HopAZ1, a type III effector of Pseudomonas amygdali pv. tabaci,induces a hypersensitive response in tobacco wildfire-resistant Nicotiana tabacum ‘N509’

Pseudomonas amygdali pv. tabaci (formerly Pseudomonas syringae pv. tabaci; Pta) is a gram-negative bacterium that causes bacterial wildfire disease in Nicotiana tabacum. The pathogen establishes infections by using a type III secretion system to inject type III effector proteins (T3Es) into cells, thereby interfering with the host__s immune system. To counteract the effectors, plants have evolved disease-resistance genes and mechanisms to induce strong resistance on effector recognition. By screening a series of Pta T3E-deficient mutants, we have identified HopAZ1 as the T3E that induces disease resistance in N. tabacum ‘N509’. Inoculation with the Pta ∆hopAZ1 mutant did not induce resistance to Pta in N509. We also found that the Pta ∆hopAZ1 mutant did not induce a hypersensitive response and promoted severe disease symptoms in N509. Furthermore, a C-terminal truncated HopAZ1 abolished HopAZ1-dependent cell death in N509. These results indicate that HopAZ1 is the avirulence factor that induces resistance to Pta by N509.     

The evolutionary and molecular features of the broad-host-range plant pathogen Sclerotinia sclerotiorum

Sclerotinia sclerotiorum is a pathogenic fungus that infects hundreds of plant species, including many of the world's most important crops. Key features of S. sclerotiorum include its extraordinary host range, preference for dicotyledonous plants, relatively slow evolution, and production of protein effectors that are active in multiple host species. Plant resistance to this pathogen is highly complex, typically involving numerous polymorphisms with infinitesimally small effects, which makes resistance breeding a major challenge. Due to its economic significance, S. sclerotiorum has been subjected to a large amount of molecular and evolutionary research. In this updated pathogen profile, we review the evolutionary and molecular features of S. sclerotiorum and discuss avenues for future research into this important species.     

The activity of the RGA5 sensor NLR from rice requires binding of its integrated HMA domain to effectors but not HMA domain self-interaction

The rice nucleotide-binding (NB) and leucine-rich repeat (LRR) domain immune receptors (NLRs) RGA4 and RGA5 form a helper NLR/sensor NLR (hNLR/sNLR) pair that specifically recognizes the effectors AVR-Pia and AVR1-CO39 from the blast fungus Magnaporthe oryzae. While RGA4 contains only canonical NLR domains, RGA5 has an additional unconventional heavy metal-associated (HMA) domain integrated after its LRR domain. This RGA5_HMA domain binds the effectors and is crucial for their recognition. Investigation of the three-dimensional structure of the AVR1-CO39/RGA5_HMA complex by X-ray crystallography identified a candidate surface for effector binding in the HMA domain and showed that the HMA domain self-interacts in the absence of effector through the same surface. Here, we investigated the relevance of this HMA homodimerization for RGA5 function and the role of the RGA5_HMA effector-binding and self-interaction surface in effector recognition. By analysing structure-informed point mutations in the RGA5_HMA-binding surface in protein interaction studies and in Nicotiana benthamiana cell death assays, we found that HMA self-interaction does not contribute to RGA5 function. However, the effector-binding surface of RGA5_HMA identified by X-ray crystallography is crucial for both in vitro and in vivo effector binding as well as effector recognition. These results support the current hypothesis that noncanonical integrated domains of NLRs act primarily as effector traps and deepen our understanding of the sNLRs' function within NLR pairs.     

