Genome-Wide Identification of Cysteine-Rich Polycomb-like Protein (CPP) Gene Family and Their Expression Profile Analysis in Stem Lettuce (Lactuca sativa)

Cysteine-rich polycomb-like protein (CPP) transcription factors (TFs) play critical roles in the process of plant growth and development, as well as stress responses. To date, no reports about CPP TFs have been published for lettuce (Lactuca sativa). In this study, six CPP TFs (LsCPP1-LsCPP6) were identified in lettuce. Phylogenetic analysis showed that LsCPP TFs were classified into two clades (Clade I and Clade II). Six LsCPP genes were distributed across four chromosomes. Cis-elements, which are involved in environmental stress, hormone response, and development processes, were identified in the promoters of LsCPP genes. LsCPP genes were induced by different tissues and the stem enlargement processes of stem lettuce. Plant hormones (SA, ABA) and abiotic stress (salt, drought) induced the expression of LsCPP genes. LsCPP4 was significantly induced after drought stress for 12 h. Notably, the expression level of LsCPP4 increased more than 10 times (12 h) and 150 times (24 h) after salt stress. ABA and SA significantly induced the expression profile of LsCPP6. This study not only provides the basis for future functional research of LsCPP genes, particularly their roles in lettuce stress resistance, but also provides a foundation for molecular breeding to enhance the agricultural traits in lettuce.