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 formation and subsequent function of the shoot apical meristem in zygotic embryos of Brassica napus L. was examined. Globular and heart stage embryos were cultured in the presence of tri-iodobenzoic acid. Only embryos at the globular stage of embryogenesis were affected by tri-iodobenzoic acid. Upon treatment, the embryos did not develop two separate cotyledons and, therefore, did not attain bilateral symmetry. Furthermore, the shoot apical meristem of these embryos was altered, as a characteristic tunica-corpus organization was not observed. The cells of the surface layer continued to maintain their meristematic characteristics and divided mainly in the anticlinal direction. The subapical cells differentiated into parenchyma cells and continued to expand such that a corpus organization never developed. Therefore, by the end of the culture period, the shoot apical meristem appeared very broad and shallow. Treated embryos that were transferred to a conversion medium exhibited a dramatic decline and delay in the formation of leaves compared with untreated embryos. Histological observations revealed that the shoot apical meristem continued to expand in width. A low percentage of treated embryos were able to convert; however, leaves were produced from a new shoot meristem that developed in the axillary position. No morphological or histological changes were evident when the embryos were treated later, at the heart stage of embryogenesis.Key words: auxin, Brassica napus, embryo, shoot apical meristem, tri-iodobenzoic acid.