The Arabidopsis mutant cev1 has constitutively active jasmonate and ethylene signal pathways and enhanced resistance to pathogens

Jasmonates (JAs) inhibit plant growth and induce plant defense responses. To define genes in the Arabidopsis JA signal pathway, we screened for mutants with constitutive expression of a luciferase reporter for the JA-responsive promoter from the vegetative storage protein gene VSP1. One mutant, named constitutive expression of VSP1 (cev1), produced plants that were smaller than wild type, had stunted roots with long root hairs, accumulated anthocyanin, had constitutive expression of the defense-related genes VSP1, VSP2, Thi2.1, PDF1.2, and CHI-B, and had enhanced resistance to powdery mildew diseases. Genetic evidence indicated that the cev1 phenotype required both COI1, an essential component of the JA signal pathway, and ETR1, which encodes the ethylene receptor. We conclude that cev1 stimulates both the JA and the ethylene signal pathways and that CEV1 regulates an early step in an Arabidopsis defense pathway.     

Ozone-sensitive arabidopsis rcd1 mutant reveals opposite roles for ethylene and jasmonate signaling pathways in regulating superoxide-dependent cell death

We have isolated a codominant Arabidopsis mutant, radical-induced cell death1 (rcd1), in which ozone (O(3)) and extracellular superoxide (O(2)(*)-), but not hydrogen peroxide, induce cellular O(2)(*)- accumulation and transient spreading lesions. The cellular O(2)(*)- accumulation is ethylene dependent, occurs ahead of the expanding lesions before visible symptoms appear, and is required for lesion propagation. Exogenous ethylene increased O(2)(*)--dependent cell death, whereas impairment of ethylene perception by norbornadiene in rcd1 or ethylene insensitivity in the ethylene-insensitive mutant ein2 and in the rcd1 ein2 double mutant blocked O(2)(*)- accumulation and lesion propagation. Exogenous methyl jasmonate inhibited propagation of cell death in rcd1. Accordingly, the O(3)-exposed jasmonate-insensitive mutant jar1 displayed spreading cell death and a prolonged O(2)(*)- accumulation pattern. These results suggest that ethylene acts as a promoting factor during the propagation phase of developing oxyradical-dependent lesions, whereas jasmonates have a role in lesion containment. Interaction and balance between these pathways may serve to fine-tune propagation and containment processes, resulting in alternate lesion size and formation kinetics.     

The Arabidopsis eer1 mutant has enhanced ethylene responses in the hypocotyl and stem

By screening for enhanced ethylene-response (eer) mutants in Arabidopsis, we isolated a novel recessive mutant, eer1, which displays increased ethylene sensitivity in the hypocotyl and stem. Dark-grown eer1 seedlings have short and thick hypocotyls even in the absence of added ethylene. This phenotype is suppressed, however, by the ethylene biosynthesis inhibitor 1-aminoethoxyvinyl-glycine. Following ethylene treatment, the dark-grown eer1 hypocotyl response is greatly exaggerated in comparison with the wild type, indicating that the eer1 phenotype is not simply due to ethylene overproduction. eer1 seedlings have significantly elevated levels of basic-chitinase expression, suggesting that eer1 may be highly sensitive to low levels of endogenous ethylene. Adult eer1 plants display exaggerated ethylene-dependent stem thickening, which is an ethylene response previously unreported in Arabidopsis. eer1 also has enhanced responsiveness to the ethylene agonists propylene and 2,5-norbornadiene. The eer1 phenotype is completely suppressed by the ethylene-insensitive mutation etr1-1, and is additive with the constitutive ethylene-response mutation ctr1-3. Our findings suggest that the wild-type EER1 product acts to oppose ethylene responses in the hypocotyl and stem.     

