The release of copper-induced phytosiderophores in barley plants is decreased by cadmium stress

A variety of factors affect the release of the mugineic acid family of phytosiderophores (PS) from the roots of graminaceous plants. We investigated the effects of copper (Cu) toxicity with or without cadmium (Cd) toxicity on PS release in barley (Hordeum vulgare L. cv. Minorimugi). To characterise the effect of Cu toxicity on PS release, 10 mu mol.L-1 Cu (a 100-fold higher concentration than the control condition) was added to the plant culture medium. The results showed that PS release from the roots was stimulated in response to Cu toxicity. In a second experiment, 5 mu mol.L-1 Cu was added to the medium with or without the addition of 5 mu mol.L-1 Cd. In this experiment, the release of PS was drastically reduced in response to Cd. We suggest that Cu toxicity in barley is a signal that activates PS release by plant roots, whereas PS release induced by Cu toxicity is strongly attenuated by Cd toxicity. We propose that inhibition of PS release could be one of the harmful effects of Cd toxicity in plants. Our results provide new information about one of the responses of graminaceous plants to heavy metal stress.