Evidence for autocatalytic cross-linking of hydroxyproline-rich glycoproteins during extracellular matrix assembly in Volvox

The microalga Volvox carteri as a cell supportive building block for tissue engineering

Background

V. carteri f. nagariensis constitutes, in its most simplified form, a cellularized spheroid built around and stabilised by a form of primitive extracellular matrix (ECM).

Methods

We developed a modular approach to soft tissue engineering, by compact stacking V. carteri-based building blocks. This approach is made possible by the structure and cell adhesive properties of these building blocks, which results from the composition of their algal ECM.

Results

A primary biocompatibility assessment demonstrated the cytocompatibility of the algal suspension, its histogenesis-promoting properties, and that it did not induce an inflammatory response in vitro. These results allowed us to consider the use of this algal suspension for soft tissue augmentation, and to initiate an in vivo biocompatibility study. V. carteri exhibited cellular fate-directing properties, causing (i) fibroblasts to take on an alkaline phosphatase+ stem-cell-like phenotype and (ii) both human adipose-derived stem cells and mouse embryonic stem cells to differentiate into preadipocytes to adipocytes. The ability of V. carteri to support histogenesis and adipogenesis was also observed in vivo by subcutaneous tissue augmentation of athymic mice, highlighting the potential of V. carteri to support or influence tissue regeneration.

Conclusions

We present for the first time V. carteri as an innovative and inspiring biomaterial for tissue engineering and soft tissue regeneration. Its strategies in terms of shape, structure and composition can be central in the design of a new generation of bio-inspired heterogeneous biomaterials recapitulating more appropriately the complexity of body tissues when guiding their regeneration.

Sanguisorba officinalis L. Ameliorates Hepatic Steatosis and Fibrosis by Modulating Oxidative Stress, Fatty Acid Oxidation, and Gut Microbiota in CDAHFD-Induced Mice

Non-alcoholic fatty liver disease (NAFLD) is a leading cause of chronic liver diseases and encompasses non-alcoholic steatosis, steatohepatitis, and fibrosis. Sanguisorba officinalis L. (SO) roots have traditionally been used for their antioxidant properties and have beneficial effects on metabolic disorders, including diabetes and obesity. However, its effects on hepatic steatosis and fibrosis remain unclear. In this study, we explored the effects of a 95% ethanolic SO extract (SOEE) on NAFLD and fibrosis in vivo and in vitro. The SOEE was orally administered to C57BL/6J mice fed a choline-deficient, L-amino-acid-defined, high-fat diet for 10 weeks. The SOEE inhibited hepatic steatosis by modulating hepatic malondialdehyde levels and the expression of oxidative stress-associated genes, regulating fatty-acid-oxidation-related genes, and inhibiting the expression of genes that are responsible for fibrosis. The SOEE suppressed the deposition of extracellular matrix hydroxyproline and mRNA expression of fibrosis-associated genes. The SOEE decreased the expression of fibrosis-related genes in vitro by inhibiting SMAD2/3 phosphorylation. Furthermore, the SOEE restored the gut microbial diversity and modulated specific bacterial genera associated with NAFLD and fibrosis. This study suggests that SOEE might be the potential candidate for inhibiting hepatic steatosis and fibrosis by modulating oxidative stress, fatty acid oxidation, and gut microbiota composition.

Cell Type-Specific Promoters of Volvox carteri for Molecular Cell Biology Studies

The multicellular green alga Volvox carteri has emerged as a valuable model organism for investigating various aspects of multicellularity and cellular differentiation, photoreception and phototaxis, cell division, biogenesis of the extracellular matrix and morphogenetic movements. While a range of molecular tools and bioinformatics resources have been made available for exploring these topics, the establishment of cell type-specific promoters in V. carteri has not been achieved so far. Therefore, here, we conducted a thorough screening of transcriptome data from RNA sequencing analyses of V. carteri in order to identify potential cell type-specific promoters. Eventually, we chose two putative strong and cell type-specific promoters, with one exhibiting specific expression in reproductive cells (gonidia), the PCY1 promoter, and the other in somatic cells, the PFP promoter. After cloning both promoter regions, they were introduced upstream of a luciferase reporter gene. By using particle bombardment, the DNA constructs were stably integrated into the genome of V. carteri. The results of the expression analyses, which were conducted at both the transcript and protein levels, demonstrated that the two promoters drive cell type-specific expression in their respective target cell types. Transformants with considerably diverse expression levels of the chimeric genes were identifiable. In conclusion, the screening and analysis of transcriptome data from RNA sequencing allowed for the identification of potential cell type-specific promoters in V. carteri. Reporter gene constructs demonstrated the actual usability of two promoters. The investigated PCY1 and PFP promoters were proven to be potent molecular tools for genetic engineering in V. carteri.

