Pierre Pétriacq

Plant perception of β-aminobutyric acid is mediated by an aspartyl-tRNA synthetase

Specific chemicals can prime the plant immune system for augmented defence. β-aminobutyric acid (BABA) is a priming agent that provides broad-spectrum disease protection. However, BABA also suppresses plant growth when applied in high doses, which has hampered its application as a crop defence activator. Here we describe a mutant of Arabidopsis thaliana that is impaired in BABA-induced disease immunity (ibi1) but hypersensitive to BABA-induced growth repression. IBI encodes an aspartyl-tRNA synthetase. Enantiomer-specific binding of R-BABA to IBI1 primed the protein for non-canonical defence signalling in the cytoplasm after pathogen attack. This priming was associated with aspartic acid accumulation and tRNA-induced phosphorylation of translation initiation factor eIF2α. However, mutation of eIF2α-phosphorylating GCN2 kinase did not affect BABA-induced immunity, but relieved BABA-induced growth repression. Hence, BABA-activated IBI1 controls plant immunity and growth via separate pathways. Our results open new opportunities to separate broad-spectrum disease resistance from the associated costs on plant growth.

Spore Density Determines Infection Strategy by the Plant Pathogenic Fungus Plectosphaerella cucumerina

A plant-pathogenic fungus alters its infection strategy, depending on initial spore density on the leaf surface and intensity of the corresponding plant immune. Necrotrophic and biotrophic pathogens are resisted by different plant defenses. While necrotrophic pathogens are sensitive to jasmonic acid (JA)-dependent resistance, biotrophic pathogens are resisted by salicylic acid (SA)- and reactive oxygen species (ROS)-dependent resistance. Although many pathogens switch from biotrophy to necrotrophy during infection, little is known about the signals triggering this transition. This study is based on the observation that the early colonization pattern and symptom development by the ascomycete pathogen Plectosphaerella cucumerina (P. cucumerina) vary between inoculation methods. Using the Arabidopsis (Arabidopsis thaliana) defense response as a proxy for infection strategy, we examined whether P. cucumerina alternates between hemibiotrophic and necrotrophic lifestyles, depending on initial spore density and distribution on the leaf surface. Untargeted metabolome analysis revealed profound differences in metabolic defense signatures upon different inoculation methods. Quantification of JA and SA, marker gene expression, and cell death confirmed that infection from high spore densities activates JA-dependent defenses with excessive cell death, while infection from low spore densities induces SA-dependent defenses with lower levels of cell death. Phenotyping of Arabidopsis mutants in JA, SA, and ROS signaling confirmed that P. cucumerina is differentially resisted by JA- and SA/ROS-dependent defenses, depending on initial spore density and distribution on the leaf. Furthermore, in situ staining for early callose deposition at the infection sites revealed that necrotrophy by P. cucumerina is associated with elevated host defense. We conclude that P. cucumerina adapts to early-acting plant defenses by switching from a hemibiotrophic to a necrotrophic infection program, thereby gaining an advantage of immunity-related cell death in the host.

NAD acts as an integral regulator of multiple defense layers

NAD-mediated defense responses stimulate PAMP-triggered immunity in Arabidopsis by stimulating redox signaling and modulating the hormonal balance. Pyridine nucleotides, such as NAD, are crucial redox carriers and have emerged as important signaling molecules in stress responses. Previously, we have demonstrated in Arabidopsis (Arabidopsis thaliana) that the inducible NAD-overproducing nadC lines are more resistant to an avirulent strain of Pseudomonas syringae pv tomato (Pst-AvrRpm1), which was associated with salicylic acid-dependent defense. Here, we have further characterized the NAD-dependent immune response in Arabidopsis. Quinolinate-induced stimulation of intracellular NAD in transgenic nadC plants enhanced resistance against a diverse range of (a)virulent pathogens, including Pst-AvrRpt2, Dickeya dadantii, and Botrytis cinerea. Characterization of the redox status demonstrated that elevated NAD levels induce reactive oxygen species (ROS) production and the expression of redox marker genes of the cytosol and mitochondrion. Using pharmacological and reverse genetics approaches, we show that NAD-induced ROS production functions independently of NADPH oxidase activity and light metabolism but depends on mitochondrial respiration, which was increased at higher NAD. We further demonstrate that NAD primes pathogen-induced callose deposition and cell death. Mass spectrometry analysis reveals that NAD simultaneously induces different defense hormones and that the NAD-induced metabolic profiles are similar to those of defense-expressing plants after treatment with pathogen-associated molecular patterns. We thus conclude that NAD triggers metabolic profiles rather similar to that of pathogen-associated molecular patterns and discuss how signaling cross talk between defense hormones, ROS, and NAD explains the observed resistance to pathogens.

