Biosynthesis of Phytosiderophores

Abstract

2'-Deoxymugineic acid (DMA), one of mugineic acid-family phytosiderophores (MAs), was synthesized in vitro both from Lmethionine and from nicotianamine (NA) with a cell-free system derived from root tips of iron-deficient barley (Hordeum vulgare L.). The reactions producing DMA from NA needed an amino group acceptor (i.e. 2-oxoglutarate, pyruvate, or oxalacetic acid) and a reductant (i.e. NADH or NADPH). The activity of the enzymes to produce NA from L-methionine was the highest at about pH 9.This biosynthetic activity was markedly induced by iron-deficiency stress. The synthesis of NA from S-adenosyl-L-methionine was more efficient than from L-methionine. From the results with the cell-free system reported here, we propose a revised biosynthetic pathway of MAs.Plants with 'Strategy 2' for iron acquisition (16) excrete iron-chelators, called phytosiderophores, from the roots to solubilize the external insoluble Fe3", and the amount of the secreted phytosiderophores increases under iron deficiency stress. MAs2 are the only examples of the phytosiderophores known so far (21). Their chemical properties (9,14,20,23) and their physiological significance (4,7,8,11,12,15,22) have already been established.The biosynthetic pathway of MAs is presently considered to be that three molecules of L-methionine are combined to form NA, which is then converted to DMA by deamination and hydroxylation at the 3"-carbon, and subsequent series of hydroxylation of DMA lead to other members of MAs. This pathway has been supported by the incorporation of L-['4C] methionine or L-['3C]methionine into MAs in vivo (5, 10) and the inhibition of deamination in vivo (6, 18). To characterize the biosynthesis furthermore, we recently developed a cellfree system for the biosynthesis of NA from L-methionine, using crude protein fraction from the roots of iron-deficient barley (18). For a larger scale preparation of the sample, we 'Partly supported by Grant-in-Aid (No. 62219004 and No. 62303014) from the Ministry of Education, Science and Culture, Japan 2Abbreviations: MAs, mugineic acid-family phytosiderophores; DMA, 2'-deoxymugineic acid; NA, nicotianamine; SAM, S-adenosyl-L-methionine; Taps, N-tris(hydroxymethyl)methyl-3-aminopropanesulfonic acid also reported the cell-free system from suspension culture of Nicotiana megalosiphon (19).On the above-mentioned hypothetical pathway of the biosynthesis of MAs, NA is a key substance, not only because its structure is much similar to MAs but also because the biosynthetic steps from NA to MAs can be thought to be evolutionally acquired by the 'strategy 2' plants, or lost by the 'strategy 1' plants: NA is widely distributed among the plants of both strategy 1 and 2, but MAs can be found only in some graminaceous plants (1,3,17). Nevertheless, there has been no direct evidence that NA is converted to DMA. The principal reason for this lack of data may be that there have been no systems to produce DMA in vitro.We thus improved our previous cell-free system in the following order. First, the cel...

Two Related Biosynthetic Pathways of Mugineic Acids in Gramineous Plants

“…On feeding four 14C-labeled amino acids and ~-[l-'~C]Met to iron-deficient barley (Hordeum vulgare L. cv Minorimugi) roots, L-Met has been found to be the most efficient precursor for mugneic acid and 2'-deoxymu@neic acid (~~~~i , , t 1988). similar in vibo findings by Shojima et al (1989Shojima et al ( , 1990 have been documented. However, experimental data on the biosynthesis of other mugineic acids such as avenic acid A and 3-epihydroxymugineic acid, and on their relationship in the biosynthetic pathway, have yet to be published, although speculation has previously been attempted (Mori and Nishizawa, 1987;Kawai et al, 1988;Shojima et al, 1989Shojima et al, , 1990.…”

“…Recently, nicotianamine has been reported as a possible previous precursor for 2'-deoxymugineic acid in the cell-free system (Shojima et al, 1990). Based on their structural features, nicotianamine might possibly be converted to 2'-deoxymugineic acid by deamination and subsequent reduction at the C-3" position, although direct verification is warranted.…”

“…Except for L-Met, which is the precursor of 2'-deoxymugineic acid and mugineic acid in barley plants (Mori and Nishizawa, 1987;Kawai et al, 1988;Shojima et al, 1989;Shojima et al, 1990), information on the biosynthesis of mugineic acid analogs is as yet limited. In the present study, biosynthesis routes of different mugineic acids in four species were compared.…”

iTRAQ Protein Profile Analysis of Arabidopsis Roots Reveals New Aspects Critical for Iron Homeostasis

“…Two enzymes with a proposed function in the salvage of L-Met from methylthioadenosine, ARD2 and At5g53850, were also induced by Fe starvation. SAM is an important methyl donor in transmethylation reactions and a substrate for the biosynthesis of NA from three molecules of SAM by NICOTIANAMINE SYNTHASE (NAS), encoded by four isogenes in Arabidopsis (Shojima et al, 1990;Herbik et al, 1999). Ectopic expression and mutations of NA synthases have detrimental effects on Fe homeostasis (Brumbarova and Bauer, 2005;Cassin et al, 2009;Klatte et al, 2009).…”

Three Nicotianamine Synthase Genes Isolated from Maize Are Differentially Regulated by Iron Nutritional Status

“…The biosynthetic pathways of MAs (Fig. 1A) have been determined Kawai et al, 1988;Shojima et al, 1990;Ma et al, 1999), and almost all the genes involved have been isolated in our laboratory (Higuchi et al, 1999b;Takahashi et al, 1999;Nakanishi et al, 2000;Kobayashi et al, 2001). NAS is a key enzyme in MA biosynthesis, catalyzing the trimerization of SAM into one molecule of NA (Higuchi et al, 1999b).…”