Host and non-host pathogens elicit different jasmonate/ethylene responses in Arabidopsis

Arabidopsis does not support the growth and asexual reproduction of the barley pathogen, Blumeria graminis f. sp. hordei Bgh). A majority of germlings fail to penetrate the epidermal cell wall and papillae. To gain additional insight into this interaction, we determined whether the salicylic acid (SA) or jasmonate (JA)/ethylene (ET) defence pathways played a role in blocking barley powdery mildew infections. Only the eds1 mutant and NahG transgenics supported a modest increase in penetration success by the barley powdery mildew. We also compared the global gene expression patterns of Arabidopsis inoculated with the non-host barley powdery mildew to those inoculated with a virulent, host powdery mildew, Erysiphe cichoracearum. Genes repressed by inoculations with non-host and host powdery mildews relative to non-inoculated control plants accounted for two-thirds of the differentially expressed genes. A majority of these genes encoded components of photosynthesis and general metabolism. Consistent with this observation, Arabidopsis growth was inhibited following inoculation with Bgh, suggesting a shift in resource allocation from growth to defence. A number of defence-associated genes were induced during both interactions. These genes likely are components of basal defence responses, which do not effectively block host powdery mildew infections. In addition, genes encoding defensins, anti-microbial peptides whose expression is under the control of the JA/ET signalling pathway, were induced exclusively by non-host pathogens. Ectopic activation of JA/ET signalling protected Arabidopsis against two biotrophic host pathogens. Taken together, these data suggest that biotrophic host pathogens must either suppress or fail to elicit the JA/ET signal transduction pathway.     

COI1 links jasmonate signalling and fertility to the SCF ubiquitin-ligase complex in Arabidopsis

Jasmonates (JAs) regulate Arabidopsis thaliana wound and defence responses, pollen development, and stress-related growth inhibition. Significantly, each of these responses requires COI1, an F-box protein. Other F-box proteins interact with SKP1 and cullin proteins to form SCF complexes that selectively recruit regulatory proteins targeted for ubiquitination. To determine whether COI1 also functions in an SCF complex, we have characterized Arabidopsis proteins that bind to COI1. An Arabidopsis cDNA expression library was screened in yeast for clones that produce proteins which can bind to COI1. We recovered two SKP1 homologues and a histone deacetylase. The Arabidopsis F-box protein TIR1 interacted with SKP1 proteins, but not with the histone deacetylase. Mutant COI1 proteins revealed that the F-box is required for interaction with SKP1s, but that sequences in leucine-rich repeat domains are required for interaction with the histone deacetylase. Epitope-tagged COI1 was introduced into Arabidopsis plants and cell cultures. Co-immunoprecipitation experiments confirmed the interaction in planta of COI1 with SKP1-like proteins and histone deacetylase, and also indicated that COI1 interacted with cullin. These results suggest that COI1 forms an SCFCOI1 complex in vivo. COI1 is therefore expected to form a functional E3-type ubiquitin ligase in plants and to regulate expression of jasmonate responsive genes, possibly by targeted ubiquitination of a histone deacetylase.     

The Arabidopsis mutant eer2 has enhanced ethylene responses in the light

By screening for ethylene response mutants in Arabidopsis, a novel mutant, eer2, was isolated which displays enhanced ethylene responses. On a low nutrient medium (LNM) light-grown eer2 seedlings showed a significant hypocotyl elongation in response to low levels of 1-amino-cyclopropane-1-carboxylate (ACC), the precursor of ethylene, compared with the wild type, indicating that eer2 is hypersensitive to ethylene. Treatment with 1-MCP (1-methylcyclopropene), a competitive inhibitor of ethylene signalling, suppressed this hypersensitive response, demonstrating that it is a bona fide ethylene effect. By contrast, roots of eer2 were less sensitive than the wild type to low concentrations of ACC. The ethylene levels in eer2 did not differ from the wild type, indicating that ethylene overproduction is not the primary cause of the eer2 phenotype. In addition to its enhanced ethylene response of hypocotyls, eer2 is also affected in the pattern of senescence and its phenotype depends on the nutritional status of the growth medium. Furthermore, linkage analysis of eer2 suggests that this mutant defines a new locus in ethylene signalling.     

