'Real time' genetic manipulation: a new tool for ecological field studies

Seed endophytic bacterium Lysinibacillus sp. (ZM1) from maize (Zea mays L.) shapes its root architecture through modulation of auxin biosynthesis and nitrogen metabolism

Seed endophytic bacteria have been shown to promote the growth and development of numerous plants. However, the underlying mechanism still needs to be better understood. The present study aims to investigate the role of a seed endophytic bacterium Lysinibacillus sp. (ZM1) in promoting plant growth and shaping the root architecture of maize seedlings. The study explores how bacteria-mediated auxin biosynthesis and nitrogen metabolism affect plant growth promotion and shape the root architecture of maize seedlings. The results demonstrate that ZM1 inoculation significantly enhances root length, root biomass, and the number of seminal roots in maize seedlings. Additionally, the treated seedlings exhibit increased shoot biomass and higher levels of photosynthetic pigments. Confocal laser scanning microscopy (CLSM) analysis revealed extensive colonization of ZM1 on root hairs, as well as in the cortical and stellar regions of the root. Furthermore, LC-MS analysis demonstrated elevated auxin content in the roots of the ZM1 treated maize seedlings compared to the uninoculated control. Inoculation with ZM1 significantly increased the levels of endogenous ammonium content, GS, and GOGAT enzyme activities in the roots of treated maize seedlings compared to the control, indicating enhanced nitrogen metabolism. Furthermore, inoculation of bacteria under nitrogen-deficient conditions enhanced plant growth, as evidenced by increased root shoot length, fresh and dry weights, average number of seminal roots, and content of photosynthetic pigments. Transcript analysis indicated upregulation of auxin biosynthetic genes, along with genes involved in nitrogen metabolism at different time points in roots of ZM1-treated maize seedlings. Collectively, our findings highlight the positive impact of Lysinibacillus sp. ZM1 inoculation on maize seeds by improving root architecture through modulation of auxin biosynthesis and affecting various nitrogen metabolism related parameters. These findings provide valuable insights into the potential utilization of seed endophytic bacteria as biofertilizers to enhance plant growth and yield in nutrient deficient soils.

A rapid and robust colorimetric method for measuring relative abundance of auxins in plant tissues

INTRODUCTION

Auxin estimation in plant tissues is a crucial component of auxin signaling studies. Despite the availability of various high-throughput auxin quantification methods like LC-MS, GC-MS, HPLC, biosensors, and DR5-gus/gfp-based assays, auxin quantification remains troublesome because these techniques are very expensive and technology intensive and they mostly involve elaborate sample preparation or require the development of transgenic plants.

OBJECTIVES

To find a solution to these problems, we made use of an old auxin detection system to quantify microbe derived auxins and modified it to effectively measure auxin levels in rice plants.

MATERIALS AND METHODS

Auxins from different tissues of rice plants, including root samples of seedlings exposed to IAA/TIBA or subjected to different abiotic stresses, were extracted in ethanol. The total auxin level was measured by the presently described colorimetric assay and counterchecked by other auxin estimation methods like LC-MS or gus staining of DR5-gus overexpressing lines.

RESULTS

The presented colorimetric method could measure (1) the auxin levels in different tissues of rice plants, thus identifying the regions of higher auxin abundance, (2) the differential accumulation of auxins in rice roots when auxin or its transport inhibitor was supplied exogenously, and (3) the levels of auxin in roots of rice seedlings subjected to various abiotic stresses. The thus obtained auxin levels correlated well with the auxin levels determined by other methods like LC-MS or gus staining and the expression pattern of auxin biosynthesis pathway genes.

CONCLUSIONS

The auxin estimation method described here is simple, rapid, cost-effective, and sensitive and allows for the efficient detection of relative auxin abundances in plant tissues.

