SummaryStrigolactones (SL) contribute to drought acclimatization in shoots, because 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. Because SL can be transported acropetally, such a drop may also affect shoots, as a systemic indication of stress.We investigated this hypothesis by analysing molecularly and physiologically wild-type (WT) tomato (Solanum lycopersicum) scions grafted onto SL-depleted rootstocks, compared with self-grafted WT and SL-depleted genotypes, during a drought time-course.Shoots receiving few SL from the roots behaved as if 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.

Grapevine (Vitis vinifera) is routinely grafted, and rootstocks inducing drought tolerance represent a source for adapting vineyards to climate change in temperate areas. Our goal was to investigate drought stress effects on microRNA (miRNA) abundance in a drought-resistant grapevine rootstock, M4 (Vitis vinifera 3 Vitis berlandieri), compared with a commercial cultivar, Cabernet Sauvignon, using their autografts and reciprocal grafts. RNA extracted from roots and leaves of droughted and irrigated plants of different graft combinations was used to prepare cDNA libraries for small RNA sequencing and to analyze miRNAs by quantitative real-time polymerase chain reaction (RT-qPCR). Measurements of leaf water potential, leaf gas exchange, and root hydraulic conductance attested that, under irrigation, M4 reduced water loss in comparison with cultivar Cabernet Sauvignon mostly through nonhydraulic, root-specific mechanisms. Under drought, stomatal conductance decreased at similar levels in the two genotypes. Small RNA sequencing allowed the identification of 70 conserved miRNAs and the prediction of 28 novel miRNAs. Different accumulation trends of miRNAs, observed upon drought and in different genotypes and organs, were confirmed by RT-qPCR. Corresponding target transcripts, predicted in silico and validated by RT-qPCR, often showed opposite expression profiles than the related miRNAs. Drought effects on miRNA abundance differed between the two genotypes. Furthermore, the concentration of drought-responsive miRNAs in each genotype was affected by reciprocal grafting, suggesting either the movement of signals inducing miRNA expression in the graft partner or, possibly, miRNA transport between scion and rootstock. These results open new perspectives in the selection of rootstocks for improving grapevine adaptation to drought.

The strigolactone analogue GR24 reduces ABA-induced anthocyanin accumulation in Vitis vinifera berries. GR24 treatment does not affect ABA biosynthesis while it activates ABA degradation and possibly ABA membrane transport.

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