Manipulation of salicylate content in Arabidopsis thaliana by the expression of an engineered bacterial salicylate synthase

Silica nanoparticles protect rice against biotic and abiotic stresses

BACKGROUND

By 2050, the world population will increase to 10 billion which urged global demand for food production to double. Plant disease and land drought will make the situation more dire, and safer and environment-friendly materials are thus considered as a new countermeasure. The rice blast fungus, Magnaporthe oryzae, causes one of the most destructive diseases of cultivated rice worldwide that seriously threatens rice production. Unfortunately, traditional breeding nor chemical approaches along control it well. Nowadays, nanotechnology stands as a new weapon against these mounting challenges and silica nanoparticles (SiO₂ NPs) have been considered as potential new safer agrochemicals recently but the systematically studies remain limited, especially in rice.

RESULTS

Salicylic acid (SA) is a key plant hormone essential for establishing plant resistance to several pathogens and its further affected a special form of induced resistance, the systemic acquired resistance (SAR), which considered as an important aspect of plant innate immunity from the locally induced disease resistance to the whole plant. Here we showed that SiO₂ NPs could stimulate plant immunity to protect rice against M. oryzae through foliar treatment that significantly decreased disease severity by nearly 70% within an appropriate concentration range. Excessive concentration of foliar treatment led to disordered intake and abnormal SA responsive genes expressions which weaken the plant resistance and even aggravated the disease. Importantly, this SA-dependent fungal resistance could achieve better results with root treatment through a SAR manner with no phytotoxicity since the orderly and moderate absorption. What's more, root treatment with SiO₂ NPs could also promote root development which was better to deal with drought.

CONCLUSIONS

Taken together, our findings not only revealed SiO₂ NPs as a potential effective and safe strategy to protect rice against biotic and abiotic stresses, but also identify root treatment for the appropriate application method since it seems not causing negative effects and even have promotion on root development.

Transcriptome profiling reveals response genes for downy mildew resistance in cucumber

We discovered a potential defense pathway of cucumber to downy mildew. The signaling that activates the pathways of ROS and lignin accumulation may play an important role in the defense response. Many resistance genes were identified by transcriptome analysis. Downy mildew (DM), caused by Pseudoperonospora cubensis, is one of the most destructive diseases and causes severe yield losses of cucumber. However, the genes and pathways involved in regulating DM resistance were still poorly understood. In our study, we observed that the highly sensitive inbred line 53 (IL53) exhibited more severe disease symptoms than the highly resistant inbred line 51 (IL51) under P. cubensis infection. Furthermore, lignin, limiting the germination and extension of P. cubensis, and H₂O₂, as a signaling molecule during the resistant process, were both shown to increase, indicating that the signaling that activates these pathways might be responsible for the resistance divergence between IL51 and IL53. Transcriptome analysis, using the resistant and susceptible pools in F₂ populations with IL51 and IL53 as parents, showed that a series of differentially expressed genes was involved in multiple functions of defense response: pathogen-associated molecular pattern recognition, signal transduction, reactive oxygen species and lignin accumulation, and transcription regulators. Combining physiological data with transcriptomes, we predicted a potential molecular mechanism of cucumber resistance to DM. Our research provided a foundation for further studies on the mechanism of cucumber resistance to DM.

Expression of a Fungal Lectin in Arabidopsis Enhances Plant Growth and Resistance Toward Microbial Pathogens and a Plant-Parasitic Nematode

Coprinopsis cinerea lectin 2 (CCL2) is a fucoside-binding lectin from the basidiomycete C. cinerea that is toxic to the bacterivorous nematode Caenorhabditis elegans as well as animal-parasitic and fungivorous nematodes. We expressed CCL2 in Arabidopsis to assess its protective potential toward plant-parasitic nematodes. Our results demonstrate that expression of CCL2 enhances host resistance against the cyst nematode Heterodera schachtii. Surprisingly, CCL2-expressing plants were also more resistant to fungal pathogens including Botrytis cinerea, and the phytopathogenic bacterium Pseudomonas syringae. In addition, CCL2 expression positively affected plant growth indicating that CCL2 has the potential to improve two important agricultural parameters namely biomass production and general disease resistance. The mechanism of the CCL2-mediated enhancement of plant disease resistance depended on fucoside-binding by CCL2 as transgenic plants expressing a mutant version of CCL2 (Y92A), compromised in fucoside-binding, exhibited wild type (WT) disease susceptibility. The protective effect of CCL2 did not seem to be direct as the lectin showed no growth-inhibition toward B. cinerea in in vitro assays. We detected, however, a significantly enhanced transcriptional induction of plant defense genes in CCL2- but not CCL2-Y92A-expressing lines in response to infection with B. cinerea compared to WT plants. This study demonstrates a potential of fungal defense lectins in plant protection beyond their use as toxins.

The Chloroplast Reactive Oxygen Species-Redox System in Plant Immunity and Disease

Pathogen infections limit plant growth and productivity, thus contributing to crop losses. As the site of photosynthesis, the chloroplast is vital for plant productivity. This organelle, communicating with other cellular compartments challenged by infection (e.g., apoplast, mitochondria, and peroxisomes), is also a key battlefield in the plant-pathogen interaction. Here, we focus on the relation between reactive oxygen species (ROS)-redox signaling, photosynthesis which is governed by redox control, and biotic stress response. We also discuss the pathogen strategies to weaken the chloroplast-mediated defense responses and to promote pathogenesis. As in the next decades crop yield increase may depend on the improvement of photosynthetic efficiency, a comprehensive understanding of the integration between photosynthesis and plant immunity is required to meet the future food demand.

Induction of resistance in rice plants using bioproducts produced from Burkholderia pyrrocinia BRM 32113

Leaf blast is the main rice disease in the world causing significant losses in productivity. Blast integrate management (BIM) requires the use of genetic resistance, cultural practices, and chemical control, although for sustainable BIM, the insertion of biological agents may be the fourth component for. The objective of this work was to test three formulations of Burkholderia pyrrocinia (BRM32113) previously selected and to verify the effectiveness in resistance induction and blast control in rice. Two experiments were carried out, in a completely randomized design with three replications, in the greenhouse (E1 and E2). E1 aimed to select the best treatment for suppressing leaf blast severity and activating plant defense mechanisms. It was composed of 8 treatments: (1) formulated 11+ B. pyrrocina × Magnaporthe oryzae; (2) formulated 17+ B. pyrrocina × M. oryzae; (3) formulated 32+ B. pyrrocina × M. oryzae; (4) formulated 11 × M. oryzae; (5) B. pyrrocinia 17 × M. oryzae; (6) formulated 32 × M. oryzae; (7) B. pyrrocina × M. oryzae; (8) M. oryzae; (9) control (water). E2 aimed to investigate the effect of the best treatments, for the promotion of plant growth and suppression of leaf blast by calculating AUDPC. It was composed of 6 treatments: (1) formulated 11+ B. pyrrocina × M. oryzae; (2) formulated 32+ B. pyrrocina × M. oryzae; (3) formulated 11 × M. oryzae; (4) formulated 32 × M. oryzae; (5) B. pyrrocina × M. oryzae; (6) water. And after, we did two assays aimed to localize this biological agent after application at seed, soil, and rice plant. In E1, formulated 11+ B. pyrrocinia and 32+ formulated and B. pyrrocina were the best, suppressing leaf blast by up to 97% and providing the significant increase of the enzymes β-1,3-glucanase, chitinase, phenylalanine ammonia lyase, lipoxygenase, and salicylic acid at 24 h and 48 h after inoculation with M. oryzae. In E2, treatments formulated 11+ B. pyrrocinia, formulated 32+ B. pyrrocinia, and B. pyrrocina provided more significant increases in growth promotion and reduced area under disease progress curve. B. pyrrocinia was detected in the rice plant for 18 days, predominantly in the root system (internal and external). The use of B. pyrrocinia formulations based on sugarcane molasses and glycerol can be an essential strategy for sustainable management. Although all the benefits come from these sustainable formulations, the adoption by commercial biological segment depends on an established formulation process. It seems that all the results showed here by this research will be readily assimilated by startups of the organic segment.

