Dickeya dadantii pectic enzymes necessary for virulence are also responsible for activation of the Arabidopsis thaliana innate immune system

Commensal lifestyle regulated by a negative feedback loop between Arabidopsis ROS and the bacterial T2SS

Despite the plant health-promoting effects of plant microbiota, these assemblages also comprise potentially detrimental microbes. How plant immunity controls its microbiota to promote plant health under these conditions remains largely unknown. We find that commensal bacteria isolated from healthy Arabidopsis plants trigger diverse patterns of reactive oxygen species (ROS) production dependent on the immune receptors and completely on the NADPH oxidase RBOHD that selectively inhibited specific commensals, notably Xanthomonas L148. Through random mutagenesis, we find that L148 gspE, encoding a type II secretion system (T2SS) component, is required for the damaging effects of Xanthomonas L148 on rbohD mutant plants. In planta bacterial transcriptomics reveals that RBOHD suppresses most T2SS gene expression including gspE. L148 colonization protected plants against a bacterial pathogen, when gspE was inhibited by ROS or mutation. Thus, a negative feedback loop between Arabidopsis ROS and the bacterial T2SS tames a potentially detrimental leaf commensal and turns it into a microbe beneficial to the host.

Host plant physiological transformation and microbial population heterogeneity as important determinants of the Soft Rot Pectobacteriaceae-plant interactions

Pectobacterium and Dickeya species belonging to the Soft Rot Pectobacteriaceae (SRP) are one of the most devastating phytopathogens. They degrade plant tissues by producing an arsenal of plant cell wall degrading enzymes. However, SRP-plant interactions are not restricted to the production of these "brute force" weapons. Additionally, these bacteria apply stealth behavior related to (1) manipulation of the host plant via induction of susceptible responses and (2) formation of heterogeneous populations with functionally specialized cells. Our review aims to summarize current knowledge on SRP-induced plant susceptible responses and on the heterogeneity of SRP populations. The review shows that SRP are capable of adjusting the host's hormonal balance, inducing host-mediated plant cell wall modification, promoting iron assimilation by the host, stimulating the accumulation of reactive oxygen species and host cell death, and activating the synthesis of secondary metabolites that are ineffective in limiting disease progression. By this means, SRP facilitate host plant susceptibility. During host colonization, SRP populations produce various functionally specialized cells adapted for enhanced virulence, increased resistance, motility, vegetative growth, or colonization of the vascular system. This enables SRP to perform self-contradictory tasks, which benefits a population's overall fitness in various environments, including host plants. Such stealthy tactical actions facilitate plant-SRP interactions and disease progression.

Plant Soft Rot Development and Regulation from the Viewpoint of Transcriptomic Profiling

Soft rot caused by Pectobacterium species is a devastating plant disease poorly characterized in terms of host plant responses. In this study, changes in the transcriptome of tobacco plants after infection with Pectobacterium atrosepticum (Pba) were analyzed using RNA-Seq. To draw a comprehensive and nontrivially itemized picture of physiological events in Pba-infected plants and to reveal novel potential molecular "players" in plant-Pba interactions, an original functional gene classification was performed. The classifications present in various databases were merged, enriched by "missed" genes, and divided into subcategories. Particular changes in plant cell wall-related processes, perturbations in hormonal and other regulatory systems, and alterations in primary, secondary, and redox metabolism were elucidated in terms of gene expression. Special attention was paid to the prediction of transcription factors (TFs) involved in the disease's development. Herewith, gene expression was analyzed within the predicted TF regulons assembled at the whole-genome level based on the presence of particular cis-regulatory elements (CREs) in gene promoters. Several TFs, whose regulons were enriched by differentially expressed genes, were considered to be potential master regulators of Pba-induced plant responses. Differential regulation of genes belonging to a particular multigene family and encoding cognate proteins was explained by the presence/absence of the particular CRE in gene promoters.

