Systemic induced resistance: a risk-spreading strategy in clonal plant networks?

Clonal integration of stress signal induces morphological and physiological response of root within clonal network

Clonal integration of defense or stress signal induced systemic resistance in leaf of interconnected ramets. However, similar effects of stress signal in root are poorly understood within clonal network. Clonal fragments of Centella asiaticas with first-young, second-mature, third-old and fourth-oldest ramets were used to investigate transportation or sharing of stress signal among interconnected ramets suffering from low water availability. Compared with control, oxidative stress in root of the first-young, second-mature and third-old ramets was significantly alleviated by exogenous ABA application to the fourth-oldest ramets as well as enhancement of antioxidant enzyme (SOD, POD, CAT and APX) activities and osmoregulation ability. Surface area and volume in root of the first-young ramets were significantly increased and total length in root of the third-old ramets was significantly decreased. POD activity in root of the fourth-oldest and third-old ramets was significantly enhanced by exogenous ABA application to the first-young ramets. Meanwhile, total length and surface area in root of the fourth-oldest and third-old ramets were significantly decreased. Ratio of belowground to aboveground biomass in the whole clonal fragments was significantly increased by exogenous ABA application to the fourth-oldest or first-young ramets. It is suggested that transportation or sharing of stress signal may induce systemic resistance in root of interconnected ramets. Specially, transportation or sharing of stress signal against phloem flow was observed in the experiment. Possible explanation is that rapid recovery of foliar photosynthesis in first-young ramets subjected to exogenous ABA application can partially reverse phloem flow within clonal network. Thus, our experiment provides insight into ecological implication on clonal integration of stress signal.

Timing of systemic resistance induced by local exogenous ABA application within clonal network of stoloniferous herb Centella asiatica subjected to low water availability

Resistance traits of plants can be activated both at the damaged site and undamaged parts. Systemic resistance induced by local exogenous abscisic acid (ABA) application alleviated negative effect of low water availability on growth performance of clonal plant. However, timing of systemic resistance was poorly understood. Timing of systemic resistance refers to its activation and decay time within clonal network. Clonal fragment of Centella asiatica with four successive ramets (including first-oldest, second-older, third-old and fourth-young ramets) subjected to low water availability (20% soil moisture content) was used to explore effects of local exogenous ABA application on the timing of resistance activation and decay. Systemic resistance activated by local exogenous ABA application after 4 days remained at least 28 days. Compared with control, biomass accumulation of whole clonal fragment, root biomass and ratio of belowground to aboveground biomass significantly increased by local exogenous ABA application after 28 days. It is suggested that rapid activation and delay of resistance response induced by local exogenous ABA application within clonal network may improve fitness of clonal plant subjected to abiotic stress.

Leaf defenses of subtropical deciduous and evergreen trees to varying intensities of herbivory

Generally, deciduous and evergreen trees coexist in subtropical forests, and both types of leaves are attacked by numerous insect herbivores. However, trees respond and defend themselves from herbivores in different ways, and these responses may vary between evergreen and deciduous species. We examined both the percentage of leaf area removed by herbivores as well as the percentage of leaves attacked by herbivores to evaluate leaf herbivore damage across 14 subtropical deciduous and evergreen tree species, and quantified plant defenses to varying intensities of herbivory. We found that there was no significant difference in mean percentage of leaf area removed between deciduous and evergreen species, yet a higher mean percentage of deciduous leaves were damaged compared to evergreen leaves (73.7% versus 60.2%). Although percent leaf area removed was mainly influenced by hemicellulose concentrations, there was some evidence that the ratio of non-structural carbohydrates:lignin and the concentration of tannins contribute to herbivory. We also highlight that leaf defenses to varying intensities of herbivory varied greatly among subtropical plant species and there was a stronger response for deciduous trees to leaf herbivore (e.g., increased nitrogen or lignin) attack than that of evergreen trees. This work elucidates how leaves respond to varying intensities of herbivory, and explores some of the underlying relationships between leaf traits and herbivore attack in subtropical forests.

Insect Herbivory on Main Stem Enhances Induced Defense of Primary Tillers in Rice (Oryza sativa L.)

