## 4 Materials and Methods
### Gene Expression Analysis  
Tissues sampled for gene expression analysis include flower buds of four different developmental stages (DP, OF, LB, and WP), three different rounds (innermost, middle, and outer) of petals in OF stages, the upper and lower halves of middle petals in DP, OF, and WP stages. Total RNA for the expression analysis was extracted using RNAprep pure kit (Tiangen, Beijing, China) according to the manufacturer's instructions. Exactly 3 \(\mu\)g of RQ1 RNase-Free DNase (Promega, Madison, WI, USA) pre-treated total RNA was reverse transcribed according to the instructions of the Primescript RT reagent kit (Takara, Tokyo, Japan). qRT-PCR was performed to determine the expression pattern of one nectary development related gene_CpCRC_, four YABBY family genes (_CpFIL_, _CpYABBY2_, _CpYABBY5-1_, and _CpYABBY5-2_) and 11 floral scent genes, such as _CpSAMT_, _CpDMAPP_, _CpIPP_, _CpGPPS1_, _CpGPPS2_, _CpGPP_, _CpLIS_, _CpMYR1_, _CpFPPS_, _CpTER3_, and _CpTER5_.  
The primers for qRT-PCR are listed in Table 1. Reactions were performed with the Sso Fast Eva Green Supermix (Bio-Rad, Hercules, CA, USA) and analyzed using Bio-Rad CFX96 (Bio-Rad CFX Manager Software Version 1.6). Thermocycler conditions were 95 \({}^{\circ}\)C for 30 s, followed by 40 cycles of 95 \({}^{\circ}\)C for 5 s and 60 \({}^{\circ}\)C for 5 s. qRT-PCR products were amplified using 5 \(\upmu\)L 2\(\times\) Sso Fast Eva Green Supermix, 0.5 \(\upmu\)L RT reaction mixture, 0.5 \(\upmu\)L of forward and reverse primer (10 \(\upmu\)mol/\(\upmu\)L) each, and RNase Free dH\({}_{2}\)O to a final volume of 10 \(\upmu\)L. Relative amounts of transcripts were calculated using the comparative CT method (2 -\({}^{\Delta\Delta\text{C}}\)), and the values were normalized. The house-keeping gene _CpTublin_ of _C. praecox_ was used as internal control. Data are shown as mean values \(\pm\) standard deviation (SD) from three replicates for each sample. Significant difference was carried out by \(t\)-test (\(p<0.05\)).  
The expression patterns of the five YABBY genes were estimated by FPKM values and were visualized using MultiExperiment Viewer (Broad Institute of MIT and Harvard University, Boston, MA, USA [38]).  
Supplementary Materials: Supplementary materials can be found at [[http://www.mdpi.com/1422-0067/19/10/3278/s1](http://www.mdpi.com/1422-0067/19/10/3278/s1)]([http://www.mdpi.com/1422-0067/19/10/3278/s1](http://www.mdpi.com/1422-0067/19/10/3278/s1)).  
**Author Contributions:** Methodology, Z.L. and Y.J.; Software, Z.L.; Validation, Z.L. and J.L.; Formal Analysis, Y.J. and Z.L.; Resources, D.L.; Data Curation, J.M.; Writing-Original Draft Preparation, Y.J. and Z.L.; Writing-Review & Editing, Z.L.; Visualization, Z.L. and Y.J.; Supervision, Z.L.; Project Administration, S.S. and M.L.; Funding Acquisition, Z.L., S.S. and J.M.  
**Funding:** This work was supported by grants from Fundamental Research Funds for the Central Universities (XDJK2017B031), the National Natural Science Foundation of China (No. 31500573, 31370698), Chongqing science and technology committee projects (estc2016 shmszx80112), Chongqing education committee projects (yjg20163042) and National undergraduate innovation and entrepreneurship training program (201410635065).  
**Conflicts of Interest:** The authors declare no conflict of interest.