Submitted to: Planta
Publication Type: Peer reviewed journal
Publication Acceptance Date: 10/22/2003
Publication Date: 5/1/2004
Citation: Cohen, C.K., Garvin, D.F., Kochian, L.V. 2004. Kinetic properties of a micronutrient transporter from pisum sativum indicate a primary function in fe uptake from the soil. Planta. 218:784-792. 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. 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 plant uptake of Cd and other heavy metals is influenced by plant Fe nutrition. In this paper, we conducted research to provide a molecular basis for our previous physiologically-based work showing that Fe deficiency stimulates Cd accumulation in pea seedlings. A putative micronutrient transporter gene was cloned from pea roots, and its ability to transport metals was studied in yeast. We showed that this transporter, which is only expressed in pea roots under Fe deficiency, primarily mediates Fe uptake. However, if other metals such as Cd or Zn are in the soil at elevated levels, this transporter can also mediate Cd and Zn uptake. These findings show that heavy metals such as Cd may enter food crop plants via transporters normally functioning in Fe nutrition. The results presented here have implications for 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.
Technical Abstract: Fe uptake in dicotyledonus plants is mediated by a root plasma membrane ferric reductase that reduces extracellular Fe(III)-chelates, releasing Fe2+ ions which are then absorbed via a metal ion transporter. Controversy exists in the literature as to whether this transporter is a specific Fe transporter or a broader specificity micronutrient transporter. We previously showed that Fe deficiency induces an increased capacity to absorb Fe and other micronutrient and heavy metals such as Zn2+ and Cd2+ into pea (Pisum sativum L.) roots To investigate this further, an Fe-regulated metal ion transporter (RIT1 for root iron transporter) was isolated from pea seedlings. RIT1 encodes a membrane protein that is induced under Fe deficiency and functionally complements yeast mutants defective in Fe uptake. Chelate buffer techniques were used to control Fe2+ in the uptake solution at the nanomolar activities found in soils, and d59Fe flux experiments were conducted to show that RIT1 is a very high-affinity 59Fe2+ uptake system (Km = 54 to 93 nM). 65Zn and 109Cd flux techniques were also used to show that RIT also can facilitate lower affinity Zn and Cd influx (Km of 4 and 100 uM, for Zn2+ and Cd2+, respectively). These findings suggest that in typical agricultural soils, RIT1 functions primarily to mediate root Fe acquisition. However, the ability of RIT1 to facilitate Zn and Cd uptake when these metals are present at elevated concentrations suggests that RIT1 may be a pathway for the entry of toxic metals into the food chain. The finding that plant Fe deficiency status may promote heavy metal uptake via increased expression of this transporter could have implications both for human nutrition, and also for the use of plants to remediate metal-contaminated soils.