Autophagy Negatively Regulates Cell Death by Controlling NPR1-Dependent Salicylic Acid Signaling during Senescence and the Innate Immune Response in Arabidopsis

Autophagy is an evolutionarily conserved intracellular process for vacuolar degradation of cytoplasmic components. In higher plants, autophagy defects result in early senescence and excessive immunity-related programmed cell death (PCD) irrespective of nutrient conditions; however, the mechanisms by which cells die in the absence of autophagy have been unclear. Here, we demonstrate a conserved requirement for salicylic acid (SA) signaling for these phenomena in autophagy-defective mutants (atg mutants). The atg mutant phenotypes of accelerated PCD in senescence and immunity are SA signaling dependent but do not require intact jasmonic acid or ethylene signaling pathways. Application of an SA agonist induces the senescence/cell death phenotype in SA-deficient atg mutants but not in atg npr1 plants, suggesting that the cell death phenotypes in the atg mutants are dependent on the SA signal transducer NONEXPRESSOR OF PATHOGENESIS-RELATED GENES1. We also show that autophagy is induced by the SA agonist. These findings imply that plant autophagy operates a novel negative feedback loop modulating SA signaling to negatively regulate senescence and immunity-related PCD.     

The Arabidopsis aberrant growth and death2 mutant shows resistance to Pseudomonas syringae and reveals a role for NPR1 in suppressing hypersensitive cell death

A novel Arabidopsis mutant has been identified with constitutive expression of GST1-GUS using plants with a pathogen-responsive reporter transgene containing the beta-glucuronidase (GUS) coding region driven by the GST1 promoter. The recessive mutant, called agd2 (aberrant growth and death2), has salicylic acid (SA)-dependent increased resistance to virulent and avirulent strains of the bacterial pathogen Pseudomonas syringae, elevated SA levels, a low level of spontaneous cell death, callose deposition, and enlarged cells in leaves. The enhanced resistance of agd2 to virulent P. syringae requires the SA signaling component NONEXPRESSOR OF PR1 (NPR1). However, agd2 renders the resistance response to P. syringae carrying avrRpt2 NPR1-independent. Thus agd2 affects both an SA- and NPR1-dependent general defense pathway and an SA-dependent, NPR1-independent pathway that is active during the recognition of avirulent P. syringae. agd2 plants also fail to show a hypersensitive cell death response (HR) unless NPR1 is removed. This novel function for NPR1 is also apparent in otherwise wild-type plants: npr1 mutants show a stronger HR, while NPR1-overproducing plants show a weaker HR when infected with P. syringae carrying the avrRpm1 gene. Spontaneous cell death in agd2 is partially suppressed by npr1, indicating that NPR1 can suppress or enhance cell death depending on the cellular context. agd2 plants depleted of SA show a dramatic exacerbation of the cell-growth phenotype and increased callose deposition, suggesting a role for SA in regulating growth and this cell-wall modification. AGD2 may function in cell death and/or growth control as well as the defense response, similarly to what has been described in animals for the functions of NFkappaB.     

Salicylic acid promotes autophagy via NPR3 and NPR4 in <Emphasis Type="Italic">Arabidopsis</Emphasis> senescence and innate immune response

In Arabidopsis thaliana, the non-expresser pathogenesis-related (NPR) multigene family members NPR1, NPR3, and NPR4 are necessary for salicylic acid (SA) perception. NPR3 and NPR4 are the CUL3 E3-ligase substrate adaptors allowing for the ubiquitination and turnover of NPR1 by the 26s proteasome. Concurrently, roots treated with the SA agonist benzothiadiazole accumulate autophagic bodies via NPR1-dependent signal pathway. However, the mechanisms by which NPR3 and NPR4 regulate autophagy remain unclear. In the present study, using single, double, and triple npr1-, npr3-, and npr4-null mutants and wild-type plants, the following results were obtained: (1) leaf senescence progressed faster in npr3/npr4 mutants than in wild type, suggesting that NPR3 and NPR4 negatively regulated leaf senescence. Moreover, npr3/npr4 promoted the expression of pathogenesis-related 1 (PR1) gene and enhanced resistance in response to avirulent pathogen infections suppressing cell death. Still, all mutants had similar SA levels, suggesting that NPR3 and NPR4 positive regulation of cell death and disease resistance was not associated with SA levels; (2) the number of autophagosomes, ATG7, and ATG8a-phosphatidylethanolamine and the concentration of free green-fluorescence protein were lower in npr3/npr4 mutants than in wild-type plants, indicating that NPR3 and NPR4 affected the two ubiquitination-like conjugation systems during the autophagosome formation and degradation of autophagic bodies.     

