Escobar-Herrera, L. | Search bioRxiv

bioRxiv2026

Exploring L-tyrosine and L-DOPA biosynthesis in faba bean (Vicia faba L.)

Background and AimsL-DOPA is an important pharmaceutical that accumulates to high levels in the legume faba bean (Vicia faba). L-DOPA is likely derived from L-tyrosine but the responsible enzyme (L-tyrosine oxidase) remains unknown. Availability of L-tyrosine may be a key factor controlling L-DOPA accumulation. In legumes, L-tyrosine is supplied via either a plastidial TyrA enzyme (ADH) or a deregulated cytosolic homolog (PDH). This study aimed at identifying L-tyrosine oxidase and TyrA genes from faba bean. MethodsWe used gene-to-metabolite correlations and homology-based searches to select fifteen L-tyrosine oxidase candidates, which were tested in yeast and in the model plant Nicotiana benthamiana. We also used isotopically labeled L-tyrosine to measure biosynthetic activity in different faba bean tissues and to test an alternative biosynthetic hypothesis. Three faba bean TyrA genes were inferred by homology and assayed in N. benthamiana by co-expression with a known L-tyrosine oxidase, CYP76AD6. Key ResultsNone of the L-tyrosine oxidase candidates produced L-DOPA upon heterologous expression. Feeding experiments showed a lack of correlation between L-DOPA accumulation and biosynthetic capacity. Feeding studies also disproved an alternative route to L-DOPA by oxidation of 4-hydroxyphenylpyruvate. Of the TyrA genes, two were able to increase L-tyrosine levels in N. benthamiana 2-3-fold (VfADH and VfPDH), and one of them was able to boost the levels of L-DOPA derivatives up to 6-fold (VfADH). ConclusionsThe faba bean L-tyrosine oxidase remains unidentified, with a possible transport of L-DOPA across tissues likely having confounded our correlation-based selection strategies. In N. benthamiana, both VfADH and VfPDH can increase the levels of L-tyrosine, while VfADH can further boost the levels of L-DOPA derivatives. Our work delivers a strategy to boost the provision of L-tyrosine in N. benthamiana and provides valuable insights in the search for the elusive L-tyrosine oxidase from faba bean.

plant biology↗

bioRxiv2020

VC1 catalyzes a key step in the biosynthesis of vicine from GTP in faba bean

Faba bean is a widely adapted and high-yielding legume cultivated for its protein-rich seeds1. However, the seeds accumulate the anti-nutritional pyrimidine glucosides vicine and convicine, which can cause haemolytic anaemia--favism--in the 400 million individuals genetically predisposed by a deficiency in glucose-6-phosphate dehydrogenase2. Here, we identify the first enzyme associated with vicine and convicine biosynthesis, which we name VC1. We show that VC1 co-locates with the major QTL for vicine and convicine content and that the expression of VC1 correlates highly with vicine content across tissues. We also show that low-vicine varieties express a version of VC1 carrying a small, frame-shift insertion, and that overexpression of wild-type VC1 leads to an increase in vicine levels. VC1 encodes a functional GTP cyclohydrolase II, an enzyme normally involved in riboflavin biosynthesis from the purine GTP. Through feeding studies, we demonstrate that GTP is a precursor of vicine both in faba bean and in the distantly related plant bitter gourd. Our results reveal an unexpected biosynthetic origin for vicine and convicine and pave the way for the development of faba bean cultivars that are free from these anti-nutrients, providing a safe and sustainable source of dietary protein.

biochemistry↗