Plant materials and growth conditions
Chrysanthemumand transgenic lines were maintained by the Chrysanthemum Institute at Yanbian University, Yanji, China. Chrysanthemums were cultivated in a greenhouse, and the temperature was maintained at 25±2 °C with a relative humidity of 70% for 12 h of photoperiod.
Cloning of ClRAP2.4 and sequence analyses
Total RNA was extracted from young leaves of C. lavandulifolium using RNAiso reagent (TaKaRa, Tokyo, Japan). First-strand cDNA synthesis was performed using 1 μg of total RNA with SuperScript III reverse transcriptase (TIANGEN) according to the manufacturer’s instructions. Based on Chrysanthemum seticuspe expressed sequence tags, a primer pair was designed to amplify the open reading frame (ORF) of ClRAP2.4. The PCR products were subjected to gel electrophoresis and the objective frame was cut and recovered using the AxyPrep™ Biospin Gel Extraction Kit (Axygen, Hangzhou, China), ligated into the pMD19-T easy vector, and cloned into Escherichia coli DH5α, followed by sequencing. The ClRAP2.4 amino acid sequence was aligned with those of other plant homologs using BLAST software (http://www.ncbi.nlm.gov/blast). Phylogenetic trees were constructed using the neighbor-joining method and MEGA software (version 5.0). The internal branching support was estimated using 1000 bootstrap replicates.
Subcellular localization and transcriptional activity of the ClRAP2.4 protein
The ClRAP2.4 ORF sequence was amplified using specific primers containing BamH I and Not I sites. The PCR products and pENTR™1A dual selection vector (Invitrogen) were digested with BamH I and Not I and ligated to generate a set of pENTR™1A-ClRAP2.4 fusions through T4 ligase. The fusion constructs were recombined with the binary vector pMDC43 to form the p35S::GFP-ClRAP2.4 constructs using the LR Clonase™ II enzyme mix (Invitrogen). The p35S::GFP-ClRAP2.4 plasmids were introduced into onion epidermal cells using biolistic bombardment (PDS-1000; Bio-Rad, USA) to enable its transient expression; the pMDC43 plasmid (containing p35S::GFP) served as the control. Onion epidermal cells containing plasmids were incubated on solidified Murashige and Skoog medium for 20 h at 25 °C in the dark, and GFP activity was detected using confocal laser scanning microscopy (LeicaSP2, Germany). Transcriptional activity assays were performed using the Matchmaker GAL4 One-Hybrid System (Clontech, Mountain View, CA, USA). The previous pENTR™1A-ClRAP2.4 plasmid was inserted into pDEST-GBKT7 to obtain GBKT7-ClRAP2.4. The pGBKT7-ClRAP2.4 plasmid was inserted into the yeast strain Y2H Gold (Clontech), following the manufacturer’s protocol, with pCL1 and pGBKT7 plasmids as the positive and negative controls, respectively. These transformed yeast cells were confirmed using PCR and streaked on synthetic dropout (SD) medium lacking histidine and adenine medium (SD/-His-Ade) for two days at 25 °C in the dark.
Plasmid construction and transformation of chrysanthemum
The previous pMDC43-ClRAP2.4 plasmids were transformed into Agrobacterium tumefaciens strain EHA105 by freeze-thaw transformation. Agrobacterium strains harboring pMDC43-ClRAP2.4 were cultured in liquid yeast mannitol (YEB) media containing kanamycin and rifampicin. The genetic transformation method of chrysanthemum was described in our previous publication (Gao et al. 2018). DNA and RNA were isolated from the putative transgenic lines using a kit to identify the transgenic plants. The primer pairs, pMDC43-F/R and ClRAP2.4-RT-F/R, were designed for PCR detection (Table S1). Three transgenic and three WT plants were planted in pots in a greenhouse.
Physiological changes of transgenic chrysanthemum under cold treatment
For the cold tolerance evaluation, the five- to six-leaf of the overexpressed chrysanthemum (OX) and the non-transgenic chrysanthemum (WT) were placed in a culture incubator (MIR-154, Sanyo). The temperature was set to 20 °C, 15 °C, 10 °C, and 5 °C for 2 h. Each treatment included three OX plants and three WT plants. After cold treatment, the second to sixth leaves from the top of each plant were collected. The samples were rapidly frozen in liquid nitrogen and stored at −80 °C for physiological and molecular index measurements. The antioxidant enzyme SOD, POD activity, proline, and MDA content were determined in accordance with previously described methods (Yue et al. 2020). OX and WT leaves were exposed to 8 °C, 4 °C, 0 °C, −4 °C, and −8 °C for 1 h. Each treatment included six leaves and three biological replicates. The relative electrical conductivity (REC) was measured using a conductivity meter and calculated using the following equation: REC = (EC before boiling/EC after boiling) × 100%.
Gene expression characterization using RNA-Seq
Three overexpressed chrysanthemum and three WT plants with five to six leaves were subjected to 10 °C for 2 h. Leaves from each plant were collected, frozen immediately in liquid nitrogen, and stored at -80 °C for RNA isolation. The library was constructed using overexpression plants and WT plants (including three replicates). The library was sequenced using an Illumina Hiseq2000 platform (Biomarker Bio-technology Co. Ltd., Beijing, China). Clean reads were checked, in terms of quality, by Q30>99.999 and GC content. Clean reads were mapped to the reference genome (http://www.amwayabrc.com/) using TopHat 2 software under the criterion of no more than two mismatches in the alignment. Gene annotations were obtained through BLAST searches using the NCBI non-redundant protein (Nr) database and the Swiss-Prot protein database. Gene expression quantification was expressed as fragments per kilobase of transcript per million fragments mapped (FPKM). Differentially expressed genes (DEGs) were identified in terms of log 2 |FC| > 2.0, and false discovery rate (FDR) < 0.05. The fold change (FC) was the ratio of the average FPKM of the two groups.
Quantitative Real-time PCR
To analyze the expression of the cold stress-responsive genes ClCOR, ClAPX, ClPX, ClNCL, ClPLK, ClFAD, and ClMYB, total RNA was extracted from chrysanthemum at 0 °C, 5 °C, and −5 °C for 2 h. cDNA synthesis was performed using the Reverse Transcriptase FastQuant RT kit (TianGen). Each 20 µL qRT-PCR sample included 200 ng cDNA, 100 nM of each primer, and 10 μl SYBR Green PCR master mix. qRT-PCR amplification was performed as described by Yue et al. (Yue et al. 2020). The specific primers for cold-related genes and elongation factor 1α (EF1α) gene primers are shown in Table 1S. Three biological replicates were used for each sample. The data were processed using the 2 -ΔΔCT method.