New salicylic acid and pyroglutamic acid conjugated derivatives confer protection to bread wheat against Zymoseptoria tritici

BACKGROUND

To promote sustainable agriculture and healthy food, research that contributes towards a new generation of eco-friendly phytosanitary compounds is increasingly encouraged. The plant hormone salicylic acid (SA) is known for its ability to induce resistance in plants against a wide range of pathogens, whereas pyroglutamic acid (PGA), a constrained analogue of γ-aminobutyric acid, has never been studied in the context of plant protection.

RESULTS

The present study investigated for the first time the protection efficacy of SA and PGA and five new conjugated derivatives against Zymoseptoria tritici, the main pathogen in wheat crops. SA and four derivatives showed significant disease severity reductions in planta (up to 49%). In vitro assays revealed that some molecules, including SA, displayed a small direct antifungal activity, whereas others, such as PGA, showed no effect. This finding suggests that, especially for molecules without any direct activity, the mode of action relies mainly on the induction of plant resistance.

CONCLUSION

Further investigations are needed to identify the defence pathways involved in plant resistance mechanisms elicited or primed by the molecules. The manufacture of these products was easily achieved on a scale of tens of grams of raw materials, and is easily scalable. The synthetic pathway is simple, short and inexpensive. For all of these reasons, the production of the target molecules is attractive for producers, whereas the prospect of a generation of non-polluting compounds with lasting efficiency against Z. tritici in wheat comes at a key moment for the sustainability of agriculture. © 2018 Society of Chemical Industry.

Salicylic acid-altering Arabidopsis plant response to cadmium exposure: Underlying mechanisms affecting antioxidation and photosynthesis-related processes

Previous studies have demonstrated that the genetic modification of basal salicylic acid (SA) level changed Arabidopsis plant response to cadmium (Cd) stress, but the mechanisms remain evaluated. In this study, Arabidopsis wild type (WT) and its SA-reducing transgenic line nahG (naphthalene hydroxylase G), SA-accumulating mutant snc1 (suppressor of nonexpressor of PR gene, constitutive 1) were exposed to 50 μM Cd²⁺ for 48 h or 7 d (just for assessing plant growth). The Cd treatment increased the expression levels of SA biosynthesis-related genes leading to enhanced SA accumulations in plant leaves, which was further confirmed by the expression patterns of SA marker genes. Cadmium accumulation was much higher in the Cd-exposed roots than in leaves, but was not affected by SA levels. Exposure to Cd inhibited plant growth of both aerial parts and roots, to a greater degree in snc1, and a lesser extent in nahG as compared with WT. Although Cd treatment increased plant antioxidative capacity, oxidative damage happened, especially to snc1 plants. Photoinhibition occurred in Cd-stressed plants leading to a decrease in photosynthetic activity, with a greater degree in snc1, while a lesser in nahG, as indicated by the changes of several key photosynthetic parameters. We comprehensively analyzed the expression profiles of photosynthesis-related genes, and observed a positive correlation between Cd tolerance and gene expression levels, wherein the transcription levels of two electron transport-related genes and two amylase-encoding genes were all up-regulated in nahG plants after Cd treatment, implying a significance of the related processes in this genotype against Cd stress.

Comparative analysis of plant isochorismate synthases reveals structural mechanisms underlying their distinct biochemical properties

Isochorismate synthase (ICS) converts chorismate into isochorismate, a precursor of primary and secondary metabolites including salicylic acid (SA). SA plays important roles in responses to stress conditions in plants. Many studies have suggested that the function of plant ICSs is regulated at the transcriptional level. In Arabidopsis thaliana, the expression of AtICS1 is induced by stress conditions in parallel with SA synthesis, and AtICS1 is required for SA synthesis. In contrast, the expression of NtICS is not induced when SA synthesis is activated in tobacco, and it is unlikely to be involved in SA synthesis. Studies on the biochemical properties of plant ICSs are limited, compared with those on transcriptional regulation. We analyzed the biochemical properties of four plant ICSs: AtICS1, NtICS, NbICS from Nicotiana benthamiana, and OsICS from rice. Multiple sequence alignment analysis revealed that their primary structures were well conserved, and predicted key residues for ICS activity were almost completely conserved. However, AtICS1 showed much higher activity than the other ICSs when expressed in Escherichia coli and N. benthamiana leaves. Moreover, the levels of AtICS1 protein expression in N. benthamiana leaves were higher than the other ICSs. Construction and analysis of chimeras between AtICS1 and OsICS revealed that the putative chloroplast transit peptides (TPs) significantly affected the levels of protein accumulation in N. benthamiana leaves. Chimeric and point-mutation analyses revealed that Thr531, Ser537, and Ile550 of AtICS1 are essential for its high activity. These distinct biochemical properties of plant ICSs may suggest different roles in their respective plant species.

Soil mixture composition alters Arabidopsis susceptibility to Pseudomonas syringae infection

Pseudomonas syringae is a gram-negative bacterial pathogen that causes disease on more than 100 different plant species, including the model plant Arabidopsis thaliana. Dissection of the Arabidopsis thaliana-Pseudomonas syringae pathosystem has identified many factors that contribute to successful infection or immunity, including the genetics of the host, the genetics of the pathogen, and the environment. Environmental factors that contribute to a successful interaction can include temperature, light, and the circadian clock, as well as the soil environment. As silicon-amended Resilience soil is advertised to enhance plant health, we sought to examine the extent to which this soil might affect the behavior of the A. thaliana-P. syringae model pathosystem and to characterize the mechanisms through which these effects may occur. We found that plants grown in Si-amended Resilience soil displayed enhanced resistance to bacteria compared to plants grown in non-Si-amended Sunshine soil, and salicylic acid biosynthesis and signaling were not required for resistance. Although silicon has been shown to contribute to broad-spectrum resistance, our data indicate that silicon is not the direct cause of enhanced resistance and that the Si-amended Resilience soil has additional properties that modulate plant resistance. Our work demonstrates the importance of environmental factors, such as soil in modulating interactions between the plant and foliar pathogens, and highlights the significance of careful annotation of the environmental conditions under which plant-pathogen interactions are studied.

The WRKY transcription factors PtrWRKY18 and PtrWRKY35 promote Melampsora resistance in Populus

WRKY transcription factors play important roles in response to diverse environmental stresses, but exact functions of these proteins in poplar defense are still largely unknown. In a previous study, we have shown that poplar WRKY89 is induced by salicylic acid (SA) treatment and plays an important role in resistance against fungi in transgenic poplars. Here, we determined an increase in transcript levels of Group IIa WRKY members in transgenic poplars overexpressing WRKY89 using quantitative real-time polymerase chain reaction analysis. Yeast one-hybrid assay showed that PtrWRKY18 and PtrWRKY35 were potential target genes of WRKY89. Furthermore, we demonstrated that PtrWRKY18 and PtrWRKY35 were localized in the nucleus, and exhibited no transcription activation activity. Constitutive overexpression of PtrWRKY18 and PtrWRKY35 in poplars activated pathogenesis-related genes, and increased resistance to the biotrophic pathogen Melampsora. The results also provided support for the involvement of SA-mediated signaling in Melampsora resistance.

