38 Organic Chemistry Research on Iron Acquisition in Graminaceous Plants(Oral Presentation)

Crops grown on alkaline soils, which cover about one-third of the world lands, are prone to iron-deficiency stress because of the low solubility of iron there. To acquire the insoluble iron efficiently, graminaceous plants have developed a unique strategy characterized by the synthesis and secretion of an iron-chelator phytosiderophore and a specific uptake system of iron(III) in its complex. Mugineic acid (MA) 1 was first identified as phytosiderophore in barley and its analogues have then been isolated and identified from various graminaceous species and cultivars; they all form water-soluble 1:1 complexes with iron(III). Owing to its significant implication in plant physiology, this iron up-take system has been a subject of intensive research since its discovery 30 years ago. However, a limited supply of phytosiderophores has been a severe bottleneck particularly for the study on the transport mechanism. Therefore, it is essential to establish an ample supply source of MA 1 and/or 2'-deoxymugineic acid (DMA) 2. In addition, supply of DMA will be potentially important to solve the worldwide problem of shortage of food supply because their use may make cultivation possible even on alkaline soils which are not favorable for farming. In this presentation, 1) the investigation of potential values of DMA as a fertilizer and synthetic studies toward an ample supply of DMA, 2) the mugineic acid derivatives as molecular probes for the mechanistic elucidation, and 3) development of 1,3a,6a-triazapentalenes as a breakthrough fluorescent molecule will be discussed. 1) A practical synthesis of L-azetidine-2-carboxylic acid, an expensive starting material for DMA, was established by using asymmetric organocatalyst. The normal rice plant was able to grow even in alkaline hydroponic culture by the addition of DMA and iron(III) salt. 2) A hydroxy group in mugineic acid (MA) that was not needed for Fe(III) complexation was used for the introduction of various labeling groups through propargylation and a click reaction. Labeled MA was incorporated as a phytosiderophore into Xenopus oocytes through the HvYS1 transporter, as determined by an electrophysiological assay and fluorescence microscopy. 3) An efficient and versatile method was established for the preparation of 1,3a,6a-triazapentalenes. The 1,3a,6a-triazapentalene skeleton without an additional fused ring system was discovered to be a compact and highly fluorescent chromophore, which exhibited various interesting fluorescent properties such as a noteworthy correlation of luminescent wavelength with the Hammett σ_p value and a strongly positive solvatofluorochromism.