Root colonization by the endophytic fungus Piriformospora indica shortens the juvenile phase of Piper nigrum L. by fine tuning the floral promotion pathways

Piriformospora indica, the mutualistic biotrophic root colonizing endosymbiotic fungus belonging to the order Sebacinales, offers host plants various benefits and enhances its growth and performance. The effect of colonization of P. indica in Piper nigrum L. cv. Panniyur1 on growth advantages, floral induction and evocation was investigated. Growth and yield benefits are credited to the alteration in the phytohormone levels fine-tuned by plants in response to the fungal colonization and perpetuation. The remarkable upregulation in the phytohormone levels, as estimated by LC- MS/MS and quantified by qRT-PCR, revealed the effectual contribution by the endophyte. qRT-PCR results revealed a significant shift in the expression of putative flowering regulatory genes in the photoperiod induction pathway (FLOWERING LOCUS T, LEAFY, APETALA1, AGAMOUS, SUPPRESSOR OF CONSTANS 1, GIGANTEA, PHYTOCHROMEA, and CRYPTOCHROME1) gibberellin biosynthetic pathway genes (GIBBERELLIN 20-OXIDASE2, GIBBERELLIN 2-OXIDASE, DELLA PROTEIN REPRESSOR OF GA1-3 1) autonomous (FLOWERING LOCUS C, FLOWERING LOCUS VE, FLOWERING LOCUS CA), and age pathway (SQUAMOSA PROMOTER LIKE9, APETALA2). The endophytic colonization had no effect on vernalization (FLOWERING LOCUS C) or biotic stress pathways (SALICYLIC ACID INDUCTION DEFICIENT 2, WRKY family transcription factor 22). The data suggest that P. nigrum responds positively to P. indica colonization, affecting preponement in floral induction as well as evocation, and thereby shortening the juvenile phase of the crop.

Insect allies-Assessment of a viral approach to plant genome editing

The Insect Allies program of the Defense Advanced Research Projects Agency has already sparked scientific debate concerning technology assessment-related issues, among which the most prevalent is that of dual use. Apart from the issues concerning peaceful applications, the technology also provides the blueprint for a potential bioweapon. However, the combination of a virus-induced genetic modification of crop plants in the field using genetically modified insect vectors poses a greater risk than the hitherto existing use of genetically modified organisms. The technology's great depth of intervention allows a number of sources for hazard and a tendency towards high exposure, but it is also encumbered with notable deficits in knowledge. These issues call for a thorough technology assessment. This article aims to provide an initial characterization from a technology assessment perspective, focusing on potential sources of risk for this novel invasive environmental biotechnology at an early stage of research and development. Integr Environ Assess Manag 2022;18:1488-1499. © 2022 The Authors. Integrated Environmental Assessment and Management published by Wiley Periodicals LLC on behalf of Society of Environmental Toxicology & Chemistry (SETAC).

Piriformospora indica recruits host-derived putrescine for growth promotion in plants

Growth promotion induced by the endosymbiont Piriformospora indica has been observed in various plants; however, except growth phytohormones, specific functional metabolites involved in P. indica-mediated growth promotion are unknown. Here, we used a gas chromatography-mass spectrometry-based untargeted metabolite analysis to identify tomato (Solanum lycopersicum) metabolites whose levels were altered during P. indica-mediated growth promotion. Metabolomic multivariate analysis revealed several primary metabolites with altered levels, with putrescine (Put) induced most significantly in roots during the interaction. Further, our results indicated that P. indica modulates the arginine decarboxylase (ADC)-mediated Put biosynthesis pathway via induction of SlADC1 in tomato. Piriformospora indica did not promote growth in Sladc1-(virus-induced gene silencing of SlADC1) lines of tomato and showed less colonization. Furthermore, using LC-MS/MS we showed that Put promoted growth by elevation of auxin (indole-3-acetic acid) and gibberellin (GA4 and GA7) levels in tomato. In Arabidopsis (Arabidopsis thaliana) adc knockout mutants, P. indica colonization also decreased and showed no plant growth promotion, and this response was rescued upon exogenous application of Put. Put is also important for hyphal growth of P. indica, indicating that it is co-adapted by both host and microbe. Taken together, we conclude that Put is an essential metabolite and its biosynthesis in plants is crucial for P. indica-mediated plant growth promotion and fungal growth.

