Hormone profiling

Plant-to-plant defence induction in cotton is mediated by delayed release of volatiles upon herbivory

Caterpillar feeding immediately triggers the release of volatile compounds stored in the leaves of cotton plants. Additionally, after 1 d of herbivory, the leaves release other newly synthesised volatiles. We investigated whether these volatiles affect chemical defences in neighbouring plants and whether such temporal shifts in emissions matter for signalling between plants. Undamaged receiver plants were exposed to volatiles from plants infested with Spodoptera caterpillars. For receiver plants, we measured changes in defence-related traits such as volatile emissions, secondary metabolites, phytohormones, gene expression, and caterpillar feeding preference. Then, we compared the effects of volatiles emitted before and after 24 h of damage on neighbouring plant defences. Genes that were upregulated in receiver plants following exposure to volatiles from damaged plants were the same as those activated directly by herbivory on a plant. Only volatiles emitted after 24 h of damage, including newly produced volatiles, were found to increase phytohormone levels, upregulate defence genes, and enhance resistance to caterpillars. These results indicate that the defence induction by volatiles is a specific response to de novo synthesised volatiles, suggesting that these compounds are honest signals of herbivore attack. These findings point to an adaptive origin of airborne signalling between plants.

Isochorismate synthase is required for phylloquinone, but not salicylic acid biosynthesis in rice

Salicylic acid (SA) is a phytohormone required for plant growth and defense signaling. There are two major SA biosynthesis pathways in plants: the isochorismate synthase (ICS) pathway and the phenylalanine ammonia-lyase (PAL) pathway. It has been demonstrated in several plant species, including the model plant Arabidopsis, that SA is derived predominantly from the ICS pathway. Here, we employed the CRISPR/Cas9 system to generate ICS knockout mutants in rice (Oryza sativa L.). The Osics mutants display severe growth defects, and are completely devoid of phylloquinone, an isochorismate-derived product. The growth defects of Osics can be rescued through exogenous application of 1,4-dihydroxy-2-naphthoic acid (NA), a precursor of phylloquinone. Remarkably, the basal SA levels are not altered in the Osics mutants. Our findings support a role of OsICS in the biosynthesis of phylloquinone, and imply that SA biosynthesis in rice may occur through an alternative route other than the ICS pathway.

Supplementary Information

The online version contains supplementary material available at 10.1007/s42994-024-00166-4.

Bacterial bioluminescence is an important regulator of multitrophic interactions in the soil

Enormous efforts have been made to understand the functions of bioluminescence; however, its relevance in soil ecosystems has barely been investigated. In addition, our understanding of the biological relevance of bioluminescence is hampered by the scarcity of tools to genetically manipulate this trait. Using the symbionts of entomopathogenic nematodes, Photorhabdus bacteria, we show that bioluminescence plays important regulatory roles in multitrophic interactions in the soil. Through genetic modifications and exploiting natural variability, we provide direct evidence for the multifunctional nature of bioluminescence. It regulates abiotic and biotic stress resistance, impacts other trophic levels, including nematodes, insects, and plants, and contributes to symbiosis. Our study contributes to understanding the factors that have driven the evolution and maintenance of this trait in belowground ecosystems.

Far-red light increases maize volatile emissions in response to volatile cues from neighbouring plants

Plants perceive the presence and defence status of their neighbours through light and volatile cues, but how plants integrate both stimuli is poorly understood. We investigated if and how low Red to Far red light (R:FR) ratios, indicative of shading or canopy closure, affect maize (Zea mays) responses to herbivore-induced plant volatiles (HIPVs), including the green leaf volatile (Z)-3-hexenyl acetate. We modulated light signalling and perception by using FR supplementation and a phyB1phyB2 mutant, and we determined volatile release as a response readout. To gain mechanistic insights, we examined expression of volatile biosynthesis genes, hormone accumulation, and photosynthesis. Exposure to a full blend of HIPVs or (Z)-3-hexenyl acetate induced maize volatile release. Short-term FR supplementation increased this response. In contrast, prolonged FR supplementation or constitutive phytochrome B inactivation in phyB1phyB2 plants showed the opposite response. Short-term FR supplementation enhanced photosynthesis and stomatal conductance and (Z)-3-hexenyl acetate-induced JA-Ile levels. We conclude that a FR-enriched light environment can prompt maize plants to respond more strongly to HIPVs emitted by neighbours, which might be explained by changes in photosynthetic processes and phytochrome B signalling. Our findings reveal interactive responses to light and volatile cues with potentially important consequences for plant-plant and plant-herbivore interactions.

Natural plant disease suppressiveness in soils extends to insect pest control

BACKGROUND

Since the 1980s, soils in a 22-km2 area near Lake Neuchâtel in Switzerland have been recognized for their innate ability to suppress the black root rot plant disease caused by the fungal pathogen Thielaviopsis basicola. However, the efficacy of natural disease suppressive soils against insect pests has not been studied.

RESULTS

We demonstrate that natural soil suppressiveness also protects plants from the leaf-feeding pest insect Oulema melanopus. Plants grown in the most suppressive soil have a reduced stress response to Oulema feeding, reflected by dampened levels of herbivore defense-related phytohormones and benzoxazinoids. Enhanced salicylate levels in insect-free plants indicate defense-priming operating in this soil. The rhizosphere microbiome of suppressive soils contained a higher proportion of plant-beneficial bacteria, coinciding with their microbiome networks being highly tolerant to the destabilizing impact of insect exposure observed in the rhizosphere of plants grown in the conducive soils. We suggest that presence of plant-beneficial bacteria in the suppressive soils along with priming, conferred plant resistance to the insect pest, manifesting also in the onset of insect microbiome dysbiosis by the displacement of the insect endosymbionts.

CONCLUSIONS

Our results show that an intricate soil-plant-insect feedback, relying on a stress tolerant microbiome network with the presence of plant-beneficial bacteria and plant priming, extends natural soil suppressiveness from soilborne diseases to insect pests. Video Abstract.

