Proteomic analysis of hybrid poplar xylem sap

Xylem sap collected from Populus trichocarpa × Populus deltoides using root pressure was estimated to contain more than 100 proteins. Ninety-seven of these proteins were identified using liquid chromatography-tandem mass spectrometry (LC-MS/MS). These proteins were classified into 10 functional categories including metabolism, signaling, stress response and cell wall functions. The majority of xylem sap proteins were metabolic enzymes involved in processes including translation, proteolysis, and glycolysis. Stress-related proteins were also prevalent. In contrast to xylem sap proteins collected from annual plants, the majority of poplar xylem sap proteins do not appear to be classically secreted since only 33 proteins were predicted to have an N-terminal signal peptide targeting them to the secretory pathway. Of the remaining 64 proteins, 27 were predicted to be secreted non-classically. While a number of proteins identified here have been previously reported in xylem sap proteomes of annual plants, many xylem sap proteins were identified in poplar which may reflect functions specific to perennial plants.