Diversified Regulation of Cytokinin Levels and Signaling During Botrytis cinerea Infection in Arabidopsis

Cytokinins (CKs) can modulate plant immunity to various pathogens, but how CKs are involved in plant defense responses to the necrotrophic pathogen Botrytis cinerea is still unknown. Here, we found that B. cinerea infection induced transcriptional changes in multiple genes involved in the biosynthesis, degradation, and signaling of CKs, as well as their contents, in pathogen-infected Arabidopsis leaves. Among the CKs, the gene expression of CYTOKININ OXIDASE/DEHYDROGENASE 5 (CKX5) was remarkably induced in the local infected leaves and the distant leaves of the same plant without pathogen inoculation. Cis-zeatin (cZ) and its riboside (cZR) accumulated considerably in infected leaves, suggesting an important role of the cis-zeatin type of CKs in the plant response to B. cinerea. Cytokinin double-receptor mutants were more susceptible to B. cinerea infection, whereas an exogenous CK treatment enhanced the expression levels of defense-related genes and of jasmonic acid (JA) and ethylene (ET), but not salicylic acid (SA), resulting in higher resistance of Arabidopsis to B. cinerea. Investigation of CK responses to B. cinerea infection in the JA biosynthesis mutant, jar1-1, and ET-insensitive mutant, ein2-1, showed that CK signaling and levels of CKs, namely, those of isopentenyladenine (iP), isopentenyladenine riboside (iPR), and trans-zeatin (tZ), were enhanced in jar1-1-infected leaves. By contrast, reductions in iP, iPR, tZ, and tZ riboside (tZR) as well as cZR contents occurred in ein2-1-infected leaves, whose transcript levels of CK signaling genes were likewise differentially regulated. The Arabidopsis Response Regulator 5 (ARR5) gene was upregulated in infected leaves of ein2-1 whereas another type-A response regulator, ARR16, was significantly downregulated, suggesting the existence of a complex regulation of CK signaling via the ET pathway. Accumulation of the cis-zeatin type of CKs in B. cinerea-infected leaves depended on ET but not JA pathways. Collectively, our findings provide evidence that CK responds to B. cinerea infection in a variety of ways that are differently modulated by JA and ET pathways in Arabidopsis.

TOC1 in Nicotiana attenuata regulates efficient allocation of nitrogen to defense metabolites under herbivory stress

The circadian clock contextualizes plant responses to environmental signals. Plants use temporal information to respond to herbivory, but many of the functional roles of circadian clock components in these responses, and their contribution to fitness, remain unknown. We investigate the role of the central clock regulator TIMING OF CAB EXPRESSION 1 (TOC1) in Nicotiana attenuata's defense responses to the specialist herbivore Manduca sexta under both field and glasshouse conditions. We utilize 15 N pulse-labeling to quantify nitrogen incorporation into pools of three defense compounds: caffeoylputrescine (CP), dicaffeoyl spermidine (DCS) and nicotine. Nitrogen incorporation was decreased in CP and DCS and increased in nicotine pools in irTOC1 plants compared to empty vector (EV) under control conditions, but these differences were abolished after simulated herbivory. Differences between EV and irTOC1 plants in nicotine, but not phenolamide production, were abolished by treatment with the ethylene agonist 1-methylcyclopropene. Using micrografting, TOC1's effect on nicotine was isolated to the root and did not affect the fitness of heterografts under field conditions. These results suggest that the circadian clock contributes to plant fitness by balancing production of metabolically expensive nitrogen-rich defense compounds and mediating the allocation of resources between vegetative biomass and reproduction.

Blumenols as shoot markers of root symbiosis with arbuscular mycorrhizal fungi

High-through-put (HTP) screening for functional arbuscular mycorrhizal fungi (AMF)-associations is challenging because roots must be excavated and colonization evaluated by transcript analysis or microscopy. Here we show that specific leaf-metabolites provide broadly applicable accurate proxies of these associations, suitable for HTP-screens. With a combination of untargeted and targeted metabolomics, we show that shoot accumulations of hydroxy- and carboxyblumenol C-glucosides mirror root AMF-colonization in Nicotiana attenuata plants. Genetic/pharmacologic manipulations indicate that these AMF-indicative foliar blumenols are synthesized and transported from roots to shoots. These blumenol-derived foliar markers, found in many di- and monocotyledonous crop and model plants (Solanum lycopersicum, Solanum tuberosum, Hordeum vulgare, Triticum aestivum, Medicago truncatula and Brachypodium distachyon), are not restricted to particular plant-AMF interactions, and are shown to be applicable for field-based QTL mapping of AMF-related genes.

