The class I TCP transcription factor AtTCP8 is a modulator of phytohormone-responsive signaling networks

The plant-specific TEOSINTE BRANCHED1/ CYCLOIDEA/ PROLIFERATING CELL FACTOR (TCP) transcription factor family is most closely associated with regulating plant developmental programs. Recently, TCPs were also shown to mediate host immune signaling, both as targets of pathogen virulence factors and regulators of plant defense genes. However, any comprehensive characterization of TCP gene targets is still lacking. Loss of the class I TCP AtTCP8 attenuates early immune signaling, and when combined with mutations in AtTCP14 and AtTCP15, additional layers of defense signaling in Arabidopsis thaliana. Here we focus on TCP8, the most poorly characterized of the three to date. We use chIP and RNA-sequencing to identify TCP8-bound gene promoters and differentially regulated genes in the tcp8 mutant, data sets that are heavily enriched in signaling components for multiple phytohormone pathways, including brassinosteroids (BRs), auxin, and jasmonic acid (JA). Using BR signaling as a representative example, we show that TCP8 directly binds and activates the promoters of the key BR transcriptional regulators BZR1 and BZR2/BES1. Furthermore, tcp8 mutant seedlings exhibit altered BR-responsive growth patterns and complementary reductions in BZR2 transcript levels, while the expressed protein demonstrates BR-responsive changes in subnuclear localization and transcriptional activity. We conclude that one explanation for the significant targeting of TCP8 alongside other TCP family members by pathogen effectors may lie in its role as a modulator of brassinosteroid and other plant hormone signaling pathways.

The structural repertoire of Fusarium oxysporum f. sp. lycopersici effectors revealed by experimental and computational studies

Plant pathogens secrete proteins, known as effectors, that function in the apoplast and inside plant cells to promote virulence. Effectors can also be detected by cell-surface and cytosolic receptors, resulting in the activation of defence pathways and plant immunity. Our understanding of fungal effector function and detection by immunity receptors is limited largely due to high sequence diversity and lack of identifiable sequence motifs precluding prediction of structure or function. Recent studies have demonstrated that fungal effectors can be grouped into structural classes despite significant sequence variation. Using protein x-ray crystallography, we identify a new structural class of effectors hidden within the secreted in xylem (SIX) effectors from Fusarium oxysporum f. sp. lycopersici (Fol). The recognised effectors Avr1 (SIX4) and Avr3 (SIX1) represent the founding members of the Fol dual-domain (FOLD) effector class. Using AlphaFold ab initio protein structure prediction, benchmarked against the experimentally determined structures, we demonstrate SIX6 and SIX13 are FOLD effectors. We show that the conserved N-domain of Avr1 and Avr3 is sufficient for recognition by their corresponding, but structurally-distinct, immunity receptors. Additional structural prediction and comparison indicate that 11 of the 14 SIX effectors group into four structural families. This revealed that genetically linked effectors are related structurally, and we provide direct evidence for a physical association between one divergently-transcribed effector pair. Collectively, these data indicate that Fol secretes groups of structurally-related molecules during plant infection, an observation that has broad implications for our understanding of pathogen virulence and the engineering of plant immunity receptors.

Prophage Activation in the Intestine: Insights Into Functions and Possible Applications

Prophage activation in intestinal environments has been frequently reported to affect host adaptability, pathogen virulence, gut bacterial community composition, and intestinal health. Prophage activation is mostly caused by various stimulators, such as diet, antibiotics, some bacterial metabolites, gastrointestinal transit, inflammatory environment, oxidative stress, and quorum sensing. Moreover, with advancements in biotechnology and the deepening cognition of prophages, prophage activation regulation therapy is currently applied to the treatment of some bacterial intestinal diseases such as Shiga toxin-producing Escherichia coli infection. This review aims to make headway on prophage induction in the intestine, in order to make a better understanding of dynamic changes of prophages, effects of prophage activation on physiological characteristics of bacteria and intestinal health, and subsequently provide guidance on prophage activation regulation therapy.

Detecting genes associated with antimicrobial resistance and pathogen virulence in three New Zealand rivers

The emergence of clinically significant antimicrobial resistance (AMR) in bacteria is frequently attributed to the use of antimicrobials in humans and livestock and is often found concurrently with human and animal pathogens. However, the incidence and natural drivers of antimicrobial resistance and pathogenic virulence in the environment, including waterways and ground water, are poorly understood. Freshwater monitoring for microbial pollution relies on culturing bacterial species indicative of faecal pollution, but detection of genes linked to antimicrobial resistance and/or those linked to virulence is a potentially superior alternative. We collected water and sediment samples in the autumn and spring from three rivers in Canterbury, New Zealand; sites were above and below reaches draining intensive dairy farming. Samples were tested for loci associated with the AMR-related group 1 CTX-M enzyme production (blaCTX-M) and Shiga toxin producing Escherichia coli (STEC). The blaCTX-M locus was only detected during spring and was more prevalent downstream of intensive dairy farms. Loci associated with STEC were detected in both the autumn and spring, again predominantly downstream of intensive dairying. This cross-sectional study suggests that targeted testing of environmental DNA is a useful tool for monitoring waterways. Further studies are now needed to extend our observations across seasons and to examine the relationship between the presence of these genetic elements and the incidence of disease in humans.

