Effects of Water Isotope Composition on Stable Isotope Distribution and Fractionation of Rice and Plant Tissues.

Rice origin authenticity is important for food safety and consumer confidence. The stable isotope composition of rice is believed to be closely related to its water source, which affects its origin characteristics. However, the influence of water availability on the distribution of rice stable isotopes (δ2H and δ18O) is not clear. In this study, three irrigation waters with different isotopic values were used to investigate isotopic water use effects of Indica and Japonica rice, using pot experiments. Under three different water isotope treatments, the δ2H values of Indica polished rice showed significant differences (-65.0 ± 2.3, -60.5 ± 0.8 and -55.8 ± 1.7‰, respectively, p < 0.05) compared to δ13C and δ15N, as did Japonica polished rice. The values of δ2H and δ18O of rice became more positive when applying more enriched (in 2H and 18O) water, and the enrichment effect was higher in rice than in the corresponding plant tissue. In addition, the δ2H and δ18O values of Indica rice leaves decreased at the heading stage, increased at the filling stage, and then decreased at the harvest stage. Japonica rice showed a similar trend. δ2H changes from stem to leaf were more negative, but δ18O changes were more positive, and δ2H and δ18O values from leaf to rice were more positive for both brown and polished rice. The results from this study will clarify different water isotopic composition effects on rice and provide useful information to improve rice origin authenticity using stable isotope-based methods.

Removal of Cd from contaminated farmland soil by washing with residues of traditional Chinese herbal medicine extracts.

Soil washing is one of the effective methods for permanent removal of heavy metals from farmland soil, and selection of washing agents determines heavy metal removal efficiency. However, there is still a lack of cost-efficient and eco-friendly washing agents. In this study, three residues of traditional Chinese herbal medicine (RTCHM) extracts: residues of Prunus mume (Sieb.) Sieb. et Zucc. (RPM), residues of Schisandra chinensis (Turcz.) Baill. (RSC), and residues of Crataegus pinnatifida Bunge (RCP), were tested for their potential of Cd removal. The variations in amounts and compositions of dissolved organic carbon (DOC) and citric acid were responsible for the difference in Cd removal efficiencies of RTCHM extracts. Fourier-transform infrared spectrophotometer (FTIR) analysis showed that hydroxyl, carboxyl, and amine were the main functional groups of RTCHM extracts to chelate with heavy metals. The optimum conditions for RTCHM extracts were 100 g L-1 concentration, solid-liquid ratio 1:10, pH 2.50, and contact time of 1 h, and the highest Cd removal efficiencies of RPM, RSC, and RCP extracts reached 35%, 11%, and 15%, respectively. The ecological risk of Cd decreased significantly due to the decrease of exchangeable and reducible Cd fractions. RTCHM extracts washing alleviated soil alkalinity and had little effect on soil cation exchange capacity. Meanwhile, the concentrations of soil organic matter and nitrogen were enhanced significantly by RPM extracts and the activities of soil catalase and urease were also improved. Overall, among the tested extracts, RPM extracts was a much more feasible and environment-friendly washing agent for the remediation of Cd-contaminated farmland soil.

One stone two birds: Bone char as a cost-effective material for stabilizing multiple heavy metals in soil and promoting crop growth.

Remediation of farmland soils contaminated with high levels of multiple heavy metals near PbZn smeltery is still a great challenge. It is of great significance to find cost-effective green remediation technologies for stabilization of multiple heavy metals in soil and reduce metal accumulation in crops with ensured yield. In this study, we demonstrated that bone char (BC) is an effective heavy metal stabilizer which can substantially increase residual fractions of heavy metals and reduce metal accumulation in pea (Pisum sativum) with its enhanced growth. We chose the soils contaminated with high levels of Pb, Zn, Cu and Cd near the Baiyin PbZn smeltery as the tested soil. After 2 months of BC application, the relative mobile fractions (non-residual fractions) of Cu, Zn, Pb and Cd in the contaminated soil decreased while the residual fraction increased significantly. The leachability of Cu, Zn, Pb and Cd decreased by 91.2%, 38.6%, 67.6% and 54.3%, respectively compared with the control. BC application remarkably promoted pea growth and reduced accumulation of heavy metals in shoots. The mechanisms for stabilization of multiple heavy metals BC include ion exchange, surface complexation and subsequent mineralization, accompanied with release of Ca and phosphate. The immobilization of heavy metals led to their reduced toxicity to plant, and thus increased pea growth. The results show that BC is a cost-effective and sustainable heavy metal stabilizer with phosphate fertilization function. It can simultaneously immobilize multiple heavy metals in soil and facilitate crop production.

