SpARF4 reduces cadmium accumulation by negatively regulating SpABCG14 and SpACO4 in the cadmium/zinc hyperaccumulator Sedum plumbizincicola

Root development and apoplastic transport are respectively important for cadmium (Cd) absorption and transportation, which profoundly influence Cd bioremediation. However, molecular mechanisms underlying the two processes are not completely understood. In this study, we demonstrated that auxin response factor 4 (SpARF4) from a Cd hyperaccumulator Sedum plumbizincicola was a negative regulator for these processes. SpARF4 positively regulated by auxin was highly expressed in xylem. Overexpression of SpARF4 significantly decreased vessel area and declined lignin content of S. plumbizincicola. Meanwhile, less adventitious roots were found, and lateral root development was delayed in transgenic plants. Furthermore, ethylene production and auxin transportation were impaired. More importantly, SpARF4 negatively regulated Cd content of xylem saps and aerial tissues. Combining dual-LUC reporter, Y1H and qRT-PCR assays, SpARF4 was a repressor for two downstream genes (SpABCG14 and SpACO4) which influenced vascular bundle development and ethylene production, respectively. PIN1, 2, 3, 7 were downregulated and slowed down local auxin accumulation rate, which suspended root development. These results indicate that SpARF4 can decelerate Cd transportation rate from roots to aerial parts and reduce Cd content of aboveground tissues by delaying the root development and decreasing vessel area.