Expression Analysis of <i>AcFT2</i> that Induces Flowering and Bolting in Field-grown Onions under Overwinter Cultivation in the Northern Hemisphere

The onion (Allium cepa) is a biennial plant, and development and size of the bulb are important factors in determining its productivity. During onion cultivation, premature flowering during crop growth, also known as bolting, must be avoided because it negatively affects bulb development and impairs quality. In many plants, day length and temperature are related to flowering, and the FLOWERING LOCUS T (FT) gene regulates flower bud differentiation at the genetic level. Onion flowering is promoted by vernalization and is correlated with upregulation of AcFT2, which may be a key signal for flowering in vernalized meristems. No studies on AcFT2 expression or its relationship with bolting under field conditions have been conducted; therefore, we investigated these points in field-grown onions under autumn-sowing (overwinter) cultivation. In this study, we revealed that relatively late-maturing cultivars for spring-sowing cultivation are not suitable for autumn-sowing cultivation due to their high bolting incidence. Based on these results, AcFT2 expression in the leaf blades of relatively late-maturing cultivars under spring-sowing cultivation was analyzed during plant growth under autumn-sowing cultivation. Our results indicated that AcFT2 expression in late-maturing cultivars gradually increases during plant growth and peaks before visible bolting occurs. This suggests that monitoring AcFT2 expression may serve as an early indicator of bolting risk in late-maturing cultivars. We also investigated the effect of seedling size at transplantation and the expression of AcFT2 and found that seedling size at transplantation affected the incidence of bolting, with higher expression of AcFT2 in large seedlings that are more susceptible to bolting.