Catalysis of 5-methyltetrahydrofolate to MeFox facilitates folate biofortification in crops

Abstract Folate deficiency is a global health problem. Biofortification has been considered a cost-effective means to tackle this problem. Here, we describe the genetic cloning and functional identification of a previously uncharacterised plant protein, designated as CTM, which functions as an enzyme in folate metabolism. Plant CTMs are capable of catalysing 5-methyl-tetrahydrofolate to MeFox, a pyrazino-s-triazine derivative of 4α-hydroxy-5-methyl-tetrahydrofolate. The natural asparagine-to-glycine substitution caused by an A-to-G single nucleotide variation in maize CTM enhances its enzymatic activity, as demonstrated by in vitro enzymatic assays and in silico analyses using a maize CTM structure model based on a monomeric sorghum CTM crystal. Loss of the CTM function led to accumulation of 5-methyl-tetrahydrofolate, and overexpression of the maize CTM carrying the G-allele boosted the metabolic flow towards MeFox, and showed no negative impacts on plant growth. Our results suggest that CTM, which has evolved 5-methyl-tetrahydrofolate-to-MeFox converting activity in plants, could be valuable for developing folate-biofortified crops to provide an alternative to the challenge presented by the global folate deficiency.