THE ROLE OF IRON-DEFICIENCY STRESS RESPONSES IN STIMULATING HEAVY-METAL TRANSPORT IN PLANTS

Plant accumulation of Fe and other metals can be enhanced under Fe def iciency. We investigated the influence of Fe status on heavy-metal and divalent-cation uptake in roots of pea (Pisum sativum L. cv Sparkle) seedlings using Cd2+ uptake as a model system. Radiotracer techniques were used to quantify unidirectional Cd-109 influx into roots of Fe-de ficient and Fe-sufficient pea seedlings. The concentration-dependent k inetics for Cd-109 influx were graphically complex and nonsaturating b ut could be resolved into a linear component and a saturable component exhibiting Michaelis-Menten kinetics. We demonstrated that the linear component was apoplastically bound Cd2+ remaining in the root cell wa ll after desorption, whereas the saturable component was transporter-m ediated Cd2+ influx across the root-cell plasma membrane. The Cd2+ tra nsport system in roots of both Fe-deficient and Fe-sufficient seedling s exhibited similar Michaelis constant values, 1.5 and 0.6 mu M, respe ctively, for saturable Cd2+ influx, whereas the maximum initial veloci ty for Cd2+ uptake in Fe-deficient seedlings was nearly 7-fold higher than that in Fe-grown seedlings. Investigations into the mechanistic b asis for this response demonstrated that Fe-deficiency-induced stimula tion of the plasma membrane H+-ATPase did not play a role in the enhan ced Cd2+ uptake. Expression studies with the Fe2+ transporter cloned f rom Arabidopsis, IRT1, indicated that Fe deficiency induced the expres sion of this transporter, which might facilitate the transport of heav y-metal divalent cations such as Cd2+ and Zn2+, in addition to Fe2+.