The biochemical properties of the two Arabidopsis thaliana isochorismate synthases

The important plant hormone salicylic acid (SA; 2-hydroxybenzoic acid) regulates several key plant responses including, most notably, defence against pathogens. A key enzyme for SA biosynthesis is isochorismate synthase (ICS), which converts chorismate into isochorismate, and for which there are two genes in Arabidopsis thaliana. One (AtICS1) has been shown to be required for increased SA biosynthesis in response to pathogens and its expression can be stimulated throughout the leaf by virus infection and exogenous SA. The other (AtICS2) appears to be expressed constitutively, predominantly in the plant vasculature. Here, we characterise the enzymatic activity of both isozymes expressed as hexahistidine fusion proteins in Escherichia coli. We show for the first time that recombinant AtICS2 is enzymatically active. Both isozymes are Mg²⁺-dependent with similar temperature optima (ca. 33°C) and similar K_m values for chorismate of 34.3 ± 3.7 and 28.8 ± 6.9 µM for ICS1 and ICS2, respectively, but reaction rates were greater for ICS1 than for ICS2, with respective values for V_max of 63.5 ± 2.4 and 28.3 ± 2.0 nM s⁻¹ and for k_cat of 38.1 ± 1.5 and 17.0 ± 1.2 min⁻¹. However, neither enzyme displayed isochorismate pyruvate lyase (IPL) activity, which would enable these proteins to act as bifunctional SA synthases, i.e. to convert chorismate into SA. These results show that although Arabidopsis has two functional ICS enzymes, it must possess one or more IPL enzymes to complete biosynthesis of SA starting from chorismate.

An incoherent feed-forward loop mediates robustness and tunability in a plant immune network

Immune signaling networks must be tunable to alleviate fitness costs associated with immunity and, at the same time, robust against pathogen interferences. How these properties mechanistically emerge in plant immune signaling networks is poorly understood. Here, we discovered a molecular mechanism by which the model plant species Arabidopsis thaliana achieves robust and tunable immunity triggered by the microbe-associated molecular pattern, flg22. Salicylic acid (SA) is a major plant immune signal molecule. Another signal molecule jasmonate (JA) induced expression of a gene essential for SA accumulation, EDS5 Paradoxically, JA inhibited expression of PAD4, a positive regulator of EDS5 expression. This incoherent type-4 feed-forward loop (I4-FFL) enabled JA to mitigate SA accumulation in the intact network but to support it under perturbation of PAD4, thereby minimizing the negative impact of SA on fitness as well as conferring robust SA-mediated immunity. We also present evidence for evolutionary conservation of these gene regulations in the family Brassicaceae Our results highlight an I4-FFL that simultaneously provides the immune network with robustness and tunability in A. thaliana and possibly in its relatives.

Production of Phloroglucinol, a Platform Chemical, in Arabidopsis using a Bacterial Gene

Phloroglucinol (1,3,5-trihydroxybenzene; PG) and its derivatives are phenolic compounds that are used for various industrial applications. Current methods to synthesize PG are not sustainable due to the requirement for carbon-based precursors and co-production of toxic byproducts. Here, we describe a more sustainable production of PG using plants expressing a native bacterial or a codon-optimized synthetic PhlD targeted to either the cytosol or chloroplasts. Transgenic lines were analyzed for the production of PG using gas and liquid chromatography coupled to mass spectroscopy. Phloroglucinol was produced in all transgenic lines and the line with the highest PhlD transcript level showed the most accumulation of PG. Over 80% of the produced PG was glycosylated to phlorin. Arabidopsis leaves have the machinery to glycosylate PG to form phlorin, which can be hydrolyzed enzymatically to produce PG. Furthermore, the metabolic profile of plants with PhlD in either the cytosol or chloroplasts was altered. Our results provide evidence that plants can be engineered to produce PG using a bacterial gene. Phytoproduction of PG using a bacterial gene paves the way for further genetic manipulations to enhance the level of PG with implications for the commercial production of this important platform chemical in plants.

Draft Genome Sequence of the Plant Growth-Promoting Cupriavidus gilardii Strain JZ4 Isolated from the Desert Plant Tribulus terrestris

We isolated the plant endophytic bacterium Cupriavidus gilardii strain JZ4 from the roots of the desert plant Tribulus terrestris, collected from the Jizan region, Saudi Arabia. We report here the draft genome sequence of JZ4, together with several enzymes related to plant growth-promoting activity, environmental adaption, and antifungal activity.

Overexpression of Poplar PtrWRKY89 in Transgenic Arabidopsis Leads to a Reduction of Disease Resistance by Regulating Defense-Related Genes in Salicylate- and Jasmonate-Dependent Signaling

The plant hormones jasmonic acid (JA) and salicylic acid (SA) play key roles in plant defenses against pathogens and several WRKY transcription factors have been shown to have a role in SA/JA crosstalk. In a previous study, overexpression of the poplar WRKY gene PtrWRKY89 enhanced resistance to pathogens in transgenic poplars. In this study, the promoter of PtrWRKY89 (ProPtrWRKY89) was isolated and used to drive GUS reporter gene. High GUS activity was observed in old leaves of transgenic Arabidopsis containing ProPtrWRKY89-GUS construct and GUS expression was extremely induced by SA solution and SA+MeJA mixture but not by MeJA treatment. Subcellular localization and transactivation assays showed that PtrWRKY89 acted as a transcription activator in the nucleus. Constitutive expression of PtrWRKY89 in Arabidopsis resulted in more susceptible to Pseudomonas syringae and Botrytis cinerea compared to wild-type plants. Quantitative real-time PCR (qRT-PCR) analysis confirmed that marker genes of SA and JA pathways were down-regulated in transgenic Arabidopsis after pathogen inoculations. Overall, our results indicated that PtrWRKY89 modulates a cross talk in resistance to P. syringe and B. cinerea by negatively regulating both SA and JA pathways in Arabidopsis.

Map-based cloning and characterization of BPH29, a B3 domain-containing recessive gene conferring brown planthopper resistance in rice

Rice (Oryza sativa L.) production, essential for global food security, is threatened by the brown planthopper (BPH). The breeding of host-resistant crops is an economical and environmentally friendly strategy for pest control, but few resistance gene resources have thus far been cloned. An indica rice introgression line RBPH54, derived from wild rice Oryza rufipogon, has been identified with sustainable resistance to BPH, which is governed by recessive alleles at two loci. In this study, a map-based cloning approach was used to fine-map one resistance gene locus to a 24kb region on the short arm of chromosome 6. Through genetic analysis and transgenic experiments, BPH29, a resistance gene containing a B3 DNA-binding domain, was cloned. The tissue specificity of BPH29 is restricted to vascular tissue, the location of BPH attack. In response to BPH infestation, RBPH54 activates the salicylic acid signalling pathway and suppresses the jasmonic acid/ethylene-dependent pathway, similar to plant defence responses to biotrophic pathogens. The cloning and characterization of BPH29 provides insights into molecular mechanisms of plant-insect interactions and should facilitate the breeding of rice host-resistant varieties.

TCP transcription factors are critical for the coordinated regulation of isochorismate synthase 1 expression in Arabidopsis thaliana

Salicylic acid (SA) plays an important role in various aspects of plant development and responses to stresses. To elucidate the sophisticated regulatory mechanism of SA synthesis and signaling, we used a yeast one-hybrid system to screen for regulators of isochorismate synthase 1 (ICS1), a gene encoding the key enzyme in SA biosynthesis in Arabidopsis thaliana. A TCP family transcription factor AtTCP8 was initially identified as a candidate regulator of ICS1. The regulation of ICS1 by TCP proteins is supported by the presence of a typical TCP binding site in the ICS1 promoter. The binding of TCP8 to this site was confirmed by in vitro and in vivo assays. Expression patterns of TCP8 and its corresponding gene TCP9 largely overlapped with ICS1 under pathogen attack. A significant reduction in the expression of ICS1 during immune responses was observed in the tcp8 tcp9 double mutant. We also detected strong interactions between TCP8 and SAR deficient 1 (SARD1), WRKY family transcription factor 28 (WRKY28), NAC (NAM/ATAF1,ATAF2/CUC2) family transcription factor 019 (NAC019), as well as among TCP8, TCP9 and TCP20, suggesting a complex coordinated regulatory mechanism underlying ICS1 expression. Our results collectively demonstrate that TCP proteins are involved in the orchestrated regulation of ICS1 expression, with TCP8 and TCP9 being verified as major representatives.

