Potassium stimulates the efflux of putrescine in roots of barley seedlings

We investigated the mechanisms for a decline of putrescine concentration associated with feeding K+ to roots of barley (Hordeum vulgare L. cv. Amagi) seedlings. The addition of K+ to roots which had fully accumulated putrescine resulted in a significant increase of K+ level and a decrease of free putrescine. The decrease was not due to conversion of free putrescine to free spermidine, spermine, or other conjugated forms in the roots. The elevated K+ supply did not cause transport of free putrescine to shoots or its conversion in the shoots. On the other hand, exogenous K+ did induce efflux of putrescine from the cytoplasm of the roots into the external medium. Activity of diamine oxidase (DAO; EC 1.4.3.6), which oxidizes putrescine, was maintained by application of K+, while its activity slightly decreased in roots without K+ application. These results suggest that putrescine efflux is largely responsible for the decrease in putrescine level associated with the supply of exogenous K+ to roots of barley seedlings.