Unique and unifying themes in the mechanisms regulating the expression of the arabidopsis thaliana PR-1 and Solanum tuberosum PR-10a inducible defense genes

Arabidopsis is a model plant used to study disease resistance; Solanum tuberosum or potato is a crop species. Both plants possess inducible defense mechanisms that are deployed upon recognition of pathogen invasion. Transcriptional reprogramming is crucial to the activation of defense responses. The Pathogenesis-Related (PR) genes are activated in these defense programs. Expression of Arabidopsis PR-l and potato PR-10a serve as markers for the deployment of defense responses in these plants. PR-l expression indicates induction of systemic acquired resistance (SAR). Activation of SAR requires accumulation of salicylic acid (SA), in addition to the interaction of the non-expressor of pathogenesis-related genes I (NPRI), with the TGA transcription factors. The PR-10a is activated in response to pathogen invasion, wounding and elicitor treatment. PR-10a induction requires recruitment of the Whirly I (Whyl) activator to the promoter. This locus is also negatively regulated by the silencer element binding factor (SEBF). We established that both the PR-l and PR-10a are occupied by repressors under non-inducing conditions. TGA2 was found to be a constitutive resident and repressor of PR-l, which mediates repression by forming an oligomeric complex on the promoter. The DNA-binding activity of this oligomer required the TGA2 N-terminus (NT). Under resting conditions we determined that the PR-10a is bound by a repressosome containing SEBF and curiously the activator Pto interacting protein 4 (Pti4). In the context of this repressosome, SEBF is responsible for PR-10a binding, yet rWe also showed that PR-l and PR-10a are activated by different means. In PR-l activation the NPRI NT domain alleviates TGA2-mediated repression by interacting with the TGA2 NT. TGA2 remains at the PR-l but adopts a dimeric conformation and forms an enhanceosome with NPRl. In contrast, the PR-10a is activated by evicting the repressosome and recruiting Why! to the promoter. These results advance our understanding of the mechanisms regulating PR-l and PR-10a expression under resting and inducing conditions. This study also revealed that the means of regulation for related genes can differ greatly between model and crop s

The structure of arabidopsis NPR1 : its function as a salicylic acid receptor and a metal-binding protein

The Arabidopsis NPRI protein regulates systemic acquired resistance dependent on salicylic acid. Analyses by plant two-hybrid analysis in vivo and pull-down assays in vitro showed that the BTB/POZ domain of NPRI at the N-terminus serves as an autoinhibitory domain to negate the function of the transactivation domain at the C-terminus through direct binding of these two domains. I t was also shown that the binding of the BTB/POZ domain to the C-terminus of NPRI was abolished by SA treatment, suggesting that SA could interfere directly with this binding. By gel filtration, it was demonstrated that SA affects the conformation of full-length NPRl , confirming the role of NPRI as an SA receptor. Gel filtration analysis also indicated that NPRI could be converted from an oligomer to a dimer with SA treatment. Furthermore, one N-terminal deletion ~513 has been shown to act as a metal-binding protein and its two Cys-521 and Cys-529 are important for binding to Ni 2 + by pull-down assays.

Regulation of Systemic Acquired Resistance through the Interaction of Arabidopsis thaliana Transcription factors TGAI and TGA2 with NPRI