Atypical E2F transcriptional repressor DEL1 acts at the intersection of plant growth and immunity by controlling the hormone salicylic acid

In plants, the activation of immunity is often inversely correlated with growth. Mechanisms that control plant growth in the context of pathogen challenge and immunity are unclear. Investigating Arabidopsis infection with the powdery mildew fungus, we find that the Arabidopsis atypical E2F DEL1, a transcriptional repressor known to promote cell proliferation, represses accumulation of the hormone salicylic acid (SA), an established regulator of plant immunity. DEL1-deficient plants are more resistant to pathogens and slightly smaller than wild-type. The resistance and size phenotypes of DEL1-deficient plants are due to the induction of SA and activation of immunity in the absence of pathogen challenge. Moreover, Enhanced Disease Susceptibility 5 (EDS5), a SA transporter required for elevated SA and immunity, is a direct repressed target of DEL1. Together, these findings indicate that DEL1 control of SA levels contributes to regulating the balance between growth and immunity in developing leaves.

Selective Interaction Between Chloroplast β-ATPase and TGB1L88 Retards Severe Symptoms Caused by Alternanthera mosaic virus Infection

The multifunctional triple gene block protein 1 (TGB1) of the Potexvirus Alternanthera mosaic virus (AltMV) has been reported to have silencing suppressor, cell-to-cell movement, and helicase functions. Yeast two hybrid screening using an Arabidopsis thaliana cDNA library with TGB1 as bait, and co-purification with TGB1 inclusion bodies identified several host proteins which interact with AltMV TGB1. Host protein interactions with TGB1 were confirmed by biomolecular fluorescence complementation, which showed positive TGB1 interaction with mitochondrial ATP synthase delta' chain subunit (ATP synthase delta'), light harvesting chlorophyll-protein complex I subunit A4 (LHCA4), chlorophyll a/b binding protein 1 (LHB1B2), chloroplast-localized IscA-like protein (ATCPISCA), and chloroplast β-ATPase. However, chloroplast β-ATPase interacts only with TGB1L88, and not with weak silencing suppressor TGB1P88. This selective interaction indicates that chloroplast β-ATPase is not required for AltMV movement and replication; however, TRV silencing of chloroplast β-ATPase in Nicotiana benthamiana induced severe tissue necrosis when plants were infected by AltMV TGB1L88 but not AltMV TGB1P88, suggesting that β-ATPase selectively responded to TGB1L88 to induce defense responses.

Salicylic acid signal transduction: the initiation of biosynthesis, perception and transcriptional reprogramming

The phytohormone salicylic acid (SA) is a small phenolic compound that regulates diverse physiological processes, in particular plant resistance against pathogens. Understanding SA-mediated signaling has been a major focus of plant research. Pathogen-induced SA is mainly synthesized via the isochorismate pathway in chloroplasts, with ICS1 (ISOCHORISMATE SYNTHASE 1) being a critical enzyme. Calcium signaling regulates activities of a subset of transcription factors thereby activating nuclear ICS1 expression. The produced SA triggers extensive transcriptional reprogramming in which NPR1 (NON-EXPRESSOR of PATHOGENESIS-RELATED GENES 1) functions as the central coactivator of TGA transcription factors. Recently, two alternative but not exclusive models for SA perception mechanisms were proposed. The first model is that NPR1 homologs, NPR3 and NPR4, perceive SA thereby regulating NPR1 protein accumulation. The second model describes that NPR1 itself perceives SA, triggering an NPR1 conformational change thereby activating SA-mediated transcription. Besides the direct SA binding, NPR1 is also regulated by SA-mediated redox changes and phosphorylation. Emerging evidence show that pathogen virulence effectors target SA signaling, further strengthening the importance of SA-mediated immunity.

An allele of Arabidopsis COI1 with hypo- and hypermorphic phenotypes in plant growth, defence and fertility

Resistance to biotrophic pathogens is largely dependent on the hormone salicylic acid (SA) while jasmonic acid (JA) regulates resistance against necrotrophs. JA negatively regulates SA and is, in itself, negatively regulated by SA. A key component of the JA signal transduction pathway is its receptor, the COI1 gene. Mutations in this gene can affect all the JA phenotypes, whereas mutations in other genes, either in JA signal transduction or in JA biosynthesis, lack this general effect. To identify components of the part of the resistance against biotrophs independent of SA, a mutagenised population of NahG plants (severely depleted of SA) was screened for suppression of susceptibility. The screen resulted in the identification of intragenic and extragenic suppressors, and the results presented here correspond to the characterization of one extragenic suppressor, coi1-40. coi1-40 is quite different from previously described coi1 alleles, and it represents a strategy for enhancing resistance to biotrophs with low levels of SA, likely suppressing NahG by increasing the perception to the remaining SA. The phenotypes of coi1-40 lead us to speculate about a modular function for COI1, since we have recovered a mutation in COI1 which has a number of JA-related phenotypes reduced while others are equal to or above wild type levels.

Inducible malondialdehyde pools in zones of cell proliferation and developing tissues in Arabidopsis

Malondialdehyde (MDA) is a natural and widespread genotoxin. Given its potentially deleterious effects, it is of interest to establish the identities of the cell types containing this aldehyde. We used in situ chemical trapping with 2-thiobarbituric acid and mass spectrometry with a deuterated standard to characterize MDA pools in the vegetative phase in Arabidopsis thaliana. In leaves, MDA occurred predominantly in the intracellular compartment of mesophyll cells and was enriched in chloroplasts where it was derived primarily from triunsaturated fatty acids (TFAs). High levels of MDA (most of which was unbound) were found within dividing cells in the root tip cell proliferation zone. The bulk of this MDA did not originate from TFAs. We confirmed the localization of MDA in transversal root sections. In addition to MDA in proliferating cells near the root tip we found evidence for the presence of MDA in pericyle cells. Remodeling of non-TFA-derived MDA pools occurred when seedlings were infected with the fungus Botrytis cinerea. Treatment of uninfected seedlings with mediators of plant stress responses (jasmonic acid or salicylic acid) increased seedling MDA levels over 20-fold. In summary, major pools of MDA are associated with cell division foci containing stem cells. The aldehyde is pathogen-inducible in these regions and its levels are increased by cellular mediators that impact defense and growth.

A dual regulatory role of Arabidopsis calreticulin-2 in plant innate immunity

Calreticulin (CRT) is an endoplasmic reticulum-resident calcium-binding molecular chaperone that is highly conserved in multi-cellular eukaryotes. Higher plants contain two distinct groups of CRTs: CRT1/CRT2 and CRT3 isoforms. Previous studies have shown that bacterial elongation factor Tu receptor (EFR), a pattern-recognition receptor that is responsible for pathogen-associated molecular pattern-triggered immunity, is a substrate for Arabidopsis CRT3, suggesting a role for CRT3 in regulating plant defense against pathogens. Here we report that Arabidopsis CRT2 is another regulator of plant innate immunity. Despite significantly increased salicylic acid levels and constitutive expression of the systemic acquired resistance-associated marker genes PR1, PR2 and PR5, transgenic plants over-expressing CRT2 displayed reduced resistance to virulent Pseudomonas syringae pv. tomato DC3000 (PstDC3000). A (45)Ca(2+) overlay assay and a domain-swapping experiment further demonstrated that the negatively charged C-terminal tail of CRT2 is responsible for its high calcium-binding capacity and function in regulating the endogenous salicylic acid level. In addition, over-expression of the His173 mutant of CRT2 greatly enhanced plant defense against PstDC3000, supporting the existence of a self-inhibition mechanism that can counteract the effects of salicylic acid-dependent immune responses. These results suggest that CRT2 functions through its N-terminal domain(s) as a self-modulator that can possibly prevent the salicylic acid-mediated runaway defense responses triggered by its C-terminal calcium-buffering activity in response to pathogen invasion.

