Proteome analysis of Ehrlichia chaffeensis containing phagosome membranes revealed the presence of numerous bacterial and host proteins

Tick-transmitted Ehrlichia chaffeensis, the causative agent for human monocytic ehrlichiosis, resides and multiplies within a host cell phagosome. Infection progression of E. chaffeensis includes internalization into a host cell by host cell membrane fusion events following engulfment leading to the formation of E. chaffeensis containing vacuole (ECV). Revealing the molecular composition of ECV is important in understanding the host cellular processes, evasion of host defense pathways and in defining host-pathogen interactions. ECVs purified from infected host cells were analyzed to define both host and bacterial proteomes associated with the phagosome membranes. About 160 bacterial proteins and 2,683 host proteins were identified in the ECV membranes. The host proteins included predominantly known phagosome proteins involved in phagocytic trafficking, fusion of vesicles, protein transport, Ras signaling pathway and pathogenic infection. Many highly expressed proteins were similar to the previously documented proteins of phagosome vacuole membranes containing other obligate pathogenic bacteria. The finding of many bacterial membrane proteins is novel; they included multiple outer membrane proteins, such as the p28-Omps, the 120 kDa protein, preprotein translocases, lipoproteins, metal binding proteins, and chaperonins, although the presence of ankyrin repeat proteins, several Type I and IV secretion system proteins is anticipated. This study demonstrates that ECV membrane is extensively modified by the pathogen. This study represents the first and the most comprehensive description of ECV membrane proteome. The identity of many host and Ehrlichia proteins in the ECV membrane will be a valuable to define pathogenic mechanisms critical for the replication of the pathogen within macrophages.

"Jumping Jack": Genomic Microsatellites Underscore the Distinctiveness of Closely Related Pseudoperonospora cubensis and Pseudoperonospora humuli and Provide New Insights Into Their Evolutionary Past

Downy mildews caused by obligate biotrophic oomycetes result in severe crop losses worldwide. Among these pathogens, Pseudoperonospora cubensis and P. humuli, two closely related oomycetes, adversely affect cucurbits and hop, respectively. Discordant hypotheses concerning their taxonomic relationships have been proposed based on host-pathogen interactions and specificity evidence and gene sequences of a few individuals, but population genetics evidence supporting these scenarios is missing. Furthermore, nuclear and mitochondrial regions of both pathogens have been analyzed using microsatellites and phylogenetically informative molecular markers, but extensive comparative population genetics research has not been done. Here, we genotyped 138 current and historical herbarium specimens of those two taxa using microsatellites (SSRs). Our goals were to assess genetic diversity and spatial distribution, to infer the evolutionary history of P. cubensis and P. humuli, and to visualize genome-scale organizational relationship between both pathogens. High genetic diversity, modest gene flow, and presence of population structure, particularly in P. cubensis, were observed. When tested for cross-amplification, 20 out of 27 P. cubensis-derived gSSRs cross-amplified DNA of P. humuli individuals, but few amplified DNA of downy mildew pathogens from related genera. Collectively, our analyses provided a definite argument for the hypothesis that both pathogens are distinct species, and suggested further speciation in the P. cubensis complex.

Comparative Transcriptome Analysis of Wheat Lines in the Field Reveals Multiple Essential Biochemical Pathways Suppressed by Obligate Pathogens

Mildew and rust are the most devastating cereal pathogens, and in wheat they can cause up to 50% yield loss every year. Wheat lines containing resistance genes are used to effectively control fungal diseases, but the molecular mechanisms underlying the interaction between wheat and its fungal pathogens are poorly understood. Here, we used RNA sequencing (RNA-Seq) to compare the transcriptomic landscape of susceptible and resistant wheat lines to identify genes and pathways that are targeted by obligate biotrophic fungal pathogens. The five lines differed in the expression of thousands of genes under infection as well as control conditions. Generally, mixed infection with powdery mildew and leaf rust resulted in downregulation of numerous genes in susceptible lines. Interestingly, transcriptomic comparison between the nearly isogenic lines Thatcher and Thatcher-Lr34 identified 753 genes that are uniquely downregulated in the susceptible line upon infection. Kyoto encyclopedia of genes and genomes (KEGG) enrichment analysis, revealed the suppression of six major biochemical pathways, namely nuclear transport, alternative splicing, DNA damage response, ubiquitin-mediated proteolysis, phosphoinositol signaling, and photosynthesis. We conclude that powdery mildew and leaf rust evade the wheat defense system by suppression of programmed cell death (PCD) and responses to cellular damage. Considering the broad range of the induced changes, we propose that the pathogen targets "master regulators" at critical steps in the respective pathways. Identification of these wheat genes targeted by the pathogen could inspire new directions for future wheat breeding.