Günter Brader

The WRKY70 Transcription Factor: A Node of Convergence for Jasmonate-Mediated and Salicylate-Mediated Signals in Plant Defense

Cross talk between salicylic acid (SA)– and jasmonic acid (JA)–dependent defense signaling has been well documented in plants, but how this cross talk is executed and the components involved remain to be elucidated. We demonstrate that the plant-specific transcription factor WRKY70 is a common component in SA- and JA-mediated signal pathways. Expression of WRKY70 is activated by SA and repressed by JA. The early induction of WRKY70 by SA is NPR1-independent, but functional NPR1 is required for full-scale induction. Epistasis analysis suggested that WRKY70 is downstream of NPR1 in an SA-dependent signal pathway. Modulation of WRKY70 transcript levels by constitutive overexpression increases resistance to virulent pathogens and results in constitutive expression of SA-induced pathogenesis-related genes. Conversely, antisense suppression of WRKY70 activates JA-responsive/COI1-dependent genes. The effect of WRKY70 is not caused by subsequent changes in SA or JA levels. We suggest that WRKY70 acts as an activator of SA-induced genes and a repressor of JA-responsive genes, integrating signals from these mutually antagonistic pathways.

Chlorophyllase 1, a Damage Control Enzyme, Affects the Balance between Defense Pathways in Plants

Accumulation of reactive oxygen species (ROS) is central to plant response to several pathogens. One of the sources of ROS is the chloroplast because of the photoactive nature of the chlorophylls. Chlorophyllase 1 (encoded by AtCLH1) of Arabidopsis thaliana is quickly induced after tissue damage (e.g., caused by the bacterial necrotroph Erwinia carotovora or the necrotrophic fungus Alternaria brassicicola). RNA interference silencing of AtCLH1 resulted in failure to degrade free chlorophyll after tissue damage and in resistance to E. carotovora. Both inoculation with E. carotovora and exposure to high light caused elevated accumulation of hydrogen peroxide in AtCLH1 silenced plants. This was accompanied by expression of marker genes for systemic acquired resistance and induction of antioxidant defenses. Interestingly, downregulation of AtCLH1 resulted in increased susceptibility to A. brassicicola, resistance to which requires jasmonate signaling. We propose that AtCLH1 is involved in plant damage control and can modulate the balance between different plant defense pathways.

The Arabidopsis Mitogen-Activated Protein Kinase Kinase MKK3 Is Upstream of Group C Mitogen-Activated Protein Kinases and Participates in Pathogen Signaling

Although the Arabidopsis thaliana genome contains genes encoding 20 mitogen-activated protein kinases (MAPKs) and 10 MAPK kinases (MAPKKs), most of them are still functionally uncharacterized. In this work, we analyzed the function of the group B MAPK kinase, MKK3. Transgenic ProMKK3:GUS lines showed basal expression in vascular tissues that was strongly induced by Pseudomonas syringae pv tomato strain DC3000 (Pst DC3000) infection but not by abiotic stresses. The growth of virulent Pst DC3000 was increased in mkk3 knockout plants and decreased in MKK3-overexpressing plants. Moreover, MKK3 overexpression lines showed increased expression of several PR genes. By yeast two-hybrid analysis, coimmunoprecipitation, and protein kinase assays, MKK3 was revealed to be an upstream activator of the group C MAPKs MPK1, MPK2, MPK7, and MPK14. Flagellin-derived flg22 peptide strongly activated MPK6 but resulted in poor activation of MPK7. By contrast, MPK6 and MPK7 were both activated by H2O2, but only MPK7 activation was enhanced by MKK3. In agreement with the notion that MKK3 regulates the expression of PR genes, ProPR1:GUS expression was strongly enhanced by coexpression of MKK3-MPK7. Our results reveal that the MKK3 pathway plays a role in pathogen defense and further underscore the importance and complexity of MAPK signaling in plant stress responses.

