The Arabidopsis SENESCENCE-ASSOCIATED GENE 13 Regulates Dark-Induced Senescence and Plays Contrasting Roles in Defense Against Bacterial and Fungal Pathogens.

SENESCENCE-ASSOCIATED GENE 13 (SAG13) of Arabidopsis is a widely conserved gene of unknown function that has been extensively used as a marker of plant senescence. SAG13 induction occurs during plant cell death processes, including senescence and hypersensitive response, a type of programmed cell death that occurs in response to pathogens. This implies that SAG13 expression is regulated through at least two different signaling pathways affecting these two different processes. Our work highlights a contrasting role for SAG13 in regulating resistance against disease-causing biotrophic bacterial and necrotrophic fungal pathogens with contrasting infection strategies. We provide further evidence that SAG13 is not only induced during oxidative stress but also plays a role in protecting the plant against other stresses. SAG13 is also required for normal seed germination, seedling growth, and anthocyanin accumulation. The work presented here provides evidence for the role of SAG13 in regulating multiple plant processes including senescence, defense, seed germination, and abiotic stress responses. SAG13 is a valuable molecular marker for these processes and is conserved in multiple plant species, and this knowledge has important implications for crop improvement.

JMJ14 encoded H3K4 demethylase modulates immune responses by regulating defence gene expression and pipecolic acid levels.

Epigenetic modifications have emerged as an important mechanism underlying plant defence against pathogens. We examined the role of JMJ14, a Jumonji (JMJ) domain-containing H3K4 demethylase, in local and systemic plant immune responses in Arabidopsis. The function of JMJ14 in local or systemic defence response was investigated by pathogen growth assays and by analysing expression and H3K4me3 enrichments of key defence genes using qPCR and ChIP-qPCR. Salicylic acid (SA) and pipecolic acid (Pip) levels were quantified and function of JMJ14 in SA- and Pip-mediated defences was analysed in Col-0 and jmj14 plants. jmj14 mutants were compromised in both local and systemic defences. JMJ14 positively regulates pathogen-induced H3K4me3 enrichment and expression of defence genes involved in SA- and Pip-mediated defence pathways. Consequently, loss of JMJ14 results in attenuated defence gene expression and reduced Pip accumulation during establishment of systemic acquired resistance (SAR). Exogenous Pip partially restored SAR in jmj14 plants, suggesting that JMJ14 regulated Pip biosynthesis and other downstream factors regulate SAR in jmj14 plants. JMJ14 positively modulates defence gene expressions and Pip levels in Arabidopsis.

JMJ27, an Arabidopsis H3K9 histone demethylase, modulates defense against Pseudomonas syringae and flowering time.

Histone methylation is known to dynamically regulate diverse developmental and physiological processes. Histone methyl marks are written by methyltransferases and erased by demethylases, and result in modification of chromatin structure to repress or activate transcription. However, little is known about how histone methylation may regulate defense mechanisms and flowering time in plants. Here we report characterization of JmjC DOMAIN-CONTAINING PROTEIN 27 (JMJ27), an Arabidopsis JHDM2 (JmjC domain-containing histone demethylase 2) family protein, which modulates defense against pathogens and flowering time. JMJ27 is a nuclear protein containing a zinc-finger motif and a catalytic JmjC domain with conserved Fe(II) and α-ketoglutarate binding sites, and displays H3K9me1/2 demethylase activity both in vitro and in vivo. JMJ27 is induced in response to virulent Pseudomonas syringae pathogens and is required for resistance against these pathogens. JMJ27 is a negative modulator of WRKY25 (a repressor of defense) and a positive modulator of several pathogenesis-related (PR) proteins. Additionally, loss of JMJ27 function leads to early flowering. JMJ27 negatively modulates the major flowering regulator CONSTANS (CO) and positively modulates FLOWERING LOCUS C (FLC). Taken together, our results indicate that JMJ27 functions as a histone demethylase to modulate both physiological (defense) and developmental (flowering time) processes in Arabidopsis.

Loss of Color Pigmentation Is Maintained at High Frequency in a Monkey Flower Population.

