Arbuscular Mycorrhizal Fungi Protect a Native Plant from Allelopathic Effects of an Invader

The allelopathic potential of the Eurasian invasive plant Alliaria petiolata has been well documented, with the bulk of the effects believed to be mediated by arbuscular mycorrhizal fungi (AMF). We exposed the herbaceous annual Impatiens pallida, which is native to North America, to fractionated A. petiolata extracts at four developmental stages (germination, presymbiosis growth, symbiosis formation, and symbiosis growth) by using exposure levels expected to be similar to field levels. Surprisingly, we found strong direct effects on I. pallida germination and growth, but no indirect effects on I. pallida growth mediated by AMF. We also observed strong synergistic effects with a complete A. petiolata extract that inhibited I. pallida germination and presymbiosis root growth more than either a glucosinolate or flavonoid enriched fraction alone. In fact, the flavonoid enriched fraction tended to stimulate germination and presymbiosis root growth. In contrast to these strong direct effects, I. pallida plant growth during both the symbiosis formation and symbiosis growth phases was unaffected by A. petiolata extracts. We also found no inhibition of AMF colonization of roots or soils by A. petiolata extracts. We show that AMF can actually ameliorate allelopathic effects of an invasive plant, and suggest that previously observed allelopathic effects of A. petiolata may be due to direct inhibition of plant and fungal growth before symbiosis formation.     

(±)-Catechin,A Root Exudate of the Invasive <Emphasis Type="Italic">Centaurea Stoebe</Emphasis> Lam. (Spotted Knapweed) Exhibits Bacteriostatic Activity Against Multiple Soil Bacterial Populations

Understanding the effects of allelopathic plant chemicals on soil microorganisms is critical to understanding their ecological roles and importance in exotic plant invasion. Centaurea stoebe Lam. (spotted knapweed), an aggressive invasive weed in North America, secretes a racemic mixture of (±)-catechin as a root exudate. This enantiomeric, polyphenolic compound has been reported to have allelopathic effects on surrounding flora and microflora. To better understand how catechin affects microbial communities in the root zone of spotted knapweed, we assessed its impact on the total culturable bacterial component and numerous individual bacterial populations from Romanian (native range) and Montana (invaded range) soils. Catechin suppressed total culturable count numbers from the bacterial community and inhibited growth of some, but not all, soil bacterial populations tested. The native soil bacterial community was significantly more resistant to inhibitory effects of catechin than either the invaded or non-invaded soils. We further show that the inhibitory effect of catechin on nine different soil bacterial strains from seven genera was reversible, demonstrating that it acts via a bacteriostatic rather than bactericidal mechanism. These findings suggest that catechin might affect bacterial community composition and activity in the root zone.     

Root Exposure to 5-Aminolevulinic Acid (ALA) Affects Leaf Element Accumulation,Isoprene Emission,Phytohormonal Balance,and Photosynthesis of Salt-Stressed Arundo donax

Arundo donax has been recognized as a promising crop for biomass production on marginal lands due to its superior productivity and stress tolerance. However, salt stress negatively impacts A. donax growth and photosynthesis. In this study, we tested whether the tolerance of A. donax to salinity stress can be enhanced by the addition of 5-aminolevulinic acid (ALA), a known promoter of plant growth and abiotic stress tolerance. Our results indicated that root exposure to ALA increased the ALA levels in leaves along the A. donax plant profile. ALA enhanced Na⁺ accumulation in the roots of salt-stressed plants and, at the same time, lowered Na⁺ concentration in leaves, while a reduced callose amount was found in the root tissue. ALA also improved the photosynthetic performance of salt-stressed apical leaves by stimulating stomatal opening and preventing an increase in the ratio between abscisic acid (ABA) and indol-3-acetic acid (IAA), without affecting leaf methanol emission and plant growth. Supply of ALA to the roots reduced isoprene fluxes from leaves of non-stressed plants, while it sustained isoprene fluxes along the profile of salt-stressed A. donax. Thus, ALA likely interacted with the methylerythritol 4-phosphate (MEP) pathway and modulate the synthesis of either ABA or isoprene under stressful conditions. Overall, our study highlights the effectiveness of ALA supply through soil fertirrigation in preserving the young apical developing leaves from the detrimental effects of salt stress, thus helping of A. donax to cope with salinity and favoring the recovery of the whole plant once the stress is removed.     

