Biosynthetic pathways of 3-epihydroxymugineic acid and 3-hydroxymugineic acid in gramineous plants

To elucidate the biosynthetic pathways of 3-epihydroxymugineic acid and 3-hydroxymugineic acid, feeding experiments using ²H- and/or ¹³C-labeled compounds were conducted with barley (Hordeum vulgare L. cv Ehimehadaka) and rye (Secale cereale L. cv Elubon), respectively. Feeding of [1, 4′, 4″¹³C₃]2′-deoxymugineic acid (25 atom% ¹³C) to iron-deficient barley yielded mugineic acid and 3-epihydroxymugineic acid enriched with ¹³C at their corresponding positions. These results enabled us to demonstrate that 2′-deoxymugineic acid acts as a precursor for mugineic acid and 3-epihydroxymugineic acid. The ²H-labeled 3-hydroxymugineic acid was obtained by feeding d, l- [3, 3, 4, 4²H₄] methionine (98.6 atom% ²H) to iron-deficient rye plants. The ²H-NMR study revealed that two deuterium atoms (one each at the C-2′ and C-3 positions) from the three ²H-labeled methionine molecules were lost in 3-hydroxymugineic acid, whereas the other 10 deuterium atoms were incorporated in a manner similar to that of 2′-deoxymugineic acid and mugineic acid. These findings suggested that 3-hydroxymugineic acid, instead of being biosynthesized from 3-epihydroxymugineic acid by epimerization, was in fact derived from mugineic acid by hydroxylation at the C-3 position. This assumption was further confirmed by the results of the incorporation of ¹³C-labeled 2’-deoxymugineic acid into 3-hydroxymugineic acid. These results revealed that the biosynthetic pathways of both 3-epihydroxymugineic acid and 3-hydroxymugineic acid are as follows: L-methionineℲ2′-deoxymugineic acid→mugineic acid→3-epihydroxymugineic acid in barley and→hydroxymugineic acid in rye.