The Arabidopsis Thiamin-Deficient Mutant pale green1 Lacks Thiamin Monophosphate Phosphatase of the Vitamin B1 Biosynthesis Pathway

Thiamin diphosphate (TDP, vitamin B1) is an essential coenzyme present in all organisms. In plants, thiamin monophosphate (TMP) is synthesized in the chloroplasts, whereas the conversion of thiamin to the active TDP occurs in the cytosol. The Arabidopsis pale green1 (pale1) mutant contained higher concentrations of TMP and less thiamin and TDP than the wild type. Supplementation with thiamin, but not the thiazole and pyrimidine precursors, rescued the mutant phenotype, indicating that the pale1 mutant is a thiamin-deficient mutant. The PALE1 gene encodes a TMP phosphatase that is localized to the mitochondrion. Thus, the biosynthesis of TDP in plant cells involves at least three different subcellular components, the chloroplasts, mitochondria, and cytosol.