Isolation and characterization of the somatic embryogenesis receptor-like kinase gene homologue (CitSERK1) from Citrus unshiu Marc.

A cDNA clone was isolated for a somatic embryogenesis receptor-like kinase (SERK) gene homologue (CitSERK1) from Citrus unshiu Marc. CitSERK1 revealed high amino acid sequence homologies to the past isolated SERK genes of Arabidopsis thaliana, Zea mays, and Daucus carota and had the common structural features of a putative leucine zipper (ZIP) with five repeat units in the extracellular domain, a proline-rich region (SPP) just upstream of the transmembrane domain, and the kinase domain in the C-terminal. The mRNA expression of CitSERK1 was relatively uniform in all plant tissues except those of mature fruit at days after flowering (DAF) 180 and abundant around the stage of nucleous division and enlargement. During callus culture, the transcripts of CitSERK1 were less than the detectable levels in proliferating non-embryogenic calli but increased until they could be detected in the calli after the initiation of embryogenesis, suggesting that CitSERK1 could be involved in embryogenesis or embryo development. In addition, genetic linkage analysis showed that the SERK1 locus was independent of both loci concerning two traits, polyembryony and seedlessness derived from Satsuma mandarin, which have been considered to be highly important characteristics in Citrus genetic improvement.