MdHB1 down-regulation activates anthocyanin biosynthesis in the white-fleshed apple cultivar 'Granny Smith'.
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'.
College of Horticulture, Northwest A&F University, Yangling, Shaanxi 712100, China.
Yonghua
Y
Jiang
College of Horticulture, Northwest A&F University, Yangling, Shaanxi 712100, China.
Cuihua
C
Liu
College of Horticulture, Northwest A&F University, Yangling, Shaanxi 712100, China.
Dan
D
Yan
College of Horticulture, Northwest A&F University, Yangling, Shaanxi 712100, China.
Xiaohong
X
Wen
College of Horticulture, Northwest A&F University, Yangling, Shaanxi 712100, China.
Yanli
Y
Liu
College of Horticulture, Northwest A&F University, Yangling, Shaanxi 712100, China.
Haojie
H
Wang
College of Horticulture, Northwest A&F University, Yangling, Shaanxi 712100, China.
Jieyu
J
Dai
College of Horticulture, Northwest A&F University, Yangling, Shaanxi 712100, China.
Yujie
Y
Zhang
College of Horticulture, Northwest A&F University, Yangling, Shaanxi 712100, China.
Yanfei
Y
Liu
College of Horticulture, Northwest A&F University, Yangling, Shaanxi 712100, China.
Bin
B
Zhou
College of Horticulture, Northwest A&F University, Yangling, Shaanxi 712100, China.
Xiaolin
X
Ren
D000595:Amino Acid Sequence; D000872:Anthocyanins / Q000096:biosynthesis*; D015536:Down-Regulation; D005638:Fruit / Q000235:genetics / Q000378:metabolism; D027845:Malus / Q000235:genetics* / Q000378:metabolism; D010802:Phylogeny; D010940:Plant Proteins / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism; D014157:Transcription Factors / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism
D000872:Anthocyanins; D010940:Plant Proteins; D014157:Transcription Factors
Anthocyanin biosynthesis; MdHB1; MdMYB10; homeobox; red-fleshed apple; subcellular localization; virus-induced gene silencing.; ‘Granny Smith’