Cell Type-Specific Pherophorins of Volvox carteri Reveal Interplay of Both Cell Types in ECM Biosynthesis

The spheroidal green algae Volvox carteri serves as a model system to investigate the formation of a complex, multifunctional extracellular matrix (ECM) in a relatively simple, multicellular organism with cell differentiation. The V. carteri ECM is mainly composed of hydroxyproline-rich glycoproteins (HRGPs) and there are diverse region-specific, anatomically distinct structures in the ECM. One large protein family with importance for ECM biosynthesis stands out: the pherophorins. The few pherophorins previously extracted from the ECM and characterized, were specifically expressed by somatic cells. However, the localization and function of most pherophorins is unknown. Here, we provide a phylogenetic analysis of 153 pherophorins of V. carteri and its unicellular relative Chlamydomonas reinhardtii. Our analysis of cell type-specific mRNA expression of pherophorins in V. carteri revealed that, contrary to previous assumptions, only about half (52%) of the 102 investigated pherophorin-related genes show stronger expression in somatic cells, whereas about one-third (34%) of the genes show significant higher expression in reproductive cells (gonidia). We fused two pherophorin genes that are expressed by different cell types to yfp, stably expressed them in Volvox and studied the tagged proteins by live-cell imaging. In contrast to earlier biochemical approaches, this genetic approach also allows the in vivo analysis of non-extractable, covalently cross-linked ECM proteins. We demonstrate that the soma-specific pherophorin SSG185 is localized in the outermost ECM structures of the spheroid, the boundary zone and at the flagellar hillocks. SSG185:YFP is detectable as early as 1.5 h after completion of embryogenesis. It is then present for the rest of the life cycle. The gonidia-specific pherophorin PhG is localized in the gonidial cellular zone 1 ("gonidial vesicle") suggesting its involvement in the protection of gonidia and developing embryos until hatching. Even if somatic cells produce the main portion of the ECM of the spheroids, ECM components produced by gonidia are also required to cooperatively assemble the total ECM. Our results provide insights into the evolution of the pherophorin protein family and convey a more detailed picture of Volvox ECM synthesis.

Unassembled cell wall proteins form aggregates in the extracellular space of Chlamydomonas reinhardtii strain UVM4

Abstract

The green microalga Chlamydomonas reinhardtii is emerging as a promising cell biofactory for secreted recombinant protein (RP) production. In recent years, the generation of the broadly used cell wall–deficient mutant strain UVM4 has allowed for a drastic increase in secreted RP yields. However, purification of secreted RPs from the extracellular space of C. reinhardtii strain UVM4 is challenging. Previous studies suggest that secreted RPs are trapped in a matrix of cell wall protein aggregates populating the secretome of strain UVM4, making it difficult to isolate and purify the RPs. To better understand the nature and behaviour of these extracellular protein aggregates, we analysed and compared the extracellular proteome of the strain UVM4 to its cell-walled ancestor, C. reinhardtii strain 137c. When grown under the same conditions, strain UVM4 produced a unique extracellular proteomic profile, including a higher abundance of secreted cell wall glycoproteins. Further characterization of high molecular weight extracellular protein aggregates in strain UVM4 revealed that they are largely comprised of pherophorins, a specific class of cell wall glycoproteins. Our results offer important new insights into the extracellular space of strain UVM4, including strain-specific secreted cell wall proteins and the composition of the aggregates possibly related to impaired RP purification. The discovery of pherophorins as a major component of extracellular protein aggregates will inform future strategies to remove or prevent aggregate formation, enhance purification of secreted RPs, and improve yields of recombinant biopharmaceuticals in this emerging cell biofactory.

Key points

• Extracellular protein aggregates hinder purification of recombinant proteins in C. reinhardtii

• Unassembled cell wall pherophorins are major components of extracellular protein aggregates

• Known aggregate composition informs future strategies for recombinant protein purification

Supplementary Information

The online version contains supplementary material available at 10.1007/s00253-022-11960-9.

Targeted migration of pherophorin-S indicates extensive extracellular matrix dynamics in Volvox carteri

Hydroxyproline-rich glycoproteins (HRGPs) constitute a major group of proteins of the extracellular matrix (ECM). The multicellular green alga Volvox carteri is a suitable model organism in which to study the evolutionary transition to multicellularity, including the basic principles and characteristics of an ECM. In Volvox, the ECM is dominated by a single HRGP family: the pherophorins. Our inventory amounts to 117 pherophorin-related genes in V. carteri. We focused on a pherophorin with an unexpected characteristic: pherophorin-S is a soluble, non-cross-linked ECM protein. Using transformants expressing a YFP-tagged pherophorin-S we observed the synthesis and secretion of pherophorin-S by somatic cells in vivo, and we then traced the protein during its conspicuous migration to the ECM around prehatching juveniles and its localized concentration there. Our results provide insights into how an ECM zone surrounding the progeny is remotely affected by distantly located parental somatic cells. In view of the properties and migration of pherophorin-S, we conclude that pherophorin-S is likely to act as an ECM plasticizer to allow for dynamic ECM remodeling.

