The outcomes of concentration-specific interactions between salicylate and jasmonate signaling include synergy, antagonism, and oxidative stress leading to cell death

Pathogen-induced Arabidopsis WRKY7 is a transcriptional repressor and enhances plant susceptibility to Pseudomonas syringae

The Arabidopsis (Arabidopsis thaliana) WRKY7 gene is induced by pathogen infection and salicylic acid (SA) treatment and may therefore play a role in plant defense responses. Here, we show that WRKY7 is localized in the nucleus, recognizes DNA molecules with the W-box (TTGAC) elements, and functions as a transcriptional repressor in plant cells. To study its biological functions directly, we have characterized both loss-of-function T-DNA insertion and RNAi mutants and gain-of-function transgenic overexpression plants for WRKY7 in Arabidopsis. The T-DNA insertion and RNAi mutant plants displayed enhanced resistance to a virulent strain of the bacterial pathogen Pseudomonas syringae as measured by significant decrease in both bacterial growth and symptom development as compared to those in wild-type plants. The enhanced resistance in the loss-of-function mutants was associated with increased induction of SA-regulated Pathogenesis-Related 1 (PR1) by the bacterial pathogen. Transgenic plants that constitutively overexpress WRKY7 have altered leaf growth and morphology strikingly similar to those observed in the previously isolated eds8 mutant plants. Like eds8 mutant plants, WRKY7-overexpressing plants supported more growth of P. syringae and developed more severe disease symptoms than wild-type plants. The enhanced susceptibility of both the WRKY7-overexpressing plants and the eds8 mutant correlated with reduced expression of defense-related genes, including PR1, but significantly increased accumulation of SA after pathogen infection, probably due to reduced negative feedback of SA synthesis. Thus, pathogen-induced WRKY7 transcription factor play a negative role in defense responses to P. syringae.

Herbivore-induced resistance against microbial pathogens in Arabidopsis

Caterpillars of the herbivore Pieris rapae stimulate the production of jasmonic acid (JA) and ethylene (ET) in Arabidopsis (Arabidopsis thaliana) and trigger a defense response that affects insect performance on systemic tissues. To investigate the spectrum of effectiveness of P. rapae-induced resistance, we examined the level of resistance against different pathogens. Although the necrotrophic fungus Alternaria brassicicola is sensitive to JA-dependent defenses, herbivore-induced resistance was not effective against this pathogen. By contrast, caterpillar feeding significantly reduced disease caused by the bacterial pathogens Pseudomonas syringae pv tomato and Xanthomonas campestris pv armoraciae. However, this effect was apparent only locally in caterpillar-damaged tissue. Arabidopsis mutants jar1, coi1, ein2, sid2, eds5, and npr1 showed wild-type levels of P. rapae-induced protection against P. syringae pv tomato, suggesting that this local, herbivore-induced defense response does not depend exclusively on either JA, ET, or salicylic acid (SA). Resistance against the biotroph Turnip crinkle virus (TCV) requires SA, but not JA and ET. Nevertheless, herbivore feeding strongly affected TCV multiplication and TCV lesion formation, also in systemic tissues. Wounding alone was not effective, but application of P. rapae regurgitate onto the wounds induced a similar level of protection. Analysis of SA-induced PATHOGENESIS RELATED-1 (PR-1) expression revealed that P. rapae grazing primed Arabidopsis leaves for augmented expression of SA-dependent defenses. Pharmacological experiments showed that ET acts synergistically on SA-induced PR-1, suggesting that the increased production of ET upon herbivore feeding sensitizes the tissue to respond faster to SA, thereby contributing to an enhanced defensive capacity toward pathogens, such as TCV, that trigger SA-dependent defenses upon infection.

Early responses in the Arabidopsis-Verticillium longisporum pathosystem are dependent on NDR1, JA- and ET-associated signals via cytosolic NPR1 and RFO1

The responses of Arabidopsis accessions and characterized genotypes were used to explore components in the early defense responses to the soilborne fungus Verticillium longisporum. V. longisporum susceptibility was found to be a complex trait, in which different disease phenotypes, such as stunting, altered flowering time, weight loss, and chlorosis were perceived differently across genotypes. A Bay-0 x Shahdara recombinant inbred line population was used to identify two loci on chromosomes 2 and 3 of Bay-0 origin that caused enhanced chlorosis after V. longisporum challenge. Furthermore, the observation that a mutation in RFO1 in Col-0 resulted in susceptibility whereas the natural rfo1 allele in Ty-0 showed a high degree of resistance to the pathogen supports the hypothesis that several resistance quantitative trait loci reside among Arabidopsis accessions. Analysis of mutants impaired in known pathogen response pathways revealed an enhanced susceptibility in ein2-1, ein4-1, ein6-1, esa1-1, and pad1-1, but not in other jasmonic acid (JA)-, ethylene (ET)-, or camalexin-deficient mutants, suggesting that V. longisporum resistance is regulated via a hitherto unknown JA- and ET-associated pathway. Pretreatments with the ET precursor 1-aminocyclo-propane-1-carboxylic acid (ACC) or methyl jasmonate (MeJA) caused enhanced resistance to V. longisporum. Mutants in the salicylic acid (SA) pathway (eds1-1, NahG, npr1-3, pad4-1, and sid2-1) did not show enhanced susceptibility to V. longisporum. In contrast, the more severe npr1-1 allele displayed enhanced V. longisporum susceptibility and decreased responses to ACC or MeJA pretreatments. This shows that cytosolic NPR1, in addition to SA responses, is required for JA- and ET-mediated V. longisporum resistance. Expression of the SA-dependent PR-1 and PR-2 and the ET-dependent PR-4 were increased 7 days postinoculation with V. longisporum. This indicates increased levels of SA and ET in response to V. longisporum inoculation. The R-gene signaling mutant ndr1-1 was found to be susceptible to V. longisporum, which could be complemented by ACC or MeJA pretreatments, in contrast to the rfo1 T-DNA mutant, which remained susceptible, suggesting that RFO1 (Fusarium oxysporum resistance) and NDR1 (nonrace specific disease resistance 1) activate two distinct signaling pathways for V. longisporum resistance.