Phytochromes regulate SA and JA signaling pathways in rice and are required for developmentally controlled resistance to Magnaporthe grisea

Old leaves of wild-type rice plants (Oryza sativa L. cv. Nipponbare) are more resistant to blast fungus (Magnaporthe grisea) than new leaves. In contrast, both old and new leaves of the rice phytochrome triple mutant (phyAphyBphyC) are susceptible to blast fungus. We demonstrate that pathogenesis-related class 1 (PR1) proteins are rapidly and strongly induced during M. grisea infection and following exogenous jasmonate (JA) or salicylic acid (SA) exposure in the old leaves, but not in the new leaves of the wild-type. In contrast, the accumulation of PR1 proteins was significantly attenuated in old and new leaves of the phyAphyBphyC mutant. These results suggest that phytochromes are required for the induction of PR1 proteins in rice. Basal transcription levels of PR1a and PR1b were substantially higher in the wild-type as compared to the phyAphyBphyC mutant, suggesting that phytochromes also are required for basal expression of PR1 genes. Moreover, the transcript levels of genes known to function in SA- or JA-dependent defense pathways were regulated by leaf age and functional phytochromes. Taken together, our findings demonstrate that phytochromes are required in rice for age-related resistance to M. grisea and may indirectly increase PR1 gene expression by regulating SA- and JA-dependent defense pathways.

Salicylic Acid biosynthesis and metabolism

Salicylic acid (SA) has been shown to regulate various aspects of growth and development; it also serves as a critical signal for activating disease resistance in Arabidopsis thaliana and other plant species. This review surveys the mechanisms involved in the biosynthesis and metabolism of this critical plant hormone. While a complete biosynthetic route has yet to be established, stressed Arabidopsis appear to synthesize SA primarily via an isochorismate-utilizing pathway in the chloroplast. A distinct pathway utilizing phenylalanine as the substrate also may contribute to SA accumulation, although to a much lesser extent. Once synthesized, free SA levels can be regulated by a variety of chemical modifications. Many of these modifications inactivate SA; however, some confer novel properties that may aid in long distance SA transport or the activation of stress responses complementary to those induced by free SA. In addition, a number of factors that directly or indirectly regulate the expression of SA biosynthetic genes or that influence the rate of SA catabolism have been identified. An integrated model, encompassing current knowledge of SA metabolism in Arabidopsis, as well as the influence other plant hormones exert on SA metabolism, is presented.

Arabidopsis putative deacetylase AtSRT2 regulates basal defense by suppressing PAD4, EDS5 and SID2 expression

The silent information regulator protein (Sir2) and its homologs are NAD(+)-dependent deacetylase enzymes that play important roles in a variety of physiological processes. However, the functions of the Sir2 family in plants are poorly understood. Here, we report that Arabidopsis AtSRT2, a homolog of yeast Sir2, negatively regulates plant basal defense against the pathogen Pseudomonas syringae pv. tomato DC3000 (PstDC3000). In response to PstDC3000 infection, the expression of AtSRT2 was down-regulated in a salicylic acid (SA)-independent manner. In addition, knock-out of AtSRT2 (srt2) enhanced resistance against PstDC3000 and increased expression of pathogenesis-related gene 1 (PR1). Conversely, overexpression of AtSRT2 resulted in hypersusceptibility to PstDC3000 and impaired PR1 induction. Consistent with this phenotype, expression of PAD4, EDS5 and SID2, three essential genes in the SA biosynthesis pathway, were increased in the srt2 mutant and decreased in AtSRT2-overexpressing plants. Taken together, these results demonstrate that AtSRT2 is a negative regulator of basal defense, possibly by suppressing SA biosynthesis.

Synergistic activation of defense responses in Arabidopsis by simultaneous loss of the GSL5 callose synthase and the EDR1 protein kinase

Loss-of-function mutations in the EDR1 gene of Arabidopsis confer enhanced resistance to Golovinomyces cichoracearum (powdery mildew). Disease resistance mediated by the edr1 mutation is dependent on an intact salicylic acid (SA) signaling pathway, but edr1 mutant plants do not constitutively express the SA-inducible gene PR-1 and are not dwarfed. To identify other components of the EDR1 signaling network, we screened for mutations that enhanced the edr1 mutant phenotype. Here, we describe an enhancer of edr1 mutant, eed3, which forms spontaneous lesions in the absence of pathogen infection, constitutively expresses both SA- and methyl jasmonate (JA)-inducible defense genes, and is dwarfed. Positional cloning of eed3 revealed that the mutation causes a premature stop codon in GLUCAN SYNTHASE-LIKE 5 (GSL5, also known as POWDERY MILDEW RESISTANT 4), which encodes a callose synthase required for pathogen-induced callose production. Significantly, gsl5 single mutants do not constitutively express PR-1 or AtERF1 (a JA-inducible gene) and are not dwarfed. Thus, loss of both EDR1 and GSL5 function has a synergistic effect. Our data suggest that EDR1 and GSL5 negatively regulate SA and JA production or signaling by independent mechanisms and that negative regulation of defense signaling by GSL5 may be independent of callose production.

Resistance and biomass in Arabidopsis: a new model for salicylic acid perception

Salicylic acid (SA) is an essential hormone for plant defence and development. SA perception is usually measured by counting the number of pathogens that grow in planta upon an exogenous application of the hormone. A biological SA perception model based on plant fresh weight reduction caused by disease resistance in Arabidopsis thaliana is proposed. This effect is more noticeable when a chemical analogue of SA is used, like Benzothiadiazole (BTH). By spraying BTH several times, a substantial difference in plant biomass is observed when compared with the mock treatment. Such difference is dose-dependent and does not require pathogen inoculation. The model is robust and allows for the comparison of different Arabidopsis ecotypes, recombinant inbreed lines, and mutants. Our results show that two mutants, non-expresser of pathogenesis-related genes 1 (npr1) and auxin resistant 3 (axr3), fail to lose biomass when BTH is applied to them. Further experiments show that axr3 responds to SA and BTH in terms of defence induction. NPR1-related genotypes also confirm the pivotal role of NPR1 in SA perception, and suggest an active program of depletion of resources in the infected tissues.

Nuclear localization of NPR1 is required for regulation of salicylate tolerance, isochorismate synthase 1 expression and salicylate accumulation in Arabidopsis

Plant systemic acquired resistance (SAR) is a broad-spectrum immune response in which pathogen infection in local tissue induces resistance in systemic leaves. Activation of SAR requires the signal molecule salicylic acid (SA), which is primarily synthesized from chorismate via isochorismate through the action of isochorismate synthase 1 (ICS1) and a putative isochorismate pyruvate lyase. The Arabidopsis transcription coactivator NPR1 is a key regulator of SAR, which functions at multiple nodes in the SA signaling network. NPR1 not only acts downstream of SA to activate SAR, but also upstream of SA to suppress the expression of ICS1, thus inhibiting SA biosynthesis. NPR1 also positively regulates SA tolerance and plays a role in SA-mediated negative regulation of jasmonic acid (JA) signaling. The NPR1 protein contains a functional bipartite nuclear localization signal (NLS). It has been shown that the NLS and nuclear localization of NPR1 are required for activation of pathogenesis-related gene expression, whereas modulation of the crosstalk between SA- and JA-dependent defense pathways is mediated by cytosolic NPR1. In this study we used two transgenic lines, one expressing a mutated npr1 with a dysfunctional NLS and the other in which NPR1 nuclear localization can be induced by dexamethasone treatment, to test whether nuclear localization is required for other functions of NPR1. We found that prevention of NPR1 nuclear localization renders transgenic seedlings sensitive to the toxicity of high levels of SA and causes over-accumulation of ICS1 transcripts and SA in response to pathogen infection. Induction of NPR1 nuclear localization restores SA tolerance and normal accumulation of ICS1 transcripts and SA. These results indicate that the NLS and nuclear localization of NPR1 are required for regulation of SA tolerance, ICS1 expression and SA accumulation.

