Manganese handling in plants: Advances in the mechanistic and functional understanding of transport pathways.

As the catalytic centre of the oxygen-evolving complex in photosystem II and a co-factor of glycosyltransferases and many other proteins, manganese (Mn) is essential for plants and a limiting factor for crop production. However, an excessive Mn availability is toxic to plants. Therefore, mechanisms need to be in place to maintain Mn homeostasis under fluctuating Mn availability. This review summarises our current understanding of the mechanisms that move Mn from the soil to its cellular targets and maintain Mn homeostasis. We zoom in from the whole-plant perspective to the intracellular allocation of the metal by transport proteins of different families acting in concert. In particular, organellar Mn supply by members of the recently identified bivalent cation transporter family and the post-translational regulation of Mn transporters by calcium-regulated phosphorylation have been a focus of current research. Finally, the emergent diversity of Mn handling beyond the Arabidopsis model will be addressed.