Making water lily flowers and tomatoes – this week in Annals of Botany

Flower and ovule morphogenesis in Nymphaea Floral biology and ovule and seed ontogeny of Nymphaea thermarum, a water lily at the brink of extinction with potential as a model system for basal angiosperms
Nymphaea thermarum is a member of the Nymphaeales, of one of the most ancient lineages of flowering plants. This species was only recently described and then declared extinct in the wild, so little is known about its reproductive biology. In general, the complete ontogeny of ovules and seeds is not well documented among species of Nymphaea and has never been studied in the subgenus Brachyceras, the clade to which N. thermarum belongs. Early male and female function indicate that N. thermarum is predisposed towards self-pollination, a phenomenon that is likely to have evolved multiple times within Nymphaea. While formation of distinct micropylar and chalazal developmental domains in the endosperm, along with a copious perisperm, characterize the seeds of most members of the Nymphaeales, seed ontogenies vary between and among the constituent families. Floral biology, life history traits and small genome size make N. thermarum uniquely promising as an early-diverging angiosperm model system for genetic and molecular studies.

 

Pollen tube cell walls of wild and domesticated tomatoes contain arabinosylated and fucosylated xyloglucan

In flowering plants, fertilization relies on the delivery of the sperm cells carried by the pollen tube to the ovule. During the tip growth of the pollen tube, proper assembly of the cell wall polymers is required to maintain the mechanical properties of the cell wall. Xyloglucan (XyG) is a cell wall polymer known for maintaining the wall integrity and thus allowing cell expansion. In most angiosperms, the XyG of somatic cells is fucosylated, except in the Asterid clade (including the Solanaceae), where the fucosyl residues are replaced by arabinose, presumably due to an adaptive and/or selective diversification. However, it has been shown recently that XyG of Nicotiana alata pollen tubes is mostly fucosylated. The objective of this paper was to determine whether such structural differences between somatic and gametophytic cells are a common feature of Nicotiana and Solanum (more precisely tomato) genera. The results show that the male gametophyte (pollen tube) and the sporophyte have structurally different XyG. This suggests that fucosylated XyG may have an important role in the tip growth of pollen tubes, and that they must have a specific set of functional XyG fucosyltransferases, which are yet to be characterized.

 

Evolution shaped the internal beauty of plants

Santa Cells

Image by Oliver Leroux.

As plants embraced the terrestrial lifestyle, they underwent a range of adaptations to increase their size and stature as well as to facilitate transport of water and/or solutes. These vegetative innovations, which include the development of vascular and mechanical tissues, increased the anatomical complexity of plants. Such specialization is reflected in the design of the cell walls which surround plant cells in each tissue and fulfill a wide range of roles that are fundamental to plant life. A recently published Special Issue of Annals of Botany features papers focusing on a diverse range of topics pertaining to cell wall diversity and evolution, as well as to cell wall biosynthesis and remodelling.

Plant microscopists take advantage of cell wall diversity and through combination of dyes with affinities for specific cell wall architectures and specialized optics one can display the beauty of the internal structure of plants. The Christmas-inspired image is composed of transverse sections of Equisetum arvense stem (eyes), Acorus calamus root (nose) and Apium graveolens petiole (mouth).

Editor’s Note: You can read Oliver Leroux’s review article Collenchyma: a versatile mechanical tissue with dynamic cell walls for free at Annals of Botany.

More to tree rings than meets the eye

Tree rings record past environmental conditions, as well as the tree’s physiological response to those conditions. Although ring widths are easily measured indicators of annual tree growth, they are limited in what they can tell us about more detailed aspects of tree physiology. Fortunately, we can use stable isotopes to help us infer the tree’s leaf-level physiology in a given year. Leaves sample C and O from the atmosphere and H2O from the soil for photosynthesis, and thereby produce sugars. Some of these sugars are then converted to cellulose and laid down in the cell walls of the xylem (which makes up the wood each growing season). So, environmental conditions such as drought or extreme temperatures, that affect how the tree sampled C and O that year, are forever captured in the annual tree ring. Therefore, we are able to use isotopic ratios of C and O of tree ring cellulose (δ13C, δ18O) to reconstruct environmental growing conditions (such as the CO2 in the atmosphere and H2O from the soil). We can also use these ratios to understand the tree’s physiology: for example, δ13C is an indication of how open the stomata were during the growing season, and reflects the amount of carbon fixed in photosynthesis compared to the amount of water lost in transpiration.