Fusarium oxysporum hijacks COI1-mediated jasmonate signaling to promote disease development in Arabidopsis

Although defense responses mediated by the plant oxylipin jasmonic acid (JA) are often necessary for resistance against pathogens with necrotrophic lifestyles, in this report we demonstrate that jasmonate signaling mediated through COI1 in Arabidopsis thaliana is responsible for susceptibility to wilt disease caused by the root-infecting fungal pathogen Fusarium oxysporum . Despite compromised JA-dependent defense responses, the JA perception mutant coronatine insensitive 1 ( coi1 ), but not JA biosynthesis mutants, exhibited a high level of resistance to wilt disease caused by F. oxysporum . This response was independent from salicylic acid-dependent defenses, as coi1/NahG plants showed similar disease resistance to coi1 plants. Inoculation of reciprocal grafts made between coi1 and wild-type plants revealed that coi1 -mediated resistance occurred primarily through the coi1 rootstock tissues. Furthermore, microscopy and quantification of fungal DNA during infection indicated that coi1 -mediated resistance was not associated with reduced fungal penetration and colonization until a late stage of infection, when leaf necrosis was highly developed in wild-type plants. In contrast to wild-type leaves, coi1 leaves showed no necrosis following the application of F. oxysporum culture filtrate, and showed reduced expression of senescence-associated genes during disease development, suggesting that coi1 resistance is most likely achieved through the inhibition of F. oxysporum -incited lesion development and plant senescence. Together, our results indicate that F. oxysporum hijacks non-defensive aspects of the JA-signaling pathway to cause wilt-disease symptoms that lead to plant death in Arabidopsis.     

The Arabidopsis mutant alh1 illustrates a cross talk between ethylene and auxin

Ethylene or its precursor 1-aminocyclopropane-1-carboxylic acid (ACC) can stimulate hypocotyl elongation in light-grown Arabidopsis seedlings. A mutant, designated ACC-related long hypocotyl 1 (alh1), that displayed a long hypocotyl in the light in the absence of the hormone was characterized. Etiolated alh1 seedlings overproduced ethylene and had an exaggerated apical hook and a thicker hypocotyl, although no difference in hypocotyl length was observed when compared with wild type. Alh1 plants were less sensitive to ethylene, as reflected by reduction of ACC-mediated inhibition of hypocotyl growth in the dark and delay in flowering and leaf senescence. Alh1 also had an altered response to auxin, whereas auxin levels in whole alh1 seedlings remained unaffected. In contrast to wild type, alh1 seedlings showed a limited hypocotyl elongation when treated with indole-3-acetic acid. Alh1 roots had a faster response to gravity. Furthermore, the hypocotyl elongation of alh1 and of ACC-treated wild type was reverted by auxin transport inhibitors. In addition, auxin up-regulated genes were ectopically expressed in hypocotyls upon ACC treatment, suggesting that the ethylene response is mediated by auxins. Together, these data indicate that alh1 is altered in the cross talk between ethylene and auxins, probably at the level of auxin transport.     

Arabidopsis RADICAL-INDUCED CELL DEATH1 belongs to the WWE protein-protein interaction domain protein family and modulates abscisic acid, ethylene, and methyl jasmonate responses

Experiments with several Arabidopsis thaliana mutants have revealed a web of interactions between hormonal signaling. Here, we show that the Arabidopsis mutant radical-induced cell death1 (rcd1), although hypersensitive to apoplastic superoxide and ozone, is more resistant to chloroplastic superoxide formation, exhibits reduced sensitivity to abscisic acid, ethylene, and methyl jasmonate, and has altered expression of several hormonally regulated genes. Furthermore, rcd1 has higher stomatal conductance than the wild type. The rcd1-1 mutation was mapped to the gene At1g32230 where it disrupts an intron splice site resulting in a truncated protein. RCD1 belongs to the (ADP-ribosyl)transferase domain-containing subfamily of the WWE protein-protein interaction domain protein family. The results suggest that RCD1 could act as an integrative node in hormonal signaling and in the regulation of several stress-responsive genes.     