鼠伤寒沙门菌入侵宿主细胞机制研究进展*

鼠伤寒沙门菌(Salmonella typhimurium)是一种人畜共患的肠道病原菌,可引起肠道炎症。该病原菌主要通过其致病岛(SPIs)编码的III型分泌系统(T3SS)分泌效应因子,包括促炎因子和抗炎因子。其在入侵肠上皮细胞时会释放促炎因子引发炎症反应,同时,为防止促炎因子过度破坏宿主细胞影响菌体的生存和繁殖,鼠伤寒沙门菌会产生一系列抗炎因子来调节细胞内信号通路,与宿主共同繁殖并最终全身扩散造成严重感染。旨在对鼠伤寒沙门菌利用T3SS效应因子入侵并调节宿主细胞信号通路机制进行概述。     

果生刺盘孢效应蛋白CfEC92靶向Mal d 1j抑制苹果免疫

由果生刺盘孢引起的炭疽叶枯病是我国苹果产区的重要病害。果生刺盘孢在侵染苹果过程中分泌效应蛋白CfEC92促进其侵染,但其作用机制仍不清楚。本研究从苹果cDNA中克隆出Mal d 1j蛋白,结构域及氨基酸序列分析显示,Mal d 1j为PR10家族成员。Mal d 1j基因过表达能显著增强苹果对炭疽叶枯病抗性,而沉默Mal d 1j显著降低其抗性。在烟草中过表达Mal d 1j提高烟草抗性,共表达CfEC92和Mal d 1j蛋白,降低Mal d 1j对疫霉菌抗性。通过酵母双杂交、BIFC和Co-IP分析证明CfEC92与Mal d 1j可以发生直接互作,且其互作定位于细胞膜和细胞核,表明CfEC92 通过与Mal d 1j互作影响植物免疫。本研究揭示了果生刺盘孢效应蛋白CfEC92通过靶向Mal d 1j抑制植物免疫促进其侵染的分子机制,为苹果炭疽叶枯病防控提供了新的思路。     

Genetically modified Streptococcus mutans for the prevention of dental caries

There are many examples of positive and negative interactions between different species of bacteria inhabiting the same ecosystem. This observation provides the basis for a novel approach to preventing microbial diseases called replacement therapy. In this approach, a harmless effector strain is permanently implanted in the host's microflora. Once established, the presence of the effector strain prevents the colonization or outgrowth of a particular pathogen. In the case of dental caries, replacement therapy has involved construction of an effector strain called BCS3-L1, which was derived from a clinical Streptococcus mutans isolate. Recombinant DNA technology was used to delete the gene encoding lactate dehydrogenase in BCS3-L1 making it entirely deficient in lactic acid production. This effector strain was also designed to produce elevated amounts of a novel peptide antibiotic called mutacin 1140 that gives it a strong selective advantage over most other strains of S. mutans. In laboratory and rodent model studies, BCS3-L1 was found to be genetically stable and to produce no apparent deleterious side effects during prolonged colonization. BCS3-L1 was significantly less cariogenic than wild-type S. mutansin gnotobiotic rats, and it did not contribute at all to the cariogenic potential of the indigenous flora of conventional Sprague-Dawley rats. And, its strong colonization properties indicated that a single application of the BCS3-L1 effector strain to human subjects should result in its permanent implantation and displacement over time of indigenous, disease-causing S. mutans strains. Thus, BCS3-L1 replacement therapy for the prevention of dental caries is an example of biofilm engineering that offers the potential for a highly efficient, cost effective augmentation of conventional prevention strategies. It is hoped that the eventual success of replacement therapy for the prevention of dental caries will stimulate the use of this approach in the prevention of other bacterial diseases.     

Mutations within a human IgG2 antibody form distinct and homogeneous disulfide isomers but do not affect Fc gamma receptor or C1q binding

Human IgG2 antibodies may exist in at least three distinct structural isomers due to disulfide shuffling within the upper hinge region. Antibody interactions with Fc gamma receptors and the complement component C1q contribute to immune effector functions. These interactions could be impacted by the accessibility and structure of the hinge region. To examine the role structural isomers may have on effector functions, a series of cysteine to serine mutations were made on a human IgG2 backbone. We observed structural homogeneity with these mutants and mapped the locations of their disulfide bonds. Importantly, there was no observed difference in binding to any of the Fc gamma receptors or C1q between the mutants and the wild‐type IgG2. However, differences were seen in the apparent binding affinity of these antibodies that were dependent on the selection of the secondary detection antibody used.