Plant immunity to insect herbivores

Role of gut symbionts of insect pests: A novel target for insect-pest control

Insects possess beneficial and nuisance values in the context of the agricultural sector and human life around them. An ensemble of gut symbionts assists insects to adapt to diverse and extreme environments and to occupy every available niche on earth. Microbial symbiosis helps host insects by supplementing necessary diet elements, providing protection from predators and parasitoids through camouflage, modulation of signaling pathway to attain homeostasis and to trigger immunity against pathogens, hijacking plant pathways to circumvent plant defence, acquiring the capability to degrade chemical pesticides, and degradation of harmful pesticides. Therefore, a microbial protection strategy can lead to overpopulation of insect pests, which can drastically reduce crop yield. Some studies have demonstrated increased insect mortality via the destruction of insect gut symbionts; through the use of antibiotics. The review summarizes various roles played by the gut microbiota of insect pests and some studies that have been conducted on pest control by targeting the symbionts. Manipulation or exploitation of the gut symbionts alters the growth and population of the host insects and is consequently a potential target for the development of better pest control strategies. Methods such as modulation of gut symbionts via CRISPR/Cas9, RNAi and the combining of IIT and SIT to increase the insect mortality are further discussed. In the ongoing insect pest management scenario, gut symbionts are proving to be the reliable, eco-friendly and novel approach in the integrated pest management.

Exchange of Microbiomes in Plant-Insect Herbivore Interactions

Prokaryotic and eukaryotic microbial symbiotic communities span through kingdoms. The vast microbial gene pool extends the host genome and supports adaptations to changing environmental conditions. Plants are versatile hosts for the symbionts, carrying microbes on the surface, inside tissues, and even within the cells. Insects are equally abundantly colonized by microbial symbionts on the exoskeleton, in the gut, in the hemocoel, and inside the cells. The insect gut is a prolific environment, but it is selective on the microbial species that enter with food. Plants and insects are often highly dependent on each other and frequently interact. Regardless of the accumulating evidence on the microbiomes of both organisms, it remains unclear how much they exchange and modify each other's microbiomes. In this review, we approach this question from the point of view of herbivores that feed on plants, with a special focus on the forest ecosystems. After a brief introduction to the subject, we concentrate on the plant microbiome, the overlap between plant and insect microbial communities, and how the exchange and modification of microbiomes affects the fitness of each host.

Transcriptional profiling reveals a critical role of GmFT2a in soybean staygreen syndrome caused by the pest Riptortus pedestris

Soybean staygreen syndrome, characterized by delayed leaf and stem senescence, abnormal pods, and aborted seeds, has recently become a serious and prominent problem in soybean production. Although the pest Riptortus pedestris has received increasing attention as the possible cause of staygreen syndrome, the mechanism remains unknown. Here, we clarify that direct feeding by R. pedestris, not transmission of a pathogen by this pest, is the primary cause of typical soybean staygreen syndrome and that critical feeding damage occurs at the early pod stage. Transcriptome profiling of soybean indicated that many signal transduction pathways, including photoperiod, hormone, defense response, and photosynthesis, respond to R. pedestris infestation. Importantly, we discovered that members of the FLOWERING LOCUS T (FT) gene family were suppressed by R. pedestris infestation, and overexpression of floral inducer GmFT2a attenuates staygreen symptoms by mediating soybean defense response and photosynthesis. Together, our findings systematically illustrate the association between pest infestation and soybean staygreen syndrome and provide the basis for establishing a targeted soybean pest prevention and control system.

Plant physiology: Untangling the roles of glutamate and turgor pressure

Long-distance wound signalling in plants involves systemic propagation of calcium waves, but the exact process of initiation and transmission of these waves remains elusive. A new study proposes a mechanism whereby pressure changes are the trigger for this response.

Do genotypes ameliorate herbivory stress through silicon amendments differently? A case study of wheat

Agricultural productivity relies on plant resistance to insect pests, with silicon (Si) being increasingly recognized as an important anti-herbivore defense. However, the processes by which Si works to counteract the effects of insect injury are not completely understood. The role of Si in mitigating the adverse effects of herbivory has been mostly studied at the species level in various crops, ignoring the sensitivity and variability at the genotypic level. Understanding such variation across genotypes is important because Si-derived benefits are associated with the amount of Si accumulated in the plant. Therefore, the present investigation was pursued to study the effect of different Si concentrations (0, 125, and 250 mg L⁻¹) on Si accumulation and plant growth using two wheat genotypes (WW-101 and SW-2) under grasshopper herbivory for 48 h. The higher Si absorption increased the concentration of leaf chlorophyll, carotenoids, soluble sugars, and proteins. Silicon application at higher concentrations increased the dry weight, antioxidant enzyme activity, total phenolics, flavonoids and shoot Si concentration, whereas it decreased the electrolyte leakage, hydrogen peroxide (H₂O₂) and malonaldehyde (MDA) levels, thereby preventing leaf damage. We infer that the higher Si concentration alleviates the adverse effects of herbivory in wheat by improving the accumulation of secondary metabolites and enhancing the antioxidant defense system. The effects were pronounced in the genotype 'WW-101' compared to 'SW-2' for most of the studied traits, indicating overall stress response to be genotype-dependent. Thus, Si acquisition efficiency of genotypes should be considered while developing efficient crop management strategies.

Non-classical digestive lipase BmTGL selected by gene amplification reduces the effects of mulberry inhibitor during silkworm domestication

Efficient utilization of dietary lipids is crucial for Bombyx mori, also known as domesticated silkworms. However, the effects of domestication on the genes encoding lipases remain unknown. In this study, we investigated the expression difference of one triacylglycerol lipase (BmTGL) between B.mori and wild (ancestor) silkworm strains (Bombyx mandarina). An immunofluorescence localization analysis showed that BmTGL was present in all parts of the gut and was released into the intestinal lumen. BmTGL expression was significantly enhanced in different domesticated silkworm strains compared to that in the B. mandarina strains. The BmTGL copy numbers in the genomes of the domesticated silkworm strains were 2-to-3 folds that of the B. mandarina strains and accounted for the enhanced expression of BmTGL in the domesticated silkworm strains. The Ser144Asn substitution in the Ser-Asp-His catalytic triads of BmTGL resulted in relatively lower lipase activity and reduced sensitivity to the lipase inhibitor morachalcone A. Moreover, BmTGL overexpression significantly increased the weights of the B. mori silkworms compared to those of the non-transgenic controls. Thus, the selection of BmTGL by gene amplification may be a trade-off between maintaining high enzymatic activity and reducing the effects of mulberry inhibitors during silkworm domestication.

The impact of spring wheat species and sowing density on soil biochemical properties, content of secondary plant metabolites and the presence of Oulema ssp

The physical and chemical properties of the soil are important factors influencing the yield of crops. One of the agrotechnical factors influencing the biochemical properties of soil is sowing density. It affects the yield components, light, moisture and thermal conditions in the canopy and the pressure of pests. Secondary metabolites, many of which are known to act as a defense mechanism against insects, are of importance in the interaction between the crop and abiotic and biotic factors of the habitat. To the best of our knowledge, the studies conducted so far do not sufficiently reveal the impacts of the wheat species and the sowing density, together with the biochemical properties of the soil, on the accumulation of bioactive ingredients in the crop plants, and the subsequent impacts on the occurrence of phytophagic entomofauna in various management systems. Explaining these processes creates an opportunity for more sustainable development of agriculture. The study aimed to determine the effect of wheat species and sowing density on the biochemical properties of the soil, concentrations of biologically active compounds in the plant and the occurrence of insect pests in organic (OPS) and conventional (CPS) production systems. The research was conducted on spring wheat species (Indian dwarf wheat-Triticum sphaerococcum Percival and Persian wheat-Triticum persicum Vavilov) grown in OPS and CPS at sowing densities 400, 500, 600 (seeds m^-2). The following analyzes were performed: (i) soil analysis: the activity of catalases (CAT), dehydrogenases (DEH), peroxidases (PER); (ii) plant analysis: total phenolic compounds (TP), chlorogenic acid (CA), antioxidant capacity (FRAP); (iii) entomological analysis of the number of insects-Oulema spp. adults and larvae. Performing analyzes in such a wide (interdisciplinary) scope will allow for a comprehensive understanding of the soil-plant-insect biological transformation evaluation. Our results showed that an increase in soil enzyme activity caused a decrease in TP contents in the wheat grown the OPS. Despite this, both the content of TP and the anti-oxidative activity of the ferric reducing ability of plasma (FRAP) were higher in these wheats. Bioactive compound contents and FRAP were most favoured by the lowest sowing density. Regardless of the production system, the occurrence of the Oulema spp. adults on T. sphaerococcum was the lowest at a sowing density of 500 seeds m^-2. The occurrence of this pest's larvae was lowest at a sowing density of 400 seeds m^-2. Research on bioactive compounds in plants, biochemical properties of soil and the occurrence of pests make it possible to comprehensively assess the impact of the sowing density of ancient wheat in the ecological and conventional production system, which is necessary for the development of environmentally sustainable agriculture.

