Analysis of auxin responses in the fern Ceratopteris richardii identifies the developmental phase as a major determinant for response properties

The auxin signaling molecule regulates a range of plant growth and developmental processes. The core transcriptional machinery responsible for auxin-mediated responses is conserved across all land plants. Genetic, physiological and molecular exploration in bryophyte and angiosperm model species have shown both qualitative and quantitative differences in auxin responses. Given the highly divergent ontogeny of the dominant gametophyte (bryophytes) and sporophyte (angiosperms) generations, however, it is unclear whether such differences derive from distinct phylogeny or ontogeny. Here, we address this question by comparing a range of physiological, developmental and molecular responses to auxin in both generations of the model fern Ceratopteris richardii. We find that auxin response in Ceratopteris gametophytes closely resembles that of a thalloid bryophyte, whereas the sporophyte mimics auxin response in flowering plants. This resemblance manifests both at the phenotypic and transcriptional levels. Furthermore, we show that disrupting auxin transport can lead to ectopic sporophyte induction on the gametophyte, suggesting a role for auxin in the alternation of generations. Our study thus identifies developmental phase, rather than phylogeny, as a major determinant of auxin response properties in land plants.

Auxin Involvement in Ceratopteris Gametophyte Meristem Regeneration

Growth and development of the Ceratopteris hermaphroditic gametophytes are dependent on cell proliferation in the marginal meristem, which when destroyed will regenerate at a new location on the body margin. We established a laser ablation method to destroy a single initial cell in the meristem. Ablation caused the cessation of cell proliferation accompanied by the disappearance of the expression of an auxin synthesis gene (CrTAA2) and a cell proliferation marker gene (CrWOXB). New meristem regeneration occurred within a predictable distance from the original two days post-ablation, signified by cell proliferation and the expression of CrTAA2. Treatment with the naturally occurring auxin indole-3-acetic acid (IAA), synthetic auxin 2,4-dichlorophenoxyacetic acid (2,4-D), or the transport inhibitor naphthylphthalamic acid (NPA) altered positioning of the original marginal meristem toward the apex of the gametophyte. IAA altered positioning of the regenerated meristem after damaging the original meristem. A model of auxin involvement in the positioning of the marginal meristem in Ceratopteris is presented to encompass these results.

An ontogenetic framework for functional studies in the model fern Ceratopteris richardii

As the sister group to seed plants, ferns are a phylogenetically informative lineage. Functional studies in representatives of the fern lineage are helping bridge the knowledge gap in developmental mechanisms between angiosperms and non-vascular plants. The fern life cycle has the advantage of combining a sizable free-living haploid gametophyte, more amenable for developmental studies than the reduced seed plant gametophyte, with an indeterminate and complex diploid sporophyte. Ceratopteris richardii has long been proposed as a model fern and has recently become tractable due to stable transgenesis and increasing genomic resources, allowing researchers to test explicit questions about gene function in a fern for the first time. As with any model system, a detailed understanding of wild-type morphology and a staged ontogeny are indispensable for the characterization of mutant phenotypes resulting from genetic manipulations. Therefore, the goal of this study is to provide a unified reference ontogeny for this emerging model fern as a tool for comparative evolutionary and developmental studies. It complements earlier research by filling gaps in major stages of development of the haploid gametophyte and diploid sporophyte generations, and provides additional descriptions of the shoot apical meristem and early leaf development. This resource is meant to facilitate not only studies of candidate genes within C. richardii, but also broader ontogenetic comparisons to other model plants.

Auxin transport in the evolution of branching forms

Contents 545 I. 545 II. 546 III. 546 IV. 548 V. 548 VI. 549 VII. 549 Acknowledgements 549 References 549 SUMMARY: Branching is one of the most striking aspects of land plant architecture, affecting resource acquisition and yield. Polar auxin transport by PIN proteins is a primary determinant of flowering plant branching patterns regulating both branch initiation and branch outgrowth. Several lines of experimental evidence suggest that PIN-mediated polar auxin transport is a conserved regulator of branching in vascular plant sporophytes. However, the mechanisms of branching and auxin transport and relationships between the two are not well known outside the flowering plants, and the paradigm for PIN-regulated branching in flowering plants does not fit bryophyte gametophytes. The evidence reviewed here suggests that divergent auxin transport routes contributed to the diversification of branching forms in distinct land plant lineages.

PIN proteins and the evolution of plant development

Many aspects of development in the model plant Arabidopsis thaliana involve regulated distribution of the hormone auxin by the PIN-FORMED (PIN) family of auxin efflux carriers. The role of PIN-mediated auxin transport in other plants is not well understood, but studies in a wider range of species have begun to illuminate developmental mechanisms across land plants. In this review, I discuss recent progress in understanding the evolution of PIN-mediated auxin transport, and its role in development across the green plant lineage. I also discuss the idea that changes in auxin biology led to morphological novelty in plant development: currently available evidence suggests major innovations in auxin transport are rare and not associated with the evolution of new developmental mechanisms.

Conserved transport mechanisms but distinct auxin responses govern shoot patterning in Selaginella kraussiana

To provide a comparative framework to understand the evolution of auxin regulation in vascular plants, the effect of perturbed auxin homeostasis was examined in the lycophyte Selaginella kraussiana. Polar auxin transport was measured by tracing tritiated IAA in excised shoots. Shoots were cultured in the presence of auxin efflux inhibitors and exogenous auxin, and developmental abnormalities were documented. Auxin transport in Selaginella shoots is exclusively basipetal, as in angiosperms. Perturbed auxin transport results in the loss of meristem maintenance and abnormal shoot architecture. Dichotomous root branching in Selaginella appears to be regulated by an antagonistic relationship between auxin and cytokinin. The results suggest that basipetal polar auxin transport occurred in the common ancestor of lycophytes and euphyllophytes. Although the mechanisms of auxin transport appear to be conserved across all vascular plants, distinct auxin responses govern shoot growth and development in lycophytes and euphyllophytes.

Embryology of Ceratopteris richardii (Pteridaceae, tribe Ceratopterideae), with emphasis on placental development

This comprehensive study of early embryology in Ceratopteris richardii combines light microscopy with the first ultrastructural evaluation of any pteridophyte embryo. Emphasis is placed on ontogeny of the foot and placental transfer cells. The embryology of C. richardii shares many similarities with that of other polypodiacious ferns while exhibiting distinctive division patterns. Formative embryonic stages have been reconstructed into three-dimensional models for ease of interpretation. The zygote divides perpendicular to the gametophyte plane and anterioposterior axis. This division establishes a prone embryological habit that maximizes rapid independent establishment of a leaf-root axis in a cordate gametophyte. After the formation of a globular eight-celled stage, initials of the first leaf, and root and shoot apical meristems are defined early by discrete formative divisions. Concomitantly, the foot expands and differentiates to transport nutrients from the gametophyte for the developing embryonic organs. Transfer cell wall ingrowth deposition begins in the gametophyte placental cells before the adjacent sporophyte cells just after the eight-celled stage. These observations provide an anatomical framework for future comparative developmental genetic studies of embryogenesis in free-sporing plants.