Interplay of signaling pathways in plant disease resistance

Plants are under constant threat of infection by pathogens armed with a diverse array of effector molecules to colonize their host. Plants have, in turn, evolved sophisticated detection and response systems that decipher pathogen signals and induce appropriate defenses. Genetic analysis of plant mutants impaired in mounting a resistance response to invading pathogens has uncovered a number of distinct, but interconnecting, signaling networks that are under both positive and negative control. These pathways operate, at least partly, through the action of small signaling molecules such as salicylate, jasmonate and ethylene. The interplay of signals probably allows the plant to fine-tune defense responses in both local and systemic tissue.     

NCS-1 is a regulator of calcium signaling in health and disease

Neuronal Calcium Sensor-1 (NCS-1) is a highly conserved calcium binding protein which contributes to the maintenance of intracellular calcium homeostasis and regulation of calcium-dependent signaling pathways. It is involved in a variety of physiological cell functions, including exocytosis, regulation of calcium permeable channels, neuroplasticity and response to neuronal damage. Over the past 30?years, continuing investigation of cellular functions of NCS-1 and associated disease states have highlighted its function in the pathophysiology of several disorders and as a therapeutic target. Among the diseases that were found to be associated with NCS-1 are neurological disorders such as bipolar disease and non-neurological conditions such as breast cancer. Furthermore, alteration of NCS-1 expression is associated with substance abuse disorders and severe side effects of chemotherapeutic agents. The objective of this article is to summarize the current body of evidence describing NCS-1 and its interactions on a molecular and cellular scale, as well as describing macroscopic implications in physiology and medicine. Particular attention is paid to the role of NCS-1 in development and prevention of chemotherapy induced peripheral neuropathy (CIPN).     

The ZAR1 resistosome is a calcium-permeable channel triggering plant immune signaling

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Response to enemies in the invasive plant Lythrum salicaria is genetically determined

Background and Aims

The enemy release hypothesis assumes that invasive plants lose their co-evolved natural enemies during introduction into the new range. This study tested, as proposed by the evolution of increased competitive ability (EICA) hypothesis, whether escape from enemies results in a decrease in defence ability in plants from the invaded range. Two straightforward aspects of the EICA are examined: (1) if invasives have lost their enemies and their defence, they should be more negatively affected by their full natural pre-invasion herbivore spectrum than their native conspecifics; and (2) the genetic basis of evolutionary change in response to enemy release in the invasive range has not been taken sufficiently into account.

Methods

Lythrum salicaria (purple loosestrife) from several populations in its native (Europe) and invasive range (North America) was exposed to all above-ground herbivores in replicated natural populations in the native range. The experiment was performed both with plants raised from field-collected seeds as well as with offspring of these where maternal effects were removed.

Key Results

Absolute and relative leaf damage was higher for introduced than for native plants. Despite having smaller height growth rate, invasive plants attained a much larger final size than natives irrespective of damage, indicating large tolerance rather than effective defence. Origin effects on response to herbivory and growth were stronger in second-generation plants, suggesting that invasive potential through enemy release has a genetic basis.

Conclusions

The findings support two predictions of the EICA hypothesis – a genetically determined difference between native and invasive plants in plant vigour and response to enemies – and point to the importance of experiments that control for maternal effects and include the entire spectrum of native range enemies.     

Structural modeling of a plant disease resistance gene product domain

Dominant plant resistance genes are involved in the protection of plants against a wide variety of pathogens. Sequence analysis has revealed a variety of classes, often having domains in common. One commonly found region has come to be known as a putative nucleotide-binding site (NBS) due to the simple presence of sequence motifs. Until now, no experimental evidence has supported this idea. Here we suggest, as an alternative hypothesis, that part of this region is structurally homologous to the receiver domain common to many proteins of His-Asp phosphotransfer pathways. This conclusion is based on sequence analysis, threading experiments, and the construction of a molecular model of one domain that performs well against structure validation tools. The new hypothesis, in contrast to the NBS hypothesis, can explain the devastating effect of a Thr-->Ala mutation in a well-characterized resistance gene product. According to the new hypothesis, regions located N-terminal and C-terminal to the modeled portion, containing highly conserved sequence motifs, could form a separate domain.     

