Plant cells are surrounded by rigid cell walls that prevent cell movement. Precisely oriented cell division is especially important in the absence of cell migration. When a plant cell divides, a new wall forms between the daughter cells; this permanently delineates their relative positions. As a consequence, the selection of the division plane is crucial for plant tissue organization and overall organ architecture.
The selection of the division plane is unique in plants compared with other organisms (for a detailed review of plant cytokinesis see Jürgens 2005
). In budding yeast, landmarks at previous bud-sites position the division plane (see Slaughter et al. 2009
), whereas in fission yeast it is positioned by the nucleus. The position of the centrosomes and the mitotic spindle determine division plane orientation in animal cells (Balasubramanian et al. 2004
; see Munro and Bowerman 2009
). In plants, the division plane is positioned by a ring of microtubules and F-actin, called the preprophase band that forms at the cell periphery (Mineyuki 1999
). Despite these striking differences, similarities are beginning to emerge. For example, in budding yeast and animals, GTPase activation plays an important role in positioning division planes (Balasubramanian et al. 2004
; see McCaffrey and Macara 2009
; Slaughter et al. 2009
; Munro and Bowerman 2009
; Prehoda 2009
). In plants, a GTPase was recently found to be localized at the preprophase band and may provide spatial information during division (Xu et al. 2008
). Therefore, common molecular mechanisms may underlie division plane selection in plants and animals.
In all organisms, asymmetric cell division is a common method to obtain distinct cell types from a single progenitor cell. Before an asymmetric division a cell must first establish an internal asymmetry; this process is termed breaking symmetry. Breaking symmetry can be generally defined as the asymmetric organization of cellular components along one axis leading to cellular polarization as a prerequisite to division. An asymmetric cell division can generate two cells of different morphology (i.e., size and/or shape) or different specialized functions or fates. Specifically, the products of asymmetric cell divisions fall into two major categories. In one case, a mother cell can divide asymmetrically to produce a daughter cell to replace it and a daughter cell with a distinct fate, as in the case of stem cell divisions. Alternatively, a mother cell can divide asymmetrically to produce two distinct daughter cells at the expense of mother cell identity. Following division, there are two basic mechanisms by which a mother cell generates daughters that are distinct from itself. One mechanism is intrinsic to the cell and is defined by segregation of cellular determinants before division. The other mechanism is extrinsic to the cell and is defined by external cues that direct daughter cell fate (Horvitz and Herskowitz 1992
). Intrinsic and extrinsic mechanisms are not necessarily independent and often work in combination to direct asymmetric division and specification of daughter cells.
The first zygotic division of fucoid brown algae, such as Fucus
, is one of the earliest studied examples of symmetry breaking in plants regulated by both extrinsic and intrinsic mechanisms. Fucus
zygotes are an excellent organism in which to examine the cellular biology underlying asymmetric cell division (for a recent review see Homble and Leonetti 2007
). In the presence of extrinsic cues, such as light, the asymmetric zygotic division is oriented perpendicular to the light gradient so that one cell resides on the “shady” side of the gradient (Kropf 1992
; Alessa and Kropf 1999
). In the absence of environmental cues, the division is oriented relative to the sperm entry site and therefore appears random within a field of cells (Hable and Kropf 2000
). This indicates that Fucus
zygotes have an intrinsic mechanism to break symmetry in the absence of extrinsic cues. This intrinsic mechanism occurs rapidly after fertilization, however it can later be overridden by extrinsic cues. Although much is known about the role of the cytoskeleton, membrane depolarization, and ion flux in Fucus
embryo polarization, the molecular components regulating these events remain unknown because of a lack of molecular tools. To address the molecular mechanisms regulating asymmetric cell divisions in plants, the use of a more genetically tractable plant is necessary.
In the model plant Arabidopsis thaliana, many genes that are involved in asymmetric cell division have been identified. Interestingly, many of these genes are thought to function in both asymmetric division and subsequent cell fate specification suggesting that, in plants, these processes are tightly linked. Despite these advances, relatively little is known about the cellular processes of breaking symmetry in plants. Because of these two caveats, the term breaking symmetry must be defined more generally in plants. We use this term to describe a process in which a cell divides asymmetrically to form a daughter cell with a distinct fate. Here we discuss three examples in which the molecular mechanisms that participate in symmetry-breaking events in plants are beginning to be elucidated. These examples occur in early embryogenesis, formation of stomata, and root development (). We confine the topics of this article to Arabidopsis thaliana because the majority of the experimental work, particularly using molecular genetics, is focused on this species.
Figure 1. An Arabidopsis plant. Schematic of an adult plant depicting roots, leaves, stems, and flowers. In this article, three symmetry-breaking events are discussed. These examples are taken from different organs of the plant. The first example is taken from (more ...)