Investigations on the strigolactones-miR156 cross-talk in tomato under drought

miRNAs are small, non-protein-coding RNAs that regulate gene expression post-transcriptionally. Strigolactones (SL) are phytohormones acting as ecological and developmental communicators, and involved in the responses to abiotic stress. In tomato (Solanum lycopersicum L.), the current model places a drought-triggered decrease of SL synthesis in roots upstream of a reduced SL flow shootward and of increased SL synthesis in above-ground organs. Perception of shoot-produced SL leads in turn to higher ABA sensitivity and to lower transpiration, thus increasing drought tolerance. A SL-miRNA cross-talk under drought is suggested, among other lines of evidence, by such role of SL in drought and by the drought inducibility of several miRNAs, among which the conserved miR156. We therefore investigated a possible SL-miR156 relationship in stressed and unstressed tomato. The results obtained so far indicate that exogenous SL are sufficient for the accumulation of mature miR156 in unstressed leaves. The comparison of WT and SL-depleted plants, both self- and hetero-grafted, shows that SL synthesis in the shoot, but not in the root, is needed for the accumulation of mature miR156 in both organs under drought; while SL synthesis in the roots is needed for drought-induced pri-miR156 transcript accumulation in the shoot. Additionally, shoot-synthesized SL seem to inhibit pri-miR156 transcription in the absence of stress; and under all conditions, to promote miR156 maturation - a process whose efficiency towards specific miRNAs is sensitive to cellular context and thus, possibly, to hormones. Plants overexpressing miR156 are being used to investigate whether miR156 is a mediator of SL-dependent water stress responses in tomato.