Achieving similar root microbiota composition in neighbouring plants through airborne signalling

The ability to recognize and respond to environmental signals is essential for plants. In response to environmental changes, the status of a plant is transmitted to other plants in the form of signals such as volatiles. Root-associated bacteria trigger the release of plant volatile organic compounds (VOCs). However, the impact of VOCs on the rhizosphere microbial community of neighbouring plants is not well understood. Here, we investigated the effect of VOCs on the rhizosphere microbial community of tomato plants inoculated with a plant growth-promoting rhizobacterium Bacillus amyloliquefaciens strain GB03 and that of their neighbouring plants. Interestingly, high similarity (up to 69%) was detected in the rhizosphere microbial communities of the inoculated and neighbouring plants. Leaves of the tomato plant treated with strain GB03-released β-caryophyllene as a signature VOC, which elicited the release of a large amount of salicylic acid (SA) in the root exudates of a neighbouring tomato seedling. The exposure of tomato leaves to β-caryophyllene resulted in the secretion of SA from the root. Our results demonstrate for the first time that the composition of the rhizosphere microbiota in surrounding plants is synchronized through aerial signals from plants.Subject terms: Microbial ecology, Soil microbiology     

Plant–plant communication and community of herbivores on tall goldenrod

1. The volatiles from damaged plants induce defense in neighboring plants. The phenomenon is called plant–plant communication, plant talk, or plant eavesdropping. Plant–plant communication has been reported to be stronger between kin plants than genetically far plants in sagebrush.
2. Why do plants distinguish volatiles from kin or genetically far plants? We hypothesize that plants respond only to important conditions; the induced defense is not free of cost for the plant. To clarify the hypothesis, we conducted experiments and investigations using goldenrod of four different genotypes.
3. The arthropod community on tall goldenrods were different among four genotypes. The response to volatiles was stronger from genetically close plants to the emitter than from genetically distant plants from the emitter. The volatiles from each genotype of goldenrods were different; and they were categorized accordingly. Moreover, the arthropod community on each genotype of goldenrods were different.
4. Synthesis: Our results support the hypothesis: Goldenrods respond to volatiles from genetically close plants because they would have similar arthropod species. These results are important clues elucidating adaptive significance of plant–plant communication.

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Short signalling distances make plant communication a soliloquy

Plants respond to attack by herbivores or pathogens with the release of volatile organic compounds. Neighbouring plants can receive these volatiles and consecutively induce their own defence arsenal. This ‘plant communication’, however, appears counterintuitive when it benefits independent and genetically unrelated receivers, which may compete with the emitter. As a solution to this problem, a role for volatile compounds in within-plant signalling has been predicted. We used wild-type lima bean (Phaseolus lunatus) to quantify under field conditions the distances over which volatile signals move, and thereby determine whether these cues will mainly trigger resistance in other parts of the same plant or in independent plants. Independent receiver plants exhibited airborne resistance to herbivores or pathogens at maximum distances of 50 cm from a resistance-expressing emitter. In undisturbed clusters of lima bean, over 80 per cent of all leaves that were located around a single leaf at this distance were other leaves of the same plant, whereas this percentage dropped below 50 per cent at larger distances. Under natural conditions, resistance-inducing volatiles of lima bean move over distances at which most leaves that can receive the signal still belong to the same plant.     

Effective distance of volatile cues for plant–plant communication in beech

In response to volatiles emitted from a plant infested by herbivorous arthropods, neighboring undamaged conspecific plants become better defended against herbivores; this is referred to as plant‒plant communication. Although plant‒plant communication occurs in a wide range of plant species, most studies have focused on herbaceous plants. Here, we investigated plant‒plant communication in beech trees in two experimental plantations in 2018 and one plantation in 2019. Approximately 20% of the leaves of a beech tree were clipped in half in the spring seasons of 2018 and 2019 (clipped tree). The damage levels to leaves in the surrounding undamaged beech trees were evaluated 90 days after the clipping (assay trees). In both years, the damage levels decreased with a reduction in the distance from the clipped tree. In 2019, we also recorded the damage levels of trees that were not exposed to volatiles (nonexposed trees) as control trees and found that those that were located <5 m away from clipped trees had significantly less leaf damage than nonexposed trees. By using a gas chromatograph–mass spectrometer, ten and eight volatile compounds were detected in the headspaces of clipped and unclipped leaves, respectively. Among them, the amount of (Z)‐3‐hexenyl acetate in clipped leaves was significantly higher than that in nonclipped leaves. Our result suggests that green leaf volatiles such as (Z)‐3‐hexenol and (Z)‐3‐hexenyl acetate and other volatile organic compounds emitted from clipped trees induced defenses in the neighboring trees within the 5 m radius. The effective distances of plant‒plant communication in trees were discussed from the viewpoint of the arthropod community structure in forest ecosystems.     

