Effect of calcium and phosphorus on ammonium and nitrate nitrogen adsorption onto iron (hydr)oxides surfaces: CD-MUSIC model and DFT computation.

Calcium (Ca2+) and phosphorous (PO43-) significantly influence the form and effectiveness of nitrogen (N), however, the precise mechanisms governing the adsorption of ammonium nitrogen (NH4+-N) and nitrate nitrogen (NO3--N) are still lacking. This study employed batch adsorption experiments, charge distribution and multi-site complexation (CD-MUSIC) models and density functional theory (DFT) calculations to elucidate the mechanism by which Ca2+ and PO43- affect the adsorption of NH4+-N and NO3--N on the goethite (GT) surface. The results showed that the adsorption of NH4+-N on the GT exhibited an initial increase followed by a decrease as pH increased, peaking at a pH of 8.5. Conversely, the adsorption of NO3--N decreased with rising pH. According to the CD-MUSIC model, Ca2+ minimally affected the NH4+-N adsorption on the GT but enhanced NO3--N adsorption via electrostatic interaction, promoting the adsorption of ≡FeOH-NO3- and ≡Fe3O-NO3- species. Similarly, PO43- inhibited the adsorption of ≡FeOH-NO3- and ≡Fe3O-NO3- species. However, PO43- boosted NH4+-N adsorption by facilitating the formation of ≡Fe3O-NH4+ via electrostatic interaction and site competition. DFT calculations indicates that although bidentate phosphate (BP) was beneficial to stabilize NH4+-N than monodentate phosphate (SP), SP-NH4+ was the main adsorption configuration at pH 5.5-9.5 owing the prevalence of SP on the GT surface under site competition of NH4+-N. The results of CD-MUSIC model and DFT calculation were verified mutually, and provide novel insights into the mechanisms underlying N fixation and migration in soil.

Dissecting the promotional effect of zinc on cadmium translocation from roots to shoots in rice.

It is often expected that Zn decreases Cd accumulation in plants due to competition for the same transporters. Here, we found that increasing Zn supply markedly increased the root-to-shoot translocation of Cd in rice. RNA sequencing showed that high Zn up-regulated expression of genes involved in glutathione biosynthesis and metabolism and the Zn/Cd transporter gene OsHMA2, but down-regulated expression of genes related to Zn uptake. Knockout of the iron or Zn transporter genes OsIRT1, OsIRT2, or OsZIP9 did not affect the Zn promotional effect on Cd translocation. Knockout of the manganese/Cd transporter gene OsNRAMP5 greatly reduced Cd uptake but did not affect the Zn promotional effect. Variation in the tonoplast transporter gene OsHMA3 affected Cd translocation but did not change the Zn promotional effect. Knockout of the Zn/Cd transporter gene OsHMA2 not only decreased Cd and Zn translocation, but also abolished the Zn promotional effect. Increased expression of OsHMA2 under high Zn conditions supports the hypothesis that this transporter participates in the promotional effect of Zn on Cd translocation. The results also show that OsIRT1, OsIRT2, and OsZIP9 made only small contributions to Cd uptake under low Zn conditions but not under high Zn conditions, whereas the dominant role of OsNRAMP5 in Cd uptake diminished under low Zn conditions.

Map-based cloning of a new total loss-of-function allele of OsHMA3 causes high cadmium accumulation in rice grain.

Rice (Oryza sativa) is a major dietary source of the toxic metal cadmium (Cd). Reducing Cd transfer from soil to the rice grain is important for food safety. Rice cultivars vary widely in their Cd accumulation, but the genetic basis for this variation is not fully understood. Based on field and pot experiments comparing 26 rice cultivars, we identified a cultivar with high Cd accumulation in grain (BG367, coded as W4) and a cultivar with low grain Cd accumulation (Huajingxian 74, coded as W0). W4 showed a higher Cd translocation from roots to shoots than W0. Using chromosome single segment substitution lines derived from the two cultivars, we mapped a quantitative trait locus for Cd accumulation in grain to a 400 kb region in chromosome 7. Using yeast expression assays and transgenic complementation, we identified OsHMA3 as the causal gene at this locus. Compared with OsHMA3W0, OsHMA3W4 has a deletion of 14 amino acids predicted to be in the ATP binding domain. OsHMA3W4 showed a complete loss of transport activity for Cd in yeast assays. Taking our findings together, we have identified a new allele of OsHMA3 with a total loss-of-function, resulting in greatly elevated Cd translocation to rice shoots and grain.