Herbivore-induced plant vaccination. Part I. The orchestration of plant defenses in nature and their fitness consequences in the wild tobacco Nicotiana attenuata

A plant's responses to attack from particular pathogens and herbivores may result in resistance to subsequent attack from the same species, but may also affect different species. Such a cross-resistance, called immunization or vaccination, can benefit the plant, if the fitness consequences of attack from the initial attacker are less than those from subsequent attackers. Here, we report an example of naturally occurring vaccination of the native tobacco plant, Nicotiana attenuata, against Manduca hornworms by prior attack from the mirid bug, Tupiocoris notatus (Dicyphus minimus), which results from the elicitation of two categories of induced plant responses. First, attack from both herbivore species causes the plants in nature to release predator-attracting volatile organic compounds (VOCs), and the attracted generalist predator, Geocoris pallens, preferentially attacks the less mobile hornworm larvae. Second, attack from both mirids and hornworms increases the accumulation of secondary metabolites and proteinase inhibitors (PIs) in the leaf tissue, which is correlated with the slow growth of Manduca larvae. Mirid damage does not significantly reduce the fitness of the plant in nature, whereas attack from the hornworm reduces lifetime seed production. Consequently, plants that are attacked by mirids realize a significant fitness advantage in environments with both herbivores. The combination of growth-slowing direct defenses and predator-attracting indirect defenses results in greater hornworm mortality on mirid-attacked plants and provides the mechanism of the vaccination phenomenon.     

Beta and habitat diversity in marine systems: a new approach to measurement, scaling and interpretation

Specialist herbivores are known to alter their host's wound-induced responses but the beneficiaries of these alterations are unknown. Nicotiana attenuata plants release a burst of ethylene specifically in response to feeding by Manduca sexta larvae, which is known to suppress wound- and methyl jasmonate (MeJA)-inducible nicotine accumulation. The ethylene burst may be a mechanism by which M. sexta larvae feed "stealthily" on their host plants or, alternatively, it may allow the plant to optimize its defense response against this specialist herbivore by reducing costs of induction. We examined the impact of the ethylene burst on defense-related fitness costs that are readily observed when plants are treated with MeJA and grown in competition with untreated plants. We elicited nicotine induction (with MeJA), the ethylene burst (with the ethylene releasing compound, ethephon) and inhibited the plant's ability to perceive ethylene (with applications of an antagonist of ethylene receptors, 1-methylcyclopropene, 1-MCP). By simultaneously applying MeJA and ethephon we mimicked the plant's hormonal response to larval attack. We hypothesized that if the ethylene burst benefited the plant, the fitness costs of MeJA induction should be reduced by ethephon and restored if the plants were additionally treated with 1-MCP. In a second experiment, we applied larval oral secretion (OS) to elicit endogenous hormone production and predicted that the 1-MCP treatment should reduce the fitness of OS-treated plants. Our measures of plant fitness, namely the rate of stalk elongation and lifetime capsule production, supported these predictions. We conclude that the ethylene burst elicited by this specialist herbivore can reduce MeJA-induced fitness costs and increase the competitive strength of OS-treated plants. Suppressed nicotine production is likely to contribute to, but is not sufficient to explain, the observed fitness outcomes. The intensity of intra-specific competition and herbivore attack will likely determine the adaptive value of the M. sexta-elicited ethylene response.     

Microbial effects on plant phenology and fitness

Plant development and the timing of developmental events (phenology) are tightly coupled with plant fitness. A variety of internal and external factors determine the timing and fitness consequences of these life-history transitions. Microbes interact with plants throughout their life history and impact host phenology. This review summarizes current mechanistic and theoretical knowledge surrounding microbe-driven changes in plant phenology. Overall, there are examples of microbes impacting every phenological transition. While most studies have focused on flowering time, microbial effects remain important for host survival and fitness across all phenological phases. Microbe-mediated changes in nutrient acquisition and phytohormone signaling can release plants from stressful conditions and alter plant stress responses inducing shifts in developmental events. The frequency and direction of phenological effects appear to be partly determined by the lifestyle and the underlying nature of a plant–microbe interaction (i.e., mutualistic or pathogenic), in addition to the taxonomic group of the microbe (fungi vs. bacteria). Finally, we highlight biases, gaps in knowledge, and future directions. This biotic source of plasticity for plant adaptation will serve an important role in sustaining plant biodiversity and managing agriculture under the pressures of climate change.     