Within-plant signalling via volatiles overcomes vascular constraints on systemic signalling and primes responses against herbivores

Plant volatiles play important roles in signalling between plants and insects, but their role in communication among plants remains controversial. Previous research on plant–plant communication has focused on interactions between neighbouring plants, largely overlooking the possibility that volatiles function as signals within plants. Here, we show that volatiles released by herbivore-wounded leaves of hybrid poplar ( Populus deltoides  ×  nigra ) prime defences in adjacent leaves with little or no vascular connection to the wounded leaves. Undamaged leaves exposed to volatiles from wounded leaves on the same stem had elevated defensive responses to feeding by gypsy moth larvae ( Lymantria dispar L.) compared with leaves that did not receive volatiles. Volatile signals may facilitate systemic responses to localized herbivory even when the transmission of internal signals is constrained by vascular connectivity. Self-signalling via volatiles is consistent with the short distances over which plant response to airborne cues has been observed to occur and has apparent benefits for emitting plants, suggesting that within-plant signalling may have equal or greater ecological significance than signalling between plants.     

Identification of olfactory receptor neurons in Uraba lugens (Lepidoptera: Nolidae) and its implications for host range

Phytophagous insects detect volatile compounds produced by host and non-host plants, using species-specific sets of olfactory receptor neurons (ORNs). To investigate the relationship between the range of host plants and the profile of ORNs, single sensillum recordings were carried out to identify ORNs and corresponding active compounds in female Uraba lugens (Lepidoptera: Nolidae), an oligophagous eucalypt feeder. Based on the response profiles to 39 plant volatile compounds, 13 classes of sensilla containing 40 classes of ORNs were identified in female U. lugens. More than 95% (163 out of 171) of these sensilla contained 16 classes of ORNs with narrow response spectra, and 62.6% (107 out of 171) 18 classes of ORNs with broad response spectra. Among the specialized ORNs, seven classes of ORNs exhibited high specificity to 1,8-cineole, (±)-citronellal, myrcene, (±)-linalool and (E)-β-caryophyllene, major volatiles produced by eucalypts, while nine other classes of ORNs showed highly specialized responses to green leaf volatiles, germacrene D, (E)-β-farnesene and geranyl acetate that are not produced by most eucalypts. We hypothesize that female U. lugens can recognize their host plants by detecting key host volatile compounds, using a set of ORNs tuned to host volatiles, and discriminate them from non-host plants using another set of ORNs specialized for non-host volatiles. The ORNs with broad response spectra may enhance the discrimination between host and non-host plants by adding moderately selective sensitivity. Based on our finding, it is suggested that phytophagous insects use the combinational input from both host-specific and non-host specific ORNs for locating their host plants, and the electrophysiological characterization of ORN profiles would be useful in predicting the range of host plants in phytophagous insects.     

Identification and quantitative analysis of the volatile substances emitted by maturing cotton in the field

When atmosphere from cotton plants (Gossypium hirsutum L., var. Deltapine Smoothleaf) was condensed by passing it over the expansion coil of an air conditioner and three 1-hour collections per day (early morning, noon, and late afternoon) were made, the total essential oils were found to consist of 50 to 60% β-bisabolol (I_k 1660) and γ-bisabolene (I_k 1550) and 30 to 40% geraniol (I_k 1250), myrtenal (I_k 1328), nerolidol (I_k 1520), and β-caryophyllene oxide (I_k 1590). As the plant matured, trans-2-hexanol was produced in concentrations of 7 to 27%. Before fruiting, β-bisabolol made up as much as 60% of the total essential oil transpired by the plants, and as the concentration of β-bisabolol increased, that of γ-bisabolene decreased.     

