Abstract
Iron (Fe) deficiency is a critical agricultural problem, especially in calcareous soil, which is distributed worldwide. Rice plants take up Fe(II) from soil through a OsIRT1 transporter (Strategy I-related system) and also take up Fe(III) via a phytosiderophore-based system (Strategy II system). However, rice plants are susceptible to low-Fe conditions because they have low Fe(III) reduction activity and low-level phytosiderophore secretion. Previously, we produced transgenic rice plants expressing a mutationally reconstructed yeast ferric chelate reductase, refre1/372, under the control of the OsIRT1 promoter. This transgenic rice line exhibited higher Fe(III) chelate reductase activity and tolerance to Fe deficiency. In addition, we produced transgenic rice overexpressing the Fe deficiency-inducible transcription factor, OsIRO2, which regulates the expression of various genes involved in the strategy II Fe(III) uptake system, including OsNAS1, OsNAAT1, OsDMAS1, OsYSL15, and TOM1. This transgenic rice exhibited improved phytosiderophore secretion ability and tolerance to Fe deficiency. In the present research, transgenic rice plants that possess both the OsIRT1 promoter-refre1/372 and the 35S promoter-OsIRO2 (RI lines) were produced to enhance both Strategy I Fe(II) reductase ability and Strategy II phytosiderophore productivity. RI lines exhibited enhanced tolerance to Fe-deficient conditions at the early and middle-late stages of growth in calcareous soil, compared to both the non-transgenic line and lines harboring either OsIRT1 promoter-refre1/372 or 35S promoter-OsIRO2 alone. RI lines also exhibited a 9-fold higher yield than the non-transgenic line. Moreover, we successfully produced Fe-deficiency-tolerant Tachisugata rice, which is a high-biomass variety used as fodder. Collectively, our results demonstrate that combined enhancement of two Fe uptake systems in rice is highly effective in conferring tolerance to low Fe availability in calcareous soil.
Highlights
Iron (Fe) deficiency is a widespread agricultural problem that is common in calcareous soils, which cover more than 30% of the earth’s surface
A construct harboring the 35S promoter-OsIRO2 and the OsIRT1 promoter-refre1/372 was introduced into rice plants by Agrobacterium-mediated transformation (Fig 1A)
We used genomic PCR to confirm the introduction of OsIRO2 and refre1/372 into 28 regenerated plant lines
Summary
Iron (Fe) deficiency is a widespread agricultural problem that is common in calcareous soils, which cover more than 30% of the earth’s surface. There are abundant minerals in soil, Fe is sparingly soluble under aerobic conditions, in particular at high pH in calcareous soil [1]. Plants often exhibit Fe-deficiency symptoms, such as chlorosis, leading to reduced crop yield and quality. Fe is a key determinant of biomass production and of plant product quality [2]. Development of crops tolerant to low Fe availability is important to meet the increased demand for food caused by rapidly increasing populations. Cultivation of high-biomass crops tolerant to low Fe availability in calcareous soil would reduce the carbon dioxide concentration in the atmosphere and, ameliorate global warming
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