|Thursday, July 08|
Hornwort biology and a new era of possibilities
* Karen Renzaglia, Southern Illinois University Carbondale, United States
Juan Carlos Villarreal , Université Laval
Although the least specious and diverse of flagellated plant groups, the hornworts present shared and unparalleled adaptations to life on land. Plants are thalloid, lack organized appendages such as hairs and leaves, and contain abundant mucilage in specialized cells and internal canals. Chloroplasts are often solitary and typically have pyrenoids, a localized site of RuBisCO for carbon concentration shared with algal streptophytes. Cell division is strictly monoplastidic and plastids serve as the focal points for the mitotic and meiotic the spindles. The cyanobacterial symbiosis is universal in all hornwort taxa and involves the development of internal elongated or globose Nostoc colonies. Gametangia are sequestered in chambers or embedded directly in thallus tissue. Male gametes are biflagellated and demonstrate little variability across hornwort diversity. The sporophyte is a single sporangium that elongates from its base and continually produces sporogenous tissue upward. Stomata are present in most taxa but have been lost in two lineages. The advent of molecular applications and genetic transformation of hornworts opens the way for fundamental and unique biological questions to be addressed based on the intriguing biology of the group.
Cell biology of hornworts
* Masaki Shimamura, Hiroshima University, Japan
The peculiarity of hornworts in land plants also extends to their characteristics at the cellular level. One of the exceptional characters is that the cells of hornworts usually contain only one chloroplast, and the chloroplast often contains pyrenoids composed of aggregated RuBisCo, the CO2 fixing enzyme. During cell division, the chloroplast divides and migrates prior to the division of the nucleus, and nuclear division follows the division axis of chloroplasts. The microtubule system extends from the chloroplast surface is involved in the reliable allotment of chloroplast to daughter cells and the determination of the cell division axis. Such cell division system is different from liverworts, in which the mitotic spindle develops from a centrosome-like structure, and mosses, in which the mitotic spindle develops without a scaffold organelle. In mature cells of hornworts, the outer shape of the chloroplast is not smooth, and many elongated protrusions are often formed. Currently, hornworts have been shown to occupy the most basal position of the bryophyte phylogeny. Therefore, hornwort cells may provide an interesting model for studying the evolution of cellular systems in land plants. I summarize the cell biology of hornworts with particular attention to chloroplasts and a view to future research.
Closing the gaps: Hornwort evolutionary developmental biology
* peter szoevenyi, University of Zurich, Switzerland
Manuel Waller, University of Zurich
Eftychios Frangedakis, University of Cambridge
Anna Neubauer, University of Zurich
Tomoaki Nishiyama, Kanazawa University
Keiko Sakakibara, Rikkyo University
Extant land plants consist of two deeply divergent monophyletic lineages, vascular plants and bryophytes, which shared a common ancestor some 500 million years ago. Land plants have evolved body plans in a way that overall complexity remained low in the bryophytes but reached greater overall complexity in the vascular plants. Comparative evolutionary developmental biology of complex and less complex lineages can help to understand how complexity evolved via the reorganization of gene regulatory networks. While information about vascular plants and the two of the three lineages of bryophytes, the mosses and liverworts, is accumulating, research on the developmental biology of hornworts have been widely neglected. Yet, as the sister group to liverworts and mosses, hornworts are critical in understanding the evolution of key land plant traits. More specifically, inference about the complexity of the common ancestor of land plants is ambiguous. This is, in part, due to the deep divergence of the three groups of bryophytes, as well as bryophytes and vascular plants, that provided ample time for independent gains/losses of genes to occur. Comparison of the developmental, physiological and molecular features of hornworts with those of mosses and liverworts will provide a more accurate picture of the nature of the common ancestor of bryophytes and that of all land plants. It will also help to understand the diversity and molecular basis of evolution and development across bryophytes and vascular plants. To this end, I will introduce the major developmental biological features of the emerging hornwort model system A. agrestis providing critical information on the evolution of key land plant traits. Furthermore, I will present results starting to resolve some questions concerning the evolution of these traits in land plants.
The lab life of the model hornwort Anthoceros agrestis, from spores to transformation
* Eftychios Frangedakis, University of Cambridge, United Kingdom
Manuel Waller, University of Zurich
Péter Szövényi, University of Zurich
Keiko Sakakibara, Rikkyo University
Hornworts are one of the most fascinating land plant groups. 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 a model species to study hornwort biology. I will introduce the emerging hornwort model system A. agrestis with a special focus on the newly developed nuclear transformation technique.
Expanding the genomic and genetic toolkits for hornwort research
* Fay-Wei Li, Boyce Thompson Institute, United States
Andika Gunadi, Boyce Thompson Institute
Peter Schafran, Boyce Thompson Institute
Tanner Robison, Boyce Thompson Institute
Hornworts have an array of unique features that can help illuminate not only the early evolution of land plants, but also the alternative paths for nitrogen and carbon assimilation (respectively via cyanobacterial symbiosis and a carbon-concentrating mechanism). Despite this, hornworts are also one of the few plant lineages having only limited genomic and genetic resources available. To address this issue, we have assembled eight new high-quality genomes from across the hornwort phylogeny, covering all the families and most of the genera. Importantly, these genomes represent a diverse trait combination—including presence/absence of pyrenoids, stomata, and sex chromosomes—and will form the basis for future comparative studies. Furthermore, a biolistic-mediated transformation has been developed that can effectively introduce genetic elements into the model hornwort Anthoceros agrestis, as well as a few other species. Building upon these new resources and tools, we are investigating the genetics of hornworts’ carbon-concentrating mechanism, and our preliminary findings will be presented here. We anticipate that enabling genetic research in hornworts will not only complement the other two bryophyte models (the moss Physcomitrium patens and the liverwort Marchantia polymorpha), but will also bring unprecedented opportunities to study a distinct plant with unparalleled biological properties.
Differential gene expression in the hornwort Anthoceros punctatus during establishment of its nitrogen-fixing symbiosis with the cyanobacterium Nostoc punctiforme
* Poulami Chatterjee, University of California, Davis, United States
Peter Schafran, Boyce Thomson Institute
Fay-Wei Li, Boyce Thomson Institute
John C Meeks, University of California, Davis
Endosymbiotic associations between hornworts and nitrogen-fixing cyanobacteria form when the plant partner is limited for combined nitrogen. We examined the patterns of differential gene expression by RNA-Seq during culture of Anthoceros punctatus in the absence of combined nitrogen and absence and presence of the model symbiotic cyano¬bacterium Nostoc punctiforme in a 6-point time-course. The raw sequencing reads were analyzed via the MaSigPro algorithm yielding 1,448 genes that were significantly differentially expressed at a P < 0.05. This data set was subjected to cluster analysis with a 9-cluster output. In 5 of the clusters, transcripts from A. punctatus nitrogen-starved in the absence of N. punctiforme increased in expression within 2 d of incubation, relative to the time zero control, and either remained elevated (2 cases) or declined at different rates; in the remaining 4 clusters, transcription declined. Co-culture with N. punctiforme resulted in either increased (3 cases) or decreased (2 cases) transcription by A. punctatus in a reciprocal manner, relative to the absence of N. punctiforme. Conversely, the presence of N. punctiforme had no substantial effect on the patterns of transcription in 4 of the clusters, relative to its absence. Functional assignments of the protein products of differentially expressed genes have been compiled and will be discussed. We conclude that a major adaptation to nitrogen starvation in A. punctatus is to decrease photosynthetic light harvesting capacity in order to minimize production of reductant and this transcriptional response can be ameliorated by dinitrogen-derived ammonium supplied endophytically by N. punctiforme.