Title : Iron deficiency in rice under alkaline conditions is induced by the suppression of iron uptake mechanisms and iron transport responses
Abstract:
Under current environmental conditions on Earth, iron (Fe) is readily oxidized from its divalent form (Fe²?) to its trivalent form (Fe³?). It predominantly exists in soil as ferric hydroxide (Fe(OH)?), which is highly insoluble in water. This insolubilization is particularly severe in alkaline soils, which cover approximately one-third of the world's land area, severely limiting iron availability to plants. Consequently, crops are often afflicted by iron deficiency symptoms such as chlorosis, and in severe cases, their growth is inhibited, or they may die. To adapt to these iron-limited environments, graminaceous plants have evolved specialized mechanisms for iron uptake called Strategy II. In this system, plants secrete iron-chelating compounds called mugineic acids (MAs) into the rhizosphere, where they solubilize ferric hydroxide into an Fe-MAs complex, enabling iron absorption by the plants. While genes associated with Strategy II are known to be induced under iron-deficient conditions, the mechanisms of iron uptake and transport mediated by MAs in alkaline environments remain poorly understood. This study aimed to investigate the iron uptake mechanism of rice under iron-limited alkaline conditions. To achieve this, we analyzed the concentrations of metals in plant tissues and xylem sap. We also measured the levels of deoxymugineic acid (DMA), a type of mugineic acids, along with its precursor, nicotianamine (NA). Additionally, we examined the secretion of DMA and NA into the rhizosphere. Oryza sativa ‘Nipponbare’ plants were grown hydroponically for 40 days and then subjected to three treatments for an additional 10 days: iron-sufficient (control), iron-deficient, and alkaline (pH 9). After the treatment period, conduit fluids, plant tissues, and hydroponic solutions were collected. Metal concentrations were measured using ICP-MS after wet decomposition of the tissues with concentrated nitric acid. Additionally, DMA and NA concentrations in the samples were determined using LC-TOF-MS. A quantitative analysis of iron concentrations revealed that, both in iron-deficient and alkaline conditions, the levels of iron in the largest and newest leaves were lower than those under control conditions. In contrast, iron concentrations in the roots exhibited different trends: they decreased under iron-deficient conditions compared to the control but increased under alkaline conditions. NA concentrations remained relatively stable across all growth conditions, but they exhibited a decreasing gradient from the hydroponic solution to the xylem sap, roots, and leaves. In contrast, concentrations of DMA increased in all samples under iron-deficient conditions compared to the control group. Additionally, under alkaline conditions, DMA concentrations rose in the leaves but significantly decreased in the hydroponic solution. These results suggest that rice plants can sense their tissue iron levels and adjust the synthesis, secretion, and transport of DMA accordingly. Under alkaline conditions, due to the high concentrations of iron retained in the roots, DMA synthesis does not increase, and neither does its translocation to the shoots or its secretion into the hydroponic solution, even though the shoots exhibit iron-deficiency symptoms. This indicates that iron deficiency under alkaline conditions is caused by an unresponsive iron absorption mechanism distinct from that observed under iron-deficient conditions.
