Isolation of ethyl methanesulfonate-induced gametophytic mutants in Arabidopsis thaliana by a segregation distortion assay using the multimarker chromosome 1.

The life cycle of plants comprises two alternating generations, the diploid sporophyte (spore-bearing plant) and the haploid gametophyte (gamete-bearing plant). In contrast to animals, the postmeiotic cells give rise to haploid organisms whose function is to produce the gametes and to mediate fertilization. Analysis of gametophyte development and function has been hampered by the difficulty of identifying haplo-phase-specific mutants in conventional mutagenesis screens. Here we use a genetic strategy that is based on segregation distortion of nearby visible markers to screen for EMS-induced gametophytic mutants in Arabidopsis thaliana. Using the multiple marker chromosome mm1 we have isolated seven lines that displayed an altered segregation of markers. Reciprocal backcrosses of these lines showed a marked reduction of the transmission of the male and/or female gametes. Phenotypic analysis revealed that different aspects of either gametophytic development or function were affected. Three male gametophytic lines showed specific arrests during pollen development. One male gametophytic line was specifically defective in pollen tube elongation. Three gametophytic lines showed variable defects in both male and female gametophytic development.

Cell morphogenesis in Arabidopsis.

Cell morphogenesis encompasses all processes required to establish a three-dimensional cell shape. Cells acquire the architecture specific to their developmental context by using the spatial information provided by internal or external cues. As a response to these signals, cells become reorganized and establish functionally distinct subcellular domains that ultimately lead to morphological changes. In its simplest form, cell morphogenesis results in the establishment of asymmetry along one axis, a cell polarity. Although cell polarity has been studied intensively in budding yeast and epithelial cells, little is known about more complex modes of cell morphogenesis involving multiple axes. In this review we compare the regulation of cell morphogenesis of different genetically well-characterized cell types in Arabidopsis thaliana.

Epidermal fate map of the Arabidopsis shoot meristem.

The shoot meristem generates all of the aerial structures of an adult plant. It is organized in three layers which produce the epidermis (L1 layer) and the subepidermal layers (L2 and L3). The origin of adult structures has previously been fate mapped to the primary meristem for L2 and L3 tissues. In this work we constructed a fate map of L1 cells in the embryonic shoot meristem. Using the trichome mutation stichel as an epidermal marker, we analyzed 153 plants that included 178 sectors. Sectors on early leaves were found to be smaller and occurred more frequently than those on late leaves. Sectors on late leaves also appeared often to affect more than one leaf. In general, the width and extent of sectors were found to be variable rather than cell lineage-restricted. Our analysis allowed us to assign the most likely fates of L1 precursor cells within the embryonic shoot meristem. The results suggest that the meristem integrates growth dynamics and patterning of all three tissues. In contrast to this coordinated growth behavior of meristematic cells, we found a difference in the lineage restrictions between the L1 and the L2 for the formation of axillary buds.