Multifunctional Roles of Zinc in Cadmium Transport in Soil-Rice Systems: Novel Insights from Stable Isotope Fractionation and Gene Expression.

The effect of Zn on Cd accumulation in rice varies under flooding and drainage conditions, and the underlying mechanism during uptake and transport from the soil to grains remains unclear. Isotope fractionation and gene expression were investigated using pot experiments under distinct water regimes and with Zn addition to gain a deeper understanding of the molecular effects of Zn on Cd uptake and transport in rice. The higher OsHMA2 expression but constitutively lower expression of zinc-regulated, iron-regulated transporter-like protein (ZIP) family genes in roots under the drainage regime than the flooding regime caused the enrichment of nonheavy Zn isotopes in the shoots relative to roots but minimally affected Cd isotopic fractionation. Drainage regime seem to exert a striking effect on the root-to-shoot translocation of Zn rather than Cd, and increased Zn transport via OsHMA2. The changes in expression patterns in response to Zn addition were similar to those observed upon switching from the flooding to drainage regime, except for OsNRAMP1 and OsNRAMP5. However, soil solution-to-rice plants and root-to-shoot fractionation toward light Zn isotopes with Zn addition (Δ66Znrice plant-soil solution = -0.49 to -0.40‰, Δ66Znshoot-root = -0.36 to -0.27‰) indicated that Zn transport occurred via nonspecific uptake pathways and OsHMA2, respectively. Accordingly, the less pronounced and minimally varied Cd isotope fractionation suggested that OsNRAMP5 and OsHMA2 are crucial for Cd uptake and root-to-shoot transport, respectively, facilitating Cd accumulation in grains. This study demonstrated that a high Zn supply promotes Cd uptake and root-to-shoot transport in rice by sharing distinct pathways, and by utilizing a non-Zn-sensitive pathway with a high affinity for Cd.

Synergistic effects of warming and humic substances on driving arsenic reduction and methanogenesis in flooded paddy soil.

Microbially-driven arsenic reduction and methane emissions in anaerobic soils are regulated by widespread humic substances (HS), while how this effect responds to climate change remains unknown. We investigated potential synergistic effects of HS in response to temperature changes in arsenic-contaminated paddy soils treated with humic acid (HA) and fulvic acid (FA) at temperatures ranging from 15 to 45 °C. Our results reveal a significant increase in arsenic reduction (5.6 times) and methane emissions (178 times) driven by HS, which can be exponentially stimulated at 45 °C. Acting as a electron shuttle, HS determines microbial arsenic reduction, further stimulated by warming. The top three sensitive genera are Geobacter, Anaeromyxobacter, and Gaiella which are responsible for enhanced arsenic reduction, as well as for the reduction of iron and HS with their functional genes; arrA and Geobacter spp. The top three sensitive methanogens are Methanosarsina, Methanocella, and Methanoculleus. Our study suggests notable synergistic effects between HS and warming in stimulating arsenic reduction and methanogenesis in paddy soils. Overall, the findings of this work highlight the high sensitivity of HS-mediated microbial arsenic transformation and methanogenesis in response to warming, which add potential value in predicting the biogeochemical cycling of arsenic and methane in soil under the context of climate change.

Impact of Flooding-Drainage Alternation on Fe Uptake and Transport in Rice: Novel Insights from Iron Isotopes.

Iron (Fe) isotopes were utilized to provide insights into the temporal changes underlying Fe uptake and translocation during rice growth (tillering, jointing, flowering, and maturity stages) in soil-rice systems under typical flooding-drainage alternation. Fe isotopic composition (δ56Fe values) of the soil solution generally decreased at vegetative stages in flooding regimes but increased during grain-filling. Fe plaques were the prevalent source of Fe uptake, as indicated by the concurrent increase in the δ56Fe values of Fe plaques and rice plants during rice growth. The increasing fractionation magnitude from stem/nodes I to flag leaves can be attributed to the preferred phloem transport of light isotopes toward grains, particularly during grain-filling. This study demonstrates that rice plants take up heavy Fe isotopes from Fe plaque and soil solution via strategy II during flooding and the subsequent drainage period, respectively, thereby providing valuable insights into improving the nutritional quality during rice production.

