CsHPt1, a putative histidine-containing phosphotransmitter protein induced during early somatic embryogenesis in Valencia sweet orange

A cDNA named CsHPt1 (for Citrus sinensis histidine phosphotransmitter protein 1) containing a conserved domain characteristic of a two-component phosphorelay intermediate associated with signal transduction mechanisms, was isolated from globular embryos of C. sinensis sweet orange cv. 'Valencia' by differential display RT-PCR. The corresponding full-length cDNA, subsequently obtained by RACE-PCR, is 772 bp with a 450 bp open reading frame encoding a 150 amino acid protein. The deduced amino acid sequence of CsHPt1 is approximately 60% similar to five AHP histidine-containing phosphotransmitter (HPt) proteins from Arabidopsis thaliana and contains the 12 amino acid motif characteristic of all known HPt domains. RT-PCR analyses showed higher CsHPt1 transcript abundance in embryogenic callus when compared to non-embryogenic callus, leaves, phloem, roots, and flavedo (rind). Immature fruits and mature flowers, both of which contain tissues with embryogenic potential, also showed increased CsHPt1 expression. Furthermore, CsHPt1 gene expression levels in globular embryos were higher than in embryogenic callus in three of the five cell lines we analyzed. Results of this study provide insight on signal transduction mechanisms that may be active during early plant embryogenesis.