Embryogenesis in Dicotyledonous Plants

Convergent occurrence of the developmental hourglass in plant and animal embryogenesis?

BACKGROUND

The remarkable similarity of animal embryos at particular stages of development led to the proposal of a developmental hourglass. In this model, early events in development are less conserved across species but lead to a highly conserved 'phylotypic period'. Beyond this stage, the model suggests that development once again becomes less conserved, leading to the diversity of forms. Recent comparative studies of gene expression in animal groups have provided strong support for the hourglass model. How and why might such an hourglass pattern be generated? More importantly, how might early acting events in development evolve while still maintaining a later conserved stage?

SCOPE

The discovery that an hourglass pattern may also exist in the embryogenesis of plants provides comparative data that may help us explain this phenomenon. Whether the developmental hourglass occurs in plants, and what this means for our understanding of embryogenesis in plants and animals is discussed. Models by which conserved early-acting genes might change their functional role in the evolution of gene networks, how networks buffer these changes, and how that might constrain, or confer diversity, of the body plan are also discused.

CONCLUSIONS

Evidence of a morphological and molecular hourglass in plant and animal embryogenesis suggests convergent evolution. This convergence is likely due to developmental constraints imposed upon embryogenesis by the need to produce a viable embryo with an established body plan, controlled by the architecture of the underlying gene regulatory networks. As the body plan is largely laid down during the middle phases of embryo development in plants and animals, then it is perhaps not surprising this stage represents the narrow waist of the hourglass where the gene regulatory networks are the oldest and most robust and integrated, limiting species diversity and constraining morphological space.

Live-cell imaging and optical manipulation of Arabidopsis early embryogenesis

Intercellular communications are essential for cell proliferation and differentiation during plant embryogenesis. However, analysis of intercellular communications in living material in real time is difficult owing to the restricted accessibility of the embryo within the flower. We established a live-embryo imaging system to visualize cell division and cell fate specification in Arabidopsis thaliana from zygote division in real time. We generated a cell-division lineage tree for early embryogenesis in Arabidopsis. Lineage analysis showed that both the direction and time course of cell division between sister cells differed along the apical-basal or radial axes. Using the Arabidopsis kpl mutant, in which single-fertilization events are frequent, we showed that endosperm development is not required for pattern formation during early embryogenesis. Optical manipulation demonstrated that damage to the embryo initial cell induces cell fate conversion of the suspensor cell to compensate for the disrupted embryo initial cell even after cell fate is specified.

Arabidopsis embryogenesis

Embryogenesis in higher plants consists of two major phases, morphogenesis and maturation. Morphogenesis involves the establishment of the embryo's body plan, whereas maturation involves cell expansion and accumulation of storage macromolecules to prepare for desiccation, germination and early seedling growth. Arabidopsis mutants showing defects in embryogenesis have provided information for understanding the events that govern embryo formation through molecular, genetic and biochemical analyses. Thus, many of the processes that underlie embryogenesis are beginning to be understood. In this chapter, we focus on genes that play key roles in the morphogenesis phase of Arabidopsis embryogenesis.

Signaling in plant embryogenesis

Embryogenesis is a critical stage of the sporophytic life cycle during which the basic body plan of the plant is established. Although positional information is implicated to play a major role in determining embryo cell fate, little is known about the nature of positional signals. Recent studies show that the monopterous and hobbit mutations reveal signaling during patterning of the embryonic axis. The LEAFY COTYLEDON1 and PICKLE genes have been implicated to play important roles in controlling embryo development.