Roots of the Invasive Species Carduus nutans L. and C. acanthoides L. Produce Large Amounts of Aplotaxene,a Possible Allelochemical

The invasive thistle Carduus nutans has been reported to be allelopathic, yet no allelochemicals have been identified from the species. In a search for allelochemicals from C. nutans and the closely related invasive species C. acanthoides, bioassay-guided fractionation of roots and leaves of each species were conducted. Only dichloromethane extracts of the roots of both species contained a phytotoxin (aplotaxene, (Z,Z,Z)-heptadeca-1,8,11,14-tetraene) with sufficient total activity to potentially act as an allelochemical. Aplotaxene made up 0.44 % of the weight of greenhouse-grown C. acanthoides roots (ca. 20 mM in the plant) and was not found in leaves of either species. It inhibited growth of lettuce 50 % (I ₅₀) in soil at a concentration of ca. 0.5 mg g^?1 of dry soil (ca. 6.5 mM in soil moisture). These values gave a total activity in soil value (molar concentration in the plant divided by the molarity required for 50 % growth inhibition in soil?=?3.08) similar to those of some established allelochemicals. The aplotaxene I ₅₀ for duckweed (Lemna paucicostata) in nutrient solution was less than 0.333 mM, and the compound caused cellular leakage of cucumber cotyledon discs in darkness and light at similar concentrations. Soil in which C. acanthoides had grown contained aplotaxene at a lower concentration than necessary for biological activity in our short-term soil bioassays, but these levels might have activity over longer periods of time and might be an underestimate of concentrations in undisturbed and/or rhizosphere soil.     

Brassica Plant Responses to Mild Herbivore Stress Elicited by Two Specialist Insects from Different Feeding Guilds

Compensation growth and chemical defense are two components of plant defense strategy against herbivores. In this study, compensation growth and the response of primary and secondary metabolites were investigated in Brassica rapa plants subjected to infestation by two herbivores from contrasting feeding guilds, the phloem-feeding aphid Brevicoryne brassicae and the leaf-feeding caterpillar Pieris brassicae. These specialist herbivores were used at two different densities and allowed to feed for seven days on a young caged leaf. Changes in growth rates were assessed for total leaf area and bulb mass, whereas changes in primary and secondary metabolites were evaluated in young and mature leaves, roots, and bulbs. Mild stress by caterpillars on young plants enhanced mean bulb mass and elicited a contrasting regulation of aliphatic and indolic glucosinolates in the leaves. In contrast, mild stress by aphids enhanced leaf growth and increased glucosinolate concentrations in the bulb, the most important storage organ of B. rapa. A similar mild stress by either herbivore to older plants did not alter plant growth parameters or concentrations of the metabolites analyzed. In conclusion, Brassica plant growth was either maintained or enhanced under mild herbivore stress, and defense patterns differed strongly in response to herbivore type and plant development stage. These results have implications for the understanding of plasticity in plant defenses against herbivores and for the management of Brassica rapa in agroecosystems.     

Different Responses of an Invasive Clonal Plant <Emphasis Type="Italic">Wedelia trilobata</Emphasis> and its Native Congener to Gibberellin: Implications for Biological Invasion

The invasive clonal plant Wedelia trilobata contains higher levels of ent-kaurane diterpenes, which are precursors of gibberellins (GAs), and higher rates of clonal growth than its native congener W. chinensis in invaded habitats. We hypothesized that the higher levels of endogenous GAs facilitate greater ramet growth in W. trilobata compared with W. chinensis. We quantified endogenous levels of GA₁₊₃ in the two species and compared their growth responses to the changes of endogenous and exogenous GA₃ by using short-term and long-term hydroponics experiments. After a period of homogeneous cultivation, levels of endogenous GA₁₊₃ were higher in W. trilobata than in W. chinensis. The reduction of endogenous GAs repressed the emergence of adventitious roots and the growth of W. trilobata in the initial cultivation stage, and inhibited its shoot elongation and biomass. Levels of endogenous GA₁₊₃ were positively correlated with the length of shoots and adventitious roots of W. trilobata. Adventitious roots of W. trilobata also emerged earlier and grew faster when treated with exogenous GA₃. In contrast, exogenous GA₃ treatment inhibited the length of adventitious roots in W. chinensis, and levels of endogenous GA₁₊₃ did not correlate with shoot or adventitious root length. Our study suggests that GAs accelerate the rapid clonal growth of W. trilobata, more than that of its native congener W. chinensis, illustrating the relationship between plant hormones and the clonal growth of invasive plants. These findings are important for understanding the mechanisms associated with the invasiveness of clonal plants and their potential management.     

