Abstract: 【Objective】 The objectives of this study are to identify citrus CEP genes (CitCEPs) based on citrus whole genome sequences, understand the phylogenetic relationship among CitCEPs as well as their expression specificity in different tissues and response to hormone and abiotic stress, and to lay a foundation for further study on the biological function of CitCEPs.【Method】The citrus CEP genes were identified by BLASTp based on Phytozome database. Gene structure, phylogenetic tree, relative molecular mass, isoelectric point and other physical and chemical properties of CitCEP members were analyzed using GSDS, ProtScale Tool, EXPASY, CLUSTALX, MEGA6.0, plant CARE and Cello. Real-time quantitative PCR (qRT-PCR) was used to detect the expression levels of CitCEPs under different treatments of ‘Citrus junos Ziyang’.【Result】The CitCEPs consists of 11 members, of which CitCEP10 and CitCEP11 contain 1 intron, and the other members have no intron. All CitCEPs contain conserved domain sequences (SPGV/IGH), and belong to hydrophilic protein. The magnitude of the most hydrophilic amino acid is 3.711 and that of the weakest hydrophilic amino acid is -2.778. Subcellular localization prediction showed that 11 CitCEPs were located in different positions of the cell. Among them, CitCEP1, CitCEP2, CitCEP4, CitCEP5 CitCEP6 and CitCEP8 were all located in extracellular. CitCEP3 and CitCEP7 existed in both extracellular and mitochondria. CitCEP9 and CitCEP10 were located in nucleus and plasma membrane, respectively. CitCEP11 was located in both mitochondria and nucleus. The phylogenetic analysis showed that the CitCEP members together with Arabidopsis thaliana CEPs were clustered into 3 groups, which indicated that the CitCEP members had different biological functions. Expression analysis showed that CitCEP2 was mainly expressed in stem, leaf and cotyledon, CitCEP3 was mainly expressed in root and cotyledon, while CitCEP10 and CitCEP11 were mainly expressed in pericarp, and the other members expressed very low or no expression in the above tissues, reflecting the difference in tissue specificity among different members. Under drought conditions, the expression of CitCEP2 was down-regulated, while that of CitCEP3, CitCEP10 and CitCEP11 was gradually increased. The expression of CitCEP2 and CitCEP10 under salt stress was similar to that under drought conditions, respectively, while the expression of CitCEP3 and CitCEP11 increased first and then decreased. The expression of CitCEP2 was significantly inhibited under ethylene (ETH) and abscisic acid (ABA) treatments, but the expression increased firstly and then decreased under methyl jasmonate (MeJA), salicylic acid (SA), indole acetic acid (IAA) and gibberellin (GA3) treatments. CitCEP3 showed a similar trend under the treatments of 6-benzylamino adenine (6-BA), IAA, GA3 and ABA. The expression of CitCEP10 was significantly up-regulated under ETH treatment, and decreased firstly and then increased under 6-BA, SA and ABA treatments, but there was no obvious regularity in the treatment of MeJA, IAA, GA₃. The expression of CitCEP11 showed a downward trend under the treatments of ETH, SA, MeJA, IAA, GA3 and ABA, which was similar to that of CitCEP3 under ETH and SA treatments. Under 6-BA treatment, the expression of CitCEP11 showed a trend of decreasing firstly and then increasing.【Conclusion】A total of 11 CitCEP members were identified from citrus whole genome sequences, and all of them are hydrophilic proteins which contain a conserved SPGV/IGH domain. The subcellular positions were varied among CitCEP members. Under different stress and hormonal treatments, CitCEP2, CitCEP3, CitCEP10 and CitCEP11 exhibited different degrees of response, while the response of the other member was not obvious or unresponsive. It is inferred that CitCEP2, CitCEP3, CitCEP10 and CitCEP11 may play an important role in citrus growth and development as well as stress response.

Key words: citrus, CitCEP genes, gene expression, hormone, stress