A bHLH transcription factor (VfTT8) underlieszt2, the locus determining zero tannin content in faba bean (Vicia faba L.)

Abstract

Faba bean (Vicia faba L.) is an important protein-rich fodder crop widely cultivated in temperate areas. However, antinutritional compounds such as condensed tannins, limit the use of this protein source in monogastric feed formulations. Previous studies demonstrated that two recessive and complementary genes, zt1 and zt2, control the absence of tannin and the white flower colour in faba bean. An ortholog of the Medicago WD40 transcription factor, (TTG1) was reported to encode the zt1 phenotypes but the responsible gene for zt2 is still unknown. A candidate gene approach combined with linkage mapping, comparative genomics and gene expression has been used in this study to fine map the zt2 genomic region and to identify the regulatory gene controlling both traits. Seventy-two genes, including 23 regulatory genes (MYB and bHLH) predicted to be associated with anthocyanin expression together with WRKY proteins were screened and genotyped in three mapping populations. The linkage groups constructed identified the regulatory gene, TRANSPARENT TESTA8 (TT8), encoding a basic helix-loop-helix transcription factor (bHLH), as the best candidate for zt2. This finding was supported by qPCR analyses and further validated in different genetic backgrounds. Accordingly, VfTT8 was down-regulated in white flowered types, in contrast to the levels of expression in wild genotypes. Our results provide new insights on the regulatory mechanisms for tannin biosynthesis in faba bean and will favour the development of an ultimate zt2 diagnostic marker for the fast generation of new value-added cultivars free of tannins and improved nutritional value.Recently, an ortholog of the Medicago WD40 transcription factors, Transparent Testa Glabra 1 (TTG1) in the faba bean chromosome (chr.) II, has been reported as the gene encoding the zt1 phenotypes [25][26][27][28] . Likewise, the locus zt2 was found to be in the distal part of the chr. III 26,28,29 but the responsible gene is still unknown. After this initial finding, the later authors 29 reported a KASP (Kompetitive Allele-specific PCR) marker (SNP marker Vf_Mt7g100500_001), located 10.5 cM apart from the flower colour (zt2 locus), as a reliable marker to discriminate low-tannin faba bean plants carrying zt2. Considering the relatively