The proteome of Populus nigra woody root: response to bending

Background and Aims

Morphological and biomechanical alterations occurring in woody roots of many plant species in response to mechanical stresses are well documented; however, little is known about the molecular mechanisms regulating these important alterations. The first forest tree genome to be decoded is that of Populus, thereby providing a tool with which to investigate the mechanisms controlling adaptation of woody roots to changing environments. The aim of this study was to use a proteomic approach to investigate the response of Populus nigra woody taproot to mechanical stress.

Methods

To simulate mechanical perturbations, the taproots of 30 one-year-old seedlings were bent to an angle of 90 ° using a steel net. A spatial and temporal two-dimensional proteome map of the taproot axis was obtained. We compared the events occurring in the above-bending, central bending and below-bending sectors of the taproot.

Key Results

The first poplar woody taproot proteome map is reported here; a total of 207 proteins were identified. Spatial and temporal proteomic analysis revealed that factors involved in plant defence, metabolism, reaction wood formation and lateral root development were differentially expressed in the various sectors of bent vs. control roots, seemingly in relation to the distribution of mechanical forces along the stressed woody taproots. A complex interplay among different signal transduction pathways involving reactive oxygen species appears to modulate these responses.

Conclusions

Poplar woody root uses different temporal and spatial mechanisms to respond to mechanical stress. Long-term bending treatment seem to reinforce the defence machinery, thereby enabling the taproot to better overcome winter and to be ready to resume growth earlier than controls.     

Comparative analysis of plant carbohydrate active enZymes and their role in xylogenesis

Background

Carbohydrate metabolism is a key feature of vascular plant architecture, and is of particular importance in large woody species, where lignocellulosic biomass is responsible for bearing the bulk of the stem and crown. Since Carbohydrate Active enZymes (CAZymes) in plants are responsible for the synthesis, modification and degradation of carbohydrate biopolymers, the differences in gene copy number and regulation between woody and herbaceous species have been highlighted previously. There are still many unanswered questions about the role of CAZymes in land plant evolution and the formation of wood, a strong carbohydrate sink.

Results

Here, twenty-two publically available plant genomes were used to characterize the frequency, diversity and complexity of CAZymes in plants. We find that a conserved suite of CAZymes is a feature of land plant evolution, with similar diversity and complexity regardless of growth habit and form. In addition, we compared the diversity and levels of CAZyme gene expression during wood formation in trees using mRNA-seq data from two distantly related angiosperm tree species Eucalyptus grandis and Populus trichocarpa, highlighting the major CAZyme classes involved in xylogenesis and lignocellulosic biomass production.

Conclusions

CAZyme domain ratio across embryophytes is maintained, and the diversity of CAZyme domains is similar in all land plants, regardless of woody habit. The stoichiometric conservation of gene expression in woody and non-woody tissues of Eucalyptus and Populus are indicative of gene balance preservation.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1571-8) contains supplementary material, which is available to authorized users.     

The Eucalyptus terpene synthase gene family

Background

Terpenoids are abundant in the foliage of Eucalyptus, providing the characteristic smell as well as being valuable economically and influencing ecological interactions. Quantitative and qualitative inter- and intra- specific variation of terpenes is common in eucalypts.

Results

The genome sequences of Eucalyptus grandis and E. globulus were mined for terpene synthase genes (TPS) and compared to other plant species. We investigated the relative expression of TPS in seven plant tissues and functionally characterized five TPS genes from E. grandis. Compared to other sequenced plant genomes, Eucalyptus grandis has the largest number of putative functional TPS genes of any sequenced plant. We discovered 113 and 106 putative functional TPS genes in E. grandis and E. globulus, respectively. All but one TPS from E. grandis were expressed in at least one of seven plant tissues examined. Genomic clusters of up to 20 genes were identified. Many TPS are expressed in tissues other than leaves which invites a re-evaluation of the function of terpenes in Eucalyptus.

