A proteomics approach to investigate the process of Z n hyperaccumulation in N occaea caerulescens ( J & C . P resl) F . K . M eyer

Summary Zinc ( Z n) is an essential trace element in all living organisms, but is toxic in excess. Several plant species are able to accumulate Z n at extraordinarily high concentrations in the leaf epidermis without showing any toxicity symptoms. However, the molecular mechanisms of this phenomenon are still poorly understood. A state‐of‐the‐art quantitative 2 D liquid chromatography/tandem mass spectrometry (2 D ‐ LC ‐ MS / MS ) proteomics approach was used to investigate the abundance of proteins involved in Z n hyperaccumulation in leaf epidermal and mesophyll tissues of N occaea caerulescens . Furthermore, the Z n speciation in planta was analyzed by a size‐exclusion chromatography/inductively coupled plasma mass spectrometer ( SEC ‐ ICP ‐ MS ) method, in order to identify the Z n‐binding ligands and mechanisms responsible for Z n hyperaccumulation. Epidermal cells have an increased capability to cope with the oxidative stress that results from excess Z n, as indicated by a higher abundance of glutathione S ‐transferase proteins. A Z n importer of the ZIP family was more abundant in the epidermal tissue than in the mesophyll tissue, but the vacuolar Z n transporter MTP 1 was equally distributed. Almost all of the Z n located in the mesophyll was stored as Z n–nicotianamine complexes. In contrast, a much lower proportion of the Z n was found as Z n–nicotianamine complexes in the epidermis. However, these cells have higher concentrations of malate and citrate, and these organic acids are probably responsible for complexation of most epidermal Z n. Here we provide evidence for a cell type‐specific adaptation to excess Z n conditions and an increased ability to transport Z n into the epidermal vacuoles.