Toby J. A. Bruce

Next-Generation Systemic Acquired Resistance

Systemic acquired resistance (SAR) is a plant immune response to pathogen attack. Recent evidence suggests that plant immunity involves regulation by chromatin remodeling and DNA methylation. We investigated whether SAR can be inherited epigenetically following disease pressure by Pseudomonas syringae pv tomato DC3000 (PstDC3000). Compared to progeny from control-treated Arabidopsis (Arabidopsis thaliana; C1), progeny from PstDC3000-inoculated Arabidopsis (P1) were primed to activate salicylic acid (SA)-inducible defense genes and were more resistant to the (hemi)biotrophic pathogens Hyaloperonospora arabidopsidis and PstDC3000. This transgenerational SAR was sustained over one stress-free generation, indicating an epigenetic basis of the phenomenon. Furthermore, P1 progeny displayed reduced responsiveness of jasmonic acid (JA)-inducible genes and enhanced susceptibility to the necrotrophic fungus Alternaria brassicicola. This shift in SA- and JA-dependent gene responsiveness was not associated with changes in corresponding hormone levels. Instead, chromatin immunoprecipitation analyses revealed that SA-inducible promoters of PATHOGENESIS-RELATED GENE1, WRKY6, and WRKY53 in P1 plants are enriched with acetylated histone H3 at lysine 9, a chromatin mark associated with a permissive state of transcription. Conversely, the JA-inducible promoter of PLANT DEFENSIN1.2 showed increased H3 triple methylation at lysine 27, a mark related to repressed gene transcription. P1 progeny from the defense regulatory mutant non expressor of PR1 (npr1)-1 failed to develop transgenerational defense phenotypes, demonstrating a critical role for NPR1 in expression of transgenerational SAR. Furthermore, the drm1drm2cmt3 mutant that is affected in non-CpG DNA methylation mimicked the transgenerational SAR phenotype. Since PstDC3000 induces DNA hypomethylation in Arabidopsis, our results suggest that transgenerational SAR is transmitted by hypomethylated genes that direct priming of SA-dependent defenses in the following generations.

Perception of plant volatile blends by herbivorous insects – Finding the right mix

Volatile plant secondary metabolites are detected by the highly sensitive olfactory system employed by insects to locate suitable plants as hosts and to avoid unsuitable hosts. Perception of these compounds depends on olfactory receptor neurones (ORNs) in sensillae, mostly on the insect antennae, which can recognise individual molecular structures. Perception of blends of plant volatiles plays a pivotal role in host recognition, non-host avoidance and ensuing behavioural responses as different responses can occur to a whole blend compared to individual components. There are emergent properties of blend perception because components of the host blend may not be recognised as host when perceived outside the context of that blend. Often there is redundancy in the composition of blends recognised as host because certain compounds can be substituted by others. Fine spatio-temporal resolution of the synchronous firing of ORNs tuned to specific compounds enables insects to pick out relevant host odour cues against high background noise and with ephemeral exposure to the volatiles at varying concentrations. This task is challenging as they usually rely on ubiquitous plant volatiles and not those taxonomically characteristic of host plants. However, such an odour coding system has the advantage of providing flexibility; it allows for adaptation to changing environments by alterations in signal processing while maintaining the same peripheral olfactory receptors.

Aphid alarm pheromone produced by transgenic plants affects aphid and parasitoid behavior

The alarm pheromone for many species of aphids, which causes dispersion in response to attack by predators or parasitoids, consists of the sesquiterpene (E)-β-farnesene (Eβf). We used high levels of expression in Arabidopsis thaliana plants of an Eβf synthase gene cloned from Mentha × piperita to cause emission of pure Eβf. These plants elicited potent effects on behavior of the aphid Myzus persicae (alarm and repellent responses) and its parasitoid Diaeretiella rapae (an arrestant response). Here, we report the transformation of a plant to produce an insect pheromone and demonstrate that the resulting emission affects behavioral responses at two trophic levels.

