Hormonal modulation of plant immunity.

Plant hormones have pivotal roles in the regulation of plant growth, development, and reproduction. Additionally, they emerged as cellular signal molecules with key functions in the regulation of immune responses to microbial pathogens, insect herbivores, and beneficial microbes. Their signaling pathways are interconnected in a complex network, which provides plants with an enormous regulatory potential to rapidly adapt to their biotic environment and to utilize their limited resources for growth and survival in a cost-efficient manner. Plants activate their immune system to counteract attack by pathogens or herbivorous insects. Intriguingly, successful plant enemies evolved ingenious mechanisms to rewire the plant's hormone signaling circuitry to suppress or evade host immunity. Evidence is emerging that beneficial root-inhabiting microbes also hijack the hormone-regulated immune signaling network to establish a prolonged mutualistic association, highlighting the central role of plant hormones in the regulation of plant growth and survival.

Architecture and dynamics of the abscisic acid gene regulatory network.

The plant hormone abscisic acid (ABA) regulates essential processes in plant development and responsiveness to abiotic and biotic stresses. ABA perception triggers a post-translational signaling cascade that elicits the ABA gene regulatory network (GRN), encompassing hundreds of transcription factors (TFs) and thousands of transcribed genes. To further our knowledge of this GRN, we performed an RNA-seq time series experiment consisting of 14 time points in the 16 h following a one-time ABA treatment of 5-week-old Arabidopsis rosettes. During this time course, ABA rapidly changed transcription levels of 7151 genes, which were partitioned into 44 coexpressed modules that carry out diverse biological functions. We integrated our time-series data with publicly available TF-binding site data, motif data, and RNA-seq data of plants inhibited in translation, and predicted (i) which TFs regulate the different coexpression clusters, (ii) which TFs contribute the most to target gene amplitude, (iii) timing of engagement of different TFs in the ABA GRN, and (iv) hierarchical position of TFs and their targets in the multi-tiered ABA GRN. The ABA GRN was found to be highly interconnected and regulated at different amplitudes and timing by a wide variety of TFs, of which the bZIP family was most prominent, and upregulation of genes encompassed more TFs than downregulation. We validated our network models in silico with additional public TF-binding site data and transcription data of selected TF mutants. Finally, using a drought assay we found that the Trihelix TF GT3a is likely an ABA-induced positive regulator of drought tolerance.

Genome-wide association study reveals WRKY42 as a novel plant transcription factor that influences oviposition preference of Pieris butterflies.

Insect herbivores are amongst the most destructive plant pests, damaging both naturally occurring and domesticated plants. As sessile organisms, plants make use of structural and chemical barriers to counteract herbivores. However, over 75% of herbivorous insect species are well adapted to their host's defenses and these specialists are generally difficult to ward off. By actively antagonizing the number of insect eggs deposited on plants, future damage by the herbivore's offspring can be limited. Therefore, it is important to understand which plant traits influence attractiveness for oviposition, especially for specialist insects that are well adapted to their host plants. In this study, we investigated the oviposition preference of Pieris butterflies (Lepidoptera: Pieridae) by offering them the choice between 350 different naturally occurring Arabidopsis accessions. Using a genome-wide association study of the oviposition data and subsequent fine mapping with full genome sequences of 164 accessions, we identified WRKY42 and AOC1 as candidate genes that are associated with the oviposition preference observed for Pieris butterflies. Host plant choice assays with Arabidopsis genotypes impaired in WRKY42 or AOC1 function confirmed a clear role for WRKY42 in oviposition preference of female Pieris butterflies, while for AOC1 the effect was mild. In contrast, WRKY42-impaired plants, which were preferred for oviposition by butterflies, negatively impacted offspring performance. These findings exemplify that plant genotype can have opposite effects on oviposition preference and caterpillar performance. This knowledge can be used for breeding trap crops or crops that are unattractive for oviposition by pest insects.

Multiple levels of crosstalk in hormone networks regulating plant defense.