Ethylene Acts as a Local and Systemic Signal to Mediate UV-B-Induced Nitrate Reallocation to Arabidopsis Leaves and Roots via Regulating the ERFs-NRT1.8 Signaling Module

Nitrate is the preferred nitrogen source for plants and plays an important role in plant growth and development. Under various soil stresses, plants reallocate nitrate to roots to promote stress tolerance through the ethylene-ethylene response factors (ERFs)-nitrate transporter (NRT) signaling module. As a light signal, ultraviolet B (UV-B) also stimulates the production of ethylene. However, whether UV-B regulates nitrate reallocation in plants via ethylene remains unknown. Here, we found that UV-B-induced expression of ERF1B, ORA59, ERF104, and NRT1.8 in both Arabidopsis shoots and roots as well as nitrate reallocation from hypocotyls to leaves and roots were impaired in ethylene signaling mutants for Ethylene Insensitive2 (EIN2) and EIN3. UV-B-induced NRT1.8 expression and nitrate reallocation to leaves and roots were also inhibited in the triple mutants for ERF1B, ORA59, and ERF104. Deletion of NRT1.8 impaired UV-B-induced nitrate reallocation to both leaves and roots. Furthermore, UV-B promoted ethylene release in both shoots and roots by enhancing the gene expression and enzymatic activities of ethylene biosynthetic enzymes only in shoots. These results show that ethylene acts as a local and systemic signal to mediate UV-B-induced nitrate reallocation from Arabidopsis hypocotyls to both leaves and roots via regulating the gene expression of the ERFs-NRT1.8 signaling module.     

Optimization of supercritical CO2 extraction of different anatomical parts of lovage (Levisticum officinale Koch.) using response surface methodology and evaluation of extracts composition

Supercritical carbon dioxide extraction (SFE-CO₂) parameters were optimized using response surface methodology and central composite design for lovage (Levisticum officinale Koch.) roots and leaves containing valuable phytoconstituents. Mathematical model predicted the highest yields of extracts from roots and leaves 2.26 and 2.29%, respectively, at 45 MPa pressure, 60 °C temperature, 90 min (roots) and 30 min (leaves) extraction time, whereas the yield of hydrodistilled essential oil was 0.24 and 0.74%, respectively. The highest relative content of the most valuable constituent Z-ligustilide in roots and leaves extracts was 77 and 50% at 10 MPa; however, the highest yields of this compound from 100 g of dry material were obtained at the highest applied pressure and constituted 1188 mg (roots) and 540 mg (leaves). This study showed that lovage is a good source of Z-ligustilide and SFE-CO₂ is a preferable technique for its isolation.     

Ectopic Expression of CsCTR1, a Cucumber CTR-Like Gene,Attenuates Constitutive Ethylene Signaling in an Arabidopsis ctr1-1 Mutant and Expression Pattern Analysis of CsCTR1 in Cucumber (Cucumis sativus)

The gaseous plant hormone ethylene regulates many aspects of plant growth, development and responses to the environment. Constitutive triple response 1 (CTR1) is a central regulator involved in the ethylene signal transduction pathway. To obtain a better understanding of this particular pathway in cucumber, the cDNA-encoding CTR1 (designated CsCTR1) was isolated from cucumber. A sequence alignment and phylogenetic analyses revealed that CsCTR1 has a high degree of homology with other plant CTR1 proteins. The ectopic expression of CsCTR1 in the Arabidopsis ctr1-1 mutant attenuates constitutive ethylene signaling of this mutant, suggesting that CsCTR1 indeed performs its function as negative regulator of the ethylene signaling pathway. CsCTR1 is constitutively expressed in all of the examined cucumber organs, including roots, stems, leaves, shoot apices, mature male and female flowers, as well as young fruits. CsCTR1 expression gradually declined during male flower development and increased during female flower development. Additionally, our results indicate that CsCTR1 can be induced in the roots, leaves and shoot apices by external ethylene. In conclusion, this study provides a basis for further studies on the role of CTR1 in the biological processes of cucumber and on the molecular mechanism of the cucumber ethylene signaling pathway.     