Whole transcriptome RNA-Seq analysis reveals extensive cell type-specific compartmentalization in Volvox carteri

Background

One of evolution’s most important achievements is the development and radiation of multicellular organisms with different types of cells. Complex multicellularity has evolved several times in eukaryotes; yet, in most lineages, an investigation of its molecular background is considerably challenging since the transition occurred too far in the past and, in addition, these lineages evolved a large number of cell types. However, for volvocine green algae, such as Volvox carteri, multicellularity is a relatively recent innovation. Furthermore, V. carteri shows a complete division of labor between only two cell types – small, flagellated somatic cells and large, immotile reproductive cells. Thus, V. carteri provides a unique opportunity to study multicellularity and cellular differentiation at the molecular level.

Results

This study provides a whole transcriptome RNA-Seq analysis of separated cell types of the multicellular green alga V. carteri f. nagariensis to reveal cell type-specific components and functions. To this end, 246 million quality filtered reads were mapped to the genome and valid expression data were obtained for 93% of the 14,247 gene loci. In the subsequent search for protein domains with assigned molecular function, we identified 9435 previously classified domains in 44% of all gene loci. Furthermore, in 43% of all gene loci we identified 15,254 domains that are involved in biological processes. All identified domains were investigated regarding cell type-specific expression. Moreover, we provide further insight into the expression pattern of previously described gene families (e.g., pherophorin, extracellular matrix metalloprotease, and VARL families). Our results demonstrate an extensive compartmentalization of the transcriptome between cell types: More than half of all genes show a clear difference in expression between somatic and reproductive cells.

Conclusions

This study constitutes the first transcriptome-wide RNA-Seq analysis of separated cell types of V. carteri focusing on gene expression. The high degree of differential expression indicates a strong differentiation of cell types despite the fact that V. carteri diverged relatively recently from its unicellular relatives. Our expression dataset and the bioinformatic analyses provide the opportunity to further investigate and understand the mechanisms of cell type-specific expression and its transcriptional regulation.

Electronic supplementary material

The online version of this article (doi:10.1186/s12915-017-0450-y) contains supplementary material, which is available to authorized users.

Ni induces the CRR1-dependent regulon revealing overlap and distinction between hypoxia and Cu deficiency responses in Chlamydomonas reinhardtii

The selectivity of metal sensors for a single metal ion is critical for cellular metal homeostasis. A suite of metal-responsive regulators is required to maintain a prescribed balance of metal ions ensuring that each apo-protein binds the correct metal. However, there are cases when non-essential metals ions disrupt proper metal sensing. An analysis of the Ni-responsive transcriptome of the green alga Chlamydomonas reinhardtii reveals that Ni artificially turns on the CRR1-dependent Cu-response regulon. Since this regulon also responds to hypoxia, a combinatorial transcriptome analysis was leveraged to gain insight into the mechanisms by which Ni interferes with the homeostatic regulation of Cu and oxygen status. Based on parallels with the effect of Ni on the hypoxic response in animals, we propose that a possible link between Cu, oxygen and Ni sensing is an as yet uncharacterized prolyl hydroxylase that regulates a co-activator of CRR1. This analysis also identified transcriptional responses to the pharmacological activation of the Cu-deficiency regulon. Although the Ni-responsive CRR1 regulon is composed of 56 genes (defined as the primary response), 259 transcripts responded to Ni treatment only when a copy of the wild-type CRR1 gene was present. The genome-wide impact of CRR1 target genes on the transcriptome was also evident from the 210 transcripts that were at least 2-fold higher in the crr1 strain, where the abundance of many CRR1 targets was suppressed. Additionally, we identified 120 transcripts that responded to Ni independent of CRR1 function. The putative functions of the proteins encoded by these transcripts suggest that high Ni results in protein damage.

The inducible nitA promoter provides a powerful molecular switch for transgene expression in Volvox carteri

BACKGROUND

The multicellular green alga Volvox carteri represents an attractive model system to study various aspects of multicellularity like cellular differentiation, morphogenesis, epithelial folding and ECM biogenesis. However, functional and molecular analyses of such processes require a wide array of molecular tools for genetic engineering. So far there are only a limited number of molecular tools available in Volvox.