Plant hormones are essential for regulating the interactions between plants and their complex biotic and abiotic environments. Each hormone initiates a specific molecular pathway and these different hormone pathways are integrated in a complex network of synergistic, antagonistic and additive interactions. This inter-pathway communication is called hormone crosstalk. By influencing the immune network topology, hormone crosstalk is essential for tailoring plant responses to diverse microbes and insects in diverse environmental and internal contexts. Crosstalk provides robustness to the immune system but also drives specificity of induced defense responses against the plethora of biotic interactors. Recent advances in dry-lab and wet-lab techniques have greatly enhanced our understanding of the broad-scale effects of hormone crosstalk on immune network functioning and have revealed underlying principles of crosstalk mechanisms. Molecular studies have demonstrated that hormone crosstalk is modulated at multiple levels of regulation, such as by affecting protein stability, gene transcription and hormone homeostasis. These new insights into hormone crosstalk regulation of plant defense are reviewed here, with a focus on crosstalk acting on the jasmonic acid pathway in Arabidopsis thaliana, highlighting the transcription factors MYC2 and ORA59 as major targets for modulation by other hormones.

Integrative omics approaches for biosynthetic pathway discovery in plants.

Covering: up to 2022With the emergence of large amounts of omics data, computational approaches for the identification of plant natural product biosynthetic pathways and their genetic regulation have become increasingly important. While genomes provide clues regarding functional associations between genes based on gene clustering, metabolome mining provides a foundational technology to chart natural product structural diversity in plants, and transcriptomics has been successfully used to identify new members of their biosynthetic pathways based on coexpression. Thus far, most approaches utilizing transcriptomics and metabolomics have been targeted towards specific pathways and use one type of omics data at a time. Recent technological advances now provide new opportunities for integration of multiple omics types and untargeted pathway discovery. Here, we review advances in plant biosynthetic pathway discovery using genomics, transcriptomics, and metabolomics, as well as recent efforts towards omics integration. We highlight how transcriptomics and metabolomics provide complementary information to link genes to metabolites, by associating temporal and spatial gene expression levels with metabolite abundance levels across samples, and by matching mass-spectral features to enzyme families. Furthermore, we suggest that elucidation of gene regulatory networks using time-series data may prove useful for efforts to unwire the complexities of biosynthetic pathway components based on regulatory interactions and events.

Mechanisms of far-red light-mediated dampening of defense against Botrytis cinerea in tomato leaves.

Plants detect neighboring competitors through a decrease in the ratio between red and far-red light (R:FR). This decreased R:FR is perceived by phytochrome photoreceptors and triggers shade avoidance responses such as shoot elongation and upward leaf movement (hyponasty). In addition to promoting elongation growth, low R:FR perception enhances plant susceptibility to pathogens: the growth-defense tradeoff. Although increased susceptibility in low R:FR has been studied for over a decade, the associated timing of molecular events is still unknown. Here, we studied the chronology of FR-induced susceptibility events in tomato (Solanum lycopersicum) plants pre-exposed to either white light (WL) or WL supplemented with FR light (WL+FR) prior to inoculation with the necrotrophic fungus Botrytis cinerea (B.c.). We monitored the leaf transcriptional changes over a 30-h time course upon infection and followed up with functional studies to identify mechanisms. We found that FR-induced susceptibility in tomato is linked to a general dampening of B.c.-responsive gene expression, and a delay in both pathogen recognition and jasmonic acid-mediated defense gene expression. In addition, we found that the supplemental FR-induced ethylene emissions affected plant immune responses under the WL+FR condition. This study improves our understanding of the growth-immunity tradeoff, while simultaneously providing leads to improve tomato resistance against pathogens in dense cropping systems.

A family of pathogen-induced cysteine-rich transmembrane proteins is involved in plant disease resistance.