Genome-wide analysis of MYB family in Nicotiana benthamiana and the functional role of the key members in resistance to Bemisia tabaci

MYB transcription factors (TFs) play a key role in plant resistance to abiotic and biotical stresses. However, little is currently known about their involvement in the plant defense to piercing-sucking insects. Here, we studied the MYB TFs that responded to and resisted Bemisia tabaci whitefly in the model plant Nicotiana benthamiana. Firstly, a total of 453 NbMYB TFs in N. benthamiana genome were identified and 182 R2R3-MYB TFs were analyzed for molecular characteristics, phylogenetic analysis, genetic structure, motif composition, and cis-elements. Then, six stress-related NbMYB genes were selected for further study. The expression pattern shows they were highly expressed in mature leaves and intensively induced upon whitefly attack. Combined with bioinformatic analysis, overexpression, β-Glucuronidase (GUS) assay, and virus-induced silencing tests, we determined the transcriptional regulation of these NbMYBs on the genes in lignin biosynthesis and SA-signaling pathways. Meanwhile, we tested the performance of whitefly on plants with increased or silenced NbMYB genes expression and found that NbMYB42, NbMYB107, NbMYB163, and NbMYB423 were resistant to whitefly. Our results contribute to a comprehensive understanding of the MYB TFs in N. benthamiana. Furthermore, our findings will facilitate further studies on the role of MYB TFs in the interaction between plants and piercing-sucking insects.

Cut to Disarm Plant Defence: A Unique Oviposition Behaviour in Rhynchites foveipennis (Coleoptera: Attelabidae)

Female weevils of the family Attelabidae (Coleoptera: Curculionoidea) possess a unique behaviour of partially cutting the branches connecting egg-bearing organs of their host plants during oviposition. However, the consequence of such behaviour remains unclear. Using Rhynchites foveipennis and its host pear (Pyrus pyrifolia), the present study tested the hypothesis that the oviposition behaviour could disarm the host plants' defence. We compared the survival rates, growth rates, and performance of eggs and larvae under two conditions: (1) the fruit stems were naturally damaged by the females before and after oviposition, and (2) the fruit stems were artificially protected from the females. When fruit stems were protected from female damage, the survival rates of eggs and larvae were only 21.3-32.6%, respectively; and the larval weight was 3.2-4.1 mg 30 days after laying eggs. When the fruit stems were damaged, the survival rates of eggs and larvae reached 86.1-94.0%, respectively; and the larval weight reached 73.0-74.9 mg 30 days after laying eggs. The contents of tannin and flavonoids in the pears did not change significantly along with the oviposition and larval feeding, but weevil eggs were crushed and killed by the callus in the pears. Once the stunted larvae in branch-growing pears were moved into the picked-off ones, the growth and development recovered. The findings indicate that the oviposition behaviour can significantly increase the survival of the offspring. Our study suggested that the oviposition behaviour of attelabid weevils is a strategy to overcome plant defence.

Overexpression of soybean trypsin inhibitor genes decreases defoliation by corn earworm (Helicoverpa zea) in soybean (Glycine max) and Arabidopsis thaliana

Trypsin inhibitors (TIs) are widely distributed in plants and are known to play a protective role against herbivores. TIs reduce the biological activity of trypsin, an enzyme involved in the breakdown of many different proteins, by inhibiting the activation and catalytic reactions of proteins. Soybean (Glycine max) contains two major TI classes: Kunitz trypsin inhibitor (KTI) and Bowman-Birk inhibitor (BBI). Both genes encoding TI inactivate trypsin and chymotrypsin enzymes, which are the main digestive enzymes in the gut fluids of Lepidopteran larvae feeding on soybean. In this study, the possible role of soybean TIs in plant defense against insects and nematodes was investigated. A total of six TIs were tested, including three known soybean trypsin inhibitors (KTI1, KTI2 and KTI3) and three genes encoding novel inhibitors identified in soybean (KTI5, KTI7, and BBI5). Their functional role was further examined by overexpression of the individual TI genes in soybean and Arabidopsis. The endogenous expression patterns of these TI genes varied among soybean tissues, including leaf, stem, seed, and root. In vitro enzyme inhibitory assays showed significant increase in trypsin and chymotrypsin inhibitory activities in both transgenic soybean and Arabidopsis. Detached leaf-punch feeding bioassays detected significant reduction in corn earworm (Helicoverpa zea) larval weight when larvae fed on transgenic soybean and Arabidopsis lines, with the greatest reduction observed in KTI7 and BBI5 overexpressing lines. Whole soybean plant greenhouse feeding bioassays with H. zea on KTI7 and BBI5 overexpressing lines resulted in significantly reduced leaf defoliation compared to non-transgenic plants. However, bioassays of KTI7 and BBI5 overexpressing lines with soybean cyst nematode (SCN, Heterodera glycines) showed no differences in SCN female index between transgenic and non-transgenic control plants. There were no significant differences in growth and productivity between transgenic and non-transgenic plants grown in the absence of herbivores to full maturity under greenhouse conditions. The present study provides further insight into the potential applications of TI genes for insect resistance improvement in plants.

Silicon Nanodots Increase Plant Resistance against Herbivores by Simultaneously Activating Physical and Chemical Defenses

Nanosilicon applications have been shown to increase plant defenses against both abiotic and biotic stresses. Silicon quantum nanodots (Si NDs), a form of nanosilicon, possess excellent biological and physiochemical properties (e.g., minimal size, high water solubility, stability, and biocompatibility), potentially making them more efficient in regulating plant responses to stress than other forms of silicon. However, to date, we still lack mechanistic evidence for how soil-applied Si NDs alter the regulation of plant physical and chemical defenses against insect herbivores. To address this gap, we compared the effect of fluorescent amine-functionalized Si NDs (5 nm) and the conventional fertilizer sodium silicate on maize (Zea mays L.) physical and chemical defenses against the oriental armyworm (Mythimna separata, Walker) caterpillars. We found that 50 mg/kg Si NDs and sodium silicate additions inhibited the growth of caterpillars the most (35.7% and 22.8%, respectively) as compared to other application doses (0, 10, and 150 mg/kg). Both Si NDs and silicate addition activated biosynthesis genes responsible for chemical (benzoxazinoids) and physical (lignin) defense production. Moreover, Si NDs upregulated the gene expression of antioxidant enzymes (SOD, CAT, and POD) and promoted the antioxidant metabolism (flavonoids) in maize leaves under M. separata attack. Finally, we show that, under field conditions, Si ND addition increased maize cob weight (28.7%), cob grain weight (40.8%), and 100-grain weight (26.5%) as compared to the control, and more so than the conventional silicon fertilizer. Altogether, our findings highlight the potential for Si NDs to be used as an effective and ecofriendly crop protection strategy in agroecosystems.

Bamboo charcoal mediated plant secondary metabolites biosynthesis in tomato against South American tomato pinworm (Tuta absoluta)

Introduction

The south American tomato pinworm (Tuta absoluta), an invasive insect pest species, has established itself in more than 33 Chinese prefectures, where it is mainly damaging tomato crops. Immediate efforts have been initiated to find strategies to control this pest. Studies have shown that biochar (BC) amendment to soil can enhance plant growth and resistance to herbivory.

Methods

First, we quantified the morphological performance of tomato plants grown in different coconut bran and bamboo charcoal (v/v) combinations and selected the most beneficial one. Then we checked the T. absoluta survival on the tomato plants grown in the selected bamboo charcoal combination. Finally, we studied the leaf metabolite accumulation and gene expression changes in tomato plants after growing in the selected bamboo charcoal combination.

Results

We found that the 30:1 ratio of bamboo charcoal and coconut bran is the most beneficial to tomato growth as its amendment to soil increased tomato plant height, stem thickness, and chlorophyll content, whereas, the T. absoluta survival decreased. The metabolome profiles of BC tomato leaves showed an increased accumulation of flavonoids, terpenoids, and phenolic acids compared to CK. Transcriptome sequencing resulted in the identification of 244 differentially expressed genes. Most of the upregulated genes were associated with stress-related hub proteins, flavonoid biosynthesis, MAPK and phytohormone signaling, and terpenoid biosynthesis. Additionally, the expression of many genes related to signaling and defense was changed in response to the bamboo charcoal amendment.

