The paracaspase MALT1 mediates T cell antigen receptor-induced signaling to the transcription factor NF-kappaB and is indispensable for T cell activation and proliferation. Enhanced expression of MALT1 or aberrant expression of a fusion protein of the apoptosis inhibitor API2 and MALT1 has been linked to mucosa-associated lymphoid tissue lymphoma. Despite the presence of a caspase-like domain, MALT1 proteolytic activity has not yet been demonstrated. Here we show that T cell antigen receptor stimulation induced recruitment of the NF-kappaB inhibitor A20 into a complex of MALT1 and the adaptor protein Bcl-10, leading to MALT1-mediated processing of A20. API2-MALT1 expression likewise resulted in cleavage of A20. MALT1 cleaved human A20 after arginine 439 and impaired its NF-kappaB-inhibitory function. Our studies identify A20 as a substrate of MALT1 and emphasize the importance of MALT1 proteolytic activity in the 'fine tuning' of T cell antigen receptor signaling.

SUMMARY Toll-like receptors (TLRs) form a major group of transmembrane receptors that are involved in the detection of invading pathogens. Double-stranded RNA is a marker for viral infection that is recognized by TLR3. TLR3 triggering activates specific signaling pathways that culminate in the activation of NF-κB and IRF3 transcription factors, as well as apoptosis, enabling the host to mount an effective innate immune response through the induction of cytokines, chemokines, and other proinflammatory mediators. In this review, we describe the paradoxical role of TLR3 in innate immunity against different viruses and in viral pathogenesis but also the evidence for TLR3 as a “danger” receptor in nonviral diseases. We also discuss the structure and cellular localization of TLR3, as well as the complex signaling and regulatory events that contribute to TLR3-mediated immune responses.

The paracaspase mucosa-associated lymphoid tissue 1 (MALT1) is central to lymphocyte activation and lymphomagenesis. MALT1 mediates antigen receptor signalling to NF-jB by acting as a scaffold protein. Furthermore, MALT1 has proteolytic activity that contributes to optimal NF-jB activation by cleaving the NF-jB inhibitor A20. Whether MALT1 protease activity is involved in other signalling pathways, and the identity of the relevant substrates, is unknown. Here, we show that T-cell receptors (TCR) activation, as well as overexpression of the oncogenic API2-MALT1 fusion protein, results in proteolytic inactivation of CYLD by MALT1, which is specifically required for c-jun N-terminal kinase (JNK) activation and the inducible expression of a subset of genes. These results indicate a novel role for MALT1 proteolytic activity in TCRinduced JNK activation and reveal CYLD cleavage as the underlying mechanism.

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.

Abscisic acid (ABA) is a sesquiterpene signaling molecule produced in all kingdoms of life. To date, the best known functions of ABA are derived from its role as a major phytohormone in plant abiotic stress resistance. Different organisms have developed different biosynthesis and signal transduction pathways related to ABA. Despite this, there are also intriguing common themes where ABA often suppresses host immune responses and is utilized by pathogens as an effector molecule. ABA also seems to play an important role in compatible mutualistic interactions such as mycorrhiza and rhizosphere bacteria with plants, and possibly also the animal gut microbiome. The frequent use of ABA in inter-species communication could be a possible reason for the wide distribution and re-invention of ABA as a signaling molecule in different organisms. In humans and animal models, it has been shown that ABA treatment or nutrient-derived ABA is beneficial in inflammatory diseases like colitis and type 2 diabetes, which confer potential to ABA as an interesting nutraceutical or pharmacognostic drug. The anti-inflammatory activity, cellular metabolic reprogramming, and other beneficial physiological and psychological effects of ABA treatment in humans and animal models has sparked an interest in this molecule and its signaling pathway as a novel pharmacological target. In contrast to plants, however, very little is known about the ABA biosynthesis and signaling in other organisms. Genes, tools and knowledge about ABA from plant sciences and studies of phytopathogenic fungi might benefit biomedical studies on the physiological role of endogenously generated ABA in humans.