Differential effector gene expression underpins epistasis in a plant fungal disease

Fungal effector–host sensitivity gene interactions play a key role in determining the outcome of septoria nodorum blotch disease (SNB) caused by Parastagonospora nodorum on wheat. The pathosystem is complex and mediated by interaction of multiple fungal necrotrophic effector–host sensitivity gene systems. Three effector sensitivity gene systems are well characterized in this pathosystem; SnToxA–Tsn1, SnTox1–Snn1 and SnTox3–Snn3. We tested a wheat mapping population that segregated for Snn1 and Snn3 with SN15, an aggressive P. nodorum isolate that produces SnToxA, SnTox1 and SnTox3, to study the inheritance of sensitivity to SnTox1 and SnTox3 and disease susceptibility. Interval quantitative trait locus (QTL) mapping showed that the SnTox1–Snn1 interaction was paramount in SNB development on both seedlings and adult plants. No effect of the SnTox3–Snn3 interaction was observed under SN15 infection. The SnTox3–Snn3 interaction was however, detected in a strain of SN15 in which SnTox1 had been deleted (tox1–6). Gene expression analysis indicates increased SnTox3 expression in tox1–6 compared with SN15. This indicates that the failure to detect the SnTox3–Snn3 interaction in SN15 is due – at least in part – to suppressed expression of SnTox3 mediated by SnTox1. Furthermore, infection of the mapping population with a strain deleted in SnToxA, SnTox1 and SnTox3 (toxa13) unmasked a significant SNB QTL on 2DS where the SnTox2 effector sensitivity gene, Snn2, is located. This QTL was not observed in SN15 and tox1–6 infections and thus suggesting that SnToxA and/or SnTox3 were epistatic. Additional QTLs responding to SNB and effectors sensitivity were detected on 2AS1 and 3AL.     

Nitrite is a signaling molecule and regulator of gene expression in mammalian tissues

Mammalian tissues produce nitric oxide (NO) to modify proteins at heme and sulfhydryl sites, thereby regulating vital cell functions. The majority of NO produced is widely assumed to be neutralized into supposedly inert oxidation products including nitrite (NO2(-)). Here we show that nitrite, also ubiquitous in dietary sources, is remarkably efficient at modifying the same protein sites, and that physiological nitrite concentrations account for the basal levels of these modifications in vivo. We further find that nitrite readily affects cyclic GMP production, cytochrome P450 activities, and heat shock protein 70 and heme oxygenase-1 expression in a variety of tissues. These cellular activities of nitrite, combined with its stability and abundance in vivo, suggest that this anion has a distinct and important signaling role in mammalian biology, perhaps by serving as an endocrine messenger and synchronizing agent. Thus, nitrite homeostasis may be of great importance to NO biology.     

Nucleolin is an Ag-NOR protein; this property is determined by its amino-terminal domain independently of its phosphorylation state.

The Ag-NOR proteins are defined as markers of "active" ribosomal genes. They correspond to a set of proteins specifically located in the nucleolar organizer regions (NORs), but have not yet been clearly identified. We adapted the specific detection method of the Ag-NOR proteins to Western blots in order to identify these proteins. Using a purified protein, Western blots, and immunological characterization, the present study brings the first direct evidence leading to the identity of one Ag-NOR protein. We found that nucleolin is specifically revealed by Ag-NOR staining. Using different nucleolin fragments generated by CNBr cleavage and by overexpression in Escherichia coli, we demonstrate that the amino-terminal domain of nucleolin and not the carboxy-part of the protein is involved in silver staining. Moreover, as the pattern of staining does not vary using casein kinase II- and cdc2-phosphorylated nucleolin or dephosphorylated nucleolin, we conclude that the reduction of the silver ions is not linked to the phosphorylation state of the molecule. We propose that the concentration of acidic amino acids in the amino-terminal domain of nucleolin is responsible for Ag-NOR staining. This hypothesis is also supported by the finding that poly L-glutamic acid peptides are silver stained. These results provide data that can be used to explain the specificity of Ag-NOR staining. Furthermore, we clearly establish that proteolysis of the amino-terminal Ag-NOR-sensitive part of nucleolin occurs in vitro, leading to the accumulation of the carboxy-terminal Ag-NOR-negative part of the protein. We argue that this cleavage occurs in vivo as already proposed, bearing in mind that nucleolin is present in the fibrillar and in the granular component of the nucleolus, whereas no Ag-NOR staining is observed in the latter nucleolar component.     