Phosphate deficiency increases plant susceptibility to Botrytis cinerea infection by inducing the abscisic acid pathway

Plants have evolved finely regulated defense systems to counter biotic and abiotic threats. In the natural environment, plants are typically challenged by simultaneous stresses and, amid such conditions, crosstalk between the activated signaling pathways becomes evident, ultimately altering the outcome of the defense response. As an example of combined biotic and abiotic stresses, inorganic phosphate (Pi) deficiency, common in natural and agricultural environments, can occur along with attack by the fungus Botrytis cinerea, a devastating necrotrophic generalist pathogen responsible for massive crop losses. We report that Pi deficiency in Arabidopsis thaliana increases its susceptibility to infection by B. cinerea by influencing the early stages of pathogen infection, namely spore adhesion and germination on the leaf surface. Remarkably, Pi-deficient plants are more susceptible to B. cinerea despite displaying the appropriate activation of the jasmonic acid and ethylene signaling pathways, as well as producing secondary defense metabolites and reactive oxygen species. Conversely, the callose deposition in response to B. cinerea infection is compromised under Pi-deficient conditions. The levels of abscisic acid (ABA) are increased in Pi-deficient plants, and the heightened susceptibility to B. cinerea observed under Pi deficiency can be reverted by blocking ABA biosynthesis. Furthermore, high level of leaf ABA induced by overexpression of NCED6 in Pi-sufficient plants also resulted in greater susceptibility to B. cinerea infection associated with increased spore adhesion and germination, and reduced callose deposition. Our findings reveal a link between the enhanced accumulation of ABA induced by Pi deficiency and an increased sensitivity to B. cinerea infection.

Facilitative and competitive interactions between mycorrhizal and nonmycorrhizal plants in an extremely phosphorus-impoverished environment: role of ectomycorrhizal fungi and native oomycete pathogens in shaping species coexistence

Nonmycorrhizal cluster root-forming species enhance the phosphorus (P) acquisition of mycorrhizal neighbours in P-impoverished megadiverse systems. However, whether mycorrhizal plants facilitate the defence of nonmycorrhizal plants against soil-borne pathogens, in return and via their symbiosis, remains unknown. We characterised growth and defence-related compounds in Banksia menziesii (nonmycorrhizal) and Eucalyptus todtiana (ectomycorrhizal, ECM) seedlings grown either in monoculture or mixture in a multifactorial glasshouse experiment involving ECM fungi and native oomycete pathogens. Roots of B. menziesii had higher levels of phytohormones (salicylic and jasmonic acids, jasmonoyl-isoleucine and 12-oxo-phytodienoic acid) than E. todtiana which further activated a salicylic acid-mediated defence response in roots of B. menziesii, but only in the presence of ECM fungi. We also found that B. menziesii induced a shift in the defence strategy of E. todtiana, from defence-related secondary metabolites (phenolic and flavonoid) towards induced phytohormone response pathways. We conclude that ECM fungi play a vital role in the interactions between mycorrhizal and nonmycorrhizal plants in a severely P-impoverished environment, by introducing a competitive component within the facilitation interaction between the two plant species with contrasting nutrient-acquisition strategies. This study sheds light on the interplay between beneficial and detrimental soil microbes that shape plant-plant interaction in severely nutrient-impoverished ecosystems.

High-resolution kinetics of herbivore-induced plant volatile transfer reveal clocked response patterns in neighboring plants

Volatiles emitted by herbivore-attacked plants (senders) can enhance defenses in neighboring plants (receivers), however, the temporal dynamics of this phenomenon remain poorly studied. Using a custom-built, high-throughput proton transfer reaction time-of-flight mass spectrometry (PTR-ToF-MS) system, we explored temporal patterns of volatile transfer and responses between herbivore-attacked and undamaged maize plants. We found that continuous exposure to natural blends of herbivore-induced volatiles results in clocked temporal response patterns in neighboring plants, characterized by an induced terpene burst at the onset of the second day of exposure. This delayed burst is not explained by terpene accumulation during the night, but coincides with delayed jasmonate accumulation in receiver plants. The delayed burst occurs independent of day:night light transitions and cannot be fully explained by sender volatile dynamics. Instead, it is the result of a stress memory from volatile exposure during the first day and secondary exposure to bioactive volatiles on the second day. Our study reveals that prolonged exposure to natural blends of stress-induced volatiles results in a response that integrates priming and direct induction into a distinct and predictable temporal response pattern. This provides an answer to the long-standing question of whether stress volatiles predominantly induce or prime plant defenses in neighboring plants, by revealing that they can do both in sequence.

Trade-offs between the accumulation of cuticular wax and jasmonic acid-mediated herbivory resistance in maize

Plants have evolved complex physical and chemical defense systems that allow them to withstand herbivory infestation. Composed of a complex mixture of very-long-chain fatty acids (VLCFAs) and their derivatives, cuticular wax constitutes the first physical line of defense against herbivores. Here, we report the function of Glossy 8 (ZmGL8), which encodes a 3-ketoacyl reductase belonging to the fatty acid elongase complex, in orchestrating wax production and jasmonic acid (JA)-mediated defenses against herbivores in maize (Zea mays). The mutation of GL8 enhanced chemical defenses by activating the JA-dependent pathway. We observed a trade-off between wax accumulation and JA levels across maize glossy mutants and 24 globally collected maize inbred lines. In addition, we demonstrated that mutants defective in cuticular wax biosynthesis in Arabidopsis thaliana and maize exhibit enhanced chemical defenses. Comprehensive transcriptomic and lipidomic analyses indicated that the gl8 mutant confers chemical resistance to herbivores by remodeling VLCFA-related lipid metabolism and subsequent JA biosynthesis and signaling. These results suggest that VLCFA-related lipid metabolism has a critical role in regulating the trade-offs between cuticular wax and JA-mediated chemical defenses.

