
We illustrated the proline synthesis and degradation in plants in schematic diagram ([Fig 7](#pone.0188514.g007){ref-type="fig"}). The expression patterns of these proline-associated enzyme encoding genes were studied in SCZ and YH9 over the treatments of CA and DA. *CsP5CS* and *CsP5CR* were up-regulated at CA2 in SCZ after 14-days cold treatment, and their levels were increased by 1.4-fold and 3.0-fold, respectively. By the contrary, no changes in transcription of *CsP5CS* were observed in YH9 over the treatments of CA and DA. While *CsP5CR* was up-regulated in CA1 in YH9. This suggested that long time induced *CsP5CR* in stage CA2 may be responsible for higher cold tolerance. In both cultivars, the transcription of *CsOAT* was decreased during CA treatments and returned to the normal level after DA treatment. In addition, the transcript level of *CsOAT* in SCZ was similar to that in YH9 ([Fig 7](#pone.0188514.g007){ref-type="fig"}). *CsP5CDH* was down-regulated in SCZ during CA1, whereas was unaffected by the CA treatments in YH9 ([Fig 7](#pone.0188514.g007){ref-type="fig"}). The transcription of *CsProDH* in both cultivars was decreased during CA treatments ([Fig 7](#pone.0188514.g007){ref-type="fig"}).

![Effect of cold treatment on gene expression in proline-metabolism in tea cultivars.\
Significant down-, up-regulation and statistically not up/down-regulation is indicated by green color, orange color, and gray color, respectively. Red arrow and green arrow indicate the flow of metabolites with biosynthetic and degradation, respectively. Gene transcript level was quantified using real-time quantitative RT-PCR approach. GAPDH was used as a control. Data are displayed as the mean of three replicates. Significant differences is based on Duncan's multiple range tests with *P* \< 0.05. SCZ and YH9 represent tea cold resistant and cold susceptible tea varieties, respectively. NA: non-acclimation; CA1: cold acclimation of 7 days at 10/4°C, day/night temperature; CA2: cold acclimation of 7 days at 4/0°C, day/nig