Category Archives: ContentSnapshots

Diversification and hybridization in Malagasy baobabs

Madagascar is the world’s fourth largest island, and is renowned for its species diversity and endemism. Due to the wide diversity of climatic and ecological conditions on the island, the native biota provides a fascinating context for the study of speciation and plant radiation. On Madagascar, the trees of the genus Adansonia (Bombacoideae, Malvaceae), the baobabs, are prominent in the dry deciduous forests and thickets of the western half of the island. Baobab trees may live for more than 1000 years and are characterized by outcrossing breeding systems with self-incompatibility.

Diversification and hybridization in Malagasy baobabs

Adansonia (Bombacoideae) comprises nine species, six of which are endemic to Madagascar and genetic relationships within these remain unresolved due to conflicting results between nuclear and plastid DNA variation. A recent paper in Annals of Botany analyses nuclear microsatellite variation using Bayesian clustering programs and find a clear interspecific differentiation. They identify early-generation hybrids in contact areas between the species showing overlapping flowering periods and sharing the same pollinators. The results reveal a new, stabilized differentiated entity originating from hybridization in the current absence of the parental species, suggesting a potential role of hybridization in the recent diversification history of the Malagasy baobabs.

 

Tsy, J. M. L. P., Lumaret, R., Flaven-Noguier, E., Sauve, M., Dubois, M. P., & Danthu, P. (2013) Nuclear microsatellite variation in Malagasy baobabs (Adansonia, Bombacoideae, Malvaceae) reveals past hybridization and introgression. Annals of botany, 112(9), 1759-1773

 

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Auxin, environmental signals and root development (free review article)

Auxin and the integration of environmental signals into plant root development Plants are extremely flexible organisms adaptable to a range of diverse environments. Their intrinsic ability to simultaneously inhabit both above- and below-ground domains makes them unique among most other living organisms, which occupy a single habitat at a given time.

In response to diverse environmental signals, plants modify their development through the perception and integration of exogenous signals into the signalling pathways of plant hormones. Auxin is one of the most versatile plant hormones and plays essential roles in growth and development. The revelation of the existence of an auxin biosynthesis, signalling and transport apparatus in single-celled green algae is a clear indication that auxin has played an important evolutionary role during the adaptation of plants to diverse land environments.

In recent years, significant progress has been made towards understanding how this hormone regulates plant growth and development. However, less is known about the roles of auxin as a regulator of biotic and abiotic stress responses. In this free review article, interesting new insights into the role of auxin as an integrator of environmental signals are highlighted.

Kazan, K. (2013) Auxin and the integration of environmental signals into plant root development. Annals of botany, 112(9), 1655-1665
Background: Auxin is a versatile plant hormone with important roles in many essential physiological processes. In recent years, significant progress has been made towards understanding the roles of this hormone in plant growth and development. Recent evidence also points to a less well-known but equally important role for auxin as a mediator of environmental adaptation in plants.
Scope: This review briefly discusses recent findings on how plants utilize auxin signalling and transport to modify their root system architecture when responding to diverse biotic and abiotic rhizosphere signals, including macro- and micro-nutrient starvation, cold and water stress, soil acidity, pathogenic and beneficial microbes, nematodes and neighbouring plants. Stress-responsive transcription factors and microRNAs that modulate auxin- and environment-mediated root development are also briefly highlighted.
Conclusions: The auxin pathway constitutes an essential component of the plant’s biotic and abiotic stress tolerance mechanisms. Further understanding of the specific roles that auxin plays in environmental adaptation can ultimately lead to the development of crops better adapted to stressful environments.

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Avoidance of interspecific pollen transfer in Pedicularis

Avoidance of interspecific pollen transfer in Pedicularis

Avoidance of interspecific pollen transfer in Pedicularis

Plants surrounded by individuals of other co-flowering species may attract more pollinators but can suffer a reproductive cost from interspecific pollen transfer. Yang et al. compare pollination and reproduction in Pedicularis densispica (lousewort) when occurring alone or together with co-flowering Astragalus pastorius. They find that mixed populations attract many more nectar-seeking bumble-bees, which move frequently between the species. However, differences in floral architecture mean that P. densispica is pollinated via the dorsum of the bees whilst A. pastorius receives pollen via the abdomen, thus avoiding interspecific transfer. The overall result is that co-flowering yields more seeds that are heavier and have higher germinability than in pure populations of P. densispica.