Conclusions

Our data indicate that terpenes in Eucalyptus may play a wider role in biotic and abiotic interactions than previously thought. Tissue specific expression is common and the possibility of stress induction needs further investigation. Phylogenetic comparison of the two investigated Eucalyptus species gives insight about recent evolution of different clades within the TPS gene family. While the majority of TPS genes occur in orthologous pairs some clades show evidence of recent gene duplication, as well as loss of function.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1598-x) contains supplementary material, which is available to authorized users.     

Ontogenetic patterns in the mechanisms of tolerance to herbivory in Plantago

Background and Aims

Herbivory and plant defence differ markedly among seedlings and juvenile and mature plants in most species. While ontogenetic patterns of chemical resistance have been the focus of much research, comparatively little is known about how tolerance to damage changes across ontogeny. Due to dramatic shifts in plant size, resource acquisition, stored reserves and growth, it was predicted that tolerance and related underlying mechanisms would differ among ontogenetic stages.

Methods

Ontogenetic patterns in the mechanisms of tolerance were investigated in Plantago lanceolata and P. major (Plantaginaceae) using the genetic sib-ship approach. Pot-grown plants were subjected to 50 % defoliation at the seedling, juvenile and mature stages and either harvested in the short-term to look at plasticity in growth and photosynthesis in response to damage or allowed to grow through seed maturation to measure phenology, shoot compensation and reproductive fitness.

Key Results

Tolerance to defoliation was high in P. lanceolata, but low in P. major, and did not vary among ontogenetic stages in either species. Mechanisms underlying tolerance did vary across ontogeny. In P. lanceolata, tolerance was significantly related to flowering (juveniles) and pre-damage shoot biomass (mature plants). In P. major, tolerance was significantly related to pre-damage root biomass (seedlings) and induction of non-photochemical quenching, a photosynthetic parameter (juveniles).

Conclusions

Biomass partitioning was very plastic in response to damage and showed associations with tolerance in both species, indicating a strong role in plant defence. In contrast, photosynthesis and phenology showed weaker responses to damage and were related to tolerance only in certain ontogenetic stages. This study highlights the pivotal role of ontogeny in plant defence and herbivory. Additional studies in more species are needed to determine how seedlings tolerate herbivory in general and whether mechanisms vary across ontogeny in consistent patterns.     

The cost of myrmecophytism: insights from allometry of stem secondary growth

Background and Aims

Plant defence traits against herbivores incur production costs that are usually difficult to measure. However, estimating these costs is a prerequisite for characterizing the plant defence strategy as a whole. Myrmecophytes are plants that provide symbiotic ants with specialized nesting cavities, called domatia, in exchange for protection against herbivores. In the particular case of stem domatia, production of extra wood seems to be the only associated cost, making this indirect defence trait a particularly suitable model for estimating the cost of defence.

Methods

Measurements were made of growth pattern and cumulative production cost of domatia over secondary growth in the myrmecophyte Leonardoxa africana subsp. africana, whose internodes display both a solid basal segment and a hollow distal part (the domatium), thus allowing paired comparison of investment in wood.

Key Results

Previous studies showed that ‘overconstruction’ of the hollow part of internodes during primary growth is needed for mechanical support. In this study, it is shown that the relationship between the woody cross-sectional area of the solid and hollow parts of internodes is negatively allometric at the beginning of secondary growth and nearly isometric later on. Thus, in hollow stems, the first phase of slow secondary growth compensates for the ‘overconstruction’ of the ring of wood during primary growth. Moreover, the cumulative production cost of a domatium (estimated as the additional volume of wood required for a hollow stem compared with a solid one) is very high at the beginning of secondary growth and then quickly tends to zero.

Conclusions

Making domatia incurs high costs early in ontogeny, costs that are then amortized later in development of stems and of individual plants. Characterizing ontogenetic variation of the net cost of this peculiar defence mechanism will help us build more accurate theoretical models of resource allocation in myrmecophytes.     

Epigenetic variation in plant responses to defence hormones

Background and Aims

There is currently much speculation about the role of epigenetic variation as a determinant of heritable variation in ecologically important plant traits. However, we still know very little about the phenotypic consequences of epigenetic variation, in particular with regard to more complex traits related to biotic interactions.

