Acylsugars protect Nicotiana benthamiana against insect herbivory and desiccation

Acylsugars, Nicotine and a Protease Inhibitor Provide Variable Protection for Nicotiana benthamiana in a Natural Setting

Plants produce an immense diversity of defensive specialized metabolites. However, despite extensive functional characterization, the relative importance of different defensive compounds is rarely examined in natural settings. Here, we compare the efficacy of three Nicotiana benthamiana defensive compounds, nicotine, acylsugars and a serine protease inhibitor, by growing plants with combinations of knockout mutations in a natural setting, quantifying invertebrate interactions and comparing relative plant performance. Among the three tested compounds, acylsugars had the greatest defensive capacity, affecting aphids, leafhoppers, spiders and flies. Nicotine mutants displayed increased leafhopper feeding and aphid colonization. Plants lacking both nicotine and acylsugars were more susceptible to flea beetles and thrips. By contrast, knockout of the serine protease inhibitor did not affect insect herbivory in the field. Complementary experiments under controlled laboratory conditions with caterpillars, grasshoppers and aphids confirmed results obtained in a natural setting. We conclude that the three metabolite groups collectively provide broad-spectrum protection to N. benthamiana. However, there is a gradient in their effects on the interacting invertebrates present in the field. Furthermore, we demonstrate that, even if individual metabolites do not have a measurable defensive benefit on their own, they can have an additive effect when combined with other defensive compounds.

Enabling biocontained plant virus transmission studies through establishment of an axenic whitefly (Bemisia tabaci) colony on plant tissue culture

Whiteflies (Bemisia tabaci) and the diseases they transmit are a major detriment to crop yields and a significant contributor to world hunger. The highly evolved interactions of host plant, phloem-feeding insect vector with endosymbionts and persistently transmitted virus represent a tremendous challenge for interdisciplinary study. Presented here is the establishment of a colony of axenic whiteflies on tissue-cultured plants. Efficient colony establishment was achieved by a surface sterilization of eggs laid on axenic phototrophically tissue-cultured plants. The transfer of emerging whiteflies through coupled tissue culture vessels to new axenic plants facilitates robust subculturing and produces hundreds of whitefly adults per month. Whitefly proliferation on more than two dozen plant species is shown as well as in vitro testing of whitefly preference for different plants. This novel multi-organism system provides the high-level of biocontainment required by Federal permitting to conduct virus transmission experiments. Axenic whitefly adults were able to acquire and transmit a begomovirus into tissue-cultured plants, indicating that culturable gut microorganisms are not required for virus transmission. The approach described enables a wide range of hypotheses regarding whitefly phytopathology without the expense, facilities, and contamination ambiguity associated with current approaches.

Detailed Profiling of 17-Hydroxygeranyllinalool Diterpene Glycosides from Nicotiana Species Reveals Complex Reaction Networks of Conjugation Isomers

BACKGROUND

Specialised anti-herbivory metabolites are abundant in the solanaceous genus Nicotiana. These metabolites include the large family of 17-hydroxygeranyllinalool diterpene glycosides (HGL-DTGs). Many HGL-DTGs occur exclusively within the Nicotiana genus, but information from the molecular model species N. tabacum, N. benthamiana, and the tree tobacco N. glauca is limited.

OBJECTIVES

We studied HGL-DTG occurrence and complexity in these species with the aim of providing in-depth reference annotations and comprehensive HGL-DTG inventories.

METHODS

We analysed polar metabolite extracts in comparison to the previously investigated wild reference species N. attenuata using positive ESI(+) and negative ESI(-) mode electrospray ionisation LC-MS and MS/MS.

RESULTS

We provide annotations of 66 HGL-DTGs with in-source and MS/MS fragmentation spectra for selected HGL-DTGs with exemplary fragment interpretations of ESI(+) as well as less studied ESI(-) spectra. We assemble a potential biosynthesis pathway comparing the presence of HGL-DTGs in N. tabacum, N. glauca, and N. benthamiana to N. attenuata. Approximately one-third of HGL-DTGs are chromatographically resolved isomers of hexose, deoxyhexose, or malonate conjugates. The number of isomers is especially high for conjugates with low numbers of deoxyhexose moieties.