Clonal plants are interconnected to form clonal plant networks with physiological integration, enabling the reassignment as well as sharing of resources among the members. The systemic induction of antiherbivore resistance via clonal integration may frequently operate in the networks. Here, we used an important food crop rice (Oryza sativa), and its destructive pest rice leaffolder (LF; Cnaphalocrocis medinalis) as a model to examine defense communication between the main stem and clonal tillers. LF infestation and MeJA pretreatment on the main stem for two days reduced the weight gain of LF larvae fed on the corresponding primary tillers by 44.5% and 29.0%, respectively. LF infestation and MeJA pretreatment on the main stem also enhanced antiherbivore defense responses in primary tillers: increased levels of a trypsin protease inhibitor, putative defensive enzymes, and jasmonic acid (JA), a key signaling compound involved in antiherbivore induced defenses; strong induction of genes encoding JA biosynthesis and perception; and rapid activation of JA pathway. However, in a JA perception OsCOI RNAi line, LF infestation on main stem showed no or minor effects on antiherbivore defense responses in primary tillers. Our work demonstrates that systemic antiherbivore defense operate in the clonal network of rice plants and JA signaling plays a crucial role in mediating defense communication between main stem and tillers in rice plants. Our findings provide a theoretical basis for the ecological control of pests by using the systemic resistance of cloned plants themselves.

Clonal integration systemically regulates leaf microstructure of Bouteloua dactyloides interconnected ramets to better adapt to different levels of simulated insect herbivory

Stolon connection of clonal plants can translocate resources and signalling molecules between interconnected ramets to enhance resistance. Plants are well known to enhance leaf anatomical structure and vein density to respond to insect herbivory. Herbivory signalling molecules are transferred through vascular system to alert distant undamaged leaves, which is called systemic defence induction. Here, we investigated how clonal integration modulates leaf vasculature and anatomical structure of Bouteloua dactyloides ramets to cope with different levels of simulated herbivory. Ramet pairs were subject to six treatments, daughter ramets were exposed to three defoliation levels (0 %, 40 % or 80 % leaf removal) and their stolon connections to mother ramets were either severed or kept intact. Local 40 % defoliation increased vein density and adaxial/abaxial cuticle thickness, decreased leaf width and areolar area of daughter ramets. However, such effects of 80 % defoliation were much smaller. Compared with remote 40 % defoliation, remote 80 % defoliation increased leaf width and areolar area and decreased vein density of interconnected undefoliated mother ramets. Without simulated herbivory, stolon connection negatively affected most leaf microstructural traits of both ramets except from denser veins of mother ramets and more bundle sheath cells of daughter ramets. The negative effect of stolon connection on leaf mechanical structures of daughter ramets was ameliorated in the 40 % defoliation treatment, but not in the 80 % defoliation treatment. Stolon connection increased vein density and decreased areolar area of daughter ramets in the 40 % defoliation treatment. In contrast, stolon connection increased areolar area and decreased bundle sheath cell number of 80 % defoliated daughter ramets. Defoliation signals were transmitted from younger ramets to older ramets to change their leaf biomechanical structure. Clonal integration can adjust leaf microstructure of younger ramets according to the degree of herbivory stress, especially leaf vasculature.

Effects of Clonal Integration on Foraging Behavior of Three Clonal Plants in Heterogeneous Soil Environments

Environments are ubiquitously heterogeneous in nature, and clonal plants commonly benefit from both clonal integration and foraging responses in heterogeneous environments. While many studies have examined clonal integration and foraging responses separately, few have tested the effect of clonal integration on the foraging response of clonal plants to environmental heterogeneity. We grew offspring ramets of each of three clonal plants (Hydrocotyle vulgaris, Duchesnea indica, and Glechoma longituba) in both homogeneous and heterogenous soil environments and severed their stem connection to a mother ramet (to prevent clonal integration from the mother ramet) or kept it intact (to allow clonal integration). Without clonal integration from the mother ramet, soil heterogeneity had no effect on biomass or number of ramets for any of the three species. With clonal integration, soil heterogeneity also had no effect on biomass or number of ramets of D. indica and G. longituba, but significantly decreased biomass and marginally significantly decreased number of ramets of H. vulgaris. Without clonal integration, offspring ramets did not demonstrate either shoot or root foraging responses in terms of total, shoot and root biomass and ramet number in the heterogeneous soil environment in any of the three species. With integration, offspring ramets of H. vulgaris also did not demonstrate either root or shoot foraging responses, but offspring ramets of G. longituba demonstrated both root and shoot foraging responses, and those of D. indica demonstrated a root foraging response when they grew in the heterogeneous soil environment. We conclude that clonal integration can alter the foraging response of clonal plants, but this effect is species-specific. Our results also suggest that foraging responses of clonal plants in heterogeneous soil environments may not necessarily benefit the growth of clonal plants.