Oxo-phytodienoic acid-containing galactolipids in Arabidopsis: jasmonate signaling dependence

The jasmonate family of phytohormones, as represented by 12-oxo-phytodienoic acid (OPDA), dinor-phytodienoic acid (dn-OPDA), and jasmonic acid in Arabidopsis (Arabidopsis thaliana), has been implicated in a vast array of different developmental processes and stress responses. Recent reports indicate that OPDA and dn-OPDA occur not only as free acids in Arabidopsis, but also as esters with complex lipids, so-called arabidopsides. Recently, we showed that recognition of the two bacterial effector proteins AvrRpm1 and AvrRpt2 induced high levels of a molecule consisting of two OPDAs and one dn-OPDA esterified to a monogalactosyl diacylglycerol moiety, named arabidopside E. In this study, we demonstrate that the synthesis of arabidopsides is mainly independent of the prokaryotic lipid biosynthesis pathway in the chloroplast, and, in addition to what previously has been reported, arabidopside E as well as an all-OPDA analog, arabidopside G, described here accumulated during the hypersensitive response and in response to wounding. We also show that different signaling pathways lead to the formation of arabidopsides during the hypersensitive response and the wounding response, respectively. However, the formation of arabidopsides during both responses is dependent on an intact jasmonate signaling pathway. Additionally, we report inhibition of growth of the fungal necrotrophic pathogen Botrytis cinerea and in planta release of free jasmonates in a time frame that overlaps with the observed reduction of arabidopside levels. Thus, arabidopsides may have a dual function: as antipathogenic substances and as storage compounds that allow the slow release of free jasmonates.     

Regulation of plant disease resistance, stress responses, cell death, and ethylene signaling in Arabidopsis by the EDR1 protein kinase

ENHANCED DISEASE RESISTANCE 1 (EDR1) encodes a CTR1-like kinase and was previously reported to function as a negative regulator of disease resistance and ethylene-induced senescence. Here, we report that the edr1 mutant displays enhanced stress responses and spontaneous necrotic lesions under drought conditions in the absence of pathogen, suggesting that EDR1 is also involved in stress response signaling and cell death regulation. Double mutant analysis revealed that these drought-induced phenotypes require salicylic acid but not ethylene signaling pathways. In addition, the edr1-mediated ethylene-induced senescence phenotype was suppressed by mutations in EIN2, but not by mutations in SID2, PAD4, EDS1, or NPR1, suggesting that EDR1 functions at a point of cross talk between ethylene and salicylic acid signaling that impinges on senescence and cell death. Two edr1-associated phenotypes, drought-induced growth inhibition and ethylene-induced senescence, were suppressed by mutations in ORE9, implicating ubiquitin-mediated protein degradation in the regulation of these phenotypes. However, the ore9 mutation did not suppress edr1-mediated enhanced disease resistance to powdery mildew or spontaneous lesions, indicating that these phenotypes are controlled by separate signaling pathways. To investigate the function of the EDR1 kinase domain, we expressed the C-terminal third of EDR1 in wild-type Columbia and edr1 backgrounds under the control of a dexamethasone-inducible promoter. Overexpression of the EDR1 kinase domain in an edr1 background had no obvious effect on edr1-associated phenotypes. However, overexpression of the EDR1 kinase domain in a wild-type Columbia background caused dominant negative phenotypes, including enhanced disease resistance to powdery mildew and enhanced ethylene-induced senescence; thus, the overexpressed EDR1 kinase domain alone does not exert EDR1 function, but rather negatively affects the function of native EDR1 protein.     

