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.

Sexually deceptive orchids

Chiloglottis trapeziformis Plants use a diverse range of visual and olfactory cues to attract animal pollinators, with floral volatiles often particularly critical for securing insect pollination. More than 1700 different compounds have been documented in plant floral odours, however relatively few studies have determined the precise role of these compounds in pollinator attraction. There are even fewer cases where we have a comprehensive understanding of the physiology, biochemistry and biosynthesis of the floral volatile compounds involved in pollination.

The sexually deceptive orchids are a group of plants for which specific semiochemicals play a key role in pollination. These orchids lure specific male insect pollinators to their flower by emitting volatile semiochemicals that mimic the female sex pheromone. Remarkably, this pollination strategy has evolved independently in orchids on at least four continents (Africa, Australia, Europe and South America), and new discoveries continue to be made within this family.

A recent paper in Annals of Botany studies the sexually deceptive Chiloglottis trapeziformis orchid, which attracts its specific male wasp pollinators by means of chiloglottones, a newly discovered class of volatiles with unique structures. The authors find that while initiation of chiloglottone biosynthesis requires only UV-B light, sustained production requires both UV-B and de novo protein synthesis. The amount of chiloglottone in flowers reflects the interplay between developmental stage, time and intensity of UV-B exposure, de novo protein synthesis, and feedback loops that cap the internal amount.

 

Amarasinghe, R., Poldy, J., Matsuba, Y., Barrow, R. A., Hemmi, J. M., Pichersky, E., & Peakall, R. (2015) UV-B light contributes directly to the synthesis of chiloglottone floral volatiles. Annals of Botany, 115(4): 693-703
Australian sexually deceptive Chiloglottis orchids attract their specific male wasp pollinators by means of 2,5-dialkylcyclohexane-1,3-diones or ‘chiloglottones’, representing a newly discovered class of volatiles with unique structures. This study investigated the hypothesis that UV-B light at low intensities is directly required for chiloglottone biosynthesis in Chiloglottis trapeziformis. Chiloglottone production occurs only in specific tissue (the callus) of the labellum. Cut buds and flowers, and whole plants with buds and flowers, sourced from the field, were kept in a growth chamber and interactions between growth stage of the flowers and duration and intensity of UV-B exposure on chiloglottone production were studied. The effects of the protein synthesis inhibitor cycloheximide were also examined. Chiloglottone was not present in buds, but was detected in buds that were manually opened and then exposed to sunlight, or artificial UV-B light for ≥5 min. Spectrophotometry revealed that the sepals and petals blocked UV-B light from reaching the labellum inside the bud. Rates of chiloglottone production increased with developmental stage, increasing exposure time and increasing UV-B irradiance intensity. Cycloheximide did not inhibit the initial production of chiloglottone within 5 min of UV-B exposure. However, inhibition of chiloglottone production by cycloheximide occurred over 2 h of UV-B exposure, indicating a requirement for de novo protein synthesis to sustain chiloglottone production under UV-B. The sepals and petals of Chiloglottis orchids strongly block UV-B wavelengths of light, preventing chiloglottone production inside the bud. While initiation of chiloglottone biosynthesis requires only UV-B light, sustained chiloglottone biosynthesis requires both UV-B and de novo protein biosynthesis. The internal amounts of chiloglottone in a flower reflect the interplay between developmental stage, duration and intensity of UV-B exposure, de novo protein synthesis, and feedback loops linked to the starting amount of chiloglottone. It is concluded that UV-B light contributes directly to chiloglottone biosynthesis. These findings suggest an entirely new and unexpected biochemical reaction that might also occur in taxa other than these orchids.

Climate change and wood formation

Tibetan Plateau In cold climates, seasonal changes in trees (phenology) is primarily controlled by temperature. Experiments inducing artificial heating of the stem in evergreen conifers during the quiescent stage showed that it is possible to induce reactivation of cell division, demonstrating that temperature is a trigger for the formation of wood (xylogenesis). The higher temperatures predicted for the future are expected to advance xylem formation in spring, although photoperiodic constraints could restrict the responses of some boreal and temperate species to climate warming. Most studies in temperate and cold climates have been performed in ecosystems in which snowmelt provides abundant water especially at the beginning of the growing seasons, precipitation is frequent in spring and summer, and water availability is not a limiting factor for xylem formation. Little is known about the effect of precipitation on the onset of xylogenesis in cold drought-prone areas.