CONCLUSIONS

We extend the number of known HGL-DTGs with a focus on Nicotiana model species and demonstrate that the HGL-DTG family of N. tabacum plants can be surprisingly complex. Our study provides an improved basis with detailed references to previous studies of wild Nicotiana species and enables inference of HGL-DTG pathways with required enzymes for the biosynthesis of this important family of specialised defence metabolites.

The nicotine demethylase CYP82E4 is essential for the formation of red dapples on flue-cured leaves of cherry-red tobacco

Common flue-cured tobacco (Nicotiana tabacum L.) primarily accumulates nicotine, and its flue-cured leaves exhibit a lemon appearance. In contrast, a spontaneous cherry-red variant (CR60) primarily accumulates nornicotine, accompanied by distinctive red dapples on the cured leaves. In this study, suppression of conversion of nicotine to nornicotine by genome editing resulted in decreased nornicotine and N-acyl nornicotines (NacNNs), and the subsequent disappearance of red dapples in CR60. Conversely, overexpression of CYP82E4 increased nornicotine and NacNNs accumulation, inducing a red dapple phenotype in common tobacco. Notably, nicotine conversion triggered significant alterations in leaf total sugars, alkaloids, and nitrogens. Metabolome analyses using 1352 identified compounds indicated nicotine conversion dramatically affected the entire metabolic network and induced unique metabolic responses across diverse genetic backgrounds. Further WGCNA analysis revealed that nicotine conversion caused substantial contents variation of alkaloids, flavonoids and amino acids and derivatives in cured leaves. Overall, this research provides valuable insights into the mechanisms underlying red dapple formation in cherry-red tobacco, elucidating profound influence of nicotine conversion on entire metabolic network.

Metabolic and Molecular Insights Into Nicotiana benthamiana Trichome Exudates: An Ammunition Depot for Plant Resistance Against Insect Pests

Nicotiana benthamiana, a widely acknowledged laboratory model plant for molecular studies, exhibits lethality to certain insect pests and can serve as a dead-end trap plant for pest control in the field. However, the underlying mechanism of N. benthamiana's resistance against insects remains unknown. Here, we elucidate that the lethal effect of N. benthamiana on the whitefly Bemisia tabaci arises from the toxic glandular trichome exudates. By comparing the metabolite profiles of trichome exudates, we found that 51 metabolites, including five O-acyl sugars (O-AS) with medium-chain acyl moieties, were highly accumulated in N. benthamiana. Silencing of two O-AS biosynthesis genes, branched-chain keto acid dehydrogenase (BCKD) and Isopropyl malate synthase-C (IPMS-C), significantly reduced the O-AS levels in N. benthamiana and its resistance against whiteflies. Additionally, we demonstrated that the higher expression levels of BCKD and IPMS-C in the trichomes of N. benthamiana contribute to O-AS synthesis and consequently enhance whitefly resistance. Furthermore, overexpression of NbBCKD and NbIPMS-C genes in the cultivated tobacco Nicotiana tabacum enhanced its resistance to whiteflies. Our study revealed the metabolic and molecular mechanisms underlying the lethal effect of N. benthamiana on whiteflies and presents a promising avenue for improving whitefly resistance.

Serine proteinase inhibitors from Nicotiana benthamiana, a nonpreferred host plant, inhibit the growth of Myzus persicae (green peach aphid)

BACKGROUND

The green peach aphid (Myzus persicae) is a severe agricultural crop pest that has developed resistance to most current control methods, requiring the urgent development of novel strategies. Plant proteinase inhibitors (PINs) are small proteins that protect plants against pathogens and/or herbivores, likely by preventing efficient protein digestion.

