|Thomson, James - Jim|
Submitted to: Acta Horticulturae
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/23/2016
Publication Date: 10/17/2017
Citation: Belknap, W.R., Thomson, J.G., Thilmony, R.L., McCue, K.F., Hao, G., Stover, E.W. 2017. Small cyclic amphipathic peptides (SCAmpPs) genes in citrus provide promising tools for more effective tissue specific transgenic expression. Acta Horticulturae. 1172:85-90. https://doi.org/10.17660/ActaHortic.2017.1172.15.
Interpretive Summary: A family of 100-150 closely related genes has been identified in citrus. These encode a set of small cyclic peptides, Small Cyclic Amphipathic Peptides (SCAmpPs), some of which are present in high concentrations in certain tissues, including phloem, roots and ovaries. Thus far, the function of these proteins remains undetermined. The gene structures are very similar within the family, with the small protein coding regions interrupted by a single intron, and remarkable sequence conservation among the components that control gene expression. These gene promoter components were attached to coding sequences of reporter genes and transformed into Arabidopsis and citrus plants. The expression of the reporter genes matched the predicted tissue-specific profiles of the SCAmpPs genes from which the promoters were derived. Previously studied tissue-specific promoters have provided only modest expression, and it appears that these new citrus promoters will provide much greater expression with strong tissue specificity. Studies are underway to determine how to use these sequences to express anti-microbial compounds in plants to control the growth of phloem-limited pathogenic bacteria.
Technical Abstract: A gene family encoding Small Cyclic Amphipathic Peptides (SCAmpPs) has been identified in citrus. Citrus genomes include 100-150 SCAmpPs genes, and about fifty transcripts are represented in the citrus EST database. These genes encode small ~50 residue precursor proteins that are post-translationally processed, releasing 5-10 residue cyclic peptides. Thus far, the function of SCAmpPs remains undetermined, though their structures identified through LC-MS-MS are consistent with membrane binding and K+ ionophore function. The SCAmpPs gene structures are highly conserved, with the small coding domains interrupted by a single intron, and relatively extended untranslated regions. Some family members are very highly transcribed in specific tissues such as phloem, root or ovary, as determined by their representation in tissue-specific cDNA libraries. High concentrations of phloem-specific SCAmpPs have been verified in citrus phloem tissue using LC-MS. Remarkably, even though the final predicted peptide sequences of the phloem SCAmpPs are quite variable, there is almost complete identity in the promoter, first exon and introns of their genes. Constructs using phloem, root and ovary SCAmpPs’ transcriptional control sequences have been used to express GUS reporter genes in Arabidopsis and citrus. Reporter gene expression matched the tissue-specific profiles predicted by the abundance of individual SCAmpPs ESTs. It appears that SCAmpPs promoters will provide much greater expression than previously studied tissue-specific promoters. Studies are underway to further delineate the functional SCAmpPs gene promoter sequences and to determine their value for high levels of phloem expression of anti-microbial peptides to control phloem-limited bacteria.