Bisymmetry in the embryonic root is dependent on cotyledon number and position.

In the shoot pole of Arabidopsis embryos, radial symmetry is broken by cotyledon specification. Subsequently, the radial pattern of the embryo axis is converted to bisymmetric. In a recent publication, we showed that distinct boundaries of hormonal signalling output specify the vascular pattern in the root meristem through a mutually inhibitory feedback loop between the hormones auxin and cytokinin. We observed that during embryogenesis, symmetry breakage in the root pole coincided with an influx of auxin from the cotyledons. In this manuscript, we provide genetic data to support the role of the cotyledons in initiating symmetry breaking in the embryonic root pole. Mutants with alterations in cotyledon number fail to establish bisymmetry in the embryo axis. These data further support the idea that input from the cotyledons may be required for the propagation of bisymmetry from the cotyledons to the embryonic root.