MdHB1 down-regulation activates anthocyanin biosynthesis in the white-fleshed apple cultivar ‘Granny Smith’

Abstract

Coloration in apple (Malus×domestica) flesh is mainly caused by the accumulation of anthocyanin. Anthocyanin is biosynthesized through the flavonoid pathway and regulated by MYB, bHLH, and WD40 transcription factors (TFs). Here, we report that the HD-Zip I TF MdHB1 was also involved in the regulation of anthocyanin accumulation. MdHB1 silencing caused the accumulation of anthocyanin in 'Granny Smith' flesh, whereas its overexpression reduced the flesh content of anthocyanin in 'Ballerina' (red-fleshed apple). Moreover, flowers of transgenic tobacco (Nicotiana tabacum 'NC89') overexpressing MdHB1 showed a remarkable reduction in pigmentation. Transient promoter activation assays and yeast one-hybrid results indicated that MdHB1 indirectly inhibited expression of the anthocyanin biosynthetic genes encoding dihydroflavonol-4-reductase (DFR) and UDP-glucose:flavonoid 3-O-glycosyltransferase (UFGT). Yeast two-hybrid and bimolecular fluorescence complementation determined that MdHB1 acted as a homodimer and could interact with MYB, bHLH, and WD40 in the cytoplasm, consistent with its cytoplasmic localization by green fluorescent protein fluorescence observations. Together, these results suggest that MdHB1 constrains MdMYB10, MdbHLH3, and MdTTG1 to the cytoplasm, and then represses the transcription of MdDFR and MdUFGT indirectly. When MdHB1 is silenced, these TFs are released to activate the expression of MdDFR and MdUFGT and also anthocyanin biosynthesis, resulting in red flesh in 'Granny Smith'.

Transcriptomic and metabolomic analysis provides insights into anthocyanin and pyocyanin accumulation in pear

“…In this study, the core set of candidate genes for anthocyanin and PA accumulation included four R2R3-MYB genes: PcMYB3, PcMYB86, PcMYB111 and PcMYB114, which were strongly positively correlated with anthocyanins and PAs. This result provides evidence that MdUFGT indirectly by interacting with MdMYB10, MdbHLH3, and MdTTG1 [50]. Consistent with the results in apple, silencing of RhHB1 in rose led to higher anthocyanin levels in the petals [51].…”

Transcriptomic and metabolomic analysis provides insights into anthocyanin and procyanidin accumulation in pear

“…Overexpression of MdHB1 led to reduced anthocyanin accumulation in apple flesh. MdHB1 was found to repress the transcription of MdDFR and MdUFGT indirectly by interacting with MdMYB10, MdbHLH3, and MdTTG1 [37]. Consistent with the results in apple, silencing of RhHB1 in rose led to higher anthocyanin levels in the petals [38].…”

Genome-wide identification and expression analyses of the homeobox transcription factor family during ovule development in seedless and seeded grapes

“…In a recent study, a tomato HD-ZIP I family gene designated LeHB1 was shown to play key roles in carpel development and fruit maturation 15 . MdHB1 , the homologous gene of LeHB1 in apple, has been reported to be involved in the regulation of anthocyanin accumulation 63 . In our study, some VvHB genes ( VvHB62 , VvHB54 and VvHB7 ) belonging to the HD-ZIP I subfamily showed markedly differential expression during ovule development between seedless and seeded grape.…”