Siderophore-mediated upregulation of Arabidopsis ferritin expression in response to Erwinia chrysanthemi infection

Cadmium and Zn hyperaccumulation provide efficient constitutive defense against Turnip yellow mosaic virus infection in Noccaea caerulescens

To understand the role of Zn and Cd in anti-viral defence, Zn/Cd hyperaccumulator Noccaea caerulescens plants grown with deficient (0.3 µM), replete (10 µM) and excess (100 µM) Zn2+ and Cd (10 µM Zn2+ + 1 µM Cd2+) were infected with Turnip yellow mosaic virus (TYMV). Gas exchange and chlorophyll fluorescence kinetics analyses demonstrated direct TYMV effects on photosynthetic light reactions but N. caerulescens was more resistant against TYMV than the previously studied non-hyperaccumulator N. ochroleucum. Virus abundance and photosynthesis inhibition were the lowest in the high Zn and Cd treatments. RNAseq analysis of 10 µM Zn2+ plants revealed TYMV-induced upregulation of Ca transporters, chloroplastic ZTP29 and defence genes, but none of those that are known to be strongly involved in hyperaccumulation. Synchrotron µ-XRF tomography, however, showed that Zn hyperaccumulation remained strongest in vacuoles of epidermal storage cells regardless of infection. This was in contrast to N. ochroleucum, where apoplastic Zn drastically increased in response to TYMV. These results suggest that the antiviral response of N. caerulescens is less induced by the onset of this biotic stress, but it is rather a permanent resistant state of the plant. Real-time qPCR revealed upregulation of ferritin in Zn10 infected plants, suggesting Fe deprivation as a virus defence strategy under suboptimal Zn supply.

The Role of Iron in Phytopathogenic Microbe-Plant Interactions: Insights into Virulence and Host Immune Response

Iron is an essential element required for the growth and survival of nearly all forms of life. It serves as a catalytic component in multiple enzymatic reactions, such as photosynthesis, respiration, and DNA replication. However, the excessive accumulation of iron can result in cellular toxicity due to the production of reactive oxygen species (ROS) through the Fenton reaction. Therefore, to maintain iron homeostasis, organisms have developed a complex regulatory network at the molecular level. Besides catalyzing cellular redox reactions, iron also regulates virulence-associated functions in several microbial pathogens. Hosts and pathogens have evolved sophisticated strategies to compete against each other over iron resources. Although the role of iron in microbial pathogenesis in animals has been extensively studied, mechanistic insights into phytopathogenic microbe-plant associations remain poorly understood. Recent intensive research has provided intriguing insights into the role of iron in several plant-pathogen interactions. This review aims to describe the recent advances in understanding the role of iron in the lifestyle and virulence of phytopathogenic microbes, focusing on bacteria and host immune responses.

Multiple roles of wheat ferritin genes during stress treatment and TaFER5D-1 as a positive regulator in response to drought and salt tolerance

Ferritin not only regulates the plant's iron content but also plays a significant role in the plant's development and resistance to oxidative damage. However, the role of the FER family in wheat has not been systematically elucidated. In this study, 39 FERs identified from wheat and its ancestral species were clustered into two subgroups, and gene members from the same group contain relatively conservative protein models. The structural analyses indicated that the gene members from the same group contained relatively conserved protein models. The cis-acting elements and expression patterns analysis suggested that TaFERs might play an important role combating to abiotic and biotic stresses. In the transcriptional analysis, the TaFER5D-1 gene was found to be significantly up-regulated under drought and salt stresses and was, therefore, selected to further explore the biological functions Moreover, the GFP expression assay revealed the subcellular localization of TaFER5D-1 proteins in the chloroplast, nucleus, membrane and cytoplasm. Over-expression of TaFER5D-1 in transgenic Arabidopsis lines conferred greater tolerance to drought and salt stress. According to the qRT-PCR data, TaFER5D-1 gene over-expression increased the expression of genes related to root development (Atsweet-17 and AtRSL4), iron storage (AtVIT1 and AtYSL1), and stress response (AtGolS1 and AtCOR47). So it is speculated that TaFER5D-1 could improve stress tolerance by promoting root growth, iron storage, and stress-response ability. Thus, the current study provides insight into the role of TaFER genes in wheat.

Control of nutrient metal availability during host-microbe interactions: beyond nutritional immunity

The control of nutrient availability is an essential ecological function of the host organism in host-microbe systems. Although often overshadowed by macronutrients such as carbohydrates, micronutrient metals are known as key drivers of host-microbe interactions. The ways in which host organisms control nutrient metal availability are dictated by principles in bioinorganic chemistry. Here I ponder about the actions of metal-binding molecules from the host organism in controlling nutrient metal availability to the host microbiota. I hope that these musings will encourage new explorations into the fundamental roles of metals in the ecology of diverse host-microbe systems.

Fe deficiency-induced ethylene synthesis confers resistance to Botrytis cinerea

Although iron (Fe) deficiency is an adverse condition to growth and development of plants, it increases the resistance to pathogens. How Fe deficiency induces the resistance to pathogens is still unclear. Here, we reveal that the inoculation of Botrytis cinerea activates the Fe deficiency response of plants, which further induces ethylene synthesis and then resistance to B. cinerea. FIT and bHLH Ib are a pair of bHLH transcription factors, which control the Fe deficiency response. Both the Fe deficiency-induced ethylene synthesis and resistance are blocked in fit-2 and bhlh4x-1 (a quadruple mutant for four bHLH Ib members). SAM1 and SAM2, two ethylene synthesis-associated genes, are induced by Fe deficiency in a FIT-bHLH Ib-dependent manner. Moreover, SAM1 and SAM2 are required for the increased ethylene and resistance to B. cinerea under Fe-deficient conditions. Our findings suggest that the FIT-bHLH Ib module activates the expression of SAM1 and SAM2, thereby inducing ethylene synthesis and resistance to B. cinerea. This study uncovers that Fe signaling also functions as a part of the plant immune system against pathogens.

Diproline-induced resistance to parasitic nematodes in the same and subsequent rice generations: Roles of iron, nitric oxide and ethylene

Induced resistance (IR) is a plant phenotype characterized by lower susceptibility to biotic challenges upon elicitation by so-called IR stimuli. Earlier, we identified diproline (cyclo(l-Pro-l-Pro)) as IR stimulus that protects rice (Oryza sativa) against the root-knot nematode Meloidogyne graminicola (Mg). In the current study, detailed transcriptome analyses at different time points, and under uninfected and nematode-infected conditions revealed that this rice IR phenotype is correlated with induction of genes related to iron (Fe), ethylene (ET) and reactive oxygen species (ROS)/reactive nitrogen species (RNS) metabolism. An infection experiment under Fe limiting conditions confirmed that diproline-IR is only effective under optimal Fe supply. Although total root Fe levels were not affected in diproline-treated plants, phytosiderophore secretion was found to be induced by this treatment. Experiments on mutant and transgenic rice lines impaired in ET or ROS/RNS metabolism confirmed that these metabolites are involved in diproline-IR. Finally, we provide evidence for transgenerational inheritance of diproline-IR (diproline-TIR), as two successive generations of diproline-treated ancestors exhibited an IR phenotype while themselves never being exposed to diproline. Transcriptome analyses on the offspring plants revealed extensive overlap between the pathways underpinning diproline-IR and diproline-TIR. Although diproline induces significant systemic changes in global DNA methylation levels early after treatment, such changes in DNA methylation were not detected in the descendants of these plants. To our knowledge, this is the first report of TIR in rice and the first transcriptional assessment of TIR in monocots.

