Figure 1

Thiamin biosynthesis in plants. Figure adapted from Strobbe and Van Der Straeten¹⁷. Biosynthetic pathway is depicted in blue, while enzymes are shown in black. Black boxes depict transporters, interrogation points represent unknown enzymes. Only one TDP transporter is known²²; other transporters are proposed. The synthesis of the pyrimidine moiety (HMP-P) requires 5-aminoimidazole ribonucleotide (AIR) and S-adenosylmethionine (SAM) as substrates and is catalyzed by 4-amino-2-methyl-5-hydroxymethylpyrimidine phosphate (HMP-P) synthase (THIC)^6,7. THIC mRNA precursor is regulated by a riboswitch in its 3’ untranslated region (UTR). This riboswitch functions as a TDP sensor controlling THIC mRNA stability and consequently, TDP production^23,24,25,26. The synthesis of the thiazole moiety (HET-P) is mediated by 4-methyl-5-b-hydroxyethylthiazole phosphate (HET-P) synthase (THI1)^8,9, the substrates of this reaction are nicotinamide adenine dinucleotide (NAD) and glycine²⁷. TH1 (HMP-P kinase/thiamin monophosphate (TMP) pyrophosphorylase) phosphorylates HMP-P to HMP-PP and also condenses HMP-PP and HET-P to form TMP^3,10. TH2 dephosphorylates TMP^5,11,12 which is then pyrophosphorylated to TDP by TDP kinases (TDPKs)¹³.