Characterization of the Root Exudation Pattern in Tomato Plants Exposed to Single and Combined Fe and S Deficiency

The availability of Fe can be a major constraint to plant growth and yield in many environments worldwide being even worse when the lack of Fe co-occurs with S deficiency. In fact, several studies identified the direct impact of S deficiency on plant capability to cope with Fe starvation or viceversa highlighting the importance of the interaction between the two nutrients. Low molecular weight organic compounds released by roots, named root exudates, trigger the mobilization and acquisition of mineral nutrients both directly and indirectly; indeed, root exudates are involved in both mechanisms (Strategy I and Strategy II) that plants have evolved to maintain Fe homeostasis.

This work aims at characterizing the exudomic profile of hydroponically grown tomato (Solanum lycopersicum L.) plants subjected to single or combined Fe and S deficiency and its influence on the mechanisms of regulation modulating Fe and S mobilization and uptake.

Root exudates were collected and their metabolites were determined by liquid chromatography coupled to QTOF mass spectrometry (UHPLC-ESI/QTOF-MS), as well as by gas chromatography mass spectrometry (GC/MS) following derivatization.

As expected, the root exudation pattern varied depending on the plant nutrient status revealing a high heterogeneity and complexity. In fact, three distinct clusters could be distinguished by the partial least-squares discriminant analysis (PLS-DA) suggesting that plants react differently under combined deficiencies rather than to single ones. In particular, both by GC/MS and UHPLC-ESI/QTOF-MS we identified 217 metabolites including mainly organic acids, amino acids, sugars, flavonoids and other phenolics. Venn diagrams following Volcano analysis (P< 0.001, fold-change cut-off=3) revealed that 23% (51), 10% (21) and 21% (45) of metabolites were differentially modulated by single Fe deficiency, single S deficiency and combined Fe/S deficiency, respectively. These results suggest that Fe deficiency both alone or in combination with S starvation modulate most of the primary and secondary metabolites.