Color polymorphisms have long been of evolutionary interest for their diverse roles, including mate choice, predator avoidance, and pollinator attraction. While color variation is often under strong selection, some taxa demonstrate unexpectedly high frequencies of presumed deleterious color forms. Here we show that a genetic variant underlying complete loss of anthocyanin pigmentation has risen to an unexpectedly high frequency of >0.2 in a natural population of the plant Mimulus guttatus. Decreased expression of MYB5 transcription factor is associated with unpigmented morphs. While the allele was found only in heterozygote adults in the wild, suggesting negative selection, experiments were unable to demonstrate a fitness cost for unpigmented plants, suggesting a cryptic selection pressure in the wild. However, life-history differences among morphs suggests that unpigmented individuals benefit from later flowering and clonal growth. Overall, our study highlights the complex interplay of factors maintaining variation in nature, even for genes of major effect.

Grassland root communities: species distributions and how they are linked to aboveground abundance.

There is little comprehensive information on the distribution of root systems among coexisting species, despite the expected importance of those distributions in determining the composition and diversity of plant communities. This gap in knowledge is particularly acute for grasslands, which possess large numbers of species with morphologically indistinguishable roots. In this study we adapted a molecular method, fluorescent fragment length polymorphism, to identify root fragments and determine species root distributions in two grasslands in Yellowstone National Park (YNP). Aboveground biomass was measured, and soil cores (2 cm in diameter) were collected to depths of 40 cm and 90 cm in an upland, dry grassland and a mesic, slope-bottom grassland, respectively, at peak foliar expansion. Cores were subdivided, and species that occurred in each 10-cm interval were identified. The results indicated that the average number of species in 10-cm intervals (31 cm3) throughout the sampled soil profile was 3.9 and 2.8 species at a dry grassland and a mesic grassland, respectively. By contrast, there was an average of 6.7 and 14.1 species per 0.5 m2, determined by the presence of shoot material, at dry and mesic sites, respectively. There was no relationship between soil depth and number of species per 10-cm interval in either grassland, despite the exponential decline of root biomass with soil depth at both sites. There also was no relationship between root frequency (i.e., the percentage of samples in which a species occurred) and soil depth for the vast majority of species at both sites. The preponderance of species were distributed throughout the soil profile at both sites. Assembly analyses indicated that species root occurrences were randomly assorted in all soil intervals at both sites, with the exception that Festuca idahoensis segregated from Artemisia tridentata and Pseudoroegnaria spicata in 10-20 cm soil at the dry grassland. Root frequency throughout the entire sampled soil profile was positively associated with shoot biomass among species. Together these results indicated the importance of large, well-proliferated root systems in establishing aboveground dominance. The findings suggest that spatial belowground segregation of species probably plays a minor role in fostering resource partitioning and species coexistence in these YNP grasslands.

Overexpression of Arabidopsis microRNA167 induces salicylic acid-dependent defense against Pseudomonas syringae through the regulation of its targets ARF6 and ARF8.

microRNAs are powerful regulators of growth, development, and stress responses in plants. The Arabidopsis thaliana microRNA miR167 was previously found to regulate diverse processes including flower development, root development, and response to osmotic stress by controlling the patterns of expression of its target genes AUXIN RESPONSE FACTOR 6 (ARF6), ARF8, and IAA-Ala RESISTANT 3. Here, we report that miR167 also modulates defense against pathogens through ARF6 and ARF8. miR167 is differentially expressed in response to the bacterial pathogen Pseudomonas syringae, and overexpression of miR167 confers very high levels of resistance. This resistance appears to be due to suppression of auxin responses and is partially dependent upon salicylic acid signaling, and also depends upon altered stomatal behavior in these plants. Closure of stomata upon the detection of P. syringae is an important aspect of the basal defense response, as it prevents bacterial cells from entering the leaf interior and causing infection. Plants overexpressing miR167 constitutively maintain small stomatal apertures, resulting in very high resistance when the pathogen is inoculated onto the leaf surface. Additionally, the systemic acquired resistance (SAR) response is severely compromised in plants overexpressing miR167, in agreement with previous work showing that the activation of SAR requires intact auxin signaling responses. This work highlights a new role for miR167, and also emphasizes the importance of hormonal balance in short- and long-term defense and of stomata as an initial barrier to pathogen entry.