Fitness consequences of a clock pollinator filter in Nicotiana attenuata flowers in nature

Nicotiana attenuata flowers, diurnally open,emit scents and move vertically to interact with nocturnal hawkmoth and day-active hummingbird pollinators. To examine the fitness consequences of these floral rhythms, we conducted pollination trials in the plant's native habitat with phase-shifted flowers of plants silenced in circadian clock genes. The results revealed that some pollination benefits observed under glasshouse conditions were not reproduced under natural field conditions. Floral arrhythmicity increased pollination success by hummingbirds, while reducing those by hawkmoths in the field. Thus, floral circadian rhythms may influence a plant's fitness by filtering pollinators leading to altered seed set from outcrossed pollen.     

Foliar resistance to simulated extreme temperature events in contrasting plant functional and chorological types

We analysed leaf resistance of 41 Angiosperms belonging to a wide range of plant functional (PFTs) and chorological types (PCTs) to simulated frost and high‐temperature extreme events (EE). Leaf resistance was estimated as percentage of membrane electrolyte leakage under heating and freezing treatments in the lab. Leaf resistance to heating or freezing was not significantly correlated with the main resource‐use characteristics that defined PFTs, such as leaf specific area, toughness, N concentration or thickness. Leaf resistance to heating differed among PFTs (graminoids and bromeliads were the more resistant groups), but not among PCTs. In contrast, leaf resistance to freezing significantly differed among PCTs. Along a steep regional climatic gradient, climate variables (annual mean temperature, mean minimum temperature, mean maximum temperature and number of frost‐free months) at the locations where the given species were most abundant were also significantly correlated with freezing resistance. Species from colder habitats both at the sub‐continental and regional scales showed the highest leaf resistance to freezing. Our work indicates that leaf resistance to climatic EE and resource‐use strategy (assessed in previous studies) represent two different, partially decoupled axes of plant specialisation. It also suggests that changes in the frequency of very low temperature events might have regional‐scale impacts on vegetation, whereas changes in the frequency of very high temperature events might have more influence at the local scale.     

COEVOLUTION BETWEEN COOPERATORS AND CHEATS IN A MICROBIAL SYSTEM

In many circumstances organisms invest in cooperative activities to increase their mutual fitness but are susceptible to cheats that obtain the benefits of cooperation without investment. Natural selection may favor cooperators that resist cheats, and cheats that avoid such resistance; in theory the coevolutionary interaction may be sustained and dynamic. Here, we report evidence of antagonistic coevolution between cooperators and cheats involved in biofilm formation by Pseudomonas fluorescens bacteria. Two distinct phenotypes occur in static culture tubes: one that can form a biofilm at the air–broth interface and thus obtain improved access to oxygen, and one that colonizes the broth phase but which can also invade, and weaken, the biofilm produced by the other type. Over serial passage, biofilm producers (considered here as cooperators) evolve to become better at resisting invasion, and biofilm nonproducers (cheats) evolve to be more efficient invaders. Each type has higher performance (resistance in the case of cooperators and biofilm invasion for cheats) in competition with isolates of the other type from their past compared to that from their future, indicating a dynamic coevolutionary interaction. Such coevolution may have important consequences for the maintenance of cooperation.     