RESULTS

Here, we show that the promoter of the V. carteri nitrate reductase gene (nitA) is a powerful molecular switch for induction of transgene expression. Strong expression is triggered by simply changing the nitrogen source from ammonium to nitrate. We also show that the luciferase (g-luc) gene from the marine copepod Gaussia princeps, which previously was engineered to match the codon usage of the unicellular alga Chlamydomonas reinhardtii, is a suitable reporter gene in V. carteri. Emitted light of the chemiluminescent reaction can be easily detected and quantified with a luminometer. Long-term stability of inducible expression of the chimeric nitA/g-luc transgenes after stable nuclear transformation was demonstrated by transcription analysis and bioluminescence assays.

CONCLUSION

Two novel molecular tools for genetic engineering of Volvox are now available: the nitrate-inducible nitA promoter of V. carteri and the codon-adapted luciferase reporter gene of G. princeps. These novel tools will be useful for future molecular research in V. carteri.

De novo sequencing, assembly, and analysis of the root transcriptome of Persea americana (Mill.) in response to Phytophthora cinnamomi and flooding

Avocado is a diploid angiosperm containing 24 chromosomes with a genome estimated to be around 920 Mb. It is an important fruit crop worldwide but is susceptible to a root rot caused by the ubiquitous oomycete Phytophthora cinnamomi. Phytophthora root rot (PRR) causes damage to the feeder roots of trees, causing necrosis. This leads to branch-dieback and eventual tree death, resulting in severe losses in production. Control strategies are limited and at present an integrated approach involving the use of phosphite, tolerant rootstocks, and proper nursery management has shown the best results. Disease progression of PRR is accelerated under high soil moisture or flooding conditions. In addition, avocado is highly susceptible to flooding, with even short periods of flooding causing significant losses. Despite the commercial importance of avocado, limited genomic resources are available. Next generation sequencing has provided the means to generate sequence data at a relatively low cost, making this an attractive option for non-model organisms such as avocado. The aims of this study were to generate sequence data for the avocado root transcriptome and identify stress-related genes. Tissue was isolated from avocado infected with P. cinnamomi, avocado exposed to flooding and avocado exposed to a combination of these two stresses. Three separate sequencing runs were performed on the Roche 454 platform and produced approximately 124 Mb of data. This was assembled into 7685 contigs, with 106 448 sequences remaining as singletons. Genes involved in defence pathways such as the salicylic acid and jasmonic acid pathways as well as genes associated with the response to low oxygen caused by flooding, were identified. This is the most comprehensive study of transcripts derived from root tissue of avocado to date and will provide a useful resource for future studies.

Oxygen levels and the regulation of cell adhesion in the nervous system: a control point for morphogenesis in development, disease and evolution?

In this article, I discuss the hallmarks of hypoxia in vitro and in vivo and review work showing that many types of stem cell proliferate more robustly in lowered oxygen. I then discuss recent studies showing that alterations in the levels and the types of cell and substrate adhesion molecules are a notable response to reduced O(2) levels in both cultured primary neural stem cells and brain tissues in response to hypoxia in vivo. The ability of O(2) levels to regulate adhesion molecule expression is linked to the Wnt signaling pathway, which can control and be controlled by adhesion events. The ability of O(2) levels to influence cell adhesion also has far-reaching implications for development, ischemic trauma and neural regeneration, as well as for cancer and other diseases. Finally I discuss the possibility that the fluctuations in O(2) levels known to have occurred over evolutionary time could, by influencing adhesion systems, have contributed to early symbiotic events in unicellular organisms and to the emergence of multicellularity. It is not my intention to be exhaustive in these domains, which are far from my own field of study. Rather this article is meant to provoke and stimulate thinking about molecular evolution involving O(2) sensing and signaling during eras of geologic and atmospheric change that might inform modern studies on development and disease.

Genomics of Volvocine Algae

Volvocine algae are a group of chlorophytes that together comprise a unique model for evolutionary and developmental biology. The species Chlamydomonas reinhardtii and Volvox carteri represent extremes in morphological diversity within the Volvocine clade. Chlamydomonas is unicellular and reflects the ancestral state of the group, while Volvox is multicellular and has evolved numerous innovations including germ-soma differentiation, sexual dimorphism, and complex morphogenetic patterning. The Chlamydomonas genome sequence has shed light on several areas of eukaryotic cell biology, metabolism and evolution, while the Volvox genome sequence has enabled a comparison with Chlamydomonas that reveals some of the underlying changes that enabled its transition to multicellularity, but also underscores the subtlety of this transition. Many of the tools and resources are in place to further develop Volvocine algae as a model for evolutionary genomics.

