Was the Chlamydial Adaptative Strategy to Tryptophan Starvation an Early Determinant of Plastid Endosymbiosis?

Chlamydiales were recently proposed to have sheltered the future cyanobacterial ancestor of plastids in a common inclusion. The intracellular pathogens are thought to have donated those critical transporters that triggered the efflux of photosynthetic carbon and the consequent onset of symbiosis. Chlamydiales are also suspected to have encoded glycogen metabolism TTS (Type Three Secretion) effectors responsible for photosynthetic carbon assimilation in the eukaryotic cytosol. We now review the reasons underlying other chlamydial lateral gene transfers evidenced in the descendants of plastid endosymbiosis. In particular we show that half of the genes encoding enzymes of tryptophan synthesis in Archaeplastida are of chlamydial origin. Tryptophan concentration is an essential cue triggering two alternative modes of replication in Chlamydiales. In addition, sophisticated tryptophan starvation mechanisms are known to act as antibacterial defenses in animal hosts. We propose that Chlamydiales have donated their tryptophan operon to the emerging plastid to ensure increased synthesis of tryptophan by the plastid ancestor. This would have allowed massive expression of the tryptophan rich chlamydial transporters responsible for symbiosis. It would also have allowed possible export of this valuable amino-acid in the inclusion of the tryptophan hungry pathogens. Free-living single cell cyanobacteria are devoid of proteins able to transport this amino-acid. We therefore investigated the phylogeny of the Tyr/Trp transporters homologous to E. coli TyrP/Mre and found yet another LGT from Chlamydiales to Archaeplastida thereby considerably strengthening our proposal.

Impaired Chloroplast Biogenesis in Immutans, an Arabidopsis Variegation Mutant, Modifies Developmental Programming, Cell Wall Composition and Resistance to Pseudomonas syringae.

The immutans (im) variegation mutation of Arabidopsis has green- and white- sectored leaves due to action of a nuclear recessive gene. IM codes for PTOX, a plastoquinol oxidase in plastid membranes. Previous studies have revealed that the green and white sectors develop into sources (green tissues) and sinks (white tissues) early in leaf development. In this report we focus on white sectors, and show that their transformation into effective sinks involves a sharp reduction in plastid number and size. Despite these reductions, cells in the white sectors have near-normal amounts of plastid RNA and protein, and surprisingly, a marked amplification of chloroplast DNA. The maintenance of protein synthesis capacity in the white sectors might poise plastids for their development into other plastid types. The green and white im sectors have different cell wall compositions: whereas cell walls in the green sectors resemble those in wild type, cell walls in the white sectors have reduced lignin and cellulose microfibrils, as well as alterations in galactomannans and the decoration of xyloglucan. These changes promote susceptibility to the pathogen Pseudomonas syringae. Enhanced susceptibility can also be explained by repressed expression of some, but not all, defense genes. We suggest that differences in morphology, physiology and biochemistry between the green and white sectors is caused by a reprogramming of leaf development that is coordinated, in part, by mechanisms of retrograde (plastid-to-nucleus) signaling, perhaps mediated by ROS. We conclude that variegation mutants offer a novel system to study leaf developmental programming, cell wall metabolism and host-pathogen interactions.

Blurred pictures from the crime scene: the growing case for a function of Chlamydiales in plastid endosymbiosis.

A number of recent papers have brought suggestive evidence for an active role of Chlamydiales in the establishment of the plastid. Chlamydiales define a very ancient group of obligate intracellular bacterial pathogens that multiply in vesicles within eukaryotic phagotrophic host cells such as animals, amoebae or other protists, possibly including the hypothetical phagotroph that internalized the cyanobacterial ancestor of the plastid over a billion years ago. We briefly survey the case for an active role of these ancient pathogens in plastid endosymbiosis. We argue that a good understanding of the Chlamydiales infection cycle and diversity may help to shed light on the process of metabolic integration of the evolving plastid.

Toward an understanding of the function of Chlamydiales in plastid endosymbiosis.

Plastid endosymbiosis defines a process through which a fully evolved cyanobacterial ancestor has transmitted to a eukaryotic phagotroph the hundreds of genes required to perform oxygenic photosynthesis, together with the membrane structures, and cellular compartment associated with this process. In this review, we will summarize the evidence pointing to an active role of Chlamydiales in metabolic integration of free living cyanobacteria, within the cytosol of the last common plant ancestor.

