Fern spore germination in response to environmental factors

Two distinct light-induced reactions are needed to promote germination in spores of Ceratopteris richardii

Germination of Ceratopteris richardii spores is initiated by light and terminates 3-4 days later with the emergence of a rhizoid. Early studies documented that the photoreceptor for initiating this response is phytochrome. However, completion of germination requires additional light input. If no further light stimulus is given after phytochrome photoactivation, the spores do not germinate. Here we show that a crucial second light reaction is required, and its function is to activate and sustain photosynthesis. Even in the presence of light, blocking photosynthesis with DCMU after phytochrome photoactivation blocks germination. In addition, RT-PCR showed that transcripts for different phytochromes are expressed in spores in darkness, and the photoactivation of these phytochromes results in the increased transcription of messages encoding chlorophyll a/b binding proteins. The lack of chlorophyll-binding protein transcripts in unirradiated spores and their slow accumulation makes it unlikely that photosynthesis is required for the initial light reaction. This conclusion is supported by the observation that the transient presence of DCMU, only during the initial light reaction, had no effect on germination. Additionally, the [ATP] in Ceratopteris richardii spores increased coincidentally with the length of light treatment during germination. Overall, these results support the conclusion that two distinct light reactions are required for the germination of Ceratopteris richardii spores.

Providing the missing links in fern life history: Insights from a phenological survey of the gametophyte stage

PREMISE

The entire life cycle of ferns has been documented, yet their life histories are still poorly understood. In particular, the phenology of fern gametophytes remains largely unknown. To address this issue, we demonstrated a new ecological approach to explore the phenological link between spore release and gametophyte maturation within the life history of a tree fern species.

METHODS

We conducted a serial survey of Alsophila podophylla gametophyte abundance in the field, and recorded the time of its spore release. Every two months for one year, all terrestrial fern gametophytes in an unsampled subplot were collected and identified using tissue-direct PCR.

RESULTS

We found temporal differences in gametophyte abundances, with a sevenfold difference between the highest and lowest months. The number of spores released was linked to the gametophyte abundance two months later. The switch from gametophyte to juvenile sporophyte was found to be most correlated with precipitation.

DISCUSSION

The observed fluctuation in gametophyte abundance and population structure was likely associated with the phenology of spore release and environmental factors. Importantly, these findings provide the first evidence of phenological links between different developmental stages in a fern's life history.

Light- and hormone-mediated development in non-flowering plants: An overview

Light, hormones and their interaction regulate different aspects of development in non-flowering plants. They might have played a role in the evolution of different plant groups by conferring specific adaptive evolutionary changes. Plants are sessile organisms. Unlike animals, they lack the opportunity to abandon their habitat in unfavorable conditions. They respond to different environmental cues and adapt accordingly to control their growth and developmental pattern. While phytohormones are known to be internal regulators of plant development, light is a major environmental signal that shapes plant processes. It is plausible that light-hormone crosstalk might have played an important role in plant evolution. But how the crosstalk between light and phytohormone signaling pathways might have shaped the plant evolution is unclear. One of the possible reasons is that flowering plants have been studied extensively in context of plant development, which cannot serve the purpose of evolutionary comparisons. In order to elucidate the role of light, hormone and their crosstalk in the evolutionary adaptation in plant kingdom, one needs to understand various light- and hormone-mediated processes in diverse non-flowering plants. This review is an attempt to outline major light- and phytohormone-mediated responses in non-flowering plant groups such as algae, bryophytes, pteridophytes and gymnosperms.