Genetic engineering of algae for enhanced biofuel production

There are currently intensive global research efforts aimed at increasing and modifying the accumulation of lipids, alcohols, hydrocarbons, polysaccharides, and other energy storage compounds in photosynthetic organisms, yeast, and bacteria through genetic engineering. Many improvements have been realized, including increased lipid and carbohydrate production, improved H(2) yields, and the diversion of central metabolic intermediates into fungible biofuels. Photosynthetic microorganisms are attracting considerable interest within these efforts due to their relatively high photosynthetic conversion efficiencies, diverse metabolic capabilities, superior growth rates, and ability to store or secrete energy-rich hydrocarbons. Relative to cyanobacteria, eukaryotic microalgae possess several unique metabolic attributes of relevance to biofuel production, including the accumulation of significant quantities of triacylglycerol; the synthesis of storage starch (amylopectin and amylose), which is similar to that found in higher plants; and the ability to efficiently couple photosynthetic electron transport to H(2) production. Although the application of genetic engineering to improve energy production phenotypes in eukaryotic microalgae is in its infancy, significant advances in the development of genetic manipulation tools have recently been achieved with microalgal model systems and are being used to manipulate central carbon metabolism in these organisms. It is likely that many of these advances can be extended to industrially relevant organisms. This review is focused on potential avenues of genetic engineering that may be undertaken in order to improve microalgae as a biofuel platform for the production of biohydrogen, starch-derived alcohols, diesel fuel surrogates, and/or alkanes.

Genes related to immunity, as expressed in the alfalfa leafcutting bee, Megachile rotundata, during pathogen challenge

Virtually nothing is known about disease resistance in solitary bees, so expressed sequence tag (EST) databases were developed to search for immune response genes in the alfalfa leafcutting bee. We identified 104 putative immunity-related genes from both healthy and pathogen-challenged bee larvae, and 12 more genes using PCR amplification. The genes identified coded for proteins with a wide variety of innate immune response functions, including pathogen recognition, phagocytosis, the prophenoloxidase cascade, melanization, coagulation and several signalling pathways. Some immune response genes were highly conserved with honey bee genes, and more distantly related to other insects. The data presented provides the first analysis of immune function in a solitary bee and provides a foundation for the further analysis of gene expression patterns in bees.

Chemical and in situ characterization of macromolecular components of the cell walls from the green seaweed Codium fragile

A comprehensive analysis of the carbohydrate-containing macromolecules from the coencocytic green seaweed Codium fragile and their arrangement in the cell wall was carried out. Cell walls in this seaweed are highly complex structures composed of 31% (w/w) of linear (1-->4)-beta-D-mannans, 9% (w/w) of pyruvylated arabinogalactan sulfates (pAGS), and low amounts of hydroxyproline rich-glycoprotein epitopes (HRGP). In situ chemical imaging by synchrotron radiation Fourier transform infrared (SR-FTIR) microspectroscopy and by immunolabeling using antibodies against specific cell wall carbohydrate epitopes revealed that beta-d-mannans and pAGS are placed in the middle part of the cell wall, whereas HRGP epitopes (arabinogalactan proteins (AGPs) and extensins) are located on the wall boundaries, especially in the utricle apical zone. pAGS are sulfated at C-2 and/or C-4 of the 3-linked beta-L-arabinopyranose units and at C-4 and/or C-6 of the 3-linked beta-D-galactopyranose residues. In addition, high levels of ketals of pyruvic acid were found mainly at 3,4- of some terminal beta-D-Galp units forming a five-membered ring. Ramification was found at some C-6 of the 3-linked beta-D-Galp units. In agreement with the immunolabeled AGP epitopes, a nonsulfated branched furanosidic arabinan with 5-linked alpha-L-Araf, 3,5-linked alpha-L-Araf, and terminal alpha-L-Araf units and a nonsulfated galactan structure composed of 3-(3,6)-linked beta-D-Galp residues, both typical of type-II AG glycans were found, suggesting that AGP structures are present at low levels in the cell walls of this seaweed. Based on this study, it is starting to emerge that Codium has developed unique cell wall architecture, when compared, not only with that of vascular plants, but also with other related green seaweeds and algae.

CELL WALL CARBOHYDRATE EPITOPES IN THE GREEN ALGA OEDOGONIUM BHARUCHAE F. MINOR (OEDOGONIALES, CHLOROPHYTA)(1)

Cell wall changes in vegetative and suffultory cells (SCs) and in oogonial structures from Oedogonium bharuchae N. D. Kamat f. minor Vélez were characterized using monoclonal antibodies against several carbohydrate epitopes. Vegetative cells and SCs develop only a primary cell wall (PCW), whereas mature oogonial cells secrete a second wall, the oogonium cell wall (OCW). Based on histochemical and immunolabeling results, (1→4)-β-glucans in the form of crystalline cellulose together with a variable degree of Me-esterified homogalacturonans (HGs) and hydroxyproline-rich glycoprotein (HRGP) epitopes were detected in the PCW. The OCW showed arabinosides of the extensin type and low levels of arabinogalactan-protein (AGP) glycans but lacked cellulose, at least in its crystalline form. Surprisingly, strong colabeling in the cytoplasm of mature oogonia cells with three different antibodies (LM-5, LM-6, and CCRC-M2) was found, suggesting the presence of rhamnogalacturonan I (RG-I)-like structures. Our results are discussed relating the possible functions of these cell wall epitopes with polysaccharides and O-glycoproteins during oogonium differentiation. This study represents the first attempt to characterize these two types of cell walls in O. bharuchae, comparing their similarities and differences with those from other green algae and land plants. This work represents a contribution to the understanding of how cell walls have evolved from simple few-celled to complex multicelled organisms.