Effect of Centaurea maculosa on Sheep Rumen Microbial Activity and Mass in Vitro

Spotted knapweed, Centaurea maculosa, a herbaceous weed from Eurasia, is altering the composition of native rangeland communities across western North America. Herbivore use of this plant is limited, possibly because glandular trichomes covering the epidermal surfaces of aerial tissues produce cnicin, a biologically active sesquiterpene lactone. We determined the concentrations of cnicin in plant parts from different growth stages (initial, mature, regrowth) of C. maculosa and the effects of these plant parts on sheep rumen microbial activity and mass (in vitro), when mixed in different proportions with grass hay. Leaves had higher crude protein and lower neutral and acid detergent fiber than stems or grass hay. Cnicin concentrations were highest in leaves, intermediate in flower buds, and lowest in stems. Cnicin concentrations in leaves increased from June to July, but decreased in stems. Regrowth had slightly lower cnicin concentrations that mature growth. High percentages (70% and 100%) of mature and regrowth leaves and flowers of C. maculosa in the mixtures depressed the rate and amount of microbial activity, whereas high percentages of stems from initial growth enhanced the rates of microbial activity. Microbial activity was more responsive to the different mixtures, plant parts, and growth stages than microbial mass, possibly because microbial populations cannot adjust rapidly to changes in diet. After cnicin was extracted from leaves, microbial activity was greater from these leaves than from grass hay. In contrast, after cnicin was extracted from flower buds, microbial activity from these flower buds was still depressed, indicating other compounds or the remaining cnicin were still affecting microbial activity. In summary, sheep rumen microbial activity was reduced significantly by mature and regrowth leaves that contained high concentrations of cnicin. Since most herbivores selectively graze leaves, the bitter-tasting cnicin could deter large ruminant feeding of C. maculosa by altering their behavior and/or by affecting rumen function.     

Density-Dependent Phytotoxicity of <Emphasis Type="Italic">Impatiens pallida</Emphasis> Plants Exposed to Extracts of <Emphasis Type="Italic">Alliaria petiolata</Emphasis>

Invasive plants are by definition excellent competitors, either indirectly through competition for resources or directly through allelopathic inhibition of neighboring plants. Although both forms of competition are commonly studied, attempts to explore the interactions between direct and indirect competition are rare. We monitored the effects of several doses of extracts of Alliaria petiolata, a Eurasian invader in North America, on the growth of Impatiens pallida, a North American native, at several planting densities. The density-dependent phytotoxicity model predicts that as plant density increases, individual plant size will decrease, unless a toxin is present in the soil. In this case, individual plant size is predicted to increase as plant density increases, as plants share a limited toxin dose. We tested this model using fractions of an A. petiolata extract enriched in flavonoids or glucosinolates, as well as a combined fraction. The flavonoid-enriched fraction and the combined fraction suppressed I. pallida growth but only when applied at a dose eight times higher than that expected in the field. When treated with a dose equivalent to estimated field exposure levels, I. pallida growth was not distinguishable from that of control plants that received no extract, showing that indirect competition for resources was more important for determining the growth of I. pallida than direct allelopathic inhibition by A. petiolata. This is an important reminder that, even though many plants have the demonstrated potential to exert strong allelopathic effects, those effects may not always be apparent when other forms of competition are considered as well.     

Allelochemicals Isolated from Tissues of the Invasive Weed Garlic Mustard (Alliaria petiolata)

Garlic mustard (Alliaria petiolata) is a naturalized Eurasian species that has invaded woodlands and degraded habitats in the eastern United States and Canada. Several phytotoxic hydrolysis products of glucosinolates, principally allyl isothiocyanate (AITC) and benzyl isothiocyanate (BzITC), were isolated from dichloromethane extracts of garlic mustard tissues. AITC and BzITC were much more phytotoxic to wheat (Triticum aestivum) than their respective parent glucosinolates sinigrin and glucotropaeolin. However, garden cress (Lepidium sativum) growth was inhibited to a greater degree by glucotropaeolin than BzITC, possibly due to conversion to BzITC by endogenous myrosinase. Sinigrin and glucotropaeolin were not detected in leaf/stem tissues harvested at the initiation of flowering, but were present in leaves and stems harvested in the autumn. Sinigrin levels in roots were similar for both sampling dates, but autumn-harvested roots contained glucotropaeolin at levels over three times higher than spring-harvested roots. The dominance of garlic mustard in forest ecosystems may be attributable in part to release of these phytotoxins, especially from root tissues.     