Discussion

We conclude that bamboo charcoal induces biosynthesis of flavonoids, terpenoids, and phenolic acids, which improve plant growth and tolerance against T. absoluta, thus reducing the survival of destructive pests.

Plant Microbiome: An Ocean of Possibilities for Improving Disease Resistance in Plants

Plant diseases pose a serious threat to crop production and the agricultural economy across the globe. Currently, chemical pesticides are frequently employed to combat these infections, which cause environmental toxicity and the emergence of resistant pathogens. Moreover, the genetic manipulation of plant defense pathways and the breeding of resistant genes has attained limited success due to the rapid evolution of pathogen virulence and resistance, together with host range expansion. Additionally, due to climate change and global warming, the occurrence of multiple stresses during disease outbreak has further impacted overall crop growth and productivity, posing a serious threat to food security. In this regard, harnessing the plant beneficial microbiome and its products can provide novel avenues for disease resistance in addition to boosting agricultural output, soil fertility and environmental sustainability. In plant-beneficial microbiome interactions, induced systemic resistance (ISR) has emerged as a key mechanism by which a beneficial microbiome primes the entire plant system for better defense against a wide range of phytopathogens and pests. In this review, we provide the recent developments on the role of plant beneficial microbiomes in disease resistance. We also highlight knowledge gaps and discuss how the plant immune system distinguishes pathogens and beneficial microbiota. Furthermore, we provide an overview on how immune signature hormones, such as salicylic acid (SA), jasmonic acid (JA) and ethylene (ET), shape plant beneficial microbiome. We also discuss the importance of various high-throughput tools and their integration with synthetic biology to design tailored microbial communities for disease resistance. Finally, we conclude by highlighting important themes that need future attention in order to fill the knowledge gaps regarding the plant immune system and plant-beneficial-microbiome-mediated disease resistance.

Effects of Methyl Jasmonate Fumigation on the Growth and Detoxification Ability of Spodoptera litura to Xanthotoxin

Methyl jasmonate (MeJA) is a volatile substance derived from jasmonic acid (JA), and it responds to interbiotic and abiotic stresses by participating in interplant communication. Despite its function in interplant communication, the specific role of MeJA in insect defense responses is poorly understood. In this study, we found that carboxylesterase (CarE) activities, glutathione-S-transferase (GSTs) activities, and cytochrome mono-oxygenases (P450s) content increased more after the feeding of diets containing xanthotoxin, while larvae exposed to MeJA fumigation also showed higher enzyme activity in a dose-dependent manner: lower and medium concentrations of MeJA induced higher detoxification enzyme activities than higher concentrations of MeJA. Moreover, MeJA improved the growth of larvae fed on the control diet without toxins and diets with lower concentrations of xanthotoxin (0.05%); however, MeJA could not protect the larvae against higher concentrations of xanthotoxin (0.1%, 0.2%). In summary, we demonstrated that MeJA is effective at inducing S. litura defense response, but the enhanced detoxifying ability could not overcome the strong toxins.

Assessment of the Molecular Responses of an Ancient Angiosperm against Atypical Insect Oviposition: The Case of Hass Avocados and the Tephritid Fly Anastrepha ludens

Anastrepha spp. (Diptera: Tephritidae) infestations cause significant economic losses in commercial fruit production worldwide. However, some plants quickly counteract the insertion of eggs by females by generating neoplasia and hindering eclosion, as is the case for Persea americana Mill., cv. Hass (Hass avocados). We followed a combined transcriptomics/metabolomics approach to identify the molecular mechanisms triggered by Hass avocados to detect and react to the oviposition of the pestiferous Anastrepha ludens (Loew). We evaluated two conditions: fruit damaged using a sterile pin (pin) and fruit oviposited by A. ludens females (ovi). We evaluated both of the conditions in a time course experiment covering five sampling points: without treatment (day 0), 20 min after the treatment (day 1), and days 3, 6, and 9 after the treatment. We identified 288 differentially expressed genes related to the treatments. Oviposition (and possibly bacteria on the eggs' surface) induces a plant hypersensitive response (HR), triggering a chitin receptor, producing an oxidative burst, and synthesizing phytoalexins. We also observed a process of cell wall modification and polyphenols biosynthesis, which could lead to polymerization in the neoplastic tissue surrounding the eggs.

Revisiting plant defense-fitness trade-off hypotheses using Solanum as a model genus

Plants possess physical and chemical defenses which have been found to deter herbivores that feed and oviposit on them. Despite having wide variety of defenses which can be constitutive or induced, plants are attacked and damaged by insects associated with different mouthparts and feeding habits. Since these defenses are costly, trade-offs for growth and defense traits play an important role in warding off the herbivores, with consequences for plant and herbivore growth, development and fitness. Solanum is a diverse and rich genus comprising of over 1,500 species with economic and ecological importance. Although a large number of studies on Solanum species with different herbivores have been carried out to understand plant defenses and herbivore counter defenses, they have primarily focused on pairwise interactions, and a few species of economic and ecological importance. Therefore, a detailed and updated understanding of the integrated defense system (sum of total defenses and trade-offs) is still lacking. Through this review, we take a closer look at the most common plant defense hypotheses, their assumptions and trade-offs and also a comprehensive evaluation of studies that use the genus Solanum as their host plant, and their generalist and specialist herbivores from different feeding guilds. Overall, review emphasizes on using ubiquitous Solanum genus and working toward building an integrated model which can predict defense-fitness-trade-offs in various systems with maximum accuracy and minimum deviations from realistic results.

Density-Dependent Effects of Simultaneous Root and Floral Herbivory on Plant Fitness and Defense

Plants are attacked by multiple herbivores, and depend on a precise regulation of responses to cope with a wide range of antagonists. Simultaneous herbivory can occur in different plant compartments, which may pose a serious threat to plant growth and reproduction. In particular, plants often face co-occurring root and floral herbivory, but few studies have focused on such interactions. Here, we investigated in the field the combined density-dependent effects of root-chewing cebrionid beetle larvae and flower-chewing pierid caterpillars on the fitness and defense of a semiarid Brassicaceae herb. We found that the fitness impact of both herbivore groups was independent and density-dependent. Increasing root herbivore density non-significantly reduced plant fitness, while the relationship between increasing floral herbivore density and the reduction they caused in both seed number and seedling emergence was non-linear. The plant defensive response was non-additive with regard to the different densities of root and floral herbivores; high floral herbivore density provoked compensatory investment in reproduction, and this tolerance response was combined with aboveground chemical defense induction when also root herbivore density was high. Plants may thus prioritize specific trait combinations in response to varying combined below- and aboveground herbivore densities to minimize negative impacts on fitness.

Novel aldo-keto reductase AKR2E9 regulates aldehyde content in the midgut and antennae of the silkworm (Bombyx mori)

We studied the effects of green leaf volatiles (including reactive aldehydes) emitted by plants on insects that feed on these plants. The silkworm (Bombyx mori) is a model lepidopteran that eats mulberry leaves. Defense-related enzymes in silkworms can be targeted for developing new pest control methods. The aldo-keto reductase (AKR) superfamily catalyzes aldehyde reduction by converting a carbonyl group into an alcohol group. Here, we characterized a novel silkworm AKR, designated as AKR2E9. Recombinant AKR2E9 was overexpressed in Escherichia coli. The recombinant protein was used, along with nicotinamide adenine dinucleotide phosphate as a coenzyme, to reduce aldehydes present in mulberry (Morus alba) leaves. The catalytic efficiency of AKR2E9 toward various aldehyde substrates and its inhibitor sensitivity was lower than those of AKR2E8. High expression levels of akr2e9 messenger RNA (mRNA) were detected in the midgut and antennae of silkworms. In the antennae of adult silkworms, akr2e9 mRNA was more abundant than akr2e8 mRNA. The catalytic efficiency of AKR2E9 was low because of steric hindrance, due to which its active site is blocked. High expression levels of AKR2E9 in the midgut and antennae suggest that it may regulate the detoxification of toxic aldehydes in silkworms.

Elevating herbivore-induced JA-Ile enhances potato resistance to the polyphagous beet armyworm but not to the oligophagous potato tuber moth

BACKGROUND

The oligophagous potato tuber moth (PTM), Phthorimaea operculella, and the polyphagous beet armyworm (BAW), Spodoptera exigua, are two destructive pests of potato, and infestations can lead to serious reduction in potato yield. However, potato plant responses to the two herbivories are only poorly understood. Endogenous jasmonoyl-isoleucine (JA-Ile) is a signal responsible for the induction of plant anti-herbivore defenses. Elevation of JA-Ile by blocking its catabolism is considered to be an effective and sustainable approach to enhance plant resistance to insect pests. However, it is not clear whether this approach can enhance potato resistance to PTM and BAW.

