Abstract
BackgroundAnthocyanins, which have important biological functions and have a beneficial effect on human health, notably account for pigmentation in purple-fleshed sweet potato tuberous roots. Individual regulatory factors of anthocyanin biosynthesis have been identified; however, the regulatory network of anthocyanin biosynthesis in purple-fleshed sweet potato is unclear.ResultsWe functionally determined that IbMYB340 cotransformed with IbbHLH2 in tobacco and strawberry receptacles induced anthocyanin accumulation, and the addition of IbNAC56a or IbNAC56b caused increased pigmentation. Furthermore, we confirmed the interaction of IbMYB340 with IbbHLH2 and IbNAC56a or IbNAC56b via yeast two-hybrid and firefly luciferase complementation assays; these proteins could form a MYB340-bHLH2-NAC56a or MYB340-bHLH2-NAC56b transcriptional complex to regulate anthocyanin biosynthesis by binding to the IbANS promoter rather than the IbUFGT promoter. Furthermore, it was found by a transient expression system in tobacco leaves that IbMYB44 could decrease anthocyanin accumulation. Moreover, the interaction of IbMYB44 with IbMYB340 and IbNAC56a or IbNAC56b was verified. This result suggested that IbMYB44 acts as a repressor of anthocyanin in sweet potato.ConclusionsThe repressor IbMYB44 affected anthocyanin biosynthesis by competitively inhibiting the IbMYB340-IbbHLH2-IbNAC56a or IbMYB340-IbbHLH2-IbNAC56b regulatory complex formation. Overall, the present study proposed a novel regulatory network whereby several vital TFs play key roles in regulating anthocyanin biosynthesis, and it provides strong insight into the potential mechanism underlying anthocyanin biosynthesis in sweet potato tuberous roots with purple color.
Highlights
Anthocyanins, which have important biological functions and have a beneficial effect on human health, notably account for pigmentation in purple-fleshed sweet potato tuberous roots
Phylogenetic analysis indicated that IbMYB340 belongs to the activator clade, together with other known MYB transcription factors with the R2R3 domain (Fig. 1a)
Transcription factor (TF) IbMYB44, which harbors the transcriptional repressor domain LxLxL (Fig. 1d), was phylogenetically related to FaMYB44.1 (Fig. 1a), which acts as a transcriptional repressor in strawberry [19]
Summary
Anthocyanins, which have important biological functions and have a beneficial effect on human health, notably account for pigmentation in purple-fleshed sweet potato tuberous roots. Among the different colored sweet potato types, purple-fleshed sweet potato is widely popular for its high anthocyanin contents in its storage roots. The anthocyanin biosynthesis pathway has been extensively documented in different plant species and includes several key structural genes, such as phenylalanine ammonia lyase (PAL), flavanone 3-hydroxylase (F3H), dihydroflavonol 4-reductase (DFR), anthocyanidin synthase (ANS), UDP-glucose flavonoid 3-O-glucosyltransferase (UFGT) and glutathione S-transferase (GST) [6,7,8]. In addition to MYB activators, R3-MYB and some R2R3-MYB repressors, including the chrysanthemum CmMYB#7 (an R3-MYB), strawberry FaMYB1 and FaMYB44.1, peach PpMYB18 and potato StMYB44 (R2R3-MYBs) ones, have been identified as participating in the flavonoid biosynthetic pathway [19,20,21,22]
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