Biosynthetic pathway of phytosiderophores in iron-deficient graminaceous plants

Abstract

Graminaceous plants secrete iron-chelators, called phytosiderophores, from the roots to solubilize the external insoluble iron, and the amount of the secreted phytosiderophores increases under iron deficiency stress (Takagi 1976). The only phytosiderophores known so far are mugineic acid-family (MAs). We developed a cell-free system derived from root tips of iron-deficient barley for the biosynthesis of 2′-deoxymugineic acid (DMA) from L-methionine (Mori and Nishizawa 1987; Shojima et al. 1990). At the same time, we showed that nicotianamine (NA) was converted to DMA, in vitro, in the presence of 2-oxoglutarate and NADH. We suggested that the synthesis of DMA from NA involves of the following two steps. At first, the amino group at the 3″-carbon of NA may be transferred to amino group acceptors (2-oxoglutarate, pyruvate, or oxalacetate), then the resulting 3″-oxo form may be reduced with reductants (NADH or NADPH) (Fig. 1). Since the putative intermediate of the 3″-oxo form was not available until now, we were unable to distinguish the enzyme activities involved in these two steps from each other in this cell-free system. Therefore, we developed a new assay system to detect selectively the enzyme activity of NA aminotransferase which catalyzes transamination at the 3″-carbon of NA by using NaBH₄ as a chemical reductant at the 3″-oxo group of a putative intermediate. Here we report the new assay system for the detection of NA aminotransferase activity.