Recently, Nature Communications published an online publication titled”A molecular switch in sulfur metabolism to reduce arsenic and enrich selenium in” by Professor Zhao Fangjie’s team from Nanjing Agricultural University.”Rice grain” research paper. This paper clarified that an arsenic-tolerant semi-dominant mutant astol1 (arsenite tolerant 1) was selected from the rice mutant library. It was found that allelic mutations of this gene can regulate the absorption of sulfur and selenium in rice, enhance rice sulfur metabolism, and promote Cysteine and plant chelating peptides are synthesized to achieve multiple effects of arsenic tolerance in rice, arsenic reduction in rice, sulfur and selenium enrichment in rice.
Arsenic is a metal-like metal widely found in the soil and is very toxic to organisms. The arsenic in the soil is easily activated under flooding conditions in rice fields, and the strong ability of rice roots to absorb arsenic makes it easier for rice to accumulate arsenic than other cereal crops. Rice is the main source of human arsenic intake. Therefore, controlling the accumulation of arsenic in rice is of great significance for ensuring the quality and safety of agricultural products and human health. Soil arsenic pollution may also cause arsenic poisoning in rice and reduce rice production. Contrary to arsenic, selenium is an essential trace element for the human body, which plays an important role in enhancing the body’s immune function and anti-oxidation to delay aging. Nearly three-quarters of rice in the world has low selenium content, which cannot meet the human body’s demand for selenium. Increasing the selenium content of rice is of great significance to improving the selenium nutritional status of the human body.
Zhao Fangjie’s team cloned the arsenic-tolerant mutant astol1 gene OsASTOL1 through forward genetics. This gene encodes a chloroplast-based cysteine synthase (OAS-TL). In the astol1 mutant, the 189th serine (S) of the protein is mutated to asparagine (N). A series of experiments proved that OsASTOL1S189N lost OAS-TL enzyme activity, but due to the presence of other isoenzymes, it did not affect cysteine synthesis. Compared with the wild type, OsASTOL1S189N enhances the stability of the cysteine synthase complex (CSC) formed with serine acetyltransferase (SAT), improves the activity of SAT enzyme in rice, and leads to the key to cysteine synthesis The substrate O-acetylserine (OAS) accumulates, and OAS is a signal substance that regulates sulfur metabolism, thereby positively regulating the absorption and assimilation of sulfur and selenium, increasing the content of sulfur and selenium in rice, and increasing sulfur metabolites including cysteine The synthesis of glutathione, glutathione, and phytochelin enhances the detoxification ability of rice to arsenic, and makes more arsenic trapped in the roots, and finally results in the phenotype of rice tolerant to arsenic and reducing arsenic and selenium enrichment in rice (Figure 1 ). The study also found that the mutation site is conserved in all bacterial and plant OAS-TL proteins, and the targeted mutation of Arabidopsis AtOAS-TL AS102N also has the characteristics of enhancing the stability of the CSC complex. Therefore, the S189N mutation of the rice OsASTOL1 protein gained new functions while losing its own function, making the switch of sulfur absorption and metabolism in an open state, which played a powerful role in regulating the absorption and metabolism pathways of sulfur and selenium (Figure 2). The study also showed that too much OsASTOL1S189N will affect the growth of rice. Overexpression of wild-type OsASTOL1 in mutant astol1 can balance the effect and reverse the negative effect on growth.
This study reveals that rice cysteine synthase complex affects sulfur metabolism and arsenic detoxification, which in turn affects the molecules that accumulate arsenic, sulphur and selenium in rice grains The mechanism provides new genetic resources for cultivating new varieties of low-arsenic and selenium-enriched rice. Cysteine synthesis is one of the basic metabolic pathways of organisms. The results of this study provide an important research tool for further clarifying the physiological and biochemical functions and regulatory mechanisms of plant cysteine synthase complexes.
The research work was completed by the research team of Fangjie Zhao from Nanjing Agricultural University, and the research team of Professor Rüdiger Hell of Heidelberg University in Germany participated in part of the research work. Sun Shengkai, a doctoral student in Zhao Fangjie’s research group, is the first author of the paper, and Zhao Fangjie is the corresponding author. This research work was funded by key international cooperation projects of the National Natural Science Foundation of China.