Studies have investigated the interactions between plants through competition and resource sharing to understand the mechanisms behind the positive effects of plant diversity on productivity. Volatile organic compounds (VOCs) are important info-chemicals in plant-plant interactions, but they have so far rarely been considered in this context. Here, we measured VOC emissions at the community scale and for one species (Plantago lanceolata) in experimental plant communities of varying diversity (The Jena Experiment) to understand the role of VOCs in driving biodiversity-ecosystem functioning relationships. We show that plant diversity determines the release of plant VOCs at both scales. At the community level, plant species richness directly enhanced VOC emission and increased VOC richness both directly and indirectly by altering leaf area index. At the species level, plant diversity did not directly affect the VOC emissions of P. lanceolata but indirectly affected it by influencing the VOC emissions from the surrounding community. P. lanceolata individuals in communities with high concentrations of green leaf volatiles decreased their VOC emission, while those in communities with high concentrations of terpenoids increased their VOC diversity. Our results provide evidence that plant diversity shapes community-level plant VOC emission and thus influences focal plant VOC emission inside the community.

Diterpene resin acids are abundant oleoresin compounds in conifer trees and persistent pollutants in pulp mill effluents. The bacterium Pseudomonas abietaniphila BKME-9 has previously been reported to metabolize abietane-type diterpene resin acids, but the complete catabolic pathway and the identities of downstream metabolites are not yet fully elucidated. In this study, time-resolved metabolomics was used to characterize the metabolism of dehydroabietic acid (1) by P. abietaniphila. In addition to the known intermediates 7β-hydroxy-dehydroabietic acid (2), 7-oxo-dehydroabietic acid (3), and 11,12-dihydroxy-7-oxoabieta-8,13-dien-18-oic acid (4), four previously undescribed diterpenoid acids were detected at later incubation times. These new metabolites were identified as 5-hydroxy-dehydroabietic acid (5), 5,7β-dihydroxyabietan-18-oic acid (6), 5-hydroxy-pimara-8-en-18-oic acid (7), and 5-hydroxy-7-oxo-pimara-8-en-18-oic acid (8) by spectroscopic analysis. Time-course data revealed sequential formation and accumulation of these compounds through new oxidations and rearrangements of dehydroabietic acid. These findings expand the known diversity of microbial diterpene metabolites and provide insights into the metabolic network of P. abietaniphila involved in transforming plant-derived diterpenoids.

Terpenoids, also known as isoprenoids, constitute the largest and most structurally diverse class of bioactive natural products, with diverse applications in medicine, food, cosmetics, and bioenergy. However, large-scale extraction of bioactive terpenoids from plants is hampered by low inherent yields and the limited, unsustainable nature of plant resources. Developing high-efficiency microbial cell factories via synthetic biology has emerged as a promising and sustainable alternative for terpenoids production. Cyanobacteria, as photoautotrophic prokaryotes that utilize sunlight and CO2, naturally harbor the methylerythritol 4-phosphate (MEP) pathway for terpenoid precursors synthesis, making them ideal microbial chassis for sustainable terpenoid biosynthesis. Here, we summarize recent advances in cyanobacterial terpenoid biosynthesis, identify critical bottlenecks limiting efficient terpenoid synthesis, and discuss cutting-edge synthetic biology strategies to enhance terpenoids productivity, including CRISPR-based gene editing, RNA interference (RNAi), synthetic microbial consortia, enzyme engineering, and systems metabolic engineering. This work aims to provide valuable insights for advancing the development of high-performance cyanobacterial cell factories, ultimately facilitating the sustainable biosynthesis of terpenoid-based bioactive compounds and other high-value-added natural products.

