Phosphorylation of the alpha-I motif in SYMRK drives root nodule organogenesis.

Symbiosis receptor-like kinase SYMRK is required for root nodule symbiosis between legume plants and nitrogen-fixing bacteria. To understand symbiotic signaling from SYMRK, we determined the crystal structure to 1.95 Å and mapped the phosphorylation sites onto the intracellular domain. We identified four serine residues in a conserved "alpha-I" motif, located on the border between the kinase core domain and the flexible C-terminal tail, that, when phosphorylated, drives organogenesis. Substituting the four serines with alanines abolished symbiotic signaling, while substituting them with phosphorylation-mimicking aspartates induced the formation of spontaneous nodules in the absence of bacteria. These findings show that the signaling pathway controlling root nodule organogenesis is mediated by SYMRK phosphorylation, which may help when engineering this trait into non-legume plants.

Chemoreceptors in Sinorhizobium meliloti require minimal pentapeptide tethers to provide adaptational assistance.

Sensory adaptation in bacterial chemotaxis is mediated by posttranslational modifications of methyl-accepting chemotaxis proteins (MCPs). In Escherichia coli, the adaptation proteins CheR and CheB tether to a conserved C-terminal receptor pentapeptide. Here,we investigated the function of the pentapeptide motif (N/D)WE(E/N)F in Sinorhizobium meliloti chemotaxis. Isothermal titration calorimetry revealed stronger affinity of the pentapeptides to CheR and activated CheB relative to unmodified CheB. Strains with mutations of the conserved tryptophan in one or all four MCP pentapeptides resulted in a significant decrease or loss of chemotaxis to glycine betaine, lysine, and acetate, chemoattractants sensed by pentapeptide-bearing McpX and pentapeptide-lacking McpU and McpV, respectively. Importantly, we discovered that the pentapeptide mediates chemotaxis when fused to the C-terminus of pentapeptide-lacking chemoreceptors via a flexible linker. We propose that adaptational assistance and a threshold number of available sites enable the efficient docking of adaptation proteins to the chemosensory array. Altogether, these results demonstrate that S. meliloti effectively utilizes a pentapeptide-dependent adaptation system with a minimal number of tethering units to assist pentapeptide-lacking chemoreceptors and hypothesize that the higher abundance of CheR and CheB in S. meliloti compared to E. coli allows for ample recruitment of adaptation proteins to the chemosensory array.

RALF22 is a key modulator of the root hair growth responses to fungal ethylene emissions in Arabidopsis.

In Arabidopsis (Arabidopsis thaliana (L.) Heynh), exposure to volatile compounds (VCs) emitted by Penicillium aurantiogriseum promotes root hair (RH) proliferation and hyper-elongation through mechanisms involving ethylene, auxin and photosynthesis signaling. In addition, this treatment enhances the levels of the small signalling peptide RAPID ALKALINIZATION FACTOR 22 (RALF22). Here we used genetics to address the role of RALF22 in fungal VC-promoted RH growth and to identify the bioactive fungal VC. We found that RHs of ralf22 and feronia (fer-4) plants impaired in the expression of RALF22 and its receptor FERONIA, respectively, responded weakly to fungal VCs. Unlike in WT roots, fungal VC exposure did not enhance RALF22 transcript levels in roots of fer-4 and ethylene- and auxin- insensitive mutants. In ralf22 and fer-4 roots, this treatment did not enhance the levels of ERS2 transcripts encoding one member of the ethylene receptor family and those of some RH-related genes. RHs of ers2-1 and the rsl2rsl4 double mutants impaired in the expression of ERS2 and the ethylene- and auxin-responsive ROOT HAIR DEFECTIVE 6-LIKE 2 and 4 transcription factors, respectively, weakly responded to fungal VCs. Moreover, roots of plants defective in photosynthetic responsiveness to VCs exhibited weak RALF22 expression and RH growth responses to fungal VCs. VCs of ΔefeA strains of P. aurantiogriseum cultures impaired in ethylene synthesis weakly promoted RH proliferation and elongation in exposed plants. We conclude that RALF22 simultaneously functions as a transcriptionally regulated signaling molecule that participates in the ethylene, auxin and photosynthesis signaling-mediated RH growth response to fungal ethylene emissions and regulation of ethylene perception in RHs.

Mobility and growth in confined spaces are important mechanisms for the establishment of Bacillus subtilis in the rhizosphere.