Arabidopsis RTE1 is essential to ethylene receptor ETR1 amino-terminal signaling independent of CTR1

The Arabidopsis (Arabidopsis thaliana) ethylene receptor Ethylene Response1 (ETR1) can mediate the receptor signal output via its carboxyl terminus interacting with the amino (N) terminus of Constitutive Triple Response1 (CTR1) or via its N terminus (etr1^1-349 or the dominant ethylene-insensitive etr1-1^1-349) by an unknown mechanism. Given that CTR1 is essential to ethylene receptor signaling and that overexpression of Reversion To Ethylene Sensitivity1 (RTE1) promotes ETR1 N-terminal signaling, we evaluated the roles of CTR1 and RTE1 in ETR1 N-terminal signaling. The mutant phenotype of ctr1-1 and ctr1-2 was suppressed in part by the transgenes etr1^1-349 and etr1-1^1-349, with etr1-1^1-349 conferring ethylene insensitivity. Coexpression of 35S:RTE1 and etr1^1-349 conferred ethylene insensitivity in ctr1-1, whereas suppression of the ctr1-1 phenotype by etr1^1-349 was prevented by rte1-2. Thus, RTE1 was essential to ETR1 N-terminal signaling independent of the CTR1 pathway. An excess amount of the CTR1 N terminus CTR1^7-560 prevented ethylene receptor signaling, and the CTR1^7-560 overexpressor CTR1-Nox showed a constitutive ethylene response phenotype. Expression of the ETR1 N terminus suppressed the CTR1-Nox phenotype. etr1^1-349 restored the ethylene insensitivity conferred by dominant receptor mutant alleles in the ctr1-1 background. Therefore, ETR1 N-terminal signaling was not mediated by full-length ethylene receptors; rather, full-length ethylene receptors acted cooperatively with the ETR1 N terminus to mediate the receptor signal independent of CTR1. ETR1 N-terminal signaling may involve RTE1, receptor cooperation, and negative regulation by the ETR1 carboxyl terminus.The gaseous plant hormone ethylene is perceived by a small family of ethylene receptors. Arabidopsis (Arabidopsis thaliana) has five ethylene receptors that are structurally similar to prokaryotic two-component histidine kinase (HK) proteins. Mutants defective in multiple ethylene receptor genes show a constitutive ethylene response phenotype, which indicates a negative regulation of ethylene responses by the receptor genes (Hua and Meyerowitz, 1998).The receptor N terminus has three or four transmembrane domains that bind ethylene. The GAF (for cGMP-specific phosphodiesterases, adenylyl cyclases, and FhlA) domain, which follows the transmembrane helices, mediates noncovalent receptor heterodimerization and may have a role in receptor cooperation (Gamble et al., 2002; O’Malley et al., 2005; Xie et al., 2006; Gao et al., 2008). The subfamily I receptors Ethylene Response1 (ETR1) and Ethylene Response Sensor1 (ERS1) have a conserved HK domain following the GAF domain. For subfamily II members ETR2, Ethylene Insensitive4 (EIN4), and ERS2, the HK domain is less conserved, and they lack most signature motifs essential for HK activity (Chang et al., 1993; Gamble et al., 1998; Hua et al., 1998; Qu and Schaller, 2004; Xie et al., 2006). Among the five receptors, ETR1, ETR2, and EIN4 have a receiver domain following the HK domain. The ETR1 HK domain may have a role in mediating the receptor signal to downstream components, and the HK activity facilitates the ethylene signaling (Clark et al., 1998; Huang et al., 2003; Hall et al., 2012). The receiver domain can dimerize and could involve receptor cooperation (Müller-Dieckmann et al., 1999). However, differential receptor cooperation occurs between the receiver domain-lacking ERS1 and the other ethylene receptors, which does not support the hypothesis that the domains involve receptor cooperation (Liu and Wen, 2012).Acting downstream of the ethylene receptors is Constitutive Triple Response1 (CTR1), a MEK kinase (mitogen-activated protein kinase kinase kinase) with Ser/Thr kinase activity, and the kinase domain locates at the C terminus. The CTR1 N terminus does not share sequence similarity to known domains and can physically interact with the ethylene-receptor HK domain (Clark et al., 1998; Huang et al., 2003). ctr1 mutants showing attenuated CTR1 kinase activity or the ETR1-CTR1 association exhibit various degrees of the constitutive ethylene-response phenotype. For example, the ctr1-1 and ctr1^btk mutations result from the D694E and E626K substitutions, respectively, in the CTR1 kinase domain, and ctr1-1 shows a stronger ethylene-response phenotype than ctr1^btk, with ctr1-1 having much weaker kinase activity than ctr1^btk (Kieber et al., 1993; Huang et al., 2003; Ikeda et al., 2009). The ctr1-8 mutation results in the G354E substitution that prevents the ETR1-CTR1 association, and the mutant exhibits a constitutive ethylene-response phenotype. Overexpression of the CTR1 