Methods

Here, a test was carried out to determine whether variation in DNA methylation alone can cause heritable variation in plant growth responses to jasmonic acid and salicylic acid, two key hormones involved in induction of plant defences against herbivores and pathogens. In order to be able to ascribe phenotypic differences to epigenetic variation, the hormone responses were studied of epigenetic recombinant inbred lines (epiRILs) of Arabidopsis thaliana – lines that are highly variable at the level of DNA methylation but nearly identical at the level of DNA sequence.

Key Results

Significant heritable variation was found among epiRILs both in the means of phenotypic traits, including growth rate, and in the degree to which these responded to treatment with jasmonic acid and salicylic acid. Moreover, there was a positive epigenetic correlation between the responses of different epiRILs to the two hormones, suggesting that plant responses to herbivore and pathogen attack may have a similar molecular epigenetic basis.

Conclusions

This study demonstrates that epigenetic variation alone can cause heritable variation in, and thus potentially microevolution of, plant responses to defence hormones. This suggests that part of the variation of plant defences observed in natural populations may be due to underlying epigenetic, rather than entirely genetic, variation.     

Specificity of extrafloral nectar induction by herbivores differs among native and invasive populations of tallow tree

Background and Aims

Invasive plants can be released from specialist herbivores and encounter novel generalists in their introduced ranges, leading to variation in defence among native and invasive populations. However, few studies have examined how constitutive and induced indirect defences change during plant invasion, especially during the juvenile stage.

Methods

Constitutive extrafloral nectar (EFN) production of native and invasive populations of juvenile tallow tree (Triadica sebifera) were compared, and leaf clipping, and damage by a native specialist (Noctuid) and two native generalist caterpillars (Noctuid and Limacodid) were used to examine inducible EFN production.

Key results

Plants from introduced populations had more leaves producing constitutive EFN than did native populations, but the content of soluble solids of EFN did not differ. Herbivores induced EFN production more than simulated herbivory. The specialist (Noctuid) induced more EFN than either generalist for native populations. The content of soluble solids in EFN was higher (2·1 times), with the specialist vs. the generalists causing the stronger response for native populations, but the specialist response was always comparable with the generalist responses for invasive populations.

Conclusions

These results suggest that constitutive and induced indirect defences are retained in juvenile plants of invasive populations even during plant establishment, perhaps due to generalist herbivory in the introduced range. However, responses specific to a specialist herbivore may be reduced in the introduced range where specialists are absent. This decreased defence may benefit specialist insects that are introduced for classical biological control of invasive plants.     

Foliar aphid feeding recruits rhizosphere bacteria and primes plant immunity against pathogenic and non-pathogenic bacteria in pepper

Background and Aims

Plants modulate defence signalling networks in response to different biotic stresses. The present study evaluated the effect of a phloem-sucking aphid on plant defence mechanisms in pepper (Capsicum annuum) during subsequent pathogen attacks on leaves and rhizosphere bacteria on roots.

Methods

Plants were pretreated with aphids and/or the chemical trigger benzothiadiazol (BTH) 7 d before being challenged with two pathogenic bacteria, Xanthomonas axonopodis pv. vesicatoria (Xav) as a compatible pathogen and X. axonopodis pv. glycines (Xag) as an incompatible (non-host) pathogen.

Key Results

Disease severity was noticeably lower in aphid- and BTH + aphid-treated plants than in controls. Although treatment with BTH or aphids alone did not affect the hypersensitive response (HR) against Xag strain 8ra, the combination treatment had a synergistic effect on the HR. The aphid population was reduced by BTH pretreatment and by combination treatment with BTH and bacterial pathogens in a synergistic manner. Analysis of the expression of the defence-related genes Capsicum annum pathogenesis-related gene 9 (CaPR9), chitinase 2 (CaCHI2), SAR8·2 and Lipoxygenase1 (CaLOX1) revealed that aphid infestation resulted in the priming of the systemic defence responses against compatible and incompatible pathogens. Conversely, pre-challenge with the compatible pathogen Xav on pepper leaves significantly reduced aphid numbers. Aphid infestation increased the population of the beneficial Bacillus subtilis GB03 but reduced that of the pathogenic Ralstonia solanacearum SL1931. The expression of defence-related genes in the root and leaf after aphid feeding indicated that the above-ground aphid infestation elicited salicylic acid and jasmonic acid signalling throughout the whole plant.