Oil body-mediated defense against fungi: From tissues to ecology

Oil bodies are localized in the seed cells and leaf cells of many land plants. They have a passive function as storage organelles for lipids. We recently reported that the leaf oil body has an active function as a subcellular factory that produces an antifungal oxylipin during fungal infection in Arabidopsis thaliana. Here, we propose a model for oil body-mediated plant defense. Remarkably, senescent leaves develop oil bodies and accumulate α-dioxygenase1 (α-DOX1) and caleosin3 (CLO3) on the oil-body membrane, which catalyze the conversion of α-linolenic acid to the phytoalexin 2-hydroxy-octadecatrienoic acid (2-HOT). The model proposes that senescent leaves actively produce antifungal oxylipins and phytoalexins, and abscised leaves contain a mixture of antifungal compounds. In natural settings, the abscised leaves with antifungal compounds accumulate in leaf litter and function to protect healthy tissues and young plants from fungal infection. Plants might have evolved this ecological function for dead leaves.     

Heterotrimeric G protein signalling in the plant kingdom

In animals, heterotrimeric G proteins, comprising α-, β-and γ-subunits, perceive extracellular stimuli through cell surface receptors, and transmit signals to ion channels, enzymes and other effector proteins to affect numerous cellular behaviours. In plants, G proteins have structural similarities to the corresponding molecules in animals but transmit signals by atypical mechanisms and effector proteins to control growth, cell proliferation, defence, stomate movements, channel regulation, sugar sensing and some hormonal responses. In this review, we summarize the current knowledge on the molecular regulation of plant G proteins, their effectors and the physiological functions studied mainly in two model organisms: Arabidopsis thaliana and rice (Oryza sativa). We also look at recent progress on structural analyses, systems biology and evolutionary studies.     

Floral volatiles play a key role in specialized ant pollination

Chemical signals emitted by plants are crucial to understand the ecology and evolution of plant–animal interactions. Scent is an important component of floral phenotype and represents a decisive communication channel between plants and floral visitors. Floral volatiles promote attraction of mutualistic pollinators and, in some cases, serve to prevent flower visitation by antagonists such as ants. Despite ant visits to flowers have been suggested to be detrimental to plant fitness, in recent years there has been a growing recognition of the positive role of ants in pollination. Nevertheless, the question of whether floral volatiles mediate mutualisms between ants and ant-pollinated plants still remains largely unexplored. Here we review the documented cases of ant pollination and investigate the chemical composition of the floral scent in the ant-pollinated plant Cytinus hypocistis. By using chemical-electrophysiological analyses and field behavioural assays, we examine the importance of olfactory cues for ants, identify compounds that stimulate antennal responses, and evaluate whether these compounds elicit behavioural responses. Our findings reveal that floral scent plays a crucial role in this mutualistic ant–flower interaction, and that only ant species that provide pollination services and not others occurring in the habitat are efficiently attracted by floral volatiles. 4-oxoisophorone, (E)-cinnamaldehyde, and (E)-cinnamyl alcohol were the most abundant compounds in Cytinus flowers, and ant antennae responded to all of them. Four ant pollinator species were significantly attracted to volatiles emitted by Cytinus inflorescences as well as to synthetic mixtures and single antennal-active compounds. The small amount of available data so far suggest that there is broad interspecific variation in floral scent composition among ant-pollinated plants, which could reflect differential responses and olfactory preferences among different ant species. Many exciting discoveries will be made as we enter into further research on chemical communication between ants and plants.     

The Arabidopsis pi4kIIIβ1β2 double mutant is salicylic acid-overaccumulating: a new example of salicylic acid influence on plant stature

Growth is the best visible sign of plant comfort. If plants are under stress, a difference in growth with control conditions can indicate that something is going wrong (or better). Phytohormones such as auxin, cytokinins, brassinosteroids or giberellins, are important growth regulators and their role in plant growth was extensively studied. On the other hand the role of salicylic acid (SA), a phytohormone commonly connected with plant defense responses, in plant growth is under-rated. However, studies with SA-overaccumulating mutants directly showed an influence of SA on plant growth. Recently we characterized an Arabidopsis SA-overaccumulating mutant impaired in phosphatidylinositol-4-kinases (pi4kIIIβ1β2). This mutant is dwarf. The crossing with mutants impaired in SA signaling revealed that pi4kIIIβ1β2 stunted rosette is due to high SA, while the short root length is not. This brings into evidence that upper and lower parts of the plants, even though they may share common phenotypes, are differently regulated.     

