Primed plants do not forget

In their struggle for life, plants can employ sophisticated strategies to defend themselves against potentially harmful pathogens and insects. One mechanism by which plants can increase their level of resistance is by intensifying the responsiveness of their immune system upon recognition of selected signals from their environment. This so-called priming of defence can provide long-lasting resistance, which is based on a faster and/or stronger defence reaction upon pathogen or pest attack. Priming can target various layers of induced defence that are active during different stages of the plant–attacker interaction. Recent discoveries have extended our knowledge about the mechanistic basis of defence priming and suggest that a primed defence state can be inherited epi-genetically from defence-expressing plants. In this review, we provide an overview of the latest insights about defence priming, ranging from early responses controlled by adjustments in hormone-dependent signalling pathways and availability of signal transduction proteins, to longer lasting mechanisms that involve possible regulation chromatin modification or DNA methylation.     

Priming of antiherbivore defensive responses in plants

Defense priming is defined as increased readiness of defense induction. A growing body of literature indicates that plants (or intact parts of a plant) are primed in anticipation of impending environmental stresses, both biotic and abiotic, and upon the following stimulus, induce defenses more quickly and strongly. For instance, some plants previously exposed to herbivore‐inducible plant volatiles (HIPVs) from neighboring plants under herbivore attack show faster or stronger defense activation and enhanced insect resistance when challenged with secondary insect feeding. Research on priming of antiherbivore defense has been limited to the HIPV‐mediated mechanism until recently, but significant advances were made in the past three years, including non‐HIPV‐mediated defense priming, epigenetic modifications as the molecular mechanism of priming, and others. It is timely to consider the advances in research on defense priming in the plant–insect interactions.     

Dispensing synthetic green leaf volatiles in maize fields increases the release of sesquiterpenes by the plants, but has little effect on the attraction of pest and beneficial insects

Maize plants respond to feeding by arthropod herbivores by producing a number of secondary plant compounds, including volatile organic compounds (VOCs). These herbivore-induced VOCs are not only known to attract natural enemies of the herbivores, but they may also prime inducible defences in neighbouring plants, resulting in stronger and faster defence responses in these VOC-exposed plants. Among the compounds that cause this priming effect, green leaf volatiles (GLVs) have received particular attention, as they are ubiquitous and rapidly emitted upon damage. In this study, we investigated their effects under realistic conditions by applying specially devised dispensers to release four synthetic GLVs at physiologically relevant concentrations in a series of experiments in maize fields. We compared the VOC emission of GLV-exposed maize plants to non-exposed plants and monitored the attraction of herbivores and predators, as well as parasitism of the caterpillar Spodoptera frugiperda, the most common herbivore in the experimental maize fields. We found that maize plants that were exposed to GLVs emitted increased quantities of sesquiterpenes compared to non-exposed plants. In several replicates, herbivorous insects, such as adult Diabrotica beetles and S. frugiperda larvae, were observed more frequently in GLV-treated plots and caused more damage to GLV-exposed plants than to non-exposed plants. Parasitism of S. frugiperda was only weakly affected by GLVs and overall parasitism rates of S. frugiperda were similar in GLV-exposed and non-exposed plots. The effects on insect presence depended on the distance from the GLV-dispensers at which the plants were located. The results are discussed in the context of strategies to improve biological control by enhancing plant-mediated attraction of natural enemies.     

Mining the plant-herbivore interface with a leafmining Drosophila of Arabidopsis

Experimental infections of Arabidopsis thaliana (Arabidopsis) with genomically characterized plant pathogens such as Pseudomonas syringae have facilitated the dissection of canonical eukaryotic defence pathways and parasite virulence factors. Plants are also attacked by herbivorous insects, and the development of an ecologically relevant genetic model herbivore that feeds on Arabidopsis will enable the parallel dissection of host defence and reciprocal resistance pathways such as those involved in xenobiotic metabolism. An ideal candidate is Scaptomyza flava, a drosophilid fly whose leafmining larvae are true herbivores that can be found in nature feeding on Arabidopsis and other crucifers. Here, we describe the life cycle of S.?flava on Arabidopsis and use multiple approaches to characterize the response of Arabidopsis to S.?flava attack. Oviposition choice tests and growth performance assays on different Arabidopsis ecotypes, defence-related mutants, and hormone and chitin-treated plants revealed significant differences in host preference and variation in larval performance across Arabidopsis accessions. The jasmonate and glucosinolate pathways in Arabidopsis are important in mediating quantitative resistance against S.?flava, and priming with jasmonate or chitin resulted in increased resistance. Expression of xenobiotic detoxification genes was reduced in S.?flava larvae reared on Arabidopsis jasmonate signalling mutants and increased in plants pretreated with chitin. These results and future research directions are discussed in the context of developing a genetic model system to analyse insect-plant interactions.     