Conclusions

The findings of this study show that aphid feeding elicits plant resistance responses and attracts beneficial bacterial populations to help the plant cope with subsequent pathogen attacks.     

Environmental influences on growth and defence responses of the invasive shrub,Lonicera maackii,to simulated and real herbivory in the juvenile stage

Background and Aims

Tolerance and defence against herbivory are among the many mechanisms attributed to the success of invasive plants in their novel ranges. Because tolerance and defence against herbivory differ with the ontogeny of a plant, the effects of herbivore damage on plant fitness vary with ontogenetic stage and are compounded throughout a plant''s lifetime. Environmental stresses such as light and nutrient limitations can further influence the response of the plant. Much is known about the response of plants in the seedling and reproductive adult stages, but less attention has been given to the pre-reproductive juvenile stage.

Methods

Juvenile plants of the North American invasive Lonicera maackii were exposed to simulated herbivory under high and low light and nitrogen availability and growth, allocation patterns and foliar defensive chemistry were measured. In a second experiment, complete nutrient availability and damage type (generalist caterpillar or simulated) were manipulated.

Key Results

Juvenile plants receiving 50 % defoliation had lower total biomass and a higher root^:^shoot ratio than controls for all treatment combinations except low nitrogen/low light. Low light and defoliation increased root^:^shoot ratio. Light, fertilization and defoliation had little impact on foliar defensive chemistry. In the second experiment, there was a reduction in total biomass when caterpillar damage was applied. The root^:^shoot ratio increased under low soil fertility and was not affected by defoliation. Stem-diameter growth rates and specific leaf area did not vary by damage type or fertilization. Foliar protein increased through time, and more strongly in defoliated plants than in controls, while peroxidase activity and total flavonoids decreased with time. Overall, resource limitations were more influential than damage in the growth of juvenile L. maackii plants.

Conclusions

The findings illustrate that even when resources are limited, the tolerance and defence against herbivory of a woody invasive plant in the juvenile stage may contribute to the establishment and persistence of some species in a variety of habitats.     

Association between border cell responses and localized root infection by pathogenic Aphanomyces euteiches

Background and Aims

The oomycete Aphanomyces euteiches causes up to 80 % crop loss in pea (Pisum sativum). Aphanomyces euteiches invades the root system leading to a complete arrest of root growth and ultimately to plant death. To date, disease control measures are limited to crop rotation and no resistant pea lines are available. The present study aims to get a deeper understanding of the early oomycete–plant interaction at the tissue and cellular levels.

Methods

Here, the process of root infection by A. euteiches on pea is investigated using flow cytometry and microscopic techniques. Dynamic changes in secondary metabolism are analysed with high-performance liquid chromatography with diode-array detection.

Key Results

Root infection is initiated in the elongation zone but not in the root cap and border cells. Border-cell production is significantly enhanced in response to root inoculation with changes in their size and morphology. The stimulatory effect of A. euteiches on border-cell production is dependent on the number of oospores inoculated. Interestingly, border cells respond to pathogen challenge by increasing the synthesis of the phytoalexin pisatin.

Conclusions

Distinctive responses to A. euteiches inoculation occur at the root tissue level. The findings suggest that root border cells in pea are involved in local defence of the root tip against A. euteiches. Root border cells constitute a convenient quantitative model to measure the molecular and cellular basis of plant–microbe interactions.     

Host responses and metabolic profiles of wood components in Dutch elm hybrids with a contrasting tolerance to Dutch elm disease

Background and Aims

Changes occurring in the macromolecular traits of cell wall components in elm wood following attack by Ophiostoma novo-ulmi, the causative agent of Dutch elm disease (DED), are poorly understood. The purpose of this study was to compare host responses and the metabolic profiles of wood components for two Dutch elm (Ulmus) hybrids, ‘Groeneveld’ (a susceptible clone) and ‘Dodoens’ (a tolerant clone), that have contrasting survival strategies upon infection with the current prevalent strain of DED.

