Joris J. Glas

Plant Glandular Trichomes as Targets for Breeding or Engineering of Resistance to Herbivores

Glandular trichomes are specialized hairs found on the surface of about 30% of all vascular plants and are responsible for a significant portion of a plant’s secondary chemistry. Glandular trichomes are an important source of essential oils, i.e., natural fragrances or products that can be used by the pharmaceutical industry, although many of these substances have evolved to provide the plant with protection against herbivores and pathogens. The storage compartment of glandular trichomes usually is located on the tip of the hair and is part of the glandular cell, or cells, which are metabolically active. Trichomes and their exudates can be harvested relatively easily, and this has permitted a detailed study of their metabolites, as well as the genes and proteins responsible for them. This knowledge now assists classical breeding programs, as well as targeted genetic engineering, aimed to optimize trichome density and physiology to facilitate customization of essential oil production or to tune biocide activity to enhance crop protection. We will provide an overview of the metabolic diversity found within plant glandular trichomes, with the emphasis on those of the Solanaceae, and of the tools available to manipulate their activities for enhancing the plant’s resistance to pests.

Spider mites suppress tomato defenses downstream of jasmonate and salicylate independently of hormonal crosstalk.

Plants respond to herbivory by mounting a defense. Some plant-eating spider mites (Tetranychus spp.) have adapted to plant defenses to maintain a high reproductive performance. From natural populations we selected three spider mite strains from two species, Tetranychus urticae and Tetranychus evansi, that can suppress plant defenses, using a fourth defense-inducing strain as a benchmark, to assess to which extent these strains suppress defenses differently. We characterized timing and magnitude of phytohormone accumulation and defense-gene expression, and determined if mites that cannot suppress defenses benefit from sharing a leaf with suppressors. The nonsuppressor strain induced a mixture of jasmonate- (JA) and salicylate (SA)-dependent defenses. Induced defense genes separated into three groups: ‘early’ (expression peak at 1 d postinfestation (dpi)); ‘intermediate’ (4 dpi); and ‘late’, whose expression increased until the leaf died. The T. evansi strains suppressed genes from all three groups, but the T. urticae strain only suppressed the late ones. Suppression occurred downstream of JA and SA accumulation, independently of the JA–SA antagonism, and was powerful enough to boost the reproductive performance of nonsuppressors up to 45%. Our results show that suppressing defenses not only brings benefits but, within herbivore communities, can also generate a considerable ecological cost when promoting the population growth of a competitor.

Salivary proteins of spider mites suppress defenses in Nicotiana benthamiana and promote mite reproduction

Spider mites (Tetranychidae sp.) are widely occurring arthropod pests on cultivated plants. Feeding by the two-spotted spider mite T. urticae, a generalist herbivore, induces a defense response in plants that mainly depends on the phytohormones jasmonic acid and salicylic acid (SA). On tomato (Solanum lycopersicum), however, certain genotypes of T. urticae and the specialist species T. evansi were found to suppress these defenses. This phenomenon occurs downstream of phytohormone accumulation via an unknown mechanism. We investigated if spider mites possess effector-like proteins in their saliva that can account for this defense suppression. First we performed an in silico prediction of the T. urticae and the T. evansi secretomes, and subsequently generated a short list of candidate effectors based on additional selection criteria such as life stage-specific expression and salivary gland expression via whole mount in situ hybridization. We picked the top five most promising protein families and then expressed representatives in Nicotiana benthamiana using Agrobacterium tumefaciens transient expression assays to assess their effect on plant defenses. Four proteins from two families suppressed defenses downstream of the phytohormone SA. Furthermore, T. urticae performance on N. benthamiana improved in response to transient expression of three of these proteins and this improvement was similar to that of mites feeding on the tomato SA accumulation mutant nahG. Our results suggest that both generalist and specialist plant-eating mite species are sensitive to SA defenses but secrete proteins via their saliva to reduce the negative effects of these defenses.

Defense suppression benefits herbivores that have a monopoly on their feeding site but can backfire within natural communities

BackgroundPlants have inducible defenses to combat attacking organisms. Hence, some herbivores have adapted to suppress these defenses. Suppression of plant defenses has been shown to benefit herbivores by boosting their growth and reproductive performance.ResultsWe observed in field-grown tomatoes that spider mites (Tetranychus urticae) establish larger colonies on plants already infested with the tomato russet mite (Aculops lycopersici). Using laboratory assays, we observed that spider mites have a much higher reproductive performance on russet mite-infested plants, similar to their performance on the jasmonic acid (JA)-biosynthesis mutant def-1. Hence, we tested if russet mites suppress JA-responses thereby facilitating spider mites. We found that russet mites manipulate defenses: they induce those mediated by salicylic acid (SA) but suppress those mediated by JA which would otherwise hinder growth. This suppression of JA-defenses occurs downstream of JA-accumulation and is independent from its natural antagonist SA. In contrast, spider mites induced both JA- and SA-responses while plants infested with the two mite species together display strongly reduced JA-responses, yet a doubled SA-response. The spider mite-induced JA-response in the presence of russet mites was restored on transgenic tomatoes unable to accumulate SA (nahG), but russet mites alone still did not induce JA-responses on nahG plants. Thus, indirect facilitation of spider mites by russet mites depends on the antagonistic action of SA on JA while suppression of JA-defenses by russet mites does not. Furthermore, russet mite-induced SA-responses inhibited secondary infection by Pseudomonas syringae (Pst) while not affecting the mite itself. Finally, while facilitating spider mites, russet mites experience reduced population growth.ConclusionsOur results show that the benefits of suppressing plant defenses may diminish within communities with natural competitors. We show that suppression of defenses via the JA-SA antagonism can be a consequence, rather than the cause, of a primary suppression event and that its overall effect is determined by the presence of competing herbivores and the distinct palette of defenses these induce. Thus, whether or not host-defense manipulation improves an herbivore’s fitness depends on interactions with other herbivores via induced-host defenses, implicating bidirectional causation of community structure of herbivores sharing a plant.