Motherly care: How Leymus chinensis ramets support their offspring exposed to saline-alkali and clipping stresses

BACKGROUND

While clonal integration can improve the performance of rhizomatous plants, it remains unclear whether their clonal integration strategy changes under contrasting clipping and saline-alkali homogeneous and heterogeneous environments. Leymus chinensis is a clonal grass native to the Songnen grassland where heavy grazing and patchy saline-alkali stress are serious environmental and ecological problems. We hypothesized that L. chinensis overcomes these stresses through clonal integration, in particular the transfer of nitrogen and carbohydrates.

METHODS

A pot experiment was carried out with ¹⁵N isotope soil labeling method to study clonal integration strategy in the connected mother and daughter ramets of L. chinensis. The connected ramet pairs were grown in homogeneous (both connected ramets were treated) and heterogeneous (only daughter ramets were treated) environments with four treatments: control, clipping (60% aboveground biomass removal), saline-alkali (3.45 g of NaCl, NaHCO₃, and Na₂CO₃ per pot), and clipping × saline-alkali.

RESULTS

A significant amount (22.5%) of ¹⁵N was transferred from mother to daughter ramets under non-stressed conditions. When homogeneously stressing both mother and daughter ramets, N transfer was significantly reduced to 8.5--14.6%, independent of the nature of the stress. When only daughters were stressed (heterogeneous stress), saline-alkali stress led to a division of labor where daughters had enhanced photosynthesis, and mother ramets had increased ¹⁵N uptake and growth. Clipping only daughters reduced biomass and ¹⁵N uptake of both daughter and mother ramets.

CONCLUSIONS

Our results demonstrated that clonal integration also occurs in homogeneous favorable environments but is reduced under homogeneous stress. In heterogeneous environments, clonal integration is used to translocate resource after clipping and a division of labor is established to overcome saline-alkali stress. Clonal integration continued even when daughters were severely stressed by the combined treatments. Our findings suggest that these mechanisms are key to the success of L. chinensis in the Songnen grassland.

Ultraviolet B Radiation Triggers DNA Methylation Change and Affects Foraging Behavior of the Clonal Plant Glechoma longituba

Clonal plants in heterogeneous environments can benefit from their habitat selection behavior, which enables them to utilize patchily distributed resources efficiently. It has been shown that such behavior can be strongly influenced by their memories on past environmental interactions. Epigenetic variation such as DNA methylation was proposed to be one of the mechanisms involved in the memory. Here, we explored whether the experience with Ultraviolet B (UV-B) radiation triggers epigenetic memory and affects clonal plants' foraging behavior in an UV-B heterogeneous environment. Parental ramets of Glechoma longituba were exposed to UV-B radiation for 15 days or not (controls), and their offspring ramets were allowed to choose light environment enriched with UV-B or not (the species is monopodial and can only choose one environment). Sizes and epigenetic profiles (based on methylation-sensitive amplification polymorphism analysis) of parental and offspring plants from different environments were also analyzed. Parental ramets that have been exposed to UV-B radiation were smaller than ramets from control environment and produced less and smaller offspring ramets. Offspring ramets were placed more often into the control light environment (88.46% ramets) than to the UV-B light environment (11.54% ramets) when parental ramets were exposed to UV-B radiation, which is a manifestation of "escape strategy." Offspring of control parental ramets show similar preference to the two light environments. Parental ramets exposed to UV-B had lower levels of overall DNA methylation and had different epigenetic profiles than control parental ramets. The methylation of UV-B-stressed parental ramets was maintained among their offspring ramets, although the epigenetic differentiation was reduced after several asexual generations. The parental experience with the UV-B radiation strongly influenced foraging behavior. The memory on the previous environmental interaction enables clonal plants to better interact with a heterogeneous environment and the memory is at least partly based on heritable epigenetic variation.

