Abstract
The Tiller Angle Control 1 (TAC1) gene belongs to the IGT family, which mainly controls plant branch angle, thereby affecting plant form. Two members of MdTAC1 are identified in apple; the regulation of apple branch angle by MdTAC1 is still unclear. In this study, a subcellular localization analysis detected MdTAC1a in the nucleus and cell membrane, but MdTAC1b was detected in the cell membrane. Transgenic tobacco by overexpression of MdTAC1a or MdTAC1b showed enlarged leaf angles, the upregulation of several genes, such as GA 2-oxidase (GA2ox), and a sensitive response to light and gravity. According to a qRT-PCR analysis, MdTAC1a and MdTAC1b were strongly expressed in shoot tips and vegetative buds of weeping cultivars but were weakly expressed in columnar cultivars. In the MdTAC1a promoter, there were losses of 2 bp in spur cultivars and 6 bp in weeping cultivar compared with standard and columnar cultivars. An InDel marker specific to the MdTAC1a promoter was developed to distinguish apple cultivars and F1 progeny. We identified a protein, MdSRC2, that interacts with MdTAC1a, whose encoding gene which was highly expressed in trees with large branch angles. Our results indicate that differences in the MdTAC1a promoter are major contributors to branch-angle variation in apple, and the MdTAC1a interacts with MdSRC2 to affect this trait.
Highlights
Apple (Malus x domestica Borkh.) is widely cultivated and ranks third with respect to global fruit production according to FAO statistics in 2019 (https://www.fao.org)
The MdTAC1a was detected in the cell nucleus and cell membrane, whereas the MdTAC1b was localized to the cell membrane (Figure 1)
We found that an InDel marker developed by specific primers based on promoter sequences of TAC1a co-segregated with large branch angles (>65◦), either in analyzed apple cultivars, or in a F1 population
Summary
Apple (Malus x domestica Borkh.) is widely cultivated and ranks third with respect to global fruit production according to FAO statistics in 2019 (https://www.fao.org (accessed on 5 November 2020)). Strigolactones can interact with auxins and cytokinins to reduce the extent of plant branching and play an important role in branch development [10]. There was no difference in the cDNA sequences, but variation in the promoters of two genes were detected among the analyzed ideotypes [20] This preliminary work involved few apple cultivars, and a correlation with branch angle could not be determined. We performed subcellular localization experiments and used full-length MdTAC1a/b cDNA sequences to construct overexpression vectors of MdTAC1a/b driven by a strong promoter, CaMV35S, in genetically transformed tobacco (Nicotiana benthamiana) This approach allowed us to observe changes in the leaf angle of the transgenic lines and verify the gene functions of MdTAC1a/b. Our findings, including the function of MdTAC1a/b genes, lay a foundation for analysis of the molecular mechanism of MdTAC1a regulation of branch angle in apple, thereby contributing to apple genetic improvement
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