The rhizosphere hosts complex and abundant microbiomes whose structure and composition are now well described by metagenomic studies. However, the dynamic mechanisms that enable micro-organisms to establish along a growing plant root are poorly characterized. Here, we studied how a motile bacterium utilizes the microhabitats created by soil pore space to establish in the proximity of plant roots. We have established a model system consisting of Bacillus subtilis and lettuce seedlings co-inoculated in transparent soil microcosms. We carried out live imaging experiments and developed image analysis pipelines to quantify the abundance of the bacterium as a function of time and position in the pore space. Results showed that the establishment of the bacterium in the rhizosphere follows a precise sequence of events where small islands of mobile bacteria were first seen forming near the root tip within the first 12-24 h of inoculation. Biofilm was then seen forming on the root epidermis at distances of about 700-1000 µm from the tip. Bacteria accumulated predominantly in confined pore spaces within 200 µm from the root or the surface of a particle. Using probabilistic models, we could map the complete sequence of events and propose a conceptual model of bacterial establishment in the pore space. This study therefore advances our understanding of the respective role of growth and mobility in the efficient colonization of bacteria in the rhizosphere.

The interplay of suppressive soil bacteria and plant root exudates determines germination of microsclerotia of Verticillium longisporum.

Dormant microsclerotia play a vital role in the survival and spread of Verticillium longisporum, as they can stay viable in the soil and maintain their infectivity for many years. In our previous work, we revealed that soil bacterial volatiles are a key inhibitory factor causing microsclerotia dormancy in the soil. In this study, we further demonstrate that root exudates collected from both host and non-host plants can effectively rescue microsclerotia from bacterial suppression and initiate germination. To identify the specific compounds in root exudates responsible for microsclerotia germination, we fractionated the collected root exudates into polar and non-polar compounds. Subsequently, we conducted comprehensive bioassays with each fraction on germination-suppressed microsclerotia. The result revealed a pivotal role of primary metabolites in root exudates, particularly glutamic acid, in triggering microsclerotia germination and overcoming bacterial inhibition. Moreover, our studies revealed a decrease in inhibitory bacterial volatile fatty acids when bacteria were cultured in the presence of root exudates or glutamic acid. This suggests a potential mechanism, by which root exudates set-off bacterial suppression on microsclerotia. Here, we reveal for the first time that plant root exudates, instead of directly inducing the germination of microsclerotia, enact a set-off effect by counteracting the suppressive impact of soil bacteria on the microsclerotia germination process. This nuanced interaction advances our understanding of the multifaceted dynamics governing microsclerotia dormancy and germination in the soil environment. IMPORTANCE: Our research provides first-time insights into the crucial interaction between plant root exudates and soil bacteria in regulating the germination of Verticillium longisporum microsclerotia, a significant structure in the survival and proliferation of this soil-borne pathogen. We describe so far unknown mechanisms, which are key to understand how root infections on oilseed rape can occur. By pinpointing primary metabolites in root exudates as key factors in overcoming bacteria-induced dormancy and promote microsclerotia germination, our study highlights the potential for exploiting plant - as well as soil microbe-derived - compounds to control V. longisporum. This work underscores the importance of elucidating the nuanced interactions within the soil ecosystem to devise innovative strategies for managing root infective plant diseases, thereby contributing to the resilience and health of cropping systems.

Transfer RNA and ribosomal RNA fragments - emerging players in plant-microbe interactions.

According to current textbooks, the principal task of transfer and ribosomal RNAs (tRNAs and rRNAs, respectively) is synthesizing proteins. During the last decade, additional cellular roles for precisely processed tRNA and rRNAs fragments have become evident in all kingdoms of life. These RNA fragments were originally overlooked in transcriptome datasets or regarded as unspecific degradation products. Upon closer inspection, they were found to engage in a variety of cellular processes, in particular the modulation of translation and the regulation of gene expression by sequence complementarity- and Argonaute protein-dependent gene silencing. More recently, the presence of tRNA and rRNA fragments has also been recognized in the context of plant-microbe interactions, both on the plant and the microbial side. While most of these fragments are likely to affect endogenous processes, there is increasing evidence for their transfer across kingdoms in the course of such interactions; these processes may involve mutual exchange in association with extracellular vesicles. Here, we summarize the state-of-the-art understanding of tRNA and rRNA fragment's roles in the context of plant-microbe interactions, their potential biogenesis, presumed delivery routes, and presumptive modes of action.