Transcriptional analysis of the sweet orange interaction with the citrus canker pathogens Xanthomonas axonopodis pv. citri and Xanthomonas axonopodis pv. aurantifolii

Xanthomonas axonopodis pv. citri (Xac) and Xanthomonas axonopodis pv. aurantifolii pathotype C (Xaa) are responsible for citrus canker disease; however, while Xac causes canker on all citrus varieties, Xaa is restricted to Mexican lime, and in sweet oranges it triggers a defence response. To gain insights into the differential pathogenicity exhibited by Xac and Xaa and to survey the early molecular events leading to canker development, a detailed transcriptional analysis of sweet orange plants infected with the pathogens was performed. Using differential display, suppressed subtractive hybridization and microarrays, we identified changes in transcript levels in approximately 2.0% of the approximately 32,000 citrus genes examined. Genes with altered expression in response to Xac/Xaa surveyed at 6 and 48 h post-infection (hpi) were associated with cell-wall modifications, cell division and expansion, vesicle trafficking, disease resistance, carbon and nitrogen metabolism, and responses to hormones auxin, gibberellin and ethylene. Most of the genes that were commonly modulated by Xac and Xaa were associated with basal defences triggered by pathogen-associated molecular patterns, including those involved in reactive oxygen species production and lignification. Significantly, we detected clear changes in the transcriptional profiles of defence, cell-wall, vesicle trafficking and cell growth-related genes in Xac-infected leaves between 6 and 48 hpi. This is consistent with the notion that Xac suppresses host defences early during infection and simultaneously changes the physiological status of the host cells, reprogramming them for division and growth. Notably, brefeldin A, an inhibitor of vesicle trafficking, retarded canker development. In contrast, Xaa triggered a mitogen-activated protein kinase signalling pathway involving WRKY and ethylene-responsive transcriptional factors known to activate downstream defence genes.

Sexual pheromone induces diffusion of the pheromone-homologous polypeptide in the extracellular matrix of Volvox carteri

The C-terminal domain of pherophorin II is homologous to the sexual pheromone of Volvox carteri and is released from other domains during sexual induction. Green fluorescent protein fused to the C terminus of pherophorin II was located at the extracellular matrix directly surrounding the gonidium, the final target of the sexual-induction signal.

A small cysteine-rich extracellular protein, VCRP, is inducible by the sex-inducer of Volvox carteri and by wounding

The green alga Volvox carteri represents one of the simplest multicellular organisms: it is composed of only two cell types, somatic and reproductive. Volvox is capable of both vegetative and sexual reproduction. Sexual development of males and females is triggered by a sex-inducer at concentrations as low as 10(-16) M. By differential screenings of cDNA libraries, a novel gene was identified that is under the control of this sex-inducer and that encodes a small cysteine-rich extracellular protein, named VCRP. Analysis of the VCRP polypeptide sequence suggests ten disulfide bonds and a dimetal-binding capacity. VCRP mRNA is detectable in males and females approximately 1 h after the spheroids' first contact with the sex-inducer, but transcription is restricted to the somatic cell-type. mRNA and protein synthesis is triggered not only by the sex-inducer, but also by wounding. VCRP does not share significant sequence similarity with any known protein sequence, but a potential EGF-like calcium-binding motif and a potential plant metallothionein family-15 motif have been identified. The characteristics of VCRP suggest a function as a signal transducer molecule, an extracellular second messenger from somatic cells to reproductive cells, or a role within the stress response.

Genes specifically expressed in sexually differentiated female spheroids of Volvox carteri

Volvox carteri is a multicellular green alga with only two cell types, somatic cells and reproductive cells. Phylogenetic analysis suggests that this organism has evolved from a Chlamydomonas-like unicellular ancestor along with multicellularity, cellular differentiation, and a change in the mode of sexual reproduction from isogamy to oogamy. To examine the mechanism of sexual differentiation and the evolution of oogamy, we isolated 6 different cDNA sequences specifically expressed in sexually differentiated female spheroids. The genes for the cDNAs were designated SEF1 to SEF6. The time course of accumulation of each mRNA was shown to be distinct. The expression of some of these genes was not significantly affected when the sexual inducer was removed after the induction of sexual development. Sequence analysis indicates that SEF5 and SEF6 encode pherophorin-related proteins. Of these, SEF5 has the unique structural feature of a polyproline stretch in the C-terminal domain in addition to the one found in the central region.

