Characteristics of Hormogonia Formation by Symbiotic Nostoc spp. in Response to the Presence of Anthoceros punctatus or Its Extracellular Products

Biotic interactions, evolutionary forces and the pan-plant specialized metabolism

Plant specialized metabolism has a complex evolutionary history. Some aspects are conserved across the green lineage, but many metabolites are unique to certain lineages. The network of specialized metabolism continuously diversified, simplified or reshaped during the evolution of streptophytes. Many routes of pan-plant specialized metabolism are involved in plant defence. Biotic interactions are recalled as major drivers of lineage-specific metabolomic diversification. However, the consequences of this diversity of specialized metabolism in the context of plant terrestrialization and land plant diversification into the major lineages of bryophytes, lycophytes, ferns, gymnosperms and angiosperms remain only little explored. Overall, this hampers conclusions on the evolutionary scenarios that shaped specialized metabolism. Recent efforts have brought forth new streptophyte model systems, an increase in genetically accessible species from distinct major plant lineages, and new functional data from a diversity of land plants on specialized metabolic pathways. In this review, we will integrate the recent data on the evolution of the plant immune system with the molecular data of specialized metabolism and its recognition. Based on this we will provide a contextual framework of the pan-plant specialized metabolism, the evolutionary aspects that shape it and the impact on adaptation to the terrestrial environment.This article is part of the theme issue 'The evolution of plant metabolism'.

Phylometagenomics of cycad coralloid roots reveals shared symbiotic signals

Cycads are known to host symbiotic cyanobacteria, including Nostocales species, as well as other sympatric bacterial taxa within their specialized coralloid roots. Yet, it is unknown if these bacteria share a phylogenetic origin and/or common genomic functions that allow them to engage in facultative symbiosis with cycad roots. To address this, we obtained metagenomic sequences from 39 coralloid roots sampled from diverse cycad species and origins in Australia and Mexico. Culture-independent shotgun metagenomic sequencing was used to validate sub-community co-cultures as an efficient approach for functional and taxonomic analysis. Our metanalysis shows a host-independent microbiome core consisting of seven bacterial orders with high species diversity within the identified taxa. Moreover, we recovered 43 cyanobacterial metagenome-assembled genomes, and in addition to Nostoc spp., symbiotic cyanobacteria of the genus Aulosira were identified for the first time. Using this robust dataset, we used phylometagenomic analysis to reveal three monophyletic cyanobiont clades, two host-generalist and one cycad-specific that includes Aulosira spp. Although the symbiotic clades have independently arisen, they are enriched in certain functional genes, such as those related to secondary metabolism. Furthermore, the taxonomic composition of associated sympatric bacterial taxa remained constant. Our research quadruples the number of cycad cyanobiont genomes and provides a robust framework to decipher cyanobacterial symbioses, with the potential of improving our understanding of symbiotic communities. This study lays a solid foundation to harness cyanobionts for agriculture and bioprospection, and assist in conservation of critically endangered cycads.

Symbiosis between cyanobacteria and plants: from molecular studies to agronomic applications

Nitrogen-fixing cyanobacteria from the order Nostocales are able to establish symbiotic relationships with diverse plant species. They are promiscuous symbionts, as the same strain of cyanobacterium is able to form symbiotic biological nitrogen-fixing relationships with different plants species. This review will focus on the different types of cyanobacterial-plant associations, both endophytic and epiphytic, and provide insights from a structural viewpoint, as well as our current understanding of the mechanisms involved in the symbiotic crosstalk. In all these symbioses, the benefit for the plant is clear; it obtains from the cyanobacterium fixed nitrogen and other bioactive compounds, such as phytohormones, polysaccharides, siderophores, or vitamins, leading to enhanced plant growth and productivity. Additionally, there is increasing use of different cyanobacterial species as bio-inoculants for biological nitrogen fixation to improve soil fertility and crop production, thus providing an eco-friendly, alternative, and sustainable approach to reduce the over-reliance on synthetic chemical fertilizers.

Hormogonium Development and Motility in Filamentous Cyanobacteria

Filamentous cyanobacteria exhibit some of the greatest developmental complexity observed in the prokaryotic domain. This includes the ability to differentiate nitrogen-fixing cells known as heterocysts, spore-like akinetes, and hormogonia, which are specialized motile filaments capable of gliding on solid surfaces. Hormogonia and motility play critical roles in several aspects of the biology of filamentous cyanobacteria, including dispersal, phototaxis, the formation of supracellular structures, and the establishment of nitrogen-fixing symbioses with plants. While heterocyst development has been investigated extensively at the molecular level, much less is known about akinete or hormogonium development and motility. This is due, in part, to the loss of developmental complexity during prolonged laboratory culture in commonly employed model filamentous cyanobacteria. In this review, recent progress in understanding the molecular level regulation of hormogonium development and motility in filamentous cyanobacteria is discussed, with a focus on experiments performed using the genetically tractable model filamentous cyanobacterium Nostoc punctiforme, which retains the developmental complexity of field isolates.

Diversity and regulation of symbiotic nitrogen fixation in plants

Plants associate with nitrogen-fixing bacteria to secure nitrogen, which is generally the most limiting nutrient for plant growth. Endosymbiotic nitrogen-fixing associations are widespread among diverse plant lineages, ranging from microalgae to angiosperms, and are primarily one of three types: cyanobacterial, actinorhizal or rhizobial. The large overlap in the signaling pathways and infection components of arbuscular mycorrhizal, actinorhizal and rhizobial symbioses reflects their evolutionary relatedness. These beneficial associations are influenced by environmental factors and other microorganisms in the rhizosphere. In this review, we summarize the diversity of nitrogen-fixing symbioses, key signal transduction pathways and colonization mechanisms relevant to such interactions, and compare and contrast these interactions with arbuscular mycorrhizal associations from an evolutionary standpoint. Additionally, we highlight recent studies on environmental factors regulating nitrogen-fixing symbioses to provide insights into the adaptation of symbiotic plants to complex environments.

What can hornworts teach us?

The hornworts are a small group of land plants, consisting of only 11 families and approximately 220 species. Despite their small size as a group, their phylogenetic position and unique biology are of great importance. Hornworts, together with mosses and liverworts, form the monophyletic group of bryophytes that is sister to all other land plants (Tracheophytes). It is only recently that hornworts became amenable to experimental investigation with the establishment of Anthoceros agrestis as a model system. In this perspective, we summarize the recent advances in the development of A. agrestis as an experimental system and compare it with other plant model systems. We also discuss how A. agrestis can help to further research in comparative developmental studies across land plants and to solve key questions of plant biology associated with the colonization of the terrestrial environment. Finally, we explore the significance of A. agrestis in crop improvement and synthetic biology applications in general.

