Energy Flow from Root to Shoot: A Comprehensive In silico Analysis

BACKGROUND: Root to shoot connection and transfer of information seems to be taken place mostly via the transmissions of signal molecules, secondary metabolites, amino acids, hormones and proteins, through xylem sap. Examination of earlier reports is indicative of relatively high levels of conservation in xylem sap protein compositions. Apparently these protein molecules are being synthesized in roots in response to environmental changes and get transported to aerial plant parts after secretion into xylem sap. OBJECTIVES: In order to comprehend this so-called passive signaling, some questions need to be answered: 1) Do these proteins have the capability to act as signals? 2) How much energy does root spend for the biosynthesis of the secreted proteins? How similar is the amount of energy that root cells spent for the biosynthesis of intra- and extra-cellular proteins? MATERIALS AND METHODS: Reported xylem sap proteins curated from Arabidopsis, maize and soybean. Their sequences were put under scrutiny in terms of considering their mobility, and physical and chemical properties. Metabolic energy required for their biosynthesis along with the energy hidden in their peptide bonds were calculated and compared with random non-xylem sap proteins as control. RESULTS: Xylem sap proteins were significantly smaller than the root proteins, while they were bigger in size when compared to the leaf group. Xylem protein pIs were significantly higher than the control proteins in different plants. Similarly, the protein stability was higher for xylem sap proteins in comparison with roots and leaves in all analyzed plants, except for soybean that the stability was indifferent between xylem and root. The data were suggestive a significantly lower energy consumption for the synthesis of xylem sap proteins. CONCLUSIONS: Lower energy consumption may suggest an economical route of communication between roots and shoots in plants that mainly rely on symplastic signaling.