Translocation pattern of heavy metals in soil-rice systems at different growth stages: A case study in the Taihu region, Eastern China

Uptake and translocation of cadmium and trace metals in common rice varieties at different growth stages

Rice (Oryza sativa) is an important nutritional grain for the majority of Asian countries, but it is also a major source of cadmium (Cd) accumulation. A pot experiment was carried out to investigate the Cd uptake and translocation of high Cd (IR-50) and low Cd (White Ponni) rice cultivars in Cd-contaminated soils. The findings revealed that Cd impacts on rice development and growth differed depending on rice cultivars. Soil Cd levels in the seedling stage exceeded the critical levels (3-6 mg kg-1) only 5.0 mg kg-1 Cd treatment for the IR-50 (7.47 mg kg-1). At higher Cd treatments (1.0 and 5.0 mg kg-1), morphometric characteristics and yield of grains showed a declining and increasing trend in both rice varieties, respectively. The accumulation of Cd was higher in soil and roots during seedling and tillering stages, whereas in booting and maturity stages increased in stems and leaves in IR-50 and WP rice varieties. Cd levels in rice grains above the maximum allowable limit (0.4 mg kg-1) only in IR-50 (0.51 mg kg-1) rice cultivar at maturity stage. The EF of Cd were classified as minor enrichment to 'moderate enrichment' in both rice cultivars. TF values exhibited > 1 in booting and maturity stages in both rice cultivars at higher Cd treatments. The study concluded that the IR-50 rice variety exhibited increased Cd intake and transported to various parts of rice plants, particularly grains. The findings indicate that WP rice cultivar is more resistant to Cd toxicity, reducing health hazards for persons who preferred the staple food rice.

Integrated morphological, physiological and transcriptomic analyses reveal response mechanisms of rice under different cadmium exposure routes

Rice (Oryza sativa) is one of the major cereal crops and takes up cadmium (Cd) more readily than other crops. Understanding the mechanism of Cd uptake and defense in rice can help us avoid Cd in the food chain. However, studies comparing Cd uptake, toxicity, and detoxification mechanisms of leaf and root Cd exposure at the morphological, physiological, and transcriptional levels are still lacking. Therefore, experiments were conducted in this study and found that root Cd exposure resulted in more severe oxidative and photosynthetic damage, lower plant biomass, higher Cd accumulation, and transcriptional changes in rice than leaf Cd exposure. The activation of phenylpropanoids biosynthesis in both root and leaf tissues under different Cd exposure routes suggests that increased lignin is the response mechanism of rice under Cd stress. Moreover, the roots of rice are more sensitive to Cd stress and their adaptation responses are more pronounced than those of leaves. Quantitative PCR revealed that OsPOX, OsCAD, OsPAL and OsCCR play important roles in the response to Cd stress, which further emphasize the importance of lignin. Therefore, this study provides theoretical evidence for future chemical and genetic regulation of lignin biosynthesis in crop plants to reduce Cd accumulation.

Temperature-induced changes in DTPA-extractable trace elements: Predicting the potential impact of climate change on the availability of soil elements

The extraction of trace elements from soil with DTPA is a widely used protocol across laboratories. There is a possible "hidden" discrepancy regarding the results obtained from the extractions, i.e., ambient laboratory temperature and soil properties. In this study, the possible influence of these factors on the extractability of the available forms of Cu, Fe, Pb, Mn, Ni, and Zn, measured with DTPA were studied. Α series of extractions was carried out on a soil sample under normal laboratory temperatures, which fluctuated throughout the year, from 15 to 33.9 °C. In other 144 soil samples, the prevailing physico-chemical properties of soil were evaluated (pH, organic C, clay, CaCO3) that affected the percentage of DTPA extractability relative to the pseudo-total determined content. A strong positive correlation of all metals versus increased ambient temperature was found. Cu had an R2 of 0.897, Fe 0.970, Mn 0.957, Ni 0.938, Pb 0.876, and, Zn 0.922, all highly significant. Extracted Mn exhibited a 6.5-fold increase at the highest temperature of 33.9 οC compared to the lowest. Similar increasing trend was observed for Fe, and Ni, and smaller for Cu, Zn, and Pb. Inherent soil properties affected the percentage of extractability relative to the total content: extractability of Cu, Fe, Mn, and Ni was affected negatively by pH, and the extractability of the studied metals with CaCO3 content. Other soil properties (organic C and clay/sand content) also had an effect, not as pronounced as that of pH and CaCO3. This signifies the necessity of employing standard conditions for routine extractions such as DTPA so that data may be comparable. Also these identified discrepancies may have consequences in the extractability and availability of soil micronutrients and toxic elements regarding climate change. This study aspires to play the role of an initial step towards more robust investigations that would suggest ways of correcting temperature and soil characteristics discrepancies across laboratories.

Auxin homeostasis in plant responses to heavy metal stress

Expeditious industrialization and anthropogenic activities have resulted in large amounts of heavy metals (HMs) being released into the environment. These HMs affect crop yields and directly threaten global food security. Therefore, significant efforts have been made to control the toxic effects of HMs on crops. When HMs are taken up by plants, various mechanisms are stimulated to alleviate HM stress, including the biosynthesis and transport of auxin in the plant. Interestingly, researchers have noted the significant potential of auxin in mediating resistance to HM stress, primarily by reducing uptake of metals, promoting chelation and sequestration in plant tissues, and mitigating oxidative damage. Both exogenous administration of auxin and manipulation of intrinsic auxin status are effective strategies to protect plants from the negative consequences of HMs stress. Regulation of genes and transcription factors related to auxin homeostasis has been shown to be related to varying degrees to the type and concentration of HMs. Therefore, to derive the maximum benefit from auxin-mediated mechanisms to attenuate HM toxicities, it is essential to gain a comprehensive understanding of signaling pathways involved in regulatory actions. This review primarily emphases on the auxin-mediated mechanisms participating in the injurious effects of HMs in plants. Thus, it will pave the way to understanding the mechanism of auxin homeostasis in regulating HM tolerance in plants and become a tool for developing sustainable strategies for agricultural growth in the future.