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

Abstract

The flowering mechanisms, especially chilling requirement-regulated flowering, in deciduous woody crops remain to be elucidated. 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” mainly in non-terminated apical buds during flower bud formation, meanwhile, most of the mature flower buds could not break until they received enough chilling hours. In this study, we highlighted two groups of differentially expressed genes (DEGs) in flower buds caused by VcFT overexpression (VcFT-OX) and full chilling. We compared the two groups of DEGs with a focus on flowering pathway genes. We found: 1) In non-chilled flower buds, VcFT-OX drove a high VcFT expression and repressed expression of a major MADS-box gene, blueberry SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (VcSOC1) resulting an increased VcFT/VcSOC1 expression ratio; 2) In fully chilled flower buds that are ready to break, the chilling upregulated VcSOC1 expression in non-transgenic “Aurora” and repressed VcFT expression in VcFT-OX “Aurora”, and each resulted in a decreased ratio of VcFT to VcSOC1; additionally, expression of a blueberry SHORT VEGETATIVE PHASE (VcSVP) was upregulated in chilled flower buds of both transgenic and non-transgenic’ “Aurora”. Together with additional analysis of VcFT and VcSOC1 in the transcriptome data of other genotypes and tissues, we provide evidence to support that VcFT expression plays a significant role in promoting floral initiation and that VcSOC1 expression is a key floral activator. We thus propose a new hypothesis on blueberry flowering mechanism, of which the ratios of VcFT-to-VcSOC1 at transcript levels in the flowering pathways 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.

Regulatory frameworks involved in the floral induction, formation and developmental programming of woody horticultural plants: a case study on blueberries

“…Notably, instances like VcFT-CX provide insight into its extensive impact, significantly elevating VcFT expression in both leaves and flower buds; intriguingly, this upregulation was notably absent in roots. Simultaneously, thousands of differentially expressed genes (DEGs) attributed to VcFT expression varied across different tissues and developmental stages, even within the same tissue ( Walworth et al., 2016 ; Song et al., 2019 ; Song et al., 2023 ).…”

“…The highbush blueberry (2n = 4x = 48), a prominent cultivated member of the Vaccinium fruit crop family, has a rather substantial chilling requirement, typically surpassing 800 chilling units, that must be met to initiate dormancy release during spring ( Song et al., 2011 ; Edger et al., 2022 ). Over the course of past decades, extensive investigations have been carried out to reveal the flowering mechanism ( Song et al., 2023 ). A summary of these blueberry studies can serve as an illustrative example, providing insights into the network of factors that underlie flowering mechanisms in woody plants.…”

“…In young leaves, it upregulated the expressions of VcAP1/VcFUL , blueberry SEPALLATA ( VcSEP ), VcLFY , VcSOC1 , and VcTFL1 , with no significant changes in VcSVP and VcFD ( Figure 3A ) ( Walworth et al., 2016 ). In mature leaves, VcFT-CX enhanced VcAP1/VcFUL and VcSEP expressions, while repressing VcSOC1 and VcSVP , and it had minimal impact on VcLFY , VcFD , and VcTFL1 ( Figure 3B ) ( Song et al., 2023 ). In nonchilled flower buds, VcFT-CX upregulated VcLFY expression, downregulated VcSEP , VcSOC1 , VcSVP , VcFD , and VcTFL1 , while VcAP1/VcFUL expression remained relatively unaffected ( Figure 3C ) ( Song et al., 2023 ).…”

Transcriptome-Based Identification of Candidate Flowering-Associated Genes of Blueberry in a Plant Factory with Artificial Lighting (PFAL) under Short-Day-Length Conditions

“…The buds then undergo a shallow dormancy, begin to swell rapidly, and bloom once the chilling requirement has been satisfied. To reveal the genetic and physiological mechanisms of blueberry flowering events, numerous studies have been conducted in recent decades, and a large number of flowering-related genes have been successfully identified, such as VcFT [ 26 , 27 ], SOC1 -like MADS-box [ 28 ], VcRR2 [ 29 ], SPL [ 13 ], and VcSVP [ 30 ]. FT is a major integrator of signaling that stimulates the transition of meristem tissue into flower buds [ 31 ], is expressed mainly in leaves, and the resultant FT protein travels through the phloem to the shoot apical meristem, where it induces flower determination [ 32 ].…”

“…Transcriptome analysis is a powerful tool to identify differentially expressed genes (DEGs) and regulatory networks involved in complex biological processes. RNA-sequencing (RNA-Seq) technology has been successfully applied in blueberry to identify candidate genes or transcripts involving multiple specific traits, such as flowering [ 30 , 33 ], adventitious root formation [ 34 ], anthocyanin accumulation [ 35 , 36 ], flavonoid biosynthetic pathway [ 37 ], and fruit development [ 38 , 39 ]. Therefore, to better understand the process of flower bud development, the effects of SD length and chilling on flower bud initiation of blueberry in a PFAL system were surveyed in this study, and transcriptomic analysis was carried out to identify the key candidate flowering genes.…”