Nostoc Talks Back: Temporal Patterns of Differential Gene Expression During Establishment of Anthoceros-Nostoc Symbiosis

Endosymbiotic associations between hornworts and nitrogen-fixing cyanobacteria form when the plant is limited for combined nitrogen (N). We generated RNA-seq data to examine temporal gene expression patterns during the culturing of N-starved Anthoceros punctatus in the absence and the presence of symbiotic cyanobacterium Nostoc punctiforme. In symbiont-free A. punctatus gametophytes, N starvation caused downregulation of chlorophyll content and chlorophyll fluorescence characteristics as well as transcription of photosynthesis-related genes. This downregulation was reversed in A. punctatus cocultured with N. punctiforme, corresponding to the provision by the symbiont of N₂-derived NH₄⁺, which commenced within 5 days of coculture and reached a maximum by 14 days. We also observed transient increases in transcription of ammonium and nitrate transporters in a N. punctiforme-dependent manner as well as that of a SWEET transporter that was initially independent of N₂-derived NH₄⁺. The temporal patterns of differential gene expression indicated that N. punctiforme transmits signals that impact gene expression to A. punctatus both prior to and after its provision of fixed N. This study is the first illustrating the temporal patterns of gene expression during establishment of an endosymbiotic nitrogen-fixing association in this monophyletic evolutionary lineage of land plants. [Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Evolutionary genomic insights into cyanobacterial symbioses in plants

Photosynthesis, the ability to fix atmospheric carbon dioxide, was acquired by eukaryotes through symbiosis: the plastids of plants and algae resulted from a cyanobacterial symbiosis that commenced more than 1.5 billion years ago and has chartered a unique evolutionary path. This resulted in the evolutionary origin of plants and algae. Some extant land plants have recruited additional biochemical aid from symbiotic cyanobacteria; these plants associate with filamentous cyanobacteria that fix atmospheric nitrogen. Examples of such interactions can be found in select species from across all major lineages of land plants. The recent rise in genomic and transcriptomic data has provided new insights into the molecular foundation of these interactions. Furthermore, the hornwort Anthoceros has emerged as a model system for the molecular biology of cyanobacteria-plant interactions. Here, we review these developments driven by high-throughput data and pinpoint their power to yield general patterns across these diverse symbioses.

Chemical Stimulation of Heterocyte Differentiation by the Feather Moss Hylocomium splendens: a Potential New Step in Plant-Cyanobacteria Symbioses

Cyanobacteria associated with mosses play a key role in the nitrogen (N) cycle in unpolluted ecosystems. Mosses have been found to release molecules that induce morphophysiological changes in epiphytic cyanobionts. Nevertheless, the extent of moss influence on these microorganisms remains unknown. To evaluate how mosses or their metabolites influence N₂ fixation rates by cyanobacteria, we assessed the nitrogenase activity, heterocyte frequency and biomass of a cyanobacterial strain isolated from the feather moss Hylocomium splendens and a non-symbiotic strain when they were either growing by themselves, together with H. splendens or exposed to H. splendens water, acetone, ethanol, or isopropanol extracts. The same cyanobacterial strains were added to another moss (Taxiphyllum barbieri) and a liverwort (Monosolenium tenerum) to assess if these bryophytes affect N₂ fixation differently. Although no significant increases in nitrogenase activity by the cyanobacteria were observed when in contact with H. splendens shoots, both the symbiotic and non-symbiotic cyanobacteria increased nitrogenase activity as well as heterocyte frequency significantly upon exposure to H. splendens ethanol extracts. Contact with T. barbieri shoots, on the other hand, did lead to increases in nitrogenase activity, indicating low host-specificity to cyanobacterial activity. These findings suggest that H. splendens produces heterocyte-differentiating factors (HDFs) that are capable of stimulating cyanobacterial N₂ fixation regardless of symbiotic competency. Based on previous knowledge about the chemical ecology and dynamics of moss-cyanobacteria interactions, we speculate that HDF expression by the host takes place in a hypothetical new step occurring after plant colonization and the repression of hormogonia.

Genome Evolution of Filamentous Cyanobacterium Nostoc Species: From Facultative Symbiosis to Free Living

In contrast to obligate bacteria, facultative symbiotic bacteria are mainly characterized by genome enlargement. However, the underlying relationship of this feature with adaptations to various habitats remains unclear. In this study, we used the global genome data of Nostoc strains, including 10 novel genomes sequenced in this study and 26 genomes available from public databases, and analyzed their evolutionary history. The evolutionary boundary of the real clade of Nostoc species was identified and was found to be consistent with the results of polyphasic taxonomy. The initial ancestral species of Nostoc was demonstrated to be consistent with a facultative symbiotic population. Further analyses revealed that Nostoc strains tended to shift from facultative symbiosis to a free-living one, along with an increase in genome sizes during the dispersal of each exterior branch. Intracellular symbiosis was proved to be essentially related to Nostoc evolution, and the adaptation of its members to free-living environments was coupled with a large preference for gene acquisition involved in gene repair and recombination. These findings provided unique evidence of genomic mechanisms by which homologous microbes adapt to distinct life manners and external environments.

Comparative genomic insights into culturable symbiotic cyanobacteria from the water fern Azolla

Species of the floating, freshwater fern Azolla form a well-characterized symbiotic association with the non-culturable cyanobacterium Nostoc azollae, which fixes nitrogen for the plant. However, several cyanobacterial strains have over the years been isolated and cultured from Azolla from all over the world. The genomes of 10 of these strains were sequenced and compared with each other, with other symbiotic cyanobacterial strains, and with similar strains that were not isolated from a symbiotic association. The 10 strains fell into three distinct groups: six strains were nearly identical to the non-symbiotic strain, Nostoc (Anabaena) variabilis ATCC 29413; three were similar to the symbiotic strain, Nostoc punctiforme, and one, Nostoc sp. 2RC, was most similar to non-symbiotic strains of Nostoc linckia. However, Nostoc sp. 2RC was unusual because it has three sets of nitrogenase genes; it has complete gene clusters for two distinct Mo-nitrogenases and an alternative V-nitrogenase. Genes for Mo-nitrogenase, sugar transport, chemotaxis and pili characterized all the symbiotic strains. Several of the strains infected the liverwort Blasia, including N. variabilis ATCC 29413, which did not originate from Azolla but rather from a sewage pond. However, only Nostoc sp. 2RC, which produced highly motile hormogonia, was capable of high-frequency infection of Blasia. Thus, some of these strains, which grow readily in the laboratory, may be useful in establishing novel symbiotic associations with other plants.