Calm Before the Storm: A Glimpse into the Secondary Metabolism of Aspergillus welwitschiae, the Etiologic Agent of the Sisal Bole Rot

Aspergillus welwitschiae is a species of the Nigri section of the genus Aspergillus. In nature, it is usually a saprotroph, decomposing plant material. However, it causes the bole rot disease of Agave sisalana (sisal), a plant species used for the extraction of hard natural fibers, causing great economic loss to this culture. In this study, we isolated and sequenced one genome of A. welwitschiae (isolate CCMB 674 (Collection of Cultures of Microorganisms of Bahia)) from the stem tissues of sisal and performed in silico and wet lab experimental strategies to describe its ability to produce mycotoxins. CCMB 674 possesses 64 secondary metabolite gene clusters (SMGCs) and, under normal conditions, it produces secondary metabolism compounds that could disturb the cellular cycle of sisal or induce abnormalities in plant growth, such as malformin C. This isolate also produces a pigment that might explain the characteristic red color of the affected tissues. Additionally, this isolate is defective for the production of fumonisin B1, and, despite bearing the full cluster for the synthesis of this compound, it did not produce ochratoxin A. Altogether, these results provide new information on possible strategies used by the fungi during the sisal bole rot, helping to better understand this disease and how to control it.

Pectate lyases: Their role in plants and importance in fruit ripening

Plant cell walls are complex structures that are modified throughout development. They are a major contributor to the properties of plant structure and act as barriers against pathogens. The primary cell walls of plants are composed of polysaccharides and proteins. The polysaccharide fraction is divided into components cellulose, hemicelluloses and pectin, are all modified during fruit ripening. Pectin plays an important role in intercellular adhesion and controlling the porosity of the wall. A large number of pectin degrading enzymes have been characterised from plants and they are involved in numerous aspects of plant development. The role of pectate lyases in plant development has received little attention, probably because they are normally associated with the action of plant pathogenic organisms. However their importance in plant development and ripening is now becoming well established and new information about the role of pectate lyases in plant development forms the focus of this review.

Oligogalacturonide production upon Arabidopsis thaliana-Botrytis cinerea interaction

Despite an ever-increasing interest for the use of pectin-derived oligogalacturonides (OGs) as biological control agents in agriculture, very little information exists-mainly for technical reasons-on the nature and activity of the OGs that accumulate during pathogen infection. Here we developed a sensitive OG profiling method, which revealed unsuspected features of the OGs generated during infection of Arabidopsis thaliana with the fungus Botrytis cinerea Indeed, in contrast to previous reports, most OGs were acetyl- and methylesterified, and 80% of them were produced by fungal pectin lyases, not by polygalacturonases. Polygalacturonase products did not accumulate as larger size OGs but were converted into oxidized GalA dimers. Finally, the comparison of the OGs and transcriptomes of leaves infected with B. cinerea mutants with reduced pectinolytic activity but with decreased or increased virulence, respectively, identified candidate OG elicitors. In conclusion, OG analysis provides insights into the enzymatic arms race between plant and pathogen and facilitates the identification of defense elicitors.

Regulation of Protein Secretion Systems Mediated by Cyclic Diguanylate in Plant-Interacting Bacteria

The ubiquitous second messenger cyclic diguanylate (c-di-GMP) is involved in the regulation of different processes in bacteria. In phytopathogens, intracellular fluctuations in the concentration of this molecule contribute to the lifestyle switching from a motile and virulent stage to a sessile and biofilm-forming phase. Among the virulence mechanisms used by bacterial pathogens, different specific type secretion systems (TSSs) and the effector proteins that they translocate are included. Some of these TSS are conceived to suppress host immune responses during bacterial colonization. The modulation of the expression of secretion systems components and/or effector proteins can be influenced by c-di-GMP levels at transcriptional, translational, or post-translational levels and can take place directly by binding to specific or global regulators, or via transducer proteins. Different genera of plant-interacting bacteria have been analyzed to shed some light in the implications of c-di-GMP in the regulation of host plant colonization through protein secretion systems. Expression of (1) adhesins secreted by Type 1 secretion systems to bind the host plant in Pectobacterium (formerly Erwinia) and some beneficial Pseudomonas strains; (2) catalytic exoproteins delivered by Type 2 secretion systems to break plant cell wall in Dickeya; (3) effectors secreted by Type 3 secretion systems to suppress plant immunity in Xanthomonas; or (4) the activity of Type 6 secretion systems to export an ATPase in Pseudomonas, are finely tuned by c-di-GMP levels. In this minireview, we summarize the knowledge available about the implications of c-di-GMP in the regulation of protein secretion in different plant-interacting bacteria. Topic: Secretion systems and effector proteins of phytopathogenic and beneficial bacteria regulated by NSM.