Cell-autonomous defense, re-organization and trafficking of membranes in plant-microbe interactions.

Plant cells dynamically change their architecture and molecular composition following encounters with beneficial or parasitic microbes, a process referred to as host cell reprogramming. Cell-autonomous defense reactions are typically polarized to the plant cell periphery underneath microbial contact sites, including de novo cell wall biosynthesis. Alternatively, host cell reprogramming converges in the biogenesis of membrane-enveloped compartments for accommodation of beneficial bacteria or invasive infection structures of filamentous microbes. Recent advances have revealed that, in response to microbial encounters, plasma membrane symmetry is broken, membrane tethering and SNARE complexes are recruited, lipid composition changes and plasma membrane-to-cytoskeleton signaling is activated, either for pre-invasive defense or for microbial entry. We provide a critical appraisal on recent studies with a focus on how plant cells re-structure membranes and the associated cytoskeleton in interactions with microbial pathogens, nitrogen-fixing rhizobia and mycorrhiza fungi.

Taxonomía y filogenética de Rhinosporidium seeberi, un patógeno para el ser humano y los animales: revisión crítica / The taxonomy and phylogenetics of the human and animal pathogen Rhinosporidium seeberi: a critical review

Rhinosporidum seeberi es el agente etiológico de la rinosporidiosis, una enfermedad de las membranas mucosas y, con menos frecuencia, de la piel y otros tejidos. Debido a que se resiste a crecer en los medios de cultivo desde hace más de 100 años, la identidad taxonómica de R. seeberi ha sido motivo de controversia. Tres nuevas hipótesis en una larga lista de puntos de vista similares han sido introducidas: 1) la cianobacteria Microcystis es el agente etiológico de la rinosporidiosis, 2) R. seeberi es un patógeno eucariota en los Mesomycetozoa, y 3) R. seeberi es un hongo. La literatura revisada sobre los estudios realizados con microscopia electrónica, los datos histopatológico y, más recientemente, los datos de varios estudios moleculares, apoyan fuertemente la idea de que R. seeberi es un patógeno eucariota, pero no un hongo. La semejanza morfológica propuesta por algunos de que R. seeberi es similar a los miembros de los géneros Microcystis (bacteria), Synchytrium y Colletotrichum (hongos) es meramente hipotética y no tiene el rigor científico necesario para validar el sistema propuesto. Un aspecto fundamental en contra de la teoría procariota es la presencia de núcleos descrita por numerosos autores y que actualizamos en esta revisión. Además, las características ultra-estructurales de los géneros Microcystis y Synchytrium y de sus ciclos celulares no han sido encontradas en la fase parasitaria de R. seeberi. La amplificación por PCR de una secuencia del rADN 16S típica de las cianobacterias en muestras de casos de rinosporidiosis, aunque interesante, será considerada en esta revisión como una anomalía debido a la contaminación con el medio ambiente (Microcystis) o tal vez como una adquisición endosimbiótica de plastidios a partir de cianobacterias ancestrales. Así pues, aunque R. seeberi podría poseer ADN procariota, esto no demuestra necesariamente que R. seeberi sea una cianobacteria. La clasificación de R. seeberi dentro de los hongos es insostenible. El aislamiento de un hongo, los análisis de ADN realizados, y la ausencia de controles apropiados son los problemas más importantes de esta teoría. Más estudios serán necesarios para validar la adquisición de plastidios procariotas en R. seeberi, y otros temas que requieren un cuidadoso escrutinio(AU)