Recently in Tree Physiology, Hartl-Meier et al. (2014) used tree ring widths and tree ring δ13C and δ18O to compare climate responses of spruce, larch, and beech, which have a range of drought tolerances, at three sites with varying soil moisture availability. δ13C and δ18O help paint a more complete picture of the species’ physiological responses to climate and these parameters showed a higher sensitivity to climate than the tree ring width signal. Cloud cover, which had the strongest correlations with δ13C and δ18O, was linked to climatic factors that influence stomatal behavior. The uniform sensitivity between climate and δ13C and δ18O was linked to differing hydraulic strategies across the species related to rooting depth, growing season length, stomatal behavior, and growth among the evergreen and deciduous species. Studies such as Hartl-Meier et al. (2014) show us the benefit of combining stable isotope analyses with traditional tree ring research, to generate a more nuanced picture of plant-environment interactions, and make it clear that tree cores can offer much more information about past climates and tree physiology than initially meets the eye.

Figure 1 from Hartl-Meier et al. 2014 describing the study sites

Figure 1 from Hartl-Meier et al. 2014 describing the study sites

Hartl-Meier C., Zang C., Buntgen U., Esper J., Rothe A., Gottlein A., Dirnbock T. & Treydte K. (2014). Uniform climate sensitivity in tree-ring stable isotopes across species and sites in a mid-latitude temperate forest, Tree Physiology, DOI: http://dx.doi.org/10.1093/treephys/tpu096

Pteris – the most diverse ferns

Pteris Pteris L. (Pteridaceae) is a cosmopolitan fern genus growing either terrestrially or lithophytically (on rocks) in forests, coastal areas and xeric niches. Most species of this genus occur in tropical and subtropical areas, but a few live in temperate regions. Species of Pteris are usually distributed at lower altitudes, below 2500 m, but Pteris coriacea Desv. can be found up to 3500 m. Some species have ornamental value, especially those with pale marks on the leaves. A few species are cultivated worldwide and some have become naturalized. With 250–300 species, Pteris is one of the most diverse fern genera. The uncertainty about the total number of species highlights the need for further taxonomic and phylogenetic studies.

A recent article in Annals of Botany finds that the biogeographic history of Pteris highlights long-distance dispersal as a major process shaping the worldwide distribution of the genus. Colonizing into different niches was followed by subsequent morphological diversification, and dispersal events followed by allopatric and parapatric speciation have contributed to the species diversity. This phylogeny should contribute to a new, more reliable infrageneric classification, based not just on a few morphological characters but also on ecological traits and geographic distribution.

 

Chao, Y.S., Rouhan, G., Amoroso, V.B., and Chiou, W. L. (2014) Molecular phylogeny and biogeography of the fern genus Pteris (Pteridaceae). Annals of Botany, 114(1): 109-124.

 

Stem extension and mechanical stability in Xanthium

Stem extension and mechanical stability in <i>Xanthium</i>

Stem extension and mechanical stability in Xanthium

There seems to be a trade-off between height growth and mechanical stability in plants growing in crowded habitats. Watari et al. measure extension growth and mechanical properties of internodes in Xanthium canadense plants grown at different densities and show that tissue stiffness (Young’s modulus of elasticity) and strength (modulus of rupture) play crucial roles in maintaining stability in herbaceous species that lack the capacity for secondary growth. This differs from woody species where diameter growth has been considered more important.

Temporal variation in gender in an annual plant

Temporal variation in gender in an annual plant

Temporal variation in gender in an annual plant

Sex allocation can vary widely among flowers on a plant and among plants within a population. Austen and Weis develop a numerical model and use it to demonstrate that the widespread tendency towards declining fruit set from first to last flowers on plants may contribute to temporal variation in allocation optima. Temporal trends in relative pollen and ovule investment measured in Brassica rapa, however, do not match the predicted trends in functional gender, but some findings of the model, namely decreasing male reproductive success with later flowering onset, may nonetheless apply in this taxon.

Ecophysiology of Nostoc – pioneer and permanent resident

Nostoc The cyanobacterial genus Nostoc includes many species that are highly diverse with respect to morphology, functional properties, biotic relations and habitat distribution. Nostoc species have filaments with normal photosynthetic cells and N2-fixing heterocysts and they periodically form resistant akinetes for survival and short motile filaments (hormogonia) for reproduction. Some species are free-living and many species engage in loose or obligate cooperation with land plants and fungi (e.g. lichens). A third, fascinating Nostoc type forms large gelatinous colonies of variable shape and structure in rice fields, freshwater lakes, ponds and streams and on alternating wet and dry soils or rock surfaces. The large gelatinous species require special adaptations to obtain sufficient light, nutrients and dissolved inorganic carbon in water and to survive the extreme variations in temperature, water supply and irradiance on naked soils and rock surfaces.

The gelatinous Nostoc species have filaments with normal photosynthetic cells and N2-fixing heterocysts embedded in an extensive gelatinous matrix of polysaccharides and many other organic substances providing biological and environmental protection. Large colony size imposes constraints on the use of external resources and the gelatinous matrix represents extra costs and reduced growth rates.

