Submitted to: Planta
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/5/2011
Publication Date: 1/19/2011
Citation: Krishnan, H.B., Natarajan, S.S., Bennett, J.O., Sicher Jr, R.C. 2011. Protein and metabolite composition of xylem sap from field-grown soybeans (Glycine max). Planta. 233:921-931. Interpretive Summary: Transport of nutrients and signal molecules is facilitated by vascular bundles which are made up of xylem and phloem. Xylem is involved in the transport of water and inorganic nutrients from the soil to the aerial parts of the plant. Xylem sap has been shown to contain a number of proteins involved in defense response and environmental stresses. Xylem sap protein composition of soybean seedlings has been previously investigated. However, the previous studies were conducted with soybean seedlings grown in a controlled environment. Plants in the field are subjected to high light and to environmental stress that is not normally found in controlled environmental chambers. In this study, we have collected xylem sap from field grown soybeans and identified proteins potentially involved in defense-related reactions, nutrient translocation, signal-transduction and cell wall metabolism. Results from this basic study indicates that the dynamic changes in the composition of xylem sap proteins and metabolites can be utilized as a useful indicator of plants response to environmental stresses. This research will be valuable to plant physiologists, biochemists, and geneticists studying plant response to biotic and abiotic stresses.
Technical Abstract: The xylem, in addition to transporting water, nutrients and metabolites, is also involved in long-distance signaling in response to pathogens, symbionts and environmental stresses. Xylem sap has been shown to contain a number of proteins including metabolic enzymes, stress-related proteins, signal transduction proteins and putative transcription factors. Previous studies on xylem sap have mostly utilized plants grown in controlled environmental chambers. However, plants in the field are subjected to high light and to environmental stress that is not normally found in growth chambers. In this study, we have examined the protein and metabolite composition of xylem sap from field-grown cultivated soybean plants. One-dimensional gel electrophoresis of xylem sap from determinate, indeterminate, nodulating and non-nodulating soybean cultivars revealed similar protein profiles consisting of about 8-10 prominent polypeptides. Two-dimensional gel electrophoresis of soybean xylem sap resulted in the visualization of about 60 distinct protein spots. A total of 45 protein spots were identified using MALDI-TOF MS and LC-MS/MS. The most abundant proteins present in the xylem sap were identified as 31 kDa and 28 kDa vegetative storage proteins. In addition, several proteins that are conserved among different plant species were also identified. Diurnal changes in the metabolite profile of xylem sap collected during a 24h cycle revealed that asparagine and aspartate were the two predominant amino acids irrespective of the time collected. Pinitol (D-3-O-methyl-chiro-inositol) was the most abundant carbohydrate present. The possible roles of xylem sap proteins and metabolites as nutrient reserves for sink tissue and as an indicator of biotic stress are also discussed.