Ford, S. A.; Franks, A. E.; Hu, A.; De Melo, C.; Evans, S. E.; Bergman, M. E.; Phillips, M.

The shikimate pathway provides precursors for phenolic metabolites in plant primary and secondary metabolism. Carbon flux measurements in intact Arabidopsis leaves using a novel isotopologue MS/MS methodology revealed unexpected dynamics between chorismate and isochorismate pools. 13CO2 labeling kinetics point to chloroplast-derived shikimate as a major bifurcation point, with approximately one third diverted from the chloroplast. In contrast, the small pool of chorismate was mostly chloroplast-localized and turned over rapidly, reaching more than 70% labeling within minutes. Isochorismate, a precursor to salicylic acid (SA) in the Brassicales, was present at 50-fold molar excess over chorismate but labeled more slowly and appeared primarily extra-chloroplastic. Total isochorismate declined by 90% in the Arabidopsis enhanced disease susceptibility mutant, which lacks the isochorismate exporter. Non-aqueous fractionation further supported a primarily chloroplast-localized chorismate pool but extra-plastidic isochorismate. Populus trichocarpa and Nicotiana benthamiana leaves contained chorismate but only trace isochorismate, consistent with use of the benzoyl-CoA route to SA. Carbon commitment calculations indicated that one third of the total chorismate pool in Arabidopsis leaves is diverted to isochorismate. Global leaf calculations based on elemental analysis-isotope ratio mass spectrometry and targeted isotope recovery indicate that only 0.03% of total assimilated carbon (5.27 pmol*mg-1 D.W.*min-1) enters the shikimate pathway in photosynthetic mesophyll. For comparison ~0.05% (7.75 pmol*mg-1 D.W.*min-1) enters the 2-C-methyl-D-erythritol 4-phosphate pathway, which provides precursors for photosynthetic pigments and electron carriers. The compartmentalization and turnover dynamics of shikimate, chorismate, and isochorismate suggest continuous demand for aromatic precursors in mesophyll tissue is comparable to demand for MEP-pathway derived pigments.