Genomic and transcriptomic insights into the habitat adaptation of the diazotrophic paddy-field cyanobacterium Nostoc sphaeroides

Nitrogen-fixing cyanobacteria are common in paddy fields, one of the most productive wetland ecosystems. Here, we present the complete genome of Nostoc sphaeroides, a paddy-field diazotroph used for food and medicine for more than 1700 years and deciphered the transcriptional regulation during the developmental transition from hormogonia to vegetative filaments with heterocysts. The genome of N. sphaeroides consists of one circular chromosome (6.48 Mb), one of the largest ever reported megaplasmids (2.34 Mb), and seven plasmids. Multiple gene families involved in the adaption to high solar radiation and water fluctuation conditions were found expanded, while genes involved in anoxic adaptation and phosphonate utilization are located on the megaplasmid, suggesting its indispensable role in environmental adaptation. Distinct gene expression patterns were observed during the light-intensity-regulated transition from hormogonia to vegetative filaments, specifically, genes encoding proteins involved in photosynthetic light reaction, carbon fixation, nitrogen metabolism and heterocyst differentiation were significantly upregulated, whereas genes related to cell motility were down-regulated. Our results provide genomic and transcriptomic insights into the adaptation of a filamentous nitrogen-fixing cyanobacterium to the highly dynamic paddy-field habitat, suggesting N. sphaeroides as an excellent system to understand the transition from aquatic to terrestrial habitats and to support sustainable rice production.

Structural Determinants and Their Role in Cyanobacterial Morphogenesis

Cells have to erect and sustain an organized and dynamically adaptable structure for an efficient mode of operation that allows drastic morphological changes during cell growth and cell division. These manifold tasks are complied by the so-called cytoskeleton and its associated proteins. In bacteria, FtsZ and MreB, the bacterial homologs to tubulin and actin, respectively, as well as coiled-coil-rich proteins of intermediate filament (IF)-like function to fulfil these tasks. Despite generally being characterized as Gram-negative, cyanobacteria have a remarkably thick peptidoglycan layer and possess Gram-positive-specific cell division proteins such as SepF and DivIVA-like proteins, besides Gram-negative and cyanobacterial-specific cell division proteins like MinE, SepI, ZipN (Ftn2) and ZipS (Ftn6). The diversity of cellular morphologies and cell growth strategies in cyanobacteria could therefore be the result of additional unidentified structural determinants such as cytoskeletal proteins. In this article, we review the current advances in the understanding of the cyanobacterial cell shape, cell division and cell growth.

Cyanobacteria-Derived Proline Increases Stress Tolerance in Arabidopsis thaliana Root Hairs by Suppressing Programmed Cell Death

Nitrogen-fixing heterocystous cyanobacteria are used as biofertilizer inoculants for stimulating plant growth but can also alleviate plant stress by exometabolite secretion. However, only a small number of studies have focused on elucidating the identity of said bioactives because of the wide array of exuded compounds. Here, we used the root hair assay (RHA) as a rapid programmed cell death (PCD) screening tool for characterizing the bioactivity of cyanobacteria Nostoc muscorum conditioned medium (CM) on Arabidopsis thaliana root hair stress tolerance. We found that heat-stressed A. thaliana pre-treated with N. muscorum CM fractions exhibited significantly lower root hair PCD levels compared to untreated seedlings. Treatment with CM increased stress tolerance by suppressing PCD in root hairs but not necrosis, indicating the bioactive compound was specifically modulating the PCD pathway and not a general stress response. Based on documented N. muscorum exometabolites, we identified the stress-responsive proline as a compound of interest and strong evidence from the ninhydrin assay and HPLC indicate that proline is present in N. muscorum CM. To establish whether proline was capable of suppressing PCD, we conducted proline supplementation experiments. Our results showed that exogenous proline had a similar effect on root hairs as N. muscorum CM treatment, with comparable PCD suppression levels and insignificant necrosis changes. To verify proline as one of the biologically active compounds in N. muscorum CM, we used three mutant A. thaliana lines with proline transporter mutations (lht1, aap1 and atprot1-1::atprot2-3::atprot3-2). Compared with the wild-type seedlings, PCD-suppression in lht1and aap1 mutants was significantly reduced when supplied with low proline (1-5 μM) levels. Similarly, pre-treatment with N. muscorum CM resulted in elevated PCD levels in all three mutant lines compared to wild-type seedlings. Our results show that plant uptake of cyanobacteria-derived proline alters their root hair PCD sensitivity threshold. This offers evidence of a novel biofertilizer mechanism for reducing stress-induced PCD levels, independent of the existing mechanisms documented in the literature.

The role of the cytoskeletal proteins MreB and FtsZ in multicellular cyanobacteria

Multiseriate and true‐branching cyanobacteria are at the peak of prokaryotic morphological complexity. However, little is known about the mechanisms governing multiplanar cell division and morphogenesis. Here, we study the function of the prokaryotic cytoskeletal proteins, MreB and FtsZ in Fischerella muscicola PCC 7414 and Chlorogloeopsis fritschii PCC 6912. Vancomycin and HADA labeling revealed a mixed apical, septal, and lateral trichome growth mode in F. muscicola, whereas C. fritschii exhibits septal growth. In all morphotypes from both species, MreB forms either linear filaments or filamentous strings and can interact with FtsZ. Furthermore, multiplanar cell division in F. muscicola likely depends on FtsZ dosage. Our results lay the groundwork for future studies on cytoskeletal proteins in morphologically complex cyanobacteria.

The Role of the Cyanobacterial Type IV Pilus Machinery in Finding and Maintaining a Favourable Environment

Type IV pili (T4P) are proteinaceous filaments found on the cell surface of many prokaryotic organisms and convey twitching motility through their extension/retraction cycles, moving cells across surfaces. In cyanobacteria, twitching motility is the sole mode of motility properly characterised to date and is the means by which cells perform phototaxis, the movement towards and away from directional light sources. The wavelength and intensity of the light source determine the direction of movement and, sometimes in concert with nutrient conditions, act as signals for some cyanobacteria to form mucoid multicellular assemblages. Formation of such aggregates or flocs represents an acclimation strategy to unfavourable environmental conditions and stresses, such as harmful light conditions or predation. T4P are also involved in natural transformation by exogenous DNA, secretion processes, and in cellular adaptation and survival strategies, further cementing the role of cell surface appendages. In this way, cyanobacteria are finely tuned by external stimuli to either escape unfavourable environmental conditions via phototaxis, exchange genetic material, and to modify their surroundings to fit their needs by forming multicellular assemblies.