Plants emit volatile compounds that orchestrate complex ecological interactions, with methylated catabolites of interaction-induced phytohormones being common examples. Salicylic acid (SA) mediates plant antipathogen responses, while its methylated derivative, MeSA, broadly mediates plant-insect interactions without specificity. Here, we identified dimethyl salicylate (DMSA), an unappreciated dimethylated SA catabolite, emitted by rice when attacked by the major destructive pest, the brown planthopper. DMSA biosynthesis requires an O-methyltransferase cascade, BSMT1 (benzoic acid/salicylic acid carboxyl methyltransferase 1)-MSOMT (methyl salicylate O-methyltransferase), which is directly activated by a jasmonate (JA)-responsive MYC2-JAMYB transcriptional cascade. Natural variation in the MSOMT promoter confers its herbivory-induced expression in indica but not japonica cultivars. Functionally, DMSA acts as a specific volatile signal attracting the egg-parasitoid wasps of brown planthoppers (BPHs) without mediating direct resistance, which demonstrably suppresses BPH populations in paddy fields. DMSA is an optimized advance in SA signaling derived plant "alarm calls" with great potential in sustainable rice pest management.

Aboveground induction of plant defense pathways can shape root-associated microbial communities. However, whether these changes are pathway-specific and how they affect plant growth and resistance remains unclear. We evaluated how induction of the Jasmonic Acid (JA) and Salicylic Acid (SA) defense pathways shapes the root microbiome of Brassica oleracea, and whether these soil-mediated shifts affect plant growth and resistance to herbivory in a subsequent generation using a plant-soil feedback (PSF) approach. In the conditioning phase, defense pathways were induced either through foliar application of methyl jasmonate (MeJA) and SA solutions, or through herbivory by caterpillars (JA) and aphids (SA). Both pathways led to distinct shifts in microbial communities, with bacterial and fungal composition varying by pathway identity and induction method. JA induction resulted in more differentially abundant ASVs than SA, particularly with Proteobacteria depletion. Conversely, Planctomycetota (bacteria) and Mortierellomycota (fungi) were enriched under both pathways, suggesting that these represent general stress-responsive groups. In the feedback phase, JA- and SA-conditioned soils had no effect on resistance under high aphid pressure, whereas under low aphid density, plants grown in SA-conditioned soil exhibited reduced phloem feeding and lower aphid population development. Together, our results indicate that benefits provided by the defense-shaped root microbiome depend on pest pressure intensity and arise from overall community shifts rather than specific taxa enrichment. Our findings underscore the complex interactions between plant-defense pathways, rhizosphere microbes, and herbivores.

Healthy plant leaves potentially host both commensal bacteria and opportunistic pathogens, which, under some circumstances, may cause disease. The interactions between commensals and opportunistic pathogens are generally poorly understood, but such understanding is crucial for developing effective biocontrol strategies. In Arabidopsis thaliana, isothiocyanates (ITCs) are defense metabolites that suppress most bacteria; commensals are especially affected as they do not express ITC resistance genes. The ITC hydrolase SaxA detoxifies ITCs, making it an important virulence factor for bacterial and fungal pathogens. To investigate pathogen-commensal interactions based on SaxA-mediated ITC degradation, we used five ITC-sensitive bacterial commensals and the opportunistic pathogen Pseudomonas viridiflava 3D9 (PS). All strains were isolated from healthy A. thaliana leaves. PS degrades 4-methylsulfinylbutyl-ITC (4MSOB-ITC) with SaxA. We examined commensal growth in the presence of 4MSOB-ITC, both in monoculture and in coculture with PS or a saxA-deficient mutant (PSKO). We used the growth data to develop a generalizable consumer-resource mathematical model incorporating ITC toxicity, ITC degradation, and nutrient use. We predicted and confirmed experimentally that the extent to which SaxA benefits the pathogen depends on its effects on commensals. In some contexts, commensal rescue and the resultant nutrient competition limit pathogen growth. In addition, we tested in silico how commensal ITC susceptibility, pathogen ITC degradation rates, and growth parameters affect the trade-off between SaxA-mediated virulence (strong pathogen growth) and commensal rescue (commensal growth). Our findings suggest that the effects of microbial traits-traditionally viewed as either virulence or plant-beneficial factors-are constrained in the microbiome context. This underscores the need to reconsider how such traits are classified in the context of plant-microbiome interactions.IMPORTANCEHealthy plant leaves host a variety of bacteria; these can be beneficial, but some (opportunistic pathogens) can also be harmful under certain conditions. To design effective biocontrol strategies to sustainably protect plants, it is important to understand how opportunistic pathogens thrive as part of a healthy leaf microbiome. Plant defense metabolites, such as isothiocyanates (ITCs), which kill commensal leaf bacteria, and bacterial ITC resistance mechanisms, such as the ITC hydrolase SaxA, which are often expressed in pathogens and degrade ITCs, may play key roles in the plant microbiome composition. In this study, we explore how SaxA-mediated ITC degradation by a pathogen also benefits diverse ITC-sensitive commensals and how this, in turn, could shape microbiome stability and plant health. Using mathematical modeling based on growth data from Pseudomonas viridiflava with diverse commensals, we find that interaction dynamics can be explained by ITC detoxification and nutrient competition. We predict and experimentally confirm that conditions exist under which SaxA favors commensal growth so strongly that the pathogen is outcompeted for resources, thus not benefiting from its own virulence factor. Our findings suggest that the effects of microbial traits, including virulence factors, are context-dependent, especially when functioning as a public good in a community context like SaxA. Moreover, we propose that this concept, which has been known from antibiotic-degrading microbes, may be worth considering as well when studying plant-pathogen interactions under natural conditions where the commensal microbiome might play an important role in plant disease outcomes.