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Hydrochoric gene flow in invasive riparian Impatiens

Hydrochoric gene flow in invasive riparian Impatiens

Hydrochoric gene flow in invasive riparian Impatiens

Riparian systems are prone to invasion by alien plant species, which may be facilitated by hydrochory, the transport of seeds by water. Love et al. study gene flow associated with hydrochoric dispersal of the invasive riparian plant Impatiens glandulifera (Himalayan balsam) in two contrasting river systems and find a significant increase in levels of genetic diversity downstream, consistent with the accumulation of propagules from upstream source populations. There is strong evidence for organisation of this diversity between different tributaries, reflecting the dendritic organisation of the river systems studied. The results indicate that hydrochory, rather than anthropogenic dispersal, is primarily responsible for the spread of I. glandulifera in these river systems.

 

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Evolutionary history of Afro-Madagascan Ixora

Ixora regalis

Ixora regalis

The pantropical genus Ixora is one of the largest genera in Rubiaceae, with approximately 530 species. Tosh et al. conduct phylogenetic analyses based on four plastid and two nuclear ribosomal markers to infer the historical biogeography of Afro-Madagascan Ixora species. They find that Madagascan Ixora do not form a monophyletic group, but are represented by two separate lineages of different ages, with at least one dispersal event occurring from East Africa into Madagascar in the late Pliocene. Both lineages of Madagascan Ixora exhibit morphological characters that are rare throughout the rest of the genus, including pauciflorous inflorescences and extreme corolla tube length.

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Biomechanics of aquatic plants under aerial conditions

Biomechanics of aquatic plants under aerial conditions

Biomechanics of aquatic plants under aerial conditions

Terrestrial plants don’t like being underwater for long periods of time – as farmers across the UK have found out to their cost in the past few months. But aquatic plants are not designed to “work” out of the water either. Normally, the aquatic environment supports much of the weight of the plant and consequently, aquatic plants don’t devote as much of their resources to the sort of structural tissues required to hold up land plants.

But what about plants from environments where the water level routinely fluctuates? Wetlands are impacted by hydrological regimes that can lead to periods of low water levels. During these periods, aquatic plants experience a drastic change in the mechanical conditions that they encounter, from low gravitational and tensile hydrodynamic forces when exposed to flow under aquatic conditions, to high gravitational and bending forces under terrestrial conditions. The objective of this study was to test the capacity of aquatic plants to produce self-supporting growth forms when growing under aerial conditions by assessing their resistance to terrestrial mechanical conditions and the associated morpho-anatomical changes.

A recent paper in Annals of Botany investigates the capacity of aquatic plants from eight genera to produce self-supportive phenotypes capable of resisting terrestrial mechanical conditions.

They find that six species show higher stiffness in bending, either as the result of an increased allocation to strengthening tissues or by an increase in cross-sectional area in the organs bearing the mechanical forces. These plastic responses may play a key role in the ability of the species to colonize highly fluctuating environments, but reduced capacity for plants to tolerate aquatic mechanical conditions when water level rises again could represent a cost of producing a growth form adapted to aerial conditions.

Hamann, E., & Puijalon, S. (2013). Biomechanical responses of aquatic plants to aerial conditions. Annals of botany, 112(9), 1869-1878.

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Abscisic acid underlies genotypic variation in stomatal responses

Abscisic acid underlies genotypic variation in stomatal responses

Abscisic acid underlies genotypic variation in stomatal responses

Stomata formed at high relative humidity (RH) are less responsive to abscisic acid (ABA), an effect that varies widely between genotypes. Giday et al. study four rose cultivars (Rosa hybrida) grown at 60% and 90% RH and find stomatal responsiveness to desiccation and ABA feeding to be attenuated in two of them at high RH. [ABA] is lower in plants grown at high RH, an effect that is more pronounced in these sensitive cultivars. They determine that the sensitive cultivars undergo a larger decrease in [ABA], rather than having a higher [ABA] threshold for inducing stomatal functioning. However, the cultivar differences in stomatal closure following ABA feeding are not apparent in response to H2O2 and downstream elements, indicating that signalling events prior to H2O2 generation are involved in the observed genotypic variation.

