IAA accumulation and ZmPIN1 and BETL1 transcript localization in de*-B18 endosperm transfer layer cells. Longitudinal sections of kernels at 12 DAP of B73 (A–C) and de*-B18 (D–F) were labeled with an antisense mRNA probe that hybridizes to all three ZmPIN1 transcripts (A and D) and with an antisense BETL1 probe (B and E; Hueros et al., 1995). Anti-IAA monoclonal antibody was used to determine the auxin distribution and accumulation in developing BETL cells (C and F). Unlike in B73 wild-type kernels (A), de*-B18 maize mutant kernels lack ZmPIN1 expression at the level of BETL cells (D) and show an altered differentiation pattern of these cells, which are round shaped and not polarized (D). Compared with the wild type (B), also the BETL1 gene is not expressed in mutant transfer and conductive cells (E). These alterations are accompanied by a reduction of auxin accumulation in BETL cells (C and F), while IAA accumulates as in the wild type in the chalazal maternal region. cha, Chalaza; se, starchy endosperm. Bars = 100 μ m in B, C, E, and F; and 50 μ m in A and D.