The Hybrid Histidine Kinase HrmK Is an Early-Acting Factor in the Hormogonium Gene Regulatory Network

Filamentous, heterocyst-forming cyanobacteria belonging to taxonomic subsections IV and V are developmentally complex multicellular organisms capable of differentiating an array of cell and filament types, including motile hormogonia. Hormogonia exhibit gliding motility that facilitates dispersal, phototaxis, and the establishment of nitrogen-fixing symbioses. The gene regulatory network (GRN) governing hormogonium development involves a hierarchical sigma factor cascade, but the factors governing the activation of this cascade are currently undefined. Here, using a forward genetic approach, we identified hrmK, a gene encoding a putative hybrid histidine kinase that functions upstream of the sigma factor cascade. The deletion of hrmK produced nonmotile filaments that failed to display hormogonium morphology or accumulate hormogonium-specific proteins or polysaccharide. Targeted transcriptional analyses using reverse transcription-quantitative PCR (RT-qPCR) demonstrated that hormogonium-specific genes both within and outside the sigma factor cascade are drastically downregulated in the absence of hrmK and that hrmK may be subject to indirect, positive autoregulation via sigJ and sigC Orthologs of HrmK are ubiquitous among, and exclusive to, heterocyst-forming cyanobacteria. Collectively, these results indicate that hrmK functions upstream of the sigma factor cascade to initiate hormogonium development, likely by modulating the phosphorylation state of an unknown protein that may serve as the master regulator of hormogonium development in heterocyst-forming cyanobacteria.IMPORTANCE Filamentous cyanobacteria are morphologically complex, with several representative species amenable to routine genetic manipulation, making them excellent model organisms for the study of development. Furthermore, two of the developmental alternatives, nitrogen-fixing heterocysts and motile hormogonia, are essential to establish nitrogen-fixing symbioses with plant partners. These symbioses are integral to global nitrogen cycles and could be artificially recreated with crop plants to serve as biofertilizers, but to achieve this goal, detailed understanding and manipulation of the hormogonium and heterocyst gene regulatory networks may be necessary. Here, using the model organism Nostoc punctiforme, we identify a previously uncharacterized hybrid histidine kinase that is confined to heterocyst-forming cyanobacteria as the earliest known participant in hormogonium development.

Perspectives on Endosymbiosis in Coralloid Roots: Association of Cycads and Cyanobacteria

Past endosymbiotic events allowed photosynthetic organisms to flourish and evolve in terrestrial areas. The precursor of chloroplasts was an ancient photosynthetic cyanobacterium. Presently, cyanobacteria are still capable of establishing successful symbioses in a wide range of hosts. One particular host plant among the gymnosperms is cycads (Order Cycadales) in which a special type of root system, referred to as coralloid roots, develops to house symbiotic cyanobacteria. A number of studies have explained coralloid root formation and cyanobiont invasion but the questions on mechanisms of this host-microbe association remains vague. Most researches focus on diversity of symbionts in coralloid roots but equally important is to explore the underlying mechanisms of cycads-Nostoc symbiosis as well. Besides providing an overview of relevant areas presently known about this association and citing putative genes involved in cycad-cyanobacteria symbioses, this paper aims to identify the limitations that hamper attempts to get to the root of the matter and suggests future research directions that may prove useful.

Isolation and characterization of 1-palmitoyl-2-linoleoyl-sn-glycerol as a hormogonium-inducing factor (HIF) from the coralloid roots of Cycas revoluta (Cycadaceae)

Coralloid roots are specialized tissues of cycads (Cycas revoluta) that are involved in symbioses with nitrogen-fixing Nostoc cyanobacteria. We found that a crude methanolic extract of coralloid roots induced differentiation of the filamentous cell aggregates of Nostoc species into motile hormogonia. Hence, the hormogonium-inducing factor (HIF) was chased using bioassay-based isolation, and the active principle was characterized as a mixture of diacylglycerols (DAGs), mainly composed of 1-palmitoyl-2-linoleoyl-sn-glycerol (1), 1-palmitoyl-2-oleoyl-sn-glycerol (2), 1-stearoyl-2-linolenoyl-sn-glycerol (3), and 1-stearoyl-2-linoleoyl-sn-glycerol (4). Enantioselectively synthesised compound 1 showed a clear HIF activity at 1 nmol (0.6 µg) disc⁻¹ for the filamentous cells, whereas synthesised 2-linoleoyl-3-palmitoyl-sn-glycerol (1′) and 1-palmitoyl-2-linoleoyl-rac-glycerol (1/1′) were less active than 1. Conversely, synthesised 1-linoleoyl-2-palmitoyl-rac-glycerol (8/8′) which is an acyl positional isomer of compound 1 was inactive. In addition, neither 1-monoacylglycerols nor phospholipids structurally related to 1 showed HIF-like activities. As DAGs are protein kinase C (PKC) activators, 12-O-tetradecanoylphorbol-13-acetate (12), urushiol C15:3-Δ^10,13,16 (13), and a skin irritant anacardic acid C15:1-Δ⁸ (14) were also examined for HIF-like activities toward the Nostoc cells. Neither 12 nor 13 showed HIF-like activities, whereas 14 showed an HIF-like activity at 1 nmol/disc. These findings appear to indicate that some DAGs act as hormogonium-inducing signal molecules for filamentous Nostoc cyanobacteria.

A partner-switching regulatory system controls hormogonium development in the filamentous cyanobacterium Nostoc punctiforme

Filamentous cyanobacteria exhibit developmental complexity, including the transient differentiation of motile hormogonia in many species. Using a forward genetic approach, a trio of genes unique to filamentous cyanobacteria encoding a putative Rsb-like partner-switching regulatory system (PSRS) was implicated in regulating hormogonium development in the model filamentous cyanobacterium Nostoc punctiforme. Analysis of in-frame deletion strains indicated that HmpU (putative serine phosphatase) and HmpV (STAS domain) enhance, while HmpW (putative serine kinase) represses motility and persistence of the hormogonium state. Protein-protein interaction studies demonstrated specificity between HmpW and HmpV. Epistasis analysis between hmpW and hmpV was consistent with HmpV acting as the downstream effector of the system, rather than regulation of a sigma factor by HmpW. Deletion of hmpU or hmpV reduced accumulation of extracellular PilA and hormogonium polysaccharide (HPS), and expression of type IV pilus- and HPS-specific genes was reduced in the ΔhmpV strain. Expression of the Hmp PSRS is induced in hormogonia, and the cytoplasmic localization of HmpV-GFPuv implies that its downstream target is probably cytoplasmic as well. Collectively, these results support a model where HmpU and HmpW antagonistically regulate the phosphorylation state of HmpV, and subsequently, unphosphorylated HmpV positively regulates an undefined downstream target to affect hormogonium-specific gene expression.