Galleria mellonella as a Good Model to Study Acinetobacter baumannii Pathogenesis

The invertebrate model, Galleria mellonella, has been widely used to study host–pathogen interactions due to its cheapness, ease of handling, and similar mammalian innate immune system. G. mellonella larvae have been proven to be useful and a reliable model for analyzing pathogenesis mechanisms of multidrug resistant Acinetobacter baumannii, an opportunistic pathogen difficult to kill. This review describes the detailed experimental design of G. mellonella/A. baumannii models, and provides a comprehensive comparison of various virulence factors and therapy strategies using the G. mellonella host. These investigations highlight the importance of this host–pathogen model for in vivo pathogen virulence studies. On the long term, further development of the G. mellonella/A. baumannii model will offer promising insights for clinical treatments of A. baumannii infection.

Influence of flagellin polymorphisms, gene regulation, and responsive memory on the motility of Xanthomonas species that cause bacterial spot disease of solanaceous plants

Increasingly, new evidence has demonstrated variability in the epitope regions of bacterial flagellin, including in regions harboring the microbe-associated molecular patterns flg22 and flgII-28 that are recognized by the pattern recognition receptors FLS2 and FLS3, respectively. Additionally, since bacterial motility is known to contribute to pathogen virulence and chemotaxis, reductions in or loss of motility can significantly reduce bacterial fitness. In this study, we determined that variations in flg22 and flgII-28 epitopes allow some, but not all, Xanthomonas species to evade both FLS2-and FLS3-mediated oxidative burst responses. We observed variation in the motility for many isolates, irrespective of their flagellin sequence. Instead, we determined that past growth conditions may have a significant impact on the motility status of isolates, as we could minimize this variability by inducing motility using chemoattractant assays. Additionally, motility could be significantly suppressed under nutrient-limited conditions, and bacteria could “remember” its prior motility status after storage at ultra-cold temperatures. Finally, we observed larger bacterial populations of strains with flagellin variants predicted not to be recognized by either FLS2 or FLS3, suggesting that these bacteria can evade flagellin recognition in tomato plants. While some flagellin variants may impart altered motility and differential recognition by the host immune system, external growth parameters and gene expression regulation appear to have more significant impacts on the motility phenotypes for these Xanthomonas species.

Understanding plant–pathogen interactions in net blotch infection of cereals

An economically important disease of barley that causes significant yield and quality losses is net blotch caused by the necrotrophic fungal pathogen Pyrenophora teres. To reduce the impact of net blotch the research community is engaging in basic and applied research to enhance genetic resistances, as it is the most economic and sustainable management strategy. Durable resistance against P. teres will be a major achievement towards the goal of developing widely adapted barley varieties that have high yields and quality across dynamic environments. This chapter focuses on a thorough review of the latest knowledge of both host resistance/susceptibility and pathogen virulence/avirulence in this important pathosystem, and the implications this knowledge will have on deploying sustainable resistances to this destructive pathogen of barley.

Nep1-like Proteins from Valsa mali Differentially Regulate Pathogen Virulence and Response to Abiotic Stresses

Necrosis and ethylene-inducing peptide 1(Nep1)-like protein (NLP) is well known for its cytotoxicity and immunogenicity on dicotyledonous, and it has attracted large attention due to its gene expansion and functional diversification in numerous phytopathogens. Here, two NLP family proteins, VmNLP1 and VmNLP2, were identified in the pathogenic fungus Valsa mali. We showed that VmNLP2 but not VmNLP1 induced cell death when transiently expressed in Nicotiana benthamiana. VmNLP2 was also shown to induce cell death in apple leaves via the treatment of the Escherichia coli-produced recombinant protein. VmNLP1 and VmNLP2 transcripts were drastically induced at the early stage of V. mali infection, whereas only VmNLP2 was shown to be essential for pathogen virulence. We also found that VmNLP1 and VmNLP2 are required for maintaining the integrity of cell membranes, and they differentially contribute to V. mali tolerance to salt- and osmo-stresses. Notably, multiple sequence alignment revealed that the second histidine (H) among the conserved heptapeptide (GHRHDWE) of VmNLP2 is mutated to tyrosine (Y). When this tyrosine (Y) was substituted by histidine (H), the variant displayed enhanced cytotoxicity in N. benthamiana, as well as enhanced virulence on apple leaves, suggesting that the virulence role of VmNLP2 probably correlates to its cytotoxicity activity. We further showed that the peptide among VmNLP2, called nlp25 (VmNLP2), triggered strong immune response in Arabidopsis thaliana. This work demonstrates that NLPs from V. mali involve multiple biological roles, and shed new light on how intricately complex the functions of NLP might be.