Bacterial leaf symbiosis in angiosperms: host specificity without co-speciation

Bacterial leaf symbiosis is a unique and intimate interaction between bacteria and flowering plants, in which endosymbionts are organized in specialized leaf structures. Previously, bacterial leaf symbiosis has been described as a cyclic and obligate interaction in which the endosymbionts are vertically transmitted between plant generations and lack autonomous growth. Theoretically this allows for co-speciation between leaf nodulated plants and their endosymbionts. We sequenced the nodulated Burkholderia endosymbionts of 54 plant species from known leaf nodulated angiosperm genera, i.e. Ardisia, Pavetta, Psychotria and Sericanthe. Phylogenetic reconstruction of bacterial leaf symbionts and closely related free-living bacteria indicates the occurrence of multiple horizontal transfers of bacteria from the environment to leaf nodulated plant species. This rejects the hypothesis of a long co-speciation process between the bacterial endosymbionts and their host plants. Our results indicate a recent evolutionary process towards a stable and host specific interaction confirming the proposed maternal transmission mode of the endosymbionts through the seeds. Divergence estimates provide evidence for a relatively recent origin of bacterial leaf symbiosis, dating back to the Miocene (5-23 Mya). This geological epoch was characterized by cool and arid conditions, which may have triggered the origin of bacterial leaf symbiosis.     

Bacterial leaf nodule symbiosis in angiosperms with emphasis on Rubiaceae and Myrsinaceae

Over 400 species of three genera of Rubiaceae and one genus of Myrsinaceae reportedly have bacterial leaf nodules. Light and/or electron microscope studies of a few species have shown that bacteria exist in spaces within buds filled with mucilage secreted by glands. These bacteria enter substomatal chambers (Rubiaceae) or marginal hydathodes (Myrsinaceae) and establish short-lived colonies, in intercellular spaces, that die out almost before full leaf expansion. Bacteria occur in seeds between endosperm and embryo, but only two studies have followed bacteria into flowers and ovules. Previous work on the physical relations of bacteria and host plants is discussed critically. Reviewing work done on isolation and identification of presumed endophytes leads to the conclusion that there is no agreement whether one or several bacterial taxa are the endophyte, and no unambiguous identifications, although four genera are suggested as possibilities. Nitrogen fixation was considered as the bacterial contribution until quite recently, but a review of such studies reveals that fixation has been detected almost exclusively in isolated presumed endophytes, whereas almost all studies involving the bacterium in intact leaves have failed to detect nitrogen fixation. Studies of particular substances (besides combined nitrogen) contributed by the endophyte have been inconclusive, although the most recent works suggest that cytokinins are involved. Host plants lacking the endophyte have been reportedly produced many times, either spontaneously or by seed treatment. Such “cripples”, used for several aspects of symbiosis study, frequently revert to a nodulated condition, and a more reliable method of producing them is needed. Tissue culture may offer the best potential, but this approach has not yet produced whole bacteria-free plants. A proposed scheme for the evolution of the symbiosis suggests that a variety of bacteria entered buds first, and only in rare instances were compatible with the host bud mucilage. In a few of these cases, specific bacteria, compatible with the microenvironment, contributed a useful substance to the host, and bud mucilage and those bacteria co-evolved until large numbers of bacteria thrived in the buds. Nodules may have resulted from accidental entry of bacteria into leaves, with the possibility that some host plant nodules are merely pathogenic responses, whereas in others the bacteria are beneficial and further selection has resulted in numerous, regularly produced nodules. This review deals with taxonomy of host plants and endophytes, morphology of the symbiosis, its physiology, and speculation on the evolution of the symbiosis.     

豆科植物凝集素及其对根瘤菌的识别作用

本文讨论了豆科植物凝集素的性质、分布、基因及其表达;近年来研究表明识别根瘤菌的因子是豆科植物根上的凝集素。将一种豆科植物的凝集素基因转化到另一种豆科植物后,再接种前一种豆科植物的根瘤菌,可以使其被侵染和结瘤。由此人们提出了扩大根瘤菌宿主范围到非豆科植物,特别是粮食作物范围的可能性。     

