Effetti degli strigolattoni sull'induzione fiorale in pomodoro

Strigolactones (SL) are a class of plant hormones that affect plant morphology at various levels, including inhibition of shoot branching, promotion of shoot secondary growth and modulation of root architecture. Exuded in soil, SL will take part in the molecular dialogue between the plant and rhizosphere (micro)flora. They also help acclimatisation to abiotic stress, notably low nutrients and drought; during the latter, they act in concert with abscisic acid to promote stomata closure. SL also affect reproduction in several plant species, among which tomato (Solanum lycopersicum L.). SL-deficient tomato lines vegetate profusely and set few flowers, fruits and seeds; and treating wt plants with GR24 (a synthetic SL) leads to early flowering and more flowers resulting in higher yield, compared to mock-treated plants. The first hypothesis tested to explain these results has been that SL negatively affect the concentration of mature miR156. This miRNA inhibits flowering in juvenile plants, and its levels decrease with age thus allowing for increase of miR172 (a flowering promoter), phase transition and flowering initiation. However, mature miR156 is induced instead of repressed by exogenous SL treatment in tomato; an induction that seems rather connected to the stress-dependent effects of miR156, since miR156 increase during drought is totally SL-dependent. Therefore, the effect of SL on flowering should follow miR156-independent pathways. In order to go deeper into these aspects, the effects of exogenous SL treatment on flowering induction were investigated at the molecular level by targeted transcript quantification, while a RNA-seq analysis will follow. To this goal, we contrasted GR24 with mock-treated plants for transcript concentrations from genes encoding putative or demonstrated flowering activators and repressors, namely those involved in the gibberellin-dependent pathway. Among the first, we targeted SFT (Single Flower Truss), and MIR172a; among the second, SlSP5G (Self Pruning 5G), SlCDF3 (Dof Zinc Finger 3) and SlRAV2 (Related to Abi3/Vp1 2). The latter was identified using as a query the amino acidic sequence of Arabidopsis thaliana TEMPRANILLO2 (AtTEM2), also called AtRAV2; phylogenetic analysis confirmed SlRAV2 to be a putative AP2/B3 DNA-binding factor. Transcript abundance from these loci was quantified at different time points: 1, 2, 6 and 24 hours after treatment, in order to profile gene expression kinetics.The results show a fast and transient burst of expression for flowering activator genes in plants treated with GR24, and lower transcript abundance for inhibitors. This substantiates, at the molecular level, the effect on flowering observed in SL-related mutants and SL-treated plants. It also suggests that SL may play a significant role in flowering induction in tomato via a miR172-dependent, miR156-independent pathway, possibly linked to gibberellin action. Given the fact that SL are induced by drought and sufficient to promote flowering, a further, intriguing hypothesis is that SL may promote the drought escape response, i.e. the early flowering after stress allowing for life cycle completion before stress occurs again. Indeed, drought escape is enabled by the convergence of environmental acclimatization and flowering induction, and SL play a role in both processes. Further research will be necessary to explore SL -promoted flowering induction and the genetic network underlying drought escape in tomato.