Phytochemicals of Brassicaceae in plant protection and human health – Influences of climate, environment and agronomic practice

In this review, we provide an overview of the role of glucosinolates and other phytochemical compounds present in the Brassicaceae in relation to plant protection and human health. Current knowledge of the factors that influence phytochemical content and profile in the Brassicaceae is also summarized and multi-factorial approaches are briefly discussed. Variation in agronomic conditions (plant species, cultivar, developmental stage, plant organ, plant competition, fertilization, pH), season, climatic factors, water availability, light (intensity, quality, duration) and CO(2) are known to significantly affect content and profile of phytochemicals. Phytochemicals such as the glucosinolates and leaf surface waxes play an important role in interactions with pests and pathogens. Factors that affect production of phytochemicals are important when designing plant protection strategies that exploit these compounds to minimize crop damage caused by plant pests and pathogens. Brassicaceous plants are consumed increasingly for possible health benefits, for example, glucosinolate-derived effects on degenerative diseases such as cancer, cardiovascular and neurodegenerative diseases. Thus, factors influencing phytochemical content and profile in the production of brassicaceous plants are worth considering both for plant and human health. Even though it is known that factors that influence phytochemical content and profile may interact, studies of plant compounds were, until recently, restricted by methods allowing only a reductionistic approach. It is now possible to design multi-factorial experiments that simulate their combined effects. This will provide important information to ecologists, plant breeders and agronomists.

Underground signals carried through common mycelial networks warn neighbouring plants of aphid attack.

The roots of most land plants are colonised by mycorrhizal fungi that provide mineral nutrients in exchange for carbon. Here, we show that mycorrhizal mycelia can also act as a conduit for signalling between plants, acting as an early warning system for herbivore attack. Insect herbivory causes systemic changes in the production of plant volatiles, particularly methyl salicylate, making bean plants, Vicia faba, repellent to aphids but attractive to aphid enemies such as parasitoids. We demonstrate that these effects can also occur in aphid-free plants but only when they are connected to aphid-infested plants via a common mycorrhizal mycelial network. This underground messaging system allows neighbouring plants to invoke herbivore defences before attack. Our findings demonstrate that common mycorrhizal mycelial networks can determine the outcome of multitrophic interactions by communicating information on herbivore attack between plants, thereby influencing the behaviour of both herbivores and their natural enemies.

Volatiles functioning as host cues in a blend become nonhost cues when presented alone to the black bean aphid.

Herbivorous insects recognize and locate their hosts by detecting characteristic blends of volatile compounds these plants emit. The possibility that insects may use the same compounds in a different context as nonhost cues has received relatively little attention. Volatiles normally emitted by the host but encountered without other host volatiles could theoretically function as nonhost cues. We hypothesized that insects might show a positive response to host volatile compounds when encountered together in a blend but avoid the same volatiles when encountered individually. To test this we examined the behavioural responses of the black bean aphid, Aphis fabae, to physiologically relevant doses of volatile compounds emitted by its host, Vicia faba, which had been previously implicated in host recognition. Of 15 volatiles tested for behavioural activity, 10 caused aphids to respond negatively, suggesting they were repellent. We then made a blend comprising each of these compounds at the concentration at which they elicited the most negative behavioural response. The resultant blend elicited a positive response, suggesting it was attractive/arrestant. This demonstrated that the same volatile compounds can function as both host and nonhost cues, depending upon the context in which they are perceived. Thus, background odour context needs to be considered for successful use of behaviourally active volatile compounds in integrated pest management strategies. Furthermore, the finding that odorants are perceived differently when combined suggests that there is an emergent property of odour perception whereby discrimination of odour quality can occur according to blend properties.

cis-Jasmone induces Arabidopsis genes that affect the chemical ecology of multitrophic interactions with aphids and their parasitoids.

It is of adaptive value for a plant to prepare its defenses when a threat is detected, and certain plant volatiles associated with insect damage, such as cis-jasmone (CJ), are known to switch-on defense metabolism. We used aphid and aphid parasitoid responses to Arabidopsis thaliana as a model system for studying gene expression and defense chemistry and its impact at different trophic levels. Differential responses to volatiles of induced Arabidopsis occurred for specialist and generalist insects: the generalist aphid, Myzus persicae, was repelled, whereas the specialist, Lipaphis erysimi, was attracted; the generalist aphid parasitoid Aphidius ervi was attracted, but the specialist parasitoid Diaeretiella rapae was not affected. A. ervi also spent longer foraging on induced plants than on untreated ones. Transcriptomic analyses of CJ-induced Arabidopsis plants revealed that a limited number of genes, including a gene for a cytochrome P450, CYP81D11, were strongly up-regulated in the treated plants. We examined transgenic Arabidopsis lines constitutively overexpressing this gene in bioassays and found insect responses similar to those obtained for wild-type plants induced with CJ, indicating the importance of this gene in the CJ-activated defense response. Genes involved in glucosinolate biosynthesis and catabolism are unaffected by CJ and, because these genes relate to interactions with herbivores and parasitoids specific to this family of plants (Brassicaceae), this finding may explain the differences in behavioral response of specialist and generalist insects.

