Evolution of the plant body plan.

Abstract

Land plants evolved about 470 million years ago or even earlier, in a biological crust-dominated terrestrial flora. The origin of land plants was probably one of the most significant events in Earth's history, which ultimately contributed to the greening of the terrestrial environment and opened up the way for the diversification of both plant and non-plant lineages. Fossil and phylogenetic evidence suggest that land plants have evolved from fresh-water charophycean algae, which were physiologically, genetically, and developmentally potentiated to make the transition to land. Since all land plants have biphasic life cycles, in contrast to the haplontic life cycle of Charophytes, the evolution of land plants was linked to the origin of a multicellular sporophytic phase. Land plants have evolved complex body plans in a way that overall complexity increased toward the tip of the land plant tree of life. Early forms were unbranched, with terminal sporangia and simple rhizoid rooting structures but without vasculature and leaves. Later on, branched forms with lateral sporangia appeared and paved the route for the evolution for indeterminacy. Finally, leaves and roots evolved to enable efficient nutrient transport to support a large plant body. The fossil record also suggests that almost all plant organs, such as leaves and roots, evolved multiple times independently over the course of land plant evolution. In this review, we summarize the current knowledge on the evolution of the land plant body plan by combining evidence of the fossil record, phylogenetics, and developmental biology.