3.4 ǀ Retention of Fe in roots
In healthy roots, most of the Fe2+ absorbed is retained in the roots in metabolically-inactive forms (Yoshida, 1981). This is likely to involve sequestration in root-cell vacuoles (Moore et al., 2014; Stein, Ricachenevsky & Fett, 2014) and in the ferritin protein in plastids (Briat et al., 2010; da Silveira et al., 2009; Stein, Ricachenevsky & Fett, 2009). Rice ferritin genes (OsFER1 and OsFER2) are strongly up-regulated in roots and shoots by excess Fe (Aung et al., 2018a; Finatto et al., 2015; Quintet et al., 2012). Regulation of ferritin genes in response to Fe excess occurs at the transcriptional level (Stein et al., 2009; Briat et al., 2010), and involves regulatory pathways mediated by ABA, ROS and ethylene. However, Majerus, Bertin & Lutts (2009) showed that in O. glaberrima a signalling pathway leading to the induction of ferritin synthesis depended neither on ABA nor ROS.
In plants taking up chelated Fe(III), increased apoplastic pH depresses ferric chelate reductase activity, restricting Fe(III) reduction to Fe(II) and thereby restricting Fe2+ mobility (Kosegarten et al., 2004). Failure to regulate apoplastic pH or supress ferric chelate reductase activity may result in uncontrolled accumulation of Fe2+ in shoots (Becker & Asch, 2005). Acidification of the rhizosphere as a result of Fe2+oxidation and excess cation over anion intake (Section 3.1) will tend to lower the pH in the root apoplast. The cation-exchange capacity of root cell walls is also a factor. Cell-wall components with negative surface charges (pectin and hemicelluloses) possess cation exchange sites, for which Fe2+ ions compete with other cations (Li et al., 2016b). The cation exchange capacity will decrease as the apoplastic pH decreases.
The effectiveness of this adaptation is limited by the Fe2+ storage capacity of the root tissues, particularly towards the end of the growth cycle (Becker & Asch, 2005). Wu et al. (2017) studied regulation of genes controlling Fe uptake, partitioning and storage in a tolerant and a sensitive genotype in nutrient culture, and found no genotype differences in Fe concentration and speciation in different plant tissues, nor in sub-cellular partitioning genes.