Abscisic acid (ABA) is a defense hormone with influence on callose-dependent and -independent resistance against Leptosphaeria maculans acting in the RLMcol pathway. ABA-deficient and -insensitive mutants in Ler-0 background (abal-3 and abil-1) displayed susceptibility to L. maculans, along with a significantly decreased level of callose depositions, whereas abi2-1 and abi3-1 remained resistant, together with the abi5-1 mutant of Ws-0 background. Suppressor mutants of abil-1 confirmed that the L. maculans-susceptible response was due to the dominant negative nature of the abil-1 mutant. Highly induced camalexin levels made ABA mutants in Col-0 background (aba2-1, aba3-1, and abi4-1) appear resistant, but displayed enhanced susceptibility as double mutants with pad3-1, impaired in camalexin biosynthesis. beta-Aminobutyric acid (BABA) pretreatment of Ler-0 contributed to an elevated level of endogenous ABA after L. maculans inoculation. Comparisons between (RLM1co1)pad3 and rlmlLerpad3 showed that ABA and BABA enhancement of callose deposition requires induction from RLM1col. ABII, but not ABI2, was found to be involved in a feedback mechanism that modulates RLM1co, expression. Genetic analysis showed further that this feedback occurs upstream of ABI4 and that components downstream of ABI4 modulate ABIJ activity. ABA and BABA treatments of the L. maculans-susceptible callose synthase mutant pmr4 showed that ABA also induces a callose-independent resistance. Similar treatments enhanced callose depositions and induced resistance to L. maculans in oilseed rape, and BABA-induced resistance was found to be independent of salicylic acid.

SummaryIn a cross between the two resistant accessions Col-0 and Ler-0, a 15:1 segregation was found in F 2 , suggesting the presence of unlinked resistance loci to Leptosphaeria maculans. One hundred Col-4 · Ler-0, and 50 Ler-2 · Cvi-1 recombinant inbred lines, and seven susceptible Ler-0 · Ws-0 F 2 progenies were examined to identify the two loci. Resistance in Col-4, Ws-0 and Cvi-1 (RLM1) was mapped to the marker m305 on chromosome 1. Col-4 · Ler-0 and Ler-2 · Cvi-1 mapping populations located RLM2 Ler on the same arm of chromosome 4. A tight physical location of RLM2 was established through near-isogenic lines. This region was found to correspond to an ancient duplication event between the RLM1 and RLM2 loci. Two independent T-DNA mutants in a TIR-NB-LRR R gene (At1g64070) displayed susceptibility, and L. maculans susceptible mutant phenotypes were confirmed to be allelic for rlm1 in F 1 after crosses with susceptible rlm1 Ler rlm2 Col plants. Complementation of rlm1 Ler rlm2 Col with the genomic Col-0 sequence of At1g64070 conferred resistance. In addition, two T-DNA mutants in a neighbouring homologous TIR-NB-LRR gene (At1g63880) displayed moderate susceptibility to L. maculans. Sequence analysis revealed that At1g64070 was truncated by a premature stop codon, and that At1g63880 was absent in Ler-0. RNA interference confirmed that Ler-0 resistance is dependent on genes structurally related to RLM1. Camalexin was identified as a quantitative co-dominant resistance factor of Col-0 origin, but independent of RLM1. RLM1/RLM2 resistance was, however, found to require RAR1 and partially HSP90.1.

SummaryOut of 168 Arabidopsis accessions screened with isolates of Leptosphaeria maculans, one (An-1) showed clear disease symptoms. In order to identify additional components involved in containment of L. maculans in Arabidopsis, a screen for L. maculans-susceptible (lms) mutants was performed. Eleven lms mutants were isolated, which displayed differential susceptibility responses to L. maculans. lms1 was crossed with Columbia (Col-0) and Ws-0, and mapping data for both populations showed the highest linkage to a region on chromosome 2. Reduced levels of PR-1 and PDF1.2 expression were found in lms1 compared to wild-type plants 48 h after pathogen inoculation. In contrast, the lms1 mutant displayed upregulation of either marker gene upon chemical treatment, possibly as an effect of an altered ethylene (ET) response. To assess the contribution of different defence pathways, genotypes implicated in salicylic acid (SA) signalling plants expressing the bacterial salicylate hydroxylase (nahG) gene, non-expressor of PR1 (npr1 )-1 and phytoalexin-de®cient (pad4-1 ), jasmonic acid (JA) signalling (coronatine insensitive (coi )1-16, enhanced disease susceptibility (eds )8-1 and jasmonic acid resistant (jar)1-1 ) and ET signalling (eds4-1, ethylene insensitive (ein )2, ein3-1 and ethylene resistant (etr)1-1 ) were screened. All the genotypes screened were as resistant as wild-type plants, demonstrating the dispensability of the pathways in L. maculans resistance. When mutants implicated in cell death responses were assayed, responsive to antagonist 1 (ran1 )-1 exhibited a weak susceptible phenotype, whereas accelerated cell death (acd )1-20 showed a rapid lesion development. Camalexin is only partially responsible for L. maculans containment in Arabidopsis, as pad3-1 and enhanced susceptibility to Alternaria (esa )1 clearly showed a susceptible response while wild-type levels of camalexin were present in An-1 and lms1. The data presented point to the existence of multiple defence mechanisms controlling the containment of L. maculans in Arabidopsis.