MAIN CONCLUSION: Overexpression of pathogen-induced cysteine-rich transmembrane proteins (PCMs) in Arabidopsis thaliana enhances resistance against biotrophic pathogens and stimulates hypocotyl growth, suggesting a potential role for PCMs in connecting both biological processes. Plants possess a sophisticated immune system to protect themselves against pathogen attack. The defense hormone salicylic acid (SA) is an important player in the plant immune gene regulatory network. Using RNA-seq time series data of Arabidopsis thaliana leaves treated with SA, we identified a largely uncharacterized SA-responsive gene family of eight members that are all activated in response to various pathogens or their immune elicitors and encode small proteins with cysteine-rich transmembrane domains. Based on their nucleotide similarity and chromosomal position, the designated Pathogen-induced Cysteine-rich transMembrane protein (PCM) genes were subdivided into three subgroups consisting of PCM1-3 (subgroup I), PCM4-6 (subgroup II), and PCM7-8 (subgroup III). Of the PCM genes, only PCM4 (also known as PCC1) has previously been implicated in plant immunity. Transient expression assays in Nicotiana benthamiana indicated that most PCM proteins localize to the plasma membrane. Ectopic overexpression of the PCMs in Arabidopsis thaliana resulted in all eight cases in enhanced resistance against the biotrophic oomycete pathogen Hyaloperonospora arabidopsidis Noco2. Additionally, overexpression of PCM subgroup I genes conferred enhanced resistance to the hemi-biotrophic bacterial pathogen Pseudomonas syringae pv. tomato DC3000. The PCM-overexpression lines were found to be also affected in the expression of genes related to light signaling and development, and accordingly, PCM-overexpressing seedlings displayed elongated hypocotyl growth. These results point to a function of PCMs in both disease resistance and photomorphogenesis, connecting both biological processes, possibly via effects on membrane structure or activity of interacting proteins at the plasma membrane.

Carbonic anhydrases CA1 and CA4 function in atmospheric CO2-modulated disease resistance.

MAIN CONCLUSION: Carbonic anhydrases CA1 and CA4 attenuate plant immunity and can contribute to altered disease resistance levels in response to changing atmospheric CO2 conditions. ß-Carbonic anhydrases (CAs) play an important role in CO2 metabolism and plant development, but have also been implicated in plant immunity. Here we show that the bacterial pathogen Pseudomonas syringae and application of the microbe-associated molecular pattern (MAMP) flg22 repress CA1 and CA4 gene expression in Arabidopsis thaliana. Using the CA double-mutant ca1ca4, we provide evidence that CA1 and CA4 play an attenuating role in pathogen- and flg22-triggered immune responses. In line with this, ca1ca4 plants exhibited enhanced resistance against P. syringae, which was accompanied by an increased expression of the defense-related genes FRK1 and ICS1. Under low atmospheric CO2 conditions (150 ppm), when CA activity is typically low, the levels of CA1 transcription and resistance to P. syringae in wild-type Col-0 were similar to those observed in ca1ca4. However, under ambient (400 ppm) and elevated (800 ppm) atmospheric CO2 conditions, CA1 transcription was enhanced and resistance to P. syringae reduced. Together, these results suggest that CA1 and CA4 attenuate plant immunity and that differential CA gene expression in response to changing atmospheric CO2 conditions contribute to altered disease resistance levels.

Far-red light promotes Botrytis cinerea disease development in tomato leaves via jasmonate-dependent modulation of soluble sugars.

Plants experience a decrease in the red:far-red light ratio (R:FR) when grown at high planting density. In addition to eliciting the shade avoidance response, low R:FR also enhances plant susceptibility to pathogens via modulation of defense hormone-mediated responses. However, other mechanisms, also affected by low R:FR, have not been considered as potential components in FR-induced susceptibility. Here, we identify FR-induced accumulation of leaf soluble sugars as a novel component of FR-induced susceptibility. We observed that phytochrome inactivation by FR or phytochrome B mutation was associated with elevated leaf glucose and fructose levels and enhanced disease severity caused by Botrytis cinerea. By experimentally manipulating internal leaf sugar levels, we found that the FR-induced susceptibility in tomato was partly sugar-dependent. Further analysis revealed that the observed sugar accumulation in supplemental FR occurred in a jasmonic acid (JA)-dependent manner, and the JA biosynthesis mutant def1 also displayed elevated soluble sugar levels, which was rescued by exogenous methyl jasmonate (MeJA) application. We propose that the reduced JA responsiveness under low R:FR promotes disease symptoms not only via dampened induction of defense responses, but also via increased levels of soluble sugars that supports pathogen growth in tomato leaves.