Wound-response jasmonate dynamics in the primary vasculature

The links between wound-response electrical signalling and the activation of jasmonate synthesis are unknown. We investigated damage-response remodelling of jasmonate precursor pools in the Arabidopsis thaliana leaf vasculature. Galactolipids and jasmonate precursors in primary veins from undamaged and wounded plants were analysed using MS-based metabolomics and NMR. In parallel, DAD1-LIKE LIPASEs (DALLs), which control the levels of jasmonate precursors in veins, were identified. A novel galactolipid containing the jasmonate precursor 12-oxo-phytodienoic acid (OPDA) was identified in veins: sn-2-O-(cis-12-oxo-phytodienoyl)-sn-3-O-(β-galactopyranosyl) glyceride (sn-2-OPDA-MGMG). Lower levels of sn-1-OPDA-MGMG were also detected. Vascular OPDA-MGMGs, sn-2-18:3-MGMG and free OPDA pools were reduced rapidly in response to damage-activated electrical signals. Reduced function dall2 mutants failed to build resting vascular sn-2-OPDA-MGMG and OPDA pools and, upon wounding, dall2 produced less jasmonoyl-isoleucine (JA-Ile) than the wild-type. DALL3 acted to suppress excess JA-Ile production after wounding, whereas dall2 dall3 double mutants strongly reduce jasmonate signalling in leaves distal to wounds. LOX6 and DALL2 function to produce OPDA and the non-bilayer-forming lipid sn-2-OPDA-MGMG in the primary vasculature. Membrane depolarizations trigger rapid depletion of these molecules. We suggest that electrical signal-dependent lipid phase changes help to initiate vascular jasmonate synthesis in wounded leaves.

Immature leaves are the dominant volatile-sensing organs of maize

Plants perceive herbivory-induced volatiles and respond to them by upregulating their defenses. To date, the organs responsible for volatile perception remain poorly described. Here, we show that responsiveness to the herbivory-induced green leaf volatile (Z)-3-hexenyl acetate (HAC) in terms of volatile emission, transcriptional regulation, and jasmonate defense hormone activation is largely constrained to younger maize leaves. Older leaves are much less sensitive to HAC. In a given leaf, responsiveness to HAC is high at immature developmental stages and drops off rapidly during maturation. Responsiveness to the non-volatile elicitor ZmPep3 shows an opposite pattern, demonstrating that this form of hyposmia (i.e., decreased sense of smell) is not due to a general defect in jasmonate defense signaling in mature leaves. Neither stomatal conductance nor leaf cuticle composition explains the unresponsiveness of older leaves to HAC, suggesting perception mechanisms upstream of jasmonate signaling as driving factors. Finally, we show that hyposmia in older leaves is not restricted to HAC and extends to the full blend of herbivory-induced volatiles. In conclusion, our work identifies immature maize leaves as dominant stress volatile-sensing organs. The tight spatiotemporal control of volatile perception may facilitate within plant defense signaling to protect young leaves and may allow plants with complex architectures to explore the dynamic odor landscapes at the outer periphery of their shoots.

The Arabidopsis endosperm is a temperature-sensing tissue that implements seed thermoinhibition through phyB

Seed thermoinhibition, the repression of germination under high temperatures, prevents seedling establishment under potentially fatal conditions. Thermoinhibition is relevant for phenology and agriculture, particularly in a warming globe. The temperature sensing mechanisms and signaling pathways sustaining thermoinhibition are unknown. Here we show that thermoinhibition in Arabidopsis thaliana is not autonomously controlled by the embryo but is rather implemented by the endosperm. High temperature is sensed through endospermic phyB by accelerating its reversion from the active signaling Pfr form into the inactive Pr form, as previously described in seedlings. This leads to thermoinhibition mediated by PIFs, mainly PIF1, PIF3 and PIF5. Endospermic PIF3 represses the expression of the endospermic ABA catabolic gene CYP707A1 and promotes endospermic ABA accumulation and release towards the embryo to block its growth. Furthermore, endospermic ABA represses embryonic PIF3 accumulation that would otherwise promote embryonic growth. Hence, under high temperatures PIF3 exerts opposite growth responses in the endosperm and embryo.

Abscisic acid modulates neighbor proximity-induced leaf hyponasty in Arabidopsis

Leaves of shade-avoiding plants such as Arabidopsis (Arabidopsis thaliana) change their growth pattern and position in response to low red to far-red ratios (LRFRs) encountered in dense plant communities. Under LRFR, transcription factors of the phytochrome-interacting factor (PIF) family are derepressed. PIFs induce auxin production, which is required for promoting leaf hyponasty, thereby favoring access to unfiltered sunlight. Abscisic acid (ABA) has also been implicated in the control of leaf hyponasty, with gene expression patterns suggesting that LRFR regulates the ABA response. Here, we show that LRFR leads to a rapid increase in ABA levels in leaves. Changes in ABA levels depend on PIFs, which regulate the expression of genes encoding isoforms of the enzyme catalyzing a rate-limiting step in ABA biosynthesis. Interestingly, ABA biosynthesis and signaling mutants have more erect leaves than wild-type Arabidopsis under white light but respond less to LRFR. Consistent with this, ABA application decreases leaf angle under white light; however, this response is inhibited under LRFR. Tissue-specific interference with ABA signaling indicates that an ABA response is required in different cell types for LRFR-induced hyponasty. Collectively, our data indicate that LRFR triggers rapid PIF-mediated ABA production. ABA plays a different role in controlling hyponasty under white light than under LRFR. Moreover, ABA exerts its activity in multiple cell types to control leaf position.