A recent paper in Annals of Botany monitors xylogenesis in Juniperus przewalskii under extreme dry conditions on the north-eastern Tibetan Plateau and finds that precipitation in the early growing season can be a critical trigger of xylogenesis when the thermal conditions are favourable. Xylem growth shows a positive and significant response to precipitation but not to temperature. The delay in the initiation of xylogenesis under extremely dry conditions seems to be a stress-avoidance strategy against hydraulic failure.

Rising temperatures since the late 1970s have increased aridity in many continental regions of the globe, exposing semi-arid forests in particular to additional stress. Such drought stress may postpone the initiation of xylogenesis, reduce cell production or contribute to radial growth decline. Long-term monitoring is necessary to detect potential thresholds in precipitation or soil moisture for the onset of xylogenesis.

 

Ren, P., Rossi, S., Gricar, J., Liang, E., & Cufar, K. (2015) Is precipitation a trigger for the onset of xylogenesis in Juniperus przewalskii on the north-eastern Tibetan Plateau? Annals of botany, 115(4): 629-639
A series of studies have shown that temperature triggers the onset of xylogenesis of trees after winter dormancy. However, little is known about whether and how moisture availability influences xylogenesis in spring in drought-prone areas.
Xylogenesis was monitored in five mature Qilian junipers (Juniperus przewalskii) by microcore sampling from 2009 to 2011 in a semi-arid area of the north-eastern Tibetan Plateau. A simple physical model of xylem cell production was developed and its sensitivity was analysed. The relationship between climate and growth was then evaluated, using weekly wood production data and climatic data from the study site.
Delayed onset of xylogenesis in 2010 corresponded to a negative standardized precipitation evapotranspiration index (SPEI) value and a continuous period without rainfall in early May. The main period of wood formation was in June and July, and drier conditions from May to July led to a smaller number of xylem cells. Dry conditions in July could cause early cessation of xylem differentiation. The final number of xylem cells was mainly determined by the average production rate rather than the duration of new cell production. Xylem growth showed a positive and significant response to precipitation, but not to temperature.
Precipitation in late spring and summer can play a critical role in the onset of xylogenesis and xylem cell production. The delay in the initiation of xylogenesis under extremely dry conditions seems to be a stress-avoidance strategy against hydraulic failure. These findings could thus demonstrate an evolutionary adaptation of Qilian juniper to the extremely dry conditions of the north-eastern Tibetan Plateau.

Why are carnivorous plants?

Carnivorous plants

Carnivorous plants have long fascinated scientists, and were described by Charles Darwin in the book Insectivorous plants (Darwin, 1875). Carnivorous plants typically attract, capture and digest animal prey by modified leaves called traps. No carnivorous plant is able to capture prey by its flower. Givnish et al. (1984) proposed that a plant must fulfil two basic requirements to be considered as carnivorous. First, it must be able to absorb nutrients from dead prey, and thereby obtain some increment to fitness in terms of increased growth, pollen production or seed set. Secondly, the plant must have some adaptation or resource allocation whose primary result is the active attraction, capture and/or digestion of prey. The first is needed to differentiate carnivory from defensive adaptation that immobilizes or kills animal enemies without leading to substantial nutrient absorption and thus increased plant survival. The second is required because many plants can passively profit by absorbing some nutrients from dead animals decomposing in the soil or on leaf surfaces. A plant must have at least one adaptation (i.e. active attraction, capture and digestion) in combination with nutrient absorption to be qualified as carnivorous, because many genera of carnivorous plants lack some of these attributes.

A long-standing problem in evolutionary biology, i.e. an explanation for the ecological conditions under which botanical carnivory is likely to evolve repeatedly, was resolved by Givnish et al. (1984). Several comprehensive reviews of the rise of carnivorous plants have been published over the past decade, all focusing on trade-offs among physiological and morphological traits. A new paper in Annals of Botany reviews the cost–benefit model for the evolution of botanical carnivory in view of new data on the molecular biology of trap leaves and highlights the importance of energetic costs of active trapping mechanisms. It also address the similarities between carnivory and plant defence mechanisms and the role of jasmonate signalling in carnivory and extends the intepretation of the cost–benefit model to alternative nutrient sequestration strategies in carnivorous plants.