Host plant physiological transformation and microbial population heterogeneity as important determinants of the Soft Rot Pectobacteriaceae-plant interactions

Pectobacterium and Dickeya species belonging to the Soft Rot Pectobacteriaceae (SRP) are one of the most devastating phytopathogens. They degrade plant tissues by producing an arsenal of plant cell wall degrading enzymes. However, SRP-plant interactions are not restricted to the production of these "brute force" weapons. Additionally, these bacteria apply stealth behavior related to (1) manipulation of the host plant via induction of susceptible responses and (2) formation of heterogeneous populations with functionally specialized cells. Our review aims to summarize current knowledge on SRP-induced plant susceptible responses and on the heterogeneity of SRP populations. The review shows that SRP are capable of adjusting the host's hormonal balance, inducing host-mediated plant cell wall modification, promoting iron assimilation by the host, stimulating the accumulation of reactive oxygen species and host cell death, and activating the synthesis of secondary metabolites that are ineffective in limiting disease progression. By this means, SRP facilitate host plant susceptibility. During host colonization, SRP populations produce various functionally specialized cells adapted for enhanced virulence, increased resistance, motility, vegetative growth, or colonization of the vascular system. This enables SRP to perform self-contradictory tasks, which benefits a population's overall fitness in various environments, including host plants. Such stealthy tactical actions facilitate plant-SRP interactions and disease progression.

Techniques to Study Common Root Responses to Beneficial Microbes and Iron Deficiency

Iron (Fe) plays a central role in the vital processes of a plant. The Fe status of a plant influences growth and immunity, but it also dictates interactions of roots with soil microbiota through the production of Fe mobilizing, antimicrobial fluorescent phenolic compounds called coumarins. To adapt to low Fe availability in the soil, plants deploy an efficient Fe deficiency response. Interestingly, this Fe deficiency response is hijacked by root-colonizing microbes in the root microbiome to establish a mutually beneficial relationship. In this chapter, we describe how we cultivate plants and microbes to study the interaction between plants, beneficial rhizobacteria, and the plant's Fe deficiency response. We describe (a) how we study activity and localization of these responses by assessing gene-specific promoter activities using GUS assays, (b) how we visualize root-secreted coumarins in response to Fe deficiency and colonization by beneficial rhizobacteria, and (c) how we prepare our samples for metabolite extraction and reverse-transcriptase quantitative PCR to analyze the expression of marker genes.

A novel phytopathogen Erwinia sorbitola sp. nov., isolated from the feces of ruddy shelducks

The species in the genus Erwinia are Gram-stain-negative, facultatively anaerobic, motile, and rod-shaped. Most species in the genus Erwinia are phytopathogens. Also, Erwinia persicina was involved in several human infections. Based on the reverse microbial etiology principles, it is worth analyzing the pathogenicity of species in this genus. In this study, we isolated and sequenced two species of Erwinia. Phylogenetic, phenotypic, biochemical, and chemotaxonomic analyses were performed to identify its taxonomy position. The virulence tests on plant leaves and pear fruits were used to identify the plant pathogenicity of two species of Erwinia. Bioinformatic methods predicted the possible pathogenic determinants based on the genome sequence. Meanwhile, adhesion, invasion, and cytotoxicity assays on RAW 264.7 cells were applied to identify animal pathogenicity. We isolated two Gram-stain-negative, facultatively anaerobic, motile, and rod-shaped strains from the feces of ruddy shelducks in the Tibet Plateau of China, designated J780T and J316. Distinct phylogenetic, genomic, phenotypic, biochemical, and chemotaxonomic characters of J780T and J316 identified they were novel species and belonged to the genus Erwinia, for which the name Erwinia sorbitola sp. nov. was proposed, the type strain was J780T (= CGMCC 1.17334T = GDMCC 1.1666T = JCM 33839T). Virulence tests showed blight and rot on the leaves and pear fruits confirmed Erwinia sorbitola sp. nov. was a phytopathogen. Predicted gene clusters of motility, biofilm formation, exopolysaccharides, stress survival, siderophores, and Type VI secretion system might be the causes of pathogenicity. In addition, predicted polysaccharide biosynthesis gene clusters on the genome sequence, and the high capacity for adhesion, invasion, and cytotoxicity to animal cells confirmed it has pathogenicity on animals. In conclusion, we isolated and identified a novel phytopathogen Erwinia sorbitola sp. nov. in ruddy shelducks. A predefined pathogen is beneficial for preventing from suffering potential economic losses caused by this new pathogen.

Trace metals at the frontline of pathogen defence responses in non-hyperaccumulating plants

Metal hyperaccumulation is an exclusive evolutionary trait contributing to efficient plant defence against biotic stress. The defence can be based on direct metal toxicity or the joint effects of accumulated metal and organic compounds, the latter being based on integrated signalling networks. While the role of metals in biotic stress defence of hyperaccumulators has been intensively studied, their role in the pathogen immunity of non-accumulator plants is far less understood. New findings show that in metal non-hyperaccumulating plants, localized hot spots of zinc, manganese, and iron increase plant immunity, while manipulation of nutrient availability may be used for priming against subsequent pathogen attack. Recent findings on the role of metals in plant-pathogen interactions are discussed considering the narrow line between deficiency and toxicity, host-pathogen nutrient competition and synergistic effects of simultaneous metal and biotic stress. We discuss the suitability of the direct-defence and joint-effects hypotheses in non-hyperaccumulating plants, and the involvement of metals as active centres of immunity-related enzymes. We also consider future challenges in revealing the mechanisms underlying metal-mediated plant immunity.

Rice iron storage protein ferritin 2 (OsFER2) positively regulates ferroptotic cell death and defense responses against Magnaporthe oryzae

Ferritin is a ubiquitous iron storage protein that regulates iron homeostasis and oxidative stress in plants. Iron plays an important role in ferroptotic cell death response of rice (Oryza sativa) to Magnaporthe oryzae infection. Here, we report that rice ferritin 2, OsFER2, is required for iron- and reactive oxygen species (ROS)-dependent ferroptotic cell death and defense response against the avirulent M. oryzae INA168. The full-length ferritin OsFER2 and its transit peptide were localized to the chloroplast, the most Fe-rich organelle for photosynthesis. This suggests that the transit peptide acts as a signal peptide for the rice ferritin OsFER2 to move into chloroplasts. OsFER2 expression is involved in rice resistance to M. oryzae infection. OsFER2 knock-out in wild-type rice HY did not induce ROS and ferric ion (Fe³⁺) accumulation, lipid peroxidation and hypersensitive response (HR) cell death, and also downregulated the defense-related genes OsPAL1, OsPR1-b, OsRbohB, OsNADP-ME2-3, OsMEK2 and OsMPK1, and vacuolar membrane transporter OsVIT2 expression. OsFER2 complementation in ΔOsfer2 knock-out mutants restored ROS and iron accumulation and HR cell death phenotypes during infection. The iron chelator deferoxamine, the lipid-ROS scavenger ferrostatin-1, the actin microfilament polymerization inhibitor cytochalasin E and the redox inhibitor diphenyleneiodonium suppressed ROS and iron accumulation and HR cell death in rice leaf sheaths. However, the small-molecule inducer erastin did not trigger iron-dependent ROS accumulation and HR cell death induction in ΔOsfer2 mutants. These combined results suggest that OsFER2 expression positively regulates iron- and ROS-dependent ferroptotic cell death and defense response in rice-M. oryzae interactions.

Crosstalk of nitro-oxidative stress and iron in plant immunity

Accumulation of oxygen and nitrogen radicals and their derivatives, known as reactive oxygen species (ROS) and reactive nitrogen species (RNS), occurs throughout various phases of plant growth in association with biotic and abiotic stresses. One of the consequences of environmental stresses is disruption of homeostasis between production and scavenging of ROS and RNS, which leads to nitro-oxidative burst and affects other defense-related mechanisms, such as polyamines levels, phenolics, lignin and callose as defense components related to plant cell wall reinforcement. Although this subject has attracted huge interest, the cross-talk between these signaling molecules and iron, as a main metal element involved in the activity of various enzymes and numerous vital processes in the living cells, remains largely unexplored. Therefore, it seems necessary to pay more in depth attention to the mechanisms of plant resistance against various environmental stimuli for designing novel and effective plant protection strategies. This review is focused on advances in recent knowledge related to the role of ROS, RNS, and association of these signaling molecules with iron in plant immunity. Furthermore, the role of cell wall fortification as a main physical barrier involved in plant defense have been discussed in association with reactive species and iron ions.

