When farmers grow rice with less water to conserve freshwater, the rice grains can end up with more of a toxic metal called cadmium. For years, scientists around the world have wondered why this happens. Now, researchers in Nanjing, China have found the answer — and more importantly, they found a way to fix it.
A team led by professors Shen Renfang and Zhu Xiaofang from the Institute of Soil Science at the Chinese Academy of Sciences discovered the molecular chain of events that causes cadmium to build up in rice when farmers use water-saving irrigation methods. Their findings were published in the journal Current Biology.
The problem works like this: to save water, many rice farmers now use intermittent irrigation, periodically draining their fields instead of keeping them flooded. But when soil dries out and oxygen enters, it changes the soil chemistry in a way that makes cadmium — a heavy metal that occurs naturally in some soils — more easy for rice roots to absorb. The result: rice grains with higher cadmium levels, which can be harmful to human health if eaten regularly.
What surprised the research team was that this isn't just a chemical reaction happening in the soil. The rice plant itself is actively pulling in more cadmium through a biological pathway inside its cells.
The researchers identified a three-part mechanism: a protein called OsSAPK2 activates another protein called OsNAC4, which then switches on a gene called OsNRAMP1. That gene makes a transporter protein that acts like a doorway, letting cadmium flow from soil into the plant's roots and eventually into the grains.
But here's where the discovery becomes hopeful news: when the scientists used CRISPR-Cas9 gene editing to disable OsNAC4 in rice plants, the plants absorbed 30% to 50% less cadmium — even when grown under water-saving irrigation conditions. And critically, the edited rice plants still produced the same amount of grain and maintained normal levels of other essential metals like manganese and iron that plants need to grow.
"Our work demonstrates that elevated grain Cd under drainage is not merely a passive consequence of soil redox shifts; rather, plants actively amplify Cd absorption via endogenous ABA signaling cascades in response to aerobic environments," said Zhu, one of the study's lead authors.
This discovery gives rice breeders a precise target for developing new rice varieties that can be grown with less water while still producing grains safe for people to eat. As freshwater shortages grow more common around the world, this research offers a path forward for sustainable rice farming that doesn't compromise food safety.