Architecture and Dynamics of the Jasmonic Acid Gene Regulatory Network.

Jasmonic acid (JA) is a critical hormonal regulator of plant growth and defense. To advance our understanding of the architecture and dynamic regulation of the JA gene regulatory network, we performed a high-resolution RNA-seq time series of methyl JA-treated Arabidopsis thaliana at 15 time points over a 16-h period. Computational analysis showed that methyl JA (MeJA) induces a burst of transcriptional activity, generating diverse expression patterns over time that partition into distinct sectors of the JA response targeting specific biological processes. The presence of transcription factor (TF) DNA binding motifs correlated with specific TF activity during temporal MeJA-induced transcriptional reprogramming. Insight into the underlying dynamic transcriptional regulation mechanisms was captured in a chronological model of the JA gene regulatory network. Several TFs, including MYB59 and bHLH27, were uncovered as early network components with a role in pathogen and insect resistance. Analysis of subnetworks surrounding the TFs ORA47, RAP2.6L, MYB59, and ANAC055, using transcriptome profiling of overexpressors and mutants, provided insights into their regulatory role in defined modules of the JA network. Collectively, our work illuminates the complexity of the JA gene regulatory network, pinpoints and validates previously unknown regulators, and provides a valuable resource for functional studies on JA signaling components in plant defense and development.

Arabidopsis JASMONATE-INDUCED OXYGENASES down-regulate plant immunity by hydroxylation and inactivation of the hormone jasmonic acid.

The phytohormone jasmonic acid (JA) is vital in plant defense and development. Although biosynthesis of JA and activation of JA-responsive gene expression by the bioactive form JA-isoleucine have been well-studied, knowledge on JA metabolism is incomplete. In particular, the enzyme that hydroxylates JA to 12-OH-JA, an inactive form of JA that accumulates after wounding and pathogen attack, is unknown. Here, we report the identification of four paralogous 2-oxoglutarate/Fe(II)-dependent oxygenases in Arabidopsis thaliana as JA hydroxylases and show that they down-regulate JA-dependent responses. Because they are induced by JA we named them JASMONATE-INDUCED OXYGENASES (JOXs). Concurrent mutation of the four genes in a quadruple Arabidopsis mutant resulted in increased defense gene expression and increased resistance to the necrotrophic fungus Botrytis cinerea and the caterpillar Mamestra brassicae In addition, root and shoot growth of the plants was inhibited. Metabolite analysis of leaves showed that loss of function of the four JOX enzymes resulted in overaccumulation of JA and in reduced turnover of JA into 12-OH-JA. Transformation of the quadruple mutant with each JOX gene strongly reduced JA levels, demonstrating that all four JOXs inactivate JA in plants. The in vitro catalysis of 12-OH-JA from JA by recombinant enzyme could be confirmed for three JOXs. The identification of the enzymes responsible for hydroxylation of JA reveals a missing step in JA metabolism, which is important for the inactivation of the hormone and subsequent down-regulation of JA-dependent defenses.

Mining the natural genetic variation in Arabidopsis thaliana for adaptation to sequential abiotic and biotic stresses.