N terminus CTR1^7-560, which is responsible for interaction with ethylene receptors, leads to constitutive ethylene responses, possibly by titrating out available ethylene receptors (Kieber et al., 1993; Huang et al., 2003). These studies suggest that CTR1 kinase activity and the interaction of CTR1 with the receptor HK domain may be important to the ethylene receptor signal output in suppressing constitutive ethylene responses.Although the ETR1-CTR1 interaction via the HK domain is essential to the ethylene receptor signal output, evidence suggests that the ETR1 receptor signal output can also be independent of the HK activity or domain. The etr1 ers1 loss-of-function mutant displays extreme growth defects. The etr1[HGG] mutation inactivates ETR1 HK activity, and expression of the getr1[HGG] transgene rescues the etr1 ers1 growth defects, which indicates a lack of association of ETR1 receptor signaling and its kinase activity (Wang et al., 2003). The dominant etr1-1 mutation results in the C65Y substitution and confers ethylene insensitivity (Chang et al., 1993), and the expression of the HK domain-lacking etr1^1-349 and ethylene-insensitive etr1-1^1-349 isoforms partially suppresses the growth defects of etr1 ers1-2. Loss-of-function mutations of subfamily II members do not affect etr1-1^1-349 functions. Therefore, etr1-1^1-349 predominantly cooperates with subfamily I receptors to mediate the ethylene receptor signal output (Xie et al., 2006). Biochemical and transformation studies showing that ethylene receptors can form heterodimers and that each receptor is a component of high-molecular-mass complexes explain how ethylene receptors may act cooperatively (Gao et al., 2008; Gao and Schaller, 2009; Chen et al., 2010).Reversion To Ethylene Sensitivity1 (RTE1), a Golgi/endoplasmic reticulum protein, was isolated from a suppressor screen of the dominant ethylene-insensitive etr1-2 mutation. The cross-species complementation of the rte1-2 loss-of-function mutation by the rice (Oryza sativa) RTE Homolog1 (OsRTH1) suggests a conserved mechanism that modulates the ethylene receptor signaling across higher plant species (Zhang et al., 2012). RTE1 and OsRTH1 overexpression led to ethylene insensitivity in wild-type Arabidopsis but not the etr1-7 loss-of-function mutant, and expression of etr1^1-349 restored ethylene insensitivity with RTE1 overexpression in etr1-7 (Resnick et al., 2006; Zhou et al., 2007; Zhang et al., 2010). Coimmunoprecipitation of epitope-tagged ETR1 and RTE1 and Trp fluorescence spectroscopy revealed the physical interaction of RTE1 and ETR1 (Zhou et al., 2007; Dong et al., 2008, 2010). Therefore, RTE1 may directly promote ETR1 receptor signal output through the ETR1 N terminus, but whether RTE1 has an essential role in ETR1 N-terminal signaling remains to be addressed.Currently, the biochemical nature of the ethylene receptor signal is unknown, and the underlying mechanisms of mediation of the ethylene receptor signal output remain uninvestigated. Genetic and biochemical studies suggest that activation of CTR1 by ethylene receptors may suppress constitutive ethylene responses; upon ethylene binding, the receptors are converted to an inactive state and fail to activate CTR1, and the suppression of ethylene responses by CTR1 is alleviated (Hua and Meyerowitz, 1998; Klee, 2004; Wang et al., 2006; Hall et al., 2007). However, this model does not address how the ETR1 N terminus, which does not have the CTR1-interacting site, mediates the receptor signal to suppress constitutive ethylene responses. The receptor signal of the truncated etr1 isoforms may be mediated by other full-length ethylene receptors and then activate CTR1; alternatively, the ETR1 N-terminal signal may be mediated by a pathway independent of CTR1 (Gamble et al., 2002; Qu and Schaller, 2004; Xie et al., 2006). Results showing that mutants defective in multiple ethylene receptor genes exhibit a more severe ethylene-response phenotype than ctr1 and that ctr1 mutants are responsive to ethylene support the presence of a CTR1-independent pathway (Hua and Meyerowitz, 1998; Cancel and Larsen, 2002; Huang et al., 2003; Liu et al., 2010).In this study, we investigated whether mediation of ETR1 N-terminal signaling is independent of CTR1 and whether RTE1 is essential to the CTR1-independent ETR1 N-terminal signaling. The ETR1 N-terminal signaling was not mediated via other full-length ethylene receptors, but the signal of full-length ethylene receptors could be mediated by the ETR1 N terminus independent of CTR1. The ETR1 C terminus may inhibit ETR1 N-terminal signaling, whereby deletion of the C terminus facilitates N-terminal signaling. We propose a model for the possible modulation of ETR1 receptor signaling.     