Fungi readily colonize the inner bark (phloem) of spruce and other conifers despite these tissues having a high concentration of antifungal defense metabolites. These compounds include stilbenes, flavonoids and other phenolic substances, mostly present as glucosides. Yet the underlying biochemical mechanisms by which fungi resist conifer phenolics remain largely unresolved. Using untargeted metabolomics, structural elucidation and biological and biochemical assays, we investigated how and why fungi metabolize the major phenolics of Norway spruce (Picea abies). Various fungi, including those associated with bark beetles, were found to hydrolyze stilbene glucosides to their corresponding aglucones. Two basidiomycetes, the saprotroph Coprinellus radians and Cylindrobasidium ipidophilum, a symbiont of the Eurasian spruce bark beetle Ips typographus, then converted the stilbene aglucones into α-ribofuranosylated derivatives, revealing a previously unrecognized pathway in tree-colonizing fungi. Ribosylation markedly reduced the antifungal activity of the aglucones and stabilized them against hydrolysis by fungal and I. typographus enzymes, preventing regeneration of the toxic aglucones. Ribosylation was also correlated with increased growth on spruce bark-containing medium. Hence, the ability to overcome major conifer bark defenses by conversion of toxic stilbene aglucones to non-toxic α-ribosides may explain the successful colonization of this tissue by fungi, some of which support I. typographus attack.

Conifers are a challenging host for herbivores since their tissues are very low in essential nutrients but high in chemical defenses. For herbivorous insects, such as phloem-colonizing bark beetles, mutualistic fungi may improve their diet by providing a nutritious mycelium. A recent study revealed that two filamentous fungi are mutualists of the European fir engraver beetle Pityokteines vorontzowi, but a potential nutritional contribution of the fungi, as well as their capability to degrade plant antiherbivore defenses remains unknown. We analyzed the nutrient content of the fungal mutualists Ophiostoma piceae and Geosmithia sp. F1 and examined their ability to degrade the constitutive chemical defenses of silver fir phloem in comparison to other fungi. Both mutualists turned out to be rich in amino acids, sugars, and B vitamins and were found to efficiently deplete their phloem media of several defenses. Strikingly, O. piceae not only accumulated the highest amounts of the B vitamin nicotinic acid of the 17 tested fungi but also showed a high ability to deplete its medium of chemical defenses, similar to the behavior of the Ips typographus mutualist Endoconidiophora polonica. Beetle-vectored, non-mutualistic fungi isolated from P. vorontzowi showed similar capacities to deplete defensive compounds, whereas non-fir-associated fungi were less effective in reducing their concentrations in the phloem medium. The nutritious mycelium of O. piceae and Geosmithia sp. F1 and the ability of these fungi to deplete the medium of major fir defense compounds likely facilitates the colonization of silver fir phloem by P. vorontzowi.

