Comparative metabolite profiling of the phloem saps of maize, tomato and Arabidopsis plants

In higher plants, two vascular tissues, phloem and xylem, are responsible for the transport of water, minerals and nutrients between organs. The xylem transports water and minerals absorbed by the roots and the phloem transports from source to sink organs photoassimilates synthesized in the photosynthetic leaves. The phloem is essential for higher plants in many developmental and physiological processes. It not only provides the route for the distribution of assimilates but also redistribute mineral nutrients. Additionally, the phloem is essential for sending information between distant plant organs and steering developmental and defence processes. For example, flowering and tuberization time are controlled by phloem-mobile signals and important defence reactions on the whole plant level, like systemic acquired resistance or systemic gene silencing, are spread through the phloem. In addition, recent results demonstrate that also the allocation of mineral nutrients is coordinated by phloem mobile signaling molecules. However, in many studies the important analysis of phloem sap is neglected, probably because the content of sieve tubes is not easy to access. We used the EDTA-facilitated exudation method to collect the phloem sap from three plant species, Maize, Tomato and Arabidopsis. The metabolites present in the samples were analysed by GC-MS or GC-TOF to determine the range of metabolites present in the phloem sap. We detected more than 50 metabolites, including sugars, amino acids and organic acids. Because of an intrinsic biological variability in the efficiency of exudation depending on the plant samples, we developed a method for the normalisation and the statistical analysis of the phloem sap metabolite profiles. This study showed that for each species, there is a significant conservation of the phloem sap content, revealing the strong physical and physiological constraints on phloem transport. However we observed important differences in the phloem sap metabolic profiles between plant species, showing different strategies for the allocation of sugars and amino acids between organs.