Control of plant development and gene expression by sugar signaling

Coordination of development with the availability of nutrients, such as soluble sugars, may help ensure an adequate supply of building materials and energy with which to carry out specific developmental programs. For example, in-vivo and in-vitro experiments suggest that increasing sugar levels delay seed germination and stimulate the induction of flowering and senescence in at least some plant species. Higher sugar concentrations can also increase the number of tubers formed by potatoes and can stimulate the formation of adventitious roots by Arabidopsis. New insights into the mechanisms by which sugar-response pathways interact with other response pathways have been provided by microarray experiments examining sugar-regulated gene expression under different light and nitrogen conditions.     

Controlling hormone signaling is a plant and pathogen challenge for growth and survival

Plants and pathogens have continuously confronted each other during evolution in a battle for growth and survival. New advances in the field have provided fascinating insights into the mechanisms that have co-evolved to gain a competitive advantage in this battle. When plants encounter an invading pathogen, not only responses signaled by defense hormones are activated to restrict pathogen invasion, but also the modulation of additional hormone pathways is required to serve other purposes, which are equally important for plant survival, such as re-allocation of resources, control of cell death, regulation of water stress, and modification of plant architecture. Notably, pathogens can counteract both types of responses as a strategy to enhance virulence. Pathogens regulate production and signaling responses of plant hormones during infection, and also produce phytohormones themselves to modulate plant responses. These results indicate that hormone signaling is a relevant component in plant-pathogen interactions, and that the ability to dictate hormonal directionality is critical to the outcome of an interaction.     

The common metabolite glycerol-3-phosphate is a novel regulator of plant defense signaling

