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Summary Iron acquisition of graminaceous plants is characterized by the synthesis and secretion of the iron‐chelating phytosiderophore, mugineic acid (MA), and by a specific uptake system for iron(III)–phytosiderophore complexes. We identified a gene specifically encoding an iron–phytosiderophore transporter (HvYS1) in barley, which is the most tolerant species to iron deficiency among graminaceous plants. HvYS1 was predicted to encode a polypeptide of 678 amino acids and to have 72.7% identity with ZmYS1, a first protein identified as an iron(III)–phytosiderophore transporter in maize. Real‐time RT‐PCR analysis showed that the HvYS1 gene was mainly expressed in the roots, and its expression was enhanced under iron deficiency. In situ hybridization analysis of iron‐deficient barley roots revealed that the mRNA of HvYS1 was localized in epidermal root cells. Furthermore, immunohistological staining with anti‐HvYS1 polyclonal antibody showed the same localization as the mRNA. HvYS1 functionally complemented yeast strains defective in iron uptake on media containing iron(III)–MA, but not iron–nicotianamine (NA). Expression of HvYS1 in Xenopus oocytes showed strict specificity for both metals and ligands: HvYS1 transports only iron(III) chelated with phytosiderophore. The localization and substrate specificity of HvYS1 is different from those of ZmYS1, indicating that HvYS1 is a specific transporter for iron(III)–phytosiderophore involved in primary iron acquisition from soil in barley roots.

An internal detoxification mechanism for AI was investigated in an AI-accumulating plant, hydrangea (Hydrangea macrophylla), focusing on AI forms present in the cells. l h e leaves of hydrangea contained as much as 15.7 mmol AI kg-' fresh weight, and more than two-thirds of the AI was found in the cell sap. Using "AInuclear magnetic resonance, the dominant peak of AI was observed at a chemical shift of 11 to 12 parts per million in both intact leaves and the extracted cell sap, which is in good accordance with the chemical shift for the 1 :1 AI-citrate complex. Purification of cell sap by molecular sieve chromatography (Sephadex C-1 O) combined with ion-exclusion chromatography indicated that AI in fractions with the same retention time as citric acid contributed to the observed "AI peak,in the intact leaves. l h e molar ratio of AI to citric acid in the crude and purified cell sap approximated 1. The structure of the ligand chelated with AI was identified to be citric acid. Bioassay experiments showed that the purified AI complex from the cell sap did not inhibit root elongation of corn (Zea mays L.) and the viability of cells on the root tip surface was also not affected. lhese observations indicate that AI is bound to citric acid in the cells of hydrangea leaves.A1 toxicity is primarily characterized by the inhibition of root elongation, with no appearance of clearly identifiable symptoms in plant tops. This is because A13+, a toxic ionic species, has a high binding ability with cellular components of roots, and usually shows little translocation to the upper parts of plants. Most plants contain not more than 0.2 mg AI 8-l dry weight. However, some plants, known as "A1 accumulators," may contain more than 10 times this leve1 of A1 without any AI injury. Tea plants are typical AI accumulators; the A1 content in these plants can reach as high as 30 mg 8-l dry weight in old leaves, although

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The biosynthesis of 2'-deoxymugineic acid, a key phytosiderophore, was examined in association with the putative methionine recycling pathway in the roots of wheat using labeling experiments and structural analysis. Feeding with D-[1-13C]ribose did not result in 13C enrichment of 2'-deoxymugineic acid, while D-[2-13C]ribose resulted in 13C enrichment at the C-4", -1, -4' positions, and D-[5-13C]ribose did in C-1', -4, and -1" positions of 2'-deoxymugineic acid, respectively. Furthermore, two isotope-labeled intermediates of the methionine recycling pathway, 5-[5-2H2]methylthioribose and 2-[1-13C]keto-4-methylthiobutyric acid, were synthesized, and their incorporation into 2'-deoxymugineic acids was investigated. Six deuterium atoms at the C-4, -1', and -1" positions of 2'-deoxymugineic acid were observed after feeding with 5-[5-2H2]methylthioribose. Feeding with 2-[1-13C]keto-4-methylthiobutyric acid yielded 2'-deoxymugineic acid enriched with 13C at the C-4', -1, and -4" positions. These results demonstrated for the first time that the biosynthesis of 2'-deoxymugineic acid is associated with the methionine recycling pathway. This association system functions to recycle methionine required for continued synthesis of mugineic acids in the roots of gramineous plants.

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Tachykinins (TKs) constitute the largest vertebrate neuropeptide family with multifunctions in central and peripheral tissues. In several invertebrate species, two types of structurally related peptides, 'tachykinin-related peptides (TKRPs)' and 'invertebrate tachykinins (inv-TKs)' have been identified. TKRPs, isolated from the nerve and/or gut tissues, contain the common C-terminal sequence -Phe-X-Gly-Y-Arg-NH(2) (X and Y are variable) analogous to the vertebrate TK consensus -Phe-X-Gly-Leu-Met-NH(2), and exhibit vertebrate TK-like contractile activity on invertebrate gut tissues. Inv-TKs have been shown to be present exclusively in the salivary gland of several species, to share vertebrate TK consensus motif, and to possess TK-like potencies on vertebrate, not invertebrate tissues. However, the functional and evolutionary relevance of TKRPs and inv-TKs to vertebrate TKs remains to be understood. Recent studies have revealed that TKRP precursors dramatically differ from vertebrate preprotachykinins in structural organization and that TKRP receptors share structural and functional properties with vertebrate TK receptors. Moreover, the C-terminal arginine in TKRPs has been shown to play an essential role in discriminating their receptors from vertebrate TK receptors. Such recent marked progress is expected to enhance further investigation of biological roles of TKRPs. This review provides an overview of the basic findings obtained previously and a buildup of new knowledge regarding TKRPs and inv-TKs. We also compare TKRPs and inv-TKs to vertebrate TKs with regard to evolutionary relationships in structure and function among these structurally related peptides.

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