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