Cytokinin transfer by a free-living mirid to Nicotiana attenuata recapitulates a strategy of endophytic insects

Endophytic insects provide the textbook examples of herbivores that manipulate their host plant's physiology, putatively altering source/sink relationships by transferring cytokinins (CK) to create 'green islands' that increase the nutritional value of infested tissues. However, unambiguous demonstrations of CK transfer are lacking. Here we show that feeding by the free-living herbivore Tupiocoris notatus on Nicotiana attenuata is characterized by stable nutrient levels, increased CK levels and alterations in CK-related transcript levels in attacked leaves, in striking similarity to endophytic insects. Using 15N-isotope labeling, we demonstrate that the CK N6-isopentenyladenine (IP) is transferred from insects to plants via their oral secretions. In the field, T. notatus preferentially attacks leaves with transgenically increased CK levels; plants with abrogated CK-perception are less tolerant of T. notatus feeding damage. We infer that this free-living insect uses CKs to manipulate source/sink relationships to increase food quality and minimize the fitness consequences of its feeding.

Ecological Roles and Biological Activities of Specialized Metabolites from the Genus Nicotiana

Species of Nicotiana grow naturally in different parts of the world and have long been used both medicinally and recreationally by human societies. More recently in our history, Nicotiana tabacum has attracted interest as one of the most economically important industrial crops. Nicotiana species are frequently investigated for their bioactive natural products, and the ecological role of their specialized metabolites in responses to abiotic stress or biotic stress factors like pathogens and herbivores. The interest of tobacco companies in genetic information as well as the success of a few wild tobacco species as experimental model organisms have resulted in growing knowledge about the molecular biology and ecology of these plants and functional studies of the plant's natural products. Although a large number of reviews and books on biologically active natural products already exists, mostly from N. tabacum, we focus our attention on the ecological roles and biological activity of natural products, versus products from cured and processed material, in this Review. The studied compounds include alkaloids, aromatic compounds, flavonoids, volatiles, sesquiterpenoids, diterpenes alcohols, and sugar esters from trichomes of the plants, and recently characterized acyclic hydroxygeranyllinalool diterpene glycosides (HGL-DTGs). In this Review (1800s-2017), we describe the above-mentioned classes of natural products, emphasizing their biological activities and functions as they have been determined either in bioassay-guided purification approaches or in bioassays with plants in which the expression of specific biosynthetic genes has been genetically manipulated. Additionally, a review on the history, taxonomy, ecology, and medicinal application of different Nicotiana species growing around the globe presented in this Review may be of interest for pharmacognosists, natural products, and ecological chemists.

Real-Time Genetic Manipulations of the Cytokinin Pathway: A Tool for Laboratory and Field Studies

Although many established tools for cytokinin (CK) pathway manipulations are well suitable for the analysis of molecular interactions, their use on a whole plant scale is often limited by the induction of severe developmental defects. To circumvent this problem, different methods were developed that allow for a more precise manipulation of the CK pathway. Here we present one of these systems, the pOp6/LhGR system for chemically inducible gene expression. This system allows regulation on a spatial, temporal, and quantitative scale and therefore provides a superior tool for analyzing the role of CKs in the interactions of plants with their environment. The pOp6/LhGR system was tested for RNAi-mediated gene silencing and heterologous gene expression and was successfully used for CK pathway manipulations in different model organisms (Arabidopsis thaliana, Nicotiana tabaccum, Nicotiana attenuata, Citrus sinensis × C. trifoliate). Here we describe specific aspects of the screening procedure and present an experimental setup that can not only be used in the laboratory but is also applicable under field conditions.

