Strigolactones are crucial for miR156 inducibility by drought in tomato

MicroRNAs (miRNAs) are a set of small, non-protein-coding RNAs that regulate gene expression by negatively affecting transcript stability and translatability. A cross-talk between strigolactones (SL) and miRNAs is suggested by 1) potential target sequences for the conserved miR156/166 families in some SL-related genes in Arabidopsis; 2) partial phenotypic overlap between plants overexpressing miR156 (oe-miR156) and SL-related mutants; 3) lower SL content in axillary meristems of oe-miR156 plants; 4) the importance of SL dynamics for acclimatization to drought, coupled to the drought inducibility of several miRNAs including miR156/166. We therefore set to investigate the existence of a SL-miR156/166 cross-talk in tomato under drought. The results obtained so far indicate that exogenous SL are sufficient for the induction of mature miR156 accumulation 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 (but not miR166) in both organs under drought; while SL synthesis in the roots is needed for drought-induced pri-miR156 (primary 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. Oe-miR156 plants are being used to investigate whether miR156 is a mediator of SL-dependent water stress resilience in tomato; and also, whether miR156 is part of the negative feedback regulation known to occur on SL biosynthesis.