豆科植物凝集素及其对根瘤菌的识别作用

本文讨论了豆科植物凝集素的性质、分布、基因及其表达;近年来研究表明识别根瘤菌的因子是豆科植物根上的凝集素。将一种豆科植物的凝集素基因转化到另一种豆科植物后,再接种前一种豆科植物的根瘤菌,可以使其被侵染和结瘤。由此人们提出了扩大根瘤菌宿主范围到非豆科植物,特别是粮食作物范围的可能性。     

植物内生细菌测定方法的研究进展

伴随全功能体(holobiont)和全基因组(hologenome)概念的出现,植物微生物群落被看作植物全功能体的重要组成部分,其结构和功能逐渐得到研究和解析.内生细菌是植物微生物群落的成员之一,由于其定殖在组织内部而与宿主的接触更为紧密,因而其与植物的相互作用也更加直接、高效且不容易受到环境条件变化的影响.本文介绍了...     

The symbiosis of Bacillus subtilis L-forms with Chinese cabbage seedlings inhibits conidial germination of Botrytis cinerea

AIMS: To establish whether germination of Botrytis cinerea was affected by the symbiosis of Bacillus subtilis L-form bacteria with Chinese cabbage. METHODS AND RESULTS: Germinating seeds of Chinese cabbage were co-cultivated with either L-forms of Bacillus subtilis or 5% (w/v) mannitol by soaking for 3 h. Seeds were then washed in sterile water, sown on a minimal medium and incubated in controlled conditions. L-form symbiosis was detected over a time course by ELISA. Conidial germination of Botrytis cinerea was significantly reduced on cotyledonous leaves of L-form-treated plants compared with controls. CONCLUSIONS: Symbiosis of B. subtilis L-form bacteria during seed germination of Chinese cabbage inhibits conidial germination in plants on subsequent exposure to Botrytis cinerea. SIGNIFICANCE AND IMPACT OF THE STUDY: This is the first account of plant symbiosis with L-form bacteria showing antagonism to a fungal plant pathogen. This has promising implications for the use of this L-form as a biocontrol agent.     

白蚁与动物及微生物的共生类型及其机制

综述了白蚁螱客的主要种类、共生关系及相关机制的研究进展。白蚁螱客中,已报道的动物种类达170种。在与动物的共生关系中存在偏利共生（宾主共栖和异种共栖）、互利共生和无关共生三种;在与微生物的共生关系中,存在与内生菌（原生动物、细菌、真菌和放线菌）和外生菌（蚁巢伞菌等）间的互利关系。指出了白蚁与螱客研究中存在的问题,给出了解决方案,并提出了今后可能的研究热点或方向,为白蚁的综合利用（如纤维素酶）及今后研究物种间的协同进化提供了基础资料。     

Genetic resources of nodule bacteria

Nodule bacteria (rhizobia) form highly specific symbiosis with leguminous plants. The efficiency of accumulation of biological nitrogen depends on molecular-genetic interaction between the host plant and rhizobia. Genetic characteristics of microsymbiotic strains are crucial in developing highly productive and stress-resistant symbiotic pairs: rhizobium strain-host plant cultivar (species). The present review considers the issue of studying genetic resources of nodule bacteria to identify genes and their blocks, responsible for the ability of rhizobia to form highly effective symbiosis in various agroecological conditions. The main approaches to investigate of intraspecific and interspecific genetic and genomic diversity of nodule bacteria are considered, from MLEE analysis to the recent methods of genomic DNA analysis using biochips. The data are presented showing that gene centers of host plants are centers of genetic diversification of nodule bacteria, because the intraspecific polymorphism of genetic markers of the core and the accessory rhizobial genomes is extremely high in them. Genotypic features of trapped and nodule subpopulations of alfalfa nodule bacteria are discussed. A survey of literature showed that the genomes of natural strains in alfalfa gene centers exhibit significant differences in genes involved in control of metabolism, replication, recombination, and the formation of defense response (hsd genes). Natural populations of rhizobia are regarded as a huge gene pool serving as a source of evolutionary innovations.     

