Iron (Fe) is an essential nutrient, but is poorly bioavailable because of its low solubility in alkaline soils; this leads to reduced agricultural productivity. To overcome this problem, we first showed that the soil application of synthetic 2′-deoxymugineic acid, a natural phytosiderophore from the Poaceae, can recover Fe deficiency in rice grown in calcareous soil. However, the high cost and poor stability of synthetic 2′-deoxymugineic acid preclude its agricultural use. In this work, we develop a more stable and less expensive analog, proline-2′-deoxymugineic acid, and demonstrate its practical synthesis and transport of its Fe-chelated form across the plasma membrane by Fe(III)•2’-deoxymugineic acid transporters. Possibility of its use as an iron fertilizer on alkaline soils is supported by promotion of rice growth in a calcareous soil by soil application of metal free proline-2’-deoxymugineic acid.

To acquire iron (Fe), graminaceous plants secrete mugineic acid family phytosiderophores (MAs) from their roots into the rhizosphere. MAs are biosynthesized from methionine via nicotianamine (NA). It is thought that only graminaceous plants synthesize MAs, whereas non-graminaceous plants synthesize NA which is involved in translocation of Fe within plants. In this report, using liquid chromatography time-of-flight mass spectrometry (LC-TOF-MS), we detected 2ʹ-deoxymugineic acid (DMA), and NA in xylem sap from olive (Olea europaea L.), which is a non-graminaceous dicot plant. High-performance liquid chromatography (HPLC) confirmed the presence of these compounds in olive leaves. A transcriptome analysis of young leaves revealed several contigs encoding the putative enzymes of NA synthase, NA aminotransferase and aldo-keto reductase, which are thought to catalyze the sequential synthesis of DMA from S-adenosyl methionine. Additionally, five contigs encoding YSL (yellow stripe 1 like) transporters were identified in olive leaves. Based on these results, we propose that both NA and DMA are synthesized in olive plants and contribute to maintaining Fe homeostasis in olive trees.

Citrus greening (CG) is among the most devastating citrus diseases worldwide. CG-infected trees exhibit interveinal chlorotic leaves due to iron (Fe) deficiency derived from CG; thus, Fe content is lower in infected leaves than in healthy leaves. In this study, we demonstrated that the foliar application of Fe²⁺ relieves the symptom of CG infection in citrus trees. We applied Fe²⁺ and citrate to the leaves of infected rough lemon plants. Following this treatment, a reduction in the number of yellow symptomatic leaves was observed, and their growth was restored. Using chlorophyll content as an index, we screened for effective Fe complexes and found that a high ratio of citrate to Fe²⁺ in the applied solution led to effects against CG in Shikuwasa trees. A high proportion of Fe²⁺ to total Fe was another key factor explaining the effectiveness of the solution in CG infection, indicating the importance of Fe²⁺ absorption into plant cells. We confirmed the proportion of Fe²⁺ to total Fe through the high correlation of reflectometry data via a triazine reaction and X-ray absorption fine structure analysis. These results demonstrate that the foliar application of a high-Fe²⁺ citrate solution can restore the growth of CG diseased trees.

Purpose: Phytosiderophores (PS) from grasses solubilize sparingly soluble iron (Fe), and the resultant PS-Fe is an Fe source even for dicots. Recently, the synthetic PS proline-2′-deoxymugineic acid (PDMA) has been developed as a moderately biodegradable Fe fertilizer for grasses. We aimed to investigate whether PDMA-Fe is also a good Fe source for dicots. Methods: The availability of PDMA-Fe to cucumber was evaluated in a calcareous substrate and hydroponic cultures at pH 7.0–9.0 by determining chlorophyll level, PSII activity, and Fe uptake. EDDHA-Fe, EDTA-Fe, and citrate-Fe were used as controls. The reducibility of Fe chelates by roots was measured to determine the mechanism underlying differences in availability. Expressions of Fe deficiency-inducible genes were analyzed to estimate the Fe status in plants. Results: The application of PDMA-Fe and EDDHA-Fe to a calcareous substrate reduced Fe-deficient chlorosis to a similar extent; however, the shoot Fe concentration was higher in the PDMA-Fe treatment. In the hydroponic culture, the availability of PDMA-Fe was higher than that of the other chelates at all pH levels, and this was confirmed by higher PSII activity and lower expression of Fe deficiency-inducible genes. The reducibility assay revealed that the reduction level of PDMA-Fe was greater than that of EDTA-Fe and citrate-Fe under alkaline pH. Conclusions: PDMA-Fe is utilized by cucumber roots more efficiently than traditional synthetic chelates in both calcareous substrate and hydroponic cultures. The higher availability of PDMA-Fe may be attributed to its higher reducibility. Our findings suggest that PDMA-Fe could be a good Fe fertilizer for dicots.

High-temperature stress poses a significant threat to agricultural productivity and ecological diversity. Here, we show the effects of prolonged high-temperature stress on wheat (Triticum aestivum) and the model grass Brachypodium distachyon and demonstrate that heat stress induces iron deficiency in newly emerged leaves. Quantitative trait locus analysis of B. distachyon reveals a genomic region associated with heat resilience that includes the transporter of mugineic acid family phytosiderophores 1 gene (BdTOM1). Iron-deficiency-related genes including BdTOM1 are more highly expressed in a high-temperature-tolerant B. distachyon accession at high temperature than in a sensitive accession, resulting in greater secretion of deoxymugineic acid. Treatment with proline-2′-deoxymugineic acid mitigates heat-induced growth inhibition, but excess iron treatment leads to toxicity in both species. Our findings highlight the role of heat-induced nutritional stress in prolonged high-temperature stress and suggest that iron homeostasis could provide a promising target for improving crop resilience to climate extremes.