No abstract

Faba bean ( Vicia faba L.) is a widely adapted and high-yielding legume cultivated for its protein-rich seeds ( 1 ). However, the seeds accumulate the pyrimidine glucosides vicine and convicine, which can cause hemolytic anemia (favism) in 400 million genetically predisposed individuals ( 2 ). Here, we use gene-to-metabolite correlations, gene mapping, and genetic complementation to identify VC1 as a key enzyme in vicine and convicine biosynthesis. We demonstrate that VC1 has GTP cyclohydrolase II activity and that the purine GTP is a precursor of both vicine and convicine. Finally, we show that cultivars with low vicine and convicine levels carry an inactivating insertion in the coding sequence of VC1 . Our results reveal an unexpected, purine rather than pyrimidine, biosynthetic origin for vicine and convicine and pave the way for the development of faba bean cultivars that are free of these anti-nutrients.

25Faba bean is a widely adapted and high-yielding legume cultivated for its protein-rich seeds 1 . 26 However, the seeds accumulate the anti-nutritional pyrimidine glucosides vicine and convicine, which 27 can cause haemolytic anaemia-favism-in the 400 million individuals genetically predisposed by a 28 deficiency in glucose-6-phosphate dehydrogenase 2 . Here, we identify the first enzyme associated with 29 vicine and convicine biosynthesis, which we name VC1. We show that VC1 co-locates with the major QTL 30 for vicine and convicine content and that the expression of VC1 correlates highly with vicine content 31 across tissues. We also show that low-vicine varieties express a version of VC1 carrying a small, frame-32 shift insertion, and that overexpression of wild-type VC1 leads to an increase in vicine levels. VC1 33 encodes a functional GTP cyclohydrolase II, an enzyme normally involved in riboflavin biosynthesis from 34 the purine GTP. Through feeding studies, we demonstrate that GTP is a precursor of vicine both in faba 35 bean and in the distantly related plant bitter gourd. Our results reveal an unexpected biosynthetic origin 36 for vicine and convicine and pave the way for the development of faba bean cultivars that are free from 37 these anti-nutrients, providing a safe and sustainable source of dietary protein. 38 39 Main Text 40 According to the UN's Intergovernmental Panel on Climate Change (IPCC), switching to a plant-based 41 diet can reduce carbon emissions, especially in the West 3 . The suggested change in diet will require a wider 42and more varied cultivation of locally adapted protein crops. On a worldwide basis, faba bean ( Fig. 1a) has 43 the highest yield of the legumes after soybean (1.92 Mg/ha in 2013-2017) 4 and the highest seed protein 44 content of the starch-containing legumes (29% dry-matter basis) 5 . Furthermore, faba bean is adapted to 45 cool climates such as Mediterranean winters and northern European summers, where soybean performs 46 poorly 6 . The main factor restricting faba bean cultivation and consumption is the presence of the anti-47 2, Supplementary File 6). Using seed coat cDNA and PCR primers able to distinguish between VC1 and vc1, 132