Heavy metal stress can prime for herbivore-induced plant volatile emission

Heavy metals are important pollutants that can severely impact ecological foodwebs. In addition to direct toxic effects, these contaminants have been suggested to disrupt chemical communication channels between plants and insects that rely on volatile organic compounds (VOCs). We investigated how different concentrations of copper (Cu) and cadmium (Cd) stress affect the capacity of Zea mays to synthesize VOCs in the presence and absence of herbivorous insects. Hydroponically grown maize exposed to a high and low concentration of either Cu or Cd showed stunted growth and lower photosynthetic capacities. Herbivores feeding on stressed plants also had attenuated growth rates. Heavy metal treatment alone did not induce VOC emission in maize plants; however, the higher Cu dose was found to prime for enhanced volatile production that can be triggered by caterpillar feeding. Cu stress correlated with increased levels of reactive oxygen species in roots and priming of herbivore-induced jasmonic acid in leaves. Plants challenged with Cd and herbivory did not differ in responses compared with herbivore-damaged controls with no heavy metals added to the substrate. For Cu stress, our results support the 'single biochemical mechanism for multiple stressors' model which predicts overlapping signalling and responses to abiotic and biotic stress factors.     

Treating seeds with activators of plant defence generates long-lasting priming of resistance to pests and pathogens

? Priming of defence is a strategy employed by plants exposed to stress to enhance resistance against future stress episodes with minimal associated costs on growth. Here, we test the hypothesis that application of priming agents to seeds can result in plants with primed defences. ? We measured resistance to arthropod herbivores and disease in tomato (Solanum lycopersicum) plants grown from seed treated with jasmonic acid (JA) and/or β-aminobutryric acid (BABA). ? Plants grown from JA-treated seed showed increased resistance against herbivory by spider mites, caterpillars and aphids, and against the necrotrophic fungal pathogen, Botrytis cinerea. BABA seed treatment provided primed defence against powdery mildew disease caused by the biotrophic fungal pathogen, Oidium neolycopersici. Priming responses were long-lasting, with significant increases in resistance sustained in plants grown from treated seed for at least 8 wk, and were associated with enhanced defence gene expression during pathogen attack. There was no significant antagonism between different forms of defence in plants grown from seeds treated with a combination of JA and BABA. ? Long-term defence priming by seed treatments was not accompanied by reductions in growth, and may therefore be suitable for commercial exploitation.     

Priming: getting ready for battle

Infection of plants by necrotizing pathogens or colonization of plant roots with certain beneficial microbes causes the induction of a unique physiological state called "priming." The primed state can also be induced by treatment of plants with various natural and synthetic compounds. Primed plants display either faster, stronger, or both activation of the various cellular defense responses that are induced following attack by either pathogens or insects or in response to abiotic stress. Although the phenomenon has been known for decades, most progress in our understanding of priming has been made over the past few years. Here, we summarize the current knowledge of priming in various induced-resistance phenomena in plants.     

Differential effectiveness of microbially induced resistance against herbivorous insects in Arabidopsis

Rhizobacteria-induced systemic resistance (ISR) and pathogen-induced systemic acquired resistance (SAR) have a broad, yet partly distinct, range of effectiveness against pathogenic microorganisms. Here, we investigated the effectiveness of ISR and SAR in Arabidopsis against the tissue-chewing insects Pieris rapae and Spodoptera exigua. Resistance against insects consists of direct defense, such as the production of toxins and feeding deterrents and indirect defense such as the production of plant volatiles that attract carnivorous enemies of the herbivores. Wind-tunnel experiments revealed that ISR and SAR did not affect herbivore-induced attraction of the parasitic wasp Cotesia rubecula (indirect defense). By contrast, ISR and SAR significantly reduced growth and development of the generalist herbivore S. exigua, although not that of the specialist P. rapae. This enhanced direct defense against S. exigua was associated with potentiated expression of the defense-related genes PDF1.2 and HEL. Expression profiling using a dedicated cDNA microarray revealed four additional, differentially primed genes in microbially induced S. exigua-challenged plants, three of which encode a lipid-transfer protein. Together, these results indicate that microbially induced plants are differentially primed for enhanced insect-responsive gene expression that is associated with increased direct defense against the generalist S. exigua but not against the specialist P. rapae.     

Multiple functions of inducible plant volatiles

A considerable amount of the carbon fixed by plants is emitted back into the atmosphere as volatile organic compounds (VOCs). Novel inducible VOCs released from plants after biotic or abiotic stresses temporarily increase total emissions of carbon substantially. As well as having a role in attracting the natural enemies of herbivores, inducible VOCs are also involved in plant-to-plant signalling, pathogen defence and ozone quenching, as well as tropospheric ozone and fine-particle aerosol formation. To relate these diverse observations to active plant defence, a conceptual framework of four functional levels (plant cellular interspace, leaf boundary layer, ecosystem and atmosphere) of inducible VOCs is proposed to aid understanding of the evolutionary role of inducible plant volatiles.     