Functional characterization of MANNOSE-BINDING LECTIN 1, a G-type lectin gene family member, in response to fungal pathogens of strawberry

The mannose-binding lectin gene MANNOSE-BINDING LECTIN 1 (MBL1) is a member of the G-type lectin family and is involved in defense in strawberry (Fragaria × ananassa). Genome-wide identification of the G-type lectin family was carried out in woodland strawberry, F. vesca, and 133 G-lectin genes were found. Their expression profiles were retrieved from available databases and indicated that many are actively expressed during plant development or interaction with pathogens. We selected MBL1 for further investigation and generated stable transgenic FaMBL1-overexpressing plants of F. ×ananassa to examine the role of this gene in defense. Plants were selected and evaluated for their contents of disease-related phytohormones and their reaction to biotic stresses, and this revealed that jasmonic acid decreased in the overexpressing lines compared with the wild-type (WT). Petioles of the overexpressing lines inoculated with Colletotrichum fioriniae had lower disease incidence than the WT, and leaves of these lines challenged by Botrytis cinerea showed significantly smaller lesion diameters than the WT and higher expression of CLASS II CHITINASE 2-1. Our results indicate that FaMBL1 plays important roles in strawberry response to fungal diseases caused by C. fioriniae and B. cinerea.

Leafminer attack accelerates the development of soil-dwelling conspecific pupae via plant-mediated changes in belowground volatiles

Herbivore population dynamics are strongly influenced by the interactions established through their shared host. Such plant-mediated interactions can occur between different herbivore species and different life developmental stages of the same herbivore. However, whether these interactions occur between leaf-feeding herbivores and their soil-dwelling pupae is unknown. We studied whether tomato (Solanum lycopersicum) leaf herbivory by the American serpentine leafminer Liriomyza trifolii affects the performance of conspecific pupae exposed to the soil headspace of the plant. To gain mechanistic insights, we performed insect bioassays with the jasmonate-deficient tomato mutant def-1 and its wild-type, along with phytohormones, gene expression and root volatiles analyses. Belowground volatiles accelerated leafminer metamorphosis when wild-type plants were attacked aboveground by conspecifics. The opposite pattern was observed for def-1 plants, in which aboveground herbivory slowed metamorphosis. Leafminer attack induced jasmonate and abscisic acid accumulation and modulated volatile production in tomato roots in a def-1-dependent manner. Our results demonstrate that aboveground herbivory triggers changes in root defence signalling and expression, which can directly or indirectly via changes in soil or microbial volatiles, alter pupal development time. This finding expands the repertoire of plant-herbivore interactions to herbivory-induced modulation of metamorphosis, with potential consequences for plant and herbivore community dynamics.

The MIK2/SCOOP Signaling System Contributes to Arabidopsis Resistance Against Herbivory by Modulating Jasmonate and Indole Glucosinolate Biosynthesis

Initiation of plant immune signaling requires recognition of conserved molecular patterns from microbes and herbivores by plasma membrane-localized pattern recognition receptors. Additionally, plants produce and secrete numerous small peptide hormones, termed phytocytokines, which act as secondary danger signals to modulate immunity. In Arabidopsis, the Brassicae-specific SERINE RICH ENDOGENOUS PEPTIDE (SCOOP) family consists of 14 members that are perceived by the leucine-rich repeat receptor kinase MALE DISCOVERER 1-INTERACTING RECEPTOR LIKE KINASE 2 (MIK2). Recognition of SCOOP peptides elicits generic early signaling responses but knowledge on how and if SCOOPs modulate specific downstream immune defenses is limited. We report here that depletion of MIK2 or the single PROSCOOP12 precursor results in decreased Arabidopsis resistance against the generalist herbivore Spodoptera littoralis but not the specialist Pieris brassicae. Increased performance of S. littoralis on mik2-1 and proscoop12 is accompanied by a diminished accumulation of jasmonic acid, jasmonate-isoleucine and indolic glucosinolates. Additionally, we show transcriptional activation of the PROSCOOP gene family in response to insect herbivory. Our data therefore indicate that perception of endogenous SCOOP peptides by MIK2 modulates the jasmonate pathway and thereby contributes to enhanced defense against a generalist herbivore.

Soil composition and plant genotype determine benzoxazinoid-mediated plant-soil feedbacks in cereals

Plant-soil feedbacks refer to effects on plants that are mediated by soil modifications caused by the previous plant generation. Maize conditions the surrounding soil by secretion of root exudates including benzoxazinoids (BXs), a class of bioactive secondary metabolites. Previous work found that a BX-conditioned soil microbiota enhances insect resistance while reducing biomass in the next generation of maize plants. Whether these BX-mediated and microbially driven feedbacks are conserved across different soils and response species is unknown. We found the BX-feedbacks on maize growth and insect resistance conserved between two arable soils, but absent in a more fertile grassland soil, suggesting a soil-type dependence of BX feedbacks. We demonstrated that wheat also responded to BX-feedbacks. While the negative growth response to BX-conditioning was conserved in both cereals, insect resistance showed opposite patterns, with an increase in maize and a decrease in wheat. Wheat pathogen resistance was not affected. Finally and consistent with maize, we found the BX-feedbacks to be cultivar-specific. Taken together, BX-feedbacks affected cereal growth and resistance in a soil and genotype-dependent manner. Cultivar-specificity of BX-feedbacks is a key finding, as it hides the potential to optimize crops that avoid negative plant-soil feedbacks in rotations.

Phosphate Suppression of Arbuscular Mycorrhizal Symbiosis Involves Gibberellic Acid Signaling

Most land plants entertain a mutualistic symbiosis known as arbuscular mycorrhiza with fungi (Glomeromycota) that provide them with essential mineral nutrients, in particular phosphate (Pi), and protect them from biotic and abiotic stress. Arbuscular mycorrhizal (AM) symbiosis increases plant productivity and biodiversity and is therefore relevant for both natural plant communities and crop production. However, AM fungal populations suffer from intense farming practices in agricultural soils, in particular Pi fertilization. The dilemma between natural fertilization from AM symbiosis and chemical fertilization has raised major concern and emphasizes the need to better understand the mechanisms by which Pi suppresses AM symbiosis. Here, we test the hypothesis that Pi may interfere with AM symbiosis via the phytohormone gibberellic acid (GA) in the Solanaceous model systems Petunia hybrida and Nicotiana tabacum. Indeed, we find that GA is inhibitory to AM symbiosis and that Pi may cause GA levels to increase in mycorrhizal roots. Consistent with a role of endogenous GA as an inhibitor of AM development, GA-defective N. tabacum lines expressing a GA-metabolizing enzyme (GA methyltransferase-GAMT) are colonized more quickly by the AM fungus Rhizoglomus irregulare, and exogenous Pi is less effective in inhibiting AM colonization in these lines. Systematic gene expression analysis of GA-related genes reveals a complex picture, in which GA degradation by GA2 oxidase plays a prominent role. These findings reveal potential targets for crop breeding that could reduce Pi suppression of AM symbiosis, thereby reconciling the advantages of Pi fertilization with the diverse benefits of AM symbiosis.

