Alkaline conditions not only decrease iron availability but also affect iron homeostasis mediated by mugineic acids...

Under alkaline conditions, iron concentrations differed among rice plant tissues, and 2'-deoxymugineic acid biosynthesis was not induced in the root. About 30% of the world's soil is alkaline, restricting iron (Fe) availability in plants. Grasses have a unique mechanism called Strategy II to acquire Fe from the soil by secreting mugineic acids (MAs). MAs are suggested to facilitate not only Fe uptake but also internal metal transport within the plant. We investigated how MAs, which are involved in Fe homeostasis, are affected by Fe deficiency induced under alkaline conditions. At pH 9, rice leaves were chlorotic and had reduced Fe concentrations, whereas the roots had ~ 8 × the Fe level of the control. The expression of genes involved in the synthesis and transport of 2'-deoxymugineic acid (DMA) that responded to Fe deficiency (OsNAS1, OsNAS2, OsNAAT1, OsDMAS1, OsTOM1, and OsYSL15) was induced in the leaves at pH 9, as in Fe deficiency, but not in the roots, indicating different Fe-related responses between tissues. At pH 9, the concentrations of DMA in leaves and roots and in xylem sap were comparable to those in the control, inconsistent with the observed changes in gene expression. These findings suggest that Strategy II is regulated differently between alkaline conditions and typical Fe-deficient conditions, influenced by Fe status in different tissues. Furthermore, under alkaline conditions, the problem for plants may lie not only in the reduced availability of Fe but also in the failure of roots to properly sense Fe deficiency, even when the shoots are experiencing it.