MAIN CONCLUSION: In this genome-wide association study, we obtained novel insights into the genetic basis of the effect of herbivory or drought stress on the level of resistance against the fungus Botrytis cinerea. In nature, plants function in complex environments where they encounter different biotic and abiotic stresses individually, sequentially or simultaneously. The adaptive response to a single stress does not always reflect how plants respond to such a stress in combination with other stresses. To identify genetic factors that contribute to the plant's ability to swiftly adapt to different stresses, we investigated the response of Arabidopsis thaliana to infection by the necrotrophic fungus B. cinerea when preceded by Pieris rapae herbivory or drought stress. Using 346 natural A. thaliana accessions, we found natural genetic variation in the level of resistance against single B. cinerea infection. When preceded by herbivory or drought stress, the level of B. cinerea resistance was differentially influenced in the 346 accessions. To study the genetic factors contributing to the differential adaptation of A. thaliana to B. cinerea infection under multi-stress conditions, we performed a genome-wide association study supported by quantitative trait loci mapping and fine mapping with full genome sequences of 164 accessions. This yielded several genes previously associated with defense to B. cinerea and additional candidate genes with putative roles in the plant's adaptive response to a combination of herbivory, drought and B. cinerea infection.

Genome-wide association study reveals novel players in defense hormone crosstalk in Arabidopsis.

Jasmonic acid (JA) regulates plant defenses against necrotrophic pathogens and insect herbivores. Salicylic acid (SA) and abscisic acid (ABA) can antagonize JA-regulated defenses, thereby modulating pathogen or insect resistance. We performed a genome-wide association (GWA) study on natural genetic variation in Arabidopsis thaliana for the effect of SA and ABA on the JA pathway. We treated 349 Arabidopsis accessions with methyl JA (MeJA), or a combination of MeJA and either SA or ABA, after which expression of the JA-responsive marker gene PLANT DEFENSIN1.2 (PDF1.2) was quantified as a readout for GWA analysis. Both hormones antagonized MeJA-induced PDF1.2 in the majority of the accessions but with a large variation in magnitude. GWA mapping of the SA- and ABA-affected PDF1.2 expression data revealed loci associated with crosstalk. GLYI4 (encoding a glyoxalase) and ARR11 (encoding an Arabidopsis response regulator involved in cytokinin signalling) were confirmed by T-DNA insertion mutant analysis to affect SA-JA crosstalk and resistance against the necrotroph Botrytis cinerea. In addition, At1g16310 (encoding a cation efflux family protein) was confirmed to affect ABA-JA crosstalk and susceptibility to Mamestra brassicae herbivory. Collectively, this GWA study identified novel players in JA hormone crosstalk with potential roles in the regulation of pathogen or insect resistance.

Thrips advisor: exploiting thrips-induced defences to combat pests on crops.

Plants have developed diverse defence mechanisms to ward off herbivorous pests. However, agriculture still faces estimated crop yield losses ranging from 25% to 40% annually. These losses arise not only because of direct feeding damage, but also because many pests serve as vectors of plant viruses. Herbivorous thrips (Thysanoptera) are important pests of vegetable and ornamental crops worldwide, and encompass virtually all general problems of pests: they are highly polyphagous, hard to control because of their complex lifestyle, and they are vectors of destructive viruses. Currently, control management of thrips mainly relies on the use of chemical pesticides. However, thrips rapidly develop resistance to these pesticides. With the rising demand for more sustainable, safer, and healthier food production systems, we urgently need to pinpoint the gaps in knowledge of plant defences against thrips to enable the future development of novel control methods. In this review, we summarize the current, rather scarce, knowledge of thrips-induced plant responses and the role of phytohormonal signalling and chemical defences in these responses. We describe concrete opportunities for breeding resistance against pests such as thrips as a prototype approach for next-generation resistance breeding.

VIH2 Regulates the Synthesis of Inositol Pyrophosphate InsP8 and Jasmonate-Dependent Defenses in Arabidopsis.

Diphosphorylated inositol polyphosphates, also referred to as inositol pyrophosphates, are important signaling molecules that regulate critical cellular activities in many eukaryotic organisms, such as membrane trafficking, telomere maintenance, ribosome biogenesis, and apoptosis. In mammals and fungi, two distinct classes of inositol phosphate kinases mediate biosynthesis of inositol pyrophosphates: Kcs1/IP6K- and Vip1/PPIP5K-like proteins. Here, we report that PPIP5K homologs are widely distributed in plants and that Arabidopsis thaliana VIH1 and VIH2 are functional PPIP5K enzymes. We show a specific induction of inositol pyrophosphate InsP8 by jasmonate and demonstrate that steady state and jasmonate-induced pools of InsP8 in Arabidopsis seedlings depend on VIH2. We identify a role of VIH2 in regulating jasmonate perception and plant defenses against herbivorous insects and necrotrophic fungi. In silico docking experiments and radioligand binding-based reconstitution assays show high-affinity binding of inositol pyrophosphates to the F-box protein COI1-JAZ jasmonate coreceptor complex and suggest that coincidence detection of jasmonate and InsP8 by COI1-JAZ is a critical component in jasmonate-regulated defenses.