Iron redistribution induces oxidative burst and resistance in maize against Curvularia lunata

ΔClnps6 induced iron redistribution in maize B73 leaf cells and resulted in reactive oxygen species (ROS) burst to enhance plant resistance against Curvularia lunata. Iron is an indispensable co-factor of various crucial enzymes that are involved in cellular metabolic processes and energy metabolism in eukaryotes. For this reason, plants and pathogens compete for iron to maintain their iron homeostasis, respectively. In our previous study, ΔClnps6, the extracellular siderophore biosynthesis deletion mutant of Curvularia lunata, was sensitive to exogenous hydrogen peroxide and virulence reduction. However, the mechanism was not studied. Here, we report that maize B73 displayed highly resistance to ΔClnps6. The plants recruited more iron at cell wall appositions (CWAs) to cause ROS bursts. Intracellular iron deficiency induced by iron redistribution originated form up-regulated expression of genes involved in intracellular iron consumption in leaves and absorption in roots. The RNA-sequencing data also showed that the expression of respiratory burst oxidase homologue (ZmRBOH4) and NADP-dependent malic enzyme 4 (ZmNADP-ME4) involved in ROS production was up-regulated in maize B73 after ΔClnps6 infection. Simultaneously, jasmonic acid (JA) biosynthesis genes lipoxygenase (ZmLOX), allene oxide synthase (ZmAOS), GA degradation gene gibberellin 2-beta-dioxygenase (ZmGA2OX6) and ABA degradation genes abscisic acid hydroxylase (ZmABH1, ZmABH2) involved in iron homeostasis were up-regulated expression. Ferritin1 (ZmFER1) positive regulated maize resistance against C. lunata via ROS burst under Fe-limiting conditions. Overall, our results showed that iron played vital roles in activating maize resistance in B73-C. lunata interaction.

Belowground plant-microbe communications via volatile compounds

Volatile compounds play important roles in rhizosphere biological communications and interactions. The emission of plant and microbial volatiles is a dynamic phenomenon that is affected by several endogenous and exogenous signals. Diffusion of volatiles can be limited by their adsorption, degradation, and dissolution under specific environmental conditions. Therefore, rhizosphere volatiles need to be investigated on a micro and spatiotemporal scale. Plant and microbial volatiles can expand and specialize the rhizobacterial niche not only by improving the root system architecture such that it serves as a nutrient-rich shelter, but also by inhibiting or promoting the growth, chemotaxis, survival, and robustness of neighboring organisms. Root volatiles play an important role in engineering the belowground microbiome by shaping the microbial community structure and recruiting beneficial microbes. Microbial volatiles are appropriate candidates for improving plant growth and health during environmental challenges and climate change. However, some technical and experimental challenges limit the non-destructive monitoring of volatile emissions in the rhizosphere in real-time. In this review, we attempt to clarify the volatile-mediated intra- and inter-kingdom communications in the rhizosphere, and propose improvements in experimental design for future research.

The Knockout of Enterobactin-Related Gene in Pectobacterium atrosepticum Results in Reduced Stress Resistance and Virulence towards the Primed Plants

Siderophores produced by microorganisms to scavenge iron from the environment have been shown to contribute to virulence and/or stress resistance of some plant pathogenic bacteria. Phytopathogenic bacteria of Pectobacterium genus possess genes for the synthesis of siderophore enterobactin, which role in plant-pathogen interactions has not been elucidated. In the present study we characterized the phenotype of the mutant strain of Pba deficient for the enterobactin-biosynthetic gene entA. We showed that enterobactin may be considered as a conditionally beneficial virulence factor of Pba. The entA knockout did not reduce Pba virulence on non-primed plants; however, salicylic acid-primed plants were more resistant to ΔentA mutant than to the wild type Pba. The reduced virulence of ΔentA mutant towards the primed plants is likely explained by its compromised resistance to oxidative stress.

RNA Chaperones Hfq and ProQ Play a Key Role in the Virulence of the Plant Pathogenic Bacterium Dickeya dadantii

Dickeya dadantii is an important pathogenic bacterium that infects a number of crops including potato and chicory. While extensive works have been carried out on the control of the transcription of its genes encoding the main virulence functions, little information is available on the post-transcriptional regulation of these functions. We investigated the involvement of the RNA chaperones Hfq and ProQ in the production of the main D. dadantii virulence functions. Phenotypic assays on the hfq and proQ mutants showed that inactivation of hfq resulted in a growth defect, a modified capacity for biofilm formation and strongly reduced motility, and in the production of degradative extracellular enzymes (proteases, cellulase, and pectate lyases). Accordingly, the hfq mutant failed to cause soft rot on chicory leaves. The proQ mutant had reduced resistance to osmotic stress, reduced extracellular pectate lyase activity compared to the wild-type strain, and reduced virulence on chicory leaves. Most of the phenotypes of the hfq and proQ mutants were related to the low amounts of mRNA of the corresponding virulence factors. Complementation of the double mutant hfq-proQ by each individual protein and cross-complementation of each chaperone suggested that they might exert their effects via partially overlapping but different sets of targets. Overall, it clearly appeared that the two Hfq and ProQ RNA chaperones are important regulators of pathogenicity in D. dadantii. This underscores that virulence genes are regulated post-transcriptionally by non-coding RNAs.

Interplay between Coumarin Accumulation, Iron Deficiency and Plant Resistance to Dickeya spp

Coumarins belong to a group of secondary metabolites well known for their high biological activities including antibacterial and antifungal properties. Recently, an important role of coumarins in plant resistance to pathogens and their release into the rhizosphere upon pathogen infection was discovered. It is also well documented that coumarins play a crucial role in the Arabidopsis thaliana growth under Fe-limited conditions. However, the mechanisms underlying interplay between plant resistance, accumulation of coumarins and Fe status, remain largely unknown. In this work, we investigated the effect of both mentioned factors on the disease severity using the model system of Arabidopsis/Dickeya spp. molecular interactions. We evaluated the disease symptoms in Arabidopsis plants, wild-type Col-0 and its mutants defective in coumarin accumulation, grown in hydroponic cultures with contrasting Fe regimes and in soil mixes. Under all tested conditions, Arabidopsis plants inoculated with Dickeya solani IFB0099 strain developed more severe disease symptoms compared to lines inoculated with Dickeya dadantii 3937. We also showed that the expression of genes encoding plant stress markers were strongly affected by D. solani IFB0099 infection. Interestingly, the response of plants to D. dadantii 3937 infection was genotype-dependent in Fe-deficient hydroponic solution.

Mechanisms underlying iron deficiency-induced resistance against pathogens with different lifestyles

Iron (Fe) is a poorly available mineral nutrient which affects the outcome of many cross-kingdom interactions. In Arabidopsis thaliana, Fe starvation limits infection by necrotrophic pathogens. Here, we report that Fe deficiency also reduces disease caused by the hemi-biotrophic bacterium Pseudomonas syringae and the biotrophic oomycete Hyaloperonospora arabidopsidis, indicating that Fe deficiency-induced resistance is effective against pathogens with different lifestyles. Furthermore, we show that Fe deficiency-induced resistance is not caused by withholding Fe from the pathogen but is a plant-mediated defense response that requires activity of ethylene and salicylic acid. Because rhizobacteria-induced systemic resistance (ISR) is associated with a transient up-regulation of the Fe deficiency response, we tested whether Fe deficiency-induced resistance and ISR are similarly regulated. However, Fe deficiency-induced resistance functions independently of the ISR regulators MYB72 and BGLU42, indicating that both types of induced resistance are regulated in a different manner. Mutants opt3 and frd1, which display misregulated Fe homeostasis under Fe-sufficient conditions, show disease resistance levels comparable with those of Fe-starved wild-type plants. Our results suggest that disturbance of Fe homeostasis, through Fe starvation stress or other non-homeostatic conditions, is sufficient to prime the plant immune system for enhanced defense.

