All posts by AJ Cann

About AJ Cann

Alan Cann is a Senior Lecturer in the Department of Biology at the University of Leicester and Internet Consulting Editor for AoB.

Plant evolution: The inevitability of C4 photosynthesis

Photosynthesis Although atmospheric carbon dioxide (CO2) levels are currently rising, the last 30 million years witnessed great declines in CO2, which has limited the efficiency of photosynthesis. Rubisco, the critical photosynthetic enzyme that catalyses the fixation of CO2 into carbohydrate, also reacts with oxygen when CO2 levels are low and temperatures are high. When this occurs, plants activate a process known as photorespiration, an energetically expensive set of reactions that release one molecule of CO2.

C4 photosynthesis is a clever solution to the problem of low atmospheric CO2. It is an internal plant carbon-concentrating mechanism that largely eliminates photorespiration: a ‘fuel-injection’ system for the photosynthetic engine. C4 plants differ from plants with the more typical ‘C3′ photosynthesis because they restrict Rubisco activity to an inner compartment, typically the bundle sheath, with atmospheric CO2 being fixed into a 4-carbon acid in the outer mesophyll. This molecule then travels to the bundle sheath, where it is broken down again, bathing Rubisco in CO2 and limiting the costly process of photorespiration.

The evolution of the C4 pathway requires many changes. These include the recruitment of multiple enzymes into new biochemical functions, massive shifts in the spatial distribution of proteins and organelles, and a set of anatomical modifications to cell size and structure. It is complex, and it is also highly effective: C4 plants include many of our most important and productive crops (maize, sorghum, sugarcane, millet) and are responsible for around 25% of global terrestrial photosynthesis. A new paper in eLife examines how this process may have evolved, first to correct an intercellular nitrogen imbalance, and only later evolved a central role in carbon fixation.

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Understanding and forecasting the response of plants to climate change

Climate change effects on range and diversity of a fern

Climate change effects on range and diversity of a fern

Understanding and forecasting the response of plant species to climatic fluctuation is one of the top priorities for current biodiversity research because of the critical need to conserve and manage natural resources and biodiversity. Climate fluctuations are not a new phenomenon. Plants have responded to global, regional and local climate change via migration and/or adaptation since their origin. In turn, slow and/or little response to climate change (e.g. slow migration rate) increases the probability of local or global extinction. By constructing the spatio-temporal dynamics of plant response to climate change from the past, it may be possible to improve our ability to predict future changes in the range and distribution of species and their genetic diversity. Low diversity coincides with high climate change velocity.

Recently researchers have tried to untangle the response of plants to changing climates at the microevolutionary scale, by integrating species distribution models and statistical phylogeography. Combining these two techniques will not only overcome their individual limitations, but will also improve our understanding of the spatio-temporal population dynamics involved.

A recent paper in Annals of Botany uses species distribution modelling and population genetic analysis to assess how Asplenium fontanum, a fern species with high migration capacity, has responded to environmental change since the last ice-age and to predict possible future implications under global warming. The results show the importance of climatically stable areas for maintenance of populations and accumulation of genetic diversity, and indicate that such areas are vulnerable to extinction under future scenarios of climate change, resulting in possible permanent loss of historic genetic variation.

 

Bystriakova, N., Ansell, S.W., Russell, S.J., Grundmann, M., Vogel, J.C., & Schneider, H. Present, past and future of the European rock fern Asplenium fontanum: combining distribution modelling and population genetics to study the effect of climate change on geographic range and genetic diversity. (2014) Annals of Botany, 113(3), 453-465.
Climate change is expected to alter the geographic range of many plant species dramatically. Predicting this response will be critical to managing the conservation of plant resources and the effects of invasive species. The aim of this study was to predict the response of temperate homosporous ferns to climate change. Genetic diversity and changes in distribution range were inferred for the diploid rock fern Asplenium fontanum along a South–North transect, extending from its putative last glacial maximum (LGM) refugia in southern France towards southern Germany and eastern-central France. This study reconciles observations from distribution models and phylogeographic analyses derived from plastid and nuclear diversity. Genetic diversity distribution and niche modelling propose that genetic diversity accumulates in the LGM climate refugium in southern France with the formation of a diversity gradient reflecting a slow, post-LGM range expansion towards the current distribution range. Evidence supports the fern’s preference for outcrossing, contradicting the expectation that homosporous ferns would populate new sites by single-spore colonization. Prediction of climate and distribution range change suggests that a dramatic loss of range and genetic diversity in this fern is possible. The observed migration is best described by the phalanx expansion model. The results suggest that homosporous ferns reproducing preferentially by outcrossing accumulate genetic diversity primarily in LGM climate refugia and may be threatened if these areas disappear due to global climate change.