RESULTS

We identified 67 protease genes in the transcriptomes of three M. persicae lineages (USDA-Red, G002 and G006). Comparison of gene expression levels in aphid guts and whole aphids showed that several proteases, including a highly expressed serine protease, are significantly overexpressed in the guts. Furthermore, we identified three genes encoding serine protease inhibitors (SerPIN-II1, 2 and 3) in Nicotiana benthamiana, which is a nonpreferred host for M. persicae. Using virus-induced gene silencing (VIGS) with a tobacco rattle virus (TRV) vector and overexpression with a turnip mosaic virus (TuMV) vector, we demonstrated that N. benthamiana SerPIN-II1 and SerPIN-II2 cause reduced survival and growth, but do not affect aphid protein content. Likewise, SerPIN-II3 overexpression reduced survival and growth, and serpin-II3 knockout mutations, which we generated using CRISPR/Cas9, increased survival and growth. Protein content was significantly increased in aphids fed on SerPIN-II3 overexpressing plants, yet it was decreased in aphids fed on serpin-II3 mutants.

CONCLUSION

Our results show that three PIN-IIs from N. benthamiana, a nonpreferred host plant, effectively inhibit M. persicae survival and growth, thereby representing a new resource for the development of aphid-resistant crop plants. © 2024 Society of Chemical Industry.

Tomato root specialized metabolites evolved through gene duplication and regulatory divergence within a biosynthetic gene cluster

Tremendous plant metabolic diversity arises from phylogenetically restricted specialized metabolic pathways. Specialized metabolites are synthesized in dedicated cells or tissues, with pathway genes sometimes colocalizing in biosynthetic gene clusters (BGCs). However, the mechanisms by which spatial expression patterns arise and the role of BGCs in pathway evolution remain underappreciated. In this study, we investigated the mechanisms driving acylsugar evolution in the Solanaceae. Previously thought to be restricted to glandular trichomes, acylsugars were recently found in cultivated tomato roots. We demonstrated that acylsugars in cultivated tomato roots and trichomes have different sugar cores, identified root-enriched paralogs of trichome acylsugar pathway genes, and characterized a key paralog required for root acylsugar biosynthesis, SlASAT1-LIKE (SlASAT1-L), which is nested within a previously reported trichome acylsugar BGC. Last, we provided evidence that ASAT1-L arose through duplication of its paralog, ASAT1, and was trichome-expressed before acquiring root-specific expression in the Solanum genus. Our results illuminate the genomic context and molecular mechanisms underpinning metabolic diversity in plants.

Discovery of Three Bemisia tabaci Effectors and Their Effect on Gene Expression in Planta

Bemisia tabaci (whitefly) is a polyphagous agroeconomic pest species complex. Two members of this species complex, Mediterranean (MED) and Middle-East-Asia Minor 1 (MEAM1), have a worldwide distribution and have been shown to manipulate plant defenses through effectors. In this study, we used three different strategies to identify three MEAM1 proteins that can act as effectors. Effector B1 was identified using a bioinformatics-driven effector-mining strategy, whereas effectors S1 and P1 were identified in the saliva of whiteflies collected from artificial diet and in phloem exudate of tomato on which nymphs were feeding, respectively. These three effectors were B. tabaci specific and able to increase whitefly fecundity when transiently expressed in tobacco plants (Nicotiana tabacum). Moreover, they reduced growth of Pseudomonas syringae pv. tabaci in Nicotiana benthamiana. All three effectors changed gene expression in planta, and B1 and S1 also changed phytohormone levels. Gene ontology and KEGG pathway enrichment analysis pinpointed plant-pathogen interaction and photosynthesis as the main enriched pathways for all three effectors. Our data thus show the discovery and validation of three new B. tabaci MEAM1 effectors that increase whitefly fecundity and modulate plant immunity. [Formula: see text] Copyright © 2024 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Molecular basis of methyl-salicylate-mediated plant airborne defence

