Polyethylene Glycol-Induced Osmotic Stress Promotes Flowering by Up-Regulating Dehydroascorbic Acid-to-Ascorbic Acid Ratio in Tomato

Drought stress can promote early flowering in plants. However, the mechanism remains not well understood. Here, we investigated the role of the ascorbate redox ratio in regulating tomato flowering under polyethylene glycol-6000 (PEG)-induced osmotic stress, which mimics drought. The results showed that compared with the control, a drought stress at 45% substrate water content promoted tomato flowering, while 25% water content did not affect flowering. Under simulated drought induced by PEG, early flowering was only observed under 10% PEG, while lower or higher PEG concentrations did not affect flowering. Under 10% PEG, the expressions of flowering integrator genes (SUPPRESSOR OF OVEREXPRESSION OF CONSTANS1 and SINGLE FLOWER TRUSS) and meristem identity genes (APETALA1 and LEAFY) were up-regulated, while they were largely unchanged or down-regulated under 20% PEG. The dehydroascorbic acid (DHA)/reduced ascorbic acid (AsA) ratio was increased under 10% PEG but remained unchanged under 20% PEG. Foliar DHA treatment increased the DHA/AsA ratio in leaves, promoted tomato flowering, and up-regulated the expressions of flowering integrator and meristem identify genes in the control conditions, while foliar AsA treatment demonstrated the opposite phenotype under 10% PEG. In addition, foliar treatment with N,N′-dimethylthiourea, a reactive oxygen species scavenger, decreased the DHA/AsA ratio in leaves and delayed tomato flowering under 10% PEG. The expression patterns of genes in the photoperiod, vernalization, autonomous, ambient temperature, and age pathways did not provide evidence for their involvement in osmotic stress-induced early flowering in the present sampling conditions. Our results suggest that osmotic stress-induced H2O2 accumulation increased the DHA/AsA ratio in plants, which promoted tomato flowering. The study may help clarify the mechanism for drought-induced flowering in plants.