INFLUENCE OF AUXIN ON THE ESTABLISHMENT OF BILATERAL SYMMETRY IN MONOCOTS

To study the influence of auxin on the shift from radial to bilateral symmetry during monocot embryogenesis, the fate of young wheat (Tritic om aestivum L.) zygotic embryos has been manipulated in vitro by addin g auxins, an auxin transport inhibitor and an auxin antagonist to the culture medium. The two synthetic auxins used, 2,4-dichlorophenoxyacet ic acid (2,4-D) and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T), induc ed identical phenotypes. In the most severe cases, the shift from radi al to bilateral symmetry was blocked resulting in continuous uniform r adial growth. The natural auxin indole-3-acetic acid (IAA) induced the same phenotype. The effect of 2,4,5-T and 2,4-D depended on their con centrations and on the developmental stage of the isolated embryos. In the presence of 2,3,5-triiodobenzoic acid (TIBA), an auxin transport inhibitor, the overall embryo symmetry was abnormal. The relative posi tion of the shoot apical meristem in comparison with the scutellum was anomalous. The quality of shoot apical meristem and the scutellum dif ferentiation was altered compared with normal developed embryos. No ro ot meristem was differentiated. The effect of TIBA depends on its conc entration and on the developmental stage of the isolated embryos. By c ontrast, 2-(p-chlorophenoxy)-2-methylpropionic acid (PCIB) which is de scribed as an auxin antagonist, has no visible direct effect on the em bryonic symmetry. These observations indicate that auxin influences th e change from radial symmetry to embryonic polarity during monocot emb ryogenesis. A model of auxin action during early wheat embryo developm ent is proposed.