Metabolic profile of phloem sap of tomato plants in response to phytoplasma infection

In higher plants, phloem and xylem are the vascular tissues that permit the transport of photosynthates, water and other nutrients among the organs. Carbon is mainly fixed in photosynthetic leaves through the conversion of light into carbohydrates that are thus transported via the phloem into sink organs. The phloem sap content is mainly composed of sugars, such as sucrose, in addition to amino acids, organic acids and a range of secondary metabolites involved in signaling and defense. Phloem functions can be altered by plant pathogens. Notably phytoplasmas, which are phloem-restricted pathogens, can induce a switch in the source-sink balance between organs altering the normal pattern of sugar partitioning. Phytoplasmas affect many crops on which they cause severe diseases. Interactions between tomato and ‘Candidatus Phytoplasma solani’, which belongs to stolbur group 16SrXII, have been investigated in order to determine the effects of phytoplasmas infection on carbon allocation, using several approaches. In particular we set up an EDTA-facilitated exudation method to collect the phloem sap from tomato leaves. Sap metabolite contents were analyzed by Gas Chromatography (GC-FID). We were able to detect more than 50 metabolites, including sugars, amino acids and organic acids. Due to the high biological variability among plant samples we have developed a method of normalization before performing statistical analysis on data. Comparing phloem saps from healthy and infected plants, we observed significant changes in few metabolites belonging to different classes of compounds. For example, the rate of exudation of some organic acids such as glyoxylate has been found to be increased in infected plants while sucrose flow was found decreased. Profiling phloem sap content has appeared as an interesting tool to monitor plant status in the case of disease.