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.

How orchids feed specialized bee pollinators

A significant proportion of orchids in the subtribe Oncidiinae produce floral oil as a food reward that attracts specialized bee pollinators. This oil is produced either by glands (epithelial elaiophores) or by tufts of secretory hairs (trichomal elaiophores). Although the structure of epithelial elaiophores has been well documented, trichomal elaiophores are less common and have not received as much attention.

Variation in floral morphology in the genus Lockhartia

The flowers of Lockhartia are 5–30 mm in length and lack fragrance perceptible to humans. Oil secretion by flowers of Lockhartia was first reported by Silvera (2002), but the morphology and anatomy of their elaiophores have not previously been studied in detail. A recent paper in Annals of Botany surveys the flowers of 16 species of Lockhartia and shows that all have elaiophores (oil glands) of the trichomal type.

Specialized hairs on the legs or abdomen (but not the mouthparts) of oil-gathering bees are used to collect oils, and the latter are then used as food for larvae. Pollinaria of Lockhartia are small (typically 0·7–1·3 mm long) and their attachment to the bodies of bees has not been reported. This may be due to the fact that the thin stipe collapses upon drying and this obfuscates identification of the pollinarium to generic level. The situation is further exacerbated by the fast-flying and extremely timid nature of oil-collecting bees. As a result, they are much more difficult to capture or observe from short distances than male euglossine bees, for which an abundance of observational data exists.

Blanco, M. A., Davies, K. L., Stpiczyńska, M., Carlsward, B. S., Ionta, G. M., & Gerlach, G. (2013). Floral elaiophores in Lockhartia Hook. (Orchidaceae: Oncidiinae): their distribution, diversity and anatomy. Annals of Botany, 112(9), 1775-1791.

 

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Morphology of lateral root development – free review article

Morphology of lateral root development

Morphology of lateral root development

The crucial role of roots in plant nutrition, and consequently in plant productivity, is a strong motivation to study the growth and functioning of various aspects of the root system. Numerous studies on lateral roots mostly focus on the physiological and molecular bases of developmental processes. Unfortunately, little attention is paid either to the morphological changes accompanying the formation of a lateral root or to morphological defects occurring in lateral root primordia. The latter are observed in some mutants and occasionally in wild-type plants, but may also result from application of external factors.

A recent free review article in Annals of Botany discusses morphological aspects of lateral branching in roots are analysed, examining studies that have looked at developmental changes in lateral root morphology in order to understand better the process of lateral root development.

Our knowledge of the molecular bases of lateral root initiation and development has increased rapidly within recent decades. Building on these advances, we may try to widen our knowledge of the probable relation between auxin and root system morphology, based in part on the auxin-related mutants whose root growth and development are altered in comparison with wild-type plants. Yet it is important to remember that, as a physical object, the lateral root (as well as other plant organs) also has characteristic physical properties. A change of form of such an object implies either a change in the distribution of mechanical stress or a change in mechanical properties. Direct measurement of both of these remains a challenge, mostly because of technical difficulties. However, the few reports examining the mechanical parameters of tissues of roots show that no challenge in science is so great that it is not taken up.

Szymanowska-Pułka, J. (2013). Form matters: morphological aspects of lateral root development. Annals of botany, 112(9), 1643-1654.

 

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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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Revving up photosynthesis with nanotechnology – take that, greenhouse effect!

Carbon nanotube What happens when you insert single-walled carbon nanotubes into the leaves of Arabidopsis? The semiconducting nanotubes integrate themselves into the chloroplasts’ outer envelope and triple photosynthetic activity by enhancing electron transport.

So should we be making genetically modified plants containing carbon nanotubes? Well probably not – you have to believe that 3.5 billion years of evolution has optimised photosynthesis pretty well to achieve a nice balance. But that doen’t mean that this research is without applications, such as making living leaves that perform non-biological functions (for example, detecting pollutants or pesticides), or constructing artificial energy harvesting systems which don’t contribute to climate change.

Plant nanobionics approach to augment photosynthesis and biochemical sensing. (2014) Nature Materials 13, 400–408 doi:10.1038/nmat3890 [Subscription]
Abstract: The interface between plant organelles and non-biological nanostructures has the potential to impart organelles with new and enhanced functions. Here, we show that single-walled carbon nanotubes (SWNTs) passively transport and irreversibly localize within the lipid envelope of extracted plant chloroplasts, promote over three times higher photosynthetic activity than that of controls, and enhance maximum electron transport rates. The SWNT–chloroplast assemblies also enable higher rates of leaf electron transport in vivo through a mechanism consistent with augmented photoabsorption. Concentrations of reactive oxygen species inside extracted chloroplasts are significantly suppressed by delivering poly(acrylic acid)–nanoceria or SWNT–nanoceria complexes. Moreover, we show that SWNTs enable near-infrared fluorescence monitoring of nitric oxide both ex vivo and in vivo, thus demonstrating that a plant can be augmented to function as a photonic chemical sensor. Nanobionics engineering of plant function may contribute to the development of biomimetic materials for light-harvesting and biochemical detection with regenerative properties and enhanced efficiency.