Metabolite profiling of non-sterile rhizosphere soil

Summary Rhizosphere chemistry is the sum of root exudation chemicals, their breakdown products and the microbial products of soil‐derived chemicals. To date, most studies about root exudation chemistry are based on sterile cultivation systems, which limits the discovery of microbial breakdown products that act as semiochemicals and shape microbial rhizosphere communities. Here, we present a method for untargeted metabolic profiling of non‐sterile rhizosphere soil. We have developed an experimental growth system that enables the collection and analysis of rhizosphere chemicals from different plant species. High‐throughput sequencing of 16S rRNA genes demonstrated that plants in the growth system support a microbial rhizosphere effect. To collect a range of (a)polar chemicals from the system, we developed extraction methods that do not cause detectable damage to root cells or soil‐inhabiting microbes, thus preventing contamination with cellular metabolites. Untargeted metabolite profiling by UPLC‐Q‐TOF mass spectrometry, followed by uni‐ and multivariate statistical analyses, identified a wide range of secondary metabolites that are enriched in plant‐containing soil, compared with control soil without roots. We show that the method is suitable for profiling the rhizosphere chemistry of Zea mays (maize) in agricultural soil, thereby demonstrating the applicability to different plant–soil combinations. Our study provides a robust method for the comprehensive metabolite profiling of non‐sterile rhizosphere soil, which represents a technical advance towards the establishment of causal relationships between the chemistry and microbial composition of the rhizosphere.

Photoperiod Affects the Phenotype of Mitochondrial Complex I Mutants

Respiratory complex I mutants do not properly acclimate to long-day conditions in Arabidopsis, demonstrating the importance of mitochondria for the photoperiod response. Plant mutants for genes encoding subunits of mitochondrial complex I (CI; NADH:ubiquinone oxidoreductase), the first enzyme of the respiratory chain, display various phenotypes depending on growth conditions. Here, we examined the impact of photoperiod, a major environmental factor controlling plant development, on two Arabidopsis (Arabidopsis thaliana) CI mutants: a new insertion mutant interrupted in both ndufs8.1 and ndufs8.2 genes encoding the NDUFS8 subunit and the previously characterized ndufs4 CI mutant. In the long day (LD) condition, both ndufs8.1 and ndufs8.2 single mutants were indistinguishable from Columbia-0 at phenotypic and biochemical levels, whereas the ndufs8.1 ndufs8.2 double mutant was devoid of detectable holo-CI assembly/activity, showed higher alternative oxidase content/activity, and displayed a growth retardation phenotype similar to that of the ndufs4 mutant. Although growth was more affected in ndufs4 than in ndufs8.1 ndufs8.2 under the short day (SD) condition, both mutants displayed a similar impairment of growth acceleration after transfer to LD compared with the wild type. Untargeted and targeted metabolomics showed that overall metabolism was less responsive to the SD-to-LD transition in mutants than in the wild type. The typical LD acclimation of carbon and nitrogen assimilation as well as redox-related parameters was not observed in ndufs8.1 ndufs8. Similarly, NAD(H) content, which was higher in the SD condition in both mutants than in Columbia-0, did not adjust under LD. We propose that altered redox homeostasis and NAD(H) content/redox state control the phenotype of CI mutants and photoperiod acclimation in Arabidopsis.

Pyridine nucleotides induce changes in cytosolic pools of calcium in Arabidopsis.

ABSTRACT NAD is a pyridine nucleotide that is involved in cell metabolism and signaling of plant growth and stress. Recently, we reported on the multifaceted nature of NAD-inducible immunity in Arabidopsis. We identified NAD as an integral regulator of multiple defense layers such as production of ROS, deposition of callose, stimulation of cell death and modulation of defense metabolism including the defense hormones SA, JA and ABA, and other defense-associated metabolites. Altogether, NAD-induced immune effects confer resistance to diverse pathogenic microbes. Our addendum to this work further demonstrates an impact of NAD on the cytosolic calcium pool, a well-known component of early plant defense response.

Long-lasting β-aminobutyric acid-induced resistance protects tomato fruit against Botrytis cinerea

Minimising losses to pests and diseases is essential for producing sufficient food to feed our rapidly growing population. The necrotrophic fungus Botrytis cinerea triggers devastating pre- and post-harvest yield losses in tomato (Solanum lycopersicum). Current control methods are based on the pre-harvest use of fungicides, which are limited by strict legislation. Here, we have tested whether induction of resistance by β-aminobutyric acid (BABA) at different developmental stages, provides an alternative strategy to protect tomato fruit post-harvest against B. cinerea. Soil-drenching plants with BABA once fruit had already formed, had no impact on tomatoes susceptibility to B. cinerea. Whereas BABA application to seedlings was found to significantly reduce the post-harvest infection of fruit. This resistance response was not associated with a yield reduction, however there was a delay in fruit ripening. Untargeted metabolomics unravelled differences between fruit from water and BABA-treated plants, demonstrating that BABA triggered a defence-associated metabolomics profile that was long-lasting. Targeted analysis of defence hormones suggested a role of abscisic acid (ABA) in the resistance phenotype. Post-harvest application of ABA to the fruit of water-treated plants induced susceptibility to B. cinerea. This phenotype was absent from the ABA exposed fruit of BABA-treated plants, suggesting a complex role of ABA in the BABA-induced resistance phenotype. A final targeted metabolomic analysis detected trace residues of BABA accumulated in the red fruit. Overall, we have demonstrated that β-aminobutyric acid induces post-harvest resistance in tomato fruit against B. cinerea with no penalties in yield. This article is protected by copyright. All rights reserved.

