Magnesium deficiency damages the youngest mature leaf in rice through tissue-specific iron toxicity

Magnesium deficiency can cause starch accumulation, photosynthesis inhibition and senescence particularly in young mature leaves. This study was performed to identify the initial process leading to leaf senescence under Mg deficiency. Gene expression in the young leaf was analyzed at days 2, 4, 5 of Mg deficiency using microarray analysis, and several Fe responsive genes were identified. Therefore, the effect of lowering Fe supply on gene expression and oxidative stress under Mg deficiency was evaluated. Transcriptome analysis revealed that 7 of the 30 most upregulated genes and 11 of the 30 most downregulated genes were Fe-responsive. Particularly, the upregulation of OsFER2 and downregulation of OsMIR and OsIRO2 hinted at the induction of excess Fe stress under Mg deficiency. Both lowering of Fe concentration in Mg-free solutions and resupply of Mg without modifying Fe concentrations at day 4 rescued leaves from senescence by inhibiting oxidative stress and normalising the expression of Fe-responsive genes. Meanwhile, Fe content was equal between control, Mg-deficient and Mg-resupplied plants. Mg shortage can induce excess Fe stress, which in turn causes oxidative stress before inhibition of photosynthesis. It is proposed that Mg deficiency disrupts a mechanism for storing toxic Fe ions into the vacuole in the expanding young leaf cells.