Conversion of glycerol to glycerol-3-phosphate (G3P) is one of the highly conserved steps of glycerol metabolism in evolutionary diverse organisms. In plants, G3P is produced either via the glycerol kinase (GK)-mediated phosphorylation of glycerol, or via G3P dehydrogenase (G3Pdh)-mediated reduction of dihydroxyacetone phosphate (DHAP). We have recently shown that G3P levels contribute to basal resistance against the hemibiotrophic pathogen, Colletotrichum higginsianum. Since a mutation in the GLY1-encoded G3Pdh conferred more susceptibility compared to a mutation in the GLI1-encoded GK, we proposed that GLY1 is the major contributor of the total G3P pool that participates in defense against C. higginsianum.Key words: glycerol-3-phosphate, glycerol metabolism, defense, signalingGlycerol and its metabolites are involved in a variety of physiopathological processes in both prokaryotes and eukaryotes, most of which appear to be highly conserved,¹ signifying the fundamental importance of these molecules. Glycerol-3-phosphate (G3P), an obligatory component of energy-producing reactions including glycolysis and glycerolipid biosynthesis, participates in the disease-related physiologies of many organisms. In humans, deficiencies in glycerol kinase activity (catalyzing the phosphorylation of glycerol to G3P) result in a variety of metabolic and neurological disorders, while mutations in G3P dehydrogenase (G3Pdh, catalyzing the oxidation of dihydroxyacetone phosphate, DHAP, to G3P) have been linked to sudden infant death syndrome and decreased cardiac Na²⁺ current resulting in ventricular arrhythmias and sudden death.^2,3 Given the fact that glycerol metabolism is conserved between plants and animals, it is conceivable that glycerol and/or G3P might also participate in disease physiology of plants. However, such a role for glycerol and/or G3P remains unexplored.Previous work from our laboratory and others has shown that GLY1-encoded G3Pdh plays an important role in plastidal oleic acid-mediated signaling^4–7 and systemic acquired resistance.⁸ This group of enzymes also plays an important role in fungi and it was recently shown that the disruption of a G3Pdh gene in Colletotrichum gloeosporioides eliminated the ability of the mutant fungus to grow on most carbon sources in vitro, including amino acids and glucose.⁹ However, the G3Pdh knockout (KO) fungus grew normally in the presence of glycerol. The G3Pdh KO fungus also developed normally in its plant host (the round-leaved mallow), prompting the suggestion that glycerol, rather than glucose or sucrose, was the primary transferred source of carbon in planta. This was an unexpected finding, but direct analysis of infected host leaves revealed that their glycerol content did decrease by 40% within 48 hours of infection with C. gloeosporioides.⁹ Since the hemibiotroph C. gloeosporioides appears to be able to utilize glycerol for growth and conidiation in planta, it was possible that glycerol metabolism and associated pathways in the host played an important role in the establishment of infections by Colletotrichum fungi. Furthermore, it was possible that the host had evolved to sense these pathogen-mediated changes in glycerol levels and utilize them as signal(s) to initiate defense.We tested these possibilities by characterizing the role of glycerol metabolism in the Arabidopsis—C. higginsianum interaction (Fig. 1). Infection with C. higginsianum reduced the glycerol content while concomitantly increasing the G3P content in Arabidopsis plants.¹⁰ Mutations in G3P-synthesizing genes gly1 (a G3Pdh) and gli1 (a glycerol kinase),¹¹ resulted in enhanced susceptibility to C. higginsianum. The gly1 plants were much more susceptible than the gli1 plants, suggesting that GLY1-encoded G3Pdh played a more important role in basal resistance to C. higginsianum. Conversely, the act1 mutant, which is impaired in the acylation of G3P with oleic acid (18:1) (Fig. 1), was more resistant to the fungus. The phenotypes seen in the infected gly1 and act1 plants correlated with pathogen-induced G3P levels; C. higginsianum inoculation induced ∼2-fold higher accumulation of G3P in the act1 plants, and ∼2-fold lower G3P in the susceptible gly1 plants, as compared to wild-type plants.