Vitamin B(1) diversity and characterization of biosynthesis genes in cassava

Vitamin B(1), which consists of the vitamers thiamin and its phosphorylated derivatives, is an essential micronutrient for all living organisms because it is required as a metabolic cofactor in several enzymatic reactions. Genetic diversity of vitamin B(1) biosynthesis and accumulation has not been investigated in major crop species other than rice and potato. We analyzed cassava germplasm for accumulation of B(1) vitamers. Vitamin B(1) content in leaves and roots of 41 cassava accessions showed significant variation between accessions. HPLC analyses of B(1) vitamers revealed distinct profiles in cassava leaves and storage roots, with nearly equal relative levels of thiamin pyrophosphate and thiamin monophosphate in leaves, but mostly thiamin pyrophosphate in storage roots. Unusually, the cassava genome has two genes encoding the 4-amino-2-methyl-5-hydroxymethylpyrimidine phosphate synthase, THIC (MeTHIC1 and MeTHIC2), both of which carry a riboswitch in the 3ʹ-UTR, as well as the adenylated thiazole synthase, THI1 (MeTHI1a and MeTHI1b). The THIC and THI1 genes are expressed at very low levels in storage roots compared with the accumulation of vitamin B(1), indicating only limited biosynthesis de novo therein. In leaves, vitamin B(1) content is negatively correlated with THIC and THI1 expression levels, suggesting post-transcriptional regulation of THIC by the riboswitch present in the 3ʹ-UTR of the THIC mRNA and regulation of THI1 by promoter activity or alternative post-transcriptional mechanisms.