How does natural variation impact the metabolite profile of phloem and xylem saps in Arabidopsis thaliana?
Résumé
An important step for land colonization by plants has been the emergence of the vascular tissues, which spatialized into phloem and xylem the transport of water, minerals and photoassimilates. Both systems evolved into critical compartments for the allocation of nutrients in addition to the long distance transport of signal molecules acting on shoot-root communication. Although their anatomy and the biophysics of transport are very different, both vascular transport systems are based on the sieve elements and the vessels that form composed by a network of conducting cells for transporting fluids, and on a subset of highly specialized cells acting in loading, retrieval, unloading and storage. The range of metabolites transported by the xylem and phloem mirrors the physiology of the plant in response to its environment and is intimately associated to its stage of development. Therefore, changes in the phloem and xylem sap compositions can rapidly occur in response to abiotic stresses for the acclimation of plants to the osmotic and metabolic constraints. They provide to the plant an efficient framework to integrate shoot and root stimuli and to coordinate the metabolism of distant organs. With the aim of identifying key factors acting in the long distance integration of shoot and root constraints, we analysed the metabolite profiles of the phloem and xylem saps sampled from Arabidopsis mature plants. Five Arabidopsis accessions grown either in plethoric or limiting nitrogen nutritional conditions were used for this study, with the objective to address the question of the potential plasticity in the sap compositions in response to abiotic stress.The collect of the phloem sap was performed by EDTA-facilitated exudation method from the petiole of mature leaves and the collect of xylem sap was performed from sections of petioles on adult rosettes. The metabolites were analysed by GC-MS to determine the range and the relative content of metabolites present in the exudates. This study showed a significant variability in the sap compositions and in the shoot to root flows, indicating several levels of adaptive mechanisms. The implications for the metabolic bottlenecks occurring either in the phloem or in the xylem will be presented.