Summary: Extant land plants consist of two deeply divergent groups, tracheophytes and bryophytes, which shared a common ancestor some 500 million years ago. While information about vascular plants and the two of the three lineages of bryophytes, the mosses and liverworts, is steadily accumulating, the biology of hornworts remains poorly explored. Yet, as the sister group to liverworts and mosses, hornworts are critical in understanding the evolution of key land plant traits. Until recently, there was no hornwort model species amenable to systematic experimental investigation, which hampered detailed insight into the molecular biology and genetics of this unique group of land plants. The emerging hornwort model species, Anthoceros agrestis, is instrumental in our efforts to better understand not only hornwort biology but also fundamental questions of land plant evolution. To this end, here we provide an overview of hornwort biology and current research on the model plant A. agrestis to highlight its potential in answering key questions of land plant biology and evolution.

Summary: Despite their key phylogenetic position and their unique biology, hornworts have been widely overlooked. Until recently there was no hornwort model species amenable to systematic experimental investigation.Anthoceros agrestishas been proposed as the model species to study hornwort biology. We have developed anAgrobacterium‐mediated method for the stable transformation ofA. agrestis, a hornwort model species for which a genetic manipulation technique was not yet available. High transformation efficiency was achieved by using thallus tissue grown under low light conditions. We generated a total of 274 transgenicA. agrestislines expressing the β‐glucuronidase (GUS), cyan, green, and yellow fluorescent proteins under control of the CaMV 35S promoter and several endogenous promoters. Nuclear and plasma membrane localization with multiple color fluorescent proteins was also confirmed. The transformation technique described here should pave the way for detailed molecular and genetic studies of hornwort biology, providing much needed insight into the molecular mechanisms underlying symbiosis, carbon‐concentrating mechanism, RNA editing and land plant evolution in general.

SUMMARY: Land plants comprise two large monophyletic lineages, the vascular plants and the bryophytes, which diverged from their most recent common ancestor approximately 480 million years ago. Of the three lineages of bryophytes, only the mosses and the liverworts are systematically investigated, while the hornworts are understudied. Despite their importance for understanding fundamental questions of land plant evolution, they only recently became amenable to experimental investigation, with Anthoceros agrestis being developed as a hornwort model system. Availability of a high‐quality genome assembly and a recently developed genetic transformation technique makes A. agrestis an attractive model species for hornworts. Here we describe an updated and optimized transformation protocol for A. agrestis, which can be successfully used to genetically modify one more strain of A. agrestis and three more hornwort species, Anthoceros punctatus, Leiosporoceros dussii, and Phaeoceros carolinianus. The new transformation method is less laborious, faster, and results in the generation of greatly increased numbers of transformants compared with the previous method. We have also developed a new selection marker for transformation. Finally, we report the development of a set of different cellular localization signal peptides for hornworts providing new tools to better understand the hornwort cell biology.

Premise: A detailed protocol for the protoplast transformation of hornwort tissue is not yet available, limiting molecular biological investigations of these plants and comparative analyses with other bryophytes, which display a gametophyte‐dominant life cycle and are critical to understanding the evolution of key land plant traits.Methods and Results: We describe a detailed protocol to isolate and transiently transform protoplasts of the model hornwort Anthoceros agrestis. The digestion of liquid cultures with Driselase yields a high number of viable protoplasts suitable for polyethylene glycol (PEG)‐mediated transformation. We also report early signs of protoplast regeneration, such as chloroplast division and cell wall reconstitution.Conclusions: This protocol represents a straightforward method for isolating and transforming A. agrestis protoplasts that is less laborious than previously described approaches. In combination with the recently developed stable genome transformation technique, this work further expands the prospects of functional studies in this model hornwort.

Summary: Biophysical CO ₂ concentrating mechanisms (CCMs) operating at the single‐cell level have evolved independently in eukaryotic algae and a single land plant lineage, hornworts. An important component for an efficient eukaryotic CCM is a pyrenoid whose biology is well‐characterized in the unicellular green alga, Chlamydomonas reinhardtii . By contrast, pyrenoids and CCM are little understood in hornworts. Here, we investigate the molecular underpinnings and dynamics of hornwort pyrenoids. We do so by studying the subcellular localization of candidate proteins homologous to essential CCM genes in C. reinhardtii and assessing their mobility kinetics in the hornwort model Anthoceros agrestis Paton. We provide evidence that an essential pyrenoid component 1 analog and the ribulose 1,5‐bisphosphate carboxylase/oxygenase colocalize in the pyrenoid, but pyrenoids seem less dynamic in A. agrestis than in C. reinhardtii . We further found that a carbonic anhydrase homolog localizes to the thylakoid membranes between the pyrenoid subunits, while an limiting CO ₂ ‐inducible B‐like homolog is less intimately linked to the pyrenoid than in C. reinhardtii . Our results imply that the pyrenoid‐based CCM of hornworts is characterized by a mixture of Chlamydomonas ‐like and hornwort‐specific features, which is in line with its independent evolutionary origin. Using these observations, we provide a mechanistic model of hornwort CCM.

Background: While genome evolutionary processes of seed plants are intensively investigated, very little is known about seed-free plants in this respect. Here, we use one of the largest groups of seed-free plants, the mosses, and newly generated chromosome-scale genome assemblies to investigate three poorly known aspects of genome dynamics and their underlying processes in seed-free plants: (i) genome size variation, (ii) genomic collinearity/synteny, and (iii) gene set differentiation. Results: Comparative genomic analyses on the model moss Physcomitrium (Physcomitrella) patens and two genomes of Funaria hygrometrica reveal that, like in seed plants, genome size change (approx. 140 Mbp) is primarily due to transposable element expansion/contraction. Despite 60 million years of divergence, the genomes of P. patens and F. hygrometrica show remarkable chromosomal stability with the majority of homologous genes located in conserved collinear blocks. In addition, both genomes contain a relatively large set of lineage-specific genes with no detectible homologs in the other species’ genome, suggesting a highly dynamic gene space fueled by the process of de novo gene birth and loss rather than by gene family diversification/duplication. Conclusions: These, combined with previous observations suggest that genome dynamics in mosses involves the coexistence of a collinear homologous and a highly dynamic species-specific gene sets. Besides its significance for understanding genome evolution, the presented chromosome-scale genome assemblies will provide a foundation for comparative genomic and functional studies in the Funariaceae, a family holding historical and contemporary model taxa in the evolutionary biology of mosses.