Diagrammatic representation of heavy-metal transport and accumulation in Nicotiana leaves. Left circle: cadmium is transported by IRT from the environment in the root, and then by HMA into the phloem. Middle: NA and PCS transport cadmium from the root to the leaves. Right circle: in roots and leaves, cadmium enters cells, where it is transported to the vacuole by HMA, ABC or CAX. ABC also exports cadmium outside of the cell. ABC, ATP-binding cassette transporter; CAX, cation/proton exchanger; HMA, heavy metal ATPase; IRT, iron transporter protein; NA, nicotinamine synthase; PCS, phytochelatin synthase.