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MADS-box genes similar to Arabidopsis thaliana SHORT VEGETATIVE PHASE (SVP) have been implicated in regulation of flowering in annual species and winter dormancy in perennial species. However, the underlying regulatory mechanisms remain to be identified. In this study, the role of kiwifruit SVP2 was explored using ectopic transgenic expression in kiwifruit species with different chilling requirements and the model species tobacco, followed by transcriptomic analysis of transgenic kiwifruit plants. Ectopic expression of SVP2 affected the duration of dormancy in a high-chill kiwifruit Actinidia deliciosa. This effect could be overcome by sufficient winter chilling. SVP2 had a minimal effect on the duration of dormancy in a low-chill kiwifruit A. eriantha. Expression in a tobacco cultivar with photoperiodic regulation of flowering resulted in retarded vegetative growth but no impact on flowering. Transcriptomic analyses of the kiwifruit SVP2 transgenic and control lines identified 92 significantly differentially expressed genes potentially involved in SVP2-mediated growth repression during dormancy, suggesting a role complementary to abscisic acid (ABA). This study has demonstrated that kiwifruit SVP2 has an integrative role in suppression of meristem activity to prevent precocious budbreak before the fulfilment of winter chilling requirements. [ABSTRACT FROM AUTHOR]

Highlights • AcSVP2 regulates the timing of budbreak in a low-chill kiwifruit A. chinensis. • Histone modification plays a role in regulation of AcSVP2 expression. • Candidate transcription factors that may regulate AcSVP2 expression include homologs of SOC1. • Aspects of SVP and SOC1 interactions are conserved across distant taxa but may have no role in regulation of flowering time. Abstract The SHORT VEGETATIVE PHASE (SVP)- like and DORMANCY ASSOCIATED MADS-BOX (DAM) genes have been shown to regulate winter dormancy in woody perennials. In kiwifruit, AcSVP2 affects the duration of dormancy in cultivars that require high chill for dormancy release. In this study, we used a low-chill kiwifruit Actinidia chinensis 'Hort16A' to further study the function and regulation of AcSVP2. Overexpression of AcSVP2 in transgenic A. chinensis delayed budbreak in spring. A reduction in the active trimethylation histone marks of the histone H3K4 and acetylation of histone H3 contributed to the reduction of AcSVP2 expression towards dormancy release, while the inactive histone marks of trimethylation of the histone H3K27 and H3K9 in AcSVP2 locus did not show significant enrichment at the end of winter dormancy. Analysis of expression in shoot buds showed that AcSVP2 transcript was elevated in dormant buds during winter months and declined prior to budbreak, which was coordinated with expression of some of kiwifruit SUPPRESSOR OF OVEREXPRESSION OF CONSTANS1 (SOC1) -like genes. Screening of 101 transcription factors in an assay with a 2.3 kb promoter region of AcSVP2 found that kiwifruit SOC1 -like genes are able to activate the AcSVP2 promoter. We further identified additional transcription factors associated with drought/osmotic stress and dormancy which may regulate AcSVP2 expression. [ABSTRACT FROM AUTHOR]

Kiwifruit SHORT VEGETATIVE PHASE (AcSVP) genes have been implicated in regulation of dormancy and flowering. Previously, we have shown that AcSVP2 and AcSVP3 have different effects on dormancy and flowering in transgenic kiwifruit and model plants. The role of two homologous genes, AcSVP1 and AcSVP4, remained unclear. Here, AcSVP1 and AcSVP4 were functionally characterized by ectopic expression in Actinidia species which show different chilling requirements for budbreak, as well as the model plant tobacco. Overexpression of AcSVP1 and AcSVP4 delayed spring budbreak in a high-chill A. chinensis var. deliciosa , but did not affect budbreak and flowering in a low-chill A. eriantha . Overexpression of AcSVP1 delayed flowering and seed germination in tobacco, while overexpression of AcSVP4 had only minor effects. Analysis of cis -regulatory motifs in the promoter regions of AcSVP1 and AcSVP4 and promoter transactivation assays revealed that AcSVP1 and AcSVP4 may be controlled by similar transcription factors, suggesting a conserved regulatory mechanism and a synergistic role for AcSVP1 and AcSVP4 in preventing premature kiwifruit budbreak during winter. [ABSTRACT FROM AUTHOR]