Nutrition acquisition strategies during fungal infection of plants

In host-pathogen interactions, efficient pathogen nutrition is a prerequisite for successful colonization and fungal fitness. Filamentous fungi have a remarkable capability to adapt and exploit the external nutrient environment. For phytopathogenic fungi, this asset has developed within the context of host physiology and metabolism. The understanding of nutrient acquisition and pathogen primary metabolism is of great importance in the development of novel disease control strategies. In this review, we discuss the current knowledge on how plant nutrient supplies are utilized by phytopathogenic fungi, and how these activities are controlled. The generation and use of auxotrophic mutants have been elemental to the determination of essential and nonessential nutrient compounds from the plant. Considerable evidence indicates that pathogen entrainment of host metabolism is a widespread phenomenon and can be accomplished by rerouting of the plant's responses. Crucial fungal signalling components for nutrient-sensing pathways as well as their developmental dependency have now been identified, and were shown to operate in a coordinate cross-talk fashion that ensures proper nutrition-related behaviour during the infection process.     

Great Spotted Cuckoos Frequently Lay Their Eggs While Their Magpie Host is Incubating

Species that suffer from brood parasitism face a considerable reduction in their fitness which selects for the evolution of host defences. To prevent parasitism, hosts can mob or attack brood parasites when they approach the host nest and block the access to the nest by sitting on the clutch. In turn, as a counter‐adaptation, brood parasites evolved secretive behaviours near their host nests. Here, we have studied great spotted cuckoo (Clamator glandarius) egg‐laying behaviour and defence by their magpie (Pica pica) hosts inside the nest using continuous video recordings. We have found several surprising results that contradict some general assumptions. The most important is that most (71%) of the parasitic events by cuckoo females are completed while the magpie females are incubating. By staying in the nest, magpies force cuckoo females to lay their egg facing the high risk of being attacked by the incubating magpie (attack occurred in all but one of the events, n = 15). During these attacks, magpies pecked the cuckoo violently, but could never effectively avoid parasitism. These novel observations expand the sequence of adaptations and counter‐adaptations in the arms race between brood parasites and their hosts during the pre‐laying and laying periods.     

Generalized movement strategies for constrained consumers: ignoring fitness can be adaptive

Abstract Movements made by real organisms--such as movements involved in dispersal, migration, and habitat selection--are expected to occasionally be suboptimal because of realistic constraints imposed by incomplete information, perceptual limitations, and stochasticity. Previous theory considering such constraints has shown that movements appropriately conditioned on habitat or resource characteristics can balance out suboptimal components of movement and thereby lead organisms to ideal free distributions and fitness maxima, whereas movements conditioned on fitness differentials cannot. These findings suggest a somewhat paradoxical hypothesis: even if organisms have information about their fitness, movement strategies that maximize fitness may be conditioned on something other than fitness per se. We test this hypothesis by investigating the evolutionary stability of generalized, conditional movement strategies that vary in their use of information on fitness versus information on habitat characteristics. We show that when costs of sensory machinery are included, natural selection should favor movement strategies that completely ignore fitness information. Finally, we synthesize previous work by showing how several previous important theoretical results for adaptive movement strategies are united under our one general model.     

COSTS OF INDUCED RESPONSES AND TOLERANCE TO HERBIVORY IN MALE AND FEMALE FITNESS COMPONENTS OF WILD RADISH

Theory predicts that plant defensive traits are costly due to trade-offs between allocation to defense and growth and reproduction. Most previous studies of costs of plant defense focused on female fitness costs of constitutively expressed defenses. Consideration of alternative plant strategies, such as induced defenses and tolerance to herbivory, and multiple types of costs, including allocation to male reproductive function, may increase our ability to detect costs of plant defense against herbivores. In this study we measured male and female reproductive costs associated with induced responses and tolerance to herbivory in annual wild radish plants (Raphanus raphanistrum). We induced resistance in the plants by subjecting them to herbivory by Pieris rapae caterpillars. We also induced resistance in plants without leaf tissue removal using a natural chemical elicitor, jasmonic acid; in addition, we removed leaf tissue without inducing plant responses using manual clipping. Induced responses included increased concentrations of indole glucosinolates, which are putative defense compounds. Induced responses, in the absence of leaf tissue removal, reduced plant fitness when five fitness components were considered together; costs of induction were individually detected for time to first flower and number of pollen grains produced per flower. In this system, induced responses appear to impose a cost, although this cost may not have been detected had we only quantified the traditionally measured fitness components, growth and seed production. In the absence of induced responses, 50% leaf tissue removal, reduced plant fitness in three out of the five fitness components measured. Induced responses to herbivory and leaf tissue removal had additive effects on plant fitness. Although plant sibships varied greatly (49–136%) in their level of tolerance to herbivory, costs of tolerance were not detected, as we did not find a negative association between the ability to compensate for damage and plant fitness in the absence of damage. We suggest that consideration of alternative plant defense strategies and multiple costs will result in a broader understanding of the evolutionary ecology of plant defense.     

