C-13 NUCLEAR-MAGNETIC-RESONANCE DETECTION OF INTERACTIONS OF SERINE HYDROXYMETHYLTRANSFERASE WITH C1-TETRAHYDROFOLATE SYNTHASE AND GLYCINE DECARBOXYLASE COMPLEX ACTIVITIES IN ARABIDOPSIS

In C-3 plants, serine synthesis is associated with photorespiratory gl ycine metabolism involving the tetrahydrofolate (THF)-dependent activi ties of the glycine decarboxylase complex (GDC) and serine hydroxymeth yl transferase (SHMT). Alternatively, THF-dependent serine synthesis c an occur via the C1-THF synthase/SHMT pathway. We used C-13 nuclear ma gnetic resonance to examine serine biosynthesis by these two pathways in Arabidopsis thaliana (L.) Heynh. Columbia wild type. We confirmed t he tight coupling of the GDC/SHMT system and observed directly in a hi gher plant the flux of formate through the C1-THF synthase/SHMT system . The accumulation of C-13-enriched serine over 24 h from the GDC/SHMT activities was 4-fold greater than that from C1-THF synthase/SHMT act ivities. Our experiments strongly suggest that the two pathways operat e independently in Arabidopsis. Plants exposed to methotrexate and sul fanilamide, powerful inhibitors of THF biosynthesis, reduced serine sy nthesis by both pathways. The results suggest that continuous supply o f THF is essential to maintain high rates of serine metabolism. Nuclea r magnetic resonance is a powerful tool for the examination of THF-med iated metabolism in its natural cellular environment.