Metal ligands in micronutrient acquisition and homeostasis

Acquisition and homeostasis of micronutrients such as iron (Fe) and zinc (Zn) pose specific challenges. Poor solubility and high reactivity require controlled synthesis and supply of ligands to complex these metals extracellularly and intracellularly. Cytosolic labile pools represent only a minute fraction of the total cellular content. Several low‐molecular‐weight ligands are known in plants, including sulfur ligands (cysteine and peptides), nitrogen/oxygen ligands (S‐adenosyl‐l‐methionine‐derived molecules and histidine), and oxygen ligands (phenolics and organic acids). Some ligands are secreted into the extracellular space and influence the phytoavailability of metal ions. A second principal function is the intracellular buffering of micronutrients as well as the facilitation of long‐distance transport in xylem and phloem. Furthermore, low‐molecular‐weight ligands are involved in the storage of metals, predominantly in vacuoles. A detailed molecular understanding is hampered by technical limitations, in particular the difficulty to detect and quantify cellular metal–ligand complexes. More, but still too little, is known about ligand synthesis and the transport across membranes, either with or without a complexed metal. Metal ligands have an immediate impact on human well‐being. Engineering metal ligand synthesis and distribution in crops has tremendous potential to improve the nutritional quality of food and to tackle major human health risks. This review highlights the important functions of low‐molecular‐weight ligands in the acquisition and homeostasis of micronutrients such as Fe and Zn. Recent progress in understanding the roles of major plant metal ligands is discussed, as well as the main limitations of our knowledge and the potential to improve food quality and human health via the engineering of metal ligand synthesis.