Iron in plant-pathogen interactions

Iron is an essential element for most organisms. As an indispensable co-factor of many enzymes, iron is involved in various crucial metabolic processes that are required for the survival of plants and pathogens. Conversely, excessive iron produces highly active reactive oxygen species, which are toxic to the cells of plants and pathogens. Therefore, plants and pathogens have evolved sophisticated mechanisms to modulate iron status at a moderate level for maintaining their fitness. Over the past decades, many efforts have been made to reveal these mechanisms, and some progress has been made. In this review, we describe recent advances in understanding the roles of iron in plant-pathogen interactions and propose prospects for future studies.

Proteomics reveals synergy between biomass degrading enzymes and inorganic Fenton chemistry in leaf-cutting ant colonies

The symbiotic partnership between leaf-cutting ants and fungal cultivars processes plant biomass via ant fecal fluid mixed with chewed plant substrate before fungal degradation. Here we present a full proteome of the fecal fluid of Acromyrmex leaf-cutting ants, showing that most proteins function as biomass degrading enzymes and that ca. 85% are produced by the fungus and ingested, but not digested, by the ants. Hydrogen peroxide producing oxidoreductases were remarkably common in the proteome, inspiring us to test a scenario in which hydrogen peroxide reacts with iron to form reactive oxygen radicals after which oxidized iron is reduced by other fecal-fluid enzymes. Our biochemical assays confirmed that these so-called Fenton reactions do indeed take place in special substrate pellets, presumably to degrade plant cell wall polymers. This implies that the symbiotic partnership manages a combination of oxidative and enzymatic biomass degradation, an achievement that surpasses current human bioconversion technology.

Plasmopara viticola infection affects mineral elements allocation and distribution in Vitis vinifera leaves

Plasmopara viticola is one of the most important pathogens infecting Vitis vinifera plants. The interactions among P. viticola and both susceptible and resistant grapevine plants have been extensively characterised, at transcriptomic, proteomic and metabolomic levels. However, the involvement of plants ionome in the response against the pathogen has been completely neglected so far. Therefore, this study was aimed at investigating the possible role of leaf ionomic modulation during compatible and incompatible interactions between P. viticola and grapevine plants. In susceptible cultivars, a dramatic redistribution of mineral elements has been observed, thus uncovering a possible role for mineral nutrients in the response against pathogens. On the contrary, the resistant cultivars did not present substantial rearrangement of mineral elements at leaf level, except for manganese (Mn) and iron (Fe). This might demonstrate that, resistant cultivars, albeit expressing the resistance gene, still exploit a pathogen response mechanism based on the local increase in the concentration of microelements, which are involved in the synthesis of secondary metabolites and reactive oxygen species. Moreover, these data also highlight the link between the mineral nutrition and plants' response to pathogens, further stressing that appropriate fertilization strategies can be fundamental for the expression of response mechanisms against pathogens.

A Siderophore Analog of Fimsbactin from Acinetobacter Hinders Growth of the Phytopathogen Pseudomonas syringae and Induces Systemic Priming of Immunity in Arabidopsis thaliana

Siderophores produced in soil by plant growth-promoting rhizobacteria (PGPRs) play several roles, including nutrient mobilizers and can be useful as plants defense elicitors. We investigated the role of a synthetic mixed ligand bis-catechol-mono-hydroxamate siderophore (SID) that mimics the chemical structure of a natural siderophore, fimsbactin, produced by Acinetobacter spp. in the resistance against the phytopathogen Pseudomonas syringaepv tomato DC3000 (Pst DC3000), in Arabidopsis thaliana. We first tested the antibacterial activity of SID against Pst DC3000 in vitro. After confirming that SID had antibacterial activity against Pst DC3000, we tested whether the observed in vitro activity could translate into resistance of Arabidopsis to Pst DC3000, using bacterial loads as endpoints in a plant infection model. Furthermore, using quantitative polymerase chain reaction, we explored the molecular actors involved in the resistance of Arabidopsis induced by SID. Finally, to assure that SID would not interfere with PGPRs, we tested in vitro the influence of SID on the growth of a reference PGPR, Bacillus subtilis. We report here that SID is an antibacterial agent as well as an inducer of systemic priming of resistance in A. thaliana against Pst DC3000, and that SID can, at the same time, promote growth of a PGPR.

Iron homeostasis and plant immune responses: Recent insights and translational implications

Iron metabolism and the plant immune system are both critical for plant vigor in natural ecosystems and for reliable agricultural productivity. Mechanistic studies of plant iron home-ostasis and plant immunity have traditionally been carried out in isolation from each other; however, our growing understanding of both processes has uncovered significant connections. For example, iron plays a critical role in the generation of reactive oxygen intermediates during immunity and has been recently implicated as a critical factor for immune-initiated cell death via ferroptosis. Moreover, plant iron stress triggers immune activation, suggesting that sensing of iron depletion is a mechanism by which plants recognize a pathogen threat. The iron deficiency response engages hormone signaling sectors that are also utilized for plant immune signaling, providing a probable explanation for iron-immunity cross-talk. Finally, interference with iron acquisition by pathogens might be a critical component of the immune response. Efforts to address the global burden of iron deficiency-related anemia have focused on classical breeding and transgenic approaches to develop crops biofortified for iron content. However, our improved mechanistic understanding of plant iron metabolism suggests that such alterations could promote or impede plant immunity, depending on the nature of the alteration and the virulence strategy of the pathogen. Effects of iron biofortification on disease resistance should be evaluated while developing plants for iron biofortification.

Fe2+ Ions Alleviate the Symptom of Citrus Greening Disease

Citrus greening (CG) is among the most devastating citrus diseases worldwide. CG-infected trees exhibit interveinal chlorotic leaves due to iron (Fe) deficiency derived from CG; thus, Fe content is lower in infected leaves than in healthy leaves. In this study, we demonstrated that the foliar application of Fe2+ relieves the symptom of CG infection in citrus trees. We applied Fe2+ and citrate to the leaves of infected rough lemon plants. Following this treatment, a reduction in the number of yellow symptomatic leaves was observed, and their growth was restored. Using chlorophyll content as an index, we screened for effective Fe complexes and found that a high ratio of citrate to Fe2+ in the applied solution led to effects against CG in Shikuwasa trees. A high proportion of Fe2+ to total Fe was another key factor explaining the effectiveness of the solution in CG infection, indicating the importance of Fe2+ absorption into plant cells. We confirmed the proportion of Fe2+ to total Fe through the high correlation of reflectometry data via a triazine reaction and X-ray absorption fine structure analysis. These results demonstrate that the foliar application of a high-Fe2+ citrate solution can restore the growth of CG diseased trees.

A secreted metal-binding protein protects necrotrophic phytopathogens from reactive oxygen species

Few secreted proteins involved in plant infection common to necrotrophic bacteria, fungi and oomycetes have been identified except for plant cell wall-degrading enzymes. Here we study a family of iron-binding proteins that is present in Gram-negative and Gram-positive bacteria, fungi, oomycetes and some animals. Homolog proteins in the phytopathogenic bacterium Dickeya dadantii (IbpS) and the fungal necrotroph Botrytis cinerea (BcIbp) are involved in plant infection. IbpS is secreted, can bind iron and copper, and protects the bacteria against H₂O₂-induced death. Its 1.7 Å crystal structure reveals a classical Venus Fly trap fold that forms dimers in solution and in the crystal. We propose that secreted Ibp proteins binds exogenous metals and thus limit intracellular metal accumulation and ROS formation in the microorganisms.