Taxane diterpenoids exhibit high scaffold diversity, with 6/8/6 and 5/7/6 frameworks being the most common. 5/7/6 taxoids, exemplified by taxuspine J, display promising pharmacological activities, but their chemical diversity and biosynthesis remain largely unexplored. Here, we characterized two novel 2-oxoglutarate-dependent dioxygenases, TcOGD1 and TcOGD2, which exhibit remarkable catalytic versatility, including skeleton reconstructions from 6/8/6 taxanes to 5/7/6, trinor-5/7/6, and 6/12 frameworks, and oxidation modifications at C-5, C-6, C-16, or C-20 of various taxoids. Specifically, they catalyze the skeletal rearrangement of 2-deacetoxytaxinine J (a 6/8/6 taxoid) and subsequent oxidation modifications to produce taxuspine J and a panel of 15,16,17-trinor-5/7/6 taxoids, which were identified through scale-up enzymatic reactions and NMR spectroscopy. Mechanistic investigation reveals a substrate-specific oxidative degradation for constructing the trinor-5/7/6 scaffold via radical-mediated C-C bond cleavage, accompanied by acetone release. In addition, TcOGD1 catalyzes two distinct transformations of a new Δ5,6 containing 6/8/6 taxoid isolated from Taxus chinensis var. mairei, including C-C bond cleavage coupled with aldehyde formation to form a unique highly unsaturated 6/12 taxoid, and C-16 aldehyde formation to yield a 6/8/6 taxoid. TcOGD1 also mediates the 5-OH oxidation of a 6/10/6 taxoid. Furthermore, five novel trinor-5/7/6 taxoids were isolated from the Taxus plant, and taxuspine J and trinor-5/7/6 taxoids showed effective tumor resistance reversal activity. Our findings offered new insights into the formation of taxane structural diversity, expanded the chemical space of taxane diterpenoids, and provided valuable biocatalytic tools for constructing unusual taxane architectures through enzyme engineering or synthetic biology.

Terpenoid biosynthesis involves linear prenyl diphosphate intermediates of various chain lengths. These are constructed from 2 C5 precursors, the starter unit dimethylallyl diphosphate (DMADP) and the extender unit, isopentenyl diphosphate (IDP). Isopentenyl diphosphate isomerase (IDI) alters the DMADP:IDP ratio and may furnish a specific blend of C5 precursors appropriate for the length of intermediates being formed in each cellular compartment. We studied IDI in two woody plant species, Norway spruce (Picea abies) and gray poplar (Populus × canescens), whose major terpenoid specialized metabolites are of different sizes. While the catalytic parameters of IDI from each species measured in vitro were in line with the different C5 precursor demands, the DMADP:IDP ratios of both species in vivo did not differ. Moreover, although IDI silencing in both spruce and poplar increased IDP content and significantly decreased the DMADP:IDP ratio, it caused a few significant alterations in the content of downstream terpenoid pathway intermediates or products. Taken together, these results suggest that IDI exercises a limited control over the relative amounts of different size terpenoid products. Nevertheless, the elevated IDP content of both transgenic spruce and poplar lines was associated with dramatically increased emission of isoprenol and isoprenyl acetate. Feeding experiments with cultured poplar plants indicated that these metabolites were derived directly from IDP, and their formation could serve as a metabolic mechanism to reduce high intracellular accumulation of IDP. Such a mechanism can be considered analogous to the formation of isoprene as a way to reduce high concentrations of DMADP.

Pea (Pisum sativum L.), a major legume crop, is affected by various parasites including the pea aphid (Acyrthosiphon pisum Harris). The pea aphid is composed of multiple biotypes, each one being able to feed and reproduce on one or a few legume species. To understand the pea defense mechanisms to a pea adapted and a non-adapted A. pisum biotype, we studied the early molecular responses of four pea genotypes with contrasted levels of resistance, which are controlled primarily by the ApRVII locus. We found that major defense-related phytohormones and their derivatives in pea did not show clear response to aphid infestations. Transcriptomic analyses showed that the number of differentially expressed genes (DEGs) increased over time in pea genotypes infested with pea-adapted aphids, while significantly fewer DEGs were detected in genotypes infested with non-adapted aphids. The most resistant of the four investigated pea genotypes showed the fewest DEGs to both aphid biotypes. Aphid infestation of the three other pea genotypes commonly induced down-regulation of various pathways involved in fundamental biological processes. Comparison of the transcriptional data of pea genotypes identified candidate genes potentially involved in the aphid resistance conferred by ApRVII.

