Low levels of strigolactones in roots as a component of the systemic signal of drought stress in tomato

Strigolactones (SL) contribute to drought acclimatization in shoots, since SL-depleted plants are hypersensitive to drought due to stomatal hyposensitivity to abscisic acid (ABA). However, under drought, SL biosynthesis is repressed in roots, suggesting organ specificity in their metabolism and role. Since SL can be transported acropetally, such drop may alsoaffect shoots, as a systemic indication of stress. We investigated this hypothesis by analysing molecularly and physiologically WT tomato scions grafted onto SL-depleted rootstocks, compared to self-grafted WT and SL-depleted genotypes, during a drought time-course. Shoots receiving few SL from the roots behaved as under mild stress even if irrigated. Their stomata were hypersensitive to ABA (likely via a localized enhancement of SL synthesis in shoots). Exogenous SL also enhanced stomata sensitivity to ABA. As the partial shift of SL synthesis from roots to shoots mimics what happens under drought, a reduction of root-produced SL might represent a systemic signal unlinked from shootward ABA translocation and sufficient to prime the plant for better stress avoidance.