The endophytic fungus Serendipita indica affects auxin distribution in Arabidopsis thaliana roots through alteration of auxin transport and conjugation to promote plant growth.

Plants share their habitats with a multitude of different microbes. This close vicinity promoted the evolution of interorganismic interactions between plants and many different microorganisms that provide mutual growth benefits both to the plant and the microbial partner. The symbiosis of Arabidopsis thaliana with the beneficial root colonizing endophyte Serendipita indica represents a well-studied system. Colonization of Arabidopsis roots with S. indica promotes plant growth and stress tolerance of the host plant. However, until now, the molecular mechanism by which S. indica reprograms plant growth remains largely unknown. This study used comprehensive transcriptomics, metabolomics, reverse genetics, and life cell imaging to reveal the intricacies of auxin-related processes that affect root growth in the symbiosis between A. thaliana and S. indica. Our experiments revealed the sustained stimulation of auxin signalling in fungus infected Arabidopsis roots and disclosed the essential role of tightly controlled auxin conjugation in the plant-fungus interaction. It particularly highlighted the importance of two GRETCHEN HAGEN 3 (GH3) genes, GH3.5 and GH3.17, for the fungus infection-triggered stimulation of biomass production, thus broadening our knowledge about the function of GH3s in plants. Furthermore, we provide evidence for the transcriptional alteration of the PIN2 auxin transporter gene in roots of Arabidopsis seedlings infected with S. indica and demonstrate that this transcriptional adjustment affects auxin signalling in roots, which results in increased plant growth.

Sugars, Lipids and More: New Insights Into Plant Carbon Sources During Plant-Microbe Interactions.

Heterotrophic microbes rely on host-derived carbon sources for their growth and survival. Depriving pathogens of plant carbon is therefore a promising strategy for protecting plants from disease and reducing yield losses. Importantly, this carbon starvation-mediated resistance is expected to be more broad-spectrum and durable than race-specific R-gene-mediated resistance. Although sugars are well characterized as major carbon sources for bacteria, emerging evidence suggests that plant-derived lipids are likely to be an essential carbon source for some fungal microbes, particularly biotrophs. Here, we comprehensively discuss the dual roles of carbon sources (mainly sugars and lipids) and their transport processes in immune signalling and microbial nutrition. We summarize recent findings revealing the crucial roles of lipids as susceptibility factors at all stages of pathogen infection. In particular, we discuss the potential pathways by which lipids and other plant carbon sources are delivered to biotrophs, including protein-mediated transport, vesicle trafficking and autophagy. Finally, we highlight knowledge gaps and offer suggestions for clarifying the mechanisms that underlie nutrient uptake by biotrophs, providing guidance for future research on the application of carbon starvation-mediated resistance.

Ecology and Evolution of Plant Microbiomes.

Microorganisms colonizing plant surfaces and internal tissues provide a number of life-support functions for their host. Despite increasing recognition of the vast functional capabilities of the plant microbiome, our understanding of the ecology and evolution of the taxonomically hyperdiverse microbial communities is limited. Here, we review current knowledge of plant genotypic and phenotypic traits as well as allogenic and autogenic factors that shape microbiome composition and functions. We give specific emphasis to the impact of plant domestication on microbiome assembly and how insights into microbiomes of wild plant relatives and native habitats can contribute to reinstate or enrich for microorganisms with beneficial effects on plant growth, development, and health. Finally, we introduce new concepts and perspectives in plant microbiome research, in particular how community ecology theory can provide a mechanistic framework to unravel the interplay of distinct ecological processes-i.e., selection, dispersal, drift, diversification-that structure the plant microbiome.

Signal communication during microbial modulation of root system architecture.

Every living organism on Earth depends on its interactions with other organisms. In the rhizosphere, plants and microorganisms constantly exchange signals and influence each other's behavior. Recent studies have shown that many beneficial rhizosphere microbes can produce specific signaling molecules that affect plant root architecture and therefore could have substantial effects on above-ground growth. This review examines these chemical signals and summarizes their mechanisms of action, with the aim of enhancing our understanding of plant-microbe interactions and providing references for the comprehensive development and utilization of these active components in agricultural production. In addition, we highlight future research directions and challenges, such as searching for microbial signals to induce primary root development.

