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 H-2- and/or C-13-labeled compounds were conducted with barley (Hordeum vulgare L. cv Ehimehadaka) and rye (Secale cereale L. cv Elubon), respectively. Feeding of [1,4',4''-C-13(3)] 2'-deoxymugineic acid (25 atom% C-13) to iron-deficient barley yielded mugineic acid and 3-epihydroxymugineic acid enriched with C-13 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 H-2-labeled 3-hydroxymugineic acid was obtained by feeding D,L-[3,3,4,4-H-2(4)] methionine (98.6 atom% H-2) to iron-deficient rye plants. The H-2-NMR study revealed that two deuterium atoms (one each at the C-2' and C-3 positions) from the three H-2-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 C-13-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-methionine-->2'-deoxymugineic acid-->mugineic acid-->3-epihydroxymugineic acid in barley and-->hydroxymugineic acid in rye.