Pectin Oligosaccharides Ameliorate Colon Cancer by Regulating Oxidative Stress- and Inflammation-Activated Signaling Pathways

Colon cancer (CC) is the third common neoplasm worldwide, and it is still a big challenge for exploring new effective medicine for treating CC. Natural product promoting human health has become a hot topic and attracted many researchers recently. Pectin, a complex polysaccharide in plant cell wall, mainly consists of four major types of polysaccharides: homogalacturonan, xylogalacturonan, rhamnogalacturonan I and II, all of which can be degraded into various pectin oligosaccharides (POS) and may provide abundant resource for exploring potential anticancer drugs. POS have been regarded as a novel class of potential functional food with multiple health-promoting properties. POS have antibacterial activities against some aggressive and recurrent bacterial infection and exert beneficial immunomodulation for controlling CC risk. However, the molecular functional role of POS in the prevention of CC risk and progression remains doubtful. The review focuses on antioxidant and anti-inflammatory roles of POS for promoting human health by regulating some potential oxidative and inflammation-activated pathways, such as ATP-activated protein kinase (AMPK), nuclear factor erythroid-2-related factor-2 (Nrf2), and nuclear factor-κB (NF-κB) pathways. The activation of these signaling pathways increases the antioxidant and antiinflammatory activities, which will result in the apoptosis of CC cells or in the prevention of CC risk and progression. Thus, POS may inhibit CC development by affecting antioxidant and antiinflammatory signaling pathways AMPK, Nrf2, and NF-κB. However, POS also can activate signal transduction and transcriptional activator 1 and 3 signaling pathway, which will reduce antioxidant and anti-inflammatory properties and promote CC progression. Specific structural and structurally modified POS may be associated with their functions and should be deeply explored in the future. The present review paper lacks the important information for the linkage between the specific structure of POS and its function. To further explore the effects of prebiotic potential of POS and their derivatives on human immunomodulation in the prevention of CC, the specific POS with a certain degree of polymerization or purified polymers are highly demanded to be performed in clinical practice.

Expanding Role of Type II Secretion in Bacterial Pathogenesis and Beyond

Type II secretion (T2S) is one means by which Gram-negative pathogens secrete proteins into the extracellular milieu and/or host organisms. Based upon recent genome sequencing, it is clear that T2S is largely restricted to the Proteobacteria, occurring in many, but not all, genera in the Alphaproteobacteria, Betaproteobacteria, Gammaproteobacteria, and Deltaproteobacteria classes. Prominent human and/or animal pathogens that express a T2S system(s) include Acinetobacter baumannii, Burkholderia pseudomallei, Chlamydia trachomatis, Escherichia coli, Klebsiella pneumoniae, Legionella pneumophila, Pseudomonas aeruginosa, Stenotrophomonas maltophilia, Vibrio cholerae, and Yersinia enterocolitica T2S-expressing plant pathogens include Dickeya dadantii, Erwinia amylovora, Pectobacterium carotovorum, Ralstonia solanacearum, Xanthomonas campestris, Xanthomonas oryzae, and Xylella fastidiosa T2S also occurs in nonpathogenic bacteria, facilitating symbioses, among other things. The output of a T2S system can range from only one to dozens of secreted proteins, encompassing a diverse array of toxins, degradative enzymes, and other effectors, including novel proteins. Pathogenic processes mediated by T2S include the death of host cells, degradation of tissue, suppression of innate immunity, adherence to host surfaces, biofilm formation, invasion into and growth within host cells, nutrient assimilation, and alterations in host ion flux. The reach of T2S is perhaps best illustrated by those bacteria that clearly use it for both environmental survival and virulence; e.g., L. pneumophila employs T2S for infection of amoebae, growth within lung cells, dampening of cytokines, and tissue destruction. This minireview provides an update on the types of bacteria that have T2S, the kinds of proteins that are secreted via T2S, and how T2S substrates promote infection.