Vitamin B₁ THIAMIN REQUIRING1 synthase mediates the maintenance of chloroplast function by regulating sugar and fatty acid metabolism in rice

Abstract

Vitamin B1 (VB1), including thiamin, thiamin monophosphate (TMP), and thiamin pyrophosphate (TPP), is an essential micronutrient for all living organisms. Nevertheless, the precise function of VB1 in rice remains unclear. Here, we described a VB1 auxotrophic mutant, chlorotic lethal seedling (cles) from the mutation of OsTH1, which displayed collapsed chloroplast membrane system and decreased pigment content. OsTH1 encoded a phosphomethylpyrimidine kinase/thiamin‐phosphate pyrophosphorylase, and was expressed in various tissues, especially in seedlings, leaves, and young panicles. The VB1 content in cles was markedly reduced, despite an increase in the expression of VB1 synthesis genes. The decreased TPP content affected the tricarboxylic acid cycle, pentose phosphate pathway, and de novo fatty acid synthesis, leading to a reduction in fatty acids (C16:0 and C18:0) and sugars (sucrose and glucose) of cles. Additionally, irregular expression of chloroplast membrane synthesis genes led to membrane collapse. We also found that alternative splicing and translation allowed OsTH1 to be localized to both chloroplast and cytosol. Our study revealed that OsTH1 was an essential enzyme in VB1 biosynthesis and played crucial roles in seedling growth and development by participating in fatty acid and sugar metabolism, providing new perspectives on VB1 function in rice.

Defence and nutrition synergistically contribute to the distinct tolerance of rice subspecies to the stem borer, Chilo suppressalis

“…To further test the hypothesis that changes in VB 1 titre in rice contribute to the difference of larval weight, we measured the body weight of SSB caterpillars fed on overexpression ( OxTH1 Nip ) and knockout ( th1 ) rice (ZH11 background) (Supporting Information: Figure ) (Nie et al, 2022) for 7 days. The mean body weight of SSB larvae fed on OxTH1 Nip plants was significantly higher than on WT plants (Figure 3h).…”

“…We found that the VB 1 ‐dificient rice line negatively affected SSB caterpillars which may indicate a nutrition‐based effect (Figure 3h). Previous studies have shown that a decrease in VB 1 content can lead to a reduction in sugars in rice (Nie et al, 2022) which could contribute to differences in the energy available in the two subspecies. However, when we only deleted VB 1 in artificial diets but maintained sugar levels, effects on larval growth persisted (Figure 3a).…”

“…The T3 populations were used for further bioassay. VB 1 ‐defective rice was generated as described previously (Nie et al, 2022). Primer sequences and references can be found in the Supporting Information (Table ).…”

OsTH1 is a key player in thiamin biosynthesis in rice

“…1 ). In the plastid, HMP-P is phosphorylated into HMP-PP by the action of the bifunctional HMP-P kinase/TMP synthase (TH1) which subsequently condenses HMP-PP and HET-P to form thiamin monophosphate (TMP) 3 , 10 (Fig. 1 ).…”

“…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 glycine 27 . 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 (TDPK s ) 13 .…”