Low root levels of Strigolactones as an osmotic stress signal to the shoot

Root-synthesized ABA is increased by water-related stresses and, in some plants, is considered a systemic signal of stress for the shoot. However, in other plants (e.g. tomato, Solanum lycopersicum L.) root ABA is unnecessary for appropriate shoot responses to drought, leaving uncertainty on the nature of the systemic stress signal. Recently, Strigolactones (SL) have been also proposed as mediators of resource allocation processes under environmental stress. SL are a new class of plant hormones with pervasive roles in development, from germination and reproduction to morphology and interaction with (micro)biota. Shoots of SL-depleted plants are hypersensitive to osmotic stress due to ABA hyposensitivity; a positive role for SL in the acclimatization to drought in above-ground organs is thus hypothesized. Surprisingly though, synthesis of SL is repressed in roots under drought; since these are the main SL producers under normal conditions, SL metabolism and role under osmotic stress must be organ-specific. In roots, the drop in SL levels during drought was reported to be needed for local ABA increase. However, since root SL are also distributed to the whole plant, the possibility exists that such drop effects shoots as well, as a systemic indication of stress. We investigated such possibility by analysing molecularly and physiologically WT scions grafted over SL-depleted tomato rootstocks, compared to self-grafted WT and SL-depleted genotypes, both under normal and stress conditions. The results prove that SL increase ABA sensitivity; shoots receiving less SL from the roots are indeed hypersensitive to ABA, likely via the enhancement of local SL synthesis. As this mimics what normally happens under drought, SL (or better a reduction thereof) might well represent a systemic signal of stress. We also propose the grafting of rootstocks with reduced SL content to WT shoots as a new approach for the development of crops with superior drought stress resilience.