The authors identify a family of iron-binding proteins that is present in phytopathogenic bacteria, fungi and oomycetes. Some of these proteins are secreted, bind metals, protect the pathogen from H₂O₂-induced death, and are involved in plant infection.

A Role for Zinc in Plant Defense Against Pathogens and Herbivores

Pests and diseases pose a threat to food security, which is nowadays aggravated by climate change and globalization. In this context, agricultural policies demand innovative approaches to more effectively manage resources and overcome the ecological issues raised by intensive farming. Optimization of plant mineral nutrition is a sustainable approach to ameliorate crop health and yield. Zinc is a micronutrient essential for all living organisms with a key role in growth, development, and defense. Competition for Zn affects the outcome of the host-attacker interaction in both plant and animal systems. In this review, we provide a clear framework of the different strategies involving low and high Zn concentrations launched by plants to fight their enemies. After briefly introducing the most relevant macro- and micronutrients for plant defense, the functions of Zn in plant protection are summarized with special emphasis on superoxide dismutases (SODs) and zinc finger proteins. Following, we cover recent meaningful studies identifying Zn-related passive and active mechanisms for plant protection. Finally, Zn-based strategies evolved by pathogens and pests to counteract plant defenses are discussed.

The phytopathogenic nature of Dickeya aquatica 174/2 and the dynamic early evolution of Dickeya pathogenicity

Dickeya is a genus of phytopathogenic enterobacterales causing soft rot in a variety of plants (e.g. potato, chicory, maize). Among the species affiliated to this genus, Dickeya aquatica, described in 2014, remained particularly mysterious because it had no known host. Furthermore, while D. aquatica was proposed to represent a deep-branching species among Dickeya genus, its precise phylogenetic position remained elusive. Here, we report the complete genome sequence of the D. aquatica type strain 174/2. We demonstrate the affinity of D. aquatica strain 174/2 for acidic fruits such as tomato and cucumber and show that exposure of this bacterium to acidic pH induces twitching motility. An in-depth phylogenomic analysis of all available Dickeya proteomes pinpoints D. aquatica as the second deepest branching lineage within this genus and reclassifies two lineages that likely correspond to new genomospecies (gs.): Dickeya gs. poaceaephila (Dickeya sp NCPPB 569) and Dickeya gs. undicola (Dickeya sp 2B12), together with a new putative genus, tentatively named Prodigiosinella. Finally, from comparative analyses of Dickeya proteomes, we infer the complex evolutionary history of this genus, paving the way to study the adaptive patterns and processes of Dickeya to different environmental niches and hosts. In particular, we hypothesize that the lack of xylanases and xylose degradation pathways in D. aquatica could reflect adaptation to aquatic charophyte hosts which, in contrast to land plants, do not contain xyloglucans.

Transcriptional and Chemical Changes in Soybean Leaves in Response to Long-Term Aphid Colonization

Soybean aphids (Aphis glycines Matsumura) are specialized insects that feed on soybean (Glycine max) phloem sap. Transcriptome analyses have shown that resistant soybean plants mount a fast response that limits aphid feeding and population growth. Conversely, defense responses in susceptible plants are slower and it is hypothesized that aphids block effective defenses in the compatible interaction. Unlike other pests, aphids can colonize plants for long periods of time; yet the effect on the plant transcriptome after long-term aphid feeding has not been analyzed for any plant-aphid interaction. We analyzed the susceptible and resistant (Rag1) transcriptome response to aphid feeding in soybean plants colonized by aphids (biotype 1) for 21 days. We found a reduced resistant response and a low level of aphid growth on Rag1 plants, while susceptible plants showed a strong response consistent with pattern-triggered immunity. GO-term analyses identified chitin regulation as one of the most overrepresented classes of genes, suggesting that chitin could be one of the hemipteran-associated molecular pattern that triggers this defense response. Transcriptome analyses also indicated the phenylpropanoid pathway, specifically isoflavonoid biosynthesis, was induced in susceptible plants in response to long-term aphid feeding. Metabolite analyses corroborated this finding. Aphid-treated susceptible plants accumulated daidzein, formononetin, and genistein, although glyceollins were present at low levels in these plants. Choice experiments indicated that daidzein may have a deterrent effect on aphid feeding. Mass spectrometry imaging showed these isoflavones accumulate likely in the mesophyll cells or epidermis and are absent from the vasculature, suggesting that isoflavones are part of a non-phloem defense response that can reduce aphid feeding. While it is likely that aphid can initially block defense responses in compatible interactions, it appears that susceptible soybean plants can eventually mount an effective defense in response to long-term soybean aphid colonization.

The Arabidopsis defensin gene, AtPDF1.1, mediates defence against Pectobacterium carotovorum subsp. carotovorum via an iron-withholding defence system

Plant defensins (PDFs) are cysteine-rich peptides that have a range of biological functions, including defence against fungal pathogens. However, little is known about their role in defence against bacteria. In this study, we showed that the protein encoded by ARABIDOPSIS THALIANA PLANT DEFENSIN TYPE 1.1 (AtPDF1.1) is a secreted protein that can chelate apoplastic iron. Transcripts of AtPDF1.1 were induced in both systemic non-infected leaves of Arabidopsis thaliana plants and those infected with the necrotrophic bacterium Pectobacterium carotovorum subsp. carotovorum (Pcc). The expression levels of AtPDF1.1 with correct subcellular localization in transgenic A. thaliana plants were positively correlated with tolerance to Pcc, suggesting its involvement in the defence against this bacterium. Expression analysis of genes associated with iron homeostasis/deficiency and hormone signalling indicated that the increased sequestration of iron by apoplastic AtPDF1.1 overexpression perturbs iron homeostasis in leaves and consequently activates an iron-deficiency-mediated response in roots via the ethylene signalling pathway. This in turn triggers ethylene-mediated signalling in systemic leaves, which is involved in suppressing the infection of necrotrophic pathogens. These findings provide new insight into the key functions of plant defensins in limiting the infection by the necrotrophic bacterium Pcc via an iron-deficiency-mediated defence response.

In silico prediction and characterization of secondary metabolite biosynthetic gene clusters in the wheat pathogen Zymoseptoria tritici

BACKGROUND

Fungal pathogens of plants produce diverse repertoires of secondary metabolites, which have functions ranging from iron acquisition, defense against immune perturbation, to toxic assaults on the host. The wheat pathogen Zymoseptoria tritici causes Septoria tritici blotch, a foliar disease which is a significant threat to global food security. Currently, there is limited knowledge of the secondary metabolite arsenal produced by Z. tritici, which significantly restricts mechanistic understanding of infection. In this study, we analyzed the genome of Z. tritici isolate IP0323 to identify putative secondary metabolite biosynthetic gene clusters, and used comparative genomics to predict their encoded products.

RESULTS

We identified 32 putative secondary metabolite clusters. These were physically enriched at subtelomeric regions, which may facilitate diversification of cognate products by rapid gene rearrangement or mutations. Comparative genomics revealed a four gene cluster with significant similarity to the ferrichrome-A biosynthetic locus of the maize pathogen Ustilago maydis, suggesting this siderophore is deployed by Z. tritici to acquire iron. The Z. tritici genome also contains several isoprenoid biosynthetic gene clusters, including one with high similarity to a carotenoid/opsin producing locus in several fungi. Furthermore, we identify putative phytotoxin biosynthetic clusters, suggesting Z. tritici can produce an epipolythiodioxopiperazine, and a polyketide and non-ribosomal peptide with predicted structural similarities to fumonisin and the Alternaria alternata AM-toxin, respectively. Interrogation of an existing transcriptional dataset suggests stage specific deployment of numerous predicted loci during infection, indicating an important role of these secondary metabolites in Z. tritici disease.

