Sugar metabolism mediates temperature-dependent flowering induction in saffron (Crocus sativus L.)

Saffron, which comprises the stigma of the Crocus sativus flower, is an important culinary and medicinal product. Saffron productivity depends on its flower formation, which is regulated by temperature. Inappropriate temperatures at the flowering stages result in flower atrophy and reduced or no flowering. The effect of temperature on saffron flowering was investigated using morphological studies coupled with transcriptomics and metabolic studies of meristem-enriched tissue collected from corms stored and grown at two different temperatures [high (25°C) and low (8°C)] during flowering. Morphological and histological studies suggested that high temperature, during the resting phase of the saffron corm life cycle, promotes flowering, whereas low temperature suppresses it. Low temperatures suppress flowering by inhibiting flower induction. Transcriptome analysis during the flower induction stage indicated that sugar metabolism genes were differentially regulated at different temperatures. Genes coding for starch and sucrose metabolism were enriched and differentially regulated at 25°C vs 8°C. High temperature promoted starch breakdown and soluble sugar formation, whereas low temperature had a vice-versa effect. Further, we identified that low-temperature mediated suppression of flowering involves the downregulation of the floral integrator gene CsatFT3. Exogenous sugar feeding, mainly sucrose, can rescue the low-temperature suppression of flower induction via upregulation of the CsatFT3 gene. Overall the results suggest that environmental temperatures alter sugar metabolism, which mediates temperature-dependent flower induction in saffron corms via regulation of the FT3 gene. These findings will aid in identifying temperature driven molecular factors controlling flowering in saffron.