Insight of endophytic fungi promoting the growth and development of woody plants.

Microorganisms play an important role in plant growth and development. In particular, endophytic fungi is one of the important kinds of microorganisms and has a mutually beneficial symbiotic relationship with host plants. Endophytic fungi have many substantial benefits to host plants, especially for woody plants, such as accelerating plant growth, enhancing stress resistance, promoting nutrient absorption, resisting pathogens and etc. However, the effects of endophytic fungi on the growth and development of woody plants have not been systematically summarized. In this review, the functions of endophytic fungi for the growth and development of woody plants have been mainly reviewed, including regulating plant growth (e.g., flowering, root elongation, etc.) by producing nutrients and plant hormones, and improving plant disease, insect resistance and heavy metal resistance by producing secondary metabolites. In addition, the diversity of endophytic fungi could improve the ability of woody plants to adapt to adverse environment. The components produced by endophytic fungi have excellent potential for the growth and development of woody plants. This review has systematically discussed the potential regulation mechanism of endophytic fungi regulating the growth and development of woody plants, it would be of great significance for the development and utilization of endophytic fungi resource from woody plants for the protection of forest resources.

Unveiling growth-promoting attributes of peanut root endophyte Micromonospora sp.

In this study, 20 endophytic actinobacteria were isolated from different parts of peanut plants growing in cropland with low and high salt in West Bengal, India. The endophytes underwent a rigorous morphological, biochemical, and genetic screening process to evaluate their effectiveness in enhancing plant growth. About 20% of these isolates were identified as potential plant growth-promoting endophytic actinobacteria, which showed high 16S rRNA gene sequence similarity (up to 99-100%) with different species of Micromonospora. Among these isolates, Micromonospora sp. ASENR15 produced the highest levels of indole acetic acid (IAA) and gibberellic acid (GA), while Micromonospora sp. ASENL2, Micromonospora sp. ANENR4, and Micromonospora sp. ASENR12 produced the highest level of siderophore. Among these leaf and root endophytic Micromonospora, strain ANENR4 was tested for its plant growth-promoting attributes. ANENR4 can be transmitted into the roots of a healthy peanut plant, enhances growth, and colonize the roots in abundance, suggesting the potential agricultural significance of the strain. Moreover, the study is the first report of endophytic Micromonospora in peanuts with PGP effects. The outcomes of this study open avenues for further research on harnessing the benefits of this endophytic Micromonospora for optimizing plant growth in agriculture.

Prairie restoration promotes the abundance and diversity of mutualistic arbuscular mycorrhizal fungi.

Predicting how biological communities assemble in restored ecosystems can assist in conservation efforts, but most research has focused on plants, with relatively little attention paid to soil microbial organisms that plants interact with. Arbuscular mycorrhizal (AM) fungi are an ecologically significant functional group of soil microbes that form mutualistic symbioses with plants and could therefore respond positively to plant community restoration. To evaluate the effects of plant community restoration on AM fungi, we compared AM fungal abundance, species richness, and community composition of five annually cultivated, conventionally managed agricultural fields with paired adjacent retired agricultural fields that had undergone prairie restoration 5-9 years prior to sampling. We hypothesized that restoration stimulates AM fungal abundance and species richness, particularly for disturbance-sensitive taxa, and that gains of new taxa would not displace AM fungal species present prior to restoration due to legacy effects. AM fungal abundance was quantified by measuring soil spore density and root colonization. AM fungal species richness and community composition were determined in soils and plant roots using DNA high-throughput sequencing. Soil spore density was 2.3 times higher in restored prairies compared to agricultural fields, but AM fungal root colonization did not differ between land use types. AM fungal species richness was 2.7 and 1.4 times higher in restored prairies versus agricultural fields for soil and roots, respectively. The abundance of Glomeraceae, a disturbance-tolerant family, decreased by 25% from agricultural to restored prairie soils but did not differ in plant roots. The abundance of Claroideoglomeraceae and Diversisporaceae, both disturbance-sensitive families, was 4.6 and 3.2 times higher in restored prairie versus agricultural soils, respectively. Species turnover was higher than expected relative to a null model, indicating that AM fungal species were gained by replacement. Our findings demonstrate that restoration can promote a relatively rapid increase in the abundance and diversity of soil microbial communities that had been degraded by decades of intensive land use, and community compositional change can be predicted by the disturbance tolerance of soil microbial taxonomic and functional groups.

