Cold signalling associated with vernalization in Arabidopsis thaliana does not involve CBF1 or abscisic acid

In genotypes of Arabidopsis that exhibit a winter-annual flowering habit, floral induction in response to extended cold exposure (vernalization) is mediated by repression of the flowering-inhibitor gene FLC. We are interested in identifying components of the cold signal transduction pathway leading to FLC repression. We examined the potential involvement of two factors that are known to play roles in plant cold responses: (1) CBF1, a cold-responsive transcription factor that is involved in activating the cold acclimation response, and (2) the phytohormone abscisic acid (ABA), which has traditionally been associated with plant cold responses. We introduced a transgene driving constitutive expression of CBF1 into a winter-annual genotype of Arabidopsis. In transgenic lines expressing CBF1 mRNA to high levels, FLC mRNA expression was not repressed, and flowering was not accelerated relative to control plants. We also introduced mutations that compromise ABA biosynthesis or sensitivity into a winter-annual genotype and found that the vernalization response was not affected. Finally, we found that presumed increases in ABA levels, as a result of direct application of the hormone or severe water stress, were insufficient to substitute for cold to induce flowering. Taken together, these findings indicate that vernalization involves a pathway that is distinct from cold-response mechanisms involving CBF1, cold-regulated genes under CBF1 control, and ABA.