Delay in Flowering Time in Arabidopsis thaliana Col-0 Under Water Deficit and in the ddc Triple Methylation Knockout Mutant Is Correlated with Shared Overexpression of BBX16 and BBX17

This study addresses the role of cytosine methylation in the fine-tuning of flowering time under water deficit in <i>Arabidopsis thaliana.</i> A <i>drm1 drm2 cmt3 (ddc)</i> triple methylation mutant was used together with the Col-0 wild type. The plants were grown under long-day conditions with water deficit induced by cessation of watering starting 12 days after seeding. Col-0 showed a 1-day delay in flowering as a result of the treatment. In contrast, <i>ddc</i> showed a 2-day delay regardless of the experimental conditions. We found that the two b-box domain proteins, BBX16/COL7 and BBX17/COL8, became overexpressed in the <i>ddc</i> background and in Col-0 under water deficit 24 days after seeding. Additionally, the NF-YA2 transcription factor became correspondingly down-regulated. Our results support a model where BBX16/COL7 and BBX17/COL8 interact with CONSTANS to delay the induction of <i>FT</i> under long-day conditions. <i>NF-YA2</i>, which is also recognized as a promoter of <i>FT</i> expression, with its down-regulation causes additional delay of <i>FT</i>-induced flowering. The plants overcome the BBX/NF-YA inhibition easily, resulting in a relatively small delay in flowering. The expression patterns of the three genes suggest the involvement of cytosine methylation in their regulation; however, no differential methylation could be found in <i>cis</i> that can explain these effects. The results therefore suggest a <i>trans</i> acting mechanism. Considering that the activities of <i>BBX16/COL7</i> and <i>BBX17/COL8</i> in different physiological conditions are not elucidated, this paper provides a background for future experiments targeting the role of these genes in the fine-tuning of flowering time in <i>A. thaliana</i>.