Expression of the wheat disease resistance gene Lr34 in transgenic barley leads to accumulation of abscisic acid at the leaf tip

Durable disease resistance genes such as the wheat gene Lr34 are valuable sources of resistance for agricultural breeding programs. Lr34 encodes an ATP-binding cassette transporter protein involved in the transport of the phytohormone abscisic acid. Lr34 from wheat is functionally transferable to barley, maize, rice and sorghum. A pleiotropic effect of Lr34 induces the development of a senescence-like phenotype, referred to as leaf tip necrosis. We used Lr34-expressing wheat and transgenic barley plants to elucidate the role of abscisic acid in the development of leaf tip necrosis. Leaf tips in Lr34-expressing wheat and barley showed an accumulation of abscisic acid. No increase of Lr34 expression was detected in the leaf tip. Instead, the development of ectopic, Lr34-induced leaf tip necrosis after removing the leaf tip suggests an increased flux of abscisic acid towards the tip, where it accumulates and mediates the development of leaf tip necrosis. This redistribution of abscisic acid was also observed in adult transgenic barley plants with a high Lr34 expression level growing in the field and coincided with leaf tip necrosis as well as complete field resistance against Puccinia hordei and Blumeria graminis f. sp. hordei. In a barley transgenic line with a lower Lr34 expression level, a quantitative resistance against Puccinia hordei was still observed, but without a significant redistribution of abscisic acid or apparent leaf tip necrosis. Thus, our results imply that fine-tuning the Lr34 expression level is essential to balance disease resistance versus leaf tip necrosis to deploy transgenic Lr34 in breeding programs.

Herbivore-induced plant volatiles mediate defense regulation in maize leaves but not in maize roots

Plant leaves that are exposed to herbivore-induced plant volatiles (HIPVs) respond by increasing their defenses, a phenomenon referred to as priming. Whether this phenomenon also occurs in the roots is unknown. Using maize plants, Zea mays, whose leaves respond strongly to leaf HIPVs, we measured the impact of belowground HIPVs, emanating from roots infested by the banded cucumber beetle, Diabrotica balteata, on constitutive and herbivore-induced levels of defense-related gene expression, phytohormones, volatile and non-volatile primary and secondary metabolites, growth and herbivore resistance in roots of neighbouring plants. HIPV exposure did not increase constitutive or induced levels of any of the measured root traits. Furthermore, HIPV exposure did not reduce the performance or survival of D. balteata on maize or its ancestor teosinte. Cross-exposure experiments between HIPVs from roots and leaves revealed that maize roots, in contrast to maize leaves, neither emit nor respond strongly to defense-regulating HIPVs. Together, these results demonstrate that volatile-mediated defense regulation is restricted to the leaves of maize. This finding is in line with the lower diffusibility of volatiles in the soil and the availability of other, potentially more efficient, information conduits below ground.

Indole primes defence signalling and increases herbivore resistance in tea plants

Upon herbivore attack, plants emit herbivore-induced plant volatiles (HIPVs). HIPVs can prime defences and resistance of intact plants. However, how HIPVs are decoded and translated into functional defence responses is not well understood, especially in long-lived woody plants. Here, we investigated the impact of the aromatic HIPV indole on defence-related early signalling, phytohormone accumulation, secondary metabolite biosynthesis and herbivore resistance in tea plants. We find that tea plants infested with tea geometrid caterpillars release indole at concentrations >450 ng*hr^-1 . Exposure to corresponding doses of synthetic indole primes the expression of early defence genes involved in calcium (Ca²⁺ ) signalling, MPK signalling and jasmonate biosynthesis. Indole exposure also primes the production of jasmonates and defence-related secondary metabolites. These changes are associated with higher herbivore resistance of indole-exposed tea plants. Chemical inhibition of Ca²⁺ and jasmonate signalling provides evidence that both are required for indole-mediated defence priming and herbivore resistance. Our systematic assessment of the impact of indole on defence signalling and deployment shows that indole acts by boosting Ca²⁺ signalling, resulting in enhanced jasmonate-dependent defence and resistance in a woody plant. Our work extends the molecular basis of HIPV-induced defence priming from annual plants to an economically important tree species.

VAPYRIN attenuates defence by repressing PR gene induction and localized lignin accumulation during arbuscular mycorrhizal symbiosis of Petunia hybrida

The intimate association of host and fungus in arbuscular mycorrhizal (AM) symbiosis can potentially trigger induction of host defence mechanisms against the fungus, implying that successful symbiosis requires suppression of defence. We addressed this phenomenon by using AM-defective vapyrin (vpy) mutants in Petunia hybrida, including a new allele (vpy-3) with a transposon insertion close to the ATG start codon. We explore whether abortion of fungal infection in vpy mutants is associated with the induction of defence markers, such as cell wall alterations, accumulation of reactive oxygen species (ROS), defence hormones and induction of pathogenesis-related (PR) genes. We show that vpy mutants exhibit a strong resistance against intracellular colonization, which is associated with the generation of cell wall appositions (papillae) with lignin impregnation at fungal entry sites, while no accumulation of defence hormones, ROS or callose was observed. Systematic analysis of PR gene expression revealed that several PR genes are induced in mycorrhizal roots of the wild-type, and even more in vpy plants. Some PR genes are induced exclusively in vpy mutants. Our results suggest that VPY is involved in avoiding or suppressing the induction of a cellular defence syndrome that involves localized lignin deposition and PR gene induction.