Changes in cytokinins are sufficient to alter developmental patterns of defense metabolites in Nicotiana attenuata

Plant defense metabolites are well known to be regulated developmentally. The optimal defense (OD) theory posits that a tssue's fitness values and probability of attack should determine defense metabolite allocations. Young leaves are expected to provide a larger fitness value to the plant, and therefore their defense allocations should be higher when compared with older leaves. The mechanisms that coordinate development with defense remain unknown and frequently confound tests of the OD theory predictions. Here we demonstrate that cytokinins (CKs) modulate ontogeny-dependent defenses in Nicotiana attenuata. We found that leaf CK levels highly correlate with inducible defense expressions with high levels in young and low levels in older leaves. We genetically manipulated the developmental patterns of two different CK classes by using senescence- and chemically inducible expression of CK biosynthesis genes. Genetically modifying the levels of different CKs in leaves was sufficient to alter ontogenic patterns of defense metabolites. We conclude that the developmental regulation of growth hormones that include CKs plays central roles in connecting development with defense and therefore in establishing optimal patterns of defense allocation in plants.

Sex ratio of mirid populations shifts in response to hostplant co-infestation or altered cytokinin signaling

Herbivore species sharing a host plant often compete. In this study, we show that host plant-mediated interaction between two insect herbivores - a generalist and a specialist - results in a sex ratio shift of the specialist's offspring. We studied demographic parameters of the specialist Tupiocoris notatus (Hemiptera: Miridae) when co-infesting the host plant Nicotiana attenuata (Solanaceae) with the generalist leafhopper Empoasca sp. (Hemiptera: Cicadellidae). We show that the usually female-biased sex ratio of T. notatus shifts toward a higher male proportion in the offspring on plants co-infested by Empoasca sp. This sex ratio change did not occur after oviposition, nor is it due differential mortality of female and male nymphs. Based on pyrosequencing and PCR of bacterial 16S rRNA amplicons, we concluded that sex ratio shifts were unlikely to be due to infection with Wolbachia or other known sex ratio-distorting endosymbionts. Finally, we used transgenic lines of N. attenuata to evaluate if the sex ratio shift could be mediated by changes in general or specialized host plant metabolites. We found that the sex ratio shift occurred on plants deficient in two cytokinin receptors (irCHK2/3). Thus, cytokinin-regulated traits can alter the offspring sex ratio of the specialist T. notatus.

Chemical induction of hairpin RNAi molecules to silence vital genes in plant roots

Understanding the functions encoded by plant genes can be facilitated by reducing transcript levels by hairpin RNA (hpRNA) mediated silencing. A bottleneck to this technology occurs when a gene encodes a phenotype that is necessary for cell viability and silencing the gene inhibits transformation. Here we compared the use of two chemically inducible plant promoter systems to drive hpRNA mediated gene silencing in transgenic, hairy roots. We cloned the gene encoding the Yellow Fluorescence Protein (YFP) into the dexamethasone inducible vector pOpOff2 and into the estradiol induced vector pER8. We then cloned a hpRNA targeting YFP under the regulation of the inducible promoters, transformed Medicago truncatula roots, and quantified YFP fluorescence and mRNA levels. YFP fluorescence was normal in pOpOff2 transformed roots without dexamethasone but was reduced with dexamethasone treatment. Interestingly, dexamethasone removal did not reverse YFP inhibition. YFP expression in roots transformed with pER8 was low even in the absence of inducer. We used the dexamethasone system to silence acetyl-CoA carboxylase gene and observed prolific root growth when this construct was transformed into Medicago until dexamethasone was applied. Our study shows that dexamethasone inducibility can be useful to silence vital genes in transgenic roots.

Auxin Is Rapidly Induced by Herbivore Attack and Regulates a Subset of Systemic, Jasmonate-Dependent Defenses

Plant responses to herbivore attack are regulated by phytohormonal networks. To date, the role of the auxin indole-3-acetic acid (IAA) in this context is not well understood. We quantified and manipulated the spatiotemporal patterns of IAA accumulation in herbivore-attacked Nicotiana attenuata plants to unravel its role in the regulation of plant secondary metabolism. We found that IAA is strongly, rapidly, and specifically induced by herbivore attack. IAA is elicited by herbivore oral secretions and fatty acid conjugate elicitors and is accompanied by a rapid transcriptional increase of auxin biosynthetic YUCCA-like genes. IAA accumulation starts 30 to 60 s after local induction and peaks within 5 min after induction, thereby preceding the jasmonate (JA) burst. IAA accumulation does not require JA signaling and spreads rapidly from the wound site to systemic tissues. Complementation and transport inhibition experiments reveal that IAA is required for the herbivore-specific, JA-dependent accumulation of anthocyanins and phenolamides in the stems. In contrast, IAA does not affect the accumulation of nicotine or 7-hydroxygeranyllinalool diterpene glycosides in the same tissue. Taken together, our results uncover IAA as a rapid and specific signal that regulates a subset of systemic, JA-dependent secondary metabolites in herbivore-attacked plants.