Sex as a response to oxidative stress: stress genes co-opted for sex

Despite a great deal of interest, the evolutionary origins and roles of sex remain unclear. Recently, we showed that in the multicellular green alga, Volvox carteri, sex is a response to increased levels of reactive oxygen species (ROS), which could be indicative of the ancestral role of sex as an adaptive response to stress-induced ROS. To provide additional support for the suggestion that sex evolved as a response to oxidative stress, this study addresses the hypothesis that genes involved in sexual induction are evolutionarily related to genes associated with various stress responses. In particular, this study investigates the evolutionary history of genes specific to the sexual induction process in V. carteri--including those encoding the sexual inducer (SI) and several SI-induced extracellular matrix (ECM) proteins. Surprisingly, (i) a highly diversified multigene family with similarity to the V. carteri SI and SI-induced pherophorin family is present in its unicellular relative, Chlamydomonas reinhardtii (which lacks both a SI and an ECM) and (ii) at least half of the 12 identified gene members are induced (as inferred from reported expressed sequence tags) under various stress conditions. These findings suggest an evolutionary connection between sex and stress at the gene level, via duplication and/or co-option.

The pherophorins: common, versatile building blocks in the evolution of extracellular matrix architecture in Volvocales

Green algae of the order Volvocales provide an unrivalled opportunity for exploring the transition from unicellularity to multicellularity. They range from unicells, like Chlamydomonas, through homocytic colonial forms with increasing cooperation of individual cells, like Gonium or Pandorina, to heterocytic multicellular forms with different cell types and a complete division of labour, like Volvox. A fundamental requirement for the evolution of multicellularity is the development of a complex, multifunctional extracellular matrix (ECM). The ECM has many functions, which can change under developmental control or as a result of environmental factors. Here molecular data from 15 novel proteins are presented. These proteins have been identified in Chlamydomonas reinhardtii, Gonium pectorale, Pandorina morum and Volvox carteri, and all belong to a single protein family, the pherophorins. Pherophorin-V1 is shown to be a glycoprotein localized to the 'cellular zone' of the V. carteri ECM. Pherophorin-V1 and -V2 mRNAs are strongly induced not only by the sex inducer, which triggers sexual development at extremely low concentrations, but also by mechanical wounding. Like the extensins of higher plants, which are also developmentally controlled or sometimes inducible by wounding, the pherophorins contain a (hydroxy-)proline-rich (HR) rod-like domain and are abundant within the extracellular compartment. In contrast to most extensins, pherophorins have additional globular A and B domains on both ends of the HR domains. Therefore pherophorins most closely resemble a particular class of higher plant extensin, the solanaceous lectins (e.g. potato lectin), suggesting multivalent carbohydrate-binding functions are present within the A and B domains and are responsible for cross-linking. Our results suggest that pherophorins are used as the building blocks for the extracellular scaffold throughout the Volvocales, with the characteristic mesh sizes in different ECM structures being a result of the highly diverse extensions of the HR domains. Pherophorins have therefore been a versatile element during the evolution of ECM architecture in these green algae.

Plus and minus sexual agglutinins from Chlamydomonas reinhardtii

Gametes of the unicellular green alga Chlamydomonas reinhardtii undergo sexual adhesion via enormous chimeric Hyp-rich glycoproteins (HRGPs), the plus and minus sexual agglutinins, that are displayed on their flagellar membrane surfaces. We have previously purified the agglutinins and analyzed their structural organization using electron microscopy. We report here the cloning and sequencing of the Sag1 and Sad1 genes that encode the two agglutinins and relate their derived amino acid sequences and predicted secondary structure to the morphology of the purified proteins. Both agglutinin proteins are organized into three distinct domains: a head, a shaft in a polyproline II configuration, and an N-terminal domain. The plus and minus heads are related in overall organization but poorly conserved in sequence except for two regions of predicted hydrophobic alpha-helix. The shafts contain numerous repeats of the PPSPX motif previously identified in Gp1, a cell wall HRGP. We propose that the head domains engage in autolectin associations with the distal termini of their own shafts and suggest ways that adhesion may involve head-head interactions, exolectin interactions between the heads and shafts of opposite type, and antiparallel shaft-shaft interactions mediated by carbohydrates displayed in polyproline II configurations.