Genomic Changes Associated with the Evolutionary Transitions of Nostoc to a Plant Symbiont

Cyanobacteria belonging to the genus Nostoc comprise free-living strains and also facultative plant symbionts. Symbiotic strains can enter into symbiosis with taxonomically diverse range of host plants. Little is known about genomic changes associated with evolutionary transition of Nostoc from free-living to plant symbiont. Here, we compared the genomes derived from 11 symbiotic Nostoc strains isolated from different host plants and infer phylogenetic relationships between strains. Phylogenetic reconstructions of 89 Nostocales showed that symbiotic Nostoc strains with a broad host range, entering epiphytic and intracellular or extracellular endophytic interactions, form a monophyletic clade indicating a common evolutionary history. A polyphyletic origin was found for Nostoc strains which enter only extracellular symbioses, and inference of transfer events implied that this trait was likely acquired several times in the evolution of the Nostocales. Symbiotic Nostoc strains showed enriched functions in transport and metabolism of organic sulfur, chemotaxis and motility, as well as the uptake of phosphate, branched-chain amino acids, and ammonium. The genomes of the intracellular clade differ from that of other Nostoc strains, with a gain/enrichment of genes encoding proteins to generate l-methionine from sulfite and pathways for the degradation of the plant metabolites vanillin and vanillate, and of the macromolecule xylan present in plant cell walls. These compounds could function as C-sources for members of the intracellular clade. Molecular clock analysis indicated that the intracellular clade emerged ca. 600 Ma, suggesting that intracellular Nostoc symbioses predate the origin of land plants and the emergence of their extant hosts.

Spatial organization of the three-component lichen Peltigera aphthosa in functional terms

The cephalolichen Peltigera aphthosa (L.) Willd. is characterized by lateral heterogeneity, which manifests itself in the presence of three thallus zones, referred to as the apical, basal and medial zone. These zones differ in terms of interaction between lichen bionts and their physiological activity. The apical thallus zone is more efficient in establishing a contact with cyanobacteria, because of a higher lectin content and a larger overall thallus surface area due to the presence of numerous mycobiont hyphae. Cephalodia are formed in this zone. The interaction between the mycobiont and cyanobiont is more intense in the medial zone. However, the establishment of the contact with cyanobacteria in this zone less probable. The spatial distribution of lectins in the thallus was determined. To reveal the differences in photosynthetic activity in three thallus zones, transient analysis of chlorophyll a fluorescence and the assessment of non-photochemical quenching of excited chlorophyll states were performed. Assimilation of absorbed light energy was more effective in the medial zone. The basal zone was characterized by decreased photosynthetic activity, lichen dissociation and thallus death.

A Putative O-Linked β-N-Acetylglucosamine Transferase Is Essential for Hormogonium Development and Motility in the Filamentous Cyanobacterium Nostoc punctiforme

Most species of filamentous cyanobacteria are capable of gliding motility, likely via a conserved type IV pilus-like system that may also secrete a motility-associated polysaccharide. In a subset of these organisms, motility is achieved only after the transient differentiation of hormogonia, which are specialized filaments that enter a nongrowth state dedicated to motility. Despite the fundamental importance of hormogonia to the life cycles of many filamentous cyanobacteria, the molecular regulation of hormogonium development is largely undefined. To systematically identify genes essential for hormogonium development and motility in the model heterocyst-forming filamentous cyanobacterium Nostoc punctiforme, a forward genetic screen was employed. The first gene identified using this screen, designated ogtA, encodes a putative O-linked β-N-acetylglucosamine transferase (OGT). The deletion of ogtA abolished motility, while ectopic expression of ogtA induced hormogonium development even under hormogonium-repressing conditions. Transcription of ogtA is rapidly upregulated (1 h) following hormogonium induction, and an OgtA-GFPuv fusion protein localized to the cytoplasm. In developing hormogonia, accumulation of PilA but not HmpD is dependent on ogtA Reverse transcription-quantitative PCR (RT-qPCR) analysis indicated equivalent levels of pilA transcript in the wild-type and ΔogtA mutant strains, while a reporter construct consisting of the intergenic region in the 5' direction of pilA fused to gfp produced lower levels of fluorescence in the ΔogtA mutant strain than in the wild type. The production of hormogonium polysaccharide in the ΔogtA mutant strain is reduced compared to that in the wild type but comparable to that in a pilA deletion strain. Collectively, these results imply that O-GlcNAc protein modification regulates the accumulation of PilA via a posttranscriptional mechanism in developing hormogonia.IMPORTANCE Filamentous cyanobacteria are among the most developmentally complex prokaryotes. Species such as Nostoc punctiforme develop an array of cell types, including nitrogen-fixing heterocysts, spore-like akinetes, and motile hormogonia, that function in dispersal as well as the establishment of nitrogen-fixing symbioses with plants and fungi. These symbioses are major contributors to global nitrogen fixation. Despite the fundamental importance of hormogonia to the life cycle of filamentous cyanobacteria and the establishment of symbioses, the molecular regulation of hormogonium development is largely undefined. We employed a genetic screen to identify genes essential for hormogonium development and motility in Nostoc punctiforme The first gene identified using this screen encodes a eukaryotic-like O-linked β-N-acetylglucosamine transferase that is required for accumulation of PilA in hormogonia.

Genetic analysis reveals the identity of the photoreceptor for phototaxis in hormogonium filaments of Nostoc punctiforme

In cyanobacterial Nostoc species, substratum-dependent gliding motility is confined to specialized nongrowing filaments called hormogonia, which differentiate from vegetative filaments as part of a conditional life cycle and function as dispersal units. Here we confirm that Nostoc punctiforme hormogonia are positively phototactic to white light over a wide range of intensities. N. punctiforme contains two gene clusters (clusters 2 and 2i), each of which encodes modular cyanobacteriochrome-methyl-accepting chemotaxis proteins (MCPs) and other proteins that putatively constitute a basic chemotaxis-like signal transduction complex. Transcriptional analysis established that all genes in clusters 2 and 2i, plus two additional clusters (clusters 1 and 3) with genes encoding MCPs lacking cyanobacteriochrome sensory domains, are upregulated during the differentiation of hormogonia. Mutational analysis determined that only genes in cluster 2i are essential for positive phototaxis in N. punctiforme hormogonia; here these genes are designated ptx (for phototaxis) genes. The cluster is unusual in containing complete or partial duplicates of genes encoding proteins homologous to the well-described chemotaxis elements CheY, CheW, MCP, and CheA. The cyanobacteriochrome-MCP gene (ptxD) lacks transmembrane domains and has 7 potential binding sites for bilins. The transcriptional start site of the ptx genes does not resemble a sigma 70 consensus recognition sequence; moreover, it is upstream of two genes encoding gas vesicle proteins (gvpA and gvpC), which also are expressed only in the hormogonium filaments of N. punctiforme.

