Ephedra as a gymnosperm evo-devo model lineage

The core microbiota across the green lineage

The study of plant-microbe interactions and the characterization of plant-associated microbiota has been the focus of plant researchers in the last decades due to its importance for plant health in natural conditions. Here, I explore the persistent core microbiota associated with different plant species and across different environments by performing a meta-analysis of publicly available datasets. Intra-specific analyses revealed that diverse plant genotypes growing in similar habitats interact with a common set of microbial groups but that some of these core groups are species- or environment-specific. Furthermore, interspecific meta-analysis demonstrates the conservation of seven bacterial orders across diverse photosynthetic organisms, including microalgae, suggesting a conserved capacity for interaction with these core microbes throughout evolutionary history. However, the specific functions of these core members and whether these functions are conserved across hosts remain largely unexplored. I therefore discuss the importance of understanding the roles of the core microbiota and propose future research directions, including the exploration of microbial interactions across different kingdoms. By investigating the core microbiota and its functions, it will be possible to leverage this knowledge for sustainable agricultural management and conservation goals.

Cutting the long branches: Consilience as a path to unearth the evolutionary history of Gnetales

The Gnetales are one of the most fascinating groups within seed plants. Although the advent of molecular phylogenetics has generated some confidence in their phylogenetic placement of Gnetales within seed plants, their macroevolutionary history still presents many unknowns. Here, we review the reasons for such unknowns, and we focus the discussion on the presence of “long branches” both in their molecular and morphological history. The increased rate of molecular evolution and genome instability as well as the numerous unique traits (both reproductive and vegetative) in the Gnetales have been obstacles to a better understanding of their evolution. Moreover, the fossil record of the Gnetales, though relatively rich, has not yet been properly reviewed and investigated using a phylogenetic framework. Despite these apparent blocks to progress we identify new avenues to enable us to move forward. We suggest that a consilience approach, involving different disciplines such as developmental genetics, paleobotany, molecular phylogenetics, and traditional anatomy and morphology might help to “break” these long branches, leading to a deeper understanding of this mysterious group of plants.

How was apical growth regulated in the ancestral land plant? Insights from the development of non-seed plants

Land plant life cycles are separated into distinct haploid gametophyte and diploid sporophyte stages. Indeterminate apical growth evolved independently in bryophyte (moss, liverwort, and hornwort) and fern gametophytes, and tracheophyte (vascular plant) sporophytes. The extent to which apical growth in tracheophytes co-opted conserved gametophytic gene networks, or exploited ancestral sporophytic networks, is a long-standing question in plant evolution. The recent phylogenetic confirmation of bryophytes and tracheophytes as sister groups has led to a reassessment of the nature of the ancestral land plant. Here, we review developmental genetic studies of apical regulators and speculate on their likely evolutionary history.

Fleshy or dry: transcriptome analyses reveal the genetic mechanisms underlying bract development in Ephedra

BACKGROUND

Gnetales have a key phylogenetic position in the evolution of seed plants. Among the Gnetales, there is an extraordinary morphological diversity of seeds, the genus Ephedra, in particular, exhibits fleshy, coriaceous or winged (dry) seeds. Despite this striking diversity, its underlying genetic mechanisms remain poorly understood due to the limited studies in gymnosperms. Expanding the genomic and developmental data from gymnosperms contributes to a better understanding of seed evolution and development.

RESULTS

We performed transcriptome analyses on different plant tissues of two Ephedra species with different seed morphologies. Anatomical observations in early developing ovules, show that differences in the seed morphologies are established early in their development. The transcriptomic analyses in dry-seeded Ephedra californica and fleshy-seeded Ephedra antisyphilitica, allowed us to identify the major differences between the differentially expressed genes in these species. We detected several genes known to be involved in fruit ripening as upregulated in the fleshy seed of Ephedra antisyphilitica.

CONCLUSIONS

This study allowed us to determine the differentially expressed genes involved in seed development of two Ephedra species. Furthermore, the results of this study of seeds with the enigmatic morphology in Ephedra californica and Ephedra antisyphilitica, allowed us to corroborate the hypothesis which suggest that the extra envelopes covering the seeds of Gnetales are not genetically similar to integument. Our results highlight the importance of carrying out studies on less explored species such as gymnosperms, to gain a better understanding of the evolutionary history of plants.

Heterochrony and repurposing in the evolution of gymnosperm seed dispersal units

Background

Plant dispersal units, or diaspores, allow the colonization of new environments expanding geographic range and promoting gene flow. Two broad categories of diaspores found in seed plants are dry and fleshy, associated with abiotic and biotic dispersal agents, respectively. Anatomy and developmental genetics of fleshy angiosperm fruits is advanced in contrast to the knowledge gap for analogous fleshy structures in gymnosperm diaspores. Improved understanding of the structural basis of modified accessory organs that aid in seed dispersal will enable future work on the underlying genetics, contributing to hypotheses on the origin of angiosperm fruits. To generate a structural framework for the development and evolution of gymnosperm fleshy diaspores, we studied the anatomy and histochemistry of Ephedra (Gnetales) seed cone bracts, the modified leaves surrounding the reproductive organs. We took an ontogenetic approach, comparing and contrasting the anatomy and histology of fleshy and papery-winged seed cone bracts, and their respective pollen cone bracts and leaves in four species from the South American clade.

Results

Seed bract fleshiness in Ephedra derives from mucilage accumulated in chlorenchyma cells, also found in the reduced young leaves before they reach their mature, dry stage. Cellulosic fibers, an infrequent cell type in gymnosperms, were found in Ephedra, where they presumably function as a source of supplementary apoplastic water in fleshy seed cone bracts. Papery-winged bract development more closely resembles that of leaves, with chlorenchyma mucilage cells turning into tanniniferous cells early on, and hyaline margins further extending into “wings”.

Conclusions

We propose an evolutionary developmental model whereby fleshy and papery-winged bracts develop from an early-stage anatomy shared with leaves that differs at the pollination stage. The ancestral fleshy bract state may represent a novel differentiation program built upon young leaf anatomy, while the derived dry, papery-winged state is likely built upon an existing differentiation pattern found in mature vegetative leaves. This model for the evolution of cone bract morphology in South American Ephedra hence involves a novel differentiation program repurposed from leaves combined with changes in the timing of leaf differentiation, or heterochrony, that can further be tested in other gymnosperms with fleshy diaspores.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13227-022-00191-8.