Phloem sap is the fluid that circulates between plant organs to redistribute photosynthetic assimilates, N- and S-containing compounds, and signalling molecules. As such, it is crucial for grain and fruit filling and thus primary production and yield. Despite this crucial importance, very little is known on the physiology of phloem sap flow and composition. Recent results suggest that phloem sap velocity and metabolome are controlled and might be homeostatic. We don’t know how and why. This project aims to fill this gap of knowledge, via three work packages: 

1. Highlight: we will explore phloem sap properties under varying photosynthetic and water conditions, to identify physiological situations where they may, or may not, vary. 

2. Mechanisms: we will take advantage of sap metabolomics, sap ionomics and genetics to delineate potential mechanisms behind phloem homeostasis, such as possible metabolic compensations, regulation of sugar and water transport, or dynamic coregulation with cations. 

3. Data integration and modelling: we will analyse links between data blocks to identify condition-specific sap omics patterns (via machine learning) and use experimental data to design and perform a model of phloem sap generation and transport. For each WP, we exploit new technologies uniquely available in our consortium: isotopic method to estimate phloem sap concentration and flow, magnetic resonance imaging (MRI) for in vivo metabolic profiling of phloem sap, high resolution mass spectrometry (Orbitrap®) coupled to gas chromatography with an associated exact mass database, and a model to describe phloem sap concentration, pressure and velocity. The project gathers the national experts on phloem and uses three plant species of recognised importance for French agronomy. It will be essential to understand phloem sap homeostasis and a first step to identifying sap biomarkers of crop monitoring in a context of climatic events impacting photosynthesis and phloem loading.

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