Phospholipase D δ knock-out mutants are tolerant to severe drought stress

Phospholipase D (PLD) is involved in different plant processes, ranging from responses to abiotic and biotic stress to plant development. Phospholipase Dδ (PLDδ) is activated in dehydration and salt stress, producing the lipid second messenger phosphatidic acid. In this work we show that pldδ Arabidopsis mutants were more tolerant to severe drought than wild-type plants. PLDδ has been shown to be required for ABA regulation of stomatal closure of isolated epidermal peels. However, there was no significant difference in stomatal conductance at the whole plant level between wild-type and pldδ mutants. Since PLD hydrolyses structural phospholipids, then we looked at membrane integrity. Ion leakage measurements showed that during dehydration of leaf discs pldδ mutant has less membrane degradation compared to the wild-type. We further analyzed the mutants and showed that pldδ have higher mRNA levels of RAB18 and RD29A compared to wild-type plants under normal growth conditions. Transient expression of AtPLDδ in Nicotiana benthamiana plants induced a wilting phenotype. These findings suggest that, in wt plants PLDδ disrupt membranes in severe drought stress and, in the absence of the protein (PLDδ knock-out) might drought-prime the plants, making them more tolerant to severe drought stress. The results are discussed in relation to PLDδ role in guard cell signaling and drought tolerance.     

Root–shoot growth responses during interspecific competition quantified using allometric modelling

Background and Aims

Plant competition studies are restricted by the difficulty of quantifying root systems of competitors. Analyses are usually limited to above-ground traits. Here, a new approach to address this issue is reported.

Methods

Root system weights of competing plants can be estimated from: shoot weights of competitors; combined root weights of competitors; and slopes (scaling exponents, α) and intercepts (allometric coefficients, β) of ln-regressions of root weight on shoot weight of isolated plants. If competition induces no change in root : shoot growth, α and β values of competing and isolated plants will be equal. Measured combined root weight of competitors will equal that estimated allometrically from measured shoot weights of each competing plant. Combined root weights can be partitioned directly among competitors. If, as will be more usual, competition changes relative root and shoot growth, the competitors'' combined root weight will not equal that estimated allometrically and cannot be partitioned directly. However, if the isolated-plant α and β values are adjusted until the estimated combined root weight of competitors matches the measured combined root weight, the latter can be partitioned among competitors using their new α and β values. The approach is illustrated using two herbaceous species, Dactylis glomerata and Plantago lanceolata.

Key Results

Allometric modelling revealed a large and continuous increase in the root : shoot ratio by Dactylis, but not Plantago, during competition. This was associated with a superior whole-plant dry weight increase in Dactylis, which was ultimately 2·5-fold greater than that of Plantago. Whole-plant growth dominance of Dactylis over Plantago, as deduced from allometric modelling, occurred 14–24 d earlier than suggested by shoot data alone.

Conclusion

Given reasonable assumptions, allometric modelling can analyse competitive interactions in any species mixture, and overcomes a long-standing problem in studies of competition.     

Effects of the virus satellite gene βC1 on host plant defense signaling and volatile emission

Tomato Yellow Leaf Curl China virus spreads together with its invasive vector, the silverleaf whitefly B biotype, which exhibits higher growth rates on infected plants. Previous studies indicate that the virus satellite gene βC1 accounts for the visible symptoms of infection and inhibits the constitutive expression of jasmonic acid (JA)—a phytohormone involved in plant defense against whiteflies—and of some JA-regulated genes. Here we present new details of the effects of on plant signaling and defense, obtained with (non-host) transgenic Arabidopsis thaliana and Nicotiana benthamiana plants. We found that JA induction in response to wounding was reduced in plants expressing βC1. This result implies that βC1 acts on conserved plant regulation mechanisms and might impair the entire JA defense pathway. Furthermore, transformed N. benthamiana plants exhibited elevated emissions of the volatile compound linalool, suggesting that βC1 also influences plant-derived olfactory cues available to vector and non-vector insects.     