Plants produce a plethora of specialized metabolites that often play important roles in their defence against pathogenic microbes or herbivorous insects. Exposure of leaf-colonizing microbes to these metabolites influences their growth, and we hypothesize that it also has consequences for microbe-microbe interactions. In Brassicaceae plants like the model plant Arabidopsis thaliana, glucosinolates and their biologically active derivatives, the isothiocyanates, are major defence metabolites. Adapted plant pathogens like Pseudomonas spp. use the hydrolase SaxA to convert the antimicrobial isothiocyanate sulforaphane to a non-toxic amine, whereas non-adapted commensal microbes are inhibited by this plant toxin. We used Plantibacter sp. 2H11-2 as a model commensal in co-culture with either Pseudomonas viridiflava 3D9 wildtype or a saxA-knock-out mutant. Both strains were isolated from the same wild A. thaliana population. Without isothiocyanate, Plantibacter grew better alone than with Pseudomonas, a potential competitor. At high isothiocyanate concentrations, however, the commensal was dependent on SaxA-mediated isothiocyanate degradation in both solid and liquid medium. At intermediate isothiocyanate concentrations, Plantibacter's transcriptome changed in response to sulforaphane in monoculture but not in co-culture with Pseudomonas, suggesting that it was fully protected from this toxin. In return, Plantibacter caused transcriptional changes in Pseudomonas, suppressing biofilm formation and increasing amino acid metabolism gene expression which might suppress virulence and so contribute to plant health. Together, we find that degradation of an antimicrobial plant metabolite can protect a commensal to depend on a pathogen-produced virulence factor, suggesting effects on community composition in environments where microbes are exposed to ITCs.

Carbon allocation plays an important role in determining tree productivity and survival under environmental change. However, our understanding of how allocation patterns and their responses to drought vary among diverse functional types in tropical forests remains limited. In a tropical forest equipped with an 80-m canopy crane, we measured leaf gas exchange and water status, leaf nonstructural carbohydrates (NSCs) and phenolics, stem growth, and crown characteristics of mature trees from 18 species spanning different canopy positions, water-use and growth strategies during both the wet and dry seasons. The results show that tall canopy trees experienced stronger VPD and water stress and greater reductions in leaf gas exchange than short understory trees, leading to declines in NSCs (particularly starch) in canopy trees but increases in understory trees in the dry season. Despite changes in carbon supply, leaf phenolic levels remained remarkably stable across species, with species-specific variation explained by tree height and herbivory. With increasing height, both whole-tree leaf phenolics and NSCs increased whereas stem growth varied among canopy species. We highlight that canopy position–driven differences in resource availability and environmental stress are key for understanding and predicting carbon balance and allocation strategies in tropical forests experiencing seasonal droughts.

Paclitaxel, a clinically potent anticancer drug derived from Taxus species, faces persistent challenges in sustainable supply. Synthetic biology presents substantial opportunities for its de novo production, particularly with recent breakthroughs in elucidating its intricate biosynthetic pathways. However, its heterologous biosynthesis is significantly constrained by key bottlenecks, including pathway complexity, poor P450 expression, and inefficient metabolic flux. In this study, we explore how synthetic biology facilitates pathway decoding and reconstruction and propose strategies involving nonclassical chassis such as plant-associated cyanobacteria and filamentous fungi to enhance P450 compatibility. We also present a pragmatic framework for the rational application of state-of-the-art tools, including cell-free systems, synthetic microbial consortia, hybrid chemoenzymatic synthesis, and machine learning, to sustainably produce paclitaxel and other natural products.