An Arabidopsis DISEASE RELATED NONSPECIFIC LIPID TRANSFER PROTEIN 1 is required for resistance against various phytopathogens and tolerance to salt stress.

Synchronous and timely regulation of multiple genes results in an effective defense response that decides the fate of the host when challenged with pathogens or unexpected changes in environmental conditions. One such gene, which is downregulated in response to multiple bacterial pathogens, is a putative nonspecific lipid transfer protein (nsLTP) of unknown function that we have named DISEASE RELATED NONSPECIFIC LIPID TRANSFER PROTEIN 1 (DRN1). We show that upon pathogen challenge, DRN1 is strongly downregulated, while a putative DRN1-targeting novel microRNA (miRNA) named DRN1 Regulating miRNA (DmiR) is reciprocally upregulated. Furthermore, we provide evidence that DRN1 is required for defense against bacterial and fungal pathogens as well as for normal seedling growth under salinity stress. Although nsLTP family members from different plant species are known to be a significant source of food allergens and are often associated with antimicrobial properties, our knowledge on the biological functions and regulation of this gene family is limited. Our current work not only sheds light on the mechanism of regulation but also helps in the functional characterization of DRN1, a putative nsLTP family member of hitherto unknown function.

Arabidopsis SMALL DEFENSE-ASSOCIATED PROTEIN 1 Modulates Pathogen Defense and Tolerance to Oxidative Stress.

Salicylic acid (SA) and reactive oxygen species (ROS) are known to be key modulators of plant defense. However, mechanisms of molecular signal perception and appropriate physiological responses to SA and ROS during biotic or abiotic stress are poorly understood. Here we report characterization of SMALL DEFENSE-ASSOCIATED PROTEIN 1 (SDA1), which modulates defense against bacterial pathogens and tolerance to oxidative stress. sda1 mutants are compromised in defense gene expression, SA accumulation, and defense against bacterial pathogens. External application of SA rescues compromised defense in sda1 mutants. sda1 mutants are also compromised in tolerance to ROS-generating chemicals. Overexpression of SDA1 leads to enhanced resistance against bacterial pathogens and tolerance to oxidative stress. These results suggest that SDA1 regulates plant immunity via the SA-mediated defense pathway and tolerance to oxidative stress. SDA1 encodes a novel small plant-specific protein containing a highly conserved seven amino acid (S/G)WA(D/E)QWD domain at the N-terminus that is critical for SDA1 function in pathogen defense and tolerance to oxidative stress. Taken together, our studies suggest that SDA1 plays a critical role in modulating both biotic and abiotic stresses in Arabidopsis (Arabidopsis thaliana) and appears to be a plant-specific stress responsive protein.

Hypersensitive response-like lesions 1 codes for AtPPT1 and regulates accumulation of ROS and defense against bacterial pathogen Pseudomonas syringae in Arabidopsis thaliana.

AIMS: Plants employ both basal and resistance gene (R gene)-mediated defenses in response to pathogens. Reactive oxygen species (ROS) are widely reported to play a central role in both basal and R gene-mediated defense; however, the nature of ROS has been less well established for basal defense. In addition, spatial distribution of redox moieties and mechanisms of plant responses during basal defense are poorly understood. We investigated redox signaling in Arabidopsis thaliana in response to virulent bacterial pathogen, focusing on the role of the mitochondria in balancing energy demands against generation of physiologically relevant ROS. RESULTS: Positional cloning of an Arabidopsis lesion mimic mutant identified a polyprenyl transferase involved in the biosynthesis of Coenzyme Q10 (CoQ), which leads to novel insights into physiological ROS levels and their role in basal resistance. Gain- and loss-of-function studies identified Coenzyme Q10 redox state to be a key determinant of ROS levels. These Coenzyme Q10 redox state-mediated ROS levels had a direct bearing on both response against pathogen and ability to thrive in high oxidative stress environments. INNOVATION: We demonstrate that Coenzyme Q10 redox state generates an ROS threshold for a successful basal resistance response. Perturbation of the Coenzyme Q10 redox state has the potential to disrupt plant defense responses against bacterial pathogens. CONCLUSIONS: Coenzyme Q10 redox state is a key regulator of Arabidopsis basal resistance against bacterial pathogens.