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Corolla morphology and diversification rates in toadflaxes

Corolla morphology and diversification rates in toadflaxes

Corolla morphology and diversification rates in toadflaxes

The role of flower specialization in plant speciation and evolution remains controversial. Fernández-Mazuecos et al. use a time-calibrated phylogeny in conjunction with morphometric analysis to study bifid toadflaxes (Linaria sect. Versicolores), which have highly specialized corollas. They determine that a restrictive character state (narrow corolla tube) is reconstructed in the most-recent common ancestor. After its early loss in the most species-rich clade, this character state has been convergently reacquired in multiple lineages of this clade in recent times, yet it seems to have exerted a negative influence on diversification rates. The results suggest that opposing individual-level and species-level selection pressures may have driven the evolution of pollinator-restrictive traits in the bifid toadflaxes.

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Stabilization of yield in plant genotype mixtures

Stabilization of yield in plant genotype mixtures

Stabilization of yield in plant genotype mixtures

Sowing crops as mixtures of varieties instead of a monoculture can result in more stable yields, especially in variable environments. Creissen et al. grow dissimilar Arabidopsis thaliana genotypes in monocultures and mixtures under strong competition and abiotic stress, and find that mixtures achieve more stable seed production through compensatory interactions. Competitive ability and performance in mixtures can be predicted from above-ground traits, even though below-ground competition appears to be more intense. The results suggest that phenotype screens of varieties could improve the choice of mixtures for agriculture in unpredictable environmental conditions.

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Measuring the flow of transgenes from GM crops into wild plants

Gene flow under contrasting levels of human disturbance

Gene flow under contrasting levels of human disturbance

Rapid development of biotechnology offers new opportunities to ensure our future food supply. Novel traits can be introduced into crops by transgene technology more efficiently than by conventional crop breeding. Since the first commercialization of a genetically modified (GM) crop in 1996, the global area of GM crops has grown steadily and reached 170·3 million hectares in 2012. Increasing numbers of GM crops with different traits are being produced and released into the environment. The introduction of new transgenes into crops has raised concerns about possible negative effects on the environment. Transgenes could move from crops into wild relatives via gene flow. Depending on the nature of the transgene and its product, such transgene flow may lead to unwanted ecological and evolutionary consequences in wild populations

The process of transgene flow from crops into wild relatives involves several steps: first, the formation of crop–wild hybrids with a transgene through hybridization between crops and wild populations; second, the establishment of the transgene in local wild populations through backcrossing with wild plants; third, the spread of the transgene across the whole metapopulation of the wild species via pollen and seed dispersal. The majority of previous studies have focused only on evaluating the first two steps of transgene introgression. A recent paper in Annals of Botany examines the role of metapopulation dynamics in transgene spread.

Wild populations close to crop fields are usually strongly affected by human disturbance. Habitat loss and fragmentation due to human disturbance may alter the level of gene flow among patches and the rate of patch turnover. If gene dispersal becomes limited under strong human disturbance, the distribution pattern of genetic diversity may change dramatically in the metapopulation. In this case, a newly emerged gene, such as a transgene in a local wild population, may not be able to spread through the metapopulation. Conversely, human-mediated dispersal may enhance connectivity among populations in areas where anthropogenic disturbance is high, which would lead to increased spread of an escaped transgene. However, it is difficult to study the effects of human disturbance and associated habitat changes on gene flow, because finding intact wild populations as controls is hard and the effects of other factors may interfere with those of human disturbance.

The authors compared historical and contemporary patterns of gene flow in a wild carrot metapopulation, testing the null hypothesis that human disturbance did not change gene flow in the metapopulation and that contemporary gene flow was similar to historical gene flow in wild carrots and aiming to answer the following questions:

  1. What is the rate of gene flow in the wild carrot metapopulation?
  2. Is contemporary gene flow equal to historical gene flow in the wild carrot metapopulation?
  3. How does the rate of gene flow affect the chance of transgene introgression into the wild carrot metapopulation?

They found that the contemporary gene flow was five times higher than the historical estimate, and the correlation between them was very low. Moreover, the contemporary gene flow in roadsides was twice that in a nature reserve, and the correlation between contemporary and historical estimates was much higher in the nature reserve. Mowing of roadsides may contribute to the increase in contemporary gene flow.

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