The distribution of climbing chalk on climbed boulders and its impact on rock-dwelling fern and moss species

Rock climbing is popular, and the number of climbers rises worldwide. Numerous studies on the impact of climbing on rock-dwelling plants have reported negative effects, which were mainly attributed to mechanical disturbances such as trampling and removal of soil and vegetation. However, climbers also use climbing chalk (magnesium carbonate hydroxide) whose potential chemical effects on rock-dwelling species have not been assessed so far. Climbing chalk is expected to alter the pH and nutrient conditions on rocks, which may affect rock-dwelling organisms. We elucidated two fundamental aspects of climbing chalk. (a) Its distribution along nonoverhanging climbing routes was measured on regularly spaced raster points on gneiss boulders used for bouldering (ropeless climbing at low height). These measurements revealed elevated climbing chalk levels even on 65% of sampling points without any visual traces of climbing chalk. (b) The impact of climbing chalk on rock-dwelling plants was assessed with four fern and four moss species in an experimental setup in a climate chamber. The experiment showed significant negative, though varied effects of elevated climbing chalk concentrations on the germination and survival of both ferns and mosses. The study thus suggests that along climbing routes, elevated climbing chalk concentration can occur even were no chalk traces are visible and that climbing chalk can have negative impacts on rock-dwelling organisms.

In vitro spore germination and phytoremediation of Hg and Pb using gametophytes of Pityrogramma calomelanos

Few pteridophytes have proven the capacity to accumulate and remediate heavy metals from contaminated soils. Pityrogramma calomelanos, a non-seasonal fast-growing, a cosmopolitan fern, is a good indicator of environmental conditions, was used in the present study. The life cycle of ferns alternates with haploid gametophyte and diploid sporophyte. The present study was undertaken to access the effect of mercury, in form of mercury (II) chloride [(HgCl₂)] and lead as lead nitrate [Pb(NO₃)₂] in developmental studies using in vitro spore germination. Periodic recording of the germination, protonemal growth, rhizoid formation and differentiation of sex organs in different concentrations of heavy metals were conducted for a period of 6 weeks. It was found that the percentage of spore germination and the number of protonemal cells reduced significantly causing developmental defects in the presence of HgCl₂ as compared to Pb(NO₃)₂ (p < 0.0001). A significant decrease in the number of archegonial count and chlorophyll content was observed in different concentrations of the heavy metals tested. Gametophytes of P. calomelanos recorded lead uptake of 646.51 ± 0.93 mg/kg in treatments of 25 ppm of lead and high mercury accumulation up to 1,885 ± 1.98 mg/kg at 10 ppm of mercury, indicating successful uptake of heavy metals. Novelty statement: Pityrogramma calomelanos is gaining interest amongst pteridologists upon proving its exclusive capacity of phytoremediation. It is superior in comparison to the most popular, patented fern Pteris vittata. Our study demonstrates the effective use of the promising fern in its simplest, nonvascular form of gametophytes as a phytoremediation agent in controlled conditions.

Synthetic Seed Technology Development and Production Studies for Storage, Transport, and Industrialization of Bracken Spores

Bracken fern (Pteridium aquilinum var. latiusculum (Desv.) Underw. ex A. Heller) has long been grown industrially in South Korea. Conventional propagation methods, including planting rhizomes and in vitro seedling culture, are labor intensive and expensive, and thus not commercially suitable. We aimed to develop a system to produce synthetic seeds using fern spores (SFS). Synthetic seeds were prepared by mixing bracken spores and alginate matrix. Spore germination and gametophyte and sporophyte growth and development from SFS proceeded normally. Spore density affected gametophyte and sporophyte numbers. SFS prepared using cold (4 °C) long-term storage spores (even 7-year-old spores) could effectively form sporophytes. The highest germination was observed at 25 °C. Soaking-treated SFS successfully formed sporophytes, even after 30 days of storage at 4 °C; indeed, sporophytes formed even after five days of storage at 25 °C during transport conditions. SFS were sown in plug trays for commercial use. Young sporophytes grown from plug seedlings were greenhouse cultivated, and transplanting within eight weeks was effective for root growth and growing-point formation. Developing synthetic seeds is a feasible solution for facilitating efficient transport and the handling of small-sized fern spores; furthermore, this SFS technology provides the basis for fern seedling culture and fern spore industrialization.

Fitness of an allopolyploid rupicolous fern compared with its diploid progenitors: from sporogenesis to sporophyte formation

PREMISE

When two populations of related cytotypes grow in sympatry, the rarer cytotype tends to be excluded due to a frequency-dependent mating disadvantage. Evolutionary models predict that polyploids, which are typically the rarer cytotype upon first formation, should have higher relative fitness and/or higher selfing rates to establish and then coexist with diploid parents.