Acquired resistance to the rice blast in transgenic rice accumulating the antimicrobial peptide thanatin

Thanatin is an antimicrobial peptide with a strong and wide-ranging antimicrobial spectrum, including certain species of fungi and Gram-negative and Gram-positive bacteria. To evaluate the application of thanatin to the generation of disease-resistant plants, we introduced a synthetic thanatin gene into rice. Several transformants that expressed the introduced gene showed significant level of antimicrobial activity. The substances showing antimicrobial activity were partially purified from these transformants and their properties were determined. The molecule with characteristics similar to those of native thanatin on the elution pattern in HPLC analysis had an identical molecular mass to that of native molecule. It should also be noted that the transformant acquired a sufficient level of resistance to the rice blast fungus, Magnaporthe oryzae, presumably due to the repressive activity of thanatin to its initial stage of infection. This result demonstrates that thanatin has antifungal activity for M. oryzae and that the introduction of the thanatin gene into rice is effective in generating a plant resistant to rice blast disease.

Dissection of salicylic acid-mediated defense signaling networks

The small phenolic molecule salicylic acid (SA) plays a key role in plant defense. Significant progress has been made recently in understanding SA-mediated defense signaling networks. Functional analysis of a large number of genes involved in SA biosynthesis and regulation of SA accumulation and signal transduction has revealed distinct but interconnecting pathways that orchestrate the control of plant defense. Further studies utilizing combinatorial approaches in genetics, molecular biology, biochemistry and genomics will uncover finer details of SA-mediated defense networks as well as further insights into the crosstalk of SA with other defense signaling pathways. The complexity of defense networks illustrates the capacity of plants to integrate multiple developmental and environmental signals into a tight control of the costly defense responses.

The chloroplast protein RPH1 plays a role in the immune response of Arabidopsis to Phytophthora brassicae

Plant immune responses to pathogens are often associated with enhanced production of reactive oxygen species (ROS), known as the oxidative burst, and with rapid hypersensitive host cell death (the hypersensitive response, HR) at sites of attempted infection. It is generally accepted that the oxidative burst acts as a promotive signal for HR, and that HR is highly correlated with efficient disease resistance. We have identified the Arabidopsis mutant rph1 (resistance to Phytophthora 1), which is susceptible to the oomycete pathogen Phytophthora brassicae despite rapid induction of HR. The susceptibility of rph1 was specific for P. brassicae and coincided with a reduced oxidative burst, a runaway cell-death response, and failure to properly activate the expression of defence-related genes. From these results, we conclude that, in the immune response to P. brassicae, (i) HR is not sufficient to stop the pathogen, (ii) HR initiation can occur in the absence of a major oxidative burst, (iii) the oxidative burst plays a role in limiting the spread of cell death, and (iv) RPH1 is a positive regulator of the P. brassicae-induced oxidative burst and enhanced expression of defence-related genes. Surprisingly, RPH1 encodes an evolutionary highly conserved chloroplast protein, indicating a function of this organelle in activation of a subset of immune reactions in response to P. brassicae. The disease resistance-related role of RPH1 was not limited to the Arabidopsis model system. Silencing of the potato homolog StRPH1 in a resistant potato cultivar caused susceptibility to the late blight pathogen Phytophthora infestans.

A rice gene of de novo origin negatively regulates pathogen-induced defense response

How defense genes originated with the evolution of their specific pathogen-responsive traits remains an important problem. It is generally known that a form of duplication can generate new genes, suggesting that a new gene usually evolves from an ancestral gene. However, we show that a new defense gene in plants may evolve by de novo origination, resulting in sophisticated disease-resistant functions in rice. Analyses of gene evolution showed that this new gene, OsDR10, had homologs only in the closest relative, Leersia genus, but not other subfamilies of the grass family; therefore, it is a rice tribe-specific gene that may have originated de novo in the tribe. We further show that this gene may evolve a highly conservative rice-specific function that contributes to the regulation difference between rice and other plant species in response to pathogen infections. Biologic analyses including gene silencing, pathologic analysis, and mutant characterization by transformation showed that the OsDR10-suppressed plants enhanced resistance to a broad spectrum of Xanthomonas oryzae pv. oryzae strains, which cause bacterial blight disease. This enhanced disease resistance was accompanied by increased accumulation of endogenous salicylic acid (SA) and suppressed accumulation of endogenous jasmonic acid (JA) as well as modified expression of a subset of defense-responsive genes functioning both upstream and downstream of SA and JA. These data and analyses provide fresh insights into the new biologic and evolutionary processes of a de novo gene recruited rapidly.

Ca(2+)/calmodulin regulates salicylic-acid-mediated plant immunity

Intracellular calcium transients during plant-pathogen interactions are necessary early events leading to local and systemic acquired resistance. Salicylic acid, a critical messenger, is also required for both of these responses, but whether and how salicylic acid level is regulated by Ca(2+) signalling during plant-pathogen interaction is unclear. Here we report a mechanism connecting Ca(2+) signal to salicylic-acid-mediated immune response through calmodulin, AtSR1 (also known as CAMTA3), a Ca(2+)/calmodulin-binding transcription factor, and EDS1, an established regulator of salicylic acid level. Constitutive disease resistance and elevated levels of salicylic acid in loss-of-function alleles of Arabidopsis AtSR1 suggest that AtSR1 is a negative regulator of plant immunity. This was confirmed by epistasis analysis with mutants of compromised salicylic acid accumulation and disease resistance. We show that AtSR1 interacts with the promoter of EDS1 and represses its expression. Furthermore, Ca(2+)/calmodulin-binding to AtSR1 is required for suppression of plant defence, indicating a direct role for Ca(2+)/calmodulin in regulating the function of AtSR1. These results reveal a previously unknown regulatory mechanism linking Ca(2+) signalling to salicylic acid level.

Salicylic acid deficiency in NahG transgenic lines and sid2 mutants increases seed yield in the annual plant Arabidopsis thaliana

Salicylic acid-deficient NahG transgenic lines and sid2 mutants were used to evaluate the role of this compound in the development of the short-lived, annual plant Arabidopsis thaliana, with a particular focus on the interplay between salicylic acid and other phytohormones. Low salicylic acid levels led to increased growth, as well as to smaller abscisic acid levels and reduced damage to PSII (as indicated by F(v)/F(m) ratios) during the reproductive stages in rosette leaves of NahG transgenic lines and sid2 mutants, compared with wild-type plants. Furthermore, salicylic acid deficiency highly influenced seed yield and composition. Seed production increased by 4.4-fold and 3.5-fold in NahG transgenic lines and sid2 mutants, respectively, compared to the wild type. Salicylic acid deficiency also improved seed composition in terms of antioxidant vitamin concentrations, seeds of salicylic acid-deficient plants showing higher levels of alpha- and gamma-tocopherol (vitamin E) and beta-carotene (pro-vitamin A) than seeds of wild-type plants. Seeds of salicylic acid-deficient plants also showed higher nitrogen concentrations than seeds of wild-type plants. It is concluded that (i) the sid2 gene, which encodes for isochorismate synthase, plays a central role in salicylic acid biosynthesis during plant development in A. thaliana, (ii) salicylic acid plays a role in the regulation of growth, senescence, and seed production, (iii) there is a cross-talk between salicylic acid and other phytohormones during plant development, and (iv) the concentrations of antioxidant vitamins in seeds may be influenced by the endogenous levels of salicylic acid in plants.