Bacterial genes induced within the nodule during the Rhizobium–legume symbiosis

During the symbiosis between the bacterium Rhizobium meliloti and plants such as alfalfa, the bacteria elicit the formation of nodules on the roots of host plants. The bacteria infect the nodule, enter the cytoplasm of plant cells and differentiate into a distinct cell type called a bacteroid, which is capable of fixing atmospheric nitrogen. To discover bacterial genes involved in the infection and differentiation stages of symbiosis, we obtained genes expressed at the appropriate time and place in the nodule by identifying promoters that are able to direct expression of the bacA gene, which is required for bacteroid differentiation. We identified 230 fusions that are expressed predominantly in the nodule. Analysis of 23 sequences indicated that only three encode proteins known to be involved in the Rhizobium-legume symbiosis, six encode proteins with homology to proteins not previously associated with symbiosis, and 14 have no significant similarity to proteins of known function. Disruption of a locus that encodes a protein with homology to a cell adhesion molecule led to a defect in the formation of nitrogen-fixing nodules, resulting in an increased number of nitrogen-starved plants. Our isolation of a large number of nodule-expressed genes will help to open the intermediate stages of nodulation to molecular analysis.     

Fungal symbiosis in rice requires an ortholog of a legume common symbiosis gene encoding a Ca2+/calmodulin-dependent protein kinase

In natural ecosystems, many plants are able to establish mutually beneficial symbioses with microorganisms. Of critical importance to sustainable agriculture are the symbioses formed between more than 80% of terrestrial plants and arbuscular mycorrhizal (AM) fungi and between legumes and nitrogen-fixing rhizobial bacteria. Interestingly, the two symbioses share overlapping signaling pathways in legumes, suggesting that the evolutionarily recent root nodule symbiosis may have acquired functions from the ancient AM symbiosis. The Medicago truncatula DMI3 (DOESN'T MAKE INFECTIONS3) gene (MtDMI3) and its orthologs in legumes are required for both bacterial and fungal symbioses. MtDMI3 encodes a Ca(2+)/calmodulin-dependent protein kinase (CCaMK) essential for the transduction of the Ca(2+) signal induced by the perception of Nod factors. Putative orthologs of MtDMI3 are also present in non-legumes, but their function in AM symbiosis has not been demonstrated in any non-legume species. Here, we combine reverse genetic approaches and a cross-species complementation test to characterize the function of the rice (Oryza sativa) ortholog of MtDMI3, namely, OsDMI3, in AM symbiosis. We demonstrate that OsDMI3 is not only required for AM symbiosis in rice but also is able to complement a M. truncatula dmi3 mutant, indicating an equivalent role of MtDMI3 orthologs in non-legumes.     

丛枝菌根真菌在寄主植物抵御生物和非生物胁迫中的作用

丛枝菌根真菌(arbuscular mycorrhiza fungi,AMF)是生态系统中普遍存在的土壤微生物,能与绝大多数植物形成共生关系,它在寄主植物抵御生物和非生物胁迫中所起的作用逐渐引起国内外学者的关注.论文综述了丛枝菌根真菌在植物抵御非生物胁迫(重金属污染、有机污染、盐胁迫和干旱胁迫)以及生物胁迫(致病菌和线虫侵染)中的作用,并在此基础上提出了未来该领域值得进一步研究的方向.     

Effect of arbuscular mycorrhizal (AM) colonization on terpene emission and content of Artemisia annua L

Plant roots interact with a wide variety of rhizospheric microorganisms, including bacteria and the symbiontic arbuscular mycorrhizal (AM) fungi. The mycorrhizal symbiosis represents a series of complex feedbacks between plant and fungus regulated by their physiology and nutrition. Despite the widespread distribution and ecological significance of AM symbiosis, little is known about the potential of AM fungi to affect plant VOC metabolism. The purpose of this study was to investigate whether colonization of plant roots by AM fungi and associated soil microorganisms affects VOC emission and content of Artemisia annua L. plants (Asteraceae). Two inoculum types were evaluated: one consisted of only an arbuscular mycorrhizal (AM) fungus species (Glomus spp.), and the other was a mixture of different Glomus species and associated soil bacteria. Inoculated plants were compared with non-inoculated plants and with plants supplemented with extra phosphorus (P) to obtain plants of the same size as mycorrhizal plants, thus excluding potentially-confounding mycorrhizal effects on shoot growth. VOC emissions of Artemisia annua plants were analyzed by leaf cuvette sampling followed by off-line measurements with pre-concentration and gas chromatography mass spectrometry (GC-MS). Measurements of CO(2) and H(2)O exchanges were conducted simultaneously. Several volatile monoterpenes were identified and characterized from leaf emissions of Artemisia annua L. by GC-MS analysis. The main components identified belong to different monoterpene structures: alpha-pinene, beta-pinene, camphor, 1,8-cineole, limonene, and artemisia ketone. A good correlation between monoterpene leaf concentration and leaf emission was found. Leaf extracts included also several sesquiterpenes. Total terpene content and emission was not affected by AM inoculation with or without bacteria, while emission of limonene and artemisia ketone was stimulated by this treatment. No differences were found among treatments for single monoterpene content, while accumulation of specific sesquiterpenes in leaves was altered in mycorrhizal plants compared to control plants. Growth conditions seemed to have mainly contributed to the outcome of the symbiosis and influenced the magnitude of the plant response. These results highlight the importance of considering the below-ground interaction between plant and soil for estimating VOC emission rates and their ecological role at multitrophic levels.     

