Genetic engineering low-arsenic and low-cadmium rice grain

Rice is prone to take up the toxic elements arsenic (As) and cadmium (Cd) from paddy soil through the transporters for other essential elements. Disruption of these essential transporters usually adversely affects the normal growth of rice and the homeostasis of essential elements. Here we report on developing low-As and low-Cd rice grain through the co-overexpression of OsPCS1, OsABCC1, and OsHMA3 genes under the control of the rice OsActin1 promoter. Co-overexpression of OsPCS1 and OsABCC1 synergistically decreased As concentration in the grain. Overexpression of OsPCS1 also decreased Cd concentration in the grain by restricting the xylem-to-phloem Cd transport in node I, but paradoxically caused Cd hypersensitivity as the overproduced phytochelatins in OsPCS1-overexpressing plants suppressed OsHMA3-dependent Cd sequestration in vacuoles and promoted Cd transport from root to shoot. Co-overexpression of OsHAM3 and OsPCS1 overcame this suppression and complemented the Cd hypersensitivity. Compared with non-transgenic rice control, co-overexpression of OsABCC1, OsPCS1, and OsHMA3 in rice decreased As and Cd concentrations in grain by 92.1% and 98%, respectively, without causing any defect in plant growth and reproduction or of mineral nutrients in grain. Our research provides an effective approach and useful genetic materials for developing low-As and low-Cd rice grain.

Multifaceted response mechanisms of Oryza sativa L. 'KDML105' to high arsenite and arsenate stress levels

Toxicity resulting from high levels of inorganic arsenic (iAs), specifically arsenite (AsIII) and arsenate (AsV), significantly induces oxidative stress and inhibits the growth of rice plants in various ways. Despite its economic importance and significance as a potent elite trait donor in rice breeding programmes, Khao Dawk Mali 105 (KDML105) has received limited attention regarding its responses to As stress. Therefore, this study aimed to comprehensively investigate how KDML105 responds to elevated AsIII and AsV stress levels. In this study, the growth, physiology, biochemical attributes and levels of As stress-associated transcripts were analysed in 45-day-old rice plants after exposing them to media containing 0, 75, 150, 300 and 600 µM AsIII or AsV for 1 and 7 days, respectively. The results revealed that AsIII had a more pronounced impact on the growth and physiological responses of KDML105 compared to AsV at equivalent concentrations. Under elevated AsIII treatment, there was a reduction in growth and photosynthetic efficiency, accompanied by increased levels of hydrogen peroxide (H2O2) and malondialdehyde (MDA). Notably, the total contents of antioxidants, such as proline, phenolics and flavonoids in the shoot, increased by 8.1-fold, 1.4-fold and 1.6-fold, respectively. Additionally, the expression of the OsABCC1 gene in the roots increased by 9.5-fold after exposure to 150 µM AsIII for 1 day. These findings suggest that KDML105's prominent responses to As stress involve sequestering AsIII in vacuoles through the up-regulation of the OsABCC1 gene in the roots, along with detoxifying excessive stress in the leaves through proline accumulation. These responses could serve as valuable traits for selecting As-tolerant rice varieties.

A perfect PCR based co-dominant marker for low grain-arsenic accumulation genotyping in rice

The development of low arsenic-accumulating varieties for the contaminated areas is one of the best options for reducing the dietary exposure of arsenic to human population through rice. In this study, grain-arsenic content in one hundred genotypes revealed a large variation ranging from 0.05 mg/kg to 0.49 mg/kg. Compared to high accumulating variety, Shatabdi, 6-8 times the transcript upregulation of Arsenic sequestering ATP binding cassette C1 type gene (ABCC1), was observed in first internode of low accumulating variety Gobindabhog when 5 mg/kg of arsenite was present in soil. A comparison of the genomic sequence of OsABCC1 identified 8 SNPs between the two genotypes; 5 in introns and 3 silent mutations in exons. We identified a PCR based co-dominant marker targeting an SNP (T/G) between the two genotypes, which clearly distinguished 100 genotypes into low (mean 0.14 mg/kg) and high (mean 0.35 mg/kg) accumulating groups. All aromatic genotypes, either long or small grain, carry the Gobindabhog-type ABCC1 allele and are low accumulators of arsenic. Gobindabhog allele carrying 62 RILs and NILs showed almost 40-50% less As-accumulation in grains relative to 84 RILs and NILs carrying Shatabdi type ABCC1-allele. The marker will be useful in introgression of low accumulating allele of OsABCC1 into high yielding photoperiod insensitive varietal backgrounds more easily and accurately.