Signaling pathways that regulate the enhanced disease resistance of Arabidopsis "defense, no death" mutants

Arabidopsis dnd1 and dnd2 mutants lack cyclic nucleotide-gated ion channel proteins and carry out avirulence or resistance gene-mediated defense with a greatly reduced hypersensitive response (HR). They also exhibit elevated broad-spectrum disease resistance and constitutively elevated salicylic acid (SA) levels. We examined the contributions of NPR1, SID2 (EDS16), NDR1, and EIN2 to dnd phenotypes. Mutations that affect SA accumulation or signaling (sid2, npr1, and ndr1) abolished the enhanced resistance of dnd mutants against Pseudomonas syringae pv. tomato and Hyaloperonospora parasitica but not Botrytis cinerea. When SA-associated pathways were disrupted, the constitutive activation of NPR1-dependent and NPR1-independent and SA-dependent pathways was redirected toward PDF1.2-associated pathways. This PDF1.2 overexpression was downregulated after infection by P. syringae. Disruption of ethylene signaling abolished the enhanced resistance to B. cinerea but not P. syringae or H. parasitica. However, loss of NPR1, SID2, NDR1, or EIN2 did not detectably alter the reduced HR in dnd mutants. The susceptibility of dnd ein2 plants to B. cinerea despite their reduced-HR phenotype suggests that cell death repression is not the primary cause of dnd resistance to necrotrophic pathogens. The partial restoration of resistance to B. cinerea in dnd1 npr1 ein2 triple mutants indicated that this resistance is not entirely EIN2 dependent. The above findings indicate that the broad-spectrum resistance of dnd mutants occurs due to activation or sensitization of multiple defense pathways, yet none of the investigated pathways are required for the reduced-HR phenotype.     

Salicylic acid-mediated cell death in the Arabidopsis len3 mutant

The Arabidopsis lesion initiation 3 (len3) mutant develops lesions on leaves without pathogen attack. len3 plants exhibit stunted growth, constitutively express pathogenesis-related (PR) genes, PR-1, PR-2, and PR-5, and accumulate elevated levels of salicylic acid (SA). Furthermore, len3 is a semidominant, male gametophytic lethal mutation with partial defects in female gametophytic development. To determine the signaling pathway activated in len3 plants, we crossed the len3 plants with nahG, npr1-1, and pad4-1 plants and analyzed the phenotypes of the double mutants. The len3-conferred phenotypes, including cell death and PR-1 expressions, were suppressed in the double mutants. Thus SA, NPR1, and PAD4 are required for the phenotypes. However, none of these double mutants could completely suppress the len3-conferred stunted growth. This result suggests that an SA-, NPR1-, and PAD4-independent pathway is also involved in the phenotype. Treatment with BTH (benzo(1,2,3)thiadiazole-7-carbothioic acid), an SA analog, induced cell death in len3 nahG plants but not in len3 npr1 or len3 pad4 plants, suggesting the involvement of the PAD4-dependent but SA-independent second signal pathway in cell death in len3 plants.     

Roles of salicylic acid, jasmonic acid, and ethylene in cpr-induced resistance in arabidopsis

Disease resistance in Arabidopsis is regulated by multiple signal transduction pathways in which salicylic acid (SA), jasmonic acid (JA), and ethylene (ET) function as key signaling molecules. Epistasis analyses were performed between mutants that disrupt these pathways (npr1, eds5, ein2, and jar1) and mutants that constitutively activate these pathways (cpr1, cpr5, and cpr6), allowing exploration of the relationship between the SA- and JA/ET-mediated resistance responses. Two important findings were made. First, the constitutive disease resistance exhibited by cpr1, cpr5, and cpr6 is completely suppressed by the SA-deficient eds5 mutant but is only partially affected by the SA-insensitive npr1 mutant. Moreover, eds5 suppresses the SA-accumulating phenotype of the cpr mutants, whereas npr1 enhances it. These data indicate the existence of an SA-mediated, NPR1-independent resistance response. Second, the ET-insensitive mutation ein2 and the JA-insensitive mutation jar1 suppress the NPR1-independent resistance response exhibited by cpr5 and cpr6. Furthermore, ein2 potentiates SA accumulation in cpr5 and cpr5 npr1 while dampening SA accumulation in cpr6 and cpr6 npr1. These latter results indicate that cpr5 and cpr6 regulate resistance through distinct pathways and that SA-mediated, NPR1-independent resistance works in combination with components of the JA/ET-mediated response pathways.     