Methods

Ten-year-old plants of the hybrid elms were inoculated with O. novo-ulmi ssp. americana × novo-ulmi. Measurements were made of the content of main cell wall components and extractives, lignin monomer composition, macromolecular traits of cellulose and neutral saccharide composition.

Key Results

Upon infection, medium molecular weight macromolecules of cellulose were degraded in both the susceptible and tolerant elm hybrids, resulting in the occurrence of secondary cell wall ruptures and cracks in the vessels, but rarely in the fibres. The ¹³C nuclear magnetic resonance spectra revealed that loss of crystalline and non-crystalline cellulose regions occurred in parallel. The rate of cellulose degradation was influenced by the syringyl:guaiacyl ratio in lignin. Both hybrids commonly responded to the medium molecular weight cellulose degradation with the biosynthesis of high molecular weight macromolecules of cellulose, resulting in a significant increase in values for the degree of polymerization and polydispersity. Other responses of the hybrids included an increase in lignin content, a decrease in relative proportions of d-glucose, and an increase in proportions of d-xylose. Differential responses between the hybrids were found in the syringyl:guaiacyl ratio in lignin.

Conclusions

In susceptible ‘Groeneveld’ plants, syringyl-rich lignin provided a far greater degree of protection from cellulose degradation than in ‘Dodoens’, but only guaiacyl-rich lignin in ‘Dodoens’ plants was involved in successful defence against the fungus. This finding was confirmed by the associations of vanillin and vanillic acid with the DED-tolerant ‘Dodoens’ plants in a multivariate analysis of wood traits.     

Profiling microRNAs in Eucalyptus grandis reveals no mutual relationship between alterations in miR156 and miR172 expression and adventitious root induction during development

Background

The change from juvenile to mature phase in woody plants is often accompanied by a gradual loss of rooting ability, as well as by reduced microRNA (miR) 156 and increased miR172 expression.

Results

We characterized the population of miRNAs of Eucalyptus grandis and compared the gradual reduction in miR156 and increase in miR172 expression during development to the loss of rooting ability. Forty known and eight novel miRNAs were discovered and their predicted targets are listed. The expression pattern of nine miRNAs was determined during adventitious root formation in juvenile and mature cuttings. While the expression levels of miR156 and miR172 were inverse in juvenile and mature tissues, no mutual relationship was found between high miR156 expression and rooting ability, or high miR172 expression and loss of rooting ability. This is shown both in E. grandis and in E. brachyphylla, in which explants that underwent rejuvenation in tissue culture conditions were also examined.

Conclusions

It is suggested that in these Eucalyptus species, there is no correlation between the switch of miR156 with miR172 expression in the stems and the loss of rooting ability.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-524) contains supplementary material, which is available to authorized users.     

The multiple fuzzy origins of woodiness within Balsaminaceae using an integrated approach. Where do we draw the line?

Background and Aims

The family Balsaminaceae is essentially herbaceous, except for some woodier species that can be described as ‘woody’ herbs or small shrubs. The family is nested within the so-called balsaminoid clade of Ericales, including the exclusively woody families Tetrameristaceae and Marcgraviaceae, which is sister to the remaining families of the predominantly woody order. A molecular phylogeny of Balsaminaceae is compared with wood anatomical observations to find out whether the woodier species are derived from herbaceous taxa (i.e. secondary woodiness), or whether woodiness in the family represents the ancestral state for the order (i.e. primary woodiness).

Methods

Wood anatomical observations of 68 Impatiens species and Hydrocera triflora, of which 47 are included in a multigene phylogeny, are carried out using light and scanning electron microscopy and compared with the molecular phylogenetic insights.