Roles of Chloride and Sulfate Ions in Controlling Cadmium Transport in a Soil-Rice System as Evidenced by the Cd Isotope Fingerprint.

Anions accompanying inorganic fertilizers, such as chloride and sulfate ions, potentially affect the solubility, uptake, and transport of Cd to rice grains. However, the role of anions in controlling Cd transport in the soil-soil solution-Fe plaque-rice plant continuum remains poorly understood. Cd isotope ratios were applied to Cd-contaminated soil pots, hydroponic rice, and adsorption experiments with or without KCl and K2SO4 treatments to decipher transport processes in the complex soil-rice system. The chloride and sulfate ions increased the Cd concentrations in the soil solution, Fe plaque, and rice plants. Accordingly, the magnitude of positive fractionation from soil to the soil solution was less pronounced, but that between soil and Fe plaque or rice plant is barely varied. The similar isotope composition of Fe plaque and soil, and the similar fractionation magnitude between Fe plaque and the solution and between goethite and the solution, suggested that desorption-sorption between iron oxides and the solution could be important at the soil-soil solution-Fe plaque continuum. This study reveals the roles of chloride and sulfate ions: (i) induce the mobility of light Cd isotopes from soil to the soil solution, (ii) chloro-Cd and sulfato-Cd complexes contribute to Cd immobilization in the Fe plaque and uptake into roots, and (iii) facilitate second leaves/node II-to-grain Cd transport within shoots. These results provide insights into the anion-induced Cd isotope effect in the soil-rice system and the roles of anions in facilitating Cd migration and transformation.

Ecological drivers and potential functions of viral communities in flooded arsenic-contaminated paddy soils.

This work revealed the profile of viral communities in paddy soils with different levels of arsenic (As) contamination during the flooded period. The structure of viral communities differed significantly in highly and moderately As-contaminated soils. The diversity of soil viral communities under high As contamination decreased. Siphoviridae, Podoviridae, Myoviridae, and Microviridae were the dominant viral families in all samples, and the relative abundances of five of the top 20 viral genera were significantly different between highly and moderately As-contaminated groups. Seventeen dissimilatory As(V)-reducing bacteria were predicted to host 161 viral operational taxonomic units (vOTUs), mainly affiliated with the genera of Sulfurospirillum, Deferribacter, Bacillus and Fusibacter. Among them, 28 vOTUs were also associated with Fe(III)-reducing bacteria, which belonged to different species of the genus Shewanella. Procrustes analysis showed that the community structure of soil viruses was strongly correlated with both prokaryotic community structure and geochemical properties. Random forest analyses revealed that the Total-Fe, DCB-Fe and oxalate-Fe were the most significant variables on viral community richness, while the total-As concentration was an important factor on the Shannon index. Furthermore, As resistance genes (ArsC, ArsR and ArsD), As methylation genes (arsM) and As transporter genes (Pst and Pit) were identified among the auxiliary metabolic genes (AMGs) of the virome. This work revealed that the viruses might influence microbial adaptation in response to As-induced stress, and provided a perspective on the potential virus-mediated biogeochemical cycling of As.

Source and Strategy of Iron Uptake by Rice Grown in Flooded and Drained Soils: Insights from Fe Isotope Fractionation and Gene Expression.

Rice can simultaneously absorb Fe2+ via a strategy I-like system and Fe(III)-phytosiderophore via strategy II from soil. Still, it remains unclear which strategy and source of Fe dominate under distinct water conditions. An isotope signature combined with gene expression was employed to evaluate Fe uptake and transport in a soil-rice system under flooded and drained conditions. Rice of flooded treatment revealed a similar δ56Fe value to that of soils (Δ56Ferice-soil = 0.05‰), while that of drained treatment was lighter than that of the soils (Δ56Ferice-soil = -0.41‰). Calculations indicated that 70.4% of Fe in rice was from Fe plaque under flooded conditions, while Fe was predominantly from soil solution under drained conditions. Up-regulated expression of OsNAAT1, OsTOM2, and OsYSL15 was observed in the root of flooded treatment, while higher expression of OsIRT1 was observed in the drained treatment. These isotopic and genetic results suggested that the Fe(III)-DMA uptake from Fe plaque and Fe2+ uptake from soil solution dominated under flooded and drained conditions, respectively.