RESULTS

We demonstrated that the transcriptional changes induced by simulated PTM and BAW feeding overlap to a large extent, and that 81.5% of the PTM- and 90.5% of the BAW-responsive genes were commonly regulated. We also generated potato transgenic lines, irStCYP94B3s, in which the three JA-Ile hydroxylases were all simultaneously silenced. These lines exhibited enhanced resistance only to BAW, but not to PTM, although levels of JA-Ile and its downstream induced defensive chemicals, including caffeoylputrescine, dicaffeoylspermidine, lyciumoside II, and the nicotianosides I, II, and VII, were all present at higher levels in PTM-infested than in BAW-infested irStCYP94B3s lines.

CONCLUSION

Our results provide support for the hypothesis that StCYP94B3 genes are able to act as potential targets for the control of polyphagous insect pests in potato, and reveal that the oligophagous PTM has evolved an effective mechanism to cope with JA-Ile-induced anti-herbivore defenses. © 2022 Society of Chemical Industry.

Priming by Insects: Differential Effects of Sympatric and Allopatric Priming upon Plant Performance and Tolerance to Herbivory

Plants have evolved multiple mechanisms to defend themselves from their multiple herbivores. Thus, being able to recognise among them and respond accordingly is fundamental for plant survival and reproduction. Defence priming prepares the plant to better or more rapidly respond to future damage; however, while it is considered an adaptive trait, to date, no studies have evaluated the extent and specificity of the priming recognition. To estimate the costs, benefits and specificity of priming, we used a highly specialist plant-insect system (Datura stramonium-Lema daturaphila) and performed a reciprocal transplant experiment with two populations where a priming stimulus (sympatric vs. allopatric) and a damage treatment (sympatric) were applied. We found no evidence of a fitness cost of priming, given that primed plants without damage showed no reduction in fitness. In contrast, our treatments affected the probability of bud abortion. That is, when damaged plants received no priming or the priming came from an allopatric insect, the likelihood of aborting the first bud was 1.9 times greater compared to plants being primed by their sympatric insect. We also found that damaged plants primed with an allopatric insect produced 14% fewer seeds compared to plants receiving a sympatric priming stimulus. Tolerance to herbivore damage was also the lowest when plants received the priming stimulus from an allopatric insect. Overall, these results suggest that, in our study system, plants recognise their local insect population reducing the negative effect of damage through a tolerance response.

Onion Thrips (Thysanoptera: Thripidae) Host Plant Preference and Performance are Mediated by a Facultative Plant Pathogen of Onion

Insect vector and phytopathogen interactions are mediated by host plants. Insects interact with pathogens directly or indirectly and they may prefer host plants based on infection status. Performance on infected hosts varies depending on the type of pathogen involved. Species specific studies of economically important insects and phytopathogens are needed to understand how these interactions impact crop yields. Onion thrips, Thrips tabaci Lindeman (Thysanoptera: Thripidae), is an economically devastating insect pest of onions (Allium cepa L., Asparagales: Amaryllidaceae) worldwide and it co-occurs simultaneously with many different pathogens. Colletotrichum coccodes (Wallr) (Glomerellales: Glomerellaceae) is a generalist fungal pathogen that attacks onion foliage, causing tan lesions and decreasing yield. Onion thrips and C. coccodes represent two important pests of onions, but the relationship between onion thrips and C. coccodes infected onions has not been studied, and it is unclear if onion thrips contribute to the spread of C. coccodes in onion fields. A four-choice test with control, artificially injured, artificially injured + symptomatic, and inoculated-symptomatic onion suggests that onion thrips distinguish between hosts based on health status. Furthermore, a two-choice test with control, inoculated-asymptomatic, and inoculated-symptomatic onion pairings revealed that onion thrips distinguish between hosts based on infection status and prefer inoculated-symptomatic hosts. In a no-choice test, onion thrips numbers increased on inoculated-symptomatic plants compared to control or inoculated-asymptomatic plants. Overall, we found that onion thrips preferred and performed best on C. coccodes infected plants.

The glutamate receptor-like 3.3 and 3.6 mediate systemic resistance to insect herbivores in Arabidopsis

Herbivory activates responses in local and systemic leaves, and the glutamate receptor-like genes GLR3.3 and GLR3.6 are critical in leaf-to-leaf systemic signalling. However, whether and how these genes mediate plant systemic resistance to insects remain largely unexplored. We show that a piercing-sucking insect Myzus persicae (green peach aphid, GPA) or chewing insect Spodoptera litura (cotton leafworm, CLW) feeding-induced systemic defences were attenuated in the glr3.3 glr3.6 mutants. In response to herbivory from either insect, glr3.3 glr3.6 mutants exhibited reduced accumulation of the hormone jasmonic acid (JA) and defensive metabolites glucosinolates (GSs) in systemic (but not local) leaves. Transcriptome analysis indicated that GLR3.3 and GLR3.6 play an important role in regulating the transcriptional responses to GPA and simulated CLW feeding in both local and systemic leaves, including JA- and GS-related genes. Metabolome analysis also revealed that in response to GPA or simulated CLW feeding, GLR3.3 and GLR3.6 are involved in the regulation of various metabolites locally and systemically, including amino acids, carbohydrates, and organic acids. Taken together, this study provides new insights into the function of GLR3.3 and GLR3.6 in mediating transcripts and metabolites in local and systemic leaves under insect attack, and highlights their role in regulating insect resistance in systemic leaves.

Wheat transcriptomic responses to extended feeding by wheat curl mites

The economic importance of wheat and its contribution to human and livestock diets has been already demonstrated. However, wheat production is impacted by pests that induce yield reductions. Among these pests, wheat curl mite (WCM, Aceria tosichella Keifer) impacts wheat all around the world. WCM are tiny pests that feed within the whorl of developing leaves, and their feeding causes leaf curling by preventing them from unfurling. The curling of the leaves provides a protective niche for the WCM. Additionally, WCM are also the vector of serious viruses in wheat. Little is known regarding the impact of the WCM on wheat transcriptome, and to date, only one article has been published describing the wheat transcriptomic changes after 1 day of WCM feeding. To better understand the wheat transcriptome variation after extended feeding by WCM [10 days post infestation (dpi)], we used an RNA-seq approach. We collected WCM-infested and uninfested leaves from two wheat cultivars: Byrd (WCM resistant) and Settler CL (WCM susceptible) at 10 dpi. Our transcriptomic analysis revealed the common and specific transcriptomic variations in WCM resistant and susceptible wheat cultivars, chromosome 3D specific location of the differentially expressed genes with functions involved in defense and stress response, and also identified the gene functions related to lipid signaling and membrane integrity, and phytohormone pathways potentially contributing to WCM resistance. Collectively, our study provides important insights on wheat defense mechanisms against WCM after extended feeding.

Herbivore-induced Ca2+ signals trigger a jasmonate burst by activating ERF16-mediated expression in tomato

Herbivory severely affects plant growth, posing a threat to crop production. Calcium ion (Ca²⁺ ) signaling and accumulation of jasmonates (JAs) are activated in plant response to herbivore attack, leading to the expression of defense pathways. However, little is known about how the Ca²⁺ signal modulates JA biosynthesis. We used diverse techniques, including CRISPR/Cas9, UPLC-MS/MS and molecular biology methods to explore the role of ETHYLENE RESPONSE FACTOR 16 in Ca²⁺ signal-triggered JA burst during herbivore defense in tomato. Here we show that simulated herbivory induces GLUTAMATE RECEPTOR LIKE3.3/3.5 (GLR3.3/3.5)-dependent increases in electrical activity, Ca²⁺ influx and increases the abundance of CALMODULIN2 (CaM2) and ERF16 transcripts in tomato. The interaction between CaM2 and ERF16 promotes JA biosynthesis by enhancing the transcriptional activity of ERF16, which increases the activation of ERF16 expression and causes expression of LIPOXYGENASE D (LOXD), AOC and 12-OXO-PHYTODIENOIC ACID REDUCTASE 3 (OPR3), the key genes in JA biosynthesis. Mutation of CaM2 results in decreased JA accumulation, together with the expression of JA biosynthesis-related genes, leading to reduced resistance to the cotton bollworm Helicoverpa armigera. These findings reveal a molecular mechanism underpinning the Ca²⁺ signal-initiated systemic JA burst and emphasize the pivotal role of Ca²⁺ signal/ERF16 crosstalk in herbivore defense.

