[en] Contribution of the root system to the flowering process remains poorly studied. Part of the problem resides in its difficult isolation from the substrate, especially on adult plants. We used an hydroponic device that allows synchronous growth and flowering of Arabidopsis and performed global transcript profiling of roots. Samples were harvested during the extension period of a single long day (LD), and in non inductive short day. Microarray data were validated by real-time RT-PCR, and the expression patterns of selected probes were further analyzed in shoots and roots. Some of the genes that were differentially expressed in the roots during the inductive LD did not show the same variations in the shoot, indicating that root transcriptome undergoes specific changes at floral transition. These genes include, for example, GIGANTEA. T-DNA mutants from selected candidate genes are being studied. Both the expression analysis and the reverse genetic approach provide new insights into the contribution of the roots to the flowering process.