Arabidopsis embryonic development: A screen for mutants disrupted in pattern formation and analysis of bobber, a gene required for proper specification of the apical region of the Arabidopsis embryo

Mutants defective in establishing fundamental patterns of gene expression during Arabidopsis embryogenesis have proven difficult to find. This may be because such mutants arrest during embryogenesis. To test this idea, we have screened among embryo arrest mutants for those that either ectopically express or fail to express a SHOOTMERISTEMLESS (STM) reporter gene. Beginning at the late globular stage of embryogenesis, this reporter marks a small group of cells that will become the shoot apical meristem; most pattern defects are expected to alter the placement or development of this domain. Several of the mutants found in the screen, including bobber, tilted axis, blueberry1,2 and 3, and button mushroom1 and 2 hold great promise for elucidating specific aspects of embryonic development. The success of this screen has proven the utility of this type of forward genetic, information-driven approach for the investigation of plant development.; bobber is an embryo defective mutant of Arabidopsis that fails to establish domains of tissue identity, properly in the apical portion of the developing embryo. bobber embryos express SHOOTMERISTEMLESS (STM), a meristem specific mRNA, in areas of the globular stage embryo that should be set aside for cotyledon fate and free of STM expression. It fails to develop past the globular stage of embryogenesis. Examination of the expression of other markers of embryonic development in bobber embryos reveals that the program for the establishment of radial and bilateral symmetry in the apical region is specifically disrupted. bobber embryos are able to achieve some hallmarks of pattern acquisition, as revealed by the proper establishment of the protoderm layer. The bobber embryo represents an uncoupling of these two programs of pattern acquisition. The BOBBER gene has been molecularly cloned and identified as a homolog of the NUDC gene of Aspergillus nidulans, a gene involved in the movement of nuclei. The fact that a disruption in this type of gene can cause misassignment of tissue fate in the apical portion of the Arabidopsis embryo has important implications for the study of mechanisms involved in the establishment of domains of identity in the developing plant embryo.