Practice question

Heavy metal resistance in Thlaspi caerulescens now Noccaea caerulescens, model zinc and cadmium hyperaccumulator thriving on calamine metalliferous soils with percent ore, relies on enhanced active efflux and compartmentalization minimizing cytosolic toxicity rather than metabolism. Central actors are P1B-type ATPases: HMA3 tonoplast localized sequestering Cd and Zn into leaf vacuoles with 100-fold higher capacity than non-accumulator due to triplication and cis-regulatory mutations driving constitutive overexpression 50 to 100 times, and HMA4 plasma membrane in root pericycle driving efflux into xylem for shoot translocation, also triplicated. Complementary MTP1 of Cation Diffusion Facilitator family aids vacuolar storage. Efflux uses ATP hydrolysis pumping cations against gradient maintaining cytoplasmic Zn below 0.5 micromolar preventing inhibition of RuBisCO and magnesium displacement in chlorophyll. Such ATP-driven efflux, not animal Na-K pumps, nor simple metabolism or volatilization, explains hyperaccumulation phenotype documented via QTL, microarray, qRT-PCR and transgenic complementation in Arabidopsis halleri and thaliana showing enhanced tolerance. Additional transcriptomic studies confirm constitutive activation under metal stress.