Clonal versus non-clonal milkweeds (Asclepias spp.) respond differently to stem damage, affecting oviposition by monarch butterflies

BACKGROUND

Oviposition decisions are critical to the fitness of herbivorous insects and are often impacted by the availability and condition of host plants. Monarch butterflies (Danaus plexippus) rely on milkweeds (Asclepias spp.) for egg-laying and as food for larvae. Previous work has shown that monarchs prefer to oviposit on recently regrown plant tissues (after removal of above-ground biomass) while larvae grow poorly on plants previously damaged by insects. We hypothesized that these effects may depend on the life-history strategy of plants, as clonal and non-clonal milkweed species differ in resource allocation and defense strategies.

METHODOLOGY/PRINCIPAL FINDINGS

We first confirmed butterfly preference for regrown tissue in a field survey of paired mowed and unmowed plots of the common milkweed A. syriaca. We then experimentally studied the effects of plant damage (comparing undamaged controls to plants clipped and regrown, or damaged by insects) on oviposition choice, larval performance, and leaf quality of two closely related clonal and non-clonal species pairs: (1) A. syriaca and A. tuberosa, and (2) A. verticillata and A. incarnata. Clonal and non-clonal species displayed different responses to plant damage, impacting the proportions of eggs laid on plants. Clonal species had similar mean proportions of eggs on regrown and control plants (≈35-40% each), but fewer on insect-damaged plants (≈20%). Meanwhile non-clonal species had similar oviposition on insect-damaged and control plants (20-30% each) but more eggs on regrown plants (40-60%). Trait analyses showed reduced defenses in regrown plants and we found some evidence, although variable, for negative effects of insect damage on subsequent larval performance.

CONCLUSIONS/SIGNIFICANCE

Overall, non-clonal species are more susceptible and preferred by monarch butterflies following clipping, while clonal species show tolerance to clipping and induced defense to insect herbivory. These results have implications for monarch conservation strategies that involve milkweed habitat management by mowing. More generally, plant life-history may mediate growth and defense strategies, explaining species-level variation in responses to different types of damage.

Associational effects of plant ontogeny on damage by a specialist insect herbivore

Intraspecific variation in plant traits is a major cause of variation in herbivore feeding and performance. Plant defensive traits change as a plant grows, such that ontogeny may account for a substantial portion of intraspecific trait variation. We tested how the ontogenic stage of an individual plant, of an individual in the context of its neighboring plants, and of a patch of plants with mixed or uniform stages affect plant-herbivore interactions. To do this, we conducted an experimental study of the interactions between Lepidium draba, a perennial brassicaceous weed, and Plutella xylostella, a common herbivore of L. draba. We found that L. draba foliar glucosinolates, secondary metabolites often implicated in defense, decreased in concentration with plant age. In single-stage patches, herbivores performed similarly on L. draba plants of different ages. Furthermore, we found no difference in the cumulative performance of herbivores reared on mixed- or even-staged patches of L. draba. However, in mixed-stage patches, the damage experienced by a focal plant depended on the stage of neighboring plants, suggesting a preference hierarchy of the herbivore among plant stages. In our study, the amount of herbivory depended on the ontogenic neighborhood in which the plant grew. However, from the herbivore's perspective, variation in plant ontogenic stage was unimportant to its success in terms of feeding rate and final weight.

Transportation or sharing of stress signals among interconnected ramets improves systemic resistance of clonal networks to water stress

Previous studies have elucidated the mechanisms, ecological implications and constraints on transportation or sharing of defence signals among interconnected ramets of clonal plants suffering from localised herbivore damage. To our knowledge, few studies have been conducted to provide insights into the ecological implications on transportation or sharing of stress signals for clonal plants subjected to water stress. As a chemical elicitor, ABA can induce resistance response in plants suffering from water stress. A pot experiment was conducted to explore transportation or sharing of stress signals among interconnected ramets by using clonal fragments of Centella asiaticas (L.) Urban with four successive ramets (oldest, old, mature and young) subjected to low water availability (20% soil moisture contents). Compared with control, foliar oxidative stress of the old, mature and young ramets significantly decreased, and antioxidant capacity was increased when exogenous ABA was applied to the oldest ramets. Meanwhile, foliar PSII activity and chlorophyll content of the old, mature and young ramets significantly increased. Compared with control, biomass accumulation and ratio of below-ground/aboveground biomass of whole clonal fragments were significantly increased by ABA application to the oldest ramets. However, similar patterns were not observed when exogenous ABA was applied to the young ramets. Our results show that transportation or sharing of stress signals among interconnected ramets improves systemic resistance of clonal networks to water stress, which is dependent on directionality of vascular flows. Compared with the old or mature ramets, the young ramets displayed stronger resistance response (such as higher antioxidant enzymes activities and proline content, lower O2•- production rate and malondialdehyde content) to water stress as well as higher PSII activity and chlorophyll content when exogenous ABA was applied to the oldest ramets. Thus, transportation or sharing of stress signals may favour young ramets that are most valuable for growth and fitness of clonal plant subjected to environmental stress. It is suggested that transportation or sharing of stress signals among interconnected ramets may confer clonal plants with considerable benefits in adapting to spatio-temporal heterogeneous habitats.

