A team of researchers from Japan and China has identified a gene duplication in rice that reduces accumulation of cadmium, a toxic heavy metal, without compromising grain quality or yield.
Eating rice with elevated levels of cadmium poses a serious health risk to humans. Diseases such as itai-itai, which cause softened bones and kidney dysfunction, are associated with high cadmium intake.
The duplicated gene is OsNramp5. In a paper published in Nature Food, “Duplication of a manganese/cadmium transporter gene reduces cadmium accumulation in rice grain,” Jian Feng Ma, PhD, professor the Institute of Plant Sciences and Resources at Okayama University, Japan, and colleagues detail the effects of the duplication.
“Global contamination of soils with toxic cadmium (Cd) is a serious health threat,” the researchers wrote in their article. Rice, a staple food for nearly half the world’s population, 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.
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 genetics. “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,” Ma said.
“[In our current study,] we found that a tandem duplication of [OsNramp5] was responsible for low-Cd accumulation in Pokkali, an old rice cultivar,” Ma’s team wrote in their paper. Pokkali is a variety of rice that has been cultivated for 3000 years in Kerala, India, and can grow in salt-laden coastal soil. It was the only cultivar, of 132 accessions examined, in which the scientists found the duplication.
According to previous reports, OsNramp5 encodes a cadmium and manganese transporter protein in rice. Ma’s team found that the duplication doubled the expression of the OsNramp5 gene but did not alter its spatial expression pattern and cellular localization.
Because of the higher expression of OsNramp5, the root cells took up more cadmium and manganese. However, because manganese competes with cadmium for translocation to the shoots, there was less cadmium in the shoots of the plants—including in the rice grain.
The scientists then introgressed the duplicated OsNramp5 gene into Koshihikari, a variety of rice that is very popular in Japan but accumulates relatively high levels of cadmium. “Introgression…through backcrossing significantly reduced Cd accumulation in the grain when cultivated in soil heavily contaminated with Cd but did not affect both grain yield and eating quality,” they wrote.
The researchers hope that through targeted breeding and other approaches, their findings may lead to healthier varieties of rice.
“Cadmium is a toxic heavy metal and threatens our health through the food chain,” Ma said. “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.”