¹⁰ To test the hypothesis that G3P synthesized via GLY1 entered the plastidial glycerolipid pathway via the ACT1 catalyzed reaction, we generated act1 gly1 plants. The results supported the hypothesis, as act1 gly1 plants were as susceptible to C. higginsianum as gly1 plants.Open in a separate windowFigure 1A condensed scheme of glycerol metabolism in plants. Glycerol is phosphorylated to glycerol-3-phosphate (G3P) by glycerol kinase (GK; GLI1). G3P can also be generated by G3P dehydrogenase (G3Pdh) via the reduction of dihydroxyacetone phosphate (DHAP) in both the cytosol and the plastids (represented by the oval). G3P generated by this reaction can be transported between the cytosol and plastid stroma. In the plastids G3P is acylated with oleic acid (18:1) by the ACT1-encoded G3P acyltransferase. This ACT1-utilized 18:1 is derived from the stearoyl-acyl carrier protein (ACP)-desaturase (SSI2)-catalyzed desaturation of stearic acid (18:0). The 18:1-ACP generated by SSI2 either enters the prokaryotic lipid biosynthetic pathway through acylation of G3P, or is exported out of the plastids as a coenzyme A (CoA)-thioester to enter the eukaryotic lipid biosynthetic pathway. Other abbreviations used are: PA, phosphatidic acid; Lyso-PA, acyl-G3P; PG, phosphatidylglycerol; MGD, monogalactosyldiacylglycerol; DGD, digalactosyl-diacylglycerol; SL, sulfolipid; DAG, diacylglycerol; DHA, dihydroxyacetone; Gl-3-P, glyceraldehyde-3-phosphate; TCA, tricarboxylic acid cycle. Enzymes as abbreviated as: ACT1, G3P acyltransferase; SSI2, stearoyl acyl carrier protein desaturase; GK, glycerol kinase; G3Pdh, G3P dehydrogenase; TPI, triose phosphate isomerase; DHAK, dihydroxyacetone kinase; F1,6-A, fructose 1,6-biphosphate aldolase; PF6P-P, pyrophosphate fructose-6-phosphate phosphotransferase; G6P-I, glucose-6-phosphate isomerase.More supporting evidence for the role of G3P in defense against C. higginsianum was obtained by overexpressing GLY1 in wild-type plants (Fig. 2A). Similar to act1, overexpression of GLY1 led to a ∼2-fold increase in G3P levels after pathogen inoculation, and these plants were also more resistant to C. higginsianum (Fig. 2B–D). Furthermore, plants overexpressing GLY1 or carrying a mutation in ACT1 exhibited enhanced resistance to C. higginsianum in the pad3 mutant background (Fig. 3).¹⁰ The pad3 plants are compromised in camalexin synthesis, and are hypersusceptible to necrotrophic pathogens.Open in a separate windowFigure 2Pathogen response and G3P levels in transgenic lines overexpressing GLY1. (A) Expression of the GLY1 gene in wild-type or 35S-GLY1 transgenic plant. RNA gel blot analysis was performed on ∼7 µg of total RNA. Ethidium bromide staining of rRNA was used as a loading control. (B) Disease symptoms in C. higginsianum-inoculated Col-0, gly1 or 35S-GLY1 plants at 5 dpi. The plants were spray-inoculated with 10⁶ spores/ml of C. higginsianum. (C) Lesion size in spot-inoculated genotypes. The plants were spot-inoculated with water or 10⁶ spores/ml and the lesion size was measured from 20–30 independent leaves at 6 dpi. Statistical significance was determined using Students t-test. Asterisks indicate data that is statistically significant from that of control (Col-0) (p < 0.05). Error bars indicate SD. (D) G3P levels in Col-0 and 35S-GLY1 plants at 0 and 72 h post-inoculation.Open in a separate windowFigure 3Pathogen response in C. higginsianum-inoculated 35S-GLY1 plants in pad3 background. (A) Disease symptoms on Col-0, pad3 or 35S-GLY1 or 35S-GLY1 pad3 plants spot-inoculated with 10⁶ spores/ml of C. higginsianum. The leaves were photographed at 7 dpi. (B) Lesion size in spot-inoculated Col-0, pad3 or 35S-GLY1 or 35S-GLY1 pad3 plants. The lesion size was measured from 20–30 independent leaves at 7 dpi. Asterisks indicate data that is statistically significant from that of control (Col-0) (p < 0.05). Error bars indicate SD.Exogenous glycerol application increased endogenous G3P and significantly enhanced the ability of the host to resist C. higginsianum.¹⁰ Glycerol-triggered synthesis of G3P also caused a decrease in 18:1 levels, which is known to induce defense signaling, resulting in enhanced basal resistance.^4–7 However, the glycerol-triggered increase in G3P precedes the reduction in 18:1 levels and confers resistance even at time points when low 18:1-mediated signaling is not induced, suggesting that the enhanced resistance after glycerol treatment was due to elevated G3P levels and not to the reduction in 18:1.Understanding the precise roles of G3P will require in-depth analysis of real-time alterations in its levels on a cellular level during pathogenesis. This is complicated by the presence of multiple isoforms of G3Pdh that contribute to the total G3P pool, and by the lack of appropriate tools for monitoring precise changes in intracellular G3P. Systematic analysis of various G3Pdh mutants, in combination with each other and with gli1, should yield novel insights into pathway(s) and steps regulating levels of G3P in the cell.     

植物抗病基因克隆研究进展

随着分子生物学及其相关技术的飞速发展,人们对植物与病原微生物相互作用的分子机制了解得越来越透彻。本文对植物过敏性反应和系统获得抗性作了简要概述,并着重讨论了植物抗病基因克隆的进展,涉及到转座子标签技术、定位克隆技术、染色体步行、染色体登陆等方法和策略,归纳了克隆到的植物抗病基因及其产物结构,概述了这些基因产物所共有的特点,并简要介绍了植物抗病基因工程的进展。     

Role of SGT1 in the regulation of plant R gene signalling

Recent important discoveries in several laboratories have identified SGT1 as an essential component of R gene-mediated disease resistance in plants. The precise molecular function of SGT1 remains unknown, although sequence analysis and structural predictions reveal that SGT1 has features of co-chaperones that associate with HSP90 in animals. This review will describe the role of SGT1 in R gene-mediated plant defence and discuss how SGT1 may regulate this process.