The ecology and evolution of inducible defenses

Inducible defenses are responses activated through a previous encounter with a consumer or competitor that confer some degree of resistance to subsequent attacks. While the importance of inducible resistance has long been known in host-parasite interactions, it is only recently that its importance has emerged in other natural systems. Although the structural defenses produced by invertebrates to their competitors and predators are by no means the same as an immune response triggered by parasites, these responses all share the properties of (1) specificity, (2) amplification and (3) memory. This review discusses the following ecological consequences and evolutionary causes of inducible defenses: (1) Inducible defenses render historical factors important in biological interactions and can affect the probability of individual survival and growth, as well as affect population dynamics of consumers in some circumstances. (2) Although the benefits of inducible defenses are often balanced by fitness costs, including reduced growth, reproductive output and survivorship, the role of costs and benefits in the evolution of inducible defenses is by no means clear. A more integrated approach would involve a multivariate analysis of the role of natural selection on the inducible characters of interest, their norms of reaction and correlated fitness characters. (3) The disproportionate representation of inducible, morphological defenses among clonal organisms may be due to both a higher rate of origination and enhanced selection to maintain these defenses in clonal taxa. (4) Inducible defenses should be most common when reliable cues are available, attacks by biological agents are unpredictable, and the fitness gains of defenses are balanced by the costs. An integrated approach to studying inducible defenses would thus combine mechanistic estimates of costs, population-level estimates of defense effectiveness, and genetic estimates of correlations between fitness and inducible characters. This will allow us to estimate rates of evolution in these phenotypically plastic threshold characters.     

Inclusive fitness,asymmetric competition and kin selection in plants

The findings that some plants alter their competitive phenotype in response to genetic relatedness of its conspecific neighbour (and presumed competitor) has spurred an increasing interest in plant kin‐interactions. This phenotypic response suggests the ability to assess the genetic relatedness of conspecific competitors, proposing kin selection as a process that can influence plant competitive interactions. Kin selection can favour restrained competitive growth towards kin, if the fitness loss from reducing own growth is compensated by increased fitness in the related neighbour. This may lead to positive frequency dependency among related conspecifics with important ecological consequences for species assemblage and coexistence. However, kin selection in plants is still controversial. First, many studies documenting a plastic response to neighbour relatedness do not estimate fitness consequences of the individual that responds, and when estimated, fitness of individuals grown in competition with kin did not necessarily exceed that of individuals grown in non‐kin groups. Although higher fitness in kin groups could be consistent with kin selection, this could also arise from mechanisms like asymmetric competition in the non‐kin groups. Here we outline the main challenges for studying kin selection in plants taking genetic variation for competitive ability into account. We emphasize the need to measure inclusive fitness in order to assess whether kin selection occurs, and show under which circumstances kin selected responses can be expected. We also illustrate why direct fitness estimates of a focal plant, and group fitness estimates are not suitable for documenting kin selection. Importantly, natural selection occurs at the individual level and it is the inclusive fitness of an individual plant – not the mean fitness of the group – that can capture if a differential response to neighbour relatedness is favoured by kin selection.     

Memorizing environmental signals through feedback and feedforward loops

Cells in diverse organisms can store the information of previous environmental conditions for long periods of time. This form of cellular memory adjusts the cell's responses to future challenges, providing fitness advantages in fluctuating environments. Many biological functions, including cellular memory, are mediated by specific recurring patterns of interactions among proteins and genes, known as ‘network motifs.’ In this review, we focus on three well-characterized network motifs — negative feedback loops, positive feedback loops, and feedforward loops, which underlie different types of cellular memories. We describe the latest studies identifying these motifs in various molecular processes and discuss how the topologies and dynamics of these motifs can enable memory encoding and storage.     