Environmentally sensitive, SA-dependent defense responses in the cpr22 mutant of Arabidopsis

To investigate the signaling pathways through which defense responses are activated following pathogen infection, we have isolated and characterized the cpr22 mutant. This plant carries a semidominant, conditional lethal mutation that confers constitutive expression of the pathogenesis-related (PR) genes PR-1, PR-2, PR-5 and the defensin gene PDF1.2. cpr22 plants also display spontaneous lesion formation, elevated levels of salicylic acid (SA) and heightened resistance to Peronospora parasitica Emco5. The cpr22 locus was mapped to chromosome 2, approximately 2 cM telomeric to the AthB102 marker. By analyzing the progeny of crosses between cpr22 plants and either NahG transgenic plants or npr1 mutants, all of the cpr22-associated phenotypes except PDF1.2 expression were found to be SA dependent. However, the SA signal transducer NPR1 was required only for constitutive PR-1 expression. A cross between cpr22 and ndr1-1 mutants revealed that enhanced resistance to P. parasitica is mediated by an NDR1-dependent pathway, while the other cpr22-induced defenses are not. Crosses between either coi1-1 or etr1-1 mutants further demonstrated that constitutive PDF1.2 expression is mediated by a JA- and ethylene-dependent pathway. Based on these results, the cpr22 mutation appears to induce its associated phenotypes by activating NPR1-dependent and NPR1-independent branches of the SA pathway, as well as an ethylene/JA signaling pathway. Interestingly, the SA-dependent phenotypes, but not the SA-independent phenotypes, are suppressed when cpr22 mutants are grown under high humidity.     

Runaway cell death, but not basal disease resistance, in lsd1 is SA- and NIM1/NPR1-dependent

LSD1 was defined as a negative regulator of plant cell death and basal disease resistance based on its null mutant phenotypes. We addressed the relationship between lsd1-mediated runaway cell death and signaling components required for systemic acquired resistance (SAR), namely salicylic acid (SA) accumulation and NIM1/NPR1. We present two important findings. First, SA accumulation and NIM1/NPR1 are required for lsd1-mediated runaway cell death following pathogen infection or application of chemicals that mimic SA action. This implies that lsd1-dependent cell death occurs 'downstream' of the accumulation of SA. As SA application triggers runaway cell death in lsd1 but not wild-type plants, we infer that LSD1 negatively regulates an SA-dependent signal leading to cell death. Thus SA is both a trigger and a required mediator of lsd1 runaway cell death. Second, neither SA accumulation nor NIM1/NPR1 function is required for the basal resistance operating in lsd1. Therefore LSD1 negatively regulates a basal defense pathway that can act upstream or independently of both NIM1/NPR1 function and SA accumulation following avirulent or virulent pathogen challenge. Our data, together with results from other studies, point to the existence of an SA-dependent 'signal potentiation loop' controlling HR. Continued escalation of signaling in the absence of LSD1 leads to runaway cell death. We propose that LSD1 is a key negative regulator of this signal potentiation.     

A role for salicylic acid and NPR1 in regulating cell growth in Arabidopsis

Salicylic acid (SA) plays a key role in activating defenses and cell death during plant-pathogen interactions. In response to some pathogens, SA also limits the extent of cell death, indicating that it acts positively or negatively depending on the host-pathogen interaction. In addition, we previously showed that SA affects cell growth in the Arabidopsis defense-related mutants accelerated cell death 6-1 (acd6-1) and aberrant growth and death 2 (agd2). Using acd6-1, agd2 and two other defense-related mutants, lesion simulating disease 6 (lsd6), suppressor of SA-insensitivity (ssi1), we show here in detail that SA regulates cell growth by specifically affecting cell enlargement, endoreduplication and/or cell division. We find that SA can act either positively or negatively to regulate cell growth depending on the context in which signaling occurs. Additionally, Nonexpressor of PR 1 (NPR1), a key SA signaling protein important for regulating defenses and cell death, also acts to promote cell division and/or suppress endoreduplication during leaf development. We propose that SA interacts with multiple receptors or signaling pathways to control cellular alterations during normal development, pathogen attack and/or stress situations. We suggest that SA and NPR1 play broader roles in cell fate control than has previously been understood.     