Rhinosporidum seeberi is the etiologic agent of rhinosporidiosis, a disease of mucous membranes and infrequent of the skin and other tissues of humans and animals. Because it resists culture, for more than 100 years true taxonomic identity of R. seeberi has been controversial. Three hypotheses in a long list of related views have been recently introduced: 1) a prokaryote cyanobacterium in the genus Microcystis is the etiologic agent of rhinosporidiosis, 2) R. seeberi is a eukaryote pathogen in the Mesomycetozoa and 3) R. seeberi is a fungus. The reviewed literature on the electron microscopic, the histopathological and more recently the data from several molecular studies strongly support the view that R. seeberi is a eukaryote pathogen, but not a fungus. The suggested morphological resemblance of R. seeberi with the genera Microcystis (bacteria), Synchytrium and Colletotrichum (fungi) by different teams is merely hypothetical and lacked the scientific rigor needed to validate the proposed systems. A fundamental aspect against the prokaryote theory is the presence of nuclei reported by numerous authors and updated in this review. Moreover, Microcystis's and Synchytrium's ultra-structural and key cell cycle traits cannot be found in R. seeberi parasitic phase. The PCR amplification of a cyanobacteria 16S rDNA sequence from cases of rhinosporidiosis, while intriguing, will be viewed here as an anomaly due to contamination with environmental Microcystis or perhaps as an endosymbiotic acquisition of plastids from cyanobacteria ancestors. Thus, even if R. seeberi possesses prokaryote DNA, this does not prove that R. seeberi is a cyanobacterium. The placement of R. seeberi within the fungi is scientifically untenable. The isolation and the DNA analysis performed in a fungal strain, and the lack of appropriate controls are the main problems of this claim. Further studies are needed to validate R. seeberi's acquisition of prokaryote plastids and other issues that still need careful scrutiny(AU)

Archaea, Bacteria, and algal plastids associated with the reef-building corals Siderastrea stellata and Mussismilia hispida from Búzios, South Atlantic Ocean, Brazil.

Reef-building corals may be seen as holobiont organisms, presenting diverse associated microbial communities. Best known is the symbiotic relationship with zooxanthellae, but Archaea, Bacteria, fungi, viruses, and algal plastids are also abundant. Until now, there is little information concerning microbial communities associated with Brazilian corals. The present study aims to describe the diversity of Archaea, Bacteria, and eukaryotic algal plastid communities associated with two sympatric species, Siderastrea stellata and Mussismilia hispida, from Southeastern Brazil, using 16S rRNA gene libraries. Since corals present a high number of other associated invertebrates, coral barcoding (COI) was performed to confirm the exclusive occurrence of coral DNA in our samples. Our analysis yielded 354 distinct microbial OTUs, represented mainly by novel phylotypes. Richness (Chao1 and ACE) and diversity (H') estimations of the microbial communities associated with both species were high and comparable to other studies. Rarefaction analyses showed that microbial diversity of S. stellata is higher than that of M. hispida. Libshuff comparative analyses showed that the highest microbial community similarity between the two coral species occurred in the bacterial libraries, while archaeal and plastidial communities were significantly different. Crenarchaeota dominated archaeal communities, while Proteobacteria was the most abundant bacterial phylum, dominated by alpha-Proteobacteria. Plastids were also represented by novel phylotypes and did not match with any 16S rRNA sequences of Cyanobacteria and zooxanthellae from GenBank. Our data improves the pool of available information on Brazilian coral microbes and shows corals as sources of diverse prokaryotic and picoeukaryotic communities.

"Chromoplast" development in arbuscular mycorrhizal roots.

The accumulation of apocarotenoids in arbuscular mycorrhizal (AM) roots suggests a dramatic reorganization of the plastids responsible for the biosynthesis of these compounds. This review describes the cytological and biochemical characterization of this phenomenon. The results presented suggest that plastids are key organelles for the establishment of the symbiotic interface of the AM symbiosis. In addition, a complex interplay of various plant cell components during the different functional phases of this interface is suggested. Arbuscule degradation appears to be of particular interest, as it correlates with the formation of the most extensive plastid structures and with apocarotenoid accumulation.

Enrichment for microbes living in association with plant tissues.

AIMS: To investigate a cultivation-independent method of enrichment for microbes living in association with plant tissues. METHODS AND RESULTS: A large quantity of leaves or seeds was enzymatically hydrolyzed, and the pellets were collected by differential centrifugation. Enzyme concentration, buffer and incubation time were optimized for release of plant-associated microbes. The relative abundance of plant nuclear DNA and bacterial DNA in the enriched sample was estimated by PCR amplification of genome-specific marker genes. The efficiency of microbe enrichment was estimated from the proportion of bacterium-derived clones and their restriction fragment length polymorphism (RFLP) types as detected by 16S rRNA gene-based techniques. With a higher ratio of bacterial to plant nuclear DNA, the enriched samples showed a considerably enhanced proportion of bacterium-derived clones and a wider sequence diversity of those clones. CONCLUSIONS: The method described here proved to be remarkably effective in enriching for bacteria living in association with plant tissues. SIGNIFICANCE AND IMPACT OF THE STUDY: The method can be applied to study plant-associated microbes in the field of environmental molecular ecology and environmental metagenomics.

