Tomato plants control leaf sodium levels to limit damage by Spodoptera littoralis larvae

Sodium is an essential element for animal growth and survival. Here we test the hypothesis that plants exposed to insect feeding can translocate sodium from the feeding site to other plant organs as a defense strategy against phytophagous insects, which need sodium in their diet. By modulating leaf sodium levels in tomato and by profiling the gene expression of key plant sodium transporters in response to insect feeding, we assessed sodium redirection pathways as affected by leaf damage and the impact of different sodium levels on food preference and intake, growth and development of Spodoptera littoralis larvae. We demonstrated that upon S. littoralis attack, tomato plants removed sodium from the feeding site by relocating it basipetally from the leaf to the root, through a coordinated regulation of the sodium transporters HKT1;1 and HKT1;2. We also showed that the leaf damage by S. littoralis larvae was positively correlated with sodium concentration and that low sodium plants negatively affected food preference, larvae growth and development. These results shed light on a novel mechanism of plant response to herbivore insect damage, providing new insights on the role of sodium in plant-insect interactions.