Bioinspired materials: Boosting plant biology. Nature Materials News & Views (2014) 13, 329–331 doi:10.1038/nmat3926 [Subscription]

 

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Another bacterial trick to manipulate plant hormone-mediated responses

Another bacterial trick to manipulate host hormone-mediated response Pseudomonas syringae is a widespread bacterial pathogen that causes disease on a broad range of economically important plant species. In order to infect, P. syringae produces a number of toxins and uses a type III secretion system to deliver effector proteins into eukaryotic cells. This mechanism is essential for successful infection by both plant- and animal-associated bacteria as bacterial mutants are no longer pathogenic. However, the molecular function and host targets of the vast majority of effectors remain largely unknown.

Plant immunity relies on a complex network of small-molecule hormone signaling pathways (see: Wasternack, C. (2007) Jasmonates: an update on biosynthesis, signal transduction and action in plant stress response, growth and development. Annals of botany, 100(4), 681-697). Classically, salicylic acid (SA) signaling mediates resistance against biotrophic and hemi-biotrophic microbes such as P. syringae, whereas a combination of jasmonic acid (JA) and ethylene (ET) pathways activates resistance against necrotrophs such as the fungus Botrytis cinerea. SA and JA/ET defense pathways generally antagonize each other – elevated resistance against biotrophs is often correlated with increased susceptibility to necrotrophs and vice versa. The collective contribution of these two hormones during plant-pathogen interactions is crucial to the success of the interaction. Remarkably, some Pseudomonas strains have evolved a sophisticated strategy for manipulating hormonal balance by producing the toxin coronatine (COR), which mimics the plant hormone jasmonate-isoleucine (JA-Ile). The JA-Ile pathway plays a key role in plant immunity by activating defenses against fungal pathogens, while promoting bacterial growth by inhibiting the salicylic acid (SA)-dependent defenses required for Pseudomonas resistance.

A recent paper in PLOS Biology reports that the effector HopX1 from a Pseudomonas syringae strain that does not produce COR exploits an alternative evolutionary strategy to activate the JA-Ile pathway. HopX1 encodes a cysteine protease that interacts with and promotes the degradation of key JA pathway repressors, the JAZ proteins. Correspondingly, ectopically expressing HopX1 in the model plant Arabidopsis induces the expression of JA-dependent genes, and natural infection with Pseudomonas producing HopX1 promotes bacterial growth in a similar fashion to COR. These results highlight a novel example by which a bacterial effector directly manipulates core regulators of hormone signaling to facilitate infection:

Gimenez-Ibanez S, Boter M, Fernández-Barbero G, Chini A, Rathjen JP, et al. (2014) The Bacterial Effector HopX1 Targets JAZ Transcriptional Repressors to Activate Jasmonate Signaling and Promote Infection in Arabidopsis. PLoS Biol 12(2): e1001792. doi:10.1371/journal.pbio.1001792

 

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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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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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Auxin distribution and nitrate in rice roots

Auxin distribution and nitrate in rice roots

Auxin distribution and nitrate in rice roots

Plants exhibit various mechanisms to adapt to different nutrient supply conditions. Among these mechanisms, the plasticity of root development is vital. Plant root systems continuously branch and form lateral roots. Lateral roots develop from founder cells in the pericycle, the outermost layer of the vascular cylinder (stele) of the root. Auxin plays a dominant role in the specification of founder cells that give rise to lateral root initiation and the later stages of development, and differential distribution of auxin is required for LR organogenesis.

Ammonium (NH4+) is the preferred form of N over NO3– in rice (Oryza sativa), due to its waterlogged growth environment. Although the predominant form of mineral N in bulk soil for paddy rice fields is likely to be NH4+, rice roots are exposed to partial NO3− nutrition by nitrification in the rice rhizosphere. The practice of intermittent flooding during rice cultivation, which causes an uneven distribution of NH4+ and NO3– within the soil horizon under field conditions, is being adopted by increasing numbers of Chinese farmers. In rice, the growth of lateral roots (LRs) is generally enhanced by partial nitrate nutrition. The roles of auxin distribution and polar transport in lateral root formation in response to localized nitrate availability are not known.

A recent paper in Annals of Botany examines two rice cultivars with differing responsiveness to nitrate and finds that initiation of lateral roots is only enhanced in the one with higher responsiveness. In this cultivar, auxin accumulation in the LR zone is greater in response to NO3compared with NH4+, and a greater number of auxin transporter genes show increased expression in the LR zone under PNN than in the less-responsive cultivar. The authors conclude that partial nitrate nutrition enhances auxin polar transport from the shoot to root and induces greater auxin flux into the lateral root zone, enhancing lateral root proliferation in the NO3-responsive ‘Nanguang’ cultivar.

 

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Where do olives come from?

Immature Green Olives Olives have a long and complex history. The origins of the Mediterranean cultivated olive (Olea europaea subsp. europaea) are hotly debated, but it is usually accepted that its domestication started in the Levant based on archaeological, historical and molecular evidence. Multiple local selections of cultivars has been suggested by genetic analyses, followed by secondary diversification of the crop followed the oleiculture diffusion over the whole Mediterranean basin. The contribution of western wild olives in this diversification process remains poorly understood.

A recent paper in Annals of Botany describes patterns of genetic differentiation in Mediterranean and Saharan olives, and tests for admixture between these taxa. Based on the results, the human-meditated diffusion of the oleiculture over the Mediterranean basin and the contribution of O. europaea subsp. laperrinei to the cultivated olive diversification are discussed. Although its genetic contribution is limited, it is clear from this work that Laperrine’s olive has been involved in the diversification of cultivated olives.

 

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