Auxin Profiling and GmPIN Expression in Phytophthora sojae-Soybean Root Interactions

Auxin (indole-3-acetic acid, IAA) has been implicated as a susceptibility factor in both beneficial and pathogenic molecular plant-microbe interactions. Previous studies have identified a large number of auxin-related genes underlying quantitative disease resistance loci (QDRLs) for Phytophthora sojae. Thus, we hypothesized that auxin may be involved the P. sojae-soybean interaction. The levels of IAA and related metabolites were measured in mycelia and media supernatant as well as in mock and inoculated soybean roots in a time course assay. The expression of 11 soybean Pin-formed (GmPIN) auxin efflux transporter genes was also examined. Tryptophan, an auxin precursor, was detected in the P. sojae mycelia and media supernatant. During colonization of roots, levels of IAA and related metabolites were significantly higher in both moderately resistant Conrad and moderately susceptible Sloan inoculated roots compared with mock controls at 48 h postinoculation (hpi) in one experiment and at 72 hpi in a second, with Sloan accumulating higher levels of the auxin catabolite IAA-Ala than Conrad. Additionally, one GmPIN at 24 hpi, one at 48 hpi, and three at 72 hpi had higher expression in inoculated compared with the mock control roots in Conrad. The ability of resistant cultivars to cope with auxin accumulation may play an important role in quantitative disease resistance. Levels of jasmonic acid (JA), another plant hormone associated with defense responses, were also higher in inoculated roots at these same time points, suggesting that JA also plays a role during the later stages of infection.

Volatile-mediated defence regulation occurs in maize leaves but not in maize root

The above article was published in error by the publisher before a final editorial decision had been reached. It has therefore been removed temporarily while the editorial process concludes. The publisher apologizes for the inconvenience.

Barley isochorismate synthase mutant is phylloquinone-deficient, but has normal basal salicylic acid level

Salicylic acid (SA) is an important signaling hormone in plant immunity. It can be synthesized by either the phenylpropanoid pathway or the isochorismate pathway, but mutant studies of this have been scarce in other species than Arabidopsis. Here we identified a mutation that introduced a stop-codon early in the barley gene for isochorismate synthase (ICS). We found that homozygous ics plants wilted if not sprayed with 1,4-dihydroxy-2-naphthoic acid, a precursor of phylloquinone, also synthesized via the isochorismate pathway. Interestingly, ics had unchanged SA, suggesting that the basal level of SA is synthesized via the phenylpropanoid pathway. Previous studies have failed seeing increased SA levels in barley after attack by the powdery mildew fungus, Blumeria graminis f.sp. hordei (Bgh), and indeed, we saw no changes in the interaction of ics with this fungus. Overall, we hope this mutant will be useful for other studies of SA in barley.

The Arabidopsis thaliana transcription factor MYB59 regulates calcium signalling during plant growth and stress response

Transcription factor MYB59 is involved in plant growth and stress responses by acting as negative regulator of Ca signalling and homeostasis. The Arabidopsis thaliana transcription factor MYB59 is induced by cadmium (Cd) and plays a key role in the regulation of cell cycle progression and root elongation, but its mechanism of action is poorly understood. We investigated the expression of MYB59 and differences between wild-type plants, the myb59 mutant and MYB59-overexpressing lines (obtained by transformation in the mutant genotype) during plant growth and in response to various forms of stress. We also compared the transcriptomes of wild-type and myb59 mutant plants to determine putative MYB59 targets. The myb59 mutant has longer roots, smaller leaves and smaller cells than wild-type plants and responds differently to stress in germination assay. Transcriptomic analysis revealed the upregulation in the myb59 mutant of multiple genes involved in calcium (Ca) homeostasis and signalling, including those encoding calmodulin-like proteins and Ca transporters. Notably, MYB59 was strongly induced by Ca deficiency, and the myb59 mutant was characterized by higher levels of cytosolic Ca in root cells and showed a modest alteration of Ca transient frequency in guard cells, associated with the absence of Ca-induced stomatal closure. These results indicate that MYB59 negatively regulates Ca homeostasis and signalling during Ca deficiency, thus controlling plant growth and stress responses.

Molecular Dissection of Early Defense Signaling Underlying Volatile-Mediated Defense Regulation and Herbivore Resistance in Rice

Herbivore-induced plant volatiles prime plant defenses and resistance, but how they are integrated into early defense signaling and whether a causal relationship exists between volatile defense priming and herbivore resistance is unclear. Here, we investigated the impact of indole, a common herbivore-induced plant volatile and modulator of many physiological processes in plants, bacteria, and animals, on early defense signaling and herbivore resistance in rice (Oryza sativa). Rice plants infested by fall armyworm (Spodoptera frugiperda) caterpillars release indole at a rate of up to 25 ng*h^-1 Exposure to equal doses of exogenous indole enhances rice resistance to S. frugiperda Screening of early signaling components revealed that indole pre-exposure directly enhances the expression of the leucine-rich repeat-receptor-like kinase OsLRR-RLK1 Pre-exposure to indole followed by simulated herbivory increases (i.e. primes) the transcription, accumulation, and activation of the mitogen-activated protein kinase OsMPK3 and the expression of the downstream WRKY transcription factor gene OsWRKY70 as well as several jasmonate biosynthesis genes, resulting in higher jasmonic acid (JA) accumulation. Analysis of transgenic plants defective in early signaling showed that OsMPK3 is required and that OsMPK6 and OsWRKY70 contribute to indole-mediated defense priming of JA-dependent herbivore resistance. Therefore, herbivore-induced plant volatiles increase plant resistance to herbivores by positively regulating early defense signaling components.

