fertilizers.

“Agriculture in many areas depends on the use of fertilizers,” Dunwell said. “Certain phosphate fertilizers in parts of the world have high levels of cadmium in them.”

The cadmium level in phosphate fertilizers varies depending on the source of the phosphate. Russian phosphate has lower cadmium levels, which creates the political risk that Russia will use that advantage as a bargaining tool.

“European agriculture doesn’t want to be dependent on Russian fertilizers,” Dunwell said.

Some of the research involves adjustment of cadmium levels in the soil.

“If you can stop the cadmium from getting into the plant that’s probably the best way to start,” Dunwell said. “Plants don’t need cadmium at all.”

Biochar binding of the soil is one potential solution.

“There is lots of agronomic interest and agronomic activity, but so far it hasn't been proven,” Dunwell said.

Adjusting the pH level of soil is complicated by the different levels throughout the world.

“There’s a very narrow pH window which you have to calculate,” Dunwell said.

A focus on cacao plant rejection of cadmium through the root stock or through transporter genes is part of the research.

“If we can do that in the long term, we can put that into a breeding program,” Dunwell said. “Cadmium comes in as a kind of hitchhiker. There is no cadmium-specific transporter.”

Most of the genetic work that has been done in the past uses rice as a model system.

“The idea was to identify the region looking at other species,” Dunwell said. “Rice is the precedent in terms of gene editing.”

Cadmium is believed to be transported into the plant through the Natural Resistance-Associated Macrophage Protein transcriptome. Cacao has five NRAMP genes.

“It’s at one gene that does one thing,” Dunwell said.

A mutagenesis process in the research utilizes downregulation of Heavy Metal ATPase genes, and when the HMA gene NtHMA4 is downregulated a reduction of more than thirtyfold is achieved.

“Mutagenesis has been used conventionally,” Dunwell said.

The NRAMP and HMA genes from were isolated in several accessions which differ in cadmium update.

“We looked at variations between the different accessions that we have,” Dunwell said.

Variants of different transcriptome length were then spliced. The researchers also used the NRAMP5 gene to transport cadmium from cacao into yeast.

“You can test these variants from different cacao accessions,” Dunwell said.

“The cadmium will have an inhibitory effect on the yeast,” Dunwell said. “It stops yeast growing, but it allows the uptake of cadmium into yeast.”

The purpose of that was for comparison rather than for adding cadmium to yeast, and the experiment confirmed that NRAMP5 encodes a protein capable of cadmium transport.

“We know they’re in the right part of the yeast cell,” Dunwell said. “We’re also interested in cadmium isotope variation. Plants can discriminate between these isotopes.”

Cadmium in nature has eight isotopes.

“We should consider cadmium not as one element but as a series of elements with different isotopes,” Dunwell said.

Joe Naiman can be reached by email at [email protected].