Transcriptome dynamics of Arabidopsis during sequential biotic and abiotic stresses.

In nature, plants have to cope with a wide range of stress conditions that often occur simultaneously or in sequence. To investigate how plants cope with multi-stress conditions, we analyzed the dynamics of whole-transcriptome profiles of Arabidopsis thaliana exposed to six sequential double stresses inflicted by combinations of: (i) infection by the necrotrophic fungus Botrytis cinerea, (ii) herbivory by chewing larvae of Pieris rapae, and (iii) drought stress. Each of these stresses induced specific expression profiles over time, in which one-third of all differentially expressed genes was shared by at least two single stresses. Of these, 394 genes were differentially expressed during all three stress conditions, albeit often in opposite directions. When two stresses were applied in sequence, plants displayed transcriptome profiles that were very similar to the second stress, irrespective of the nature of the first stress. Nevertheless, significant first-stress signatures could be identified in the sequential stress profiles. Bioinformatic analysis of the dynamics of co-expressed gene clusters highlighted specific clusters and biological processes of which the timing of activation or repression was altered by a prior stress. The first-stress signatures in second stress transcriptional profiles were remarkably often related to responses to phytohormones, strengthening the notion that hormones are global modulators of interactions between different types of stress. Because prior stresses can affect the level of tolerance against a subsequent stress (e.g. prior herbivory strongly affected resistance to B. cinerea), the first-stress signatures can provide important leads for the identification of molecular players that are decisive in the interactions between stress response pathways.

Assessing the Role of ETHYLENE RESPONSE FACTOR Transcriptional Repressors in Salicylic Acid-Mediated Suppression of Jasmonic Acid-Responsive Genes.

Salicylic acid (SA) and jasmonic acid (JA) cross-communicate in the plant immune signaling network to finely regulate induced defenses. In Arabidopsis, SA antagonizes many JA-responsive genes, partly by targeting the ETHYLENE RESPONSE FACTOR (ERF)-type transcriptional activator ORA59. Members of the ERF transcription factor family typically bind to GCC-box motifs in the promoters of JA- and ethylene-responsive genes, thereby positively or negatively regulating their expression. The GCC-box motif is sufficient for SA-mediated suppression of JA-responsive gene expression. Here, we investigated whether SA-induced ERF-type transcriptional repressors, which may compete with JA-induced ERF-type activators for binding at the GCC-box, play a role in SA/JA antagonism. We selected ERFs that are transcriptionally induced by SA and/or possess an EAR transcriptional repressor motif. Several of the 16 ERFs tested suppressed JA-dependent gene expression, as revealed by enhanced JA-induced PDF1.2 or VSP2 expression levels in the corresponding erf mutants, while others were involved in activation of these genes. However, SA could antagonize JA-induced PDF1.2 or VSP2 in all erf mutants, suggesting that the tested ERF transcriptional repressors are not required for SA/JA cross-talk. Moreover, a mutant in the co-repressor TOPLESS, that showed reduction in repression of JA signaling, still displayed SA-mediated antagonism of PDF1.2 and VSP2. Collectively, these results suggest that SA-regulated ERF transcriptional repressors are not essential for antagonism of JA-responsive gene expression by SA. We further show that de novo SA-induced protein synthesis is required for suppression of JA-induced PDF1.2, pointing to SA-stimulated production of an as yet unknown protein that suppresses JA-induced transcription.

Shifting from priming of salicylic acid- to jasmonic acid-regulated defences by Trichoderma protects tomato against the root knot nematode Meloidogyne incognita.

