Wall ultrastructure of the oldest embryophytic spores: Implications for early land plant evolution

A subclass II bHLH transcription factor in Marchantia polymorpha gives insight into the ancestral land plant trait of spore formation

Sporopollenin is often said to be one of the toughest biopolymers known to man. The shift in dormancy cell wall deposition from around the diploid zygotes of charophycean algae to sporopollenin around the haploid spores of land plants essentially imparted onto land plants the gift of passive motility, a key acquisition that contributed to their vast and successful colonization across terrestrial habitats.1,2 A putative transcription factor controlling the land plant mode of sporopollenin deposition is the subclass II bHLHs, which are conserved and novel to land plants, with mutants of genes in angiosperms and mosses divulging roles relating to tapetum degeneration and spore development.3,4,5,6,7 We demonstrate that a subclass II bHLH gene, MpbHLH37, regulates sporopollenin biosynthesis and deposition in the model liverwort Marchantia polymorpha. Mpbhlh37 sporophytes show a striking loss of secondary wall deposits of the capsule wall, the elaters, and the spore exine, all while maintaining spore viability, identifying MpbHLH37 as a master regulator of secondary wall deposits of the sporophyte. Localization of MpbHLH37 to the capsule wall and elaters of the sporophyte directly designates these tissue types as a bona fide tapetum in liverworts, giving support to the notion that the presence of a tapetum is an ancestral land plant trait. Finally, as early land plant spore walls exhibit evidence of tapetal deposition,8,9,10,11,12 a tapetal capsule wall could have provided these plants with a developmental mechanism for sporopollenin deposition.

Plant evolution: A tapetum is now effectively present in all land plant lineages

The tapetum, a tissue that elsewhere ensures correct spore development, is missing in some bryophytes. A new study shows that, in the liverwort, Marchantia polymorpha, a gene controlling spore wall deposition is expressed in the capsule lining, so these cells essentially function as a tapetum.

Green land: Multiple perspectives on green algal evolution and the earliest land plants

Green plants, broadly defined as green algae and the land plants (together, Viridiplantae), constitute the primary eukaryotic lineage that successfully colonized Earth's emergent landscape. Members of various clades of green plants have independently made the transition from fully aquatic to subaerial habitats many times throughout Earth's history. The transition, from unicells or simple filaments to complex multicellular plant bodies with functionally differentiated tissues and organs, was accompanied by innovations built upon a genetic and phenotypic toolkit that have served aquatic green phototrophs successfully for at least a billion years. These innovations opened an enormous array of new, drier places to live on the planet and resulted in a huge diversity of land plants that have dominated terrestrial ecosystems over the past 500 million years. This review examines the greening of the land from several perspectives, from paleontology to phylogenomics, to water stress responses and the genetic toolkit shared by green algae and plants, to the genomic evolution of the sporophyte generation. We summarize advances on disparate fronts in elucidating this important event in the evolution of the biosphere and the lacunae in our understanding of it. We present the process not as a step-by-step advancement from primitive green cells to an inevitable success of embryophytes, but rather as a process of adaptations and exaptations that allowed multiple clades of green plants, with various combinations of morphological and physiological terrestrialized traits, to become diverse and successful inhabitants of the land habitats of Earth.

The origin of a land flora

The origin of a land flora fundamentally shifted the course of evolution of life on earth, facilitating terrestrialization of other eukaryotic lineages and altering the planet's geology, from changing atmospheric and hydrological cycles to transforming continental erosion processes. Despite algal lineages inhabiting the terrestrial environment for a considerable preceding period, they failed to evolve complex multicellularity necessary to conquer the land. About 470 million years ago, one lineage of charophycean alga evolved complex multicellularity via developmental innovations in both haploid and diploid generations and became land plants (embryophytes), which rapidly diversified to dominate most terrestrial habitats. Genome sequences have provided unprecedented insights into the genetic and genomic bases for embryophyte origins, with some embryophyte-specific genes being associated with the evolution of key developmental or physiological attributes, such as meristems, rhizoids and the ability to form mycorrhizal associations. However, based on the fossil record, the evolution of the defining feature of embryophytes, the embryo, and consequently the sporangium that provided a reproductive advantage, may have been most critical in their rise to dominance. The long timeframe and singularity of a land flora were perhaps due to the stepwise assembly of a large constellation of genetic innovations required to conquer the terrestrial environment.

A fossil record of land plant origins from charophyte algae

Molecular time trees indicating that embryophytes originated around 500 million years ago (Ma) during the Cambrian are at odds with the record of fossil plants, which first appear in the mid-Silurian almost 80 million years later. This time gap has been attributed to a missing fossil plant record, but that attribution belies the case for fossil spores. Here, we describe a Tremadocian (Early Ordovician, about 480 Ma) assemblage with elements of both Cambrian and younger embryophyte spores that provides a new level of evolutionary continuity between embryophytes and their algal ancestors. This finding suggests that the molecular phylogenetic signal retains a latent evolutionary history of the acquisition of the embryophytic developmental genome, a history that perhaps began during Ediacaran-Cambrian time but was not completed until the mid-Silurian (about 430 Ma).