Overexpression of the Arabidopsis thaliana EDS5 gene enhances resistance to viruses

The Arabidopsis thaliana ENHANCED DISEASE SUSCEPTIBILITY 5 gene (EDS5) is required for salicylic acid (SA) synthesis in pathogen-challenged plants. SA and EDS5 have an important role in the Arabidopsis RCY1 gene-conferred resistance against the yellow strain of Cucumber mosaic virus [CMV(Y)], a Bromoviridae, and HRT-conferred resistance against the Tombusviridae, Turnip crinkle virus (TCV). EDS5 expression and SA accumulation are induced in response to CMV(Y) inoculation in the RCY1-bearing ecotype C24. To further discern the involvement of EDS5 in Arabidopsis defence against viruses, we overexpressed the EDS5 transcript from the constitutively expressed Cauliflower mosaic virus 35S gene promoter in ecotype C24. In comparison to the non-transgenic control, the basal level of salicylic acid (SA) was twofold higher in the 35S:EDS5 plant. Furthermore, viral spread and the size of the hypersensitive response associated necrotic local lesions (NLL) were more highly restricted in CMV(Y)-inoculated 35S:EDS5 than in the non-transgenic plant. The heightened restriction of CMV(Y) spread was paralleled by more rapid induction of the pathogenesis-related gene, PR-1, in the CMV(Y)-inoculated 35S:EDS5 plant. The 35S:EDS5 plant also had heightened resistance to the virulent CMV strain, CMV(B2), and TCV. These results suggest that, in addition to R gene-mediated gene-for-gene resistance, EDS5 is also important for basal resistance to viruses. However, while expression of the Pseudomonas putida nahG gene, which encodes the SA-degrading salicylate hydroxylase, completely suppressed 35S:EDS5-conferred resistance against CMV(Y) and TCV, it only partially compromised resistance against CMV(B2), indicating that SA-dependent and -independent mechanisms are associated with 35S:EDS5-conferred resistance against viruses.

Increased SA in NPR1-silenced plants antagonizes JA and JA-dependent direct and indirect defenses in herbivore-attacked Nicotiana attenuata in nature

The phytohormone jasmonic acid (JA) is known to mediate herbivore resistance, while salicylic acid (SA) and non-expressor of PR-1 (NPR1) mediate pathogen resistance in many plants. Herbivore attack on Nicotiana attenuata elicits increases in JA and JA-mediated defenses, but also increases SA levels and Na-NPR1 transcripts from the plant's single genomic copy. SA treatment of wild-type plants increases Na-NPR1 and Na-PR1 transcripts. Plants silenced in NPR1 accumulation by RNAi (ir-npr1) are highly susceptible to herbivore and pathogen attack when planted in their native habitat in Utah. They are also impaired in their ability to attract Geocorus pallens predators, due to their decreased ability to release cis-alpha-bergamotene, a JA-elicited volatile 'alarm call'. In the glasshouse, Spodoptera exigua larvae grew better on ir-npr1 plants, which had low levels of JA, JA-isoleucine/leucine, lipoxygenase-3 (LOX3) transcripts and JA-elicited direct defense metabolites (nicotine, caffeoyl putrescine and rutin), but high levels of SA and isochorismate synthase (ICS) transcripts, suggesting de novo biosynthesis of SA. A microarray analysis revealed downregulation of many JA-elicited genes and upregulation of SA biosynthetic genes. JA treatment restored nicotine levels and resistance to S. exigua in ir-npr1 plants. We conclude that, during herbivore attack, NPR1 negatively regulates SA production, allowing the unfettered elicitation of JA-mediated defenses; when NPR1 is silenced, the elicited increases in SA production antagonize JA and JA-related defenses, making the plants susceptible to herbivores.

The isochorismate pathway is negatively regulated by salicylic acid signaling in O3-exposed Arabidopsis

Ozone (O3), a major photochemical oxidant, causes leaf injury in plants. Plants synthesize salicylic acid (SA), which is reported to greatly affect O3 sensitivity. However, the mechanism of SA biosynthesis under O3 exposure remains unclear. Plants synthesize SA either by a pathway involving phenylalanine as a substrate or another involving isochorismate. To clarify how SA is produced in O3-exposed Arabidopsis, we examined the activities of phenylalanine ammonia lyase (PAL) and isochorismate synthase (ICS), which are components of the phenylalanine and isochorismate pathways, respectively. Exposure of Arabidopsis to O3 enhanced the accumulation of SA and the increase of ICS activity but did not affect PAL activity. In sid2 mutants, which have a defect in ICS1, the level of SA and the activity of ICS did not increase in response to O3 exposure. These results suggest that SA is mainly synthesized from isochorismate in Arabidopsis. Furthermore, the level of ICS1 expression and the activity of ICS during O3 exposure elevated in plants deficient for SA signaling (npr1 and eds5 mutants and NahG transgenics). Treatment of plants with SA also suppressed the enhancement of ICS1 expression by O3. These results suggest that SA synthesis is negatively regulated by SA signaling.

Comparative expression profiling of Nicotiana benthamiana leaves systemically infected with three fruit tree viruses

Plant viruses cause a wide array of disease symptoms and cytopathic effects. Although some of these changes are virus specific, many appear to be common even among diverse viruses. Currently, little is known about the underlying molecular determinants. To identify gene expression changes that are concomitant with virus symptoms, we performed comparative expression profiling experiments on Nicotiana benthamiana leaves infected with one of three different fruit tree viruses that produce distinct symptoms: Plum pox potyvirus (PPV; leaf distortion and mosaic), Tomato ringspot nepovirus (ToRSV; tissue necrosis and general chlorosis), and Prunus necrotic ringspot ilarvirus (PNRSV; subtle chlorotic mottling). The numbers of statistically significant genes identified were consistent with the severity of the observed symptoms: 1,082 (ToRSV), 744 (PPV), and 89 (PNRSV). In all, 56% of the gene expression changes found in PPV-infected leaves also were altered by ToRSV, 87% of which changed in the same direction. Both PPV- and ToRSV-infected leaves showed widespread repression of genes associated with plastid functions. PPV uniquely induced the expression of large numbers of cytosolic ribosomal genes whereas ToRSV repressed the expression of plastidic ribosomal genes. How these and other observed expression changes might be associated with symptom development are discussed.

OsWRKY13 mediates rice disease resistance by regulating defense-related genes in salicylate- and jasmonate-dependent signaling

Although 109 WRKY genes have been identified in the rice genome, the functions of most are unknown. Here, we show that OsWRKY13 plays a pivotal role in rice disease resistance. Overexpression of OsWRKY13 can enhance rice resistance to bacterial blight and fungal blast, two of the most devastating diseases of rice worldwide, at both the seedling and adult stages, and shows no influence on the fertility. This overexpression was accompanied by the activation of salicylic acid (SA) synthesis-related genes and SA-responsive genes and the suppression of jasmonic acid (JA) synthesis-related genes and JA-responsive genes. OsWRKY13 bound to the promoters of its own and at least three other genes in SA- and JA-dependent signaling pathways. Its DNA-binding activity was influenced by pathogen infection. These results suggest that OsWRKY13, as an activator of the SA-dependent pathway and a suppressor of JA-dependent pathways, mediates rice resistance by directly or indirectly regulating the expression of a subset of genes acting both upstream and downstream of SA and JA. Furthermore, OsWRKY13 will provide a transgenic tool for engineering wider-spectrum and whole-growth-stage resistance rice in breeding programs.

Expression of a beta-glucosidase gene results in increased accumulation of salicylic acid in transgenic Nicotiana tabacum cv. Xanthi-nc NN genotype

A beta-glucosidase gene (bglA) from Butyrivibrio fibrisolvens H17c was cloned into the binary vector pGA482 under the control of the 35S Cauliflower Mosaic Virus (CaMV) promoter. A second construct was generated for accumulation of the bglA gene product in the vacuole of transformed tobacco plants. Reverse transcription-polymerase chain reaction analysis demonstrated that the bglA gene was expressed in 71% of cytosol-targeted and 67% of vacuole-targeted transgenic tobacco T(1) plants. T(1) transgenic plants (pGLU100 and pGLU200) exhibited elevated levels of free salicylic acid (SA) with a concomitant significant decrease in the level of glucosylsalicylic acid (GSA) compared to the untransformed tobacco plants and tobacco plants transformed with the empty vector (pGA482). Following inoculation with Tobacco Mosaic Virus (TMV), lesion area was 51% smaller in pGLU100 plants and 60% smaller in pGLU200 plants compared to inoculated untransformed and negative control plants.