A new CULLIN 1 mutant has altered responses to hormones and light in Arabidopsis

Regulated protein degradation contributes to plant development by mediating signaling events in many hormone, light, and developmental pathways. Ubiquitin ligases recognize and ubiquitinate target proteins for subsequent degradation by the 26S proteasome. The multisubunit SCF is the best-studied class of ubiquitin ligases in Arabidopsis (Arabidopsis thaliana). However, the extent of SCF participation in signaling networks is unclear. SCFs are composed of four subunits: CULLIN 1 (CUL1), ASK, RBX1, and an F-box protein. Null mutations in CUL1 are embryo lethal, limiting insight into the role of CUL1 and SCFs in later stages of development. Here, we describe a viable and fertile weak allele of CUL1, called cul1-6. cul1-6 plants have defects in seedling and adult morphology. In addition to reduced auxin sensitivity, cul1-6 seedlings are hyposensitive to ethylene, red, and blue light conditions. An analysis of protein interactions with the cul1-6 gene product suggests that both RUB (related to ubiquitin) modification and interaction with the SCF regulatory protein CAND1 (cullin associated and neddylation dissociated) are disrupted. These findings suggest that the morphological defects observed in cul1-6 plants are caused by defective SCF complex formation. Characterization of weak cul1 mutants provides insight into the role of SCFs throughout plant growth and development.     

GDSL lipase 1 regulates ethylene signaling and ethylene-associated systemic immunity in Arabidopsis

Arabidopsis GDSL lipase 1 (GLIP1) has been shown to modulate systemic immunity through the regulation of ethylene signaling components. Here we demonstrate that the constitutive triple response mutant ctr1-1 requires GLIP1 for the ethylene response, gene expression, and pathogen resistance. The glip1-1 mutant was defective in induced resistance following primary inoculation of necrotrophic pathogens, whereas GLIP1-overexpressing plants showed resistance to multiple pathogens. Necrotrophic infection triggered the downregulation of EIN3 and the activation of ERF1 and SID2 in a GLIP1-dependent manner. These results suggest that GLIP1 positively and negatively regulates ethylene signaling, resulting in an ethylene-associated, necrotroph-induced immune response.     