 

Andrej Pavlovič and Michaela Saganová. A novel insight into the cost–benefit model for the evolution of botanical carnivory. Annals of Botany 06 May 2015 doi: 10.1093/aob/mcv050
The cost–benefit model for the evolution of botanical carnivory provides a conceptual framework for interpreting a wide range of comparative and experimental studies on carnivorous plants. This model assumes that the modified leaves called traps represent a significant cost for the plant, and this cost is outweighed by the benefits from increased nutrient uptake from prey, in terms of enhancing the rate of photosynthesis per unit leaf mass or area (AN) in the microsites inhabited by carnivorous plants. This review summarizes results from the classical interpretation of the cost–benefit model for evolution of botanical carnivory and highlights the costs and benefits of active trapping mechanisms, including water pumping, electrical signalling and accumulation of jasmonates. Novel alternative sequestration strategies (utilization of leaf litter and faeces) in carnivorous plants are also discussed in the context of the cost–benefit model. Traps of carnivorous plants have lower AN than leaves, and the leaves have higher AN after feeding. Prey digestion, water pumping and electrical signalling represent a significant carbon cost (as an increased rate of respiration, RD) for carnivorous plants. On the other hand, jasmonate accumulation during the digestive period and reprogramming of gene expression from growth and photosynthesis to prey digestion optimizes enzyme production in comparison with constitutive secretion. This inducibility may have evolved as a cost-saving strategy beneficial for carnivorous plants. The similarities between plant defence mechanisms and botanical carnivory are highlighted.

Co-ordinated carbohydrate metabolism and hormone synthesis enables plants to survive unfavourable field conditions

Chrysolaena obovata

Chrysolaena obovata. Photo by Mauricio Mercadante.

Chrysolaena obovata, an aster of the Brazilian Cerrado, presents seasonal growth, marked by senescence of aerial organs in winter and subsequent regrowth at the end of this season. The underground reserve organs, the rhizophores, accumulate inulin-type fructans and confer tolerance to drought and low temperature. Fructans and fructan-metabolizing enzymes show a characteristic spatial and temporal distribution in the rhizophores during the developmental cycle. Previous studies have shown correlations between abscisic acid (ABA) or indole acetic acid (IAA), fructans, dormancy and tolerance to drought and cold, but the signalling mechanism for the beginning of dormancy and sprouting in this species is still unknown. A new paper in Annals of Botany examines the fructan metabolism in this species in response to environmental changes.

Plants were sampled from the field across phenological phases including dormancy, sprouting and vegetative growth. Endogenous concentrations of ABA and IAA were determined and measurements made of fructan content and composition, and enzyme activities. The relative expression of corresponding genes during dormancy and sprouting were also determined. Plants showed a high fructan 1-exohydrolase activity and expression during sprouting in proximal segments of the rhizophores, indicating mobilization of fructan reserves, when ABA concentrations were relatively low and precipitation and temperature were at their minimum values. Higher IAA concentrations were consistent with the role of this regulator in promoting cell elongation and plant growth. With high rates of precipitation and high temperatures in summer, the fructan-synthesizing enzyme sucrose:sucrose 1-fructosyltransferase showed higher activity and expression in distal segments of the rhizophores, which decreased over the course of the vegetative stage when ABA concentrations were higher, possibly signalling the entry into dormancy.

These results show that fructan metabolism correlates well with endogenous hormone concentrations and environmental changes, suggesting that the co-ordinated action of carbohydrate metabolism and hormone synthesis enables C. obovata to survive unfavourable field conditions. Endogenous hormone concentrations seem to be related to regulation of fructan metabolism and to the transition between phenophases, signalling for energy storage, reserve mobilization and accumulation of oligosaccharides as osmolytes.

