Researchers Develop a Better, Less Toxic Type of Rice

Nearly half of the world's population relies on rice as a main diet. However, it takes in more cadmium from the soil than other grains like barley and wheat. According to estimates, rice makes around 40–65% of the overall amount of cadmium we consume through our diet. Consuming rice tainted with cadmium poses a serious risk to one's health because excessive cadmium use has been connected to diseases like Itai-itai sickness.

Prior efforts included regulating water, importing clean soil, mixing polluted soil with charcoal and lime, and reducing the amount of cadmium in rice. However, these methods cost a lot of money and time. Cross-breeding has been utilized to create rice that collects less cadmium as a solution to this problem.

When compared to other rice types, Pokkali's roots take in more Mn and Cd into the root cells because of duplicated OsNramp5 (for example, Koshihikari). OsHMA3 sequesters the majority of the Cd that OsNramp5 absorbs into vacuoles, whereas OsMTP9 translocates the majority of the Mn to the shoots. The loading of Cd to the xylem, and subsequently to the shoots and grains, is reduced as a result of higher Mn content in the root cells competing with Cd for an unexplained efflux transporter. The dotted arrow depicts reduced Cd loading caused by elevated Mn. Mn and Cd are represented by the hues cyan and purple, respectively. Credit goes to Okayama University's Jian Feng Ma.

Professor Jian Feng Ma of the Institute of Plant Sciences and Resources at Okayama University in Japan says, "We have been studying the mechanisms of cadmium accumulation in rice and barley for a long time and have discovered several critical genes involved in its accumulation. Professor Ma has described the genetic pathways involved in this process in an article that was published in the journal Nature Food.

In their analysis of 132 rice accessions, Professor Ma and his team found that the gene OsNramp5, when replicated simultaneously, reduced the buildup of cadmium in Pokkali, a variety of rice grown for three thousand years in Kerala, India. Previous studies have shown that OsNramp5 encodes a rice transporter protein for cadmium and manganese. The absorption of both minerals into root cells is increased when the same gene is replicated simultaneously. Manganese and cadmium consequently compete in the cells for translocation to the shoots, preventing cadmium from accumulating in these areas.

The researchers found that just one of the 132 rice accessions, Pokkali, which can thrive in salty coastal soil, naturally contains the tandem duplication of OsNramp5.

The scientists also found that the spatial expression level of OsNramp5 was consistently nearly twice as high in the roots of Pokkali as it was in Koshihikari.

The researchers reproduced the OsNramp5 gene in Koshihikari, a type of rice that is particularly well-liked in Japan but accumulates relatively high amounts of cadmium, because Pokkali keeps remarkably little cadmium in its shoots. Professor Ma explains how focused breeding might benefit people by stating, "We found a gene responsible for differential cadmium accumulation in rice grain based on natural variations in cadmium accumulation. Following that, we effectively used this gene to create rice cultivars with low cadmium buildup in the grain.

The researchers discovered that the Koshihikari cultivar with the duplicated gene greatly reduced cadmium accumulation without sacrificing grain quality or production.

Professor Ma details the advantages of a low-cadmium rice variety, saying, "Cadmium is a dangerous heavy metal that endangers our health through the food chain. Our research gave breeders of rice types with minimal cadmium buildup valuable information that will help them produce safe and wholesome food. We anticipate that many different rice cultivars with minimal cadmium accumulation will be bred using this gene. By doing this, we will avoid cadmium toxicity.

The Japan Society for the Promotion of Science provided funding for the study.

By OKAYAMA UNIVERSITY