Identification and characterisation of a novel FT orthologous gene in London plane with a distinct expression response to environmental stimuli compared to PaFT

Functional characterization of three TERMINAL FLOWER 1-like genes from Platanus acerifolia

KEY MESSAGE

TFL1-like genes of the basal eudicot Platanus acerifolia have conserved roles in maintaining vegetative growth and inhibiting flowering, but may act through distinct regulatory mechanism. Three TERMINAL FLOWER 1 (TFL1)-like genes were isolated and characterized from London plane tree (Platanus acerifolia). All genes have conserved genomic organization and characteristic of the phosphatidylethanolamine-binding protein (PEBP) family. Sequence alignment and phylogenetic analysis indicated that two genes belong to the TFL1 clade, designated as PlacTFL1a and PlacTFL1b, while another one was grouped in the BFT clade, named as PlacBFT. qRT-PCR analysis showed that all three genes primarily expressed in vegetative phase, but the expression of PlacTFL1a was much higher and wider than that of PlacTFL1b, with the latter only detected at relatively low expression levels in apical and lateral buds in April. PlacBFT was mainly expressed in young stems of adult trees followed by juvenile tissues. Ectopic expression of any TFL1-like gene in Arabidopsis showed phenotypes of delayed or repressed flowering. Furthermore, overexpression of PlacTFL1a gene in petunia also resulted in extremely delayed flowering. In non-flowering 35:PlacTFL1a transgenic petunia plants, the FT-like gene (PhFT) gene was significantly upregulated and AP1 homologues PFG, FBP26 and FBP29 were significantly down-regulated in leaves. Yeast two-hybrid analysis indicated that only weak interactions were detected between PlacTFL1a and PlacFDL, and PlacTFL1a showed no interaction with PhFDL1/2. These results indicated that the TFL1-like genes of Platanus have conserved roles in repressing flowering, but probably via a distinct regulatory mechanism.

Genome-Wide Analysis of the BBX Genes in Platanus × acerifolia and Their Relationship with Flowering and/or Dormancy

The B-BOX (BBX) gene family is widely distributed in animals and plants and is involved in the regulation of their growth and development. In plants, BBX genes play important roles in hormone signaling, biotic and abiotic stress, light-regulated photomorphogenesis, flowering, shade response, and pigment accumulation. However, there has been no systematic analysis of the BBX family in Platanus × acerifolia. In this study, we identified 39 BBX genes from the P. × acerifolia genome, and used TBtools, MEGA, MEME, NCBI CCD, PLANTCARE and other tools for gene collinearity analysis, phylogenetic analysis, gene structure, conserved domain analysis, and promoter cis-element analysis, and used the qRT-PCR and transcriptome data for analyzing expression pattern of the PaBBX genes. Collinearity analysis indicated segmental duplication was the main driver of the BBX family in P. × acerifolia, and phylogenetic analysis showed that the PaBBX family was divided into five subfamilies: I, II, III, IV and V. Gene structure analysis showed that some PaBBX genes contained super-long introns that may regulate their own expression. Moreover, the promoter of PaBBX genes contained a significant number of cis-acting elements that are associated with plant growth and development, as well as hormone and stress responses. The qRT-PCR results and transcriptome data indicated that certain PaBBX genes exhibited tissue-specific and stage-specific expression patterns, suggesting that these genes may have distinct regulatory roles in P. × acerifolia growth and development. In addition, some PaBBX genes were regularly expressed during the annual growth of P. × acerifolia, corresponding to different stages of flower transition, dormancy, and bud break, indicating that these genes may be involved in the regulation of flowering and/or dormancy of P. × acerifolia. This article provided new ideas for the study of dormancy regulation and annual growth patterns in perennial deciduous plants.