Herbivory induced non-local responses of the clonal invader Carpobrotus edulis are not mediated by clonal integration

The anthropogenic displacement of species around the world results in new environmental situations where native and exotic species coexist. Exotic plants have to face native herbivores, and interactions between introduced plants and native herbivores seem to play an important role in the invasiveness of some exotic plant species. We studied the role of clonal integration in induce morphological, physiological, and biochemical responses in the clonal invader Carpobrotus edulis against the attack of the native snail Theba pisana. Our results demonstrated the presence of labour division mediated by physiological integration, with a significant increase of photosynthesis potential (both at morphological and physiological) in un-attacked integrated ramets. This response could be especially important under herbivory, as the negative impact of T. pisana on the photosynthetic structures of attacked C. edulis ramets could be buffered by transferring the dependence of photosynthetic activity to the un-attacked ramets. Our results also showed a constitutive resistance in un-attacked apical ramets, showing a similar amount of defence compounds to those exhibited in the basal branches attacked by snails. Results reported a non-local compensatory response, which there was an increase of total biomass in apical ramets when their basal ramets were attacked by the herbivore. We interpret this result as a compensatory response, with these apical ramets increasing shoot biomass to compensate for the biomass loss due to a potential attack from herbivores. However, this non-local response was not mediated by physiological integration but probably due to belowground communication, with the presence of alarm signals released by root exudates. We conclude that the attack of this snail is not enough to be a possible biological control due to the compensatory response to this snail by C. edulis, favouring their expansion. Future studies should focus on unravelling the role of belowground communication in the defensive responses of C. edulis.

Clonal plasticity and diversity facilitates the adaptation of Rhododendron aureum Georgi to alpine environment

Four small oval populations and five large intensive populations of Rhododendron aureum growing at the alpine in Changbai Mountain (China) were studied in two types of habitat (in the tundra and in Betula ermanii forest). Identification and delimitation of genets were inferred from excavation in small populations and from amplified fragment length polymorphism (AFLP) markers by the standardized sampling design in large populations. Clonal architecture and clonal diversity were then estimated. For the four small populations, they were monoclonal, the spacer length (18.6 ± 5.6 in tundra, 29.7 ± 9.7 in Betula ermanii forest, P < 0.05) was shorter and branching intensity (136.7 ± 32.9 in tundra, 43.4 ± 12.3 in Betula ermanii forest, P < 0.05) was higher in the tundra than that in Betula ermanii forest. For the five large populations, they were composed of multiple genets with high level of clonal diversity (Simpson’s index D = 0.84, clonal richness R = 0.25, Fager's evenness E = 0.85); the spatial distribution of genets showed that the clonal growth strategy of R. aureum exhibits both guerilla and phalanx. Our results indicate that the clonal plasticity of R. aureum could enhance exploitation of resource heterogeneity and in turn greatly contribute to maintenance or improvement of fitness and the high clonal diversity of R. aureum increase the evolutionary rates to adapt the harsh alpine environment in Changbai Mountain.