Identification of Volatile Compounds Used in Host Location by the Black Bean Aphid, Aphis fabae

Behavioral and electrophysiological responses of winged Aphis fabae to volatiles of faba bean, Vicia faba (var. Sutton dwarf), plants were studied and semiochemicals used in host location were identified. In olfactometer bioassays, aphids spent significantly more time in the region of the olfactometer where V. faba volatiles from an intact plant were present than in control regions with clean air. This response also occurred when an air entrainment sample of a V. faba plant was used as the odor source. Coupled gas chromatography–electroantennography revealed the presence of 16 electrophysiologically active compounds in the air entrainment sample. Fifteen of these were identified as (Z)-3-hexen-1-ol, 1-hexanol, (E)-2-hexenal, benzaldehyde, 6-methyl-5-hepten-2-one, octanal, (Z)-3-hexen-1-yl acetate, (R)-(−)-linalool, methyl salicylate, decanal, undecanal, (E)-caryophyllene, (E)-β-farnesene, (S)-(−)-germacrene D, and (E,E,)-4,8,12-trimethyl-1,3,7,11-tridecatetraene. An olfactometer response was observed to a 15-component synthetic blend that comprised all identified compounds at the same concentration and ratio as in the natural sample, with the aphids spending significantly more time in the treated regions of the olfactometer where volatiles were present than in the control regions. These data are discussed in the context of insect host location and crop protection.

Conical Epidermal Cells Allow Bees to Grip Flowers and Increase Foraging Efficiency

The plant surface is by default flat, and development away from this default is thought to have some function of evolutionary advantage. Although the functions of many plant epidermal cells have been described, the function of conical epidermal cells, a defining feature of petals in the majority of insect-pollinated flowers, has not. The location and frequency of conical cells have led to speculation that they play a role in attracting animal pollinators. Snapdragon (Antirrhinum) mutants lacking conical cells have been shown to be discriminated against by foraging bumblebees. Here we investigated the extent to which a difference in petal surface structure influences pollinator behavior through touch-based discrimination. To isolate touch-based responses, we used both biomimetic replicas of petal surfaces and isogenic Antirrhinum lines differing only in petal epidermal cell shape. We show that foraging bumblebees are able to discriminate between different surfaces via tactile cues alone. We find that bumblebees use color cues to discriminate against flowers that lack conical cells--but only when flower surfaces are presented at steep angles, making them difficult to manipulate. This facilitation of physical handling is a likely explanation for the prevalence of conical epidermal petal cells in most flowering plants.

Responses of female orange wheat Blossom midge, Sitodiplosis mosellana, to wheat panicle volatiles.

Air entrainment samples of volatiles from panicles of intact wheat, Triticum aestivum, cultivar ‘Lynx’ were collected at the ear emergence/early anthesis growth stage. In an olfactometer bioassay, both freshly cut panicles and an air entrainment sample were found to attract female orange wheat blossom midge adults, Sitodiplosis mosellana. Coupled gas chromatography-electroantennography (GC-EAG) analyses of panicle volatiles located six electrophysiologically active components. These were identified by coupled gas chromatography-mass spectrometry and coinjection with authentic standards, on polar and nonpolar GC columns, as acetophenone, (Z)-3-hexenyl acetate, 3-carene, 2-tridecanone, 2-ethyl-1-hexanol, and 1-octen-3-ol. Although none of these was active when presented individually at the levels present in the entrainment sample, acetophenone, (Z)-3-hexenyl acetate, and 3-carene were active in the olfactometer when presented at a higher dose of 100 ng on filter paper. However, the six-component blend and a blend of acetophenone, (Z)-3-hexenyl acetate, and 3-carene, in the same ratio and concentration as in a natural sample, was as attractive to female S. mosellana as the whole air entrainment sample.