Benefits of jasmonate-dependent defenses against vertebrate herbivores in nature

Endogenous jasmonates are important regulators of plant defenses. If and how they enable plants to maintain their reproductive output when facing community-level herbivory under natural conditions, however, remains unknown. We demonstrate that jasmonate-deficient Nicotiana attenuata plants suffer more damage by arthropod and vertebrate herbivores than jasmonate-producing plants in nature. However, only damage by vertebrate herbivores translates into a significant reduction in flower production. Vertebrate stem peeling has the strongest negative impact on plant flower production. Stems are defended by jasmonate-dependent nicotine, and the native cottontail rabbit Sylvilagus nuttallii avoids jasmonate-producing N. attenuata shoots because of their high levels of nicotine. Thus, endogenous jasmonates enable plants to resist different types of herbivores in nature, and jasmonate-dependent defenses are important for plants to maintain their reproductive potential when facing vertebrate herbivory. Ecological and evolutionary models on plant defense signaling should aim at integrating arthropod and vertebrate herbivory at the community level.

High-throughput quantification of more than 100 primary- and secondary-metabolites, and phytohormones by a single solid-phase extraction based sample preparation with analysis by UHPLC-HESI-MS/MS

BACKGROUND

Plant metabolites are commonly functionally classified, as defense- or growth-related phytohormones, primary and specialized metabolites, and so forth. Analytical procedures for the quantifications of these metabolites are challenging because the metabolites can vary over several orders of magnitude in concentrations in the same tissues and have very different chemical characteristics. Plants clearly adjust their metabolism to respond to their prevailing circumstances in very sophisticated ways that blur the boundaries among these functional or chemically defined classifications. But if plant biologists want to better understand the processes that are important for a plant's adaptation to its environment, procedures are needed that can provide simultaneous quantifications of the large range of metabolites that have the potential to play central roles in these adjustments in a cost and time effective way and with a low sample consumption.

RESULTS

Here we present a method that combines well-established methods for the targeted analysis of phytohormones, including jasmonates, salicylic acid, abscisic acid, gibberellins, auxins and cytokinins, and extends it to the analysis of inducible and constitutive defense compounds, as well as the primary metabolites involved in the biosynthesis of specialized metabolites and responsible for nutritional quality (e.g., sugars and amino acids). The method is based on a single extraction of 10-100 mg of tissue and allows a broad quantitative screening of metabolites optimized by their chemical characteristics and concentrations, thereby providing a high throughput analysis unbiased by the putative functional attributes of the metabolites. The tissues of Nicotiana attenuata which accumulate high levels of nicotine and diterpene glycosides, provide a challenging matrix that thwarts quantitative analysis; the analysis of various tissues of this plant are used to illustrate the robustness of the procedure.

CONCLUSIONS

The method described has the potential to unravel various, until now overlooked interactions among different sectors of plant metabolism in a high throughput manner. Additionally, the method could be particularly beneficial as screening method in forward genetic approaches, as well as for the investigation of plants from natural populations that likely differ in metabolic traits.

Beyond the Canon: Within-Plant and Population-Level Heterogeneity in Jasmonate Signaling Engaged by Plant-Insect Interactions

Plants have evolved sophisticated communication and defense systems with which they interact with insects. Jasmonates are synthesized from the oxylipin pathway and act as pivotal cellular orchestrators of many of the metabolic and physiological processes that mediate these interactions. Many of these jasmonate-dependent responses are tissue-specific and translate from modulations of the canonical jasmonate signaling pathway. Here we provide a short overview of within-plant heterogeneities in jasmonate signaling and dependent responses in the context of plant-insect interactions as illuminated by examples from recent work with the ecological model, Nicotiana attenuata. We then discuss means of manipulating jasmonate signaling by creating tissue-specific jasmonate sinks, and the micrografting of different transgenic plants. The metabolic phenotyping of these manipulations provides an integrative understanding of the functional significance of deviations from the canonical model of this hormonal pathway. Additionally, natural variation in jasmonate biosynthesis and signaling both among and within species can explain polymorphisms in resistance to insects in nature. In this respect, insect-guided explorations of population-level variations in jasmonate metabolism have revealed more complexity than previously realized and we discuss how different "omic" techniques can be used to exploit the natural variation that occurs in this important signaling pathway.