If All Else Fails: Impact of Silicon Accumulation in Maize Leaves on Volatile Emissions and Oviposition Site Selection of Spodoptera exigua Hübner

Silicon (Si) fertilization alleviates biotic stresses in plants. Si enhances plant resistance against phytophagous insects through physical and biochemical mechanisms. In particular, Si modifies jasmonic acid levels and the emissions of herbivore-induced plant volatiles (HIPVs). Here, we investigated whether Si accumulation in the tissues of maize leaves modifies the emissions of constitutive and herbivore-induced plant volatiles, with cascade deterrent effects on oviposition site selection by Spodoptera exigua Hübner (Lepidoptera: Noctuidae). Maize plants were cultivated in a hydroponic system under three Si concentrations, resulting in three groups of plants expressing different Si concentrations in their tissues (0.31 ± 0.04, 4.69 ± 0.49, and 9.56 ± 0.30 g Si. Kg^- 1 DW). We collected volatiles from undamaged and caterpillar-infested plants, and found that Si concentration in plant tissues had no significant impact. Jasmonic acid content was high in insect-infested plants, but was similar across all Si treatments. Oviposition site selection bioassays using fertilized S. exigua females showed that Si concentration in plant tissues did not affect the number of eggs laid on Si-treated plants. In conclusion, our study shows that the Si content in maize tissues does not impact the semiochemical interactions with S. exigua.

Comparative transcriptome analysis of wheat in response to corn leaf aphid, Rhopalosiphum maidis F. infestation

Aphids are one of the most important insect pests of wheat crop in all wheat growing regions of the world. Amongst various aphid species, the corn leaf aphid (Rhopalosiphum maidis F.) is considered one of the most destructive insect pests of wheat in the North Western Plains region of India. Transcriptome profiling of highly susceptible wheat Triticum durum genotype, A-9-30-1 and tolerant wheat Triticum aestivum genotype, HD2967 was performed to investigate aphid-host interactions. The results obtained from differential gene expression analysis of R. maidis on the highly susceptible genotype, A-9-30-1 plants, when compared with the tolerant genotype, HD2967, showed that 212 genes were significantly upregulated and 1009 genes were significantly downregulated. Our findings demonstrated that the genes associated with defense were significantly higher in response to R. maidis on HD2967 as compared to A-9-30-1. Additionally, various genes with physiological attributes were expressed during aphid attack. Based on gene ontology classification, three classifications, such as, cellular components (CC), molecular function (MF), and biological processes (BP) of sequences were identified. KEGG enrichment analysis revealed that twenty-five pathway genes were differentially expressed during the infestation of wheat with R. maidis. Notable changes were observed in A-9-30-1 and HD2967 transcriptomic profiling after infestation. The results obtained in the present study will help to elucidate the mechanism governing host-pest interaction and may lead to the development of new methods for increasing the resistance level of wheat against R. maidis, including over-expression of defense-related genes.

Plant defense under Arctic light conditions: Can plants withstand invading pests?

Global warming is predicted to change the growth conditions for plants and crops in regions at high latitudes (>60° N), including the Arctic. This will be accompanied by alterations in the composition of natural plant and pest communities, as herbivorous arthropods will invade these regions as well. Interactions between previously non-overlapping species may occur and cause new challenges to herbivore attack. However, plants growing at high latitudes experience less herbivory compared to plants grown at lower latitudes. We hypothesize that this finding is due to a gradient of constitutive chemical defense towards the Northern regions. We further hypothesize that higher level of defensive compounds is mediated by higher level of the defense-related phytohormone jasmonate. Because its biosynthesis is light dependent, Arctic summer day light conditions can promote jasmonate accumulation and, hence, downstream physiological responses. A pilot study with bilberry (Vaccinium myrtillus) plants grown under different light regimes supports the hypothesis.

Transcriptome analysis of aphids exposed to glandular trichomes in tomato reveals stress and starvation related responses

Understanding the responses of insect herbivores to plant chemical defences is pivotal for the management of crops and pests. However, the mechanisms of interaction are not entirely understood. In this study, we compared the whole transcriptome gene expression of the aphid Macrosiphum euphorbiae grown on two different varieties of tomato that differ in their inducible chemical defences. We used two isogenic lines of tomato with a shared genetic background that only differ in the presence of type IV glandular trichomes and their associated acylsucrose excretions. This works also reports a de novo transcriptome of the aphid M. euphorbiae. Subsequently, we identified a unique and distinct gene expression profile for the first time corresponding to aphid´s exposure to type IV glandular trichomes and acylsugars. The analysis of the aphid transcriptome shows that tomato glandular trichomes and their associated secretions are highly efficient in triggering stress-related responses in the aphid, and demonstrating that their role in plant defence goes beyond the physical impediment of herbivore activity. Some of the differentially expressed genes were associated with carbohydrate, lipid and xenobiotic metabolisms, immune system, oxidative stress response and hormone biosynthesis pathways. Also, the observed responses are compatible with a starvation syndrome. The transcriptome analysis puts forward a wide range of genes involved in the synthesis and regulation of detoxification enzymes that reveal important underlying mechanisms in the interaction of the aphid with its host plant and provides a valuable genomic resource for future study of biological processes at the molecular level using this aphid.

Ambivalent response in pathogen defense: A double-edged sword?

Plants possess effective immune systems that defend against most microbial attackers. Recent plant immunity research has focused on the classic binary defense model involving the pivotal role of small-molecule hormones in regulating the plant defense signaling network. Although most of our current understanding comes from studies that relied on information derived from a limited number of pathosystems, newer studies concerning the incredibly diverse interactions between plants and microbes are providing additional insights into other novel mechanisms. Here, we review the roles of both classical and more recently identified components of defense signaling pathways and stress hormones in regulating the ambivalence effect during responses to diverse pathogens. Because of their different lifestyles, effective defense against biotrophic pathogens normally leads to increased susceptibility to necrotrophs, and vice versa. Given these opposing forces, the plant potentially faces a trade-off when it mounts resistance to a specific pathogen, a phenomenon referred to here as the ambivalence effect. We also highlight a novel mechanism by which translational control of the proteins involved in the ambivalence effect can be used to engineer durable and broad-spectrum disease resistance, regardless of the lifestyle of the invading pathogen.

Evolution patterns of NBS genes in the genus Dendrobium and NBS-LRR gene expression in D. officinale by salicylic acid treatment

BACKGROUND

Dendrobium officinale Kimura et Migo, which contains rich polysaccharides, flavonoids and alkaloids, is a Traditional Chinese Medicine (TCM) with important economic benefits, while various pathogens have brought huge losses to its industrialization. NBS gene family is the largest class of plant disease resistance (R) genes, proteins of which are widely distributed in the upstream and downstream of the plant immune systems and are responsible for receiving infection signals and regulating gene expression respectively. It is of great significance for the subsequent disease resistance breeding of D. officinale to identify NBS genes by using the newly published high-quality chromosome-level D. officinale genome.

RESULTS

In this study, a total of 655 NBS genes were uncovered from the genomes of D. officinale, D. nobile, D. chrysotoxum, V. planifolia, A. shenzhenica, P. equestris and A. thaliana. The phylogenetic results of CNL-type protein sequences showed that orchid NBS-LRR genes have significantly degenerated on branches a and b. The Dendrobium NBS gene homology analysis showed that the Dendrobium NBS genes have two obvious characteristics: type changing and NB-ARC domain degeneration. Because the NBS-LRR genes have both NB-ARC and LRR domains, 22 D. officinale NBS-LRR genes were used for subsequent analyses, such as gene structures, conserved motifs, cis-elements and functional annotation analyses. All these results suggested that D. officinale NBS-LRR genes take part in the ETI system, plant hormone signal transduction pathway and Ras signaling pathway. Finally, there were 1,677 DEGs identified from the salicylic acid (SA) treatment transcriptome data of D. officinale. Among them, six NBS-LRR genes (Dof013264, Dof020566, Dof019188, Dof019191, Dof020138 and Dof020707) were significantly up-regulated. However, only Dof020138 was closely related to other pathways from the results of WGCNA, such as pathogen identification pathways, MAPK signaling pathways, plant hormone signal transduction pathways, biosynthetic pathways and energy metabolism pathways.

CONCLUSION

Our results revealed that the NBS gene degenerations are common in the genus Dendrobium, which is the main reason for the diversity of NBS genes, and the NBS-LRR genes generally take part in D. officinale ETI system and signal transduction pathways. In addition, the D. officinale NBS-LRR gene Dof020138, which may have an important breeding value, is indirectly activated by SA in the ETI system.

