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Bacterial flagellin is a potent host immune activator. Parys et al. (2021) and Colaianni et al. (2021) dissected effects of flagellin epitope variants on host immune detection and bacterial motility. They report in this issue of Cell Host & Microbe that Arabidopsis-associated bacterial microbiota differentially evolved flg22 variants that allow tunability between motility and defense activation.

Background Since the 1980s, numerous mutualistic Pseudomonas spp. strains have been used in studies on the biology of plant growth-promoting rhizobacteria (PGPR) and their interactions with host plants. In 1988, a strain from the Pseudomonas fluorescens group, WCS417, was isolated from lesions of wheat roots growing in a take-all disease-suppressive soil. In subsequent trials, WCS417 limited the build-up of take-all disease in field-grown wheat and significantly increased wheat yield. In 1991, WCS417 was featured in one of the first landmark studies on rhizobacteria-induced systemic resistance (ISR), in which it was shown to confer systemic immunity in carnation (Dianthus caryophyllus) against Fusarium wilt. The discovery that WCS417 conferred systemic immunity in the model plant species Arabidopsis thaliana in 1996 incited intensive research on the molecular mechanisms by which PGPR promote plant growth and induce broad-spectrum disease resistance in plants. Since then, the strain name appeared in over 750 studies on beneficial plant-microbe interactions. Scope In this review, we will highlight key discoveries in plant-microbe interactions research that have emerged from over 30 years of research featuring WCS417 as a model rhizobacterial strain. WCS417 was instrumental in improving our understanding of the microbial determinants that are involved in root colonization and the establishment of mutually beneficial interactions with the host plant. The model strain also provided novel insight into the molecular mechanisms of plant growth promotion and the onset and expression of rhizobacteria-ISR. More recently, WCS417 has been featured in studies on host immune evasion during root colonization, and chemical communication in the rhizosphere during root microbiome assembly. Conclusions Numerous studies on the modes of action of WCS417 have provided major conceptual advances in our understanding of how free-living mutualists colonize the rhizosphere, modulate plant immunity, and promote plant growth. The concepts may prove useful in our understanding of the molecular mechanisms involved in other binary plant-beneficial microbe interactions, and in more complex microbial community contexts, such as the root microbiome.

Plants shape their rhizosphere microbiome by secreting root exudates into the soil environment. Recently, root-exuded coumarins were identified as novel players in plant–microbiome communication. Beneficial members of the root-associated microbiome stimulate coumarin biosynthesis in roots and their excretion into the rhizosphere. The iron-mobilizing activity of coumarins facilitates iron uptake from the soil environment, while their selective antimicrobial activity shapes the root microbiome, resulting in promotion of plant growth and health. Evidence is accumulating that, in analogy to strigolactones and flavonoids, coumarins may act in microbiome-to-root-to-shoot signaling events. Here, we review this multifaceted role of coumarins in bidirectional chemical communication along the microbiome–root–shoot axis.

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. Abstract β-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.

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., Crosstalk between defense‐related hormones enables plants to finely tune their immune response to the plethora of attackers that they encounter in their environment. We mined the natural genetic variation in Arabidopsis thaliana for novel genes that are associated with positive or negative interactions that the stress hormones salicylic acid and abscisic acid have on jasmonic acid‐regulated defense responses. Through genome‐wide association mapping, we identified three novel genes with putative roles in hormone crosstalk and that impact the level of resistance against the necrotrophic pathogen Botrytis cinerea or the insect herbivore Mamestra brassicae.