Aphids transmit viruses and are destructive crop pests1. Plants that have been attacked by aphids release volatile compounds to elicit airborne defence (AD) in neighbouring plants2-5. However, the mechanism underlying AD is unclear. Here we reveal that methyl-salicylate (MeSA), salicylic acid-binding protein-2 (SABP2), the transcription factor NAC2 and salicylic acid-carboxylmethyltransferase-1 (SAMT1) form a signalling circuit to mediate AD against aphids and viruses. Airborne MeSA is perceived and converted into salicylic acid by SABP2 in neighbouring plants. Salicylic acid then causes a signal transduction cascade to activate the NAC2-SAMT1 module for MeSA biosynthesis to induce plant anti-aphid immunity and reduce virus transmission. To counteract this, some aphid-transmitted viruses encode helicase-containing proteins to suppress AD by interacting with NAC2 to subcellularly relocalize and destabilize NAC2. As a consequence, plants become less repellent to aphids, and more suitable for aphid survival, infestation and viral transmission. Our findings uncover the mechanistic basis of AD and an aphid-virus co-evolutionary mutualism, demonstrating AD as a potential bioinspired strategy to control aphids and viruses.

Plant resistance against whitefly and its engineering

Plants face constant threats from insect herbivores, which limit plant distribution and abundance in nature and crop productivity in agricultural ecosystems. In recent decades, the whitefly Bemisia tabaci, a group of phloem-feeding insects, has emerged as pests of global significance. In this article, we summarize current knowledge on plant defenses against whitefly and approaches to engineer plant resistance to whitefly. Physically, plants deploy trichome and acylsugar-based strategies to restrain nutrient extraction by whitefly. Chemically, toxic secondary metabolites such as terpenoids confer resistance against whitefly in plants. Moreover, the jasmonate (JA) signaling pathway seems to be the major regulator of whitefly resistance in many plants. We next review advances in interfering with whitefly-plant interface by engineering of plant resistance using conventional and biotechnology-based breeding. These breeding programs have yielded many plant lines with high resistance against whitefly, which hold promises for whitefly control in the field. Finally, we conclude with an outlook on several issues of particular relevance to the nature and engineering of plant resistance against whitefly.

Dangerous sugars: Structural diversity and functional significance of acylsugar-like defense compounds in flowering plants

Acylsugars constitute a diverse class of secondary metabolites found in many flowering plant families. Comprising sugar cores and acyl groups connected by ester and/or ether linkages, acylsugar structures vary considerably at all taxonomic levels - from populations of the same species to across species of the same family and across flowering plants, with some species producing hundreds of acylsugars in a single organ. Acylsugars have been most well-studied in the Solanaceae family, but structurally analogous compounds have also been reported in the Convolvulaceae, Martyniaceae, Geraniaceae, Rubiaceae, Rosaceae and Caryophyllaceae families. Focusing on Solanaceae and Convolvulaceae acylsugars, this review highlights their structural diversity, the potential biosynthetic mechanisms that produce this diversity, and its functional significance. Finally, we also discuss the possibility that some of this diversity is merely "noise", arising out of enzyme promiscuity and/or non-adaptive evolutionary mechanisms.

Horizontally transferred genes as RNA interference targets for aphid and whitefly control

RNA interference (RNAi)-based technologies are starting to be commercialized as a new approach for agricultural pest control. Horizontally transferred genes (HTGs), which have been transferred into insect genomes from viruses, bacteria, fungi or plants, are attractive targets for RNAi-mediated pest control. HTGs are often unique to a specific insect family or even genus, making it unlikely that RNAi constructs targeting such genes will have negative effects on ladybugs, lacewings and other beneficial predatory insect species. In this study, we sequenced the genome of a red, tobacco-adapted isolate of Myzus persicae (green peach aphid) and bioinformatically identified 30 HTGs. We then used plant-mediated virus-induced gene silencing (VIGS) to show that several HTGs of bacterial and plant origin are important for aphid growth and/or survival. Silencing the expression of fungal-origin HTGs did not affect aphid survivorship but decreased aphid reproduction. Importantly, although there was uptake of plant-expressed RNA by Coccinella septempunctata (seven-spotted ladybugs) via the aphids that they consumed, we did not observe negative effects on ladybugs from aphid-targeted VIGS constructs. To demonstrate that this approach is more broadly applicable, we also targeted five Bemisia tabaci (whitefly) HTGs using VIGS and demonstrated that knockdown of some of these genes affected whitefly survival. As functional HTGs have been identified in the genomes of numerous pest species, we propose that these HTGs should be explored further as efficient and safe targets for control of insect pests using plant-mediated RNA interference.