Metabolic potential of endophytic bacteria

Graphical abstract

Early Responsive to Dehydration 15, a negative regulator of abscisic acid responses in Arabidopsis

EARLY RESPONSIVE TO DEHYDRATION 15 (ERD15) is rapidly induced in response to various abiotic and biotic stress stimuli in Arabidopsis (Arabidopsis thaliana). Modulation of ERD15 levels by overexpression or RNAi silencing altered the responsiveness of the transgenic plants to the phytohormone abscisic acid (ABA). Overexpression of ERD15 reduced the ABA sensitivity of Arabidopsis manifested in decreased drought tolerance and in impaired ability of the plants to increase their freezing tolerance in response to this hormone. In contrast, RNAi silencing of ERD15 resulted in plants that were hypersensitive to ABA and showed improved tolerance to both drought and freezing, as well as impaired seed germination in the presence of ABA. The modulation of ERD15 levels not only affected abiotic stress tolerance but also disease resistance: ERD15 overexpression plants showed improved resistance to the bacterial necrotroph Erwinia carotovora subsp. carotovora accompanied with enhanced induction of marker genes for systemic acquired resistance. We propose that ERD15 is a novel mediator of stress-related ABA signaling in Arabidopsis.

Chloroplast NADPH-Thioredoxin Reductase Interacts with Photoperiodic Development in Arabidopsis

Chloroplast NADPH-thioredoxin reductase (NTRC) belongs to the thioredoxin systems that control crucial metabolic and regulatory pathways in plants. Here, by characterization of T-DNA insertion lines of NTRC gene, we uncover a novel connection between chloroplast thiol redox regulation and the control of photoperiodic growth in Arabidopsis (Arabidopsis thaliana). Transcript and metabolite profiling revealed severe developmental and metabolic defects in ntrc plants grown under a short 8-h light period. Besides reduced chlorophyll and anthocyanin contents, ntrc plants showed alterations in the levels of amino acids and auxin. Furthermore, a low carbon assimilation rate of ntrc leaves was associated with enhanced transpiration and photorespiration. All of these characteristics of ntrc were less severe when plants were grown under a long 16-h photoperiod. Transcript profiling revealed that the mutant phenotypes of ntrc were accompanied by differential expression of genes involved in stomatal development, chlorophyll biosynthesis, chloroplast biogenesis, and circadian clock-linked light perception systems in ntrc plants. We propose that NTRC regulates several key processes, including chlorophyll biosynthesis and the shikimate pathway, in chloroplasts. In the absence of NTRC, imbalanced metabolic activities presumably modulate the chloroplast retrograde signals, leading to altered expression of nuclear genes and, ultimately, to the formation of the pleiotrophic phenotypes in ntrc mutant plants.

The MAP kinase kinase MKK2 affects disease resistance in Arabidopsis.

The Arabidopsis mitogen-activated protein kinase (MAPK) kinase 2 (MKK2) was shown to mediate cold and salt stress responses through activation of the two MAP kinases MPK4 and MPK6. Transcriptome analysis of plants expressing constitutively active MKK2 (MKK2-EE plants) showed altered expression of genes induced by abiotic stresses but also a significant number of genes involved in defense responses. Both MPK4 and MPK6 became rapidly activated upon Pseudomonas syringae pv. tomato DC3000 infection and MKK2-EE plants showed enhanced levels of MPK4 activation. Although MKK2-EE plants shared enhanced expression of genes encoding enzymes of ethylene (ET) and jasmonic acid (JA) synthesis, ET, JA, and salicylic acid (SA) levels did not differ dramatically from those of wild-type or mkk2-null plants under ambient growth conditions. Upon P. syringae pv. tomato DC3000 infection, however, MKK2-EE plants showed reduced increases of JA and SA levels. These results indicate that MKK2 is involved in regulating hormone levels in response to pathogens. MKK2-EE plants were more resistant to infection by P. syringae pv. tomato DC3000 and Erwinia carotovora subsp. carotovora, but showed enhanced sensitivity to the fungal necrotroph Alternaria brassicicola. Our data indicate that MKK2 plays a role in abiotic stress tolerance and plant disease resistance.

