Root cell-type specific expressions of bacterial mercury transporter MerC and plant SNARE SYP121 fusion protein differentially affect cadmium accumulation patterns of Arabidopsis

ABSTRACT There is increasing demand for solutions against cadmium pollution to secure food safety, and phytoremediation is one of the potential tools. We previously found that a bacterial mercury transporter MerC possesses cadmium uptake activity and its overexpression as a fusion protein with a plasma-membrane resident SNARE protein, SYP121 enhances the cadmium uptake ability of Arabidopsis plants. In this study, we examined whether two different root cell-type specific expression systems of MerC-SYP121 fusion protein could efficiently enhance cadmium accumulation of Arabidopsis plants, compared to the p35S-driven ubiquitous expression system. Representative transgenic lines expressing MerC-SYP121 in root surface cells (pEpi lines) or root endodermal cells (pSCR lines), established in our previous studies, were subjected to different cadmium treatments along with the p35S line. A vertical agar plate assay showed that root surface-specific line pEpi, as well as the p35S line, showed about 15% higher cadmium accumulation in shoots after one-day 10 µM cadmium treatment, compared to the wild-type Col-0. On the other hand, the endodermis-specific line pSCR accumulated 30% less cadmium in its shoots. A similar cadmium accumulation pattern in shoots was observed under the environmentally relevant much lower cadmium treatment of 0.1 µM for 4 d, using the hydroponic culture system. To further examine the potential of the MerC-SYP121 expression system for cadmium phytoremediation, the transgenic plants were hydroponically exposed to 0.1 µM for 4 weeks. The cadmium accumulation after the 4 weeks of treatment was again 16% higher in the pEpi shoots compared to that of Col-0, whereas the p35S line only showed 6% higher Cd concentration. Shoots of the pSCR line accumulated slightly less cadmium compared to Col-0. Ionomic profiles in these plants were analyzed, however, pSCR-specific patterns were not evident. Nevertheless, in our previous studies, pEpi and pSCR lines both efficiently accumulated more mercury in shoots than in the wild-type. The presented results suggest that the effects of cell-type-specific MerC-SYP121 expression differ by the target metals and its expression in root surface cells rather than that in endodermis is suitable for enhancing root cadmium uptake and subsequent shoot accumulation.