CONCLUSIONS

We were able to assign putative biosynthetic products to numerous clusters based on conservation amongst other fungi. However, analysis of the majority of secondary metabolite loci did not enable prediction of a cluster product, and consequently the capacity of these loci to play as yet undetermined roles in disease or other stages of the Z. tritici lifecycle is significant. These data will drive future experimentation for determining the role of these clusters and cognate secondary metabolite products in Z. tritici virulence, and may lead to discovery of novel bioactive molecules.

Iron and Immunity

Iron is an essential nutrient for most life on Earth because it functions as a crucial redox catalyst in many cellular processes. However, when present in excess iron can lead to the formation of harmful hydroxyl radicals. Hence, the cellular iron balance must be tightly controlled. Perturbation of iron homeostasis is a major strategy in host-pathogen interactions. Plants use iron-withholding strategies to reduce pathogen virulence or to locally increase iron levels to activate a toxic oxidative burst. Some plant pathogens counteract such defenses by secreting iron-scavenging siderophores that promote iron uptake and alleviate iron-regulated host immune responses. Mutualistic root microbiota can also influence plant disease via iron. They compete for iron with soil-borne pathogens or induce a systemic resistance that shares early signaling components with the root iron-uptake machinery. This review describes the progress in our understanding of the role of iron homeostasis in both pathogenic and beneficial plant-microbe interactions.

Overexpression of wheat ferritin gene TaFER-5B enhances tolerance to heat stress and other abiotic stresses associated with the ROS scavenging

BACKGROUND

The yield of wheat (Triticum aestivum L.), an important crop, is adversely affected by heat stress in many regions of the world. However, the molecular mechanisms underlying thermotolerance are largely unknown.

RESULTS

A novel ferritin gene, TaFER, was identified from our previous heat stress-responsive transcriptome analysis of a heat-tolerant wheat cultivar (TAM107). TaFER was mapped to chromosome 5B and named TaFER-5B. Expression pattern analysis revealed that TaFER-5B was induced by heat, polyethylene glycol (PEG), H₂O₂ and Fe-ethylenediaminedi(o-hydroxyphenylacetic) acid (Fe-EDDHA). To confirm the function of TaFER-5B in wheat, TaFER-5B was transformed into the wheat cultivar Jimai5265 (JM5265), and the transgenic plants exhibited enhanced thermotolerance. To examine whether the function of ferritin from mono- and dico-species is conserved, TaFER-5B was transformed into Arabidopsis, and overexpression of TaFER-5B functionally complemented the heat stress-sensitive phenotype of a ferritin-lacking mutant of Arabidopsis. Moreover, TaFER-5B is essential for protecting cells against heat stress associated with protecting cells against ROS. In addition, TaFER-5B overexpression also enhanced drought, oxidative and excess iron stress tolerance associated with the ROS scavenging. Finally, TaFER-5B transgenic Arabidopsis and wheat plants exhibited improved leaf iron content.

CONCLUSIONS

Our results suggest that TaFER-5B plays an important role in enhancing tolerance to heat stress and other abiotic stresses associated with the ROS scavenging.

Manipulation of ABA Content in Arabidopsis thaliana Modifies Sensitivity and Oxidative Stress Response to Dickeya dadantii and Influences Peroxidase Activity

The production of reactive oxygen species (ROS) is one of the first defense reactions induced in Arabidopsis in response to infection by the pectinolytic enterobacterium Dickeya dadantii. Previous results also suggest that abscisic acid (ABA) favors D. dadantii multiplication and spread into its hosts. Here, we confirm this hypothesis using ABA-deficient and ABA-overproducer Arabidopsis plants. We investigated the relationships between ABA status and ROS production in Arabidopsis after D. dadantii infection and showed that ABA status modulates the capacity of the plant to produce ROS in response to infection by decreasing the production of class III peroxidases. This mechanism takes place independently of the well-described oxidative stress related to the RBOHD NADPH oxidase. In addition to this weakening of plant defense, ABA content in the plant correlates positively with the production of some bacterial virulence factors during the first stages of infection. Both processes should enhance disease progression in presence of high ABA content. Given that infection increases transcript abundance for the ABA biosynthesis genes AAO3 and ABA3 and triggers ABA accumulation in leaves, we propose that D. dadantii manipulates ABA homeostasis as part of its virulence strategy.

Proteomics analysis reveals novel host molecular mechanisms associated with thermotherapy of 'Ca. Liberibacter asiaticus'-infected citrus plants

BACKGROUND

Citrus Huanglongbing (HLB), which is linked to the bacterial pathogen 'Ca. Liberibacter asiaticus' (Las), is the most devastating disease of citrus plants, and longer-term control measures via breeding or genetic engineering have been unwieldy because all cultivated citrus species are susceptible to the disease. However, the degree of susceptibility varies among citrus species, which has prompted efforts to identify potential Las resistance/tolerance-related genes in citrus plants for application in breeding or genetic engineering programs. Plant exposure to one form of stress has been shown to serendipitously induce innate resistance to other forms of stress and a recent study showed that continuous heat treatment (40 to 42 °C) reduced Las titer and HLB-associated symptoms in citrus seedlings. The goal of the present study was to apply comparative proteomics analysis via 2-DE and mass spectrometry to elucidate the molecular processes associated with heat-induced mitigation of HLB in citrus plants. Healthy or Las-infected citrus grapefruit plants were exposed to room temperature or to continuous heat treatment of 40 °C for 6 days.

RESULTS

An exhaustive total protein extraction process facilitated the identification of 107 differentially-expressed proteins in response to Las and/or heat treatment, which included a strong up-regulation of chaperones including small (23.6, 18.5 and 17.9 kDa) heat shock proteins, a HSP70-like protein and a ribulose-1,5-bisphosphate carboxylase oxygenase (RuBisCO)-binding 60 kDa chaperonin, particularly in response to heat treatment. Other proteins that were generally down-regulated due to Las infection but up-regulated in response to heat treatment include RuBisCO activase, chlorophyll a/b binding protein, glucosidase II beta subunit-like protein, a putative lipoxygenase protein, a ferritin-like protein, and a glutathione S-transferase.

CONCLUSIONS

The differentially-expressed proteins identified in this study highlights a premier characterization of the molecular mechanisms potentially involved in the reversal of Las-induced pathogenicity processes in citrus plants and are hence proposed targets for application towards the development of cisgenic Las-resistant/tolerant citrus plants.

Chickpea Ferritin CaFer1 Participates in Oxidative Stress Response, and Promotes Growth and Development

Ferritins store and sequester iron, and regulate iron homeostasis. The cDNA for a stress-responsive phytoferritin, previously identified in the extracellular matrix (ECM) of chickpea (Cicer arietinum), was cloned and designated CaFer1. The CaFer1 transcript was strongly induced in chickpea exposed to dehydration, hypersalinity and ABA treatment. Additionally, it has role in the defense against Fusarium oxysporum infection. Functional complementation of the yeast frataxin-deficient mutant, Δyfh1, indicates that CaFer1 functions in oxidative stress. The presence of CaFer1 in the extracellular space besides chloroplast establishes its inimitable nature from that of other phytoferritins. Furthermore, CaFer1 expression in response to iron suggests its differential mechanism of accumulation at two different iron conditions. CaFer1-overexpressing transgenic plants conferred improved growth and development, accompanied by altered expression of iron-responsive genes. Together, these results suggest that the phytoferritin, CaFer1, might play a key role in maintenance of iron buffering and adaptation to environmental challenges.