METHODS

Performance during early recruitment was compared among three co-occurring rupicolous fern species: the allotetraploid Cheilanthes tinaei and its diploid ancestors, C. hispanica and C. maderensis. In culture experiments, fresh spores and samples of soil spore banks were tested for variation among cytotypes in germination, survival, fecundity, and mating system of gametophytes.

RESULTS

Compared with its diploid parents, C. tinaei fresh spores had higher abortion percentages, lower dispersal ability as a result of its larger spores, and similar vigor at germination. For gametophytes from soil spore banks, C. tinaei had high survival similar to C. maderensis, but its sex expression resembled that of C. hispanica, with a high proportion of males. Patterns of sporophyte formation by females and bisexuals indicate that the polyploid does not have an increased gametophytic selfing rate. Gametophytes were larger in C. tinaei, but its reproductive success (sporophyte formation) was intermediate relative to diploids.

CONCLUSIONS

Our results show no evidence of higher selfing or fitness advantage of the allopolyploid over both diploid parents at any stage of early recruitment. These two unexpected findings suggest that further factors, such as niche differentiation, play a more important role in cytotype coexistence.

Plant species with extremely small populations (PSESP) in China: A seed and spore biology perspective

Approximately one fifth of the world's plants are at risk of extinction. Of these, a significant number exist as populations of few individuals, with limited distribution ranges and under enormous pressure due to habitat destruction. In China, these most-at-risk species are described as 'plant species with extremely small populations' (PSESP). Implementing conservation action for such listed species is urgent. Storing seeds is one of the main means of ex situ conservation for flowering plants. Spore storage could provide a simple and economical method for fern ex situ conservation. Seed and spore germination in nature is a critical step in species regeneration and thus in situ conservation. But what is known about the seed and spore biology (storage and germination) of at-risk species? We have used China's PSESP (the first group listing) as a case study to understand the gaps in knowledge on propagule biology of threatened plant species. We found that whilst germination information is available for 28 species (23% of PSESP), storage characteristics are only known for 8% of PSESP (10 species). Moreover, we estimate that 60% of the listed species may require cryopreservation for long-term storage. We conclude that comparative biology studies are urgently needed on the world's most threatened taxa so that conservation action can progress beyond species listing.

Cytological and proteomic analyses of horsetail (Equisetum arvense L.) spore germination

Spermatophyte pollen tubes and root hairs have been used as single-cell-type model systems to understand the molecular processes underlying polar growth of plant cells. Horsetail (Equisetum arvense L.) is a perennial herb species in Equisetopsida, which creates separately growing spring and summer stems in its life cycle. The mature chlorophyllous spores produced from spring stems can germinate without dormancy. Here we report the cellular features and protein expression patterns in five stages of horsetail spore germination (mature spores, rehydrated spores, double-celled spores, germinated spores, and spores with protonemal cells). Using 2-DE combined with mass spectrometry, 80 proteins were found to be abundance changed upon spore germination. Among them, proteins involved in photosynthesis, protein turnover, and energy supply were over-represented. Thirteen proteins appeared as proteoforms on the gels, indicating the potential importance of post-translational modification. In addition, the dynamic changes of ascorbate peroxidase, peroxiredoxin, and dehydroascorbate reductase implied that reactive oxygen species homeostasis is critical in regulating cell division and tip-growth. The time course of germination and diverse expression patterns of proteins in photosynthesis, energy supply, lipid and amino acid metabolism indicated that heterotrophic and autotrophic metabolism were necessary in light-dependent germination of the spores. Twenty-six proteins were involved in protein synthesis, folding, and degradation, indicating that protein turnover is vital to spore germination and rhizoid tip-growth. Furthermore, the altered abundance of 14-3-3 protein, small G protein Ran, actin, and caffeoyl-CoA O-methyltransferase revealed that signaling transduction, vesicle trafficking, cytoskeleton dynamics, and cell wall modulation were critical to cell division and polar growth. These findings lay a foundation toward understanding the molecular mechanisms underlying fern spore asymmetric division and rhizoid polar growth.