Systemic immunity

Systemic acquired resistance (SAR) provides enhanced, long-lasting systemic immunity to secondary infection by a range of biotrophic, hemibiotrophic and necrotrophic pathogens that have diverse modes of infection. Considerable effort has focussed on the conserved central positive regulator of SAR, NON-EXPRESSOR OF PATHOGENESIS-RELATED1 (NPR1), and its control by changes in cellular redox potential. Recently, genetic and genomic approaches have highlighted a critical role for nucleocytoplasmic communication and protein secretion in establishing effective systemic immunity. Identification of the mobile signals and the mechanisms by which they are perceived in distal tissues remains challenging, but emerging evidence suggests that signal translocation uses lipid-derived (possibly jasmonate-based) signals and lipid-binding chaperones. Furthermore, the demonstration that autophagy interdicts and inactivates a systemic cell death signal adds further complexity to elucidating how mobile signals are decoded and transduced for effective immunity.

Identification of a cowpea gamma-thionin with bactericidal activity

Antimicrobial peptides are an abundant group of proteinaceous compounds widely produced in the plant kingdom. Among them, the gamma-thionin family, also known as plant defensins, represents one typical family and comprises low molecular mass cysteine-rich proteins, usually cationic and distributed in different plant tissues. Here, we report the purification and characterization of a novel gamma-thionin from cowpea seeds (Vigna unguiculata), named Cp-thionin II, with bactericidal activity against Gram-positive and Gram-negative bacteria. Once the primary structure was elucidated, molecular modelling experiments were used to investigate the multimerization and mechanism of action of plant gamma-thionins. Furthermore, Cp-thionin II was also localized in different tissues in cowpea seedlings during germination in contrasting conditions, to better understand the plant protection processes. The use of plant defensins in the construction of transgenic plants and also in the production of novel drugs with activity against human pathogens is discussed.

Plant NBS-LRR proteins: adaptable guards

Plant nucleotide-binding site leucine-rich repeat (NBS-LRR) proteins are a large family involved in disease resistance; they may monitor the status of proteins targeted by pathogens.

The majority of disease resistance genes in plants encode nucleotide-binding site leucine-rich repeat (NBS-LRR) proteins. This large family is encoded by hundreds of diverse genes per genome and can be subdivided into the functionally distinct TIR-domain-containing (TNL) and CC-domain-containing (CNL) subfamilies. Their precise role in recognition is unknown; however, they are thought to monitor the status of plant proteins that are targeted by pathogen effectors.

Duplicate maize 13-lipoxygenase genes are differentially regulated by circadian rhythm, cold stress, wounding, pathogen infection, and hormonal treatments

Most plant oxylipins, a large class of diverse oxygenated polyunsaturated fatty acids and their derivatives, are produced through the lipoxygenase (LOX) pathway. Recent progress in dicots has highlighted the biological roles of oxylipins in plant defence responses to pathogens and pests. By contrast, the physiological function of LOXs and their metabolites in monocots is poorly understood. Two maize LOXs, ZmLOX10 and ZmLOX11 that share >90% amino acid sequence identity but are localized on different chromosomes, were cloned and characterized. Phylogenetic analysis revealed that ZmLOX10 and ZmLOX11 cluster together with well-characterized plastidic type 2 linoleate 13-LOXs from diverse plant species. Regio-specificity analysis of recombinant ZmLOX10 protein overexpressed in Escherichia coli proved it to be a linoleate 13-LOX with a pH optimum at approximately pH 8.0. Both predicted proteins contain putative transit peptides for chloroplast import. ZmLOX10 was preferentially expressed in leaves and was induced in response to wounding, cold stress, defence-related hormones jasmonic acid (JA), salicylic acid (SA), and abscisic acid (ABA), and inoculation with an avirulent strain of Cochliobolus carbonum. These data suggested a role for this gene in maize adaptation to abiotic stresses and defence responses against pathogens and pests. ZmLOX11 was preferentially expressed in silks and was induced in leaves only by ABA, indicating its possible involvement in responses to osmotic stress. In leaves, mRNA accumulation of ZmLOX10 is strictly regulated by a circadian rhythm, with maximal expression coinciding temporally with the highest photosynthetic activity. This study reveals the evolutionary divergence of physiological roles for relatively recently duplicated genes. Possible physiological functions of these 13-LOXs are suggested.