Natural variation in ozone sensitivity among Arabidopsis thaliana accessions and its relation to stomatal conductance

Genetic variation between naturally occurring populations provides a unique source to unravel the complex mechanisms of stress tolerance. Here, we have analysed O(3) sensitivity of 93 natural Arabidopsis thaliana accessions together with five O(3)-sensitive mutants to acute O(3) exposure. The variation in O(3) sensitivity among the natural accessions was much higher than among the O(3)-sensitive mutants and corresponding wild types. A subset of nine accessions with major variation in their O(3) responses was studied in more detail. Among the traits assayed, stomatal conductance (g(st)) was an important factor determining O(3) sensitivity of the selected accessions. The most O(3)-sensitive accession, Cvi-0, had constitutively high g(st), leading to high initial O(3) uptake rate and dose received during the first 30 min of exposure. Analyzing O(3)-induced changes in stress hormone concentrations indicated that jasmonate (JA) concentration was also positively correlated with leaf damage. Quantitative trait loci (QTL) mapping in a Col-0 x Cvi-0 recombinant inbred line (RIL) population identified three QTLs for O(3) sensitivity, and one for high water loss of Cvi-0. The major O(3) QTL mapped to the same position as the water loss QTL further supporting the role of stomata in regulating O(3) entry and damage.

Erwinia carotovora subsp. Carotovora and Erwinia-derived elicitors HrpN and peha trigger distinct but interacting defense responses and cell death in Arabidopsis

We have used an hrp-positive strain of the soft rot pathogen Erwinia carotovora subsp. carotovora to elucidate plant responses to this bacterial necrotroph. Purified virulence determinants, harpin (HrpN) and polygalacturonase (PehA), were used as tools to facilitate this analysis. We show that HrpN elicits lesion formation in Arabidopsis and tobacco and triggers systemic resistance in Arabidopsis. Establishment of resistance is accompanied by the expression of salicylic acid (SA)-dependent, but also jasmonate/ethylene (JA/ET)-dependent, marker genes PR1 and PDF1.2, respectively, suggesting that both SA-dependent and JA/ET-dependent defense pathways are activated. Use of pathway-specific mutants and transgenic NahG plants show that both pathways are required for the induction of resistance. Arabidopsis plants treated simultaneously with both elictors PehA, known to trigger only JA/ET-dependent defense signaling, and HrpN react with accelerated and enhanced induction of the marker genes PR1 and PDF1.2 both locally and systemically. This mutual amplification of defense gene expression involves both SA-dependent and JA/ET-dependent defense signaling. The two elicitors produced by E. carotovora subsp. carotovora also cooperate in triggering increased production of superoxide and lesion formation.

Cooperation of two distinct ExpR regulators controls quorum sensing specificity and virulence in the plant pathogen Erwinia carotovora

Quorum sensing, the population density‐dependent regulation mediated by N‐acylhomoserine lactones (AHSL), is essential for the control of virulence in the plant pathogen Erwinia carotovora ssp. carotovora (Ecc). In Erwinia carotovora ssp. the AHSL signal with an acyl chain of either 6 or 8 carbons is generated by an AHSL synthase, the expI gene product. This work demonstrates that the AHSL receptor, ExpR1, of Ecc strain SCC3193 has strict specificity for the cognate AHSL 3‐oxo‐C8‐HSL. We have also identified a second AHSL receptor (ExpR2) and demonstrate a novel quorum sensing mechanism, where ExpR2 acts synergistically with the previously described ExpR1 to repress virulence gene expression in Ecc. We show that this repression is released by addition of AHSLs and appears to be largely mediated via the negative regulator RsmA. Additionally we show that ExpR2 has the novel property to sense AHSLs with different acyl chain lengths. The expI expR1 double mutant is able to act in response to a number of different AHSLs, while the expI expR2 double mutant can only respond to the cognate signal of Ecc strain SCC3193. These results suggest that Ecc is able to react both to the cognate AHSL signal and the signals produced by other bacterial species.