Branching and intercellular communication in the Section V cyanobacterium Mastigocladus laminosus, a complex multicellular prokaryote

The filamentous Section V cyanobacterium Mastigocladus laminosus is one of the most morphologically complex prokaryotes. It exhibits cellular division in multiple planes, resulting in the formation of true branches, and cell differentiation into heterocysts, hormogonia and necridia. Here, we investigate branch formation and intercellular communication in M. laminosus. Monitoring of membrane rearrangement suggests that branch formation results from a randomized direction of cell growth. Transmission electron microscopy reveals cell junction structures likely to be involved in intercellular communication. We identify a sepJ gene, coding for a potential key protein in intercellular communication, and show that SepJ is localized at the septa. To directly investigate intercellular communication, we loaded the fluorescent tracer 5-carboxyfluorescein diacetate into the cytoplasm, and quantified its intercellular exchange by fluorescence recovery after photobleaching. Results demonstrate connectivity of the main trichome and branches, enabling molecular exchange throughout the filament network. Necridia formation inhibits further molecular exchange, determining the fate of a branch likely to become a hormogonium. Cells in young, narrow trichomes and hormogonia exhibited faster exchange rates than cells in older, wider trichomes. Signal transduction to co-ordinate movement of hormogonia might be accelerated by reducing cell volume.

Boreal feather mosses secrete chemical signals to gain nitrogen

The mechanistic basis of feather moss-cyanobacteria associations, a main driver of nitrogen (N) input into boreal forests, remains unknown. Here, we studied colonization by Nostoc sp. on two feather mosses that form these associations (Pleurozium schreberi and Hylocomium splendens) and two acrocarpous mosses that do not (Dicranum polysetum and Polytrichum commune). We also determined how N availability and moss reproductive stage affects colonization, and measured N transfer from cyanobacteria to mosses. The ability of mosses to induce differentiation of cyanobacterial hormogonia, and of hormogonia to then colonize mosses and re-establish a functional symbiosis was determined through microcosm experiments, microscopy and acetylene reduction assays. Nitrogen transfer between cyanobacteria and Pleurozium schreberi was monitored by secondary ion mass spectrometry (SIMS). All mosses induced hormogonia differentiation but only feather mosses were subsequently colonized. Colonization on Pleurozium schreberi was enhanced during the moss reproductive phase but impaired by elevated N. Transfer of N from cyanobacteria to their host moss was observed. Our results reveal that feather mosses likely secrete species-specific chemo-attractants when N-limited, which guide cyanobacteria towards them and from which they gain N. We conclude that this signalling is regulated by N demands of mosses, and serves as a control of N input into boreal forests.

Biological nitrogen fixation in non-legume plants

BACKGROUND

Nitrogen is an essential nutrient in plant growth. The ability of a plant to supply all or part of its requirements from biological nitrogen fixation (BNF) thanks to interactions with endosymbiotic, associative and endophytic symbionts, confers a great competitive advantage over non-nitrogen-fixing plants.

SCOPE

Because BNF in legumes is well documented, this review focuses on BNF in non-legume plants. Despite the phylogenic and ecological diversity among diazotrophic bacteria and their hosts, tightly regulated communication is always necessary between the microorganisms and the host plant to achieve a successful interaction. Ongoing research efforts to improve knowledge of the molecular mechanisms underlying these original relationships and some common strategies leading to a successful relationship between the nitrogen-fixing microorganisms and their hosts are presented.

CONCLUSIONS

Understanding the molecular mechanism of BNF outside the legume-rhizobium symbiosis could have important agronomic implications and enable the use of N-fertilizers to be reduced or even avoided. Indeed, in the short term, improved understanding could lead to more sustainable exploitation of the biodiversity of nitrogen-fixing organisms and, in the longer term, to the transfer of endosymbiotic nitrogen-fixation capacities to major non-legume crops.

A Nostoc punctiforme sugar transporter necessary to establish a Cyanobacterium-plant symbiosis

In cyanobacteria-plant symbioses, the symbiotic nitrogen-fixing cyanobacterium has low photosynthetic activity and is supplemented by sugars provided by the plant partner. Which sugars and cyanobacterial sugar uptake mechanism(s) are involved in the symbiosis, however, is unknown. Mutants of the symbiotically competent, facultatively heterotrophic cyanobacterium Nostoc punctiforme were constructed bearing a neomycin resistance gene cassette replacing genes in a putative sugar transport gene cluster. Results of transport activity assays using (14)C-labeled fructose and glucose and tests of heterotrophic growth with these sugars enabled the identification of an ATP-binding cassette-type transporter for fructose (Frt), a major facilitator permease for glucose (GlcP), and a porin needed for the optimal uptake of both fructose and glucose. Analysis of green fluorescent protein fluorescence in strains of N. punctiforme bearing frt::gfp fusions showed high expression in vegetative cells and akinetes, variable expression in hormogonia, and no expression in heterocysts. The symbiotic efficiency of N. punctiforme sugar transport mutants was investigated by testing their ability to infect a nonvascular plant partner, the hornwort Anthoceros punctatus. Strains that were specifically unable to transport glucose did not infect the plant. These results imply a role for GlcP in establishing symbiosis under the conditions used in this work.

The globins of cyanobacteria and algae

Approximately, 20 years ago, a haemoglobin gene was identified within the genome of the cyanobacterium Nostoc commune. Haemoglobins have now been confirmed in multiple species of photosynthetic microbes beyond N. commune, and the diversity of these proteins has recently come under increased scrutiny. This chapter summarizes the state of knowledge concerning the phylogeny, physiology and chemistry of globins in cyanobacteria and green algae. Sequence information is by far the best developed and the most rapidly expanding aspect of the field. Structural and ligand-binding properties have been described for just a few proteins. Physiological data are available for even fewer. Although activities such as nitric oxide dioxygenation and oxygen scavenging are strong candidates for cellular function, dedicated studies will be required to complete the story on this intriguing and ancient group of proteins.