 

Rigui,A., Gaspar,M., Oliveira, V., Purgatto, P. and Machado de Carvalho, M. Endogenous hormone concentrations correlate with fructan metabolism throughout the phenological cycle in Chrysolaena obovata. Annals of Botany 28 April 2015 doi: 10.1093/aob/mcv053

Reaction of Tibetan plants to climate change

Saussurea nigrescens Asymmetric warming is one of the distinguishing features of global climate change, in which winter and night-time temperatures are predicted to increase more than summer and diurnal temperatures. Winter warming weakens vernalization and hence decreases the potential to flower for some perennial herbs, and night warming can reduce carbohydrate concentrations in storage organs. A new paper in Annals Of Botany examines whether asymmetric warming acts to reduce flower number and nectar production per flower in a perennial herb, Saussurea nigrescens, a key nectar plant for pollinators in Tibetan alpine meadows.

A long-term (6 years) warming experiment was conducted using open-top chambers placed in a natural meadow and manipulated to achieve asymmetric increases in temperature during the growing and non-growing seasons. Measurements were taken of nectar volume and concentration (sucrose content) and leaf non-structural carbohydrate content and plant morphology. Six years of experimental warming resulted in reductions in nectar volume per floret, floret number per capitulum and capitulum number per plant, whereas nectar concentration remained unchanged. Depletion of leaf non-structural carbohydrates was significantly higher in the warmed than in the ambient condition. Overall plant density was also reduced by warming, which, when combined with reductions in flower development and nectar volumes, led to a reduction of ∼90 % in nectar production per unit area.

The negative effect of asymmetric warming on nectar yields in S. nigrescens may be explained by a depletion of leaf non-structural carbohydrates. The results highlights a novel aspect of how climate change might affect plant–pollinator interactions and plant reproduction via induction of allocation shifts for plants growing in communities subject to asymmetric warming.

 

Mu, J., Peng, Y., Xi, X., Wu, X., Li, G., Niklas, K.J. and Sun,S. Artificial asymmetric warming reduces nectar yield in a Tibetan alpine species of Asteraceae. Annals of Botany 28 April 2015 doi: 10.1093/aob/mcv042

Adventitious root formation in rice

Adventitious root formation in rice The root system is fundamentally important for plants to efficiently obtain nutrients and water. In contrast to the primary root system of plants, roots of monocot cereals consist almost entirely of a complex fibrous system and a mass of adventitious roots (ARs). AR formation is the process of root initiation from the stem base post-embryonically, which is tightly regulated to prevent the loss of valuable plant resources for non-essential root formation. A lack of stable and credible morphological data makes it difficult to study physiological and molecular mechanisms governing AR growth. However, comprehensive understanding of AR development should have important implications for manipulating root architecture, which contributes to both improving crop yield and optimizing agricultural land use.

Several plant hormones control AR formation, in which auxin plays a pivotal role. Indole-3-acetic acid (IAA) is the predominant form of active auxin in plants and it induces both AR and lateral root (LR) formation. Strigolactones (SLs) and their derivatives are plant hormones that have recently been identified as regulating root development. This study examines whether SLs play a role in mediating production of adventious roots (ARs) in rice (Oryza sativa), and also investigates possible interactions between SLs and auxin.

 

Sun, Huwei, Jinyuan Tao, Mengmeng Hou, Shuangjie Huang, Si Chen, Zhihao Liang, Tianning Xie et al. (2015) A strigolactone signal is required for adventitious root formation in rice. Annals of Botany doi: 10.1093/aob/mcv052
Strigolactones (SLs) and their derivatives are plant hormones that have recently been identified as regulating root development. This study examines whether SLs play a role in mediating production of adventious roots (ARs) in rice (Oryza sativa), and also investigates possible interactions between SLs and auxin. Wild-type (WT), SL-deficient (d10) and SL-insensitive (d3) rice mutants were used to investigate AR development in an auxin-distribution experiment that considered DR5::GUS activity, [3H] indole-3-acetic acid (IAA) transport, and associated expression of auxin transporter genes. The effects of exogenous application of GR24 (a synthetic SL analogue), NAA (α-naphthylacetic acid, exogenous auxin) and NPA (N-1-naphthylphalamic acid, a polar auxin transport inhibitor) on rice AR development in seedlings were investigated. The rice d mutants with impaired SL biosynthesis and signalling exhibited reduced AR production compared with the WT. Application of GR24 increased the number of ARs and average AR number per tiller in d10, but not in d3. These results indicate that rice AR production is positively regulated by SLs. Higher endogenous IAA concentration, stronger expression of DR5::GUS and higher [3H] IAA activity were found in the d mutants. Exogenous GR24 application decreased the expression of DR5::GUS, probably indicating that SLs modulate AR formation by inhibiting polar auxin transport. The WT and the d10 and d3 mutants had similar expression of DR5::GUS regardless of exogenous application of NAA or NPA; however, AR number was greater in the WT than in the d mutants. The results suggest that AR formation is positively regulated by SLs via the D3 response pathway. The positive effect of NAA application and the opposite effect of NPA application on AR number of WT plants also suggests the importance of auxin for AR formation, but the interaction between auxin and SLs is complex.

