Cloning, molecular characterization and expression analysis of a SOMATIC EMBRYOGENESIS RECEPTOR-LIKE KINASE gene (CitSERK1-like) in Valencia sweet orange

Somatic embryogenesis receptor-like kinase (SERK) belonging to the receptor-like kinases (RLKs) has been shown to be implicated in somatic embryogenesis (SE). In this study, a somatic embryogenesis receptor-like gene CitSERK1-like was cloned and characterized from Citrus sinensis cv. 'Valencia', a genotype with high somatic embryogenesis capacity for over 26 years. Fifteen consecutive amino acids in putative leucine zipper domain of CitSERK1-like gene were different from the reported CitSERK1 gene. Homology search and sequence analysis demonstrated that the deduced CitSERK1-like protein shared a high degree of identity with SERKs from other species in sequence and structure. Real-time PCR analysis revealed that the transcript of CitSERK1-like was enhanced during the induction of SE. At subsequent embryo-transition phase, a moderate level of expression was detected in heart-torpedo and cotyledon embryos, while low expression was detected in globular embryo. Among the different tissues, the expression of CitSERK1-like was highest in young leaves. Further analysis of its spatial expression by in situ hybridization revealed that CitSERK1-like was mainly located in the embryogenic callus and vascular cells of different embryos or tissues. The results of temporal and spatial expression of CitSERK1-like showed that it played critical roles throughout the process of SE and had a broader role in plant development. In addition, CitSERK1-like expression was up-regulated by 2, 4-D and NAA at the early stage, but down-regulated afterward. Taken together, it suggested that CitSERK1-like activated complex developmental pathways associated with somatic embryogenesis and plant growth.