Defining the Metabolic Functions and Roles in Virulence of the rpoN1 and rpoN2 Genes in Ralstonia solanacearum GMI1000.

The alternative sigma factor RpoN is a unique regulator found among bacteria. It controls numerous processes that range from basic metabolism to more complex functions such as motility and nitrogen fixation. Our current understanding of RpoN function is largely derived from studies on prototypical bacteria such as Escherichia coli. Bacillus subtilis and Pseudomonas putida. Although the extent and necessity of RpoN-dependent functions differ radically between these model organisms, each bacterium depends on a single chromosomal rpoN gene to meet the cellular demands of RpoN regulation. The bacterium Ralstonia solanacearum is often recognized for being the causative agent of wilt disease in crops, including banana, peanut and potato. However, this plant pathogen is also one of the few bacterial species whose genome possesses dual rpoN genes. To determine if the rpoN genes in this bacterium are genetically redundant and interchangeable, we constructed and characterized ΔrpoN1, ΔrpoN2 and ΔrpoN1 ΔrpoN2 mutants of R. solanacearum GMI1000. It was found that growth on a small range of metabolites, including dicarboxylates, ethanol, nitrate, ornithine, proline and xanthine, were dependent on only the rpoN1 gene. Furthermore, the rpoN1 gene was required for wilt disease on tomato whereas rpoN2 had no observable role in virulence or metabolism in R. solanacearum GMI1000. Interestingly, plasmid-based expression of rpoN2 did not fully rescue the metabolic deficiencies of the ΔrpoN1 mutants; full recovery was specific to rpoN1. In comparison, only rpoN2 was able to genetically complement a ΔrpoN E. coli mutant. These results demonstrate that the RpoN1 and RpoN2 proteins are not functionally equivalent or interchangeable in R. solanacearum GMI1000.

Transcriptional and metabolic signatures of Arabidopsis responses to chewing damage by an insect herbivore and bacterial infection and the consequences of their interaction.

Plants use multiple interacting signaling systems to identify and respond to biotic stresses. Although it is often assumed that there is specificity in signaling responses to specific pests, this is rarely examined outside of the gene-for-gene relationships of plant-pathogen interactions. In this study, we first compared early events in gene expression and later events in metabolite profiles of Arabidopsis thaliana following attack by either the caterpillar Spodoptera exigua or avirulent (DC3000 avrRpm1) Pseudomonas syringae pv. tomato at three time points. Transcriptional responses of the plant to caterpillar feeding were rapid, occurring within 1 h of feeding, and then decreased at 6 and 24 h. In contrast, plant response to the pathogen was undetectable at 1 h but grew larger and more significant at 6 and 24 h. There was a surprisingly large amount of overlap in jasmonate and salicylate signaling in responses to the insect and pathogen, including levels of gene expression and individual hormones. The caterpillar and pathogen treatments induced different patterns of expression of glucosinolate biosynthesis genes and levels of glucosinolates. This suggests that when specific responses develop, their regulation is complex and best understood by characterizing expression of many genes and metabolites. We then examined the effect of feeding by the caterpillar Spodoptera exigua on Arabidopsis susceptibility to virulent (DC3000) and avirulent (DC3000 avrRpm1) P. syringae pv. tomato, and found that caterpillar feeding enhanced Arabidopsis resistance to the avirulent pathogen and lowered resistance to the virulent strain. We conclude that efforts to improve plant resistance to bacterial pathogens are likely to influence resistance to insects and vice versa. Studies explicitly comparing plant responses to multiple stresses, including the role of elicitors at early time points, are critical to understanding how plants organize responses in natural settings.

Extracellular fibrils of pathogenic yeast Cryptococcus gattii are important for ecological niche, murine virulence and human neutrophil interactions.

