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

Abstract

Rice can simultaneously absorb Fe 2+ 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 δ 56 Fe value to that of soils (Δ 56 Fe rice−soil = 0.05‰), while that of drained treatment was lighter than that of the soils (Δ 56 Fe rice−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 Fe 2+ uptake from soil solution dominated under flooded and drained conditions, respectively.

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

“…However, the absence of citrate solubilization barely influenced the Fe accumulation in the grains . Grains were enriched in lighter Fe isotopes relative to nodes I (−0.21‰), in a similar extent to that of a previous study . A higher level of DMA at the expense of NA facilitates Fe uptake but minimally impacts Fe accumulation in grains .…”

“…5 The enrichment of lighter Fe isotopes in the stems relative to roots can be attributed to Fe translocation in the form of Fe(III)-citrate from roots to shoots via the xylem 30 and as Fe(II)-NA and Fe(III)-DMA for intercellular transportation in the phloem. 4,5 Fe(III)-phytosiderophores were heavier than Fe(III)-citrate and Fe(II)-NA by approximately 1.5 and 3‰, respectively. 13 The presence of a mixture of Fe species (including Fe(III)citrate, Fe(II)-NA, and Fe(III)-phytosiderophores) may explain the absence of fractionation from the roots to the shoots.…”

“…Fe plaque was enriched in heavier Fe isotopes compared to those in the soil solution, which is in accordance with the results of previous studies (Δ 56 Fe Fe plaque−soil solution = 2.24− 2.43‰). 4,10 This could be due to the fact that radial oxygen loss (ROL) triggers Fe(II) oxidation to form iron oxyhydroxides on the surface of the roots with stronger bonds relative to the aqueous phases. 4 Δ 56 Fe Fe plaque−soil solution values increased during vegetative stages, all of which were lower than those observed in the same soil−rice system at the maturity stage with a continuous flooding regime (2.43‰).…”

“…In contrast, strategy-II plants exhibit minimal or no heavy Fe isotope enrichment compared to soils (Δ 56 Fe plant–soil = 0.05–0.30‰), − similar in extent to the complexation of Fe from iron minerals in the soil onto phytosiderophores. The Fe isotopes in rice plants were moderately lighter than that found in the soil under drainage conditions , and unfractionated in comparison to the ones in the soil solution . However, rice plants were found to be preferentially enriched in heavy Fe isotopes relative to the soil solution in a waterlogged paddy soil. , However, under sufficient Fe supply conditions (Δ 56 Fe bulk plant–nutrient = −1.36), rice plants prefer to take up light Fe isotopes from aqueous solutions .…”

“…The repetitive measurements of U.S. Geological Survey (USGS) rock standards such as NOD-P-1 (−1.29 ± 0.04‰), BHVO-2 (0.08 ± 0.06‰), and AGV-2 (0.11 ± 0.04‰) were consistent with previously published results. 4,8,15 All acid solutions were cleaned in Teflon distills under sub-boiling conditions and prepared using high-purity water (18.2 MΩ cm, Milli-Q, Millipore). The procedural blank induced by digestion and purification processes was equivalent to 39.8 ng, and the recovery rate of Fe for the samples (∼100 μg of Fe) of the chemical purification was higher than 98%.…”