Auxin distribution and transport during embryonic pattern formation in wheat

Inhibitors of auxin polar transport disrupt normal embryogenesis and thus s pecific spatial auxin distribution due to auxin movement may be important i n establishing embryonic pattern formation in plants. In the present study, the distribution of the photoaffinity labeling agent tritiated 5-azidoindo le-3-acetic acid ([H-3],5-N(3)IAA), an analog of indole-3-acetic acid (IAA) , was visualized in zygotic wheat (Triticum aestivum L.) embryos grown in v itro and in planta, and used to deduce auxin transport pathways in these em bryos. This study provides the first direct evidence that the distribution of auxin, here [H-3],5-N(3)IAA, is heterogeneous and changes during embryo development. In particular, the shift from radial to bilateral symmetry was correlated with a redistribution of [H-3],5-N(3)IAA in the embryo. Further more, in bilaterally symmetrical embryos, that is, embryos in the late tran sition stage or older, the localization of [H-3],5-N(3)IAA was altered by N -1-naphthylphthalamic acid, a specific inhibitor of auxin polar transport. No significant effect was observed in radially symmetrical embryos, that is , globular embryos, or very early transition embryos. Thus, the shift from radial to bilateral symmetry is associated with the onset of active, direct ed auxin transport involved in auxin redistribution. A change in the distri bution of [H-3],5-N(3)IAA was also observed in morphologically abnormal emb ryos induced on media supplemented with auxin or auxin polar transport inhi bitors. By means of a microscale technique, free IAA concentration was meas ured in in vitro- and in planta-grown embryos and was found to increase dur ing development. Therefore, IAA may be synthesized or released from conjuga tes in bilaterally symmetrical embryos, although import from surrounding ti ssues cannot be excluded.