Cryptococcus gattii, an emerging fungal pathogen of humans and animals, is found on a variety of trees in tropical and temperate regions. The ecological niche and virulence of this yeast remain poorly defined. We used Arabidopsis thaliana plants and plant-derived substrates to model C. gattii in its natural habitat. Yeast cells readily colonized scratch-wounded plant leaves and formed distinctive extracellular fibrils (40-100 nm diameter x500-3000 nm length). Extracellular fibrils were observed on live plants and plant-derived substrates by scanning electron microscopy (SEM) and by high voltage- EM (HVEM). Only encapsulated yeast cells formed extracellular fibrils as a capsule-deficient C. gattii mutant completely lacked fibrils. Cells deficient in environmental sensing only formed disorganized extracellular fibrils as apparent from experiments with a C. gattii STE12alpha mutant. C. gattii cells with extracellular fibrils were more virulent in murine model of pulmonary and systemic cryptococcosis than cells lacking fibrils. C. gattii cells with extracellular fibrils were also significantly more resistant to killing by human polymorphonuclear neutrophils (PMN) in vitro even though these PMN produced elaborate neutrophil extracellular traps (NETs). These observations suggest that extracellular fibril formation could be a structural adaptation of C. gattii for cell-to-cell, cell-to-substrate and/or cell-to- phagocyte communications. Such ecological adaptation of C. gattii could play roles in enhanced virulence in mammalian hosts at least initially via inhibition of host PMN- mediated killing.

A motif extraction algorithm based on hashing and modulo-4 arithmetic.

We develop an algorithm to identify cis-elements in promoter regions of coregulated genes. This algorithm searches for subsequences of desired length whose frequency of occurrence is relatively high, while accounting for slightly perturbed variants using hash table and modulo arithmetic. Motifs are evaluated using profile matrices and higher-order Markov background model. Simulation results show that our algorithm discovers more motifs present in the test sequences, when compared with two well-known motif-discovery tools (MDScan and AlignACE). The algorithm produces very promising results on real data set; the output of the algorithm contained many known motifs.

Arabidopsis GH3-LIKE DEFENSE GENE 1 is required for accumulation of salicylic acid, activation of defense responses and resistance to Pseudomonas syringae.

In Arabidopsis, the GH3-like gene family consists of 19 members, several of which have been shown to adenylate the plant hormones jasmonic acid, indole acetic acid and salicylic acid (SA). In some cases, this adenylation has been shown to catalyze hormone conjugation to amino acids. Here we report molecular characterization of the GH3-LIKE DEFENSE GENE 1 (GDG1), a member of the GH3-like gene family, and show that GDG1 is an important component of SA-mediated defense against the bacterial pathogen Pseudomonas syringae. Expression of GDG1 is induced earlier and to a higher level in response to avirulent pathogens compared to virulent pathogens. gdg1 null mutants are compromised in several pathogen defense responses, including activation of defense genes and resistance against virulent and avirulent bacterial pathogens. Accumulation of free and glucoside-conjugated SA (SAG) in response to pathogen infection is compromised in gdg1 mutants. All defense-related phenotypes of gdg1 can be rescued by external application of SA, suggesting that gdg1 mutants are defective in the SA-mediated defense pathway(s) and that GDG1 functions upstream of SA. Our results suggest that GDG1 contributes to both basal and resistance gene-mediated inducible defenses against P. syringae (and possibly other pathogens) by playing a critical role in regulating the levels of pathogen-inducible SA. GDG1 is allelic to the PBS3 (avrPphB susceptible) gene.

Overexpression of CRK13, an Arabidopsis cysteine-rich receptor-like kinase, results in enhanced resistance to Pseudomonas syringae.

Protein kinases play important roles in relaying information from perception of a signal to the effector genes in all organisms. Cysteine-rich receptor-like kinases (CRKs) constitute a sub-family of plant receptor-like kinases (RLKs) with more than 40 members that contain the novel C-X8-C-X2-C motif (DUF26) in the extracellular domains. Here we report molecular characterization of one member of this gene family, CRK13. Expression of this gene is induced more quickly and strongly in response to the avirulent compared with the virulent strains of Pseudomonas syringae, and peaks within 4 h after pathogen infection. In response to dexamethasone (DEX) treatment, plants expressing the CRK13 gene from a DEX-inducible promoter exhibited all tested features of pathogen defense activation, including rapid tissue collapse, accumulation of high levels of several defense-related gene transcripts including PR1, PR5 and ICS1, and accumulation of salicylic acid (SA). In addition, these plants suppressed growth of virulent pathogens by about 20-fold compared with the wild-type Col-0. CRK13-conferred pathogen resistance is salicylic acid-dependent. Gene expression analysis using custom cDNA microarrays revealed a remarkable overlap between the expression profiles of the plants overexpressing CRK13 and the plants treated with Pst DC3000 (avrRpm1). Our studies suggest that upregulation of CRK13 leads to hypersensitive response-associated cell death, and induces defense against pathogens by causing increased accumulation of salicylic acid.