Spatial and Temporal Shifts of Endophytic Bacteria in Conifer Seedlings of Abies religiosa (Kunth) Schltdl. & Cham.

Endophytes play an important role in plant development, survival, and establishment, but their temporal dynamics in young conifer plants are still largely unknown. In this study, the bacterial community was determined by metabarcoding of the 16S rRNA gene in the rhizoplane, roots, and aerial parts of 1- and 5-month-old seedlings of natural populations of Abies religiosa (Kunth) Schltdl. & Cham. In 1-month-old seedlings, Pseudomonas dominated aerial parts (relative abundance 71.6%) and roots (37.9%). However, the roots exhibited significantly higher bacterial species richness than the aerial parts, with the dissimilarity between these plant sections mostly explained by the loss of bacterial amplification sequence variants. After 5 months, Mucilaginibacter dominated in the rhizoplane (9.0%), Streptomyces in the roots (12.2%), and Pseudomonas in the aerial parts (18.1%). The bacterial richness and community structure differed significantly between the plant sections, and these variations were explained mostly by 1-for-1 substitution. The relative abundance of putative metabolic pathways significantly differed between the plant sections at both 1 and 5 months. All the dominant bacterial genera (e.g., Pseudomonas and Burkholderia-Caballeronia-Paraburkholderia) have been reported to have plant growth-promoting capacities and/or antagonism against pathogens, but what defines their role for plant development has still to be determined. This investigation improves our understanding of the early plant-bacteria interactions essential for natural regeneration of A. religiosa forest.

Differential regulation of glucosinolate-myrosinase mediated defense determines host-aphid interaction in Indian mustard Brassica juncea L.

BACKGROUND: India's oilseed economy falls short of self-sufficiency and is supplemented by huge imports every year. Increasing national productivity of the major oilseeds is confronted with yield losses due to diverse biotic and abiotic stresses. The productivity of Indian mustard (Brassica juncea Linnaeus), belonging to the family Brassicaceae, is significantly reduced due to damage caused by mustard aphids (Lipaphis erysimi Kaltenbach, Hemiptera: Aphididae). Rapid colonization by the nymphs makes it difficult to protect the crop through agrochemicals. Aphids release effector molecules to modulate the host-defence responses. Glucosinolates (GSLs) extensively found in Brassicaceae family, are hydrolysed by myrosinase into toxic compounds that deter herbivore insects. METHODS: Here, we investigated the differential activation of the glucosinolate-myrosinase pathway in mustard manifesting susceptibility and resistance to different aphid species. Mustard plants were challenged by two different aphid species mustard aphid and cowpea aphid (Aphis craccivora Koch, Hemiptera: Aphididae) leading to complete host-susceptibility in one case and resistance in the other, respectively. Differential regulation of the GSL biosynthetic pathway and myrosinase activity was assessed by gene expression study and ultra-performance liquid chromatography quadrupole time-of-flight tandem mass spectrometry (UPLC- QToF-ESL-MS). RESULTS: Gene expression study identified selective transcriptional attenuation of the key GSL biosynthetic and myrosinase gene in mustard when challenged with mustard aphid. In contrary, the activation of GSL biosynthetic genes in conjunction with myrosinase at the transcriptional level was profound in mustard, when challenged with cowpea aphid. UPLC-MS analysis showed higher turnover in the hydrolysis of glucosinolates by myrosinase which led to concomitant generation of glucose as byproduct in response to cowpea aphid in mustard plants. CONCLUSION: GSL-myrosinase pathway is specifically attenuated by the successful aphid species in mustard and thus plays a pivotal role in determining the outcome of the B. juncea-aphid interaction. The results open up a new genetic modification strategy for developing resistance against aphids.

Current methods for monitoring Pseudomonas syringae biofilm development.

This work reviews biofilm investigation techniques and highlights the benefits and drawbacks of each approach focusing especially on Pseudomonas syringae and may serve as a comprehensive guide for any early-career researchers starting with the topic of biofilm. Each approach with applications of individual microscopy and spectroscopy techniques is summarized together with characterization of Pseudomonas syringae and its role in pathogenesis.

Coordinated bacterial and plant sulfur metabolism in Enterobacter sp. SA187-induced plant salt stress tolerance.