Grazing influences Stipa breviflora seed germination in desert grasslands of the Inner Mongolia Plateau

Seed germination plays an important role in determining the composition and regeneration of plant populations (Stipa breviflora). However, the influencing factors and strategies employed for seed germination in desert grasslands under grazing remain unknown. Therefore, in this study, the reproductive allocation, seed density, seed properties, and corresponding seed germination rates of S. breviflora were examined. Possible situations encountered during dispersal were also simulated to determine their effects on seed germination. The results showed that reproductive individual density not subjected to grazing were significantly higher than those subjected to moderate and heavy grazing. For seed density and seed bank in soil, the highest values were observed for the no grazing treatment, followed by the moderate and heavy grazing treatments. The seed density for germination of soil seed banks was nearly one-fourth of seed density during the growing season. In addition, grazing treatments affected the phenotypic characteristics of seeds and reduced the lower limit of the weight of germinable seeds. Awn removal significantly increased germination. The longest germination time was observed for seeds that entered the soil at an angle of 0°. Our research demonstrated that grazing negatively affected the desert grassland edificator. Individual plants adopted different adaptation strategies under different grazing intensities; for example, a fixed proportion of the seed number and seed germination number of S. breviflora in the soil seed bank was maintained by exceeding the minimum weight of a seed for seed germination. During seed dispersion, the awn effectively prevented germination under unfavourable conditions and helped seeds enter the soil at an optimal angle for promoting germination.

Effects of clonal fragmentation on intraspecific competition of a stoloniferous floating plant

Disturbance is common and can fragment clones of plants. Clonal fragmentation may affect the density and growth of ramets so that it could alter intraspecific competition. To test this hypothesis, we grew one (low density), five (medium density) or nine (high density) parent ramets of the floating invasive plant Pistia stratiotes in buckets, and newly produced offspring ramets were either severed (with fragmentation) or remained connected to parent ramets (no fragmentation). Increasing density reduced biomass of the whole clone (i.e. parent ramet plus its offspring ramets), showing intense intraspecific competition. Fragmentation decreased biomass of offspring ramets, but increased biomass of parent ramets and the whole clone, suggesting significant resource translocation from parent to offspring ramets when clones were not fragmented. There was no interaction effect of density x fragmentation on biomass of the whole clone, and fragmentation did not affect competition intensity index. We conclude that clonal fragmentation does not alter intraspecific competition between clones of P. stratiotes, but increases biomass production of the whole clone. Thus, fragmentation may contribute to its interspecific competitive ability and invasiveness, and intentional fragmentation should not be recommended as a measure to stop the rapid growth of this invasive species.

Herbivore-simulated induction of defenses in clonal networks of trembling aspen (Populus tremuloides)

Trembling aspen (Populus tremuloides Michx.) as a clonal tree species possesses a complex root system through which trees of the same or different clones are connected. Root connections have been studied with respect to resource sharing, but the nature, quantities or extent of what is shared between trees is relatively unknown. In this study, we posed the hypothesis that systemic defense induction signals could also spread through these root networks and trigger defenses in neighboring ramets before arrival of pests. Temporal expression pattern of Kunitz trypsin inhibitor (KTI) and dihydroflavonol reductase (DFR) genes, two markers of poplar defense, was followed by quantitative real-time polymerase chain reaction. The expression was quantified in systemic leaves of wounded and healthy plants that shared the same parental root and in untreated controls grown in separate pots. Untreated interconnected plants did not show induced resistance upon herbivore-simulated attack. Although wound-treated ramets induced defense genes, untreated interconnected plants produced an expression pattern similar to non-connected controls. Root connections do not automatically lead to induction of defensive traits that are expressed in plants directly under damage thought to simulate herbivory. Rather, it seems that other communication means such as airborne volatiles can serve as signal transmission pathways among neighboring plants.

Physiological integration ameliorates negative effects of drought stress in the clonal herb Fragaria orientalis

Clonal growth allows plants to spread horizontally and to establish ramets in sites of contrasting resource status. If ramets remain physiologically integrated, clones in heterogeneous environments can act as cooperative systems--effects of stress on one ramet can be ameliorated by another connected ramet inhabiting benign conditions. But little is known about the effects of patch contrast on physiological integration of clonal plants and no study has addressed its effects on physiological traits like osmolytes, reactive oxygen intermediates and antioxidant enzymes. We examined the effect of physiological integration on survival, growth and stress indicators such as osmolytes, reactive oxygen intermediates (ROIs) and antioxidant enzymes in a clonal plant, Fragaria orientalis, growing in homogenous and heterogeneous environments differing in patch contrast of water availability (1 homogeneous (no contrast) group; 2 low contrast group; 3 high contrast group). Drought stress markedly reduced the survival and growth of the severed ramets of F. orientalis, especially in high contrast treatments. Support from a ramet growing in benign patch considerably reduced drought stress and enhanced growth of ramets in dry patches. The larger the contrast between water availability, the larger the amount of support the depending ramet received from the supporting one. This support strongly affected the growth of the supporting ramet, but not to an extent to cause increase in stress indicators. We also found indication of costs related to maintenance of physiological connection between ramets. Thus, the net benefit of physiological integration depends on the environment and integration between ramets of F. orientalis could be advantageous only in heterogeneous conditions with a high contrast.