Global transcription profiles of the nitrogen stress response resulting in heterocyst or hormogonium development in Nostoc punctiforme

The filamentous cyanobacterium Nostoc punctiforme differentiates from vegetative cells into three distinct cell types, heterocysts, hormogonia, and akinetes, in response to different stimuli. Cultures growing with ammonium can be induced to form hormogonia or heterocysts upon removal of the combined nitrogen. A DNA microarray consisting of 94% of the open reading frames predicted from the 9.059-Mb N. punctiforme genome was used to generate a global transcription data set consisting of seven time points over a 24-h period of nitrogen deprivation, which results in heterocyst formation. This data set was compared to a similarly generated data set of nitrogen-starved N. punctiforme resulting in hormogonium formation that had previously been published (E. L. Campbell, H. Christman, and J. C. Meeks, J. Bacteriol. 190:7382-7391, 2008). The transition from vegetative cells to either heterocysts or hormogonia resulted in rapid and sustained expression of genes required for utilization of alternate nitrogen sources. Overall, 1,036 and 1,762 genes were found to be differentially transcribed during the heterocyst and hormogonium time courses, respectively, as analyzed with the Bayesian user-friendly software for analyzing time series microarray experiments (BATS). Successive transcription of heterocyst regulatory, structural, and functional genes occurred over the 24 h required to form a functional heterocyst. During hormogonium differentiation, some heterocyst structural and functional genes were upregulated, while the heterocyst master regulator hetR was downregulated. There are commonalities in differential expression between cells bound for differentiation into heterocysts or hormogonia, yet the two paths are distinguished by their developmentally specific transcription profiles.

Fungal lectin of Peltigera canina induces chemotropism of compatible Nostoc cells by constriction-relaxation pulses of cyanobiont cytoskeleton

A glycosylated arginase acting as a fungal lectin from Peltigera canina is able to produce recruitment of cyanobiont Nostoc cells and their adhesion to the hyphal surface. This implies that the cyanobiont would develop organelles to motility towards the chemoattractant. However when visualized by transmission electron microscopy, Nostoc cells recently isolated from P. canina thallus do not reveal any motile, superficial organelles, although their surface was covered by small spindles and serrated layer related to gliding. The use of S-(3,4-dichlorobenzyl)isothiourea, blebbistatin, phalloidin and latrunculin A provide circumstantial evidence that actin microfilaments rather than MreB, the actin-like protein from prokaryota, and, probably, an ATPase which develops contractile function similar to that of myosin II, are involved in cell motility. These experimental facts, the absence of superficial elements (fimbriae, pili or flagellum) related to cell movement, and the appearance of sunken cells during of after movement verified by scanning electron microscopy, support the hypothesis that the motility of lichen cyanobionts could be achieved by contraction-relaxation episodes of the cytoskeleton induced by fungal lectin act as a chemoattractant.

Determining cell shape: adaptive regulation of cyanobacterial cellular differentiation and morphology

Similar to other bacteria, cyanobacteria exist in a wide-ranging diversity of shapes and sizes. However, three general shapes are observed most frequently: spherical, rod and spiral. Bacteria can also grow as filaments of cells. Some filamentous cyanobacteria have differentiated cell types that exhibit distinct morphologies: motile hormogonia, nitrogen-fixing heterocysts, and spore-like akinetes. Cyanobacterial cell shapes, which are largely controlled by the cell wall, can be regulated by developmental and/or environmental cues, although the mechanisms of regulation and the selective advantage(s) of regulating cellular shape are still being elucidated. In this review, recent insights into developmental and environmental regulation of cell shape in cyanobacteria and the relationship(s) of cell shape and differentiation to organismal fitness are discussed.

DNA microarray comparisons of plant factor- and nitrogen deprivation-induced Hormogonia reveal decision-making transcriptional regulation patterns in Nostoc punctiforme

Hormogonia are nongrowing filaments, motile by means of a gliding mechanism, that are produced by certain cyanobacteria. Their differentiation is induced by positive and negative factors for growth, such as deprivation of combined nitrogen (nitrogen stress induction [NSI]). In Nostoc punctiforme, they are also induced by the exudate (hormogonium-inducing factor [HIF]) of a symbiotic plant partner. Time course (0.5 to 24 h) transcription profiles were determined by DNA microarray assays for hormogonia of N. punctiforme following induction by HIF and NSI. Clustering analysis revealed both common and distinct transcriptional patterns for the two methods of induction. By 24 h, a common set of 1,328 genes was identified. This 24-h common set of genes arose by the transition of 474 genes from an 819-member common set of genes at 1 h after induction; 405 and 51 genes unique to the HIF and NSI groups at 1 h, respectively; and 398 genes differentially transcribed at later time points. The NSI hormogonia showed a transcriptional checkpoint at 12 h following induction in which up- and downregulated genes were transiently down- or upregulated, respectively. The transient changes in these 1,043 genes appeared to reflect a switch back to a vegetative growth state. Such a checkpoint was not seen in HIF hormogonia. Genes uniquely upregulated in HIF hormogonia included those encoding proteins hypothesized to synthesize a metabolite repressor of hormogonium differentiation. Approximately 34 to 42% of the 6,893 printed genes were differentially transcribed during hormogonium differentiation; about half of those genes were upregulated, and 1,034 genes responded within 0.5 h after induction. These collective results indicate extensive and rapid global changes in the transcription of specific genes during the differentiation of these specialized filaments.

Mutation at different sites in the Nostoc punctiforme cyaC gene, encoding the multiple-domain enzyme adenylate cyclase, results in different levels of infection of the host plant Blasia pusilla

The filamentous cyanobacterium Nostoc punctiforme forms symbioses with plants. Disruption of the catalytic domain of the N. punctiforme adenylate cyclase (CyaC) significantly increased symbiotic competence, whereas reduced infectivity was observed in a mutant with a disruption close to the N terminus of CyaC. The total cellular cyclic AMP levels were significantly reduced in both mutants.