Integrated Transcriptome and Metabolome Analysis to Identify Sugarcane Gene Defense against Fall Armyworm (Spodoptera frugiperda) Herbivory

Sugarcane is the most important sugar crop, contributing ≥80% to total sugar production around the world. Spodoptera frugiperda is one of the main pests of sugarcane, potentially causing severe yield and sugar loss. The identification of key defense factors against S. frugiperda herbivory can provide targets for improving sugarcane resistance to insect pests by molecular breeding. In this work, we used one of the main sugarcane pests, S. frugiperda, as the tested insect to attack sugarcane. Integrated transcriptome and metabolomic analyses were performed to explore the changes in gene expression and metabolic processes that occurred in sugarcane leaf after continuous herbivory by S. frugiperda larvae for 72 h. The transcriptome analysis demonstrated that sugarcane pest herbivory enhanced several herbivory-induced responses, including carbohydrate metabolism, secondary metabolites and amino acid metabolism, plant hormone signaling transduction, pathogen responses, and transcription factors. Further metabolome analysis verified the inducement of specific metabolites of amino acids and secondary metabolites by insect herbivory. Finally, association analysis of the transcriptome and metabolome by the Pearson correlation coefficient method brought into focus the target defense genes against insect herbivory in sugarcane. These genes include amidase and lipoxygenase in amino acid metabolism, peroxidase in phenylpropanoid biosynthesis, and pathogenesis-related protein 1 in plant hormone signal transduction. A putative regulatory model was proposed to illustrate the sugarcane defense mechanism against insect attack. This work will accelerate the dissection of the mechanism underlying insect herbivory in sugarcane and provide targets for improving sugarcane variety resistance to insect herbivory by molecular breeding.

Plagiodera versicolora feeding induces systemic and sexually differential defense responses in poplars

Dioecious plants have evolved effective defense strategies to deal with various biotic and abiotic stresses. However, little is known regarding sexual differences in their defense against herbivores. In this study, we investigated the mechanism of systemic defense responses in male and female Populus cathayana attacked by Plagiodera versicolora Laicharting. The results revealed that P. cathayana exhibits sexually differential responses to a defoliator. The percentage of damaged leaf area was greater in males than in females. Furthermore, the observed saccharide changes imply that males and females exhibit different response times to defoliators. The contents of flavonoids and anthocyanins were significantly increased in both sexes but were higher in females. Specifically, the jasmonic acid (JA) pathway plays an important role. Expression of pest-related genes further revealed that hormones induce changes in downstream genes and metabolites, and upregulation of JA ZIM-domain (JAZ) and CORONATINE INSENSITIVE 1 (COI1) was more significant in females. In the undamaged adjacent leaves, metabolite and gene changes displayed similar patterns to the damaged local leaves, but levels of JA, JAZ1, and COI1 were higher in females. Therefore, our data confirmed that plants initiate the JA pathway to defend against herbivores, that there is systematic signal transduction, and that this ability is stronger in females than in males. This study provides new insights into the resistance of dioecious plants to herbivory and adds a new theoretical basis for the systemic signal transduction of plants in response to biotic stress.

Inhibitor of Glucosinolate Sulfatases as a Potential Friendly Insecticide to Control Plutella xylostella

The glucosinolate-myrosinase system is a two-component defense system characteristic of cruciferous plants. To evade the glucosinolate-myrosinase system, the crucifer specialist insect, Plutella xylostella, promptly desulfates the glucosinolates into harmless compounds by glucosinolate sulfatases (GSSs) in the gut. In this study, we identified an effective inhibitor of GSSs by virtual screening, molecular docking analysis, and in vitro enzyme inhibition assay. The combined effect of the GSS inhibitor with the plant glucosinolate-myrosinase system was assessed by the bioassay of P. xylostella. We show that irosustat is a GSS inhibitor and the inhibition of GSSs impairs the ability of P. xylostella to detoxify the glucosinolate-myrosinase system, leading to the systematic accumulation of toxic isothiocyanates in larvae, thereby severely affecting feeding, growth, survival, and reproduction of P. xylostella. While fed on the Arabidopsis mutants deficient in myrosinase or glucosinolates, irosustat had no significant negative effect on P. xylostella. These findings reveal that the GSS inhibitor is a novel friendly insecticide to control P. xylostella utilizing the plant glucosinolate-myrosinase system and promote the development of insecticide-plant chemical defense combination strategies.

Adverse effects of inbreeding on the transgenerational expression of herbivore-induced defense traits in Solanum carolinense

In addition to directly inducing physical and chemical defenses, herbivory experienced by plants in one generation can influence the expression of defensive traits in offspring. Plant defense phenotypes can be compromised by inbreeding, and there is some evidence that such adverse effects can extend to the transgenerational expression of induced resistance. We explored how the inbreeding status of maternal Solanum carolinense plants influenced the transgenerational effects of herbivory on the defensive traits and herbivore resistance of offspring. Manduca sexta caterpillars were used to damage inbred and outbred S. carolinense maternal plants and cross pollinations were performed to produced seeds from herbivore-damaged and undamaged, inbred and outbred maternal plants. Seeds were grown in the greenhouse to assess offspring defense-related traits (i.e., leaf trichomes, internode spines, volatile organic compounds) and resistance to herbivores. We found that feeding by M. sexta caterpillars on maternal plants had a positive influence on trichome and spine production in offspring and that caterpillar development on offspring of herbivore-damaged maternal plants was delayed relative to that on offspring of undamaged plants. Offspring of inbred maternal plants had reduced spine production, compared to those of outbred maternal plants, and caterpillars performed better on the offspring of inbred plants. Both herbivory and inbreeding in the maternal generation altered volatile emissions of offspring. In general, maternal plant inbreeding dampened transgenerational effects of herbivory on offspring defensive traits and herbivore resistance. Taken together, this study demonstrates that inducible defenses in S. carolinense can persist across generations and that inbreeding compromises transgenerational resistance in S. carolinense.

Calcium Signaling in Plant-Insect Interactions

In plant-insect interactions, calcium (Ca²⁺) variations are among the earliest events associated with the plant perception of biotic stress. Upon herbivory, Ca²⁺ waves travel long distances to transmit and convert the local signal to a systemic defense program. Reactive oxygen species (ROS), Ca²⁺ and electrical signaling are interlinked to form a network supporting rapid signal transmission, whereas the Ca²⁺ message is decoded and relayed by Ca²⁺-binding proteins (including calmodulin, Ca²⁺-dependent protein kinases, annexins and calcineurin B-like proteins). Monitoring the generation of Ca²⁺ signals at the whole plant or cell level and their long-distance propagation during biotic interactions requires innovative imaging techniques based on sensitive sensors and using genetically encoded indicators. This review summarizes the recent advances in Ca²⁺ signaling upon herbivory and reviews the most recent Ca²⁺ imaging techniques and methods.

Compatibility of mycorrhiza-induced resistance with viral and bacterial entomopathogens in the control of Spodoptera exigua in tomato

BACKGROUND

Arbuscular mycorrhizal fungi (AMF) are soil-borne microorganisms that establish mutualistic associations with roots of most terrestrial plants. This symbiosis results in nutritional and defensive benefits to the host plant, usually conferring protection against biotic stresses, but its indirect impact on third trophic levels is still unknown. In the present work, we explore whether the symbiosis of tomato plants with Funneliformis mosseae (and/or exposition to herbivory) influences the interaction of the generalist pest Spodoptera exigua (Lepidoptera: Noctuidae) with bacterial (Bacillus thuringiensis) and viral (baculovirus, SeMNPV) natural entomopathogens.

RESULTS

Symbiosis with AMF and previous herbivory reduces the relative growth of S. exigua, increases its susceptibility to a sublethal dose of B. thuringiensis and has positive or neutral impact on the lethality of SeMNPV. Reduction of the phenoloxidase activity, a marker of the insect immune response, was associated with the larval feeding on plant material previously exposed to herbivory but not to the AMF. In addition, no changes in the insect gut microbiota could be associated with the observed changes in larval growth and susceptibility to the entomopathogens.

CONCLUSION

Our findings provide the first evidence of compatibility of AMF symbiosis in tomato with the use of bacterial and viral entomopathogens, contributing to the development of novel approaches to combine the beneficial effect of AMF and entomopathogens in biological pest control. © 2022 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

MYC transcription factors coordinate tryptophan-dependent defence responses and compromise seed yield in Arabidopsis

Robust plant immunity negatively affects other fitness traits, including growth and seed production. Jasmonate (JA) confers broad-spectrum protection against plant consumers by stimulating the degradation of JASMONATE ZIM-DOMAIN (JAZ) proteins, which in turn relieves repression on transcription factors (TFs) coincident with reduced growth and fecundity. The molecular mechanisms underlying JA-mediated decreases in fitness remain largely unknown. To assess the contribution of MYC TFs to growth and reproductive fitness at high levels of defence, we mutated three MYC genes in a JAZ-deficient mutant (jazD) of Arabidopsis thaliana that exhibits strong defence and low seed yield. Genetic epistasis analysis showed that de-repression of MYC TFs in jazD not only conferred strong resistance to insect herbivory but also reduced shoot and root growth, fruit size and seed yield. We also provided evidence that the JAZ-MYC module coordinates the supply of tryptophan with the production of indole glucosinolates and the proliferation of endoplasmic reticulum bodies that metabolise glucosinolates through the action of β-glucosidases. Our results establish MYCs as major regulators of growth- and reproductive-defence trade-offs and further indicate that these factors coordinate tryptophan availability with the production of amino acid-derived defence compounds.