This review in Annals of Botany evaluates the mechanisms behind the low rates of growth and mortality, protection against environmental hazards and the persistence and longevity of gelatinous Nostoc colonies, and their ability to economize with highly limiting resources.

As free-living organisms and as symbionts in lichens, Nostoc species are both pioneers and permanent members of the vegetation of deserts, semideserts, dry grasslands and rock surfaces ranging in geographical distribution from polar to tropical regions. Their N input to these biomes can be of utmost importance. In a carefully mapped Low-Arctic tundra landscape, N2 fixation by cyanobacteria was twice the annual wet deposition of nitrogen. It remains unexplored how the high water-absorbing capacity and N2 fixation of Nostoc can facilitate the colonization of bare or newly exposed mineral surfaces by mosses and higher plants, thereby forming more stable vegetation and more organic soils. With the exposure of new mineral surfaces behind retreating glaciers on a warming Earth, this ecosystem service of Nostoc deserves future attention. Despite profound ecological differences between species, active growth of temperate specimens is mostly restricted to the same temperature range. Future studies should aim to unravel the processes behind the extreme persistence and low metabolism of Nostoc species under ambient resource supply on sediment and soil surfaces.

 

Sand-Jensen, K. (2014) Ecophysiology of gelatinous Nostoc colonies: unprecedented slow growth and survival in resource-poor and harsh environments. Annals of Botany, 114(1): 17-33.

 

Some good stuff in the recent edition of CBE Life Sciences Education

Diversity of flowering plant species
High School Students’ Learning and Perceptions of Phylogenetics of Flowering Plants. CBE Life Sci Educ vol. 13 no. 4 653-665 doi: 10.1187/cbe.14-04-0074
Basic phylogenetics and associated “tree thinking” are often minimized or excluded in formal school curricula. Informal settings provide an opportunity to extend the K–12 school curriculum, introducing learners to new ideas, piquing interest in science, and fostering scientific literacy. Similarly, university researchers participating in science, technology, engineering, and mathematics (STEM) outreach activities increase awareness of college and career options and highlight interdisciplinary fields of science research and augment the science curriculum. To aid in this effort, we designed a 6-h module in which students utilized 12 flowering plant species to generate morphological and molecular phylogenies using biological techniques and bioinformatics tools. The phylogenetics module was implemented with 83 high school students during a weeklong university STEM immersion program and aimed to increase student understanding of phylogenetics and coevolution of plants and pollinators. Student response reflected positive engagement and learning gains as evidenced through content assessments, program evaluation surveys, and program artifacts. We present the results of the first year of implementation and discuss modifications for future use in our immersion programs as well as in multiple course settings at the high school and undergraduate levels.

 

Student Interpretations of Phylogenetic Trees in an Introductory Biology Course. CBE Life Sci Educ vol. 13 no. 4 666-676 doi: 10.1187/cbe.14-01-0003
Phylogenetic trees are widely used visual representations in the biological sciences and the most important visual representations in evolutionary biology. Therefore, phylogenetic trees have also become an important component of biology education. We sought to characterize reasoning used by introductory biology students in interpreting taxa relatedness on phylogenetic trees, to measure the prevalence of correct taxa-relatedness interpretations, and to determine how student reasoning and correctness change in response to instruction and over time. Counting synapomorphies and nodes between taxa were the most common forms of incorrect reasoning, which presents a pedagogical dilemma concerning labeled synapomorphies on phylogenetic trees. Students also independently generated an alternative form of correct reasoning using monophyletic groups, the use of which decreased in popularity over time. Approximately half of all students were able to correctly interpret taxa relatedness on phylogenetic trees, and many memorized correct reasoning without understanding its application. Broad initial instruction that allowed students to generate inferences on their own contributed very little to phylogenetic tree understanding, while targeted instruction on evolutionary relationships improved understanding to some extent. Phylogenetic trees, which can directly affect student understanding of evolution, appear to offer introductory biology instructors a formidable pedagogical challenge.

 

Past climate change and population structure of Miscanthus

Past climate change and population structure of <i>Miscanthus</i>

Past climate change and population structure of Miscanthus

Miscanthus sinensis is a perennial C4 grass that is one parent of the economically important hybrid biomass species, M. ×giganteus. Clark et al. evaluate 620 M. sinensis accessions from most of its native range with >20 000 nuclear and plastid markers, and identify six genetic groups. They find that coastal south-east China was a refugium of M. sinensis during the last glacial maximum, and that the species recolonized Japan prior to recolonizing similar latitudes in mainland Asia. Ornamental cultivars originate almost exclusively from southern Japan, and many marketed as M. sinensis have hybrid ancestry from M. sacchariflorus.