The Layers of Plant Responses to Insect Herbivores

Plants collectively produce hundreds of thousands of specialized metabolites that are not required for growth or development. Each species has a qualitatively unique profile, with variation among individuals, growth stages, and tissues. By the 1950s, entomologists began to recognize the supreme importance of these metabolites in shaping insect herbivore communities. Plant defense theories arose to address observed patterns of variation, but provided few testable hypotheses because they did not distinguish clearly among proximate and ultimate causes. Molecular plant-insect interaction research has since revealed the sophistication of plant metabolic, developmental, and signaling networks. This understanding at the molecular level, rather than theoretical predictions, has driven the development of new hypotheses and tools and pushed the field forward. We reflect on the utility of the functional perspective provided by the optimal defense theory, and propose a conceptual model of plant defense as a series of layers each at a different level of analysis, illustrated by advances in the molecular ecology of plant-insect interactions.

Cytokinin concentrations and CHASE-DOMAIN CONTAINING HIS KINASE 2 (NaCHK2)- and NaCHK3-mediated perception modulate herbivory-induced defense signaling and defenses in Nicotiana attenuata

Herbivore attack elicits changes in cytokinins (CKs), but how these changes influence defense signaling remains poorly described. We investigated the influence of the CK pathway on the well-described inducible defense pathways of Nicotiana attenuata in response to wounding with and without elicitors from the specialist herbivore Manduca sexta. CK pathway manipulation often suffers from substantial side effects on plant growth and development. We therefore used multiple manipulation tools including spray application of CKs, chemically-inducible expression of the CK biosynthesis enzyme isopentenyltransferase, and transient and constitutive RNAi-mediated gene silencing of CK receptors to resolve the function of CKs in plant defense. The results demonstrated that CK concentrations in leaves and perception through CHASE-DOMAIN CONTAINING HIS KINASE 2 (NaCHK2) and NaCHK3 were important for the accumulation of jasmonic acid (JA) and phenolamides and proteinase inhibitor activity. By contrast, the CK pathway did not promote the accumulation of the active JA-isoleucine conjugate and negatively regulated the release of specific green leaf volatile esters. Interestingly, CK signaling also promotes the systemic phenolamide accumulation. We conclude that the CK pathway is an important regulator of herbivory-inducible defense signaling and chemistry, which expands its reported participation in adjusting a plant's physiology to abiotic and biotic stress responses.

The information landscape of plant constitutive and induced secondary metabolite production

Resistance against antagonist organisms, such as herbivores, has been identified as a major function of constitutive and stress-inducible production of plant secondary metabolites (PSMs). The mechanism through which constitutive expression and inducibility mediate resistance and, hence physiological and ecological factors that affect their evolution are still little understood. Here I propose information transfer as the least common denominator function of PSM production. In this framework constitutively produced PSMs represent the first line of defense through functioning as toxins, cues associated with toxicity and as detractants that interfere with antagonist host-search patterns. Information made available and utilizable by inducibility of secondary metabolite production allows plants to include the associated biological communities to cope with antagonists and to more efficiently target a specific attacker.