Stomatal density and aperture in non-vascular plants are non-responsive to above-ambient CO2 concentrations

Stomatal density Stomata are one of the crucial adaptations in the evolution of the land flora and the development of the terrestrial landscape and atmosphere on Earth. These microscopic pores on the plant epidermis first appeared in the fossil record more than 400 million years ago, some 50–60 million years after the first land plants. Today they are found on the sporophyte generations of all land plant groups with exceptions only in the liverworts, the earliest moss lineages and a few derived hornwort clades. Exposure to high atmospheric CO2 concentration ([CO2]) has been shown to consistently result in a reduction of stomatal density (number of stomata per mm2) and index (ratio of stomata to epidermal cells) in the newly developed leaves of many vascular plant species. The general consensus view is that stomatal morphology is conserved throughout land plants and that their primary function is related to the regulation of gas and water exchange, [CO2] being a key activator of stomatal frequency. However, a crucial missing piece in the jigsaw of stomatal evolution and function is the responsiveness of bryophyte stomata to [CO2]. In the only previous studies on the effects of [CO2] on stomata in non-vascular plants recorded larger apertures in the mosses Physcomitrella and Funaria grown in the absence of CO2.

A recent paper in Annals of Botany challenges the widely accepted dogma that the responsiveness of stomata to [CO2] in terms of density and opening is conserved across the land plant phylogeny through careful experimentation and cytological observation and asks: (1) Are stomatal numbers on moss and hornwort sporophytes affected in the same way as those in angiosperms by elevated [CO2] representative of atmospheric concentrations in the Palaeozoic? (2) Do guard cell lengths and apertures in bryophytes change when subjected to representative Palaeozoic [CO2] throughout development?

 

Field, K. J., Duckett, J. G., Cameron, D. D., & Pressel, S. (2015) Stomatal density and aperture in non-vascular land plants are non-responsive to above-ambient atmospheric CO2 concentrations. Annals of Botany April 8, 2015 doi: 10.1093/aob/mcv021
Following the consensus view for unitary origin and conserved function of stomata across over 400 million years of land plant evolution, stomatal abundance has been widely used to reconstruct palaeo-atmospheric environments. However, the responsiveness of stomata in mosses and hornworts, the most basal stomate lineages of extant land plants, has received relatively little attention. This study aimed to redress this imbalance and provide the first direct evidence of bryophyte stomatal responsiveness to atmospheric CO2.
A selection of hornwort (Anthoceros punctatus, Phaeoceros laevis) and moss (Polytrichum juniperinum, Mnium hornum, Funaria hygrometrica) sporophytes with contrasting stomatal morphologies were grown under different atmospheric CO2 concentrations ([CO2]) representing both modern (440 p.p.m. CO2) and ancient (1500 p.p.m. CO2) atmospheres. Upon sporophyte maturation, stomata from each bryophyte species were imaged, measured and quantified. Densities and dimensions were unaffected by changes in [CO2], other than a slight increase in stomatal density in Funaria and abnormalities in Polytrichum stomata under elevated [CO2]. The changes to stomata in Funaria and Polytrichum are attributed to differential growth of the sporophytes rather than stomata-specific responses. The absence of responses to changes in [CO2] in bryophytes is in line with findings previously reported in other early lineages of vascular plants. These findings strengthen the hypothesis of an incremental acquisition of stomatal regulatory processes through land plant evolution and urge considerable caution in using stomatal densities as proxies for paleo-atmospheric CO2 concentrations.