Damage to sagebrush attracts predators but this does not reduce herbivory

Emissions of volatiles increase following herbivory from many plant species and volatiles may serve multiple functions. Herbivore‐induced volatiles attract predators and parasitoids of herbivores and are often assumed to benefit plants by facilitating top‐down control of herbivores; this benefit of induced emissions has been tested only a few times. Volatile compounds released by experimentally clipped sagebrush shoots have been shown to reduce levels of chewing damage experienced by other shoots on the same plant and on neighboring sagebrush plants. In this study, I asked whether experimental clipping attracted predators of herbivorous insects to sagebrush shoots. I also evaluated aphid populations and chewing damage on clipped and unclipped shoots and whether predators were likely to have caused differences in aphids and chewing damage. Shoots that had been clipped recruited more generalist predators, particularly coccinellids and Geocoris spp. in visual surveys conducted during two seasons. Clipping also caused increased numbers of parasitized aphids in one season. Ants were common tending aphids but were not significantly affected by clipping. Despite the increase in generalist predators, clipped plants were more likely to support populations of aphids that increased during both seasons compared to aphids on unclipped control plants. Clipped shoots suffered less damage by chewing herbivores in the 1‐year in which this was measured. Chewing damage was not correlated with numbers of predators. These results suggest that predators and parasitoids were attracted to experimentally clipped sagebrush plants but that these predators were not effective at reducing net damage to the plant. This conclusion is not surprising as much of the herbivory is inflicted by grasshoppers and deer, herbivores that are not vulnerable to the predators attracted to sagebrush volatiles. More generally, it should not be assumed that predators that are attracted by herbivore‐induced volatiles necessarily benefit the plant without testing this hypothesis under field conditions.     

Specific herbivore-induced volatiles defend plants and determine insect community composition in the field

In response to insect attack, plants release complex blends of volatile compounds. These volatiles serve as foraging cues for herbivores, predators and parasitoids, leading to plant-mediated interactions within and between trophic levels. Hence, plant volatiles may be important determinants of insect community composition. To test this, we created rice lines that are impaired in the emission of two major signals, S-linalool and (E)-β-caryophyllene. We found that inducible S-linalool attracted predators and parasitoids as well as chewing herbivores, but repelled the rice brown planthopper Nilaparvata lugens, a major pest. The constitutively produced (E)-β-caryophyllene on the other hand attracted both parasitoids and planthoppers, resulting in an increased herbivore load. Thus, silencing either signal resulted in specific insect assemblages in the field, highlighting the importance of plant volatiles in determining insect community structures. Moreover, the results imply that the manipulation of volatile emissions in crops has great potential for the control of pest populations.     

Biogeographic,climatic and spatial drivers differentially affect α-, β- and γ-diversities on oceanic archipelagos

Island biogeographic studies traditionally treat single islands as units of analysis. This ignores the fact that most islands are spatially nested within archipelagos. Here, we took a fundamentally different approach and focused on entire archipelagos using species richness of vascular plants on 23 archipelagos worldwide and their 174 constituent islands. We assessed differential effects of biogeographic factors (area, isolation, age, elevation), current and past climate (temperature, precipitation, seasonality, climate change velocity) and intra-archipelagic spatial structure (archipelago area, number of islands, area range, connectivity, environmental volume, inter-island distance) on plant diversity. Species diversity of each archipelago (γ) was additively partitioned into α, β, nestedness and replacement β-components to investigate the relative importance of environmental and spatial drivers. Multiple regressions revealed strong effects of biogeography and climate on α and γ, whereas spatial factors, particularly number of islands, inter-island distance and area range, were key to explain β. Structural equation models additionally suggested that γ is predominantly determined by indirect abiotic effects via its components, particularly β. This highlights that β and the spatial arrangement of islands are essential to understand insular ecology and evolution. Our methodological framework can be applied more widely to other taxa and archipelago-like systems, allowing new insights into biodiversity origin and maintenance.     