Enterobacter sp. SA187 is a root endophytic bacterium that maintains growth and yield of plants under abiotic stress conditions. In this work, we compared the metabolic wirings of Arabidopsis and SA187 in the free-living and endophytic interaction states. The interaction of SA187 with Arabidopsis induced massive changes in bacterial gene expression for chemotaxis, flagellar biosynthesis, quorum sensing, and biofilm formation. Besides modification of the bacterial carbon and energy metabolism, various nutrient and metabolite transporters and the entire sulfur pathway were up-regulated. Under salt stress, Arabidopsis resembled plants under sulfate starvation but not when colonized by SA187, which reprogramed the sulfur regulon of Arabidopsis. In accordance, salt hypersensitivity of multiple Arabidopsis sulfur metabolism mutants was partially or completely rescued by SA187 as much as by the addition of sulfate, L-cysteine, or L-methionine. Many components of the sulfur metabolism that are localized in the chloroplast were partially rescued by SA187. Finally, salt-induced accumulation of reactive oxygen species as well as the hypersensitivity of LSU mutants were suppressed by SA187. LSUs encode a central regulator linking sulfur metabolism to chloroplast superoxide dismutase activity. The coordinated regulation of the sulfur metabolic pathways in both the beneficial microorganism and the host plant is required for salt stress tolerance in Arabidopsis and might be a common mechanism utilized by different beneficial microbes to mitigate the harmful effects of different abiotic stresses on plants.

Endophytic fungi improved wheat resistance to Rhopalosiphum padi by decreasing its feeding efficiency and population fitness.

The improvement of crop resistance to insect using endophytic fungi is an environmentally friendly and sustainable strategy for agricultural pest control. Clarifying the efficacy and mechanism of endophytic fungi in improving crop resistance to pest offers the opportunity for biological control. In this study, changes in the transcriptome and defense compounds of wheat inoculated with endophytic fungal strains (i.e., YC and BB) were evaluated, and the efficacy of endophytic fungi in improving wheat resistance to Rhopalosiphum padi was studied. The results showed that the numbers of upregulated differentially-expressed genes (DEGs) in wheat plants inoculated with endophytic fungal strains YC and BB were higher than those of the downregulated DEGs, irrespective of R. padi infestation. Defense-related metabolic pathways, such as plant hormone signal transduction and secondary metabolite biosynthesis pathways were significantly enriched. Endophytic fungal strains YC and BB significantly increased jasmonic acid, DIMBOA (2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one), total flavone, and tannin contents in wheat plants (P < 0.05) but decreased salicylic acid content. Variations in the contents of defense compounds were significantly correlated with decreased feeding, development, and reproduction of R. padi fed on wheat plants inoculated with strains YC and BB (|r| = 0.68-0.91, P < 0.05). The results suggested that endophytic fungi significantly decreased the feeding efficiency and population fitness [YC: (-11.13%) - (-22.07%); BB: (-10.98%) - (-22.20%)] of R. padi by altering the phytohormone pathway and secondary metabolite biosynthesis in wheat plants. This study helps in understanding of the efficacy of endophytic fungi in improving wheat resistance to insect and will be conducive to integrated pest management.

Sesquiterpenes of the ectomycorrhizal fungus Pisolithus microcarpus alter root growth and promote host colonization.

Trees form symbioses with ectomycorrhizal (ECM) fungi, maintained in part through mutual benefit to both organisms. Our understanding of the signaling events leading to the successful interaction between the two partners requires further study. This is especially true for understanding the role of volatile signals produced by ECM fungi. Terpenoids are a predominant class of volatiles produced by ECM fungi. While several ECM genomes are enriched in the enzymes responsible for the production of these volatiles (i.e., terpene synthases (TPSs)) when compared to other fungi, we have limited understanding of the biochemical products associated with each enzyme and the physiological impact of specific terpenes on plant growth. Using a combination of phylogenetic analyses, RNA sequencing, and functional characterization of five TPSs from two distantly related ECM fungi (Laccaria bicolor and Pisolithus microcarpus), we investigated the role of these secondary metabolites during the establishment of symbiosis. We found that despite phylogenetic divergence, these TPSs produced very similar terpene profiles. We focused on the role of P. microcarpus terpenes and found that the fungus expressed a diverse array of mono-, di-, and sesquiterpenes prior to contact with the host. However, these metabolites were repressed following physical contact with the host Eucalyptus grandis. Exposure of E. grandis to heterologously produced terpