Tri-iodobenzoic acid affects shoot apical meristem formation and function in zygotic embryos of Brassica napus cv. Topas

The effect of an auxin transport inhibitor, tri-iodobenzoic acid, on the fo rmation and subsequent function of the shoot apical meristem in zygotic emb ryos of Brassica napus L. was examined. Globular and heart stage embryos we re cultured in the presence of tri-iodobenzoic acid. Only embryos at the gl obular stage of embryogenesis were affected by tri-iodobenzoic acid. Upon t reatment, the embryos did not develop two separate cotyledons and, therefor e, did not attain bilateral symmetry. Furthermore, the shoot apical meriste m of these embryos was altered, as a characteristic tunica-corpus organizat ion was not observed. The cells of the surface layer continued to maintain their meristematic characteristics and divided mainly in the anticlinal dir ection. The subapical cells differentiated into parenchyma cells and contin ued to expand such that a corpus organization never developed. Therefore, b y the end of the culture period, the shoot apical meristem appeared very br oad and shallow. Treated embryos that were transferred to a conversion medi um exhibited a dramatic decline and delay in the formation of leaves compar ed with untreated embryos. Histological observations revealed that the shoo t apical meristem continued to expand in width. A low percentage of treated embryos were able to convert; however, leaves were produced from a new sho ot meristem that developed in the axillary position. No morphological or hi stological changes were evident when the embryos were treated later, at the heart stage of embryogenesis.