A recessive mutation in the Arabidopsis SSI2 gene confers SA- and NPR1-independent expression of PR genes and resistance against bacterial and oomycete pathogens

The Arabidopsis thaliana NPR1 gene is required for salicylic acid (SA)-induced expression of pathogenesis-related (PR) genes and systemic acquired resistance. However, loss-of-function mutations in NPR1 do not confer complete loss of PR gene expression or disease resistance. Thus these responses also can be activated via an NPR1-independent pathway that currently remain to be elucidated. The ssi2-1 mutant, identified in a genetic screen for suppressors of npr1-5, affects signaling through the NPR1-independent defense pathway(s). In comparison with the wild-type (SSI2 NPR1) plants and the npr1-5 mutant (SSI2 npr1-5), the ssi2-1 npr1-5 double mutant and the ssi2-1 NPR1 single mutant constitutively express PR genes [PR-1, BGL2 (PR-2) and PR-5]; accumulate elevated levels of SA; spontaneously develop lesions; and possess enhanced resistance to a virulent strain of Peronospora parasitica. The ssi2-1 mutation also confers enhanced resistance to Pseudomonas syringae pv. tomato (Pst); however, this is accomplished primarily via an NPR1-dependent pathway. Analysis of ssi2-1 NPR1 nahG and ssi2-1 npr1-5 nahG plants revealed that elevated SA levels were not essential for the ssi2-1-conferred phenotypes. However, expression of the nahG transgene did reduce the intensity of some ssi2-1-conferred phenotypes, including PR-1 expression, and disease resistance. Based on these results, SSI2 or an SSI2-generated signal appears to modulate signaling of an SA-dependent, NPR1-independent defense pathway, or an SA- and NPR1-independent defense pathway.     

Fumonisin B1-induced cell death in arabidopsis protoplasts requires jasmonate-, ethylene-, and salicylate-dependent signaling pathways

We have established an Arabidopsis protoplast model system to study plant cell death signaling. The fungal toxin fumonisin B1 (FB1) induces apoptosis-like programmed cell death (PCD) in wild-type protoplasts. FB1, however, only marginally affects the viability of protoplasts isolated from transgenic NahG plants, in which salicylic acid (SA) is metabolically degraded; from pad4-1 mutant plants, in which an SA amplification mechanism is thought to be impaired; or from jar1-1 or etr1-1 mutant plants, which are insensitive to jasmonate (JA) or ethylene (ET), respectively. FB1 susceptibility of wild-type protoplasts decreases in the dark, as does the cellular content of phenylalanine ammonia-lyase, a light-inducible enzyme involved in SA biosynthesis. Interestingly, however, FB1-induced PCD does not require the SA signal transmitter NPR1, given that npr1-1 protoplasts display wild-type FB1 susceptibility. Arabidopsis cpr1-1, cpr6-1, and acd2-2 protoplasts, in which the SA signaling pathway is constitutively activated, exhibit increased susceptibility to FB1. The cpr6-1 and acd2-2 mutants also constitutively express the JA and ET signaling pathways, but only the acd2-2 protoplasts undergo PCD in the absence of FB1. These results demonstrate that FB1 killing of Arabidopsis is light dependent and requires SA-, JA-, and ET-mediated signaling pathways as well as one or more unidentified factors activated by FB1 and the acd2-2 mutation.     

Arabidopsis local resistance to Botrytis cinerea involves salicylic acid and camalexin and requires EDS4 and PAD2, but not SID2, EDS5 or PAD4

Salicylic acid (SA) is an important regulator of plant defense responses, and a variety of Arabidopsis mutants impaired in resistance against bacterial and fungal pathogens show defects in SA accumulation, perception, or signal transduction. Nevertheless, the role of SA-dependent defense responses against necrotrophic fungi is currently unclear. We determined the susceptibility of a set of previously identified Arabidopsis mutants impaired in defense responses to the necrotrophic fungal pathogen Botrytis cinerea. The rate of development of B. cinerea disease symptoms on primary infected leaves was affected by responses mediated by the genes EIN2, JAR1, EDS4, PAD2, and PAD3, but was largely independent of EDS5, SID2/ICS1, and PAD4. Furthermore, plants expressing a nahG transgene or treated with a phenylalanine ammonia lyase (PAL) inhibitor showed enhanced symptoms, suggesting that SA synthesized via PAL, and not via isochorismate synthase (ICS), mediates lesion development. In addition, the degree of lesion development did not correlate with defensin or PR1 expression, although it was partially dependent upon camalexin accumulation. Although npr1 mutant leaves were normally susceptible to B. cinerea infection, a double ein2 npr1 mutant was significantly more susceptible than ein2 plants, and exogenous application of SA decreased B. cinerea lesion size through an NPR1-dependent mechanism that could be mimicked by the cpr1 mutation. These data indicate that local resistance to B. cinerea requires ethylene-, jasmonate-, and SA-mediated signaling, that the SA affecting this resistance does not require ICS1 and is likely synthesized via PAL, and that camalexin limits lesion development.     