Integration of two herbivore-induced plant volatiles results in synergistic effects on plant defence and resistance

Plants can use induced volatiles to detect herbivore- and pathogen-attacked neighbors and prime their defenses. Several individual volatile priming cues have been identified, but whether plants are able to integrate multiple cues from stress-related volatile blends remains poorly understood. Here, we investigated how maize plants respond to two herbivore-induced volatile priming cues with complementary information content, the green leaf volatile (Z)-3-hexenyl acetate (HAC) and the aromatic volatile indole. In the absence of herbivory, HAC directly induced defence gene expression, whereas indole had no effect. Upon induction by simulated herbivory, both volatiles increased jasmonate signalling, defence gene expression, and defensive secondary metabolite production and increased plant resistance. Plant resistance to caterpillars was more strongly induced in dual volatile-exposed plants than plants exposed to single volatiles.. Induced defence levels in dual volatile-exposed plants were significantly higher than predicted from the added effects of the individual volatiles, with the exception of induced plant volatile production, which showed no increase upon dual-exposure relative to single exposure. Thus, plants can integrate different volatile cues into strong and specific responses that promote herbivore defence induction and resistance. Integrating multiple volatiles may be beneficial, as volatile blends are more reliable indicators of future stress than single cues.

The Arabidopsis Lectin Receptor Kinase LecRK-I.8 Is Involved in Insect Egg Perception

Plants induce defense responses after insect egg deposition, but very little is known about the perception mechanisms. In Arabidopsis thaliana, eggs of the specialist insect Pieris brassicae trigger accumulation of reactive oxygen species (ROS) and salicylic acid (SA), followed by induction of defense genes and localized necrosis. Here, the involvement of the clade I L-type lectin receptor kinase LecRK-I.8 in these responses was studied. Expression of LecRK-I.8 was upregulated at the site of P. brassicae oviposition and egg extract (EE) treatment. ROS, SA, cell death, and expression of PR1 were substantially reduced in the Arabidopsis knock-out mutant lecrk-I.8 after EE treatment. In addition, EE-induced systemic resistance against Pseudomonas syringae was abolished in lecrk-I.8. Expression of ten clade I homologs of LecRK-I.8 was also induced by EE treatment, but single mutants displayed only weak alteration of EE-induced PR1 expression. These results demonstrate that LecRK-I.8 is an early component of egg perception.

Tricarboxylates Induce Defense Priming Against Bacteria in Arabidopsis thaliana

Exposure of plants to biotic stress results in an effective induction of numerous defense mechanisms that involve a vast redistribution within both primary and secondary metabolisms. For instance, an alteration of tricarboxylic acid (TCA) levels can accompany the increase of plant resistance stimulated by various synthetic and natural inducers. Moreover, components of the TCA flux may play a role during the set-up of plant defenses. In this study, we show that citrate and fumarate, two major components of the TCA cycle, are able to induce priming in Arabidopsis against the bacterial pathogen Pseudomonas syringae pv. tomato DC3000. Both citrate and fumarate show no direct antimicrobial effect and therefore enhanced bacterial resistance found in planta is solely based on the induction of the plant defense system. During the priming phase, both TCA intermediates did not induce any changes in transcript abundances of a set of defense genes, and in phytohormones and camalexin levels. However, at early time points of bacterial challenge, citrate induced a stronger salicylic acid and camalexin accumulation followed later by a boost of the jasmonic acid pathway. On the other hand, adaptations of hormonal pathways in fumarate-treated plants were more complex. While jasmonic acid was not induced, mutants impaired in jasmonic acid perception failed to mount a proper priming response induced by fumarate. Our results suggest that changes in carboxylic acid abundances can enhance Arabidopsis defense through complex signaling pathways. This highlights a promising feature of TCAs as novel defense priming agents and calls for further exploration in other pathosystems and stress situations.

Root exudate metabolites drive plant-soil feedbacks on growth and defense by shaping the rhizosphere microbiota

By changing soil properties, plants can modify their growth environment. Although the soil microbiota is known to play a key role in the resulting plant-soil feedbacks, the proximal mechanisms underlying this phenomenon remain unknown. We found that benzoxazinoids, a class of defensive secondary metabolites that are released by roots of cereals such as wheat and maize, alter root-associated fungal and bacterial communities, decrease plant growth, increase jasmonate signaling and plant defenses, and suppress herbivore performance in the next plant generation. Complementation experiments demonstrate that the benzoxazinoid breakdown product 6-methoxy-benzoxazolin-2-one (MBOA), which accumulates in the soil during the conditioning phase, is both sufficient and necessary to trigger the observed phenotypic changes. Sterilization, fungal and bacterial profiling and complementation experiments reveal that MBOA acts indirectly by altering root-associated microbiota. Our results reveal a mechanism by which plants determine the composition of rhizosphere microbiota, plant performance and plant-herbivore interactions of the next generation.

Plants can modify soil microbiota through root exudation, but how this process influences plant health in turn is often unclear. Here, Hu et al. show that maize benzoxazinoids released into the soil modify root-associated microbiota and thereby increase leaf defenses of the next plant generation.

The accumulation of β-aminobutyric acid is controlled by the plant's immune system

Endogenous levels of β-aminobutyric acid (BABA) increase after the molecular recognition of pathogen presence. BABA is accumulated differently during resistance or susceptibility to disease. The priming molecule β-aminobutyric acid has been recently shown to be a natural product of plants, and this has provided significance to the previous discovery of a perception mechanism in Arabidopsis. BABA levels were found to increase after abiotic stress or infection with virulent pathogens, but the role of endogenous BABA in defence has remained to be established. To investigate the biological significance of endogenous BABA variations during plant-pathogen interactions, we investigated how infections with virulent, avirulent (AvrRpt2), and non-pathogenic (hrpA) strains of Pseudomonas syringae pv. tomato DC3000 (Pst DC3000), as well as treatment with defence elicitors (Flg22 and AtPep2), affect the accumulation of BABA in Arabidopsis plants. We found that BABA levels increased more rapidly during resistance than susceptibility to Pst DC3000. In addition, BABA was accumulated during PAMP-triggered immunity (PTI) after infection with the non-pathogenic Pst DC3000 hrpA mutant, or treatment with elicitors. Importantly, treatment with Flg22 induced BABA rise in Columbia-0 plants but not in Wassilewskija-0 plants, which naturally possess a non-functional flagellin receptor. These results indicate that BABA levels are controlled by the plant's immune system, thus advancing the understanding of the biological role of plant produced BABA.