Regulation of alternative splicing of PaFT and PaFDL1, the FT and FD homologs in Platanus acerifolia

Alternative splicing (AS) selects different alternative splice sites and produces a variety of transcripts with different exon/intron combinations, which may result in multiple protein isoforms. The splicing signals include cis-elements and RNA structures; however, the mechanisms of AS regulation in plants have yet to be elucidated. Previous studies have shown that in Platanus acerifolia, the FLOWERING LOCUS T (FT) homolog PaFT has a unique and complex AS pattern, in which most of the splice forms of PaFT involve the first and/or second intron, and the FD homolog PaFDL1 produces two transcripts via AS, whereas the other FT homolog PaFTL is not regulated by AS. In this study, the regulatory mechanism of the AS of PaFT was demonstrated to be conserved in different plant species. To define the distribution of the AS regulatory signals, the intron-swap, site-directed mutagenesis of alternative splice sites, and deletion experiment were performed. For the PaFT gene, all the signals that regulate the AS of the first intron were located within this intron, while the usage of the first alternative splice site in the second intron was determined by the first intron. Meanwhile, the AS of PaFDL1 might be co-regulated by exons and the first intron. Additionally, the first alternative splice site and adjacent region in PaFT intron 1 might contain cis-elements and/or RNA structures that affect the use of the other sites. This study had provided a deeper insight into the distribution of AS signals in plants, namely the AS signals of different splice sites might exist in the intron where the sites were present, and might also be distributed in exons or other introns.

Two FD homologs from London plane (Platanus acerifolia) are associated with floral initiation and flower morphology

The flowering-time gene FD encodes a bZIP transcription factor that interacts with FLOWERING LOCUS T (FT) to induce flowering in Arabidopsis. Previous research has identified two FT homologs of Platanus acerifolia, PaFT and PaFTL, which each have different expression patterns and are involved in diverse developmental processes. However, it is not known whether such FT/FD complexes participate in the flowering processes in P. acerifolia. Therefore, we isolated two closely related FD homologs, PaFDL1 and PaFDL2, and investigated their functions through the analysis of expression profiles, transgenic phenotypes, their interactions with different FT proteins, and potential cis-regulatory elements in their promoters. The PaFDL genes were found to display their maximal expression levels during the stage of floral transition, and subsequent expression patterns were also seen to be related to inflorescence developmental stage. In addition, both PaFDL1 and PaFDL2 were found to be subject to post-transcriptional alternative splicing, each gene producing two transcript forms. Transgenic tobacco overexpressing each of the four resulting transcript types displayed accelerated floral initiation and produced abnormal flowers. The results suggested that the complete PaFDL proteins may interact with different PaFT/PaFTL proteins in order to fulfill both conservative and diverse functions in floral initiation and floral development.

Integrated transcriptome and proteome analysis provides insight into chilling-induced dormancy breaking in Chimonanthus praecox

Chilling has a critical role in the growth and development of perennial plants. The chilling requirement (CR) for dormancy breaking largely depends on the species. However, global warming is expected to negatively affect chilling accumulation and dormancy release in a wide range of perennial plants. Here, we used Chimonanthus praecox as a model to investigate the CR for dormancy breaking under natural and artificial conditions. We determined the minimum CR (570 chill units, CU) needed for chilling-induced dormancy breaking and analyzed the transcriptomes and proteomes of flowering and non-flowering flower buds (FBs, anther and ovary differentiation completed) with different CRs. The concentrations of ABA and GA3 in the FBs were also determined using HPLC. The results indicate that chilling induced an upregulation of ABA levels and significant downregulation of SHORT VEGETATIVE PHASE (SVP) and FLOWERING LOCUS T (FT) homologs at the transcript level in FBs when the accumulated CR reached 570 CU (IB570) compared to FBs in November (FB.Nov, CK) and nF16 (non-flowering FBs after treatment at 16 °C for -300 CU), which suggested that dormancy breaking of FBs could be regulated by the ABA-mediated SVP-FT module. Overexpression in Arabidopsis was used to confirm the function of candidate genes, and early flowering was induced in 35S::CpFT1 transgenic lines. Our data provide insight into the minimum CR (570 CU) needed for chilling-induced dormancy breaking and its underlying regulatory mechanism in C. praecox, which provides a new tool for the artificial regulation of flowering time and a rich gene resource for controlling chilling-induced blooming.