Chemical defences, such as the monoterpenes of conifer oleoresin, frequently occur as complex blends of many components, but the selective pressures that maintain these mixtures are not yet known. Several theories attempt to explain the existence of chemical defence mixtures in plants. However, due to limited empirical evidence, it is unclear which theories might best apply. Here, we tested the vapour phase activity of 12 individual Norway spruce monoterpenes and their naturally occurring mixtures to two types of natural spruce enemies, the adult Eurasian spruce bark beetles, Ips typographus, and their three major symbiotic fungi, using survival and growth bioassays. Next, we evaluated whether spruce trees could alter their monoterpene profile in response to fungal infection. Individual monoterpenes had generally opposite effects on bark beetles compared to symbiotic fungi. The compounds that were most toxic to beetles were the least inhibitory to fungal growth and vice versa. The least abundant monoterpenes had the strongest activity against beetles or fungi, while the most abundant monoterpenes showed intermediate activity against both groups of enemies. Additionally, the activity of monoterpene mixtures was significantly stronger against beetles and some symbiotic fungi than the additive effects of individual compounds. Among the symbiotic fungi tested, one (Grosmannia penicillata) exhibited high tolerance to monoterpenes, and its growth was even stimulated by the monoterpenes most toxic to the beetle. Interestingly, spruce bark responded to G. penicillata inoculation by accumulating higher concentrations of specifically fungistatic monoterpenes. Our results support the predictions of the interaction diversity hypothesis, which posits that defence mixtures are maintained in plants because the individual components target different attackers, as well as the synergy hypothesis, which predicts that mixtures will exhibit stronger activity than single compounds. Thus, these two theories may deserve increased emphasis in explaining the widespread occurrence of mixtures in plant chemical defence.Read the free Plain Language Summary for this article on the Journal blog.

Fungal endophytes of grasses and other herbaceous plants have been known to provide plants with anti-herbivore defence compounds, but there is little information about whether the endophytes of trees also engage in such mutualisms. We investigated the influence of the endophytic fungus Cladosporium sp. on the chemical defences of black poplar (Populus nigra) trees and the consequences for feeding preference and fitness of herbivorous insects and insect community assembly. Endophyte colonisation increased both constitutive- and induced poplar defences. Generalist Lymantria dispar larvae preferred and performed better on uninfected over endophyte-infected poplar leaves, most likely due to higher concentrations of salicinoids in endophyte-inoculated leaves and the endophyte-produced alkaloid stachydrine. Under field conditions, the endophytic fungus shapes insect community assembly i. a. attracting aphids, which can excrete stachydrine. Our results show that endophytic fungi play a crucial role in the defence against insects from different feeding guilds and thereby structuring insect communities.

Pea (Pisum sativum L.) is a major legume crop frequently infested by various parasites, including the pea aphid (Acyrthosiphon pisum Harris), which takes nutrients from its host and transmits phytopathogenic viruses. This aphid species comprises several biotypes, each one capable of feeding and reproducing on a limited range of specific legume species. However, little is known about the transcriptional response of the plants to different aphid biotypes. To investigate pea defense mechanisms against the pea-adapted and pea non-adapted A. pisum biotypes, we analyzed the transcriptional responses by mRNA sequencing of six pea genotypes with contrasting resistance levels and different haplotypes at the ApRVII locus. This locus confers partial resistance to the pea-adapted and non-adapted A. pisum biotypes. Transcriptomic analyses of pea genotypes with and without aphid infestation revealed 9,217 differentially expressed genes in pea genotypes infested with the pea-adapted aphids, while fewer genes, 1,561 genes total, were expressed in response to the pea non-adapted aphids. Both aphid biotypes activated immune responses and the biosynthesis of secondary metabolites, including flavonoids. However, the pea-adapted biotype appeared to suppress multiple pathways associated with photosynthesis, cell wall biosynthesis, fatty acid metabolism, and other growth-related processes. Candidate genes potentially involved in aphid resistance were identified both within and outside of the ApRVII locus. These findings provide insights into pea resistance mechanisms against both pea-adapted and pea non-adapted A. pisum biotypes, as well as the ability of the adapted biotype to modulate pea defenses leading to host susceptibility, and pave the way for follow-up studies, including metabolomic analyses.