Beneficial root endophytes such as Trichoderma spp. can reduce infections by parasitic nematodes through triggering host defences. Little is currently known about the complex hormone signalling underlying the induction of resistance. In this study, we investigated whether Trichoderma modulates the hormone signalling network in the host to induce resistance to nematodes. We investigated the role and the timing of the jasmonic acid (JA)- and salicylic acid (SA)-regulated defensive pathways in Trichoderma-induced resistance to the root knot nematode Meloidogyne incognita. A split-root system of tomato (Solanum lycopersicum) was used to study local and systemic induced defences by analysing nematode performance, defence gene expression, responsiveness to exogenous hormone application, and dependence on SA and JA signalling of Trichoderma-induced resistance. Root colonization by Trichoderma impeded nematode performance both locally and systemically at multiple stages of the parasitism, that is, invasion, galling and reproduction. First, Trichoderma primed SA-regulated defences, which limited nematode root invasion. Then, Trichoderma enhanced JA-regulated defences, thereby antagonizing the deregulation of JA-dependent immunity by the nematodes, which compromised galling and fecundity. Our results show that Trichoderma primes SA- and JA-dependent defences in roots, and that the priming of responsiveness to these hormones upon nematode attack is plastic and adaptive to the parasitism stage.

Genetic architecture of plant stress resistance: multi-trait genome-wide association mapping.

Plants are exposed to combinations of various biotic and abiotic stresses, but stress responses are usually investigated for single stresses only. Here, we investigated the genetic architecture underlying plant responses to 11 single stresses and several of their combinations by phenotyping 350 Arabidopsis thaliana accessions. A set of 214 000 single nucleotide polymorphisms (SNPs) was screened for marker-trait associations in genome-wide association (GWA) analyses using tailored multi-trait mixed models. Stress responses that share phytohormonal signaling pathways also share genetic architecture underlying these responses. After removing the effects of general robustness, for the 30 most significant SNPs, average quantitative trait locus (QTL) effect sizes were larger for dual stresses than for single stresses. Plants appear to deploy broad-spectrum defensive mechanisms influencing multiple traits in response to combined stresses. Association analyses identified QTLs with contrasting and with similar responses to biotic vs abiotic stresses, and below-ground vs above-ground stresses. Our approach allowed for an unprecedented comprehensive genetic analysis of how plants deal with a wide spectrum of stress conditions.

Airborne signals from Trichoderma fungi stimulate iron uptake responses in roots resulting in priming of jasmonic acid-dependent defences in shoots of Arabidopsis thaliana and Solanum lycopersicum.

Root colonization by Trichoderma fungi can trigger induced systemic resistance (ISR). In Arabidopsis, Trichoderma-ISR relies on the transcription factor MYB72, which plays a dual role in the onset of ISR and the activation of Fe uptake responses. Volatile compounds (VCs) from rhizobacteria are important elicitors of MYB72 in Arabidopsis roots. Here, we investigated the mode of action of VCs from Trichoderma fungi in the onset of ISR and Fe uptake responses. VCs from Trichoderma asperellum and Trichoderma harzianum were applied in an in vitro split-plate system with Arabidopsis or tomato seedlings. Locally, Trichoderma-VCs triggered MYB72 expression and molecular, physiological and morphological Fe uptake mechanisms in Arabidopsis roots. In leaves, Trichoderma-VCs primed jasmonic acid-dependent defences, leading to an enhanced resistance against Botrytis cinerea. By using Arabidopsis micrografts of VCs-exposed rootstocks and non-exposed scions, we demonstrated that perception of Trichoderma-VCs by the roots leads to a systemic signal that primes shoots for enhanced defences. Trichoderma-VCs also elicited Fe deficiency responses and shoot immunity in tomato, suggesting that this phenomenon is expressed in different plant species. Our results indicate that Trichoderma-VCs trigger locally a readjustment of Fe homeostasis in roots, which links to systemic elicitation of ISR by priming of jasmonic acid-dependent defences.