Genomic scale profiling of nutrient and trace elements in Arabidopsis thaliana

Understanding the functional connections between genes, proteins, metabolites and mineral ions is one of biology's greatest challenges in the postgenomic era. We describe here the use of mineral nutrient and trace element profiling as a tool to determine the biological significance of connections between a plant's genome and its elemental profile. Using inductively coupled plasma spectroscopy, we quantified 18 elements, including essential macro- and micronutrients and various nonessential elements, in shoots of 6,000 mutagenized M2 Arabidopsis thaliana plants. We isolated 51 mutants with altered elemental profiles. One mutant contains a deletion in FRD3, a gene known to control iron-deficiency responses in A. thaliana. Based on the frequency of elemental profile mutations, we estimate 2-4% of the A. thaliana genome is involved in regulating the plant's nutrient and trace element content. These results demonstrate the utility of elemental profiling as a useful functional genomics tool.     

?Np73 is capable of inducing apoptosis by co-ordinately activating several BH3-only proteins

Inactivation of p53 is one of the most relevant events in human cancer, since it allows transformed cells to escape their own proliferation control and leave them irresponsive to drugs that aim to damage their DNA. When p53 falls, other members of its family may become targets to attack tumoural cells. p73 has shown capacity to mediate these attacks. However, its N-terminal truncated isoforms have been associated with oncogenesis due to their capacity to act as dominant negatives of p53 and the transactivation (TA) isoforms of p73. We previously found a relationship between the overexpression of N-terminus-truncated p73 isoform (∆Np73) and that of the proapoptotic gene Bcl-2-interacting killer (BIK). In the present report we demonstrate that ∆Np73-α has the capacity to induce apoptosis through the co-ordinated activation of a group of genes harbouring GC-rich elements in their regulatory regions. ∆Np73-α synergizes with specificity protein (Sp1) on these elements but the overall response of these genes probably depends on the additional presence of consensus p53 elements. We explore the domains of ∆Np73-α involved in this transactivation capacity and found divergences with the previously described functions for them. Moreover, we found that the transforming mutation V12 of HRas impairs this transactivation capacity of ∆Np73-α, further supporting the anti-tumoural function of this later. Our data add complexity to the action of p73 on the induction of apoptosis and tumourogenesis, opening new interpretations to the expression profile of p73 isoforms in different human neoplasias.     

Interaction between ants, extrafloral nectaries and insect herbivores in Neotropical coastal sand dunes: herbivore deterrence by visiting ants increases fruit set in Opuntia stricta (Cactaceae)

1. This study examines the anti-herbivore effect of ants visiting the extrafloral nectaries (EFNs) of Opuntia stricta (Cactaceae) and its possible influence on the plant's reproductive output in Mexican coastal sand dunes. Opuntia 's EFNs are located in the areoles of the developing tissue of emerging cladodes and flower buds.
2. Ants visited the EFNs of O . stricta on a round-the-clock basis. The associated ant assemblage was formed by nine species distributed in four subfamilies, and the species composition of the principal ant visitors changed markedly from day to night period.
3. Cladodes of control (ants present) and treatment (ants excluded) plants of Opuntia were equally infested by sucking bugs and mining dipterans. Damage to buds by a pyralid moth, however, was significantly higher on treatment than on control plants. Ant visitation to Opuntia 's EFNs translated into a 50% increase in the plant's reproductive output, as expressed by the number of fruits produced by experimental control and treatment branches. Moreover, fruit production by ant-visited branches was positively and significantly associated with the mean monthly rate of ant visitation to EFNs.
4. This is the first demonstration of ant protection leading to increased fruit set in the Cactaceae under natural conditions. Although the consequences of damage by sucking and mining insects remain unclear for Opuntia , the results show how the association of EFNs with vulnerable reproductive plant organs can result in a direct ant-derived benefit to plant fitness.     