Chemical priming of immunity without costs to plant growth

β-Aminobutyric acid (BABA) induces broad-spectrum disease resistance, but also represses plant growth, which has limited its exploitation in crop protection. BABA perception relies on binding to the aspartyl-tRNA synthetase (AspRS) IBI1, which primes the enzyme for secondary defense activity. This study aimed to identify structural BABA analogues that induce resistance without stunting plant growth. Using site-directed mutagenesis, we demonstrate that the (l)-aspartic acid-binding domain of IBI1 is critical for BABA perception. Based on interaction models of this domain, we screened a small library of structural BABA analogues for growth repression and induced resistance against biotrophic Hyaloperonospora arabidopsidis (Hpa). A range of resistance-inducing compounds were identified, of which (R)-β-homoserine (RBH) was the most effective. Surprisingly, RBH acted through different pathways than BABA. RBH-induced resistance (RBH-IR) against Hpa functioned independently of salicylic acid, partially relied on camalexin, and was associated with augmented cell wall defense. RBH-IR against necrotrophic Plectosphaerella cucumerina acted via priming of ethylene and jasmonic acid defenses. RBH-IR was also effective in tomato against Botrytis cinerea. Metabolic profiling revealed that RBH, unlike BABA, does not majorly affect plant metabolism. RBH primes distinct defense pathways against biotrophic and necrotrophic pathogens without stunting plant growth, signifying strong potential for exploitation in crop protection.

Mechanisms of glacial‐to‐future atmospheric CO2 effects on plant immunity

Summary The impacts of rising atmospheric CO2 concentrations on plant disease have received increasing attention, but with little consensus emerging on the direct mechanisms by which CO2 shapes plant immunity. Furthermore, the impact of sub‐ambient CO 2 concentrations, which plants have experienced repeatedly over the past 800 000 yr, has been largely overlooked. A combination of gene expression analysis, phenotypic characterisation of mutants and mass spectrometry‐based metabolic profiling was used to determine development‐independent effects of sub‐ambient CO 2 (sa CO 2) and elevated CO 2 (eCO 2) on Arabidopsis immunity. Resistance to the necrotrophic Plectosphaerella cucumerina (Pc) was repressed at sa CO 2 and enhanced at eCO 2. This CO 2‐dependent resistance was associated with priming of jasmonic acid (JA)‐dependent gene expression and required intact JA biosynthesis and signalling. Resistance to the biotrophic oomycete Hyaloperonospora arabidopsidis (Hpa) increased at both eCO 2 and sa CO 2. Although eCO 2 primed salicylic acid (SA)‐dependent gene expression, mutations affecting SA signalling only partially suppressed Hpa resistance at eCO 2, suggesting additional mechanisms are involved. Induced production of intracellular reactive oxygen species (ROS) at sa CO 2 corresponded to a loss of resistance in glycolate oxidase mutants and increased transcription of the peroxisomal catalase gene CAT2, unveiling a mechanism by which photorespiration‐derived ROS determined Hpa resistance at saCO2. By separating indirect developmental impacts from direct immunological effects, we uncover distinct mechanisms by which CO 2 shapes plant immunity and discuss their evolutionary significance.

More to NAD+ than meets the eye: A regulator of metabolic pools and gene expression in Arabidopsis

&NA; Since its discovery more than a century ago, nicotinamide adenine dinucleotide (NAD+) is recognised as a fascinating cornerstone of cellular metabolism. This ubiquitous energy cofactor plays vital roles in metabolic pathways and regulatory processes, a fact emphasised by the essentiality of a balanced NAD+ metabolism for normal plant growth and development. Research on the role of NAD in plants has been predominantly carried out in the model plant Arabidopsis thaliana (Arabidopsis) with emphasis on the redox properties and cellular signalling functions of the metabolite. This review examines the current state of knowledge concerning how NAD can regulate both metabolic pools and gene expression in Arabidopsis. Particular focus is placed on recent studies highlighting the complexity of metabolic regulations involving NAD, more particularly in the mitochondrial compartment, and of signalling roles with respect to interactions with environmental fluctuations most specifically those involving plant immunity. Graphical abstract Figure. No caption available. HighlightsNAD+ is a fascinating cornerstone of plant cellular metabolism.NAD+ involves a complex network of metabolic and redox regulations in Arabidopsis.Beside redox properties, NAD+ is important for cell signalling including responses to stress.NAD+ acts as an integral regulator of immune responses in Arabidopsis.