Identification and Expression Analysis of the Populus trichocarpa GASA-Gene Family

The gibberellic acid-stimulated Arabidopsis (GASA) gene family plays an important regulatory role in the growth and development of plants. In this study, we identified 19 GASA genes using bioinformatics-based methods in Populus trichocarpa, and these PtGASA genes could be divided into three categories based on their phylogenetic relationships. Based on an analysis of the structure and motifs of these genes, it was concluded that PtGASA class II members are more conserved than class I and class III members are, and the results of collinearity analysis showed that members of class II are collinearly related in poplar. Expression analysis of Populus trichocarpa roots, stems, and leaves showed that most of the PtGASA genes are expressed at higher levels in the stems or roots than in the leaves; a similar expression pattern was found in Vitis vinifera, indicating that the GASA-family members mainly play a role in the morphogenesis of poplar. Considering the phenomenon of gene amplification, we found that the higher the similarity of homologous genes was, the more similar the expression patterns. This study represents the first whole-genome identification and expression-profile analysis of the GASA-gene family in poplar, a model species, laying a foundation for functional studies of poplar GASA genes and serving as a reference for related research on other woody plant species.     

Sequestration of Aristolochic Acid I from Aristolochia pilosa by Mapeta xanthomelas Walker, 1863

Sequestration of secondary plant chemicals and brightly colored bodies occur in a number of unpalatable insects. The utilization of toxic plant chemicals has been proposed as a strategy of chemical defense, while aposematic coloration may advertise unpalatability. Here, we tested for the presence of aristolochic acid I in leaves of Aristolochia pilosa and female bodies of Mapeta xanthomelas, obtained from larvae feeding on the plant, using high performance liquid chromatography with photodiode array detection and electrospray ionization mass spectrometry. The presence of aristolochic acid I in females of this conspicuous diurnal moth, an oligophagous herbivore of Aristolochia, is the first report of sequestration of aristolochic acids by an herbivore other than a species of Papilionidae.     

Heterodera schachtii Nematodes Interfere with Aphid-Plant Relations on Brassica oleracea

Aboveground and belowground herbivore species modify plant defense responses differently. Simultaneous attack can lead to non-additive effects on primary and secondary metabolite composition in roots and shoots. We previously found that aphid (Brevicoryne brassicae) population growth on Brassica oleracea was reduced on plants that were infested with nematodes (Heterodera schachtii) prior (4 weeks) to aphid infestation. Here, we examined how infection with root-feeding nematodes affected primary and secondary metabolites in the host plant and whether this could explain the increase in aphid doubling time from 3.8 to 6.7 days. We hypothesized that the effects of herbivores on plant metabolites would depend on the presence of the other herbivore and that nematode-induced changes in primary metabolites would correlate with reduced aphid performance. Total glucosinolate concentration in the leaves was not affected by nematode presence, but the composition of glucosinolates shifted, as gluconapin concentrations were reduced, while gluconapoleiferin concentrations increased in plants exposed to nematodes. Aphid presence increased 4-methoxyglucobrassicin concentrations in leaves, which correlated positively with the number of aphids per plant. Nematodes decreased amino acid and sugar concentrations in the phloem. Aphid population doubling time correlated negatively with amino acids and glucosinolate levels in leaves, whereas these correlations were non-significant when nematodes were present. In conclusion, the effects of an herbivore on plant metabolites were independent of the presence of another herbivore. Nematode presence reduced aphid population growth and disturbed feeding relations between plants and aphids.     

Costs of Jasmonic Acid Induced Defense in Aboveground and Belowground Parts of Corn (Zea mays L.)

Costs of jasmonic acid (JA) induced plant defense have gained increasing attention. In this study, JA was applied continuously to the aboveground (AG) or belowground (BG) parts, or AG plus BG parts of corn (Zea mays L.) to investigate whether JA exposure in one part of the plant would affect defense responses in another part, and whether or not JA induced defense would incur allocation costs. The results indicated that continuous JA application to AG parts systemically affected the quantities of defense chemicals in the roots, and vice versa. Quantities of DIMBOA and total amounts of phenolic compounds in leaves or roots generally increased 2 or 4?wk after the JA treatment to different plant parts. In the first 2?wk after application, the increase of defense chemicals in leaves and roots was accompanied by a significant decrease of root length, root surface area, and root biomass. Four weeks after the JA application, however, no such costs for the increase of defense chemicals in leaves and roots were detected. Instead, shoot biomass and root biomass increased. The results suggest that JA as a defense signal can be transferred from AG parts to BG parts of corn, and vice versa. Costs for induced defense elicited by continuous JA application were found in the early 2?wk, while distinct benefits were observed later, i.e., 4?wk after JA treatment.