Submitted to: Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/18/1998
Publication Date: N/A
Interpretive Summary: Iron (Fe) is an essential nutrient for both plants and humans, and Fe deficiency is a worldwide problem for both. Acquiring sufficient iron from many soils is difficult for plants; hence, not enough Fe is available in plant foods to satisfy an adult human's nutritional requirements when eaten in normal amounts. Thus, we are working to expand our understanding of the mechanisms plants use to acquire Fe from the soil in order to ultimately improve the content of bioavailable Fe in plant-based foods. Also, cadmium (Cd) is a common environmental contaminant introduced into soils by anthropogenic activities. Cd contamination poses a serious threat to human health, and uptake into plants is the primary avenue through which Cd can enter the food chain. There have been some suggestions in the literature that the uptake of Cd and other heavy metals into plants is somehow influenced by plant Fe nutrition. In this paper, we studied the relationship between plant heavy metal uptake and Fe nutrition, using pea seedlings as a model system. It was found that making seedlings Fe deficient stimulated the expression of a high affinity Cd2+ uptake into pea roots. The mechanism underlying this response was studied, and it was found that Fe deficiency turned on a gene that encodes a putative Fe transporter in root cells. It appears that this transporter also can facilitate the uptake of the toxic metal, Cd. These findings have a range of implications, including bettering our understanding of how plants acquire Fe, gaining insight into how heavy metals enter the food chain, and improving use of plants as a technology to remove toxic metals from the soil as a means of remediating certain types of environmental pollution.
Technical Abstract: Plant accumulation of iron and other metals can be enhanced under Fe deficiency. 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 109Cd influx into roots of Fe-deficient and Fe-sufficient pea seedlings. The concentration-dependent kinetics for 109Cd influx were graphically complex and non-saturating but 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 wall after desorption, whereas the saturable component was transporter-mediated Cd2+ influx across the root-cell plasma membrane. The Cd2+ transport system in roots of both Fe-deficient and Fe-sufficient seedlings exhibited similar Km values, 1.5 and 0.6 uM, respectively, for saturable Cd2+ influx, while the Vmax for Cd2+ uptake in Fe-deficient seedlings was nearly 7-fold higher than that in +Fe-grown seedlings. Investigations into the mechanistic basis for this response demonstrated that Fe deficiency-induced stimulation of the plasma membrane H+-ATPase did not play a role in the enhanced Cd2+ uptake. Expression studies with the Fe2+ transporter cloned from Arabidopsis (IRT1) indicated that Fe deficiency induced the expression of this transporter, which might facilitate the transport of heavy metal divalent cations such as Cd2+ and Zn2+, in addition to Fe2+.