Light-dependent hypersensitive response and resistance signaling against Turnip Crinkle Virus in Arabidopsis.

Resistance to Turnip Crinkle Virus (TCV) in Arabidopsis ecotype Dijon (Di)-17 is conferred by the resistance gene HRT and a recessive locus rrt. In Di-17, TCV elicits a hypersensitive response (HR), which is accompanied by increased expression of pathogenesis-related (PR) genes and high levels of salicylic acid (SA). We have previously shown that HRT-mediated resistance to TCV is dependent on SA-mediated signal transduction and that increased levels of SA confer enhanced resistance to TCV via upregulation of the HRT gene. Here we show that HRT-mediated HR and resistance are dependent on light. A dark treatment immediately following TCV inoculation suppressed HR, resistance and activation of the majority of the TCV-induced genes. However, the absence of light did not affect either TCV-induced elevated levels of free SA or the expression of HRT. Interestingly, in the dark, transgenic plants overexpressing HRT showed susceptibility, but overexpression of HRT coupled with high levels of endogenous SA resulted in pronounced resistance. Consistent with these results is the finding that exogenous application of SA prior to TCV inoculation partially overcame the requirement for light. Light was also required for N gene-mediated HR and resistance to Tobacco Mosaic Virus, suggesting that it is an important factor which may be generally required during defense signaling.

Major signaling pathways modulate Arabidopsis glucosinolate accumulation and response to both phloem-feeding and chewing insects.

Plant responses to enemies are coordinated by several interacting signaling systems. Molecular and genetic studies with mutants and exogenous signal application suggest that jasmonate (JA)-, salicylate (SA)-, and ethylene (ET)-mediated pathways modulate expression of portions of the defense phenotype in Arabidopsis (Arabidopsis thaliana), but have not yet linked these observations directly with plant responses to insect attack. We compared the glucosinolate (GS) profiles of rosette leaves of 4-week-old mutant and transgenic Arabidopsis (Columbia) plants compromised in these three major signaling pathways, and characterized responses by those plants to feeding by two phloem-feeding aphids (generalist Myzus persicae and specialist Brevicoryne brassicae) and one generalist caterpillar species (Spodoptera exigua Hubner). Blocked JA signaling in coronatine-insensitive (coi1) and enhanced expression of SA-signaled disease resistance in hypersensitive response-like (hrl1) mutants reduced constitutive GS concentrations, while blocking SA signaling at the mediator protein npr1 mutant (NPR) increased them. There was no significant impact on constitutive GS contents of blocking ET signaling (at ET resistant [etr1]) or reducing SA concentrations (nahG transgene). We found increased GS accumulation in response to insect feeding, which required functional NPR1 and ETR1 but not COI1 or SA. Insect feeding caused increases primarily in short-chain aliphatic methylsulfinyl GS. By contrast, responses to exogenous JA, a frequent experimental surrogate for insect attack, were characterized by an increase in indolyl GS. Insect performance, measured as population increase or weight increase, was negatively related to GS levels, but we found evidence that other, ET-regulated factors may also be influential. Plant resistance to (consumption by) S. exigua was not related to insect growth because some plant chemistries inhibited growth while others inhibited feeding. These major signaling pathways modulate Arabidopsis GS accumulation and response to both phloem-feeding and chewing insects, often antagonistically; NPR appears to be central to these interactions. Our results indicate that exogenous signal application and plant consumption measures may not provide useful measures of plant responses to actual insect feeding.

Preexisting systemic acquired resistance suppresses hypersensitive response-associated cell death in Arabidopsis hrl1 mutant.

