Utilization of transcription factors might be a powerful approach to modification of metabolism for a generation of crops having superior characteristics because a single transcription factor frequently regulates coordinated expression of a set of key genes for respective pathways. Here, we apply the plant-specific Dof1 transcription factor to improve nitrogen assimilation, the essential metabolism including the primary assimilation of ammonia to carbon skeletons to biosynthesize amino acids and other organic compounds involving nitrogen in plants. Expressing Dof1 induced the up-regulation of genes encoding enzymes for carbon skeleton production, a marked increase of amino acid contents, and a reduction of the glucose level in transgenic Arabidopsis. The results suggest cooperative modification of carbon and nitrogen metabolisms on the basis of their intimate link. Furthermore, elementary analysis revealed that the nitrogen content increased in the Dof1 transgenic plants (Ϸ30%), indicating promotion of net nitrogen assimilation. Most significantly, the Dof1 transgenic plants exhibit improved growth under low-nitrogen conditions, an agronomically important trait. These results highlight the great utility of transcription factors in engineering metabolism in plants.

Summary Capsaicinoids are responsible for the spicy flavor of pungent peppers (Capsicum). The cultivar CH‐19 Sweet is a non‐pungent pepper mutant derived from a pungent pepper strain, Capsicum annuum CH‐19. CH‐19 Sweet biosynthesizes capsaicinoid analogs, capsinoids. We determined the genetic and metabolic mechanisms of capsinoid biosynthesis in this cultivar. We analyzed the putative aminotransferase (pAMT) that is thought to catalyze the formation of vanillylamine from vanillin in the capsaicinoid biosynthetic pathway. Enzyme assays revealed that pAMT activity catalyzing vanillylamine formation was completely lost in CH‐19 Sweet placenta tissue. RT‐PCR analysis showed normal mRNA transcription of the pAMT gene; however, SNP analysis of the cDNA sequence showed a T nucleotide insertion at 1291 bp in the pAMT gene of CH‐19 Sweet. This insertion formed a new stop codon, TGA, that prevented normal translation of the gene, and the pAMT protein did not accumulate in CH‐19 Sweet as determined using Western blot analysis. We developed a dCAPS marker based on the T insertion in the pAMT gene of CH‐19 Sweet, and showed that the pAMT genotype co‐segregated with the capsinoid or capsaicinoid fruit phenotype in the F2 population. The T insertion was not found in other pungent and non‐pungent Capsicum lines, suggesting that it is specific to CH‐19 Sweet. CH‐19 Sweet’s pAMT gene mutation is an example of a nonsense mutation in a single gene that alters a secondary metabolite biosynthetic pathway, resulting in the biosynthesis of analogs. The dCAPS marker will be useful in selecting lines with capsinoid‐containing fruits in pepper‐breeding programs.