 

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Reproduction and invasiveness in St. John’s wort

Reproduction and invasiveness in St. John’s wort

Reproduction and invasiveness in St. John’s wort

The relative ability of different plant taxa to invade new biogeographic regions successfully is dependent upon a number of biological and physical factors, one of which is the reproductive system, which directly influences population structure, gene flow and evolutionary potential. Considering seed formation, plants can reproduce through sex (selfing and outcrossing) or apomixis (asexual reproduction through seed.

St. John’s wort (Hypericum perforatum) is such an invasive species which is indigenous to central and eastern Europe; it is self-compatible and can reproduce through sex or apomixis. H. perforatum has successfully invaded North America since the first record of introduction in Lancaster, Pennsylvania in 1793. Its high genotypic plasticity in conjunction with variable levels of facultative apomixis are hypothesized to have contributed to its rapid spread throughout the continent. For example, in an analysis of multiple phenotypic traits, Maron et al. (2004) demonstrated that the introduction of H. perforatum into North America was accompanied by rapid climatic adaptation.

Using an analysis of a collection of European native and North American invasive accessions, a recent paper in Molins Annals of Botany examines biogeographic differentiation in both natural and introduced populations, and test whether variation in apomixis traits is correlated with the propensity for H. perforatum to invade novel environments.

 

Molins, M.P., Corral, J.M., Aliyu, O.M., Koch, M.A., Betzin, A., Maron, J.L., & Sharbel, T.F. (2014) Biogeographic variation in genetic variability, apomixis expression and ploidy of St. John’s wort (Hypericum perforatum) across its native and introduced range. Annals of Botany, 113 (3): 417-427 doi: 10.1093/aob/mct268.
St. John’s wort (Hypericum perforatum) is becoming an important model plant system for investigations into ecology, reproductive biology and pharmacology. This study investigates biogeographic variation for population genetic structure and reproduction in its ancestral (European) and introduced (North America) ranges. Over 2000 individuals from 43 localities were analysed for ploidy, microsatellite variation (19 loci) and reproduction (flow cytometric seed screen). Most individuals were tetraploid (93 %), while lower frequencies of hexaploid (6 %), diploid (<1 %) and triploid (<1 %) individuals were also identified. The presence of pure and mixed populations representing all three genetic clusters in North America demonstrates that H. perforatum was introduced multiple times onto the continent, followed by gene flow between the different gene pools. Taken together, the data presented here suggest that plasticity in reproduction has no influence on the invasive potential of H. perforatum.

 

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Where do gingers some from?

Plastid genomes and relationships in Zingiberales

Plastid genomes and relationships in Zingiberales

The tropical angiosperm order Zingiberales comprises a clade of eight tropical monocot families including approximately 2500 species believed to have undergone an ancient, rapid radiation during the Cretaceous era. Zingiberales show substantial variation in floral morphology, and several members are ecologically and economically important – such as ginger, cardamom, turmeric, galangal, bananas and plantains. Deep phylogenetic relationships among primary lineages of Zingiberales have proved difficult to resolve in previous studies, representing a key region of uncertainty in the monocot tree of life. The Zingiberales comprises a diverse clade of eight families, but deep phylogenetic relationships within them are poorly understood.

A recent paper in Annals of Botany uses next-generation sequencing to generate complete plastid gene sets and finds that plastid genomes provide strong support for many relationships, but only weak support for inclusion of the Heliconiaceae order. Manipulation of various data matrix properties affects tree topology in an unpredictable fashion, suggesting that complete coding regions of the plastome do not provide sufficient character information to resolve this rapid, ancient radiation.

 

Barrett, C.F., Specht, C.D., Leebens-Mack, J., Stevenson, D.W., Zomlefer, W. B., & Davis, J.I. (2014) Resolving ancient radiations: can complete plastid gene sets elucidate deep relationships among the tropical gingers (Zingiberales)?. Annals of Botany, 113(1), 119-133.