Combined biotic stresses trigger similar transcriptomic responses but contrasting resistance against a chewing herbivore in Brassica nigra

BACKGROUND

In nature, plants are frequently exposed to simultaneous biotic stresses that activate distinct and often antagonistic defense signaling pathways. How plants integrate this information and whether they prioritize one stress over the other is not well understood.

RESULTS

We investigated the transcriptome signature of the wild annual crucifer, Brassica nigra, in response to eggs and caterpillars of Pieris brassicae butterflies, Brevicoryne brassicae aphids and the bacterial phytopathogen Xanthomonas campestris pv. raphani (Xcr). Pretreatment with egg extract, aphids, or Xcr had a weak impact on the subsequent transcriptome profile of plants challenged with caterpillars, suggesting that the second stress dominates the transcriptional response. Nevertheless, P. brassicae larval performance was strongly affected by egg extract or Xcr pretreatment and depended on the site where the initial stress was applied. Although egg extract and Xcr pretreatments inhibited insect-induced defense gene expression, suggesting salicylic acid (SA)/jasmonic acid (JA) pathway cross talk, this was not strictly correlated with larval performance.

CONCLUSION

These results emphasize the need to better integrate plant responses at different levels of biological organization and to consider localized effects in order to predict the consequence of multiple stresses on plant resistance.

The Role of the Atypical Kinases ABC1K7 and ABC1K8 in Abscisic Acid Responses

The ABC1K family of atypical kinases (activity of bc1 complex kinase) is represented in bacteria, archaea, and eukaryotes. In plants they regulate diverse physiological processes in the chloroplasts and mitochondria, but their precise functions are poorly defined. ABC1K7 and ABC1K8 are probably involved in oxidative stress responses, isoprenyl lipid synthesis and distribution of iron within chloroplasts. Because reactive oxygen species take part in abscisic acid (ABA)-mediated processes, we investigated the functions of ABC1K7 and ABC1K8 during germination, stomatal movement, and leaf senescence. Both genes were upregulated by ABA treatment and some ABA-responsive physiological processes were affected in abc1k7 and abc1k8 mutants. Germination was more severely affected by ABA, osmotic stress and salt stress in the single and double mutants; the stomatal aperture was smaller in the mutants under standard growth conditions and was not further reduced by exogenous ABA application; ABA-induced senescence symptoms were more severe in the leaves of the single and double mutants compared to wild type leaves. Taken together, our results suggest that ABC1K7 and ABC1K8 might be involved in the cross-talk between ABA and ROS signaling.

The wheat resistance gene Lr34 results in the constitutive induction of multiple defense pathways in transgenic barley

The wheat gene Lr34 encodes an ABCG-type transporter which provides durable resistance against multiple pathogens. Lr34 is functional as a transgene in barley, but its mode of action has remained largely unknown both in wheat and barley. Here we studied gene expression in uninfected barley lines transgenic for Lr34. Genes from multiple defense pathways contributing to basal and inducible disease resistance were constitutively active in seedlings and mature leaves. In addition, the hormones jasmonic acid and salicylic acid were induced to high levels, and increased levels of lignin as well as hordatines were observed. These results demonstrate a strong, constitutive re-programming of metabolism by Lr34. The resistant Lr34 allele (Lr34res) encodes a protein that differs by two amino acid polymorphisms from the susceptible Lr34sus allele. The deletion of a single phenylalanine residue in Lr34sus was sufficient to induce the characteristic Lr34-based responses. Combination of Lr34res and Lr34sus in the same plant resulted in a reduction of Lr34res expression by 8- to 20-fold when the low-expressing Lr34res line BG8 was used as a parent. Crosses with the high-expressing Lr34res line BG9 resulted in an increase of Lr34sus expression by 13- to 16-fold in progenies that inherited both alleles. These results indicate an interaction of the two Lr34 alleles on the transcriptional level. Reduction of Lr34res expression in BG8 crosses reduced the negative pleiotropic effects of Lr34res on barley growth and vigor without compromising disease resistance, suggesting that transgenic combination of Lr34res and Lr34sus can result in agronomically useful resistance.

Cell wall maturation of Arabidopsis trichomes is dependent on exocyst subunit EXO70H4 and involves callose deposition

Arabidopsis (Arabidopsis thaliana) leaf trichomes are single-cell structures with a well-studied development, but little is understood about their function. Developmental studies focused mainly on the early shaping stages, and little attention has been paid to the maturation stage. We focused on the EXO70H4 exocyst subunit, one of the most up-regulated genes in the mature trichome. We uncovered EXO70H4-dependent development of the secondary cell wall layer, highly autofluorescent and callose rich, deposited only in the upper part of the trichome. The boundary is formed between the apical and the basal parts of mature trichome by a callose ring that is also deposited in an EXO70H4-dependent manner. We call this structure the Ortmannian ring (OR). Both the secondary cell wall layer and the OR are absent in the exo70H4 mutants. Ecophysiological aspects of the trichome cell wall thickening include interference with antiherbivore defense and heavy metal accumulation. Ultraviolet B light induces EXO70H4 transcription in a CONSTITUTIVE PHOTOMORPHOGENIC1-dependent way, resulting in stimulation of trichome cell wall thickening and the OR biogenesis. EXO70H4-dependent trichome cell wall hardening is a unique phenomenon, which may be conserved among a variety of the land plants. Our analyses support a concept that Arabidopsis trichome is an excellent model to study molecular mechanisms of secondary cell wall deposition.