Salicylic acid dependent signaling promotes basal thermotolerance but is not essential for acquired thermotolerance in Arabidopsis thaliana

Salicylic acid (SA) is reported to protect plants from heat shock (HS), but insufficient is known about its role in thermotolerance or how this relates to SA signaling in pathogen resistance. We tested thermotolerance and expression of pathogenesis-related (PR) and HS proteins (HSPs) in Arabidopsis thaliana genotypes with modified SA signaling: plants with the SA hydroxylase NahG transgene, the nonexpresser of PR proteins (npr1) mutant, and the constitutive expressers of PR proteins (cpr1 and cpr5) mutants. At all growth stages from seeds to 3-week-old plants, we found evidence for SA-dependent signaling in basal thermotolerance (i.e. tolerance of HS without prior heat acclimation). Endogenous SA correlated with basal thermotolerance, with the SA-deficient NahG and SA-accumulating cpr5 genotypes having lowest and highest thermotolerance, respectively. SA promoted thermotolerance during the HS itself and subsequent recovery. Recovery from HS apparently involved an NPR1-dependent pathway but thermotolerance during HS did not. SA reduced electrolyte leakage, indicating that it induced membrane thermoprotection. PR-1 and Hsp17.6 were induced by SA or HS, indicating common factors in pathogen and HS responses. SA-induced Hsp17.6 expression had a different dose-response to PR-1 expression. HS-induced Hsp17.6 protein appeared more slowly in NahG. However, SA only partially induced HSPs. Hsp17.6 induction by HS was more substantial than by SA, and we found no SA effect on Hsp101 expression. All genotypes, including NahG and npr1, were capable of expression of HSPs and acquisition of HS tolerance by prior heat acclimation. Although SA promotes basal thermotolerance, it is not essential for acquired thermotolerance.     

Activation of an EDS1-mediated R-gene pathway in the snc1 mutant leads to constitutive, NPR1-independent pathogen resistance

The Arabidopsis NPR1 protein is an essential regulatory component of systemic acquired resistance (SAR). Mutations in the NPR1 gene completely block the induction of SAR by signals such as salicylic acid (SA). An Arabidopsis mutant, snc1 (suppressor of npr1-1, constitutive 1), was isolated in a screen for suppressors of npr1-1. In the npr1-1 background, the snc1 mutation resulted in constitutive resistance to Pseudomonas syringae maculicola ES4326 and Peronospora parasitica Noco2. High levels of SA were detected in the mutant and shown to be required for manifestation of the snc1 phenotype. The snc1 mutation was mapped to the RPP5 resistance (R) gene cluster and the eds1 mutation that blocks RPP5-mediated resistance suppressed snc1. These data suggest that a RPP5-related resistance pathway is activated constitutively in snc1. This pathway does not employ NPR1 but requires the signal molecule SA and the function of EDS1. Moreover, in snc1, constitutive resistance is conferred in the absence of cell death, which is often associated with R-gene mediated resistance.     

Vascular associated death1, a novel GRAM domain-containing protein, is a regulator of cell death and defense responses in vascular tissues

The hypersensitive response (HR) is a programmed cell death that is commonly associated with plant disease resistance. A novel lesion mimic mutant, vad1 (for vascular associated death1), that exhibits light conditional appearance of propagative HR-like lesions along the vascular system was identified. Lesion formation is associated with expression of defense genes, production of high levels of salicylic acid (SA), and increased resistance to virulent and avirulent strains of Pseudomonas syringae pv tomato. Analyses of the progeny from crosses between vad1 plants and either nahG transgenic plants, sid1, nonexpressor of PR1 (npr1), enhanced disease susceptibility1 (eds1), or non-race specific disease resistance1 (ndr1) mutants, revealed the vad1 cell death phenotype to be dependent on SA biosynthesis but NPR1 independent; in addition, both EDS1 and NDR1 are necessary for the proper timing and amplification of cell death as well as for increased resistance to Pseudomonas strains. VAD1 encodes a novel putative membrane-associated protein containing a GRAM domain, a lipid or protein binding signaling domain, and is expressed in response to pathogen infection at the vicinity of the hypersensitive lesions. VAD1 might thus represent a new potential function in cell death control associated with cells in the vicinity of vascular bundles.     