Cytokinin levels and signaling respond to wounding and the perception of herbivore elicitors in Nicotiana attenuata

Nearly half a century ago insect herbivores were found to induce the formation of green islands by manipulating cytokinin (CK) levels. However, the response of the CK pathway to attack by chewing insect herbivores remains unclear. Here, we characterize the CK pathway of Nicotiana attenuata (Torr. ex S. Wats.) and its response to wounding and perception of herbivore-associated molecular patterns (HAMPs). We identified 44 genes involved in CK biosynthesis, inactivation, degradation, and signaling. Leaf wounding rapidly induced transcriptional changes in multiple genes throughout the pathway, as well as in the levels of CKs, including isopentenyladenosine and cis-zeatin riboside; perception of HAMPs present in the oral secretions (OS) of the specialist herbivore Manduca sexta amplified these responses. The jasmonate pathway, which triggers many herbivore-induced processes, was not required for these HAMP-triggered changes, but rather suppressed the CK responses. Interestingly CK pathway changes were observed also in systemic leaves in response to wounding and OS application indicating a role of CKs in mediating long distance systemic processes in response to herbivory. Since wounding and grasshopper OS elicited similar accumulations of CKs in Arabidopsis thaliana L., we propose that CKs are integral components of wounding and HAMP-triggered responses in many plant species.

Cytokinins for immunity beyond growth, galls and green islands

Cytokinins are essential plant hormones that control almost every aspect of plant growth and development. Their function in mediating plant susceptibility to fungal biotrophs and gall-causing pathogens is well known. Here we highlight the interaction between cytokinins and salicylic acid pathways. Furthermore, we discuss ways in which cytokinin signaling could crosstalk with plant immune networks. Some of these networks are modulated by pathogens to propagate disease, whereas others help the host to mitigate an infection.

Revealing insect herbivory-induced phenolamide metabolism: from single genes to metabolic network plasticity analysis

The phenylpropanoid metabolic space comprises a network of interconnected metabolic branches that contribute to the biosynthesis of a large array of compounds with functions in plant development and stress adaptation. During biotic challenges, such as insect attack, a major rewiring of gene networks associated with phenylpropanoid metabolism is observed. This rapid reconfiguration of gene expression allows prioritized production of metabolites that help the plant solve ecological problems. Phenolamides are a group of phenolic derivatives that originate from diversion of hydroxycinnamoyl acids from the main phenylpropanoid pathway after N-acyltransferase-dependent conjugation to polyamines or aryl monoamines. These structurally diverse metabolites are abundant in the reproductive organs of many plants, and have recently been shown to play roles as induced defenses in vegetative tissues. In the wild tobacco, Nicotiana attenuata, in which herbivory-induced regulation of these metabolites has been studied, rapid elevations of the levels of phenolamides that function as induced defenses result from a multi-hormonal signaling network that re-shapes connected metabolic pathways. In this review, we summarize recent findings in the regulation of phenolamides obtained by mass spectrometry-based metabolomics profiling, and outline a conceptual framework for gene discovery in this pathway. We also introduce a multifactorial approach that is useful in deciphering metabolic pathway reorganizations among tissues in response to stress.

Cytokinin-induced phenotypes in plant-insect interactions: learning from the bacterial world

Recently, a renewed interest in cytokinins (CKs) has allowed the characterization of these phytohormones as key regulatory molecules in plant biotic interactions. They have been proved to be instrumental in microbe- and insect-mediated plant phenotypes that can be either beneficial or detrimental for the host-plant. In parallel, insect endosymbiotic bacteria have emerged as key players in plant-insect interactions mediating directly or indirectly fundamental aspects of insect nutrition, such as insect feeding efficiency or the ability to manipulate plant physiology to overcome food nutritional imbalances. However, mechanisms that regulate CK production and the role played by insects and their endosymbionts remain largely unknown. Against this backdrop, studies on plant-associated bacteria have revealed fascinating and complex molecular mechanisms that lead to the production of bacterial CKs and the modulation of plant-borne CKs which ultimately result in profound metabolic and morphological plant modifications. This review highlights major strategies used by plant-associated bacteria that impact the CK homeostasis of their host-plant, to raise parallels with strategies used by phytophagous insects and to discuss the possible role played by endosymbiotic bacteria in these CK-mediated plant phenotypes. We hypothesize that insects employ a CK-mix production strategy that manipulates the phytohormonal balance of their host-plant and overtakes plant gene expression causing a metabolic and morphological habitat modification. In addition, insect endosymbiotic bacteria may prove to be instrumental in these manipulations through the production of bacterial CKs, including specific forms that challenge the CK-degrading capacity of the plant (thus ensuring persistent effects) and the CK-mediated plant defenses.