Submitted to: Biomed Central (BMC) Plant Biology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 6/24/2010
Publication Date: 8/26/2010
Publication URL: uh7qf6fd4h.search.serialssolutions.com
Citation: Dafoe, N.J., Gowen, B., Constabel, C.P. 2010. Thaumatin-like proteins are differentially expressed and localized in phloem tissue of hybrid poplar. Biomed Central (BMC) Plant Biology. 10:191. Interpretive Summary: Sieve elements are specialized phloem cells that are important for the transport of sugars and other small organic compounds from carbon exporting (source) tissues to carbon importing (sink) tissues. At maturity, sieve elements are enucleate and therefore incapable of protein synthesis, yet hundreds of proteins have been detected in phloem exudates. In collaboration with the researchers at the University of Victoria (Centre for Forest Biology), scientists at the Center for Medical, Agricultural and Veterinary Entomology in Gainesville, FL previously conducted the first characterization of a perennial phloem exudate proteome. In our analysis of hybrid poplar phloem exudate, we detected the presence of two thaumatin-like proteins (TLPs), one of which was induced in phloem exudate 24 hours after wounding leaves suggesting a role in plant defense. In order to determine the origin of these proteins and confirm their presence in sieve elements, we characterized their localization using immunofluorescence and immunogold labeling with a TLP antiserum. TLPs were localized to sieve elements, phloem parenchyma cells, and phloem fibres. Like many TLPs, poplar TLPs were detected in cell walls, but they were also detected inside cells, associated with starch and starch-containing plastids. This pattern of localization is consistent with a role as a preformed defense against phloem-feeding insects and pathogens.
Technical Abstract: Two thaumatin-like proteins (TLPs) were previously identified in phloem exudate of hybrid poplar (Populus trichocarpa x P. deltoides) using proteomics methods, and their sieve element localization confirmed by immunofluorescence. In the current study, we analyzed different tissues to further understand TLP expression and localization in poplar, and used immunogold labeling to determine intracellular localization. Results: Immunofluorescence using a TLP antiserum confirmed the presence of TLP in punctate, organelle-like structures within sieve elements. On western blots, the antiserum labeled two constitutively expressed proteins with distinct expression patterns. Immunogold labeling suggested that TLPs are associated with starch granules and starch-containing plastids in sieve elements and phloem parenchyma cells. In addition, the antiserum recognized TLPs in the cell wall and sieve plate region of sieve elements. Conclusions: TLP localization in poplar cells and tissues is complex. TLP1 is expressed predominately in tissues with a prominent vascular system such as midveins, petioles and stems, whereas the second TLP is primarily expressed in starch-storing plastids found in young leaves and the shoot apex.