Influence of Expressing OsNRAMP5 Under the Driving of the OsLCT1 Promoter on Cadmium Migration to Rice Seeds

Cadmium (Cd) is the predominant pollutant in China's arable land, with rice cultivated on these contaminated soils being a significant dietary source of Cd for the population. This study aims to tissue-specifically express OsNRAMP5, a transporter responsible for the majority of Cd uptake in rice, to investigate strategies for developing low-Cd rice varieties and provide a reference for molecular design breeding to cope with Cd pollution.

To drive the expression of OsNRAMP5 in rice, we utilized a 2 500 bp sequence upstream of the OsLCT1 start codon as the promoter. The red fluorescent protein mRFP was fused to the C-terminus of OsNRAMP5 to visualize its tissue localization. After obtaining independent homozygous transgenic lines, the transcripts of the OsNRAMP5 were first detected using qRT-PCR, and its tissue localization in roots and nodes was observed via laser confocal microscopy. Subsequently, the accumulation and tolerance of Cd were evaluated in transgenic and wild-type rice under varying concentrations of Cd treatment. Furthermore, plants were grown in Cd-contaminated paddy soil, and the accumulation of Cd and other mineral elements in seeds and leaves, as well as related yield traits, were measured.

Under the drive of the OsLCT1 promoter, OsNRAMP5 was expressed mainly in the epidermis, exodermis and stele of roots, as well as in the phloem area of enlarged vascular bundles and diffuse vascular bundles in nodes, differing significantly from the native expression pattern of OsNRAMP5 in rice. Compared to wild-type rice, the transgenic lines exhibited increased Cd accumulation in roots, decreased Cd accumulation in shoots, and enhanced tolerance to Cd stress during the seedling stage. When cultivated in Cd-contaminated paddy soils, plant height and grain yield were unaffected by the ectopic expression of OsNRAMP5, while Cd accumulation in seeds and leaves significantly decreased in the transgenic lines. The Cd content in seeds decreased by over 80%, with a greater reduction ratio compared to that in leaves. Although the Mn content in seeds and leaves slightly decreased, the expression of OsNRAMP5 had little impact on the accumulation of other mineral elements such as Fe, Zn, and Cu.

The expression of OsNRAMP5 driven by the OsLCT1 promoter greatly decreases the Cd migration toward rice seeds by reducing Cd transport to the aboveground parts from roots and increasing the Cd transporting to leaves at nodes. Therefore, the expression of OsNRAMP5 under the control of the OsLCT1 promoter is an effective strategy to reduce Cd accumulation in rice seeds.