Microbial consortia of bacteria and fungi with focus on the lichen symbiosis

The investigation of fungal–bacterial interactions is an emerging field of research applying tools of modern microbial ecology. Studies have previously focused on the mycorrhizosphere, but in past decade, the role of bacteria in other fungal niches has been increasingly evaluated. This review presents recent progress in the understanding of fungal–bacterial interactions and contains a special focus on lichen symbioses. Lichens are traditionally considered as mutualisms between fungi and photoautotrophic species, but recent molecular approaches have revealed that lichens also harbour diverse microbial communities. Using modern DNA/RNA-based and microscopic techniques (e.g. FISH and confocal laser scanning microscopy) we are now able to analyse the abundance, composition, and structure of microbial communities in the lichen holobiont. Lichen-associated microbial communities consist of diverse taxonomic groups; the majority of bacteria belong to Alphaproteobacteria. Microbial communities can form biofilm-like structures on specific parts of the lichen thallus. Until now, the function and interaction within the microbial consortia is not fully understood. The functions displayed mainly by culturable strains suggest that bacteria have lytic activities, complement the nitrogen budget and produce bioactive substances, including hormones and antibiotics. Bacterial contribution to the lichen symbiosis is perhaps not restricted to one particular function in the lichen system, but supports a complex functional network which remains to be studied in greater detail.     

Reactive oxygen and nitrogen species and glutathione: key players in the legume-Rhizobium symbiosis

Several reactive oxygen and nitrogen species (ROS/RNS) are continuously produced in plants as by-products of aerobic metabolism or in response to stresses. Depending on the nature of the ROS and RNS, some of them are highly toxic and rapidly detoxified by various cellular enzymatic and non-enzymatic mechanisms. Whereas plants have many mechanisms with which to combat increased ROS/RNS levels produced during stress conditions, under other circumstances plants appear to generate ROS/RNS as signalling molecules to control various processes encompassing the whole lifespan of the plant such as normal growth and development stages. This review aims to summarize recent studies highlighting the involvement of ROS/RNS, as well as the low molecular weight thiols, glutathione and homoglutathione, during the symbiosis between rhizobia and leguminous plants. This compatible interaction initiated by a molecular dialogue between the plant and bacterial partners, leads to the formation of a novel root organ capable of fixing atmospheric nitrogen under nitrogen-limiting conditions. On the one hand, ROS/RNS detection during the symbiotic process highlights the similarity of the early response to infection by pathogenic and symbiotic bacteria, addressing the question as to which mechanism rhizobia use to counteract the plant defence response. Moreover, there is increasing evidence that ROS are needed to establish the symbiosis fully. On the other hand, GSH synthesis appears to be essential for proper development of the root nodules during the symbiotic interaction. Elucidating the mechanisms that control ROS/RNS signalling during symbiosis could therefore contribute in defining a powerful strategy to enhance the efficiency of the symbiotic interaction.     

Plant hormones as signals in arbuscular mycorrhizal symbiosis

Abstract

Arbuscular mycorrhizal (AM) fungi are non-specific symbionts developing mutual and beneficial symbiosis with most terrestrial plants. Because of the obligatory nature of the symbiosis, the presence of the host plant during the onset and proceeding of symbiosis is necessary. However, AM fungal spores are able to germinate in the absence of the host plant. The fungi detect the presence of the host plant through some signal communications. Among the signal molecules, which can affect mycorrhizal symbiosis are plant hormones, which may positively or adversely affect the symbiosis. In this review article, some of the most recent findings regarding the signaling effects of plant hormones, on mycorrhizal fungal symbiosis are reviewed. This may be useful for the production of plants, which are more responsive to mycorrhizal symbiosis under stress.     