Coronin-1A links cytoskeleton dynamics to TCR alpha beta-induced cell signaling

Actin polymerization plays a critical role in activated T lymphocytes both in regulating T cell receptor (TCR)-induced immunological synapse (IS) formation and signaling. Using gene targeting, we demonstrate that the hematopoietic specific, actin- and Arp2/3 complex-binding protein coronin-1A contributes to both processes. Coronin-1A-deficient mice specifically showed alterations in terminal development and the survival of alpha beta T cells, together with defects in cell activation and cytokine production following TCR triggering. The mutant T cells further displayed excessive accumulation yet reduced dynamics of F-actin and the WASP-Arp2/3 machinery at the IS, correlating with extended cell-cell contact. Cell signaling was also affected with the basal activation of the stress kinases sAPK/JNK1/2; and deficits in TCR-induced Ca2+ influx and phosphorylation and degradation of the inhibitor of NF-kappaB (I kappa B). Coronin-1A therefore links cytoskeleton plasticity with the functioning of discrete TCR signaling components. This function may be required to adjust TCR responses to selecting ligands accounting in part for the homeostasis defect that impacts alpha beta T cells in coronin-1A deficient mice, with the exclusion of other lympho/hematopoietic lineages.     

The involvement of intracellular glutathione in methyl jasmonate signaling in Arabidopsis guard cells

We examined the involvement of intracellular glutathione (GSH) in methyl jasmonate (MeJA) signaling. The chlorina1-1 (ch1-1) mutation decreased GSH in guard cells and narrowed the stomatal aperture. GSH monoethyl ester increased intracellular GSH, diminishing this phenotype. GSH did not affect MeJA-induced reactive oxygen species production or cytosolic Ca(2+) oscillation, suggesting that GSH modulates MeJA signaling downstream of production and oscillation.     

The altered responses of a new mutant long life span 1 to cytokinin in Arabidopsis thaliana

Cytokinins are a class of essential plant hormones regulating plant growth and development.Although the two-component phosphorelay pathway of cytokinin has been well characterized,the intact cytokinin responses regulation picture still needs to be fully depicted.Here we report a new mutant,long life span 1(lls1),which displays dwarf stature,curled leaves,numerous axillary branches and nearly 5-month life span.Exogenous cytokinin could not recover the phenotypes of the mutant.Moreover,mutation in lls1 suppressed the cytokinin-responsive phenotypes,including root and hypocotyl growth inhibition,anthocyanin accumulation,metaxylem promotion in primary root development.The induction of cytokinin-responsive genes,ARR5,AHP5,and CKX3,was also suppressed in lls1.According to quantitative RT-PCR(qRT-PCR) and microarray results,the basal expression of positive factors AHP5,ARR1,and ARR10 were down-regulated,while the negative factors ARR4 and ARR5 were up-regulated.Our results suggested that LLS1 gene might be involved in the regulation of cytokinin signaling.It was mapped to chromosome 4 where no other cytokinin relevant gene has been reported.     

Constitutive activation of jasmonate signaling in an Arabidopsis mutant correlates with enhanced resistance to Erysiphe cichoracearum,Pseudomonas syringae,and Myzus persicae

In Arabidopsis spp., the jasmonate (JA) response pathway generally is required for defenses against necrotrophic pathogens and chewing insects, while the salicylic acid (SA) response pathway is generally required for specific, resistance (R) gene-mediated defenses against both biotrophic and necrotrophic pathogens. For example, SA-dependent defenses are required for resistance to the biotrophic fungal pathogen Erysiphe cichoracearum UCSC1 and the bacterial pathogen Pseudomonas syringae pv. maculicola, and also are expressed during response to the green peach aphid Myzus persicae. However, recent evidence indicates that the expression of JA-dependent defenses also may confer resistance to E. cichoracearum. To confirm and to extend this observation, we have compared the disease and pest resistance of wild-type Arabidopsis plants with that of the mutants coil, which is insensitive to JA, and cev1, which has constitutive JA signaling. Measurements of the colonization of these plants by E. cichoracearum, P. syringae pv. maculicola, and M. persicae indicated that activation of the JA signal pathway enhanced resistance, and was associated with the activation of JA-dependent defense genes and the suppression of SA-dependent defense genes. We conclude that JA and SA induce alternative defense pathways that can confer resistance to the same pathogens and pests.