Plant Host Finding by Parasitic Plants: A New Perspective on Plant to Plant Communication

Plants release airborne chemicals that can convey ecologically relevant information to other organisms. These plant volatiles are known to mediate a large array of, often complex, interactions between plants and insects. It has been suggested that plant volatiles may have similar importance in mediating interactions among plant species, but there are few well-documented examples of plant-to-plant communication via volatiles, and the ecological significance of such interactions has been much debated. To date, nearly all studies of volatile-mediated interactions among plant species have focused on the reception of herbivore-induced volatiles by neighboring plants. We recently documented volatile effects in another system, demonstrating that the parasitic plant Cuscuta pentagona uses volatile cues to locate its hosts. This finding may broaden the discussion regarding plant-to-plant communication, and suggests that new classes of volatile-meditated interactions among plant species await discovery.Key Words: chemical communication, Cuscuta pentagona, host fiding, host selection, plant-plant communication, plant volatiles, parasitic plantsFor nearly 25 years, the ecological importance of plant-to-plant communication through volatiles has remained an open and much debated question. Plants exchange gases with the atmosphere and, in so doing, release plumes of volatile chemicals that can convey ecologically important information to other organisms. The potential ecological significance of these volatile cues is demonstrated by the large and growing array of interactions between plants and arthropods known to be mediated by plant volatiles. Volatiles serve as foraging cues both for insects that are beneficial to plants, such as pollinators,¹ and those that are harmful such as herbivores.^2,3 Because the volatile blends released by plants exhibit variation in response to environmental stimuli, volatiles can convey detailed information about the status of the emitting plant. Predatory and parasitic insects that feed on herbivorous insects respond preferentially to plant volatiles that are induced by insect feeding,⁴ while female herbivores use such cues to avoid laying their eggs on already-infested plants.^3,5 Moreover, the volatile blends released in response to herbivory can differ between individual herbivore species, providing highly specific cues to specialist parasitoids.⁶ Thus, plant volatiles are known to mediate complex interactions among plants and insects across multiple trophic levels.It has long been speculated that plant volatiles might have similar significance for interactions among plant species, yet there are few well-documented examples of communication between plants by way of volatile signals. Essentially all previous work on plant-to-plant communication has focused on the reception of herbivore-induced volatile signals by neighboring plants, which may use them as early warning signals to initiate their own direct and indirect defense responses. The first studies claiming to document such effects were published almost 25 years ago.^7,8 But issues with the experimental design of these early experiments and the availability of alternative explanations for their results led many ecologists to disregard the phenomenon.^9–11 Later, a number of studies demonstrated that direct and indirect plant defenses could be elicited by exposure to certain induced plant volatiles.^12–15 But many of these effects were demonstrated in airtight chambers with volatile concentrations far higher than those likely experienced in natural settings, again raising doubts about the ecological significance of plant-plant communication.^16–18 Still more recently, some researchers have provided evidence that more realistic volatile concentrations likely induce priming of the defenses of receiving plants, rather than the initiation of full scale responses,¹⁵ while others have documented volatile effects under natural conditions.^19–21 Thus, despite continuing caution about the interpretation of experiments in this area,^17,18 there is mounting evidence that plant herbivore-induced volatiles can serve as early warning signals to neighboring plants.We recently documented an entirely new class of volatile mediated interactions among plants: the role of plant volatiles in host location by parasitic plants that attach to above ground shoots of other plants. Plant parasites are important components of natural and agricultural ecosystems and play important roles in determining community structure and dynamics.^22,23 We are exploring the mechanisms of host-location and other interactions between parasitic plants in the genus Cuscuta (dodder) and their host plants. Dodder vines germinate from seeds containing limited energy reserves and, as the parasites have no roots and little photosynthetic ability, must quickly locate and attach to suitable hosts in order to survive (Fig. 1). Thus, there is presumably significant selection pressure for dodder vines to employ efficient strategies for host location, and host plant volatiles may be expected to provide relevant directional cues. Dodder seedlings exhibit a rotational growth habit (circumnutation) following germination and previous researchers have suggested that host-finding might involve random growth²⁴ or the exploitation of light cues.