In order to assess the relationship between ﬂowering time control and roots, ﬂowering time mutants and mu- tants isolated from the reverse genetic approach presented above were characterized for root architecture (ﬁg. 3a). Unexpectedly, we observed that the late ﬂowering constans (co) mutant had a striking root phenotype (ﬁg. 3b) although we did not detect expression of the CO gene in the root (data not shown). By contrast, the svp mutant did not display any root phenotype albeit the gene was highly expressed throughout the roots (ﬁg. 2c). The dormancy asso- ciated2 mutant had a signiﬁcant phenotype, including longer primary and secondary roots and an increased lateral root density. The same observations stand for the ipt3;5;7 triple mutant but not for the ipt3 single mutant. Because many ﬂowering time genes were found to be expressed in the roots, we analyzed the changes in gene expression occurring in the roots during the induction of ﬂowering. To do so, 7-week old vegetative plants grown in short days (SD) were induced to ﬂower by exposure to a single 22-h long day (LD). Roots were harvested during the photoperiod extension for transcriptomic analysis. We found that hundreds of genes were diﬀerentially expressed in the roots in LD vs SD (ﬁg. 2a). The biological signiﬁcance of these changes was addressed by recording whether loss of function mutants had a ﬂowering phenotype. Interestingly the cytokinin-biosynthesis genes IPT3, 5 & 7 (IPT; ISOPENTENYL TRANSFERASE) were all up-regulated in the roots during the during the induction of ﬂowering (ﬁg. 2b). According to the root map published by Brady et al. (2007), the expression of IPT3 and IPT5 is restricted to some tissues of the roots (ﬁg. 2c). We also looked at the expression pattern of well-characterized ﬂowering time genes detected in roots (ﬁg. 1b,c). Among them, we ob- served that SHORT VEGETATIVE PHASE (SVP) is expressed throughout the roots (ﬁg. 2c). FLORAL INDUCTION IMPACTS THE ROOT TRANSCRIPTOME FLOWERING TIME GENES ARE EXPRESSED IN ROOTS Plant Physiology ULg FLOWERING GOES UNDERGROUND F. Bouché, G. Lobet, M. D’Aloia, N. Detry, P. Tocquin, C. Périlleux. Laboratory of Plant Physiology, University of Liège, B22, Sart Tilman, B-4000 Liège, Belgium Corresponding author: f.bouche@ulg.ac.be In conclusion, complementary approaches based on published expression data, transcriptomic experiments and genetic analyses indicate that genes involved in ﬂowering time control are expressed in the roots and/or aﬀect root architecture. Further experiments are on the way to downregulate ﬂowering time genes in the roots by overexpressing artiﬁcial microRNAs in an organ-targeted manner. Acknowledgements : FB is grateful to the F.R.S.-FNRS for the award of a PhD fellowship. We thank Pr. Tatsuo Kakimoto (Osaka University, Japan) who kindly provided us with ipt3 and ipt3;5;7 seeds. This research is funded by the Interuniversity Attraction Poles Programme P7/29 initiated by the Belgian Science Policy Oﬃce. The genetic control of ﬂowering is complex : deepest knowledge has been accumulated on Arabidopsis where we identiﬁed 278 ﬂowering time genes from a littera- ture survey. We then crossed the list with transcriptomic data obtained from roots and found that 98 ﬂowering time genes were expressed in the roots of Arabidopsis plants grown in hydroponics, on soil, or in vitro (ﬁg. 1a,b). Among them, genes involved in photoperiodism or light perception were well represented whereas others – e.g. genes involved in ﬂower development - were not (ﬁg. 1c). References : Brady, S.M. et al., 2007. A high-resolution root spatiotemporal map reveals dominant expression patterns. Science, 318 ( 5851), 801-6. Lobet, G. et al., 2011. Novel Image Analysis Toolbox Enabling Quantitative Analysis of Root System Architecture. Plant Physiology,157, 29-39. AT5G18840 Figure 1. Genes controlling ﬂowering time in Arabidopsis are expressed in the roots. (a) Root transcriptomic data were obtained from plants grown in hydropo