Diverse Roles of the Salicylic Acid Receptors NPR1 and NPR3/NPR4 in Plant Immunity

The plant defense hormone salicylic acid (SA) is perceived by two classes of receptors, NPR1 and NPR3/NPR4. They function in two parallel pathways to regulate SA-induced defense gene expression. To better understand the roles of the SA receptors in plant defense, we systematically analyzed their contributions to different aspects of Arabidopsis (Arabidopsis thaliana) plant immunity using the SA-insensitive npr1-1 npr4-4D double mutant. We found that perception of SA by NPR1 and NPR4 is required for activation of N-hydroxypipecolic acid biosynthesis, which is essential for inducing systemic acquired resistance. In addition, both pattern-triggered immunity (PTI) and effector-triggered immunity (ETI) are severely compromised in the npr1-1 npr4-4D double mutant. Interestingly, the PTI and ETI attenuation in npr1-1 npr4-4D is more dramatic compared with the SA-induction deficient2-1 (sid2-1) mutant, suggesting that the perception of residual levels of SA in sid2-1 also contributes to immunity. Furthermore, NPR1 and NPR4 are involved in positive feedback amplification of SA biosynthesis and regulation of SA homeostasis through modifications including 5-hydroxylation and glycosylation. Thus, the SA receptors NPR1 and NPR4 play broad roles in plant immunity.     

Arabidopsis sfd mutants affect plastidic lipid composition and suppress dwarfing, cell death, and the enhanced disease resistance phenotypes resulting from the deficiency of a fatty acid desaturase

A loss-of-function mutation in the Arabidopsis SSI2/FAB2 gene, which encodes a plastidic stearoyl-acyl-carrier protein desaturase, has pleiotropic effects. The ssi2 mutant plant is dwarf, spontaneously develops lesions containing dead cells, accumulates increased salicylic acid (SA) levels, and constitutively expresses SA-mediated, NPR1-dependent and -independent defense responses. In parallel, jasmonic acid-regulated signaling is compromised in the ssi2 mutant. In an effort to discern the involvement of lipids in the ssi2-conferred developmental and defense phenotypes, we identified suppressors of fatty acid (stearoyl) desaturase deficiency (sfd) mutants. The sfd1, sfd2, and sfd4 mutant alleles suppress the ssi2-conferred dwarfing and lesion development, the NPR1-independent expression of the PATHOGENESIS-RELATED1 (PR1) gene, and resistance to Pseudomonas syringae pv maculicola. The sfd1 and sfd4 mutant alleles also depress ssi2-conferred PR1 expression in NPR1-containing sfd1 ssi2 and sfd4 ssi2 plants. By contrast, the sfd2 ssi2 plant retains the ssi2-conferred high-level expression of PR1. In parallel with the loss of ssi2-conferred constitutive SA signaling, the ability of jasmonic acid to activate PDF1.2 expression is reinstated in the sfd1 ssi2 npr1 plant. sfd4 is a mutation in the FAD6 gene that encodes a plastidic omega6-desaturase that is involved in the synthesis of polyunsaturated fatty acid-containing lipids. Because the levels of plastid complex lipid species containing hexadecatrienoic acid are depressed in all of the sfd ssi2 npr1 plants, we propose that these lipids are involved in the manifestation of the ssi2-conferred phenotypes.     

NPR1-dependent salicylic acid signaling is not involved in elevated CO2-induced heat stress tolerance in Arabidopsis thaliana

Elevated CO₂ can protect plants from heat stress (HS); however, the underlying mechanisms are largely unknown. Here, we used a set of Arabidopsis mutants such as salicylic acid (SA) signaling mutants nonexpressor of pathogenesis-related gene 1 (npr1-1 and npr1-5) and heat-shock proteins (HSPs) mutants (hsp21 and hsp70-1) to understand the requirement of SA signaling and HSPs in elevated CO₂-induced HS tolerance. Under ambient CO₂ (380 µmol mol⁻¹) conditions, HS (42°C, 24 h) drastically decreased maximum photochemical efficiency of PSII (Fv/Fm) in all studied plant groups. Enrichment of CO₂ (800 µmol mol⁻¹) with HS remarkably increased the Fv/Fm value in all plant groups except hsp70-1, indicating that NPR1-dependent SA signaling is not involved in the elevated CO₂-induced HS tolerance. These results also suggest an essentiality of HSP70-1, but not HSP21 in elevated CO₂-induced HS mitigation.