Rumor has it...: relay communication of stress cues in plants

Recent evidence demonstrates that plants are able not only to perceive and adaptively respond to external information but also to anticipate forthcoming hazards and stresses. Here, we tested the hypothesis that unstressed plants are able to respond to stress cues emitted from their abiotically-stressed neighbors and in turn induce stress responses in additional unstressed plants located further away from the stressed plants. Pisum sativum plants were subjected to drought while neighboring rows of five unstressed plants on both sides, with which they could exchange different cue combinations. On one side, the stressed plant and its unstressed neighbors did not share their rooting volumes (UNSHARED) and thus were limited to shoot communication. On its other side, the stressed plant shared one of its rooting volumes with its nearest unstressed neighbor and all plants shared their rooting volumes with their immediate neighbors (SHARED), allowing both root and shoot communication. Fifteen minutes following drought induction, significant stomatal closure was observed in both the stressed plants and their nearest unstressed SHARED neighbors, and within one hour, all SHARED neighbors closed their stomata. Stomatal closure was not observed in the UNSHARED neighbors. The results demonstrate that unstressed plants are able to perceive and respond to stress cues emitted by the roots of their drought-stressed neighbors and, via 'relay cuing', elicit stress responses in further unstressed plants. Further work is underway to study the underlying mechanisms of this new mode of plant communication and its possible adaptive implications for the anticipation of forthcoming abiotic stresses by plants.

Herbivory-mediated pollinator limitation: negative impacts of induced volatiles on plant-pollinator interactions

Although induced plant responses to herbivory are well studied as mechanisms of resistance, how induction shapes community interactions and ultimately plant fitness is still relatively unknown. Using a wild tomato, Solanum peruvianum, native to the Peruvian Andes, we evaluated the disruption of pollination as a potential ecological cost of induced responses. More specifically, we tested the hypothesis that metabolic changes in herbivore-attacked plants, such as the herbivore-induced emission of volatile organic compounds (VOCs), alter pollinator behavior and consequentially affect plant fitness. We conducted a series of manipulative field experiments to evaluate the role of herbivore-induced vegetative and floral VOC emissions as mechanisms by which herbivory affects pollinator behavior. In field surveys and bioassays in the plants' native habitat, we found that real and simulated herbivory (methyl jasmonate application) reduced attractiveness of S. peruvianum flowers to their native pollinators. We show that reduced pollinator preference, not resource limitation due to leaf tissue removal, resulted in reduced seed set. Solitary bee pollinators use floral plant volatiles, emitted in response to herbivory or methyl jasmonate treatment, as cues to avoid inflorescences on damaged plants. This herbivory-induced pollinator limitation can be viewed as a general cost of induced plant responses as well as a specific cost of herbivory-induced volatile emission.

Phytophagous arthropods and a pathogen sharing a host plant: evidence for indirect plant-mediated interactions

In ecological systems, indirect interactions between plant pathogens and phytophagous arthropods can arise when infestation by a first attacker alters the common host plant so that although a second attacker could be spatially or temporally separated from the first one, the former could be affected. The induction of plant defense reactions leading to the production of secondary metabolites is thought to have an important role since it involves antagonistic and/or synergistic cross-talks that may determine the outcome of such interactions. We carried out experiments under controlled conditions on young rose plants in order to assess the impact of these indirect interactions on life history traits of three pests: the necrotrophic fungus Botrytis cinerea Pers.: Fr. (Helotiales: Sclerotiniaceae), the aphid Rhodobium porosum Sanderson (Hemiptera: Aphididae) and the thrips Frankliniella occidentalis Pergande (Thysanoptera: Thripidae). Our results indicated (i) a bi-directional negative interaction between B. cinerea and R. porosum, which is conveyed by decreased aphid growth rate and reduced fungal lesion area, as well as (ii) an indirect negative effect of B. cinerea on insect behavior. No indirect effect was observed between thrips and aphids. This research highlights several complex interactions that may be involved in structuring herbivore and plant pathogen communities within natural and managed ecosystems.