Vitamin K1 accumulation in tobacco plants overexpressing bacterial genes involved in the biosynthesis of salicylic acid

Phylloquinone (Vitamin K(1)) is an essential component of the photosynthetic electron transfer. As isochorismate is required for the biosynthesis of Vitamin K(1), isochorismate synthase (ICS) activity is expected to be present in all green plants. In bacteria salicylic acid (SA) is synthesized via a two step pathway involving ICS and isochorismate pyruvate lyase (IPL). The effect of the introduction in tobacco plants of the bacterial ICS and IPL genes on the endogenous isochorismate pathway was investigated. Transgenic tobacco plants in which IPL was targeted to the chloroplast suffered severe growth retardation and had low Vitamin K(1) content. Probably because isochorismate was channeled towards SA production, the plants were no longer able to produce normal levels of Vitamin K(1). Transgenic tobacco plants in which the bacterial ICS was present in the chloroplast showed higher Vitamin K(1) contents than wild type plants.

Systemic acquired resistance

Upon infection with necrotizing pathogens many plants develop an enhanced resistance to further pathogen attack also in the uninoculated organs. This type of enhanced resistance is referred to as systemic acquired resistance (SAR). In the SAR state, plants are primed (sensitized) to more quickly and more effectively activate defense responses the second time they encounter pathogen attack. Since SAR depends on the ability to access past experience, acquired disease resistance is a paradigm for the existence of a form of "plant memory". Although the phenomenon has been known since the beginning of the 20th century, major progress in the understanding of SAR was made over the past sixteen years. This review covers the current knowledge of molecular, biochemical and physiological mechanisms that are associated with SAR.

Salicylate biosynthesis: overexpression, purification, and characterization of Irp9, a bifunctional salicylate synthase from Yersinia enterocolitica

In some bacteria, salicylate is synthesized using the enzymes isochorismate synthase and isochorismate pyruvate lyase. In contrast, gene inactivation and complementation experiments with Yersinia enterocolitica suggest the synthesis of salicylate in the biosynthesis of the siderophore yersiniabactin involves a single protein, Irp9, which converts chorismate directly into salicylate. In the present study, Irp9 was for the first time heterologously expressed in Escherichia coli as a hexahistidine fusion protein, purified to near homogeneity, and characterized biochemically. The recombinant protein was found to be a dimer, each subunit of which has a molecular mass of 50 kDa. Enzyme assays, reverse-phase high-pressure liquid chromatography and 1H nuclear magnetic resonance (NMR) spectroscopic analyses confirmed that Irp9 is a salicylate synthase and converts chorismate to salicylate with a K(m) for chorismate of 4.2 microM and a k(cat) of 8 min(-1). The reaction was shown to proceed through the intermediate isochorismate, which was detected directly using 1H NMR spectroscopy.

Chloroplasts as source and target of cellular redox regulation: a discussion on chloroplast redox signals in the context of plant physiology

During the evolution of plants, chloroplasts have lost the exclusive genetic control over redox regulation and antioxidant gene expression. Together with many other genes, all genes encoding antioxidant enzymes and enzymes involved in the biosynthesis of low molecular weight antioxidants were transferred to the nucleus. On the other hand, photosynthesis bears a high risk for photo-oxidative damage. Concomitantly, an intricate network for mutual regulation by anthero- and retrograde signals has emerged to co-ordinate the activities of the different genetic and metabolic compartments. A major focus of recent research in chloroplast regulation addressed the mechanisms of redox sensing and signal transmission, the identification of regulatory targets, and the understanding of adaptation mechanisms. In addition to redox signals communicated through signalling cascades also used in pathogen and wounding responses, specific chloroplast signals control nuclear gene expression. Signalling pathways are triggered by the redox state of the plastoquinone pool, the thioredoxin system, and the acceptor availability at photosystem I, in addition to control by oxolipins, tetrapyrroles, carbohydrates, and abscisic acid. The signalling function is discussed in the context of regulatory circuitries that control the expression of antioxidant enzymes and redox modulators, demonstrating the principal role of chloroplasts as the source and target of redox regulation.

Autoactive alleles of the flax L6 rust resistance gene induce non-race-specific rust resistance associated with the hypersensitive response

L6 is a nucleotide binding site-leucine rich repeat (NBS-LRR) gene that confers race-specific resistance in flax (Linum usitatissimum) to strains of flax rust (Melampsora lini) that carry avirulence alleles of the AvrL567 gene but not to rust strains that carry only the virulence allele. Several mutant and recombinant forms of L6 were made that altered either the methionine-histidine-aspartate (MHD) motif conserved in the NBS domain of resistance proteins or exchanged the short domain C-terminal to the LRR region that is highly variable among L allele products. In transgenic flax some of these alleles are autoactive; they cause a gene dosage-dependent dwarf phenotype and constitutive expression of genes that are markers for the plant defense response. Their effects and penetrance ranged from extreme to mild in their degree of plant stunting, survival, and reproduction. Dwarf plants were also resistant to flax rust strains virulent to wild-type L6 plants, and this nonspecific resistance was associated with a hypersensitive response (HR) at the site of rust infection. The strongest autoactive allele, expressed in Arabidopsis from an ethanol-inducible promoter, gave rise to plant death dependent on the enhanced disease susceptibility 1 (EDS1) gene, which indicates that the mutant flax (Linaceae) L6 gene can signal cell death through a defined disease-resistance pathway in a different plant family (Brassicaceae).

Host defence in a developmental context

SUMMARY Disease resistance takes place within the context of the host developmental programme. The cellular and molecular basis of the developmental control of resistance is virtually unknown. It is clear from mutant studies that developmental processes are impacted when defence factors are altered and it is equally clear that alteration of developmental factors impacts defence functions. A review of current knowledge regarding the interplay of resistance and development is presented. Stage-specific limitations on defence represent an important target for crop improvement.

High humidity suppresses ssi4-mediated cell death and disease resistance upstream of MAP kinase activation, H2O2 production and defense gene expression

The Arabidopsis ssi4 mutant, which exhibits spontaneous lesion formation, constitutive expression of pathogenesis-related (PR) genes and enhanced resistance to virulent bacterial and oomycete pathogens, contains a gain-of-function mutation in a TIR-NBS-LRR type R gene. Epistatic analyses revealed that both PR gene expression and disease resistance are activated via a salicylic acid (SA)- and EDS1-dependent, but NPR1- and NDR1-independent signaling pathway. In this study, we demonstrate that in moderate relative humidity (RH; 60%), the ssi4 mutant accumulates H(2)O(2) and SA prior to lesion formation and displays constitutive activation of the MAP kinases AtMPK6 and AtMPK3. It also constitutively expresses a variety of defense-associated genes, including those encoding the WRKY transcription factors AtWRKY29 and AtWRKY6, the MAP kinases AtMPK6 and AtMPK3, the powdery mildew R proteins RPW8.1 and RPW8.2, EDS1 and PR proteins. All of these ssi4-induced responses, as well as the chlorotic, stunted morphology and enhanced disease resistance phenotype, are suppressed by high RH (95%) growth conditions. Thus, a humidity sensitive factor (HSF) appears to function at an early point in the ssi4 signaling pathway. All ssi4 phenotypes, except for MAP kinase activation, also were suppressed by the eds1-1 mutation. Thus, ssi4-induced MAP kinase activation occurs downstream of the HSF but either upstream of EDS1 or on a separate branch of the ssi4 signaling pathway. SA is a critical signaling component in ssi4-mediated defense responses. However, exogenously supplied SA failed to restore lesion formation in high RH-grown ssi4 plants, although it induced defense gene expression. Thus, additional signals also are involved.