Biosynthesis of phytosiderophores, mugineic acids, associated with methionine cycling

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.