VcRR2 regulates chilling-mediated flowering through expression of hormone genes in a trangenic blue berry mutant

Authors: LIN, TIANYI - Michigan State University; WALWORTH, AARON - Michigan State University; ZONG, XIAOJUAN - Michigan State University; DANIAL, GHARBIA - Michigan State University; TOMASZEWSKI, ELISE - Michigan State University; CALLOW, PETE - Michigan State University; HAN, XIUMEI - National Research Council - Canada; ZAHARIA, L - National Research Council - Canada; EDGER, PATRICK - Michigan State University; Zhong, Gan-Yuan; SONG, GUO-QING - Michigan State University

Submitted to: Horticulture Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/31/2019
Publication Date: 8/22/2019
Citation: Lin, T., Walworth, A., Zong, X., Danial, G., Tomaszewski, E., Callow, P., Han, X., Zaharia, L., Edger, P., Zhong, G., Song, G. 2019. VcRR2 regulates chilling-mediated flowering through expression of hormone genes in a trangenic blue berry mutant. Horticulture Research. (2019) 6:96. https://doi.org/10.1038/s41438-019-0180-0.
DOI: https://doi.org/10.1038/s41438-019-0180-0

Interpretive Summary: Sufficient chilling is required for dormancy release and flowering through unknown molecular mechanisms in woody plants. We found a valuable transgenic blueberry mutant, named as 'Mu-legacy', which shows dwarfing, promoted flower bud formation, and flowering under nonchilling conditions. A gene, named as response regulator 2-like gene (VcRR2), in the mutant was found to express much higher than that in the non-mutant control 'Legacy'. The VcRR2 is a cytokinin gene. We conducted gene expression comparisons in leaf, chilled and nonchilled flowering bud, and late pink bud, and analyzed a total of 41 metabolites of six groups of hormones in leaf tissues of both 'Mu-legacy' and 'Legacy' plants. We found that the increased VcRR2 expression in the mutant promotes the expression of many other genes. Among them was the ENT-COPALYL DIPHOSPHATE SYNTHETASE 1 (VcGA1) gene responsible for gibberrellins synthesis. Most likely, the over expression of VcRR2 and VcGA1, which function in cytokinin responses and gibberellins synthesis, respectively, caused a reduction in plant height and an enhancement of flower bud formation of the 'Mu-legacy' plants. This work provides new insight into the mechanisms of chilling-mediated flowering in woody plants.

Technical Abstract: Sufficient chilling underpins dormancy release and flowering through unknown molecular mechanisms in woody plants. Mu-legacy is a valuable blueberry mutant, in which a transgene insertion caused an increased expression of a response regulator 2-like gene (VcRR2). Mu-legacy plants, compared to nontransgenic 'Legacy' plants, show dwarfing, promoted flower bud formation, and can flower under nonchilling conditions. We conducted transcriptomic comparisons in leaf, chilled and nonchilled flowering bud, and late pink bud, and analyzed a total of 41 metabolites of six groups of hormones in leaf tissues of both Mu-legacy and 'Legacy' plants. We find that the increased VcRR2 expression promotes the expression of an orthologue of the Arabidopsis thaliana ENT-COPALYL DIPHOSPHATE SYNTHETASE 1 (VcGA1), which induces new homeostasis of hormones in Mu-legacy leaves. Consequently, the VcRR2 and the VcGA1, which function in cytokinin responses and gibberellins synthesis, respectively, caused a reduction in plant height and an enhancement of flower bud formation of the Mu-legacy plants. In nonchilled flower buds, 29 differentially expressed transcripts of 17 genes of 5 groups of hormones were identified in transcriptome comparison between Mu-legacy and 'Legacy' plants, of which 22 were chilling responsive. The increased VcRR2 was collectively responsible for promoting flower bud formation at least in part through endogenous gibberellins in leaves and for stimulating flowering under nonchilling conditions through the expression of 5 groups of hormone genes. The overall results highlight the importance of the expression of VcRR2 and hormones in blueberry plant flowering and provide insight into the mechanisms of chilling-mediated flowering in woody plants.