Key Results

There is much continuous variation in wood development between the Impatiens species studied, making the distinction between herbaceousness and woodiness difficult. However, the most woody species, unambiguously considered as truly woody shrubs, all display paedomorphic wood features pointing to secondary woodiness. This hypothesis is further supported by the molecular phylogeny, demonstrating that these most woody species are derived from herbaceous (or less woody) species in at least five independent clades. Wood formation in H. triflora is mostly confined to the ribs of the stems and shows paedomorphic wood features as well, suggesting that the common ancestor of Balsaminaceae was probably herbaceous.

Conclusions

The terms ‘herbaceousness’ and ‘woodiness’ are notoriously difficult to use in Balsaminaceae. However, anatomical observations and molecular sequence data show that the woodier species are derived from less woody or clearly herbaceous species, demonstrating that secondary woodiness has evolved in parallel.     

Resin duct size and density as ecophysiological traits in fire scars of Pseudotsuga menziesii and Larix occidentalis

Background and Aims

Resin ducts (RDs) are features present in most conifer species as defence structures against pests and pathogens; however, little is known about RD expression in trees following fire injury. This study investigates changes in RD size and density in fire scars of Douglas fir (Pseudotsuga menziesii) and western larch (Larix occidentalis) as a means to evaluate the ecophysiological significance of traumatic resinosis for tree defence and survival.

Methods

Transverse and tangential microsections were prepared for light microscopy and image analysis in order to analyse axial and radial RDs, respectively. Epithelial cells of RDs and fusiform rays associated with radial RDs were also examined. RDs were compared between normal xylem and wound xylem at four different section heights along the fire-injured stem.

Key Results

Following fire injury, P. menziesii axial RDs narrowed by 38–43 % in the first year after injury, and the magnitude of this change increased with stem height. Larix occidentalis axial RDs widened by 46–50 % in the second year after injury. Radial RDs were of equivalent size in P. menziesii, but widened by 162–214 % in L. occidentalis. Fusiform rays were larger following fire injury, by 4–14 % in P. menziesii and by 23–38 % in L. occidentalis. Furthermore, axial RD density increased in both species due to the formation of tangential rows of traumatic RDs, especially in the first and second years after injury. However, radial RD density did not change significantly.

Conclusions

These results highlight traumatic resinosis as a species-specific response. Pseudotsuga menziesii produce RDs of equivalent or reduced size, whereas L. occidentalis produce wider RDs in both the axial and radial duct system, thereby increasing resin biosynthesis and accumulation within the whole tree. Larix occidentalis thus appears to allocate more energy to defence than P. menziesii.     

How to catch the patch? A dendrometer study of the radial increment through successive cambia in the mangrove Avicennia

Background and Aims

Successive vascular cambia are involved in the secondary growth of at least 200 woody species from >30 plant families. In the mangrove Avicennia these successive cambia are organized in patches, creating stems with non-concentric xylem tissue surrounded by internal phloem tissue. Little is known about radial growth and tree stem dynamics in trees with this type of anatomy. This study aims to (1) clarify the process of secondary growth of Avicennia trees by studying its patchiness; and (2) study the radial increment of Avicennia stems, both temporary and permanent, in relation to local climatic and environmental conditions. A test is made of the hypothesis that patchy radial growth and stem dynamics enable Avicennia trees to better survive conditions of extreme physiological drought.

Methods

Stem variations were monitored by automatic point dendrometers at four different positions around and along the stem of two Avicennia marina trees in the mangrove forest of Gazi Bay (Kenya) during 1 year.

Key Results

Patchiness was found in the radial growth and shrinkage and swelling patterns of Avicennia stems. It was, however, potentially rather than systematically present, i.e. stems reacted either concentrically or patchily to environment triggers, and it was fresh water availability and not tidal inundation that affected radial increment.

Conclusions

It is concluded that the ability to develop successive cambia in a patchy way enables Avicennia trees to adapt to changes in the prevailing environmental conditions, enhancing its survival in the highly dynamic mangrove environment. Limited water could be used in a more directive way, investing all the attainable resources in only some locations of the tree stem so that at least at these locations there is enough water to, for example, overcome vessel embolisms or create new cells. As these locations change with time, the overall functioning of the tree can be maintained.