Multiple transcriptome comparisons reveal the essential roles of flowering FLOWERING LOCUS T in floral initiation and SOC1 and SVP in floral activation in blueberry

Authors: SONG, GUO-QING - Michigan State University; CARTER, BENJAMIN - Michigan State University; Zhong, Gan-Yuan

Submitted to: Frontiers in Genetics
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/27/2023
Publication Date: 4/5/2023
Citation: Song, G., Carter, B., Zhong, G. 2023. Multiple transcriptome comparisons reveal the essential roles of FLOWERING LOCUS T in floral initiation and SOC1 and SVP in floral activation in blueberry. Frontiers in Genetics. https://doi.org/10.3389/fgene.2023.1105519.
DOI: https://doi.org/10.3389/fgene.2023.1105519

Interpretive Summary: Light and temperature are two key environmental factors that affect flower bud initiation, flowering and fruiting in deciduous fruit trees. Climate changes have caused the onset of the growing season of many trees to shift earlier. Reduced winter chill is often associated with insufficient chilling hours; warm weather sometimes leads to fruit/nut trees flowering out-of-season; and increased temperature fluctuations during plant bloom turns seasonal frost into a greater danger, often causing freezing injuries to flowers and young fruits. Flowering is generally a prerequisite for fruiting and many studies have been directed to understand flowering pathways of woody plants for developing genetic solutions to manipulation of flowering times to alleviate negative impact of climate changes. Through transcriptomic analysis, we investigated how over-expression of FLOWERING LOCUS T (FT), a major integrator of signaling that stimulates the transition of meristem tissue into flower buds, and chilling treatment would affect the expression of flowering genes and phenotypes in blueberry. We concluded that that FT is a major regulator in floral initiation and SOC1 (SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1) is a key regulator of floral activation. We further proposed that FT to SOC1 transcription ratios affect floral formation and activation in blueberry.

Technical Abstract: Flower buds of northern highbush blueberry cultivar 'Aurora' require approximately 1,000 chilling hours to bloom. Overexpression of a blueberry FLOWERING LOCUS T (VcFT) enabled precocious flowering of transgenic ‘Aurora’, but mainly in non-terminated apical shoots during flower bud formation. Most of the mature flower buds could not break until they received enough chilling hours. In this study, we identified two groups of differentially expressed genes (DEGs) in flower buds caused by VcFT overexpression (VcFT-OX) and full chilling, respectively. We compared the two groups of DEGs with a focus on flowering pathway genes. We found: 1) in nonchilled flower buds, VcFT-OX drove a high level of VcFT expression and repressed expression of a major MADS-box gene, blueberry SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (VcSOC1), which resulted an increased VcFT/VcSOC1 expression ratio; 2) In fully chilled flower buds, chilling upregulated VcSOC1 expression in nontransgenic ‘Aurora’ and repressed VcFT expression in VcFT-OX ‘Aurora’, and each resulted in a decreased ratio of VcFT to VcSOC1. In addition, expression of a blueberry SHORT VEGETATIVE PHASE (SVP) was upregulated in chilled flower buds of both transgenic and nontransgenic’ Aurora’. Based on these data and additional transcriptomic analysis of VcFT and VcSOC1 in other genotypes and tissues, we found evidence to support that VcFT expression plays a significant role in promoting floral initiation and that VcSOC1 expression is a key floral activator. We propose that the relative transcription ratios of VcFT to VcSOC1 determine flower bud formation and bud breaking. Generally, an increased VcFT/VcSOC1 ratio or increased VcSOC1 in leaf promotes precocious flowering and flower bud formation, and a decreased VcFT/VcSOC1 ratio with increased VcSOC1 in fully chilled flower buds contributes to flower bud breaking.