Fungus propagules in plastids: the mycosome hypothesis.

A stress-induced "mycosome" phase of Aureobasidium pullulans consisting of minute reproductive propagules that may revert directly to walled yeast cells is described. Mycosomes detected by light- and electron-microscopy reproduce within senescent plant plastids, and display three developmental pathways: wall-less cells (protoplasts), yeast cells, or membrane-bounded spherules that harbor plastids. Widespread in plant and algal cells, mycosomes are produced by both ascomycete and basidiomycete fungi.

Fungus propagules in plastids: the mycosome hypothesis / Propágulos fúngicos en plástidos: la hipótesis micosómica

A stress-induced «mycosome» phase of Aureobasidium pullulans consisting of minute reproductive propagules that may revert directly to walled yeast cells is described. Mycosomes detected by light- and electron-microscopy reproduce within senescent plant plastids, and display three developmental pathways: wall-less cells (protoplasts), yeast cells, or membrane-bounded spherules that harbor plastids. Widespread in plant and algal cells, mycosomes are produced by both ascomycete and basidiomycete fungi (AU)

En este artículo se describe una fase «micosómica» inducida por estrés en Aureobasidium pullulans, consistente en minúsculos propágulos reproductivos que pueden revertir directamente a células de levadura con pared. Los micosomas, detectados por microscopía óptica y electrónica, se reproducen en el interior de plástidos senescentes en plantas, y muestran tres tipos de desarrollo diferentes: células sin pared (protoplastos), células de levadura, y esférulas rodeadas por membrana que contienen plástidos. Muy extendidos en células de plantas o algas, los micosomas son producidos por hongos ascomicetes o basidiomicetes (AU)

Evolutionary analysis of Arabidopsis, cyanobacterial, and chloroplast genomes reveals plastid phylogeny and thousands of cyanobacterial genes in the nucleus.

Chloroplasts were once free-living cyanobacteria that became endosymbionts, but the genomes of contemporary plastids encode only approximately 5-10% as many genes as those of their free-living cousins, indicating that many genes were either lost from plastids or transferred to the nucleus during the course of plant evolution. Previous estimates have suggested that between 800 and perhaps as many as 2,000 genes in the Arabidopsis genome might come from cyanobacteria, but genome-wide phylogenetic surveys that could provide direct estimates of this number are lacking. We compared 24,990 proteins encoded in the Arabidopsis genome to the proteins from three cyanobacterial genomes, 16 other prokaryotic reference genomes, and yeast. Of 9,368 Arabidopsis proteins sufficiently conserved for primary sequence comparison, 866 detected homologues only among cyanobacteria and 834 other branched with cyanobacterial homologues in phylogenetic trees. Extrapolating from these conserved proteins to the whole genome, the data suggest that approximately 4,500 of Arabidopsis protein-coding genes ( approximately 18% of the total) were acquired from the cyanobacterial ancestor of plastids. These proteins encompass all functional classes, and the majority of them are targeted to cell compartments other than the chloroplast. Analysis of 15 sequenced chloroplast genomes revealed 117 nuclear-encoded proteins that are also still present in at least one chloroplast genome. A phylogeny of chloroplast genomes inferred from 41 proteins and 8,303 amino acids sites indicates that at least two independent secondary endosymbiotic events have occurred involving red algae and that amino acid composition bias in chloroplast proteins strongly affects plastid genome phylogeny.

Reorganization of tobacco root plastids during arbuscule development.

In the present paper we analyzed plastid populations labeled by the green fluorescent protein in non-mycorrhizal and mycorrhizal roots of tobacco (Nicotiana tahacum L.). We show by confocal laser scanning microscopy (i) a dramatic increase in these plastids in mycorrhizal roots and (ii) the formation of dense plastid networks covering the symbiotic interface of the arbuscular mycorrhiza, the arbuscule. These cytological observations point to an important role of root cortical cell plastids in the functioning of arbuscular mycorrhizal symbiosis.