An invaluable transgenic blueberry for studying chilling-induced flowering in woody plants

Guo-Qing Song,Aaron Walworth

doi:10.1186/s12870-018-1494-z

Abstract

BackgroundMany deciduous woody crops require a minimum level of chilling to break dormancy and allow the seasonal growth of vegetative and floral buds. In this study, we report the discovery of an invaluable transgenic event of the blueberry cultivar ‘Legacy’ (hereafter, Mu-Legacy) for studying chilling-induced flowering in woody plants. Mu-legacy and its progeny provide a unique material to study the unknown mechanism of chilling-mediated flowering in woody plants.ResultsUnlike nontransgenic ‘Legacy’ and plants of 48 other transgenic events, Mu-Legacy plants were able to flower under nonchilling conditions and had early flower bud formation, reduced plant size, and reduced chilling requirement for normal flowering. These characteristics were heritable and also observed in self-pollinated, transgenic T1 progenies of Mu-Legacy. A 47-Kbp genomic sequence surrounding the transgene insertion position was identified. RNA-sequencing data showed increased expression of a RESPONSE REGULATOR 2-like gene (VcRR2), located adjacent to the insertion position in Mu-Legacy and likely driven by the CaMV 35S promoter of the transgene. The Mu-Legacy showed 209 differentially expressed genes (DEGs) in nonchilled flower buds (compared to nontransgenic ‘Legacy’), of which only four DEGs were in the flowering pathway. This suggests altered expression of these few genes, VcRR2 and four flowering DEGs, is sufficient to significantly change flowering behavior in Mu-Legacy.ConclusionsThe significance of VcRR2 in Mu-Legacy suggests that the VcRR2-involved cytokinin pathway likely contributes to the major differences in chilling-mediated flowering between woody and herbaceous plants. More importantly, Mu-Legacy shows increased yield potential, a decreased chilling requirement, and better winter hardiness than many low-chilling cultivars growing in southern warm winter conditions.

Highlights

Many deciduous woody crops require a minimum level of chilling to break dormancy and allow the seasonal growth of vegetative and floral buds
The Mu-Legacy had an obviously shortened juvenility phase, and could flower without the typical dormancy period and subsequent release following chilling. These results suggest that the overexpressed VcDDF1 is probably not the only factor causing the changes in Mu-Legacy plants
Each data point is an average of six plants plus standard deviation bars. g, In addition to the flowering pattern observed under a short-day photoperiod (a), flowering was shown on the new shoots of Mu-Legacy under a longday photoperiod (16 h). h, A shorter Mu-legacy plant is showing more fruit production than a nontransgenic ‘Legacy’ plant

Summary

Introduction

Many deciduous woody crops require a minimum level of chilling to break dormancy and allow the seasonal growth of vegetative and floral buds. Deciduous fruit-bearing crops that grow in temperate climates come mainly from nine families, including Rosaceae (apples, pears, quinces, almonds, apricots, plums, cherries, peaches, raspberries, blackberries, loquats, and strawberries) [1], Fagaceae (chestnuts), Betulaceae (filberts), Juglandaceae (pecans and walnuts), Ebenaceae (persimmons), Moraceae (figs and mulberries), Vitaceae (grapes), Ericaceae (blueberries and cranberries), and Grossulariaceae (currants and gooseberries). These crops often require a certain period of cold exposure (chilling) to stimulate dormancy release and induce floral buds to blossom in their life cycles.

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