The architecture of flowering plants shows an enormous heterogeneity. This variability is, to a large extent, caused by the different branching patterns of the vegetative and flowering shoots. Side-shoots originate from secondary meristems, which are formed in the axils of leaves. In tomato and in Arabidopsis, the gene LATERAL SUPPRESSOR (Ls/LAS)
encodes a transcription factor controlling the initiation of axillary meristems during vegetative development. The gene is expressed in a narrow domain of the leaf axil and conditions the competence of the meristem founder cells. Transcription factors of the MYB
class are also required for axillary meristem formation in specific zones along the shoot axis. Furthermore, plant architecture is strongly influenced by differential outgrowth of the axillary buds. Recently, it has been demonstrated that, in addition to the archetypical plant hormones auxin and cytokinin, bud outgrowth is regulated by a new root derived signal.