Chromosome Interactions in DROSOPHILA MELANOGASTER. II. Total Fitness

In a large experiment, using nearly 200 population cages, we have measured the fitness of Drosophila melanogaster homozygous (1) for the second chromosome, (2) for the third chromosome, and (3) for both chromosomes. Twentyfour second chromosomes and 24 third chromosomes sampled from a natural population were tested. The mean fitness of the homozygous flies is 0.081 ± 0.014 for the second chromosome, 0.080 ± 0.017 for the third chromosome, and 0.079 ± 0.024 for both chromosomes simultaneously. Assuming that fitnesses are multiplicative (the additive fitness model makes no sense in the present case because of the large selection coefficients involved), the expected mean fitness of the homozygotes for both chromosomes is 0.0066; their observed fitness is more than ten times greater. Thus, it appears that synergistic interactions between loci are considerable; and that, consequently, the fitness function substantially departs from linearity. Two models are tentatively suggested for the fitness function: a "threshold" model and a "synergistic" model.—The experiments reported here confirm previous results showing that the concealed genetic load present in natural populations of Drosophila is sufficient to account for the selective maintenance of numerous polymorphisms (of the order of 1000).     

Phospholipase-dependent signalling during the AvrRpm1- and AvrRpt2-induced disease resistance responses in Arabidopsis thaliana

Bacterial pathogens deliver type III effector proteins into plant cells during infection. On susceptible host plants, type III effectors contribute to virulence, but on resistant hosts they betray the pathogen to the plant's immune system and are functionally termed avirulence (Avr) proteins. Recognition induces a complex suite of cellular and molecular events comprising the plant's inducible defence response. As recognition of type III effector proteins occurs inside host cells, defence responses can be elicited by in planta expression of bacterial type III effectors. We demonstrate that recognition of either of two type III effectors, AvrRpm1 or AvrRpt2 from Pseudomonas syringae , induced biphasic accumulation of phosphatidic acid (PA). The first wave of PA accumulation correlated with disappearance of monophosphatidylinosotol (PIP) and is thus tentatively attributed to activation of a PIP specific phospholipase C (PLC) in concert with diacylglycerol kinase (DAGK) activity. Subsequent activation of phospholipase D (PLD) produced large amounts of PA from structural phospholipids. This later wave of PA accumulation was several orders of magnitude higher than the PLC-dependent first wave. Inhibition of phospholipases blocked the response, and feeding PA directly to leaf tissue caused cell death and defence-gene activation. Inhibitor studies ordered these events relative to other known signalling events during the plant defence response. Influx of extracellular Ca²⁺ occurred downstream of PIP-degradation, but upstream of PLD activation. Production of reactive oxygen species occurred downstream of the phospholipases. The data presented indicate that PA is a positive regulator of RPM1- or RPS2-mediated disease resistance signalling, and that the biphasic PA production may be a conserved feature of signalling induced by the coiled-coil nucleotide binding domain leucine-rich repeat class of resistance proteins.     

Evolution of haploid–diploid life cycles when haploid and diploid fitnesses are not equal

Many organisms spend a significant portion of their life cycle as haploids and as diploids (a haploid–diploid life cycle). However, the evolutionary processes that could maintain this sort of life cycle are unclear. Most previous models of ploidy evolution have assumed that the fitness effects of new mutations are equal in haploids and homozygous diploids, however, this equivalency is not supported by empirical data. With different mutational effects, the overall (intrinsic) fitness of a haploid would not be equal to that of a diploid after a series of substitution events. Intrinsic fitness differences between haploids and diploids can also arise directly, for example because diploids tend to have larger cell sizes than haploids. Here, we incorporate intrinsic fitness differences into genetic models for the evolution of time spent in the haploid versus diploid phases, in which ploidy affects whether new mutations are masked. Life‐cycle evolution can be affected by intrinsic fitness differences between phases, the masking of mutations, or a combination of both. We find parameter ranges where these two selective forces act and show that the balance between them can favor convergence on a haploid–diploid life cycle, which is not observed in the absence of intrinsic fitness differences.