Laser‐ablation electrospray ionization mass spectrometry with ion mobility separation reveals metabolites in the symbiotic interactions of soybean roots and rhizobia

Technologies enabling in situ metabolic profiling of living plant systems are invaluable for understanding physiological processes and could be used for rapid phenotypic screening (e.g., to produce plants with superior biological nitrogen‐fixing ability). The symbiotic interaction between legumes and nitrogen‐fixing soil bacteria results in a specialized plant organ (i.e., root nodule) where the exchange of nutrients between host and endosymbiont occurs. Laser‐ablation electrospray ionization mass spectrometry (LAESI‐MS) is a method that can be performed under ambient conditions requiring minimal sample preparation. Here, we employed LAESI‐MS to explore the well characterized symbiosis between soybean (Glycine max L. Merr.) and its compatible symbiont, Bradyrhizobium japonicum. The utilization of ion mobility separation (IMS) improved the molecular coverage, selectivity, and identification of the detected biomolecules. Specifically, incorporation of IMS resulted in an increase of 153 differentially abundant spectral features in the nodule samples. The data presented demonstrate the advantages of using LAESI–IMS–MS for the rapid analysis of intact root nodules, uninfected root segments, and free‐living rhizobia. Untargeted pathway analysis revealed several metabolic processes within the nodule (e.g., zeatin, riboflavin, and purine synthesis). Compounds specific to the uninfected root and bacteria were also detected. Lastly, we performed depth profiling of intact nodules to reveal the location of metabolites to the cortex and inside the infected region, and lateral profiling of sectioned nodules confirmed these molecular distributions. Our results established the feasibility of LAESI–IMS–MS for the analysis and spatial mapping of plant tissues, with its specific demonstration to improve our understanding of the soybean‐rhizobial symbiosis.     

Rhizobien in der Pflanzenmikrobiota

Rhizobia in the plant microbiota The plant microbiota is of critical importance for plant growth and survival in soil. To explore mechanisms underlying plant‐microbiota interactions, defined commensal communities can be composed from microbiota culture collections and co‐cultivated with germ‐free plants to determine their impact on plant growth and health. The order Rhizobiales belongs to the core microbiota and includes nitrogen‐fixing bacteria that are known to engage in symbiotic interactions with legumes. Compatible host‐symbiont pairs are needed for a functional symbiosis, which involves the activation of highly specialized and interdependent signaling pathways between the two partners. Comparative genome analysis of more than 1,300 legume symbionts and rhizobial root commensals from non‐leguminous plants revealed that the most recent common ancestor of rhizobia lacked the gene repertoire needed for symbiosis and was able to colonize roots of a wide variety of plants. During evolution, key symbiosis genes were acquired multiple independent times by commensals belonging to different families of the Rhizobiales order.     

The evolution of specificity in the legume-rhizobium symbiosis

We know more about the partnership between legumes and their root-nodule bacteria than about any other symbiosis or any other plant-microbe interaction. In the light of recent research we are beginning to see details of an elaborate tapestry. The rhizobia are not a self-contained branch on the bacterial tree; their ancestry is intertwined with that of photosynthetic and pathogenic bacteria. Their host ranges, which vary enormously in breadth, overlap to form a tangled web of interconnections between plants and bacteria, and mechanisms of infection and nodule development are more diverse than we once thought. From genetic analysis of the bacteria we learn that specificity is not the province of special 'host-range determinants', but is affected by a wide range of genes with diverse modes of action. The symbiosis is a rich resource for evolutionary fact and speculation, but its complexity and diversity should warn us not to expect easy answers.     

Phosphate in the arbuscular mycorrhizal symbiosis: transport properties and regulatory roles

In response to the colonization by arbuscular mycorrhizal (AM) fungi, plants reprioritize their phosphate (Pi)-uptake strategies to take advantage of nutrient transfer via the fungus. The mechanisms underlying Pi transport are beginning to be understood, and recently, details of the regulation of plant and fungal Pi transporters in the AM symbiosis have been revealed. This review summarizes recent advances in this area and explores current data and hypotheses of how the plant Pi status affects the symbiosis. Finally, suggestions of an interrelationship of Pi and nitrogen (N) in the AM symbiosis are discussed.