²⁵Open in a separate windowFigure 1Seedling of Cuscuta pentagona (A) foraging toward a 20-day-old tomato plant, (B) attaching to and beginning to grow from stems of tomato seedlings and (C) close up of C. pentagona attachment.Using a very simple experimental design, we explored the possibility that host-plant volatiles might mediate host-location by seedlings of C. pentagona. We placed a germinated seedling in a vial of water located at the center of a dry filter paper disk. A host plant (a 20-day old tomato seedling) was placed near the edge of the disk and the dodder seedling was allowed to forage for four days. By the end of the experiment the seedling would lay horizontally on the disk and we traced its position on the filter paper in order to assess the directionality of growth relative to the host plant. This experiment was replicated 30 times and our results clearly indicated directional growth toward the tomato plant (80% of the tested seedlings grew into the disk half nearest the host) demonstrating that C. pentagona seedlings were perceiving some host-derived cue.We did not observe directed growth when we tested dodder seedling response to alternative targets including pots of moist soil, artificial plants, and vials of colored water intended to mimic possible light cues. In order to confirm a role for plant volatiles in host location by C. pentagona, we tested seedling response to host plant volatiles extracted from filtered air in a volatile collection system and then released from rubber septa in the absence of any other host-derived cues. Here we observed a directed growth response similar to that exhibited toward an intact tomato seedling, confirming that host plant volatiles do provide a cue used for host location by C. pentagona. In subsequent experiments we found directed growth toward impatiens and alfalfa plants, which are attractive hosts for C. pentagona and also toward wheat plants which are poor hosts, suggesting that the host-location mechanisms operate over a wide range of host species.Since discriminating between more and less desirable host species is likely to be important in natural settings, we next explored whether dodder seedlings could distinguish volatile signals from host and nonhost plants. Cuscuta pentagona seedlings exhibited directional growth toward tomato plants in preference to wheat plants and also to extracted volatiles from tomato in preference to those from wheat, demonstrating an ability to distinguish and choose among volatiles from more and less preferred hosts.When we tested seedling responses to individual compounds from the wheat and tomato blends, we found that three compounds from tomato, α-pinene, β-myrcene, and β-phellandrene elicited directed growth. β-myrcene was also present in the wheat blend. Unexpectedly, we also found that one compound present in the wheat blend, (Z)-3-hexenyl acetate, was repellent, providing a plausible explanation for the lower attractiveness of the wheat blend. It is interesting to note that (Z)-3-hexenyl acetate is also released by tomato in response to feeding by herbivores, and we have some data suggesting that C. pentagona seedlings may find tomato seedlings infested by Heliothis virescens caterpillars less attractive than un-attacked plants (unpublished data).The discovery that some parasitic plants exploit host plant volatiles for host location provides a new perspective on volatile mediated interactions among plant species, demonstrating that plant volatiles play a role in mediating ecologically significant interactions in at least one system other than the transfer of herbivore-induced warning signals. We think it is quite likely that plant volatiles will be found to play a role in host location by other parasitic plants and perhaps even by vining plants generally. Moreover, we think it is more likely than not that more classes of volatile mediated interactions among plants remain to be discovered given the potential availability of volatile cues and the fitness benefits to be derived by plants using such cues to gather information about the identity and condition of their neighbors.     

mTORC1 signaling and regulation of pancreatic β-cell mass

The capacity of β cells to expand in response to insulin resistance is a critical factor in the development of type 2 diabetes. Proliferation of β cells is a major component for these adaptive responses in animal models. The extracellular signals responsible for β-cell expansion include growth factors, such as insulin, and nutrients, such as glucose and amino acids. AKT activation is one of the important components linking growth signals to the regulation of β-cell expansion. Downstream of AKT, tuberous sclerosis complex 1 and 2 (TSC1/2) and mechanistic target of rapamycin complex 1 (mTORC1) signaling have emerged as prime candidates in this process, because they integrate signals from growth factors and nutrients. Recent studies demonstrate the importance of mTORC1 signaling in β cells. This review will discuss recent advances in the understanding of how this pathway regulates β-cell mass and present data on the role of TSC1 in modulation of β-cell mass. Herein, we also demonstrate that deletion of Tsc1 in pancreatic β cells results in improved glucose tolerance, hyperinsulinemia and expansion of β-cell mass that persists with aging.