Metabolic profiling analysis of genetically modified rice seedlings that overproduce tryptophan reveals the occurrence of its inter-tissue translocation

A metabolic profiling approach using high performance liquid chromatography-mass spectrometry (LC-MS) was applied to seedlings of transgenic rice plants (Oryza sativa) that over-express the OASA1D gene encoding a feedback-insensitive alpha-subunit of anthranilate synthase (AS, EC 4.1.3.27). Analysis revealed that the seedlings accumulated tryptophan (Trp) at a high concentration without marked effects on the amounts of other major metabolites. Some minor indole metabolites showed a certain degree of increase in the amounts in Trp-accumulating tissues, while no active catabolic conversion of Trp was indicated. Analysis also revealed that the distribution of Trp in the plant was uneven, with the highest level being observed in young developing tissues, despite tissue-independent expression of the OASA1D gene under the control of ubiquitin promoter. Differences in AS activity and anthranilate content among organs were small. A feeding experiment with radiolabeled Trp clearly demonstrated one-way Trp movement from old to young leaves; thus, the uneven distribution of Trp in OASA1D plants is the manifestation of Trp translocation. The negligible effects of Trp accumulation in other metabolic pathways, low metabolic activity of Trp, and efficient translocation of Trp in rice plants expressing OASA1D transgene are favorable characters from the aspect of metabolic engineering of Trp production.