Genome-Wide Analysis of the Cinnamoyl-CoA Reductase (CCR) Gene Family and Its Involvement in Lignin Biosynthesis and Stress Responses in Six Tea Plant Cultivars

Cinnamoyl-CoA reductase (CCR) is the first rate-limiting enzyme in the lignin biosynthetic pathway in higher plants. It catalyzes the conversion of cinnamoyl-CoA into the corresponding cinnamaldehydes. Tea plant (Camellia sinensis) is a perennial woody species. Systematic identification and functional characterization of the CCR gene family in tea plants is still limited. In this study, 202 CCR genes were identified from six tea plant cultivars, and a significant expansion of the CCR gene family was observed during the domestication process from wild to cultivated tea plants. A total of 50 CsCCR genes were identified in the tea cultivar ‘Shuchazao’, and they were distributed across 13 chromosomes. Multiple sequence alignment revealed that the key catalytic motifs NWYCYGK and H-X-X-K were fully conserved in CsCCR1, CsCCR2, and CsCCR3. Phylogenetic analysis showed that CsCCR1/2/3 clustered with AtCCR1/2 and PtrCCR2, which were known to be involved in lignin biosynthesis. Transcriptome data analysis showed that CsCCR3 exhibited significantly higher transcript abundance in stems than in young, mature, and old leaves. CsCCRL9, CsCCRL33, CsCCRL34, and CsCCRL36 also showed relatively high expression levels in stem. RT-qPCR further confirmed the high expression of CsCCR3 and CsCCRL33 in stems. Furthermore, comparison of CCR members derived from tandem and segmental duplication in the tea cultivar ‘Shuchazao’ showed clear differences in Ka/Ks ratios, expression correlations, and the distribution of stress-responsive cis-acting elements. This study provides new insights into the expansion and duplication-related functional divergence of the CCR gene family in tea plant and identifies key candidate genes potentially involved in lignin biosynthesis and stress responses.