Comparison of Cold Responses for Orthologs of Cabbage Vernalization-related Genes

Vernalization, a floral transition stimulated by the prolonged exposure to cold temperatures, is an undesirable feature in the breeding of leafy vegetables that impairs productivity by promoting bolting. Cabbage is categorized as a green plant vernalization type; only older plants possess vernalization competence. Despite being of agricultural importance in the breeding of bolting resistance, the molecular mechanism of age-dependent acquisition of vernalization competence is poorly understood. In Arabidopsis, FLOWERING LOCUS C (FLC) is a key vernalization response gene that functions as a floral repressor. Its expression decreases upon cold exposure, leading to floral induction. In this study, we characterized three FLC orthologs in cabbage (BoFLC1, BoFLC2, and BoFLC3). Predicted amino acid sequences of the three BoFLC cDNAs were highly conserved with the MADS-box domain. The overexpression of the three BoFLCs in Arabidopsis displayed late flowering phenotypes, indicating the functional conservation of these paralogs as floral repressors in cabbage. The expression analyses before and after constant cold treatment given at different plant ages showed that the three BoFLCs displayed distinct responses to prolonged cold exposure. Regardless of plant age, BoFLC1 declined less markedly, whereas BoFLC2 and BoFLC3 were repressed by sufficient long cold treatment. In both younger and older plants, the expression of the ortholog of floral integrator gene, FLOWERING LOCUS T (FT), was induced by cold treatment, probably due to a reduction in the repressive effect of BoFLCs, suggesting that the fate of age-dependent vernalization is determined independently or downstream of BoFT induction. Our results provide new insights into the mechanism of green plant vernalization in Brassica vegetables.