Virulence Program of a Bacterial Plant Pathogen: The Dickeya Model

The pectinolytic Dickeya spp. are Gram-negative bacteria causing severe disease in a wide range of plant species. Although the Dickeya genus was initially restricted to tropical and subtropical areas, two Dickeya species (D. dianthicola and D. solani) emerged recently in potato cultures in Europe. Soft-rot, the visible symptoms, is caused by plant cell wall degrading enzymes, mainly pectate lyases (Pels) that cleave the pectin polymer. However, an efficient colonization of the host requires many additional elements including early factors (eg, flagella, lipopolysaccharide, and exopolysaccharide) that allow adhesion of the bacteria and intermediate factors involved in adaptation to new growth conditions encountered in the host (eg, oxidative stress, iron starvation, and toxic compounds). To facilitate this adaptation, Dickeya have developed complex regulatory networks ensuring appropriate expression of virulence genes. This review presents recent advances in our understanding of the signals and genetic circuits impacting the expression of virulence determinants. Special attention is paid to integrated control of virulence functions by variations in the superhelical density of chromosomal DNA, and the global and specific regulators, making the regulation of Dickeya virulence an especially attractive model for those interested in relationships between the chromosomal dynamics and gene regulatory networks.

Immunity to plant pathogens and iron homeostasis

Iron is essential for metabolic processes in most living organisms. Pathogens and their hosts often compete for the acquisition of this nutrient. However, iron can catalyze the formation of deleterious reactive oxygen species. Hosts may use iron to increase local oxidative stress in defense responses against pathogens. Due to this duality, iron plays a complex role in plant-pathogen interactions. Plant defenses against pathogens and plant response to iron deficiency share several features, such as secretion of phenolic compounds, and use common hormone signaling pathways. Moreover, fine tuning of iron localization during infection involves genes coding iron transport and iron storage proteins, which have been shown to contribute to immunity. The influence of the plant iron status on the outcome of a given pathogen attack is strongly dependent on the nature of the pathogen infection strategy and on the host species. Microbial siderophores emerged as important factors as they have the ability to trigger plant defense responses. Depending on the plant species, siderophore perception can be mediated by their strong iron scavenging capacity or possibly via specific recognition as pathogen associated molecular patterns. This review highlights that iron has a key role in several plant-pathogen interactions by modulating immunity.

Role of Dickeya dadantii 3937 chemoreceptors in the entry to Arabidopsis leaves through wounds

Chemotaxis enables bacteria to move towards an optimal environment in response to chemical signals. In the case of plant-pathogenic bacteria, chemotaxis allows pathogens to explore the plant surface for potential entry sites with the ultimate aim to prosper inside plant tissues and to cause disease. Chemoreceptors, which constitute the sensory core of the chemotaxis system, are usually transmembrane proteins which change their conformation when sensing chemicals in the periplasm and transduce the signal through a kinase pathway to the flagellar motor. In the particular case of the soft-rot pathogen Dickeya dadantii 3937, jasmonic acid released in a plant wound has been found to be a strong chemoattractant which drives pathogen entry into the plant apoplast. In order to identify candidate chemoreceptors sensing wound-derived plant compounds, we carried out a bioinformatics search of candidate chemoreceptors in the genome of Dickeya dadantii 3937. The study of the chemotactic response to several compounds and the analysis of the entry process to Arabidopsis leaves of 10 selected mutants in chemoreceptors allowed us to determine the implications of at least two of them (ABF-0020167 and ABF-0046680) in the chemotaxis-driven entry process through plant wounds. Our data suggest that ABF-0020167 and ABF-0046680 may be candidate receptors of jasmonic acid and xylose, respectively.

Rice Xa21 primed genes and pathways that are critical for combating bacterial blight infection

Rice bacterial blight (BB) is a devastating rice disease. The Xa21 gene confers a broad and persistent resistance against BB. We introduced Xa21 into Oryza sativa L ssp indica (rice 9311), through multi-generation backcrossing, and generated a nearly isogenic, blight-resistant 9311/Xa21 rice. Using next-generation sequencing, we profiled the transcriptomes of both varieties before and within four days after infection of bacterium Xanthomonas oryzae pv. oryzae. The identified differentially expressed (DE) genes and signaling pathways revealed insights into the functions of Xa21. Surprisingly, before infection 1,889 genes on 135 of the 316 signaling pathways were DE between the 9311/Xa21 and 9311 plants. These Xa21-mediated basal pathways included mainly those related to the basic material and energy metabolisms and many related to phytohormones such as cytokinin, suggesting that Xa21 triggered redistribution of energy, phytohormones and resources among essential cellular activities before invasion. Counter-intuitively, after infection, the DE genes between the two plants were only one third of that before the infection; other than a few stress-related pathways, the affected pathways after infection constituted a small subset of the Xa21-mediated basal pathways. These results suggested that Xa21 primed critically important genes and signaling pathways, enhancing its resistance against bacterial infection.

Intracellular siderophore but not extracellular siderophore is required for full virulence in Metarhizium robertsii

Efficient iron acquisition mechanisms are fundamental for microbial survival in the environment and for pathogen virulence within their hosts. M. robertsii produces two known iron-binding natural products: metachelins, which are used to scavenge extracellular iron, and ferricrocin, which is strictly intracellular. To study the contribution of siderophore-mediated iron uptake and storage to M. robertsii fitness, we generated null mutants for each siderophore synthase gene (mrsidD and mrsidC, respectively), as well as for the iron uptake transcriptional repressor mrsreA. All of these mutants showed impaired germination speed, differential sensitivity to hydrogen peroxide, and differential ability to overcome iron chelation on growth-limiting iron concentrations. RT-qPCR data supported regulation of mrsreA, mrsidC, and mrsidD by supplied iron in vitro and during growth within the insect host, Spodoptera exigua. We also observed strong upregulation of the insect iron-binding proteins, transferrins, during infection. Insect bioassays revealed that ferricrocin is required for full virulence against S. exigua; neither the loss of metachelin production nor the deletion of the transcription factor mrsreA significantly affected M. robertsii virulence.

New insights into the role of siderophores as triggers of plant immunity: what can we learn from animals?

Microorganisms use siderophores to obtain iron from the environment. In pathogenic interactions, siderophores are involved in iron acquisition from the host and are sometimes necessary for the expression of full virulence. This review summarizes the main data describing the role of these iron scavengers in animal and plant defence systems. To protect themselves against iron theft, mammalian hosts have developed a hypoferremia strategy that includes siderophore-binding molecules called siderocalins. In addition to microbial ferri-siderophore sequestration, siderocalins are involved in triggering immunity. In plants, no similar mechanisms have been described and many fewer data are available, although recent advances have shed light on the role of siderophores in plant-pathogen interactions. Siderophores can trigger immunity in plants in several contexts. The most frequently described situation involving siderophores is induced systemic resistance (ISR) triggered by plant-growth-promoting rhizobacteria. Although ISR responses have been observed after treating roots with certain siderophores, the underlying mechanisms are poorly understood. Immunity can also be triggered by siderophores in leaves. Siderophore perception in plants appears to be different from the well-known perception mechanisms of other microbial compounds, known as microbe-associated molecular patterns. Scavenging iron per se appears to be a novel mechanism of immunity activation, involving complex disturbance of metal homeostasis. Receptor-specific recognition of siderophores has been described in animals, but not in plants. The review closes with an overview of the possible mechanisms of defence activation, via iron scavenging by siderophores or specific siderophore recognition by the plant host.

Alterations of iron distribution in Arabidopsis tissues infected by Dickeya dadantii

Dickeya dadantii is a plant-pathogenic enterobacterium responsible for plant soft rot disease in a wide range of hosts, including the model plant Arabidopsis thaliana. Iron distribution in infected A. thaliana was investigated at the cellular scale using the Perls'-diaminobenzidine-H2 O2 (PDH) method. Iron visualization during infection reveals a loss of iron from cellular compartments and plant cell walls. During symptom progression, two distinct zones are clearly visible: a macerated zone displaying weak iron content and a healthy zone displaying strong iron content. Immunolabelling of cell wall methylated pectin shows that pectin degradation is correlated with iron release from cell walls, indicating a strong relationship between cell wall integrity and iron in plant tissues. Using a D. dadantii lipopolysaccharide antibody, we show that bacteria are restricted to the infected tissue, and that they accumulate iron in planta. In conclusion, weak iron content is strictly correlated with bacterial cell localization in the infected tissues, indicating a crucial role of this element during the interaction. This is the first report of iron localization at the cellular level during a plant-microbe interaction and shows that PDH is a method of choice in this type of investigation.

