Keeping Toxic Cadmium Out of Rice, The Genetic Way

September.06.2022RESEARCH HIGHLIGHTS

Release Subtitle:
Researchers discover that the duplication of a transporter gene lowers toxic cadmium accumulation in rice without affecting quality and yield

Release Summary Text:
The consumption of rice accounts for nearly half the total intake of cadmium, a toxic heavy metal, among humans. Now, scientists have found that the duplication of a cadmium transporter gene in the Pokkali rice cultivar reduces cadmium accumulation in the shoots and grain. They introduced this trait into a different rice cultivar and saw reduced cadmium accumulation with no effect on grain yield and quality, providing a useful target to breed low-cadmium-accumulating rice cultivars.

Full text of release:
Rice is a staple food for nearly half the world’s population. However, it accumulates more cadmium from the soil than other cereals like barley and wheat. Reports estimate that 40–65% of our total intake of cadmium, a toxic heavy metal, is from rice. Eating cadmium-contaminated rice poses a serious health risk to humans, with conditions like Itai-itai disease being associated with high cadmium intake.

Efforts have been previously made to reduce the quantity of cadmium in rice through methods like importing clean soil, water management, and mixing contaminated soil with biochar and lime. However, these methods are time-consuming and expensive. To remedy this, scientists have turned to cross-breeding to cultivate rice that accumulate less cadmium.

“We have been working on the mechanisms of cadmium accumulation in rice and barley for a long time and have identified several key genes involved in its accumulation,” says Prof. Jian Feng Ma, who is affiliated to the Institute of Plant Sciences and Resources at Okayama University, Japan. Recently, Prof Ma published a paper in the journal Nature Food[New window] detailing the genetic mechanisms that play a role in this process. The paper was published online on 18 August 2022 (19 August 2022 Japan time).

After examining 132 accessions of rice, Prof Ma and the members of his research group found that the gene, OsNramp5, when duplicated in tandem, reduced the accumulation of cadmium in Pokkali, a variety of rice that has been cultivated for 3000 years in Kerala, India. According to previous reports, OsNramp5 encodes a cadmium and manganese transporter protein in rice. The same gene, when duplicated in tandem, turns to increase the uptake of both the minerals into the root cells. Consequently, manganese competes with cadmium in the cells for translocation to the shoots, which in turn reduces the accumulation of cadmium in these parts.

The scientists found that out of the 132 accessions of rice, the tandem duplication of OsNramp5 was naturally found only in Pokkali, which can grow in salt-laden coastal soil.

The researchers also noted that the spatial expression level of OsNramp5 was always around two-fold higher in the roots of Pokkali than that in the roots of Koshihikari.

As Pokkali stores extremely low cadmium in its shoots, the scientists introgressed (a term for the transfer of genetic information across species) the duplicated OsNramp5 gene in Koshihikari, a variety of rice that is very popular in Japan but accumulates relatively high levels of cadmium. Explaining how targeted breeding can help humans, Prof. Ma says, “We identified a gene responsible for differential accumulation of cadmium in rice grain based on natural variations in cadmium accumulation. Then, we applied this gene to successfully breed rice cultivars with low cadmium accumulation in the grain.” The team found that the Koshihikari cultivar with the duplicated gene accumulated significantly lower amounts of cadmium without comprising on the grain quality or yield.

Recounting the benefits of a low cadmium-accumulating rice variety, Prof. Ma explains, “Cadmium is a toxic heavy metal and threatens our health through the food chain. Our study provided a useful material for breeding varieties of rice with low cadmium accumulation, which contributes to produce safe and healthy food. We hope that this gene will be widely used in breeding different rice cultivars with low cadmium accumulation. This will protect us from cadmium poisoning.”

Release URL:
https://www.eurekalert.org/news-releases/963250

Reference:
Duplication of a manganese/cadmium transporter gene reduces cadmium accumulation in rice grain Journal: Nature Food DOI:10.1038/s43016-022-00569-w

Contact Feng Ma
Dr. Jian Feng Ma is a Professor affiliated to the Institute of Plant Sciences and Resources at Okayama University, Japan. His area of expertise is plant nutrition. He has been awarded the Japan Prize of Agricultural Science and Yomiuri Prize of Agricultural Science in 2019 and the Japanese Society of Plant Physiologists Award in 2022 for outstanding achievements in plant science research. He has published over 300 papers in his particular field and was selected as the Most Cited Researcher for seven years since 2015. In addition, based on multiple evaluation criteria, he was ranked No. 1 in Plant Biology and Botany field in Japan and No. 3 in a single year (2020) in the world.


In Pokkali, due to duplicated OsNramp5, the roots take up more Mn and Cd into the root cells compared with other rice varieties (for example, Koshihikari). Most Cd taken up through OsNramp5 is sequestered into vacuoles by OsHMA3, while most Mn is translocated to the shoots through OsMTP9. Higher Mn concentration in the root cells competes with Cd for an unidentified efflux transporter, resulting in decreased loading of Cd to the xylem and subsequently to the shoots and grains. The dotted arrow shows decreased Cd loading by high Mn. The cyan and purple colours indicate Mn and Cd, respectively.

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