Thrips are tiny, cell-content–feeding insects that are a major pest on crops and ornamentals. Besides causing direct feeding damage, thrips may also cause indirect damage by vectoring tospoviruses. Novel resistance mechanisms to thrips need to be discovered and validated. Induction of jasmonic acid–dependent defenses has been demonstrated to be essential for resistance to thrips, but underlying mechanisms still need to be discovered. For this, it is vital to use robust plant-thrips assays to analyze plant defense responses and thrips performance. In recently developed high-throughput phenotyping platforms, the feeding damage that is visible as silver spots, and the preference of thrips in a two-choice setup is assessed, using leaf discs. Here, we describe whole-plant thrips assays that are essential for (1) validation of findings obtained by the leaf disc assays, (2) assessment of longer-term effects on thrips feeding success and fecundity, (3) determination of spatial-temporal effects induced by primary thrips infestation on a secondary attack by thrips or other insects or pathogens, and (4) assessment of gene expression and metabolite changes. We present detailed methods and tips and tricks for (a) rearing and selection of thrips at different developmental stages, (b) treatment of the whole plant or an individual leaf with thrips, and (c) determination of feeding damage and visualization of thrips oviposition success in leaves.

Planten, ze zijn overal. Veruit het meeste land op aarde is bedekt met een groene laag van bossen, oerwouden of graslanden, en ook de oceanen zitten vol met primi-tieve planten, algen. Zonder planten kunnen wij niet leven; zonder planten zou er nauwelijks zuur-stof zijn en geen voedsel. Het bladgroen dat plan-ten hun groene kleur geeft, is de krachtige motor. Aangedreven door zonlicht maakt dat zuurstof uit water en suikers uit kooldioxide (CO2). Dit unieke proces heet fotosynthese. De plant zet de suikers om in eiwitten, vetten en zetmeel en slaat die op in vruchten, granen, peulen en knollen, die dieren en mensen vaak weer opeten. Planten staan dus aan de basis van de voedselpiramide.

Thrips are tiny, cell-content–feeding insects that are a major pest on crops and ornamentals. Besides causing direct feeding damage, thrips may also cause indirect damage by vectoring tospoviruses. Novel resistance mechanisms to thrips need to be discovered and validated. Induction of jasmonic acid–dependent defenses has been demonstrated to be essential for resistance to thrips, but underlying mechanisms still need to be discovered. For this, it is vital to use robust plant-thrips assays to analyze plant defense responses and thrips performance. In recently developed high-throughput phenotyping platforms, the feeding damage that is visible as silver spots, and the preference of thrips in a two-choice setup is assessed, using leaf discs. Here, we describe whole-plant thrips assays that are essential for (1) validation of findings obtained by the leaf disc assays, (2) assessment of longer-term effects on thrips feeding success and fecundity, (3) determination of spatial-temporal effects induced by primary thrips infestation on a secondary attack by thrips or other insects or pathogens, and (4) assessment of gene expression and metabolite changes. We present detailed methods and tips and tricks for (a) rearing and selection of thrips at different developmental stages, (b) treatment of the whole plant or an individual leaf with thrips, and (c) determination of feeding damage and visualization of thrips oviposition success in leaves.

Plant roots interact with an enormous diversity of commensal, mutualistic, and pathogenic microbes, which poses a big challenge to roots to distinguish beneficial microbes from harmful ones. Plants can effectively ward off pathogens following immune recognition of conserved microbe‐associated molecular patterns (MAMPs). However, such immune elicitors are essentially not different from those of neutral and beneficial microbes that are abundantly present in the root microbiome. Recent studies indicate that the plant immune system plays an active role in influencing rhizosphere microbiome composition. Moreover, it has become increasingly clear that root‐invading beneficial microbes, including rhizobia and arbuscular mycorrhiza, evade or suppress host immunity to establish a mutualistic relationship with their host. Evidence is accumulating that many free‐living rhizosphere microbiota members can suppress root immune responses, highlighting root immune suppression as an important function of the root microbiome. Thus, the gate keeping functions of the plant immune system are not restricted to warding off root‐invading pathogens but also extend to rhizosphere microbiota, likely to promote colonization by beneficial microbes and prevent growth‐defense tradeoffs triggered by the MAMP‐rich rhizosphere environment., Plant roots interact with an enormous diversity of soil‐borne microbes, which poses a big challenge to roots to distinguish friends from foes. Here, we review current knowledge on how beneficial microbes in the root microbiome actively interfere with the root immune system to establish a mutually beneficial relationship with their host plant.