Microamputation of somatic embryos of the domestic carrot reveals apical control of axis elongation and root regeneration

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Abstract
Somatic heart- and torpedo-stage embryos of the domesticated carrot, Daucus carota L., were severed at their midlengths to produce two halves termed apical and basal pieces. These pieces may be grafted or kept separate. Grafted embryos developed normally, with the exception that they tended to mature earlier than uncut control embryos. If kept separate, the apices grew at rates similar to grafted apices, while the basal ends, behaving as if they had been released from an inhibition of growth, rapidly elongated and matured (e.g. produced root hairs and a root cap) 3–4 days earlier than uncut controls. Grafted embryos treated with the transport inhibitor TIBA (2,3,5triiodobenzoic acid) had basal sections that behaved as if the sections had been kept separate. Additionally, resupplying IAA (indole-3-acetic acid) via a novel wick-bridge forced isolated basal pieces to behave as if the embryo apex were present. This apparent inhibition of root growth by the apex appears to be controlled by either the polar transport of auxin, and/or the accumulation of auxin at the root end. These experiments suggest that polar auxin transport has a greater influence on root, rather than on apex, development in these embryos.