Low-cesium rice: mutation in OsSOS2 reduces radiocesium in rice grains

Abstract

In Japan, radiocesium contamination in foods has become of great concern and it is a primary issue to reduce grain radiocesium concentration in rice (Oryza sativa L.). Here, we report a low-cesium rice mutant 1 (lcs1) with the radiocesium concentration in grain about half that in the wild-type cultivar. Genetic analyses revealed that a mutation in OsSOS2, which encodes a serine/threonine-protein kinase required for the salt overly sensitive (SOS) pathway in plants, is responsible for the decreased cesium (Cs) concentrations in lcs1. Physiological analyses showed that Cs+ uptake by lcs1 roots was significantly decreased under low-potassium (K+) conditions in the presence of sodium (Na+) (low K+/Na+). The transcript levels of several K+ and Na+ transporter genes, such as OsHAK1, OsHAK5, OsAKT1, and OsHKT2;1 were significantly down-regulated in lcs1 grown at low K+/Na+. The decreased Cs+ uptake in lcs1 might be closely related to the lower expression of these genes due to the K+/Na+ imbalance in the lcs1 roots caused by the OsSOS2 mutation. Since the lcs1 plant had no significant negative effects on agronomic traits when grown in radiocesium-contaminated paddy fields, this mutant could be used directly in agriculture for reducing radiocesium in rice grains.

Production of low‐Cs+ rice plants by inactivation of the K+ transporter OsHAK1 with the CRISPR‐Cas system

“…However, due to the available information on shoot to grain 137 Cs + transfer, with coefficients for rice in the range of 0.2–0.5 (Figure b, c) (Ohmori et al ., ,b; Kato et al ., ), including that for plants of cv. Nipponbare grown on soils displaying low contents in exchangeable K + as in our second soil (Ishikawa et al ., ), radiocesium in grains in our WT‐c plants would undoubtedly have been high (>100 times higher than the permitted level). Under such conditions, and taking into account the huge difference in shoot 137 Cs + between WT‐c and oshak1 mutant plants (Figure g), it is very likely that the grains of KO plants grown on this soil would have had far lower 137 Cs + contents than the grains of WT‐c plants (by at least a factor of 10).…”

“…Two different soils were used (Figure a). The first soil contained 7 × 10 3 Bq of 137 Cs + per kg of soil dry weight (DW), and a level of exchangeable K + (177 mg/kg DW) close to the recommended range for reducing radiocesium concentration in rice plants to levels allowing grain consumption (Figure a) (Kato et al ., ; Ishikawa et al ., ). T2 plants grown for 113 days in this batch of soil displayed low 137 Cs + in grains (Figure b), within the permitted range (<100 Bq/kg) permitted by the 2012 Food Sanitation Act (Ohmori et al ., ).…”

“…Based on the current status of knowledge, it seems however very clear that biotechnological programs aiming at breeding plants that do not significantly take up and accumulate cesium would provide the most controlled and safest solutions to reduce the consequences of nuclear accidents on crop contamination and food production. So far, the numerous programs carried out in order to characterize the mechanisms of radioactive Cs + uptake and translocation in rice plants (Fujiwara, 2013;Nemoto and Abe, 2013;Ishikawa et al, 2017) have not identified biotechnological targets as promising as OsHAK1. The present results indicate that OsHAK1 is a major contributor to Cs + accumulation in rice.…”

ATP Binding Cassette Proteins ABCG37 and ABCG33 are required for potassium-independent cesium uptake in Arabidopsis roots

“…Even the ecotype screening for cesium transporters pointed out AtCNGC1 ( CYCLIC-NUCLEOTIDE-GATED CHANNEL 1 ) as potential causal gene, which is the close homolog of AtCNGC2 , a known K + transporter (Kanter et al, 2010). In another study, OsSOS2 had been shown to transport Cs + (Ishikawa et al, 2017). However, OsSOS2 was found to be indirectly linked to K + as in OsSOS2 mutant, K + and Na + transporter genes ( OsHAK1 , OsHAK5 , OsAKT1 , OsHKT2;1 ) are downregulated at low K + /Na + (Ishikawa et al, 2017).…”

“…In another study, OsSOS2 had been shown to transport Cs + (Ishikawa et al, 2017). However, OsSOS2 was found to be indirectly linked to K + as in OsSOS2 mutant, K + and Na + transporter genes ( OsHAK1 , OsHAK5 , OsAKT1 , OsHKT2;1 ) are downregulated at low K + /Na + (Ishikawa et al, 2017). It is predicted that Cs + uptake is reduced due to the lower expression of these transporters in presence of the K + /Na + imbalance (Ishikawa et al, 2017).…”