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The apple wasn’t trying to co-operate with Isaac Newton

Ontogenetic tissue modification of fruit peduncles

Ontogenetic tissue modification of fruit peduncles

Plants are in various ways able to adapt the geometry and material properties of their organs, tissues and cells to changing conditions during development. Apple (Malus) fruit peduncles are highly modified stems with limited secondary growth because fruit ripening lasts only one season. They must reliably connect heavy fruits to the branch and cope with increasing fruit weight, which induces dynamic stresses under oscillating wind loads.

A recent study in Annals of Botany focuses on tissue modification of these small, exposed structures during fruit development. A combination of microscopic and mechanical test methods, as well as Raman spectroscopy was applied to study structure–function relationships in apple peduncles. Anatomical investigations were accompanied by biomechanical tests under static and dynamic loads to trace tissue differentiation and ontogenetic changes in properties of Malus peduncles throughout the growing season. Mechanical investigation of peduncles after successive removal of tissues revealed insights into the specific mechanical properties and function of different tissues.

The findings confirm previous assumptions that sclereids generally strengthen the plant structure. However, this work shows that brachysclereids contribute to the stiffness mostly under bending (flexural rigidity) rather than to the strength during tensile loads, and verifies their effect on viscous damping.

Horbens, M., Feldner, A., Höfer, M., & Neinhuis, C. (2014) Ontogenetic tissue modification in Malus fruit peduncles: the role of sclereids. Annals of Botany, 113(1), 105-118.

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In the right place at the right time for pollination

A key innovation in the evolution of plants was the origin of the hermaphroditic flower, where both male and female sexual functions occur in the same complex structure. However, this innovation created a significant problem: sexual conflict, in which the function of one sex is compromised by the proximity and function of the other. This led to a further fundamental challenge in the function of animal-pollinated, hermaphroditic flowers: minimizing such sexual conflict while still enabling the male and female fertile parts to contact pollinators in the same place. Two solutions to sexual conflict have been explored evolutionarily by plants: (1) spatial separation of fertile parts (herkogamy) and (2) temporal separation of sexual functions (dichogamy).

To evaluate the effect of partial dichogamy and movement herkogamy on pollination accuracy in ‘generalist’ flowers (flowers pollinated by a variety of animal species), a recent paper in Annals of Botany investigates Parnassia epunctulata, a plant with open, white flowers, from subalpine meadows. The stamens of this species show a remarkable pattern of repositioning, and dehisce one by one over several days before the female phase. This feature permitted the authors to examine whether the anthers and stigma are positioned accurately, facilitating pollen removal and receipt.

The open flowers were visited by a variety of pollinators, most of which were flies. Seed set was pollinator-dependent (bagged flowers set almost no seeds) and pollen-limited (manual pollination increased seed set over open pollination). Analyses of adaptive accuracy showed that coordinated stamen movements and style elongation (movement herkogamy) dramatically increased pollination accuracy. Specifically, dehiscing anthers and receptive stigmas were positioned accurately in the vertical and horizontal planes in relation to the opposite sexual structure and pollinator position. In contrast, the spatial correspondence between anthers and stigma was dramatically lower before the anthers dehisced and after stamens bent outwards, as well as before and after the period of stigmatic receptivity. This shows for the first time that a combination of movement herkogamy and dichogamy can maintain high pollination accuracy in flowers with generalized pollination. Staggered pollen and stigma presentation with spatial correspondence can both reduce sexual interference and improve pollination accuracy.

Armbruster, W.S., Corbet, S.A., Vey, A.J., Liu, S.J., & Huang, S.Q. (2013) In the right place at the right time: Parnassia resolves the herkogamy dilemma by accurate repositioning of stamens and stigmas. Annals of Botany, 113 (1): 97-103.

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Drosera, the hunter

Phophatase activity in Drosera capensis Carnivorous plants result from several independent evolutionary processes and are an example of convergent evolution. The American naturalist Thomas Givnish was the first to realize that terrestrial carnivorous plants are mostly restricted to sunny, nutrient-poor and wet environments, where the marginal benefit derived from carnivory exceeds the cost. He introduced a cost/benefit model of carnivory, which soon became a framework for studying functional ecological relationships in carnivorous plants. The cost of carnivory is mainly associated with carbon investment in the production of the lure, mucilage and digestive enzymes in photosynthetically inefficient traps. On the other hand, benefits derived from prey capture associated with increased nutrient uptake include an increased photosynthetic rate, higher seed production and/or direct uptake of carbon from prey.