The role of CDPKs in plant development, nutrient and stress signaling

The second messenger calcium (Ca2+) is a ubiquitous intracellular signaling molecule found in eukaryotic cells. In plants, the multigene family of calcium-dependent protein kinases (CDPKs) plays an important role in regulating plant growth, development, and stress tolerance. CDPKs sense changes in intracellular Ca2+ concentration and translate them into phosphorylation events that initiate downstream signaling processes. Several functional and expression studies on different CDPKs and their encoding genes have confirmed their multifunctional role in stress. Here, we provide an overview of the signal transduction mechanisms and functional roles of CDPKs. This review includes details on the regulation of secondary metabolites, nutrient uptake, regulation of flower development, hormonal regulation, and biotic and abiotic stress responses.

Stress-Induced Volatile Emissions and Signalling in Inter-Plant Communication

The sessile plant has developed mechanisms to survive the "rough and tumble" of its natural surroundings, aided by its evolved innate immune system. Precise perception and rapid response to stress stimuli confer a fitness edge to the plant against its competitors, guaranteeing greater chances of survival and productivity. Plants can "eavesdrop" on volatile chemical cues from their stressed neighbours and have adapted to use these airborne signals to prepare for impending danger without having to experience the actual stress themselves. The role of volatile organic compounds (VOCs) in plant-plant communication has gained significant attention over the past decade, particularly with regard to the potential of VOCs to prime non-stressed plants for more robust defence responses to future stress challenges. The ecological relevance of such interactions under various environmental stresses has been much debated, and there is a nascent understanding of the mechanisms involved. This review discusses the significance of VOC-mediated inter-plant interactions under both biotic and abiotic stresses and highlights the potential to manipulate outcomes in agricultural systems for sustainable crop protection via enhanced defence. The need to integrate physiological, biochemical, and molecular approaches in understanding the underlying mechanisms and signalling pathways involved in volatile signalling is emphasised.

Volatile Dimethyl Disulfide from Guava Plants Regulate Developmental Performance of Asian Citrus Psyllid through Activation of Defense Responses in Neighboring Orange Plants

Intercropping with guava (Psidium guajava L.) can assist with the management of Asian citrus psyllid (ACP, Diaphorina citri Kuwayama), the insect vector of the huanglongbing pathogen, in citrus orchards. Sulfur volatiles have a repellent activity and physiological effects, as well as being important components of guava volatiles. In this study, we tested whether the sulfur volatiles emitted by guava plants play a role in plant–plant communications and trigger anti-herbivore activities against ACP in sweet orange plants (Citrus sinensis L. Osbeck). Real-time determination using a proton-transfer-reaction mass spectrometer (PTR-MS) showed that guava plants continuously release methanethiol, dimethyl sulfide (DMS), and dimethyl disulfide (DMDS), and the contents increased rapidly after mechanical damage. The exposure of orange plants to DMDS resulted in the suppression of the developmental performance of ACP. The differential elevation of salicylic acid (SA) levels; the expression of phenylalanine ammonia lyase (PAL), salicylate-O-methyl transferase (SMT), and pathogenesis-related (PR1) genes; the activities of defense-related enzymes PAL, polyphenol oxidase (PPO), and peroxidase (POD); and the total polyphenol content were observed in DMDS-exposed orange plants. The emission of volatiles including myrcene, nonanal, decanal, and methyl salicylate (MeSA) was increased. In addition, phenylpropanoid and flavonoid biosynthesis, and aromatic amino acid (such as phenylalanine, tyrosine, and tryptophan) metabolic pathways were induced. Altogether, our results indicated that DMDS from guava plants can activate defense responses in eavesdropping orange plants and boost their herbivore resistance to ACP, which suggests the possibility of using DMDS as a novel approach for the management of ACP in citrus orchards.

Dichotomous Role of Jasmonic Acid in Modulating Sorghum Defense Against Aphids

The precursors and derivatives of jasmonic acid (JA) contribute to plant protective immunity to insect attack. However, the role of JA in sorghum (Sorghum bicolor) defense against sugarcane aphid (SCA) (Melanaphis sacchari), which is considered a major threat to sorghum production, remains elusive. Sorghum SC265, previously identified as a SCA-resistant genotype among the sorghum nested association mapping founder lines, transiently increased JA at early stages of aphid feeding and deterred aphid settling. Monitoring of aphid feeding behavior using electropenetrography, a technique to unveil feeding process of piercing-sucking insects, revealed that SC265 plants restricted SCA feeding from the phloem sap. However, exogenous application of JA attenuated the resistant phenotype and promoted improved aphid feeding and colonization on SC265 plants. This was further confirmed with sorghum JA-deficient plants, in which JA deficiency promoted aphid settling, however, it also reduced aphid feeding from the phloem sap and curtailed SCA population. Exogenous application of JA caused enhanced feeding and aphid proliferation on JA-deficient plants, suggesting that JA promotes aphid growth and development. SCA feeding on JA-deficient plants altered the sugar metabolism and enhanced the levels of fructose and trehalose compared with wild-type plants. Furthermore, aphid artificial diet containing fructose and trehalose curtailed aphid growth and reproduction. Our findings underscore a previously unknown dichotomous role of JA, which may have opposing effects by deterring aphid settling during the early stage and enhancing aphid proliferative capacity during later stages of aphid colonization on sorghum plants. [Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Rice Stripe Virus Modulates the Feeding Preference of Small Brown Planthopper from the Stems to Leaves of Rice Plants to Promote Virus Infection

Research on plant-virus-vector interactions has revealed that viruses can enhance their spread to new host plants by attracting nonviruliferous vectors to infected plants or driving viruliferous vectors to noninfected plants. However, whether viruses can also modulate the feeding preference of viruliferous vectors for different plant parts remains largely unknown. Here, by using rice stripe virus (RSV) and its vector, the small brown planthopper (SBPH), as a model, the effect of the virus on the feeding preference of its vector was studied by calculating the number of nonviruliferous and viruliferous SBPHs settling on different parts of rice plants. The results showed that the RSV-free SBPHs significantly preferred feeding on the stems of rice plants, whereas RSV-carrying SBPHs fed more on rice leaves. Moreover, the rice plants inoculated with RSV on the leaves showed more severe symptoms, with enhanced disease incidence and virus accumulation compared with rice plants inoculated at the top and bottom of stems, suggesting that the leaves are more susceptible to RSV than the stems of rice plants. These results demonstrate that RSV modulates the feeding preference of its transmitting vector SBPH from the stems to leaves of rice plants to promote virus infection. Interestingly, we also found that the leaves were more susceptible than the stems to rice black-streaked dwarf virus. This study proves that the feeding preference of insect vectors can be modulated by plant viruses to facilitate virus transmission.

Comparative Plant Transcriptome Profiling of Arabidopsis thaliana Col-0 and Camelina sativa var. Celine Infested with Myzus persicae Aphids Acquiring Circulative and Noncirculative Viruses Reveals Virus- and Plant-Specific Alterations Relevant to Aphid Feeding Behavior and Transmission

Evidence is accumulating that plant viruses alter host plant traits in ways that modify their insect vectors' behavior. These alterations often enhance virus transmission, which has led to the hypothesis that these effects are manipulations caused by viral adaptation. However, we lack a mechanistic understanding of the genetic basis of these indirect, plant-mediated effects on vectors, their dependence on the plant host, and their relation to the mode of virus transmission. Transcriptome profiling of Arabidopsis thaliana and Camelina sativa plants infected with turnip yellows virus (TuYV) or cauliflower mosaic virus (CaMV) and infested with the common aphid vector Myzus persicae revealed strong virus- and host-specific differences in gene expression patterns. CaMV infection caused more severe effects on the phenotype of both plant hosts than did TuYV infection, and the severity of symptoms correlated strongly with the proportion of differentially expressed genes, especially photosynthesis genes. Accordingly, CaMV infection modified aphid behavior and fecundity more strongly than did infection with TuYV. Overall, infection with CaMV, relying on the noncirculative transmission mode, tends to have effects on metabolic pathways, with strong potential implications for insect vector-plant host interactions (e.g., photosynthesis, jasmonic acid, ethylene, and glucosinolate biosynthetic processes), while TuYV, using the circulative transmission mode, alters these pathways only weakly. These virus-induced deregulations of genes that are related to plant physiology and defense responses might impact both aphid probing and feeding behavior on infected host plants, with potentially distinct effects on virus transmission. IMPORTANCE Plant viruses change the phenotype of their plant hosts. Some of the changes impact interactions of the plant with insects that feed on the plants and transmit these viruses. These modifications may result in better virus transmission. We examine here the transcriptomes of two plant species infected with two viruses with different transmission modes to work out whether there are plant species-specific and transmission mode-specific transcriptome changes. Our results show that both are the case.

