Sulfhydryl oxidase involved in the regulation of cation homeostasis. Positively regulates shoot accumulation of K(+) and inhibits accumulation of toxic cations. Acts at the level of root K(+) efflux systems involved in xylem loading (root symplast-xylem interface). O2 + 2 R'C(R)SH = H2O2 + R'C(R)S-S(R)CR' Associated with the cell wall. Highly expressed in roots. By salt and norspermidine treatments and phosphate starvation. Plants lacking QSOX1 display more sensitivity to the toxic cations lithium and sodium than wild-type plants.