Since Darwin’s work in 1875, many studies have shown the significant positive effects that prey capture has on the growth and tissue mineral nutrient content, but experimental studies expressing these benefits in terms of increased photosynthetic rate are scarce and ambiguous. A recent paper in Annals of Botany investigates the full hunting cycle of the carnivorous sundew Drosera capensis, including prey attraction, digestion and the benefit from nutrient uptake. This study tests the hypothesis that the red colour of the tentacles may lure insects onto the traps, and measures the activities of different digestive enzymes in response to prey capture and mechanical stimulation of traps. Red coloration of tentacles did not act as a signal to attract fruit flies onto the traps, but the study conforms the hypothesis that there must be a benefit in terms of increased net photosynthesis in response to feeding, prey digestion and nutrient uptake.

 

Pavlovič, A., Krausko, M., Libiaková, M., & Adamec, L. (2014) Feeding on prey increases photosynthetic efficiency in the carnivorous sundew Drosera capensis. Annals of Botany, 113(1), 69-78.

 

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Tropaeolum Tops Tobacco

Tropaeolum Tops Tobacco The soil-born bacterium Agrobacterium tumefaciens is the only organism capable of interkingdom gene transfer. It has been employed intensively for genetic manipulation of plant cells. Transformation is accomplished through the action of both bacterial and host proteins, many of which have been identified and functionally characterised. Yet, it is still impossible to predict which plant species are easily accessible and which are recalcitrant to Agrobacterium-mediated transformation.

This paper describes an Agrobacterium-mediated transformation of Tropaeolum majus (nasturtium, order Brassicales) as a convenient, cheap and efficient transient expression system. It facilitates studies in a genetic background that is closely related to the model plant Arabidopsis. In addition, it offers an alternative and complementary method to Nicotiana leaf infiltration. The accessibility of Tropaeolum to simple and fast genetic manipulation potentially drives progress in several fields of plant research, including those aimed at biotechnological and pharmacological applications. Unlike Arabidopsis, Tropaeolum is capable of engaging in endomycorrhizal associations, and is therefore also of interest to symbiosis researchers.

 

Pitzschke A. (2013) Tropaeolum Tops Tobacco – Simple and Efficient Transgene Expression in the Order Brassicales. PLoS ONE 8(9): e73355. doi:10.1371/journal.pone.0073355
Transient expression systems are valuable tools in molecular biology. Agrobacterial infiltration of leaves is well-established in tobacco, but has led to limited success in the model plant Arabidopsis thaliana. An efficient expression system combining the advantages of Arabidopsis (well-characterised) and the simplicity of leaf infiltration is desirable. Here, I describe Agrobacterium tumefaciens-mediated transformation of Tropaeolum majus (nasturtium, order Brassicales) as a remarkably simple, cheap and highly efficient transient expression system. It provides the Arabidopsis community with a tool to study subcellular localisation, protein–protein interactions and reporter gene activities (e.g. luciferase, β-glucuronidase) in a genetic background that is closely related to their primary model organism. Unlike Arabidopsis, Tropaeolum is capable of engaging in endomycorrhizal associations and is therefore relevant also to symbiosis research. RNAi-based approaches are more likely to succeed than in the distantly-related Nicotiana transformation system. Tropaeolum majus was voted the “medicinal plant of the year 2013″. Conquering this plant for genetic manipulations harbours potential for biotechnological and pharmacological applications.

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Letting Leaves Live Longer

Linking ethylene to nitrogen-dependent leaf longevity

Linking ethylene to nitrogen-dependent leaf longevity

Leaf longevity is an important plant trait associated with diverse aspects of plant function and life history, contributing to characteristic patterns of material cycling and energy flow in ecosystems. A large number of studies have shown that leaf longevity is intimately associated with leaf nitrogen (N) concentration, and that soil N addition often leads to shortened leaf longevities. Leaf longevity is thought to decrease with increased N availability because nitrogen promote photosynthesis. Despite this, we know little about the biochemical mechanisms underlying N-dependent leaf longevity. And increasing N availability does not always result in reduced leaf longevity.

Ethylene has long been recognized as a key plant hormone involved in leaf senescence and defoliation. Ethylene production from plants responds to various abiotic and biotic stresses. Nitrogen-induced ethylene production may be behind N-dependent changes in leaf longevity.