Transcriptome and Genome of the Venus Flytrap

Venus Flytrap Dionaea muscipula Darwin was fascinated by the unusual adaptations of carnivorous plants during his often frustrating studies of the evolution of flowering plants, which he referred to as an ‘abominable mystery’. Darwin’s treatise on insectivorous plants noted that the Venus flytrap (Dionaea muscipula) was ‘one of the most wonderful of the world’. Studies of carnivorous plants have continued since Darwin’s time.

An understanding of the molecular adaptations to plant carnivory has also been sought via genome size estimates. Genome sizes vary more than 2,300-fold among angiosperms, from that of Paris japonica (~149 Gbp) to that of carnivorous Genlisea margaretae (~63 Mbp). The biological significance of this massive variation is puzzling.

A new paper in PLOS ONE examines the transcriptome and genome of the Venus flytrap, Dionaea muscipula. The transcriptome provides some insight into the molecular processes occurring in a Gb-sized carnivorous plant genome. Abundant representation of processes related to the expression of genes associated with catalytic, antioxidant and electron carrier activities was observed. Future uniform meta-analyses of short-read archives, including cDNA sequences from carnivorous Utricularia and Sarracenia species will aid studies of carnivorous plants and their ecology. This underlines the importance of further expansion of sequence repositories, especially for non-model organisms, for improved understanding of molecular physiology and evolution related to Darwin’s ‘abominable mystery’.

 

Jensen MK, Vogt JK, Bressendorff S, Seguin-Orlando A, Petersen M, et al. (2015) Transcriptome and Genome Size Analysis of the Venus Flytrap. PLoS ONE 10(4): e0123887. doi:10.1371/journal.pone.0123887
Abstract
The insectivorous Venus flytrap (Dionaea muscipula) is renowned from Darwin’s studies of plant carnivory and the origins of species. To provide tools to analyze the evolution and functional genomics of D. muscipula, we sequenced a normalized cDNA library synthesized from mRNA isolated from D. muscipula flowers and traps. Using the Oases transcriptome assembler 79,165,657 quality trimmed reads were assembled into 80,806 cDNA contigs, with an average length of 679 bp and an N50 length of 1,051 bp. A total of 17,047 unique proteins were identified, and assigned to Gene Ontology (GO) and classified into functional categories. A total of 15,547 full-length cDNA sequences were identified, from which open reading frames were detected in 10,941. Comparative GO analyses revealed that D. muscipula is highly represented in molecular functions related to catalytic, antioxidant, and electron carrier activities. Also, using a single copy sequence PCR-based method, we estimated that the genome size of D. muscipula is approx. 3 Gb. Our genome size estimate and transcriptome analyses will contribute to future research on this fascinating, monotypic species and its heterotrophic adaptations.

How tree species fill geographic and ecological space in North America

Distribution of forest plots Ecologists broadly accept that the number of species present within a region balances regional processes of immigration and speciation against competitive and other interactions between populations that limit distribution and constrain diversity. Although ecological theory has, for a long time, addressed the premise that ecological space can be filled to ‘capacity’ with species, only with the availability of time-calibrated phylogenies has it been possible to test the theory that diversification slows as the number of species in a region increases. Focusing on the deciduous trees of eastern North America, this study tested predictions from competition theory concerning the distribution and abundance of species.

Local assemblages of trees tabulated in a previous study published in 1950 were analysed. Assemblages were ordinated with respect to species composition by non-metric multidimensional scaling (NMS). Most of the variance in species abundance and distribution was concentrated among closely related (i.e. congeneric) species, indicating evolutionary lability. Species distribution and abundance were unrelated to the number of close relatives, suggesting that competitive effects are diffuse. Distances between pairs of species in NMS space did not differ significantly from distances between more distantly related species, in contrast to the predictions of both competitive habitat partitioning and ecological sorting of species.

Eastern deciduous forests of North America do not appear to be saturated with species. The distributions and abundances of individual species provide little evidence of being shaped by competition from related, ecologically similar, species. Diversification is constrained by interspecific competition.

Ricklefs, R.E. How tree species fill geographic and ecological space in eastern North America (2015) Annals of Botany, April 7, 2015. doi: 10.1093/aob/mcv029