Global gene expression patterns of Nostoc punctiforme in steady-state dinitrogen-grown heterocyst-containing cultures and at single time points during the differentiation of akinetes and hormogonia

The vegetative cells of the filamentous cyanobacterium Nostoc punctiforme can differentiate into three mutually exclusive cell types: nitrogen-fixing heterocysts, spore-like akinetes, and motile hormogomium filaments. A DNA microarray consisting of 6,893 N. punctiforme genes was used to identify the global transcription patterns at single time points in the three developmental states, compared to those in ammonium-grown time zero cultures. Analysis of ammonium-grown cultures yielded a transcriptome of 2,935 genes, which is nearly twice the size of a soluble proteome. The NH(4)(+)-grown transcriptome was enriched in genes encoding core metabolic functions. A steady-state N(2)-grown (heterocyst-containing) culture showed differential transcription of 495 genes, 373 of which were up-regulated. The majority of the up-regulated genes were predicted from studies of heterocyst differentiation and N(2) fixation; other genes are candidates for more detailed genetic analysis. Three days into the developmental process, akinetes showed a similar number of differentially expressed genes (497 genes), which were equally up- and down-regulated. The down-regulated genes were enriched in core metabolic functions, consistent with entry into a nongrowth state. There were relatively few adaptive genes up-regulated in 3-day akinetes, and there was little overlap with putative heterocyst developmental genes. There were 1,827 differentially transcribed genes in 24-h hormogonia, which was nearly fivefold greater than the number in akinete-forming or N(2)-fixing cultures. The majority of the up-regulated adaptive genes were genes encoding proteins for signal transduction and transcriptional regulation, which is characteristic of a motile filament that is poised to sense and respond to the environment. The greatest fraction of the 883 down-regulated genes was involved in core metabolism, also consistent with entry into a nongrowth state. The differentiation of heterocysts (steady state, N(2) grown), akinetes, and hormogonia appears to involve the up-regulation of genes distinct for each state.

Molecular analysis of genes in Nostoc punctiforme involved in pilus biogenesis and plant infection

Hormogonia are the infective agents in many cyanobacterium-plant symbioses. Pilus-like appendages are expressed on the hormogonium surface, and mutations in pil-like genes altered surface piliation and reduced symbiotic competency. This is the first molecular evidence that pilus biogenesis in a filamentous cyanobacterium requires a type IV pilus system.

Cyanobacterial chemotaxis to extracts of host and nonhost plants

Chemotaxis may be important when forming cyanobacterial symbioses. However, knowledge of cyanobacterial attraction towards plants and factors affecting chemotaxis is limited. Chemo-attraction was observed in Nostoc strains 8964:3 and PCC 73102 towards exudate or crushed extract of the natural hosts Gunnera manicata, Cycas revoluta and Blasia pusilla, and the nonhost plants Trifolium repens, Arabidopsis thaliana and Oryza sativa. As all tested plant extracts generated chemotaxis, the possibility to attract cyanobacteria may be widespread in plants. Chemotaxis was reduced by increased temperature and darkness and was stimulated by phosphorous and iron starvation and elevated salt concentration. Sugars (arabinose, galactose, and glucose) had a positive effect on chemotaxis, whereas flavonoids (chrysin and naringenin) and amino acids (methionine, glycine, serine, phenylalanine, glutamine, and lysine) had no effect.

Nitrogen deprivation stimulates symbiotic gland development in Gunnera manicata

Gunnera is the only genus of angiosperms known to host cyanobacteria and the only group of land plants that hosts cyanobacteria intracellularly. Motile filaments of cyanobacteria, known as hormogonia, colonize Gunnera plants through cells in the plant's specialized stem glands. It is commonly held that Gunnera plants always possess functional glands for symbiosis. We found, however, that stem gland development did not occur when Gunnera manicata plants were grown on nitrogen (N)-replete medium but, rather, was initiated at predetermined positions when plants were deprived of combined N. While N status was the main determinant for gland development, an exogenous carbon source (sucrose) accelerated the process. Furthermore, a high level of sucrose stimulated the formation of callus-like tissue in place of the gland under N-replete conditions. Treatment of plants with the auxin transport inhibitor 1-naphthylphthalamic acid prevented gland development on N-limited medium, most likely by preventing resource reallocation from leaves to the stem. Optimized conditions were found for in vitro establishment of the Nostoc-Gunnera symbiosis by inoculating mature glands with hormogonia from Nostoc punctiforme, a cyanobacterium strain for which the full genome sequence is available. In contrast to uninoculated plants, G. manicata plants colonized by N. punctiforme were able to continue their growth on N-limited medium. Understanding the nature of the Gunnera plant's unusual adaptation to an N-limited environment may shed light on the evolution of plant-cyanobacterium symbioses and may suggest a route to establish productive associations between N-fixing cyanobacteria and crop plants.

Competition among symbiotic cyanobacterialNostocstrains forming artificial associations with rice (Oryza sativa)

Competition among four symbiotically competent Nostoc strains, colonizing rice roots, was examined using hetR-DGGE (denaturing gradient gel electrophoresis) as strain identification. Although mixed in various combinations, only one strain at a time associated with the rice roots. Nostoc strain 8964:3 was the most competitive and our data suggest that its competitive fitness was dependent on rapid hormogonial spreading as displayed on agar plates. Furthermore, rice roots induced hormogonia in all tested Nostoc strains, but only Nostoc strain 9104 showed positive chemotaxis towards the root. Inhibition of growth of competing cyanobacterial strains was not apparent.

Construction and use of GFP reporter vectors for analysis of cell-type-specific gene expression in Nostoc punctiforme

Two transcriptional reporter shuttle vectors were constructed for the filamentous cyanobacterium Nostoc punctiforme using the green fluorescence protein (GFP) reporter. Both the ampicillin- and kanamycin-resistant versions of the plasmid allow promoters to be directionally cloned into a multiple cloning site preceding a promoterless gfp gene using an Escherichia coli host. The ability of the self-replicating shuttle plasmids to report cell-type-specific gene expression in N. punctiforme was tested by cloning promoters expressed in normal vegetative cells, nitrogen-fixing heterocysts and spore-like akinetes. A P(psaC) reporter gene fusion was expressed in vegetative cells and not in heterocysts, whereas GFP driven from P(hetR) was found highly expressed in heterocysts. GFP expression driven by the promoter for the N. punctiforme homologue of the akinete-specific gene avaK was expressed in developing akinetes. Decreased expression of GFP from the P(psaC) reporter in hormogonia was also observed. The results demonstrate the utility of these GFP vectors to study cell-type-specific gene expression in differentiating filamentous cyanobacteria.

Cloning and expression of a putative cyclodextrin glucosyltransferase from the symbiotically competent cyanobacterium Nostoc sp. PCC 9229

A polymerase chain reaction-based method was used to isolate a Nostoc sp. PCC 9229 cDNA from infected glands of Gunnera chilensis. The complete gene sequence was isolated from a genomic Nostoc sp. PCC 9229 library. Sequence analysis showed 84% amino acid similarity to a putative cyclodextrin glycosyltransferase from Nostoc sp. PCC 7120 and the gene was therefore termed cgt. Southern blot revealed that the cgt gene was present in symbiotically competent cyanobacteria. The cgt gene was expressed in free-living nitrogen-fixing cultures in light or in darkness when supplemented with fructose. This is the first expression analysis of a cgt gene from a cyanobacterium.