Partial mechanical stimulation facilitates the growth of the rhizomatous plant Leymus secalinus: modulation by clonal integration

BACKGROUND AND AIMS

Mechanical stimulation (MS) often induces plants to undergo thigmomorphogenesis and to synthesize an array of signalling substances. In clonal plants, connected ramets often share resources and hormones. However, little is known about whether and how clonal integration influences the ability of clonal plants to withstand MS. We hypothesized that the effects of MS may be modulated by clonal integration.

METHODS

We conducted an experiment in which ramet pairs of Leymus secalinus were subjected to three treatments: (1) connected ramet pairs under a homogeneous condition [i.e. the proximal (relatively old) and distal (relatively young) ramets were not mechanically stressed]; (2) connected ramet pairs under a heterogeneous condition (i.e. the proximal ramet was mechanically stressed but the distal ramet was not); and (3) disconnected ramet pairs under the same condition as in treatment 2. At the end of the experiment, we harvested all plants and determined their biomass and allocation.

KEY RESULTS

Clonal integration had no significant influence on measured traits of distal L. secalinus ramets without MS. However, under MS, plants with distal ramets that were connected to a mother ramet produced more total plant biomass, below-ground biomass, ramets and total rhizome length than those that were not connected. Partial MS exerted local effects on stimulated ramets and remote effects on connected unstimulated ramets. Partial MS increased total biomass, root/shoot ratio, number of ramets and total rhizome length of stimulated proximal ramets, and increased total biomass, root weight ratio, number of ramets and total rhizome length of connected unstimulated ramets due to clonal integration.

CONCLUSIONS

These findings suggest that thigmomorphogenesis may protect plants from the stresses caused by high winds or trampling and that thigmomorphogenesis can be strongly modulated by the degree of clonal integration.

Timing of induced resistance in a clonal plant network

After local herbivory, plants can activate defense traits both at the damaged site and in undamaged plant parts such as in connected ramets of clonal plants. Since defense induction has costs, a mismatch in time and space between defense activation and herbivore feeding might result in negative consequences for plant fitness. A short time lag between attack and defense activation is important to ensure efficient protection of the plant. Additionally, the duration of induced defense production once the attack has stopped is also relevant in assessing the cost-benefit balance of inducible defenses, which will depend on the absence or presence of subsequent attacks. In this study we quantified the timing of induced responses in ramet networks of the stoloniferous herb Trifolium repens after local damage by Mamestra brassicae larvae. We studied the activation time of systemic defense induction in undamaged ramets and the decay time of the response after local attack. Undamaged ramets became defense-induced 38-51 h after the initial attack. Defense induction was measured as a reduction in leaf palatability. Defense induction lasted at least 28 days, and there was strong genotypic variation in the duration of this response. Ramets formed after the initial attack were also defense-induced, implying that induced defense can extend to new ramet generations, thereby contributing to protection of plant tissue that is both very vulnerable to herbivores and most valuable in terms of future plant growth and fitness.

Physiological integration in an introduced, invasive plant increases its spread into experimental communities and modifies their structure

What determines the invasiveness of introduced plants is still poorly known. Many of the most invasive plant species are clonal, and physiological integration between connected individuals (ramets) of clonal plants may contribute to their ability to spread into communities and reduce performance of existing species. This contribution of integration to the invasiveness of clonal plants may be greater in denser communities. A greenhouse study was conducted to test these two hypotheses. High- and low-density communities were created by sowing seeds of eight grassland species. Each community was planted with three ramets of the stoloniferous, introduced plant Alternanthera philoxeroides that were disconnected from or left connected to ramets growing on bare soil. Connection increased the spread of Alternanthera within a community, but did not reduce community biomass. Alternanthera grew less in high-density communities, but connection did not improve its growth more than in low-density communities. Low-density communities had higher evenness when Alternanthera was connected than when it was disconnected because shoot mass was lower in the more abundant species in the community and higher in the less abundant ones. These results partly supported the first hypothesis, but not the second. The effect of integration on community structure could be due to higher resource import by the ramets of Alternanthera closer to the dominant species. Integration therefore can increase the initial spread of new clonal plant species into communities and modify the effects of this spread on community structure.