Fig. 6.

Conceptual model summarizing selected key processes controlling manganese (Mn) efficiency in plants. Proton release: increased proton-pumping activity releases H⁺ into the rhizosphere and promotes lowering of pH, which dissolves Mn(IV) oxides into plant-available Mn²⁺ (Mizuno et al., 2006). Root exudates: exudation of organic acids such as citrate, malate, oxalate and oxaloactetate to the rhizosphere changes soil Mn solubility through their ability to reduce Mn(IV) oxides by ligand exchange and complexing the released Mn²⁺ to increase plant availability (Gherardi and Rengel, 2004; Rengel and Marschner, 2005; Chen et al., 2015; Rengel, 2015; Liu et al., 2017). Under Mn deficiency, root phytase exudation can be upregulated in plants, to solubilize Mn²⁺ complexed with inositol phosphates (George et al., 2014). Microflora: rhizosphere micro-organisms play important roles in plant tolerance towards Mn deficiency. The rhizosphere is rich in bacteria and fungi, with the ability to change the redox state of Mn and thereby influence the plant availability of Mn (Kothari et al., 1991; Posta et al., 1994; Marschner et al., 2003). Transport proteins: a range of plasma membrane-bound proteins localized at the root surface (IRT1 and NRAMP1;5) (Pedas et al., 2008; Cailliatte et al., 2010; Sasaki et al., 2012; Wu et al., 2016) and endogenous transport proteins (VIT1, CAX2, ECA1;3, ZIP1, NRAMP2;3;4 and MTP8, 11) (Pedas et al., 2014; Alejandro et al., 2017; Shao et al., 2017) are regulated by the Mn status of plants and are involved in primary uptake of Mn²⁺. Translocation and remobilization: upon Mn uptake by the roots, Mn loading and unloading of xylem and phloem is mediated by Mn transport proteins (Yamaji et al., 2013). Furthermore, the chemical environment of the vascular tissue fluid (Álvarez-Fernández et al., 2014), and leaf transpiration (Hebbern et al., 2009) influence Mn mobility within plants. Photosystem II: at the leaf level, the chloroplast Mn transporters, CMT1 (Eisenhut et al., 2018) and PAM71 (Schneider et al., 2016), are involved in Mn²⁺ homeostasis and required for Mn delivery to PSII super- and subcomplexes to fulfil the catalytic function of Mn in water splitting in photosynthesis. The peripheral proteins PsbP and PsbQ of PSII protect the Mn cluster of PSII and are important for PSII stability and functionality (Schmidt et al., 2016a, b).