Engineering rice Nramp5 modifies cadmium and manganese uptake selectivity using yeast assay system

Area of Science:

• Plant Biology
• Biochemistry
• Agricultural Science

Background:

• Cadmium (Cd) is a hazardous heavy metal in food, with stricter regulations necessitating reduced plant uptake.
• Cereals, including rice, are major sources of dietary Cd, posing risks to human health.
• Natural resistance-associated macrophage protein (Nramp) transporters, like rice OsNramp5, are key to metal ion uptake but engineering them for reduced Cd uptake without affecting essential manganese (Mn) uptake is challenging.

Purpose of the Study:

• To engineer the OsNramp5 transporter in rice to selectively reduce cadmium (Cd) uptake while preserving manganese (Mn) uptake efficiency.
• To develop strategies for low-Cd rice production to mitigate heavy metal contamination in a staple food crop.

Main Methods:

• Amino acid substitutions were introduced at key residues (Ala-232 and Met-235) in the OsNramp5 transporter, based on structural analyses of bacterial Nramps.
• Metal uptake efficiency was assessed using a yeast model assay system.
• Cd and Mn content in yeast was quantified using inductively coupled plasma optical emission spectroscopy (ICP-OES).
• Computational structural modeling was employed to understand metal transport mechanisms.

Main Results:

• Several engineered OsNramp5 mutants exhibited significantly reduced Cd uptake (less than 8.6%) while retaining substantial Mn uptake efficiency (more than 64.1%) compared to the wild-type.
• Specific mutants reduced Cd uptake to background levels while maintaining over 64.7% of Mn uptake efficiency under simulated polluted soil conditions.
• Computational modeling provided insights into the structural requirements for selective Cd/Mn transport.

Conclusions:

• Engineering OsNramp5 through targeted amino acid substitutions is a viable strategy for reducing Cd accumulation in rice.
• This approach offers a promising avenue for developing low-Cd rice varieties, enhancing food safety.
• Understanding the structural basis of metal selectivity in Nramp transporters can guide future crop improvement efforts.