Monitoring dynamic expression of nuclear genes in Chlamydomonas reinhardtii by using a synthetic luciferase reporter gene

For monitoring the expression profile of selected nuclear genes in Chlamydomonas reinhardtii in response to altered environmental parameters or during cell cycle, in the past many RNA or protein samples had to be taken and analyzed by RNA hybridization or protein immunoblotting. Here we report the synthesis of a gene that codes for the luciferase of Renilla reniformis (RLuc) and is adapted to the nuclear codon usage of C. reinhardtii . This crluc gene was expressed alone or as a fusion to the zeocin resistance gene ble under control of different promoter variants. Luciferase activity was monitored in living cells, increased with the promoter strength and paralleled the amount of expressed protein. Under control of the Lhcb-1 promoter the Luc-activity in synchronized cultures was dependent on the dark-light cycle. Additionally, crluc was placed under control of the Chop-2 promoter and activity was measured under different light conditions. Chop-2 promoter activity was found to be most pronouced under low-light and dark conditions, further supporting that channelrhodopsin-2 is most active in dark-adapted cells. We conclude that crluc is a reliable tool for convenient monitoring of nuclear gene expression in C. reinhardtii .

A biochemical and molecular characterization of LEP1, an extensin peroxidase from lupin

An analysis of apoplastic extensin cross-linking activity in vegetative organs of Lupinus albus indicated that leaves contained the highest specific activity. Assays of peroxidases fractionated from this material demonstrated that this activity could be largely attributed to a soluble and apoplastic 51-kDa peroxidase, denoted LEP1. Relative to other purified peroxidases, LEP1 demonstrates high extensin cross-linking activity and can be classified as an extensin peroxidase (EP). Optimal conditions for the in vitro oxidation of other phenolic substrates included 1.5-3.0 mm peroxide at pH 5.0. EP activity of LEP1 was low under these conditions but optimal and substantially higher with 100 microm peroxide and neutral pH, suggesting that physiological changes in pH and peroxide in muro could heavily influence the extensin cross-linking activity of LEP1 in vivo. Analysis of LEP1 glycans indicated 11-12 N-linked glycans, predominantly the heptasaccharide Man3XylFucGlcNAc2, but also larger structures showing substantial heterogeneity. Comparative assays with horseradish peroxidase isoform C and peanut peroxidases suggested the high level of glycosylation in LEP1 may be responsible for the high solubility of this EP in the apoplastic space. A full-length cDNA corresponding to LEP1 was cloned. Quantitative reverse transcriptase-PCR demonstrated LEP1 induction in apical portions of etiolated hypocotyls 30-60 min after exposure to white light, prior to the onset of growth inhibition. Comparative modeling of the translated sequence indicated an unusually unobstructed equatorial cleft across the substrate access channel, which might facilitate interaction with extensin and confer higher EP activity.

14-3-3 proteins are constituents of the insoluble glycoprotein framework of the chlamydomonas cell wall

The cell wall of the unicellular green alga Chlamydomonas reinhardtii consists predominantly of Hyp-rich glycoproteins, which also occur in the extracellular matrix of multicellular green algae and higher plants. In addition to the Hyp-rich polypeptides, the insoluble glycoprotein framework of the Chlamydomonas cell wall contains minor amounts of 14-3-3 proteins, as revealed by immunochemical studies and mass spectroscopic analysis of tryptic peptides. Polypeptides immunologically related to the 14-3-3 proteins also were found in the culture medium of Chlamydomonas. The levels of two of these 14-3-3-related polypeptides were decreased in the culture medium of the wall-deficient mutant cw-15. These findings indicate that 14-3-3 proteins are involved in the cross-linking of Hyp-rich glycoproteins in the Chlamydomonas cell wall.

Extracellular matrix and sex-inducing pheromone in Volvox

During evolution of multicellularity it was imperative to create a complex, multifunctional extracellular matrix (ECM) out of the simple cell wall of a unicellular ancestor. The green alga Volvox represents one of the simplest multicellular organisms, but even so, it already has a highly developed ECM. This ECM is mainly composed of an assortment of glycoproteins, many of which are hydroxyproline rich and extensively sulfated. Several ECM proteins are cross-linked and might have only structural functions. However, the ECM does not represent a static but rather a dynamic and multifunctional interface between a cell and its neighboring cells or its environment. It not only provides protection and structural support for the shape of each cell and the organism as a whole, but also plays a broad range of biological roles in growth, development, reproduction, and responses to environmental stress or wounding. The variety of functions of the ECM requires many glycoproteins to do the work. To attain a high flexibility and adaptability, almost all ECM glycoproteins from Volvox consist of modules, defined as functional subunits that form modular mosaic proteins with an outstanding combinatorial potential. The ECM's functions are not only extensive but also change under developmental control or by environmental incidents. The changing scope of duties necessitates a permanent ECM turnover and remodeling. In Volvox carteri one particularly challenging trigger of such ECM modifications is a sex-inducing pheromone, which is one of the most potent biological effector molecules known: the glycoprotein pheromone is fully effective for inducing sexual development in males and females at concentrations as low as 10(-16) M. The earliest detectable response to the pheromone is the synthesis of ECM glycoproteins.