Natural variation meets synthetic biology: Promiscuous trichome-expressed acyltransferases from Nicotiana

Acylsugars are defensive, trichome-synthesized sugar esters produced in plants across the Solanaceae (nightshade) family. Although assembled from simple metabolites and synthesized by a relatively short core biosynthetic pathway, tremendous within- and across-species acylsugar structural variation is documented across the family. To advance our understanding of the diversity and the synthesis of acylsugars within the Nicotiana genus, trichome extracts were profiled across the genus coupled with transcriptomics-guided enzyme discovery and in vivo and in vitro analysis. Differences in the types of sugar cores, numbers of acylations, and acyl chain structures contributed to over 300 unique annotated acylsugars throughout Nicotiana. Placement of acyl chain length into a phylogenetic context revealed that an unsaturated acyl chain type was detected in a few closely related species. A comparative transcriptomics approach identified trichome-enriched Nicotiana acuminata acylsugar biosynthetic candidate enzymes. More than 25 acylsugar variants could be produced in a single enzyme assay with four N. acuminata acylsugar acyltransferases (NacASAT1-4) together with structurally diverse acyl-CoAs and sucrose. Liquid chromatography coupled with mass spectrometry screening of in vitro products revealed the ability of these enzymes to make acylsugars not present in Nicotiana plant extracts. In vitro acylsugar production also provided insights into acyltransferase acyl donor promiscuity and acyl acceptor specificity as well as regiospecificity of some ASATs. This study suggests that promiscuous Nicotiana acyltransferases can be used as synthetic biology tools to produce novel and potentially useful metabolites.

Constitutive and Induced Defenses in Long-lived Pines Do Not Trade Off but Are Influenced by Climate

Plants resist herbivores and pathogens by using constitutive (baseline) and inducible (change in defense after an attack) defenses. Inducibility has long been predicted to trade off with constitutive defense, reflecting the economic use of resources. However, empirical evidence for such tradeoffs is variable, and we still lack understanding about when and where defense trade-offs occur. We tested for tradeoffs between constitutive and induced defenses in natural populations of three species of long-lived pines (Pinus balfouriana, P. flexilis, P. longaeva) that differ greatly in constitutive defense and resistance to mountain pine beetle (MPB, Dendroctonus ponderosae). We also assessed how climate influenced constitutive and inducible defenses. At seven high-elevation sites in the western U.S., we simulated MPB attack to induce defenses and measured concentrations of terpene-based phloem defenses on days 0, 15, and 30. Constitutive and induced defenses did not trade off among or within species. Simulated MPB attack induced large increases in defense concentrations in all species independent of constitutive levels. MPB and its symbiotic fungi typically kill trees and thus could be selective forces maintaining strong inducibility within and among species. The contrasting constitutive concentrations in these species could be driven by the adaptation for specializing in harsh, high-elevation environments (e.g., P. balfouriana and P. longaeva) or by competition (e.g., P. flexilis), though these hypotheses have not been empirically examined. Climate influenced defenses, with the greatest concentrations of constitutive and induced defenses occurring at the coldest and driest sites. The interactions between climate and defenses have implications for these species under climate change.

Plant Volatile Compounds of the Invasive Alligatorweed, Alternanthera philoxeroides (Mart.) Griseb, Infested by Agasicles hygrophila Selman and Vogt (Coleoptera: Chrysomelidae)

Plants release a variety of volatiles and herbivore-induced plant volatiles (HIPVs) after being damaged by herbivorous insects, which play multiple roles in the interactions with other plants and insects. Agasicles hygrophila Selman and Vogt (Coleoptera: Chrysomelidae) is a monophagous natural enemy and an effective biocontrol agent for Alternanthera philoxeroides (Mart.) Griseb. Here, we reported differences among the volatiles of A. philoxeroides by solid phase microextraction (SPME) using a gas chromatography-mass spectrometer (GC-MS). We compared the volatile emission of: (1) clean plants (CK); (2) A. philoxeroides plants with mechanical damage treatment (MD); and (3) A. philoxeroides plants infested with A. hygrophila 1st, 2nd, and 3rd larvae and female and male adults. A total of 97 volatiles were recorded, of which 5 occurred consistently in all treatments, while 61 volatiles were only observed in A. philoxeroides infested by A. hygrophila, such as trans-nerolidol, (E)-β-farnesene, and (3E,7E)-4,8,12-trimethyltrideca-1,3,7,11-tetraene (E, E-TMTT), etc. Among the 97 volatile compounds, 37 compounds belong to alkenes, 29 compounds belong to alkanes, and there were 8 esters, 8 alcohols and 6 ketones. Orthogonal partial least squares-discrimination analysis (OPLS-DA) showed that the different treatments were separated from each other, especially insect feeding from CK and MD treatments, and 19 volatiles contributed most to the separation among the treatments, with variable importance for the projection (VIP) values > 1. Our findings indicated that the alligatorweed plants could be induced to release volatiles by different stages of A. hygrophila, and the volatile compounds released differ quantitatively and qualitatively. The results from this study laid an important foundation for using volatile organic compounds (VOCs) and HIPVs of alligatorweed to improve the control effect of A. hygrophila on A. philoxeroides.

Multi-Omics Approaches to Improve Clubroot Resistance in Brassica with a Special Focus on Brassica oleracea L

Brassica oleracea is an agronomically important species of the Brassicaceae family, including several nutrient-rich vegetables grown and consumed across the continents. But its sustainability is heavily constrained by a range of destructive pathogens, among which, clubroot disease, caused by a biotrophic protist Plasmodiophora brassicae, has caused significant yield and economic losses worldwide, thereby threatening global food security. To counter the pathogen attack, it demands a better understanding of the complex phenomenon of Brassica-P. brassicae pathosystem at the physiological, biochemical, molecular, and cellular levels. In recent years, multiple omics technologies with high-throughput techniques have emerged as successful in elucidating the responses to biotic and abiotic stresses. In Brassica spp., omics technologies such as genomics, transcriptomics, ncRNAomics, proteomics, and metabolomics are well documented, allowing us to gain insights into the dynamic changes that transpired during host-pathogen interactions at a deeper level. So, it is critical that we must review the recent advances in omics approaches and discuss how the current knowledge in multi-omics technologies has been able to breed high-quality clubroot-resistant B. oleracea. This review highlights the recent advances made in utilizing various omics approaches to understand the host resistance mechanisms adopted by Brassica crops in response to the P. brassicae attack. Finally, we have discussed the bottlenecks and the way forward to overcome the persisting knowledge gaps in delivering solutions to breed clubroot-resistant Brassica crops in a holistic, targeted, and precise way.

Antimicrobial diterpene induced by two gall-inducing adelgids coexisting on Picea koraiensis

The mechanism by which closely related species can coexist is a central factor in the stability of ecological communities. The larch adelgid (Adelges laricis laricis) and the eastern spruce adelgid (Adelges (Sacchiphantes) abietis) have both been found on the branches of Picea koraiensis in China. These two adelgids exhibit strong infectivity and readily induce the formation of 'fish scale-like' and 'pineapple-like' galls with branch parasitism rates of between 75.01 ± 7.03 and 88.02 ± 4.39%. Interestingly, the gall tissues in which these two gall-inducing insects were found to be coexisting were discovered at a rate of ~0.2% in the studied populations. The weight and number of gall chambers as well as the number of adelgids in the 'fish scale-like' side were higher than those in the 'pineapple-like' side. Furthermore, compared with the normal branches, a diterpene neoabietic acid was found at elevated concentrations in the gall tissues, with especially high concentrations seen in the tissues of the co-occupied galls. Neoabietic acid exhibited strong antibacterial activities against Bacillus spp. isolated from the branches of P. koraiensis, as well as potent antifungal activity against the hyphal growth of Fusarium graminearum JMY-1, which was obtained from the gall tissues. Our result provides evidence that the coexistence of the two closely related species could be explained by alterations of the host tissues by the insects resulting in increased concentrations of the antimicrobial agent.