Arabidopsis thaliana is an established model plant system for studying plantpathogen interactions. The knowledge garnered from examining the mechanism of induced disease resistance in this model system can be applied to eliminate the cost and danger associated with current means of crop protection. A specific defense pathway, known as systemic acquired resistance (SAR), involves whole plant protection from a wide variety of bacterial, viral and fungal pathogens and remains induced weeks to months after being triggered. The ability of Arabidopsis to mount SAR depends on the accumulation of salicylic acid (SA), the NPRI (non-expressor of pathogenesis related gene 1) protein and the expression of a subset of pathogenesis related (PR) genes. NPRI exerts its effect in this pathway through interaction with a closely related class of bZIP transcription factors known as TGA factors, which are named for their recognition of the cognate DNA motif TGACG. We have discovered that one of these transcription factors, TGA2, behaves as a repressor in unchallenged Arabidopsis and acts to repress NPRI-dependent activation of PRJ. TGA1, which bears moderate sequence similarity to TGA2, acts as a transcriptional activator in unchallenged Arabidopsis, however the significance of this activity is J unclear. Once SAR has been induced, TGAI and TGA2 interact with NPRI to form complexes that are capable of activating transcription. Curiously, although TGAI is capable of transactivating, the ability of the TGAI-NPRI complex to activate transcription results from a novel transactivation domain in NPRI. This transactivation domain, which depends on the oxidation of cysteines 521 and 529, is also responsible for the transactivation ability of the TGA2-NPRI complex. Although the exact mechanism preventing TGA2-NPRI interaction in unchallenged Arabidopsis is unclear, the regulation of TGAI-NPRI interaction is based on the redox status of cysteines 260 and 266 in TGAl. We determined that a glutaredoxin, which is an enzyme capable of regulating a protein's redox status, interacts with the reduced form of TGAI and this interaction results .in the glutathionylation of TGAI and a loss of interaction with NPRl. Taken together, these results expand our understanding of how TGA transcription factors and NPRI behave to regulate events and gene expression during SAR. Furthermore, the regulation of the behavior of both TGAI and NPRI by their redox status and the involvement of a glutaredoxin in modulating TGAI-NPRI interaction suggests the redox regulation of proteins is a general mechanism implemented in SAR.

The Molecular Consequences of CK2-mediated Phosphorylation of the TGA2 Transcription Factor within Systemic Acquired Resistance of Arabidopsis thaliana

During infection, the model plant Arabidopsis thaliana is capable of activating long lasting defence responses both in tissue directly affected by the pathogen and in more distal tissue. Systemic acquired resistance (SAR) is a type of systemic defence response deployed against biotrophic pathogens resulting in altered plant gene expression and production of antimicrobial compounds. One such gene involved in plant defence is called pathogenesis-related 1 (PR1) and is under the control of several protein regulators. TGA II-clade transcription factors (namely TGA2) repress PR1 activity prior to infection by forming large oligomeric complexes effectively blocking gene transcription. After pathogen detection, these complexes are dispersed by a mechanism unknown until now and free TGA molecules interact with the non-expressor of pathogenesis-related gene 1 (NPR1) protein forming an activating complex enabling PR1 transcription. This study elucidates the TGA2 dissociation mechanism by introducing protein kinase CK2 into this process. This enzyme efficiently phosphorylates TGA2 resulting in two crucial events. Firstly, the DNA-binding ability of this transcription factor is completely abolished explaining how the large TGA2 complexes are quickly evicted from the PR1 promoter. Secondly, a portion of TGA2 molecules dissociate from the complexes after phosphorylation which likely makes them available for the formation of the TGA2-NPR1 activating complex. We also show that phosphorylation of a multiserine motif found within TGA2’s N terminus is responsible for the change of affinity to DNA, while modification of a single threonine in the leucine zipper domain seems to be responsible for deoligomerization. Despite the substantial changes caused by phosphorylation, TGA2 is still capable of interacting with NPR1 and these proteins together form a complex on DNA promoting PR1 transcription. Therefore, we propose a change in the current model of how PR1 is regulated by adding CK2 which targets TGA2 displacing it’s complexes from the promoter and providing solitary TGA2 molecules for assembly of the activating complex. Amino acid sequences of regions targeted by CK2 in Arabidopsis TGA2 are similar to those found in TGA2 homologs in rice and tobacco. Therefore, the molecular mechanism that we have identified may be conserved among various plants, including important crop species, adding to the significance of our findings.