Herbivory-associated degradation of tomato trichomes and its impact on biological control of Aculops lycopersici

Tomato plants have their leaves, petioles and stems covered with glandular trichomes that protect the plant against two-spotted spider mites and many other herbivorous arthropods, but also hinder searching by phytoseiid mites and other natural enemies of these herbivores. This trichome cover creates competitor-free and enemy-free space for the tomato russet mite (TRM) Aculops lycopersici (Acari: Eriophyidae), being so minute that it can seek refuge and feed inbetween the glandular trichomes on tomato cultivars currently used in practice. Indeed, several species of predatory mites tested for biological control of TRM have been reported to feed and reproduce when offered TRM as prey in laboratory experiments, yet in practice these predator species appeared to be unable to prevent TRM outbreaks. Using the phytoseiid mite, Amblydromalus limonicus, we found exactly the same, but also obtained evidence for successful establishment of a population of this predatory mite on whole plants that had been previously infested with TRM. This successful establishment may be explained by our observation that the defensive barrier of glandular plant trichomes is literally dropped some time after TRM infestation of the tomato plants: the glandular trichome heads first rapidly develop a brownish discoloration after which they dry out and fall over onto the plant surface. Wherever TRM triggered this response, predatory mites were able to successfully establish a population. Nevertheless, biological control was still unsuccessful because trichome deterioration in TRM-infested areas takes a couple of days to take effect and because it is not a systemic response in the plant, thereby enabling TRM to seek temporary refuge from predation in pest-free trichome-dense areas which continue to be formed while the plant grows. We formulate a hypothesis unifying these observations into one framework with an explicit set of assumptions and predictions to be tested in future experiments.

The Impact of Induced Plant Volatiles on Plant-Arthropod Interactions

Plants release volatile organic compounds from their vegetative tissues into their environment during most of their life cycle. The functions of these volatiles are diverse and not always known but some of these volatiles repel foraging herbivores while others, in turn, attract them and are feeding stimuli. Upon herbivory the amount of volatiles increases dramatically while, simultaneously, also the composition of the blend changes thereby enhancing the attractiveness of the plant to foraging natural enemies and in some cases increasing repellency to herbivores. Hence, herbivore-induced volatiles promote a natural form of biological pest control referred to as “indirect plant defense” and it has often been suggested that this phenomenon could be exploited to enhance crop protection. Here we will introduce the concept of indirect plant defense via volatiles and via other means and outline the current state of knowledge to the extent in which it contributes to protecting a plant to maximize its fitness under natural conditions in an evolutionary and ecological context. Moreover we will summarize the different approaches that have been undertaken to manipulate indirect defenses, either via application of synthetic volatiles or via transgenic manipulation of plant-volatile production, to control the movements of foraging arthropods to improve biological control. Finally, we will discuss to which extent IPM can be improved or even be disrupted via manipulation of plant volatiles.

Induction and Suppression of Herbivore-Induced Indirect Defenses

Plants release volatiles into the air. Upon herbivory, the amounts they release from the vegetative tissues increases dramatically. Although the physiological necessity for this increased emission is not fully understood, it has interesting consequences, the most important one being that foraging predators and host-searching parasitoids use these signals to track down plants with prey. This process is referred to as “indirect defense” since these responses can augment the plant’s own “direct” defenses, such as structural barriers and toxins, when they result in decreased herbivory via increased predation. Here we will describe how plants organize indirect defenses and how herbivores have adapted to interfere with these processes.

Distinct Signatures of Host Defense Suppression by Plant-Feeding Mites

Tomato plants are attacked by diverse herbivorous arthropods, including by cell-content-feeding mites, such as the extreme generalist Tetranychus urticae and specialists like Tetranychus evansi and Aculops lycopersici. Mite feeding induces plant defense responses that reduce mite performance. However, T. evansi and A. lycopersici suppress plant defenses via poorly understood mechanisms and, consequently, maintain a high performance on tomato. On a shared host, T. urticae can be facilitated by either of the specialist mites, likely due to the suppression of plant defenses. To better understand defense suppression and indirect plant-mediated interactions between herbivorous mites, we used gene-expression microarrays to analyze the transcriptomic changes in tomato after attack by either a single mite species (T. urticae, T. evansi, A. lycopersici) or two species simultaneously (T. urticae plus T. evansi or T. urticae plus A. lycopersici). Additionally, we assessed mite-induced changes in defense-associated phytohormones using LC-MS/MS. Compared to non-infested controls, jasmonates (JAs) and salicylate (SA) accumulated to higher amounts upon all mite-infestation treatments, but the response was attenuated after single infestations with defense-suppressors. Strikingly, whereas 8 to 10% of tomato genes were differentially expressed upon single infestations with T. urticae or A. lycopersici, respectively, only 0.1% was altered in T. evansi-infested plants. Transcriptome analysis of dual-infested leaves revealed that A. lycopersici primarily suppressed T. urticae-induced JA defenses, while T. evansi dampened T. urticae-triggered host responses on a transcriptome-wide scale. The latter suggests that T. evansi not solely down-regulates plant gene expression, but rather directs it back towards housekeeping levels. Our results provide valuable new insights into the mechanisms underlying host defense suppression and the plant-mediated facilitation of competing herbivores.