Detoxification of cadmium in tobacco plants: formation and active excretion of crystals containing cadmium and calcium through trichomes

In tobacco (Nicotiana tabacum L.), long and short trichomes can be distingu ished morphologically. The established function of long trichomes is to exu de a sticky gum containing diterpenes, whereas that of short trichomes is n ot known. When tobacco seedlings were exposed to toxic levels of cadmium (C d), growth was retarded, but trichome number was increased up to 2-fold in comparison with untreated samples. Observation by variable-pressure scannin g electron microscopy (VP-SEM) indicated that large crystals of 150 mum in size were formed on head cells of both short and long trichomes. An energy- dispersive X-ray analysis system fitted with VP-SEM revealed the crystals t o contain amounts of Cd and calcium (Ca) at much higher concentrations than in the head cells themselves. Transmission electron microscopy demonstrate d crystal formation in amorphous osmiophilic deposits in vacuoles. When see dlings were treated with Cd in the presence of Ca, tolerance was increased in proportion to the increase in Ca concentration. These results indicate t hat tobacco plants actively exclude toxic Cd by forming and excreting Cd/Ca -containing crystals through the head cells of trichomes.