Saffron CsSEP3 regulates flowering time and flower organ morphogenesis.

Saffron (Crocus sativus L.), known as "red gold", is a highly valued medicinal herb with significant economic benefits. Its medicinal value comes from the stigma, and increasing the weight of the stigma can enhance its yield. SEPALLATA3 (SEP3), a key MADS-box transcription factor, plays crucial roles in floral development and organ identity determination. However, the specific role of SEP3 in saffron has remained unclear. In this study, the CsSEP3 gene was cloned from saffron. Bioinformatics analysis identified highly conserved MADS-box and K-box domains within the protein. Phylogenetic analysis indicated that CsSEP3 clusters closely with the ortholog of SEP3 in Asparagus officinalis. CsSEP3 is specifically activated during the floral induction stage and rapidly up-regulated during the flower organ differentiation stage. It is primarily expressed in petals, stamens and stigmas during the yellow- and orange-stigma stages, while in the red-stigma stage and on the day of flowering, its expression is mainly detected in petals and stigmas. Subcellular localization experiments confirmed that CsSEP3 is localized in the nucleus. When CsSEP3 was ectopically expressed in Arabidopsis thaliana, it significantly accelerated flowering and led to enlarged flower organs of petals, stamens, and pistils. Protein-protein interaction predictions indicate that CsSEP3 likely forms complexes through interactions with MADS-box proteins such as CsAP3, CsSOC1, and CsAG. These findings not only enhance our understanding of CsSEP3's role in saffron but also provide a theoretical basis for boosting saffron stigma yield through molecular breeding strategies by manipulating the expression levels of CsSEP3.