Phosphate Transporter PvPht1;2 Enhances Phosphorus Accumulation and Plant Growth without Impacting Arsenic Uptake in Plants.

Phosphorus is an important macronutrient for plant growth and is acquired by plants mainly as phosphate (P). Phosphate transporters (Phts) are responsible for P and arsenate (AsV) uptake in plants including arsenic-hyperaccumulator Pteris vittata. P. vittata is efficient in AsV uptake and P utilization, but the molecular mechanism of its P uptake is largely unknown. In this study, a P. vittata Pht, PvPht1;2, was cloned and transformed into tobacco ( Nicotiana tabacum). In hydroponic experiments, all transgenic lines displayed markedly higher P content and better growth than wild type, suggesting that PvPht1;2 mediated P uptake in plants. In addition, expressing PvPht1;2 also increased the shoot/root 32P ratio by 69-92% and enhanced xylem sap P by 46-62%, indicating that PvPht1;2 also mediated P translocation in plants. Unlike many Phts permeable to AsV, PvPht1;2 showed little ability to transport AsV. In soil experiments, PvPht1;2 also significantly increased shoot biomass without elevating As accumulation in PvPht1;2 transgenic tobacco. Taken together, our results demonstrated that PvPht1;2 is a specific P transporter responsible for P acquisition and translocation in plants. We envisioned that PvPht1;2 can enhance crop P acquisition without impacting AsV uptake, thereby increasing crop production without compromising food safety.

Knocking Out OsPT4 Gene Decreases Arsenate Uptake by Rice Plants and Inorganic Arsenic Accumulation in Rice Grains.

Arsenic (As) accumulation in rice grains poses health risk to humans. Plants including rice take up arsenate (AsV) by phosphate transporters. In this study, rice phosphate transporter OsPT4 (OsPht1;4) was investigated based on two independent T-DNA insertion mutants of OsPT4 (M1 and M2), which displayed stronger AsV resistance than wild types WT1 and WT2. When cultivated in medium (+P or -P) with AsV, ospt4 mutants accumulated 16-32% lower As in plants, suggesting that OsPT4 mediates AsV uptake. Analysis of the xylem sap showed that AsV concentrations in ospt4 mutants was 20-40% lower than WT controls under -P condition, indicating OsPT4 may also mediate AsV translocation. Moreover, kinetics analysis showed that ospt4 mutants had lower AsV uptake rates than the WT controls, further proving that OsPT4 functions as an AsV transporter in rice. When grown in flooded soils with As, AsV concentrations in rice grains of ospt4 mutants decreased by 50-55%. More importantly, knocking out OsPT4 in M1 and M2 reduced inorganic As accumulation in rice grains by 20-44%, significant for controlling As exposure risk from rice. Taken together, our findings revealed a critical role of OsPT4 in AsV uptake and translocation in rice. Knocking out OsPT4 effectively decreased inorganic As accumulation in rice grains, shedding light on engineering low-As rice to enhance food safety.

Arsenic-hyperaccumulator Pteris vittata efficiently solubilized phosphate rock to sustain plant growth and As uptake.

Phosphorus (P) is one of the most important nutrients for phytoremediation of arsenic (As)-contaminated soils. In this study, we demonstrated that As-hyperaccumulator Pteris vittata was efficient in acquiring P from insoluble phosphate rock (PR). When supplemented with PR as the sole P source in hydroponic systems, P. vittata accumulated 49% and 28% higher P in the roots and fronds than the -P treatment. In contrast, non-hyperaccumulator Pteris ensiformis was unable to solubilize P from PR. To gain insights into PR solubilization by plants, organic acids in plant root exudates were analyzed by HPLC. The results showed that phytic acid was the predominant (>90%) organic acid in P. vittata root exudates whereas only oxalic acid was detected in P. ensiformis. Moreover, P. vittata secreted more phytic acid in -P and PR treatments. Compared to oxalic acid, phytic acid was more effective in solubilizing PR, suggesting that phytic acid was critical for PR utilization. Besides, secretion of phytic acid by P. vittata was not inhibited by arsenate. Our data indicated that phytic acid played an important role in efficient use of insoluble PR by P. vittata, shedding light on using insoluble PR to enhance phytoremediation of As-contaminated soils.