Abstract
Brassica napus L. (rapeseed, oilseed rape, and canola) and varieties of its two diploid parents, B. oleracea and B. rapa, display a large amount of variation in anthocyanin pigmentation of the leaf, stem, and fruit. Here, we demonstrate that BnaPAP2.A7, an ortholog of the B. oleracea anthocyanin activator BoMYB2 that confers purple traits, positively regulates anthocyanin biosynthesis in leaves of B. napus. Sequencing of BnaPAP2.A7 and transgenic analysis suggests that activation of this gene in purple rapeseed may result from a single nucleotide and/or 2bp insertion in its promoter region. BnaPAP2.A7 gives rise to three splice variants, designated BnaPAP2.A7-744, BnaPAP2.A7-910, and BnaPAP2.A7-395 according to the length of the transcripts. While BnaPAP2.A7-744 encodes a full-length R2R3-MYB, both BnaPAP2.A7-910 and BnaPAP2.A7-395 encode truncated proteins that lack both a partial R3 repeat and the complete C terminal domain, and so in vitro are unable to interact with the Arabidopsis bHLH protein AtTT8. Although expression of either BnaPAP2.A7-910 or BnaPAP2.A7-395 in green rapeseed does not result in purple leaves, both genes do modify genome-wide gene expression, with a strong repression of anthocyanin-related genes. We have demonstrated that BnaPAP.A7 regulates anthocyanin accumulation in leaves of B. napus and propose a potential mechanism for modulation of anthocyanin biosynthesis by alternative splicing.
Highlights
Anthocyanins are a group of plant secondary metabolite flavonoid pigments
These results suggested that allelic variation of one of the MYB-bHLHWD40 (MBW) components may result in the activation of late-biosynthesis genes (LBGs) and lead to the purple color formation of purple rapeseed (PR)
BnaA07g25800D was considered the key transcription factor involved in the regulation of anthocyanin biosynthesis in leaves of PR
Summary
Anthocyanins are a group of plant secondary metabolite flavonoid pigments. They play the role of “nature’s Swiss army knife” in conferring tolerance to various stressors as well as in defense against herbivores and pathogens (Gould, 2004). Within the MBW complex R2R3-MYBs are the major determinants involved in anthocyanin biosynthesis (Lai et al, 2014; Liu et al, 2015; Jin et al, 2016), either as activators or repressors of structural genes (Pérez-Dıá z et al, 2016) They represent the largest subgroup of the plant MYB TF family, and they have two MYB repeats in the N terminal DNA-binding domain that are most similar to R2 and R3 from c-MYB (Feller et al, 2011) as well as an activation or repression domain usually located at the C terminus (Zimmermann et al, 2004; Dubos et al, 2010; Xu et al, 2014). R2R3MYB transcription activators have subsequently been reported in various plant taxa, such as AtMYB75 (PAP1), AtMYB90 (PAP2), AtMYB113, and AtMYB114 in Arabidopsis (Borevitz et al, 2000; Stracke et al, 2001; Zimmermann et al, 2004; Stracke et al, 2007; Gonzalez et al, 2008), ROSEA1 and ROSEA2 in Antirrhinum (Schwinn et al, 2006; Shang et al, 2011), VvMYBA1 and VvMYBA2 in Vitis grapevine (Walker et al, 2007), and Ruby in Citrus (Butelli et al, 2012; Huang et al, 2018)
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