The hypersensitive response (HR) displayed by resistant plants against invading pathogens is a prominent feature of plant-pathogen interactions. The Arabidopsis hypersensitive response like lesions1 (hrl1) mutant is characterized by heightened defense responses that make it more resistant to virulent pathogens. However, hrl1 suppresses avirulent pathogen-induced HR cell death. Furthermore, the high PR-1 expression observed in hrl1 remains unaltered after avirulent and virulent pathogen infections. The suppressed HR phenotype in hrl1 is observed even when an elicitor is expressed endogenously from an inducible promoter, suggesting that an impaired transfer of avirulent factors is not the reason. Interestingly, the lack of HR phenotype in hrl1 is reversed if the constitutive defense responses are compromised either by a mutation in NON EXPRESSOR OF PR-1 (NPR1) or by depleting salicylic acid due to the expression of the nahG gene. The rescue of HR cell death in hrl1 npr1 and in hrl1 nahG depends on the extent to which the constitutive systemic acquired response (SAR) is compromised. Pretreating Arabidopsis wild-type plants with SAR-inducers, before pathogen infection resulted in a significant decrease in HR cell death. Together, these results demonstrate that the preexisting SAR may serve as one form of negative feedback loop to regulate HR-associated cell death in hrl1 mutant and in the wild-type plants.

The Arabidopsis hrl1 mutation reveals novel overlapping roles for salicylic acid, jasmonic acid and ethylene signalling in cell death and defence against pathogens.

Defence against pathogens in Arabidopsis is orchestrated by at least three signalling molecules: salicylic acid (SA), jasmonic acid (JA) and ethylene (ET). The hrl1 (hypersensitive response-like lesions 1) mutant of Arabidopsis is characterized by spontaneous necrotic lesions, accumulation of reactive oxygen species, constitutive expression of SA- and ET/JA-responsive defence genes, and enhanced resistance to virulent bacterial and oomycete pathogens. Epistasis analyses of hrl1 with npr1, etr1, coi1 and SA-depleted nahG plants revealed novel interactions between SA and ET/JA signalling pathways in regulating defence gene expression and cell death. RNA gel-blot analysis of RNA isolated separately from the lesion+ and the lesion- leaves of double mutants of hrl1 revealed different signalling requirements for the expression of defence genes in these tissues. Expression of the ET/JA-responsive PDF1.2 gene was markedly reduced in hrl1 npr1 and in SA-depleted hrl1 nahG plants. In hrl1 nahG plants, expression of PDF1.2 was regulated by benzathiadiazole in a concentration-dependent manner: induced at low concentration and suppressed at high concentration. The hrl1 etr1 plants lacked systemic PR-1 expression, and exhibited compromised resistance to virulent Pseudomonas syringae and Peronospora parasitica. Inhibiting JA responses in hrl1 coi1 plants lead to exaggerated cell death and severe stunting of plants. Finally, the hrl1 mutation lead to elevated expression of AtrbohD, which encodes a major subunit of the NADPH oxidase complex. Our results indicate that defence gene expression and resistance against pathogens in hrl1 is regulated synergistically by SA and ET/JA defence pathways.

The Arabidopsis gain-of-function mutant dll1 spontaneously develops lesions mimicking cell death associated with disease.

We describe the characterization of a novel gain-of-function Arabidopsis mutant, dll1 (disease-like lesions1), which spontaneously develops lesions mimicking bacterial speck disease and constitutively expresses biochemical and molecular markers associated with pathogen infection. Despite the constitutive expression of defense-related responses, dll1 is unable to suppress the growth of virulent pathogens. However, dll1 elicits normal hypersensitive response in response to avirulent pathogens, thus indicating that dll1 is not defective in the induction of normal resistance responses. The lesion+ leaves of dll1 support the growth of hrcC mutant of Pseudomonas syringae, which is defective in the transfer of virulence factors into the plant cells, and therefore non-pathogenic to wild-type Col-0 plants. This suggests that dll1 intrinsically expresses many of the cellular processes that are required for pathogen growth during disease. Epistasis analyses reveal that salicylic acid and NPR1 are required for lesion formation, while ethylene modulates lesion development in dll1, suggesting that significant overlap exist between the signalling pathways leading to resistance- and disease-associated cell death. Our results suggest that host cell death during compatible interactions, at least in part, is genetically controlled by the plant and DLL1 may positively regulate this process.