The Arabidopsis NPR1 Protein Is a Receptor for the Plant Defense Hormone Salicylic Acid

Systemic Acquired Resistance (SAR) is a type of plant systemic resistance occurring against a broad spectrum of pathogens. It can be activated in response to pathogen infection in the model plant Arabidopsis thaliana and many agriculturally important crops. Upon SAR activation, the infected plant undergoes transcriptional reprogramming, marked by the induction of a battery of defense genes, including Pathogenesis-related (PR) genes. Activation of the PR-1 gene serves as a molecular marker for the deployment of SAR. The accumulation of a defense hormone, salicylic acid (SA) is crucial for the infected plant to mount SAR. Increased cellular levels of SA lead to the downstream activation of the PR-1 gene, triggered by the combined action of the Non-expressor of Pathogenesis-related Gene 1 (NPR1) protein and the TGA II-clade transcription factor (namely TGA2). Despite the importance of SA, its receptor has remained elusive for decades. In this study, we demonstrated that in Arabidopsis the NPR1 protein is a receptor for SA. SA physically binds to the C-terminal transactivation domain of NPR1. The two cysteines (Cys521 and Cys529), which are important for NPR1’s coactivator function, within this transactivation domain are critical for the binding of SA to NPR1. The interaction between SA and NPR1 requires a transition metal, copper, as a cofactor. Our results also suggested a conformational change in NPR1 upon SA binding, releasing the C-terminal transactivation domain from the N-terminal autoinhibitory BTB/POZ domain. These results advance our understanding of the plant immune function, specifically related to the molecular mechanisms underlying SAR. The discovery of NPR1 as a SA receptor enables future chemical screening for small molecules that activate plant immune responses through their interaction with NPR1 or NPR1-like proteins in commercially important plants. This will help in identifying the next generation of non-biocidal pesticides.

Novel glutathione disulfide transferase function of CC-glutaredoxins involved in disease resistance and flower development

Glutaredoxins are oxidoreductases capable of reducing protein disulfide bridges and glutathione mixed disulfides through the process of deglutathionylation and glutathionylation. Lately, redox-mediated modifications of functional cysteine residues of TGA1 and TGA8 transcription factors have been postulated. Namely, GRX480 and ROXY1 glutaredoxins have been previously shown to interact with TGA proteins and have been suggested to regulate redox state of these proteins. TGA1, together with TGA2, is involved in systemic acquired resistance (SAR) establishment in the plant Arabidopsis thaliana through PR1 (Pathogenesis related 1) gene activation. They both form an enhanceosome complex with the NPR1 protein (non-expressor of pathogenesis related gene 1) which leads to PR1 transcription. Although TGA1 is capable of activating PR1 transcription, the ability of the TGA1 NPR1 enhanceosome complex to assembly is based on the redox status of TGA1. We identified GRX480 as a glutathionylating enzyme that catalyzes the TGA1 glutathione disulfide transferase reaction with a Km of around 20μM GSSG (oxidized glutathione). Out of four cysteine residues found within TGA1, C172 and C266 were found to be glutathionylated by this enzyme. We also confirmed TGA1 glutathionylation in vivo and showed that this modification takes place while TGA1 is associated with the PR1 promoter enzymatically via GRX480. Furthermore, we show that glutathionylation via GRX480 abolishes TGA1's interaction with NPR1 and consequently prevents the TGA1-NPR1 transcription activation of PR1. When glutathionylated, TGA1 is recruited to the PR1 promoter and acts as a repressor. Therefore, glutathionylation is a mechanism that prevents TGA1 NPR1 interaction, allowing TGA1 to function as a repressor of PR1 transcription. Surprisingly, GRX480 was not able to deglutathionylate proteins demonstrating the irreversible nature of the reaction. Moreover, we demonstrate that other members of CC-class glutaredoxins, namely ROXY1 and ROXY2, can also catalyze protein glutathionylation. The TGA8 protein was previously shown to interact with NPR1 analogs, BOP1 and BOP2 proteins. However, unlike the case of TGA1 NPR1 interaction, here we demonstrate that TGA8-BOP1 interaction is not redox regulated and that TGA8 glutathionylation by ROXY1 and ROXY2 enzymes does not abolish this interaction in vitro. However, TGA8 glutathionylation results in TGA8 oligomer disassembly into smaller complexes and monomers. Our results suggest that CC-Grxs are unable to reduce mixed disulfides, instead they efficiently catalyze the opposite reaction which distinguishes them from traditional glutaredoxins. Therefore, they should not be classified as glutaredoxins but as protein glutathione disulfide transferases.