Volatile-mediated signalling in barley induces metabolic reprogramming and resistance against the biotrophic fungus Blumeria hordei

• Plants have evolved diverse secondary metabolites to counteract biotic stress. Volatile organic compounds (VOCs) are released upon herbivore attack or pathogen infection. Recent studies suggest that VOCs can act as signalling molecules in plant defence and induce resistance in distant organs and neighbouring plants. However, knowledge is lacking on the function of VOCs in biotrophic fungal infection on cereal plants.
• We analysed VOCs emitted by 13 ± 1-day-old barley plants (Hordeum vulgare L.) after mechanical wounding using passive absorbers and TD-GC/MS. We investigated the effect of pure VOC and complex VOC mixtures released from wounded plants on the barley–powdery mildew interaction by pre-exposure in a dynamic headspace connected to a powdery mildew susceptibility assay. Untargeted metabolomics and lipidomics were applied to investigate metabolic changes in sender and receiver barley plants.
• Green leaf volatiles (GLVs) dominated the volatile profile of wounded barley plants, with (Z)-3-hexenyl acetate (Z3HAC) as the most abundant compound. Barley volatiles emitted after mechanical wounding enhanced resistance in receiver plants towards fungal infection. We found volatile-mediated modifications of the plant–pathogen interaction in a concentration-dependent manner. Pre-exposure with physiologically relevant concentrations of Z3HAC resulted in induced resistance, suggesting that this GLV is a key player in barley anti-pathogen defence.
• The complex VOC mixture released from wounded barley and Z3HAC induced e.g. accumulation of chlorophyll, linolenic acid and linolenate-conjugated lipids, as well as defence-related secondary metabolites, such as hordatines in receiving plants. Barley VOCs hence induce a complex physiological response and disease resistance in receiver plants.

     

Reduced levels of volatile emissions in jasmonate-deficient spr2 tomato mutants favour oviposition by insect herbivores

Tomato plants release volatile organic compounds (VOCs) following insect or mechanical damage. In this study, the constitutive and wound-induced emission levels of VOCs in suppressor of prosystemin-mediated responses2 (spr2) mutant plants, compromised in linolenic acid (LA) and jasmonic acid (JA) synthesis, and in 35S::prosystemin (35S::prosys) plants, having upregulated direct defence responses, were compared. The spr2 mutants produced constitutively lower levels of VOCs, which were nonetheless increased in response to (a)biotic damage, although at lower levels than wild-type (Wt) and 35S::prosys plants. No significant differences in VOC emissions were detected between the latter two genotypes, thereby suggesting that systemin does not regulate indirect defence responses, whereas differences in fatty acid composition in spr2 plants led to the predominant emission of saturated C6 volatiles in response to wounding. The expression of 1-deoxy-D-xylulose 5-phosphate synthase (DXS2), a key gene involved in VOC synthesis in the chloroplast, was only upregulated in Manduca sexta L.-damaged Wt and 35S::prosys plants. However, its expression was restored in spr2 plants by exogenous LA or JA, suggesting that abated VOC emissions in spr2 plants are correlated with lowered DXS2 expression. Bioassays with two different insects showed that adult females significantly preferred spr2 plants, indicating that lowered levels of VOCs in tomato influence plant selection by insects during oviposition.     

Effects of amount and recurrence of leaf herbivory on the induction of direct and indirect defences in wild cotton

• The induction of defences in response to herbivory is a key mechanism of plant resistance. While a number of studies have investigated the time course and magnitude of plant induction in response to a single event of herbivory, few have looked at the effects of recurrent herbivory. Furthermore, studies measuring the effects of the total amount and recurrence of herbivory on both direct and indirect plant defences are lacking. To address this gap, here we asked whether insect leaf herbivory induced changes in the amount and concentration of extrafloral nectar (an indirect defence) and concentration of leaf phenolic compounds (a direct defence) in wild cotton (Gossypium hirsutum).
• We conducted a greenhouse experiment where we tested single event or recurrent herbivory effects on defence induction by applying mechanical leaf damage and caterpillar (Spodoptera frugiperda) regurgitant.
• Single events of 25% and 50% leaf damage did not significantly influence extrafloral nectar production or concentration. Extrafloral nectar traits did, however, increase significantly relative to controls when plants were exposed to recurrent herbivory (two episodes of 25% damage). In contrast, phenolic compounds increased significantly in response to single events of  leaf damage but not to recurrent damage. In addition, we found. that local induction of extrafloral nectar production was stronger than systemic induction, whereas the reverse pattern was observed for phenolics.
• Together, these results reveal seemingly inverse patterns of induction of direct and indirect defences in response to herbivory in wild cotton.