Facing the challenges of Cu, Fe and Zn homeostasis in plants

Abstract

Plants have recently moved into the spotlight owing to the growing realization that the world needs solutions to energy and food production that are sustainable and environmentally sound. Iron, copper and zinc are essential for plant growth and development, yet the same properties that make these transition metals indispensable can also make them deadly in excess. Iron and copper are most often used for their redox properties, whereas zinc is primarily used for its ability to act as a Lewis acid. Here we review recent advances in the field of metal homeostasis and integrate the findings on uptake and transport of these three metals.

Figure 1. Intercellular metal transport

Fe, Zn, and Cu are taken up into the symplast by transporters in the epidermis. Reduction of Fe and possibly Cu by FRO2 and acidification of the soil by an AHA contribute to increased metal uptake. Metals can then travel through the symplastic space to the vasculature, bypassing the waxy Casparian strip on the endodermis. Transport into the xylem is still not fully characterized but is thought to involve members of the HMA family and the citrate effluxer FRD3. In the xylem, metals are carried to the shoot through the transpiration stream where they are unloaded into the shoot, most likely by a member of the YSL family. YSLs may also translocate metals to the phloem, where they can then be delivered to the seed. Dark brown boxes represents the Casparian strip.

Figure 2. Intracellular metal transport

Fe and Zn are transported into the vacuole by VIT1 and MTP1/3 respectively, and Fe is remobilized from the vacuole by NRAMP3/4. Transport into the chloroplast is best characterized for Cu, which is transported into the chloroplast by HMA1, PAA1 and possibly PIC1. PAA2 is thought to transport Cu across the thylakoid membrane. Transport of Fe into the chloroplast is known to require reduction by FRO7 and may involve transport by PIC1. Very little is known about transport in and out of the mitochondria, thought ATM3 is well established as an Fe-S exporter.