Individual and combined roles of malonichrome, ferricrocin, and TAFC siderophores in Fusarium graminearum pathogenic and sexual development

Intra- and extracellular iron-chelating siderophores produced by fungal non-ribosomal peptide synthetases have been shown to be involved in reproductive and pathogenic developmental processes and in iron and oxidative stress management. Here we report individual and combined contributions of three of these metabolites to developmental success of the destructive cereal pathogen Fusarium graminearum. In previous work, we determined that deletion of the NPS2 gene, responsible for intracellular siderophore biosynthesis, results in inability to produce sexual spores when mutants of this homothallic ascomycete are selfed. Deletion of the NPS6 gene, required for extracellular siderophore biosynthesis, does not affect sexual reproduction but results in sensitivity to iron starvation and oxidative stress and leads to reduced virulence to the host. Building on this, we report that double mutants lacking both NPS2 and NPS6 are augmented in all collective phenotypes of single deletion strains (i.e., abnormal sexual and pathogenic development, hypersensitivity to oxidative and iron-depletion stress), which suggests overlap of function. Using comparative biochemical analysis of wild-type and mutant strains, we show that NPS1, a third gene associated with siderophore biosynthesis, is responsible for biosynthesis of a second extracellular siderophore, malonichrome. nps1 mutants fail to produce this metabolite. Phenotypic characterization reveals that, although single nps1 mutants are like wild-type with respect to sexual development, hypersensitivity to ROS and iron-depletion stress, and virulence to the host, triple nps1nps2nps6 deletion strains, lacking all three siderophores, are even more impaired in these attributes than double nps2nps6 strains. Thus, combinatorial mutants lacking key iron-associated genes uncovered malonichrome function. The intimate connection between presence/absence of siderophores and resistance/sensitivity to ROS is central to sexual and pathogenic development.

β-Aminobutyric acid (BABA)-induced resistance in Arabidopsis thaliana: link with iron homeostasis

β-Aminobutyric acid (BABA) is a nonprotein amino acid inducing resistance in many different plant species against a wide range of abiotic and biotic stresses. Nevertheless, how BABA primes plant natural defense reactions remains poorly understood. Based on its structure, we hypothesized and confirmed that BABA is able to chelate iron (Fe) in vitro. In vivo, we showed that it led to a transient Fe deficiency response in Arabidopsis thaliana plants exemplified by a reduction of ferritin accumulation and disturbances in the expression of genes related to Fe homeostasis. This response was not correlated to changes in Fe concentrations, suggesting that BABA affects the availability or the distribution of Fe rather than its assimilation. The phenotype of BABA-treated plants was similar to those of plants cultivated in Fe-deficient conditions. A metabolomic analysis indicated that both BABA and Fe deficiency induced the accumulation of common metabolites, including p-coumaroylagmatine, a metabolite previously shown to be synthesized in several plant species facing pathogen attack. Finally, we showed that the protective effect induced by BABA against Botrytis cinerea was mimicked by Fe deficiency. In conclusion, the Fe deficiency response caused by BABA could bring the plant to a defense-ready state, participating in the plant resistance against the pathogens.

An adequate Fe nutritional status of maize suppresses infection and biotrophic growth of Colletotrichum graminicola

Iron (Fe) is an essential element for plant pathogens as well as for their host plants. As Fe plays a central role in pathogen virulence, most plants have evolved Fe-withholding strategies to reduce Fe availability to pathogens. On the other hand, plants need Fe for an oxidative burst in their basal defense response against pathogens. To investigate how the plant Fe nutritional status affects plant tolerance to a hemibiotrophic fungal pathogen, we employed the maize-Colletotrichum graminicola pathosystem. Fungal infection progressed rapidly via biotrophic to necrotrophic growth in Fe-deficient leaves, while an adequate Fe nutritional status suppressed the formation of infection structures of C. graminicola already during the early biotrophic growth phase. As indicated by Prussian blue and 3,3'-diaminobenzidine (DAB) staining, the retarding effect of an adequate Fe nutritional status on fungal development coincided temporally and spatially with the recruitment of Fe to infection sites and a local production of H2 O2 . A similar coincidence between local Fe and H2 O2 accumulation was found in a parallel approach employing C. graminicola mutants affected in Fe acquisition and differing in virulence. These results indicate that an adequate Fe nutritional status delays and partially suppresses the fungal infection process and the biotrophic growth phase of C. graminicola, most likely via the recruitment of free Fe to the fungal infection site for a timely oxidative burst.

Biotrophy-specific downregulation of siderophore biosynthesis in Colletotrichum graminicola is required for modulation of immune responses of maize

The hemibiotrophic maize pathogen Colletotrichum graminicola synthesizes one intracellular and three secreted siderophores. eGFP fusions with the key siderophore biosynthesis gene, SID1, encoding l-ornithine-N(5) -monooxygenase, suggested that siderophore biosynthesis is rigorously downregulated specifically during biotrophic development. In order to investigate the role of siderophores during vegetative development and pathogenesis, SID1, which is required for synthesis of all siderophores, and the non-ribosomal peptide synthetase gene NPS6, synthesizing secreted siderophores, were deleted. Mutant analyses revealed that siderophores are required for vegetative growth under iron-limiting conditions, conidiation, ROS tolerance, and cell wall integrity. Δsid1 and Δnps6 mutants were hampered in formation of melanized appressoria and impaired in virulence. In agreement with biotrophy-specific downregulation of siderophore biosynthesis, Δsid1 and Δnps6 strains were not affected in biotrophic development, but spread of necrotrophic hyphae was reduced. To address the question why siderophore biosynthesis is specifically downregulated in biotrophic hyphae, maize leaves were infiltrated with siderophores. Siderophore infiltration alone did not induce defence responses, but formation of biotrophic hyphae in siderophore-infiltrated leaves caused dramatically increased ROS formation and transcriptional activation of genes encoding defence-related peroxidases and PR proteins. These data suggest that fungal siderophores modulate the plant immune system.

Scavenging iron: a novel mechanism of plant immunity activation by microbial siderophores

Siderophores are specific ferric iron chelators synthesized by virtually all microorganisms in response to iron deficiency. We have previously shown that they promote infection by the phytopathogenic enterobacteria Dickeya dadantii and Erwinia amylovora. Siderophores also have the ability to activate plant immunity. We have used complete Arabidopsis transcriptome microarrays to investigate the global transcriptional modifications in roots and leaves of Arabidopsis (Arabidopsis thaliana) plants after leaf treatment with the siderophore deferrioxamine (DFO). Physiological relevance of these transcriptional modifications was validated experimentally. Immunity and heavy-metal homeostasis were the major processes affected by DFO. These two physiological responses could be activated by a synthetic iron chelator ethylenediamine-di(o-hydroxyphenylacetic) acid, indicating that siderophores eliciting activities rely on their strong iron-chelating capacity. DFO was able to protect Arabidopsis against the pathogenic bacterium Pseudomonas syringae pv tomato DC3000. Siderophore treatment caused local modifications of iron distribution in leaf cells visible by ferrocyanide and diaminobenzidine-H₂O₂ staining. Metal quantifications showed that DFO causes a transient iron and zinc uptake at the root level, which is presumably mediated by the metal transporter iron regulated transporter1 (IRT1). Defense gene expression and callose deposition in response to DFO were compromised in an irt1 mutant. Consistently, plant susceptibility to D. dadantii was increased in the irt1 mutant. Our work shows that iron scavenging is a unique mechanism of immunity activation in plants. It highlights the strong relationship between heavy-metal homeostasis and immunity.