Combinatorial use of sulfur-responsive regions of sulfate transporters provides a highly sensitive plant-based system for detecting selenate and chromate in the environment

Selenium and chromium are heavy metals that are highly toxic to living organisms when they are present in excess amounts. The major forms of selenium and chromium in the environment are selenate and chromate, which act as toxic analogs of sulfate in plants, inhibiting its uptake and causing sulfur deficiency. To provide simple and inexpensive means of environmental analysis for selenate and chromate, we previously developed a model system for detecting and quantifying their levels by measuring the accumulation of green fluorescent protein (GFP) in transgenic plants under the control of the sulfur-responsive promoter region of the high-affinity sulfate transporter SULTR1;2 from Arabidopsis (P_SULTR1;2-GFP). However, the detection limit for both selenate and chromate was 10 μmol L⁻¹, which was higher than the environmental standards. Here, we report a more sensitive monitoring system for selenium and chromium by combining P_SULTR1;2 with another sulfur-responsive region of the sulfate transporter SULTR2;1 located in the 3ʹ-non-transcribed intergenic region (T_SULTR2;1). The fusion gene construct consisting of P_SULTR1;2, GFP and T_SULTR2;1 (P_SULTR1;2-GFP-T_SULTR2;1) was introduced into Arabidopsis plants, and P_SULTR1;2-GFP-T_SULTR2;1 transgenic plants showed significant increases in GFP under sulfur-deficient conditions or with the addition of selenate or chromate to the growth medium. The increase in GFP depended on selenate concentration. Overall, the use of P_SULTR1;2-GFP-T_SULTR2;1 plants decreased the detection limits to 1 and 3 μmol L⁻¹ for selenate and chromate, respectively, providing a highly sensitive plant-based system for detecting selenium and chromium in the environment.