Acylsugar-mediated resistance as part of a multilayered defense against thrips, orthotospoviruses, and beyond

Resistant varieties are critical tools for crop production, and single-resistance genes providing strong protection against pests or pathogens are deployed in agriculture. Durability of these traits is threatened by emergence of resistance-breaking pests and pathogens. This review focuses on acylsugar-mediated resistance in tomato. Wild tomatoes have type-IV trichomes that exude chemically complex mixtures of acylsugars altering behavior and suppressing multiple pest species, and with thrips and whiteflies (WF), suppressing virus transmission, for example, Tomato spotted wilt orthotospovirus and Tomato yellow leaf curl virus, respectively. Marker-assisted selection and bioassays led to development of advanced cultivated tomato breeding lines rich in acylsugar variations, allowing acylsugar-mediated resistance to be combined with other resistance traits providing a layered defense system that reduces pest populations and virus disease prevalence. This strategy also holds promise for enhancing durability of virus resistance genes by reducing the intensity of selection for resistance-breaking variants.

Acylsugar protection of Nicotiana benthamiana confers mortality and transgenerational fitness costs in Spodoptera litura

Acylsugars are secondary metabolites that are produced in the trichomes of some solanaceous species and can help control several herbivorous insect pests. Previously, knockout mutations (asat2 mutants) were shown to significantly reduce the acylsugar content of Nicotiana benthamiana, and significantly improve the fitness of six generalist insect herbivores. The current study compared the significant mortality and fitness costs in Spodoptera litura conferred by acylsugar protection of N. benthamiana (wild-type plants) compared to S. litura strains reared in acylsugar-deficient plants with depleted acylsugar biosynthesis. Acylsugar protection prolonged the developmental duration and decreased viability in the larval stages. Further, the fecundity of females and the hatching rate of eggs significantly decreased under acylsugar protection. For F₁ offspring, acylsugar protection still exerted significant negative effects on larval survival rate and fecundity per female. The net reproductive rate and relative fitness of the S. litura strain were strongly affected by acylsugar. Altogether, these results indicate that acylsugar could contribute to plant protection due to toxicity to pests, diffused availability, and low environmental persistence. This could represent a complementary and alternative strategy to control populations of insect pests.

Characterization of trichome-specific BAHD acyltransferases involved in acylsugar biosynthesis in Nicotiana tabacum

Glandular trichomes of tobacco (Nicotiana tabacum) produce blends of acylsucroses that contribute to defence against pathogens and herbivorous insects, but the mechanism of assembly of these acylsugars has not yet been determined. In this study, we isolated and characterized two trichome-specific acylsugar acyltransferases that are localized in the endoplasmic reticulum, NtASAT1 and NtASAT2. They sequentially catalyse two additive steps of acyl donors to sucrose to produce di-acylsucrose. Knocking out of NtASAT1 or NtASAT2 resulted in deficiency of acylsucrose; however, there was no effect on acylsugar accumulation in plants overexpressing NtASAT1 or NtASAT2. Genomic analysis and profiling revealed that NtASATs originated from the T subgenome, which is derived from the acylsugar-producing diploid ancestor N. tomentosiformis. Our identification of NtASAT1 and NtASAT2 as enzymes involved in acylsugar assembly in tobacco potentially provides a new approach and target genes for improving crop resistance against pathogens and insects.