As the dominant vegetation type in the semiarid regions of Eurasia, the temperate steppe of Inner Mongolia in northern China is reported to be sensitive to environmental change. As N is one of the key limiting factors for plant growth in this area, projected increases in N deposition will presumably impact plant physiological process. A recent paper in Annals of Botany explores the role of ethylene in the mechanisms underlying how N addition decreases leaf longevity. Three main results emerged: (1) N addition enhanced leaf ethylene production; (2) increased leaf ethylene production was associated with decreased leaf longevity; and (3) N addition reduced leaf longevity mainly through altering leaf ethylene production. This study has produced the first experimental evidence that ethylene modulates the N-induced decrease in leaf longevity, providing a novel yet intuitive explanation for a widely observed ecological phenomenon.

 

Ren, H., Xu, Z., Zhang, W., Jiang, L., Huang, J., Chen, S., Wang, L. & Han, X. (2013. Linking ethylene to nitrogen-dependent leaf longevity of grass species in a temperate steppe. Annals of Botany, 112(9), 1879-1885.
Leaf longevity is an important plant functional trait that often varies with soil nitrogen supply. Ethylene is a classical plant hormone involved in the control of senescence and abscission, but its role in nitrogen-dependent leaf longevity is largely unknown. Pot and field experiments were performed to examine the effects of nitrogen addition on leaf longevity and ethylene production in two dominant plant species, Agropyron cristatum and Stipa krylovii, in a temperate steppe in northern China. Nitrogen addition increased leaf ethylene production and nitrogen concentration but shortened leaf longevity; the addition of cobalt chloride, an ethylene biosynthesis inhibitor, reduced leaf nitrogen concentration and increased leaf longevity. Path analysis indicated that nitrogen addition reduced leaf longevity mainly through altering leaf ethylene production. These findings provide the first experimental evidence in support of the involvement of ethylene in nitrogen-induced decrease in leaf longevity.

 

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Shoot development in alfalfa plants competing for light

Shoot development in alfalfa plants competing for light

Shoot development in alfalfa plants competing for light

Leaf area largely determines light interception and transpiration in plants. An increase in crop leaf area over time depends on variables at different levels of organization: plant density at the population level; the number of shoots per plant and shoot development at the organism level; and ultimately individual leaf expansion at the organ level. However, these variables are seldom considered in crop models.

Alfalfa–grass mixtures are among the most widespread forage crops in many temperate areas. In such communities, alfalfa leaf area expansion has been shown to be the main attribute that explains light interception by the legume component and its biomass production at both the canopy and plant scales. A recent paper in Annals of Botany assesses the relative sensitivity of the morphogenetic processes of alfalfa involved in plant leaf area expansion (primary shoot development, branching and leaf expansion) in response to light availability.

 

Baldissera, T.C., Frak, E., de Faccio Carvalho, P.C., & Louarn, G. Plant development controls leaf area expansion in alfalfa plants competing for light. (2014) Annals of Botany, 113(1), 145-157.
The growth of crops in a mixture is more variable and difficult to predict than that in pure stands. Light partitioning and crop leaf area expansion play prominent roles in explaining this variability. However, in many crops commonly grown in mixtures, including the forage species alfalfa, the sensitivity and relative importance of the physiological responses involved in the light modulation of leaf area expansion are still to be established. This study was designed to assess the relative sensitivity of primary shoot development, branching and individual leaf expansion in alfalfa in response to light availability.
Two experiments were carried out. The first studied isolated plants to assess the potential development of different shoot types and growth periods. The second consisted of manipulating the intensity of competition for light using a range of canopies in pure and mixed stands at two densities so as to evaluate the relative effects on shoot development, leaf growth, and plant and shoot demography.
Shoot development in the absence of light competition was deterministic (constant phyllochrons of 32·5 °Cd and 48·2 °Cd for primary axes and branches, branching probability of 1, constant delay of 1·75 phyllochron before axillary bud burst) and identical irrespective of shoot type and growth/regrowth periods. During light competition experiments, changes in plant development explained most of the plant leaf area variations, with average leaf size contributing to a lesser extent. Branch development and the number of shoots per plant were the leaf area components most affected by light availability. Primary axis development and plant demography were only affected in situations of severe light competition.
Plant leaf area components differed with regard to their sensitivity to light competition. The potential shoot development model presented in this study could serve as a framework to integrate light responses in alfalfa crop models.

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