Healthy plant leaves host both commensal bacteria, which usually do not cause harm, and opportunistic pathogens, which under the right circumstances can cause disease. Microbial and plant-derived factors can potentially govern the balance between commensals and pathogens; understanding this dynamic is crucial for developing effective biocontrol strategies. In Arabidopsis thaliana , isothiocyanates (ITCs) are toxic defense metabolites that suppress most bacteria. An important virulence factor for bacterial and fungal pathogens of A. thaliana is the ITC hydrolase SaxA, which detoxifies ITCs. To investigate microbial interactions based on SaxA-mediated ITC degradation, we used five ITC-sensitive bacterial commensals and the opportunistic pathogen Pseudomonas viridiflava 3D9 (PS). All strains were isolated from healthy A. thaliana leaves and PS degrades 4-methylsulfinylbutyl-ITC (4MSOB-ITC) with SaxA. We examined their growth in the presence of 4MSOB-ITC, both in monoculture and in coculture with PS or a saxA -deficient mutant (PSKO). Based on experimental growth data, we developed a generalizable consumer-resource mathematical model incorporating ITC toxicity, ITC degradation, and nutrient use. We predicted conditions and confirmed them experimentally under which SaxA not only benefits the pathogen but also indirectly favors commensal growth, which then can limit pathogen proliferation by competing for nutrients. In addition, we tested in silico how commensal ITC susceptibility, pathogen ITC degradation rates, and growth parameters affect the trade-off between SaxA-mediated virulence (strong pathogen growth) and high commensal rescue (commensal growth). Our findings suggest that the effects of microbial traits - traditionally viewed as either virulence or plant-beneficial factors - are context-dependent. This underscores the need to reconsider how such traits are classified in the context of plant-microbiome interactions. Author Summary Healthy plant leaves host a variety of bacteria; these can be beneficial, but some (opportunistic pathogens) can also be harmful under certain conditions. To design effective biocontrol strategies to sustainably protect plants, it is important to understand how opportunistic pathogens thrive as part of a healthy, balanced leaf microbiome. Plant defense metabolites such as isothiocyanates (ITCs) and bacterial ITC resistance mechanisms such as the ITC hydrolase SaxA may play key roles in maintaining this balance. In this study, we explore how SaxA-mediated ITC degradation by a pathogen also benefits diverse ITC-sensitive commensals and how this in turn could shape microbiome stability and plant health. Using mathematical modeling based on growth data from PS with diverse commensals, we find that interaction dynamics can be explained by ITC detoxification and nutrient competition. We predict and experimentally confirm that conditions exist under which SaxA favors commensal growth so strongly that the pathogen is outcompeted for resources, thus not benefiting from its own virulence factor. Our findings suggest that the effects of microbial traits are context-dependent, especially when functioning as public good in a community context like SaxA. Taken together, quantitative modeling of these interactions may inform strategies to maintain healthy plant microbiomes and control disease. ### Competing Interest Statement The authors have declared no competing interest. Jena School for Microbial Communication Deutsche Forschungsgemeinschaft, https://ror.org/018mejw64, Project-ID 390713860 Max Planck Society, https://ror.org/01hhn8329

After consumption by herbivores, plant antimicrobial defense compounds may enhance herbivore immunity to pathogenic microbes. In conifer-bark beetle interactions, beetles ingest large quantities of phloem tissue containing high concentrations of antimicrobial phenolic glucosides, such as stilbenes and flavonoids. It is not known, however, if these compounds increase bark beetle resistance to pathogens. We showed that Eurasian spruce bark beetles (Ips typographus) attacking Norway spruce (Picea abies) hydrolyze phenolic glucosides to their corresponding aglucones increasing their antifungal activity. However, the entomopathogen Beauveria bassiana, a natural fungal parasite of these beetles, detoxifies stilbene and flavonoid aglucones by forming methylglucoside derivatives. A two-step pathway involving a UDP-glycosyltransferase and an O-methyltransferase produces phenolic O-methylglucosides that are no longer toxic to B. bassiana and are stable to β-glucosidase action. Compared to wild-type strains of B. bassiana, mutant strains knocked out in the genes of this pathway exhibited decreased methylglucoside formation, slower growth on medium containing phenolic compounds, and reduced virulence toward bark beetles. Hence, methylglucosylation of plant-derived phenolics is a detoxification process that significantly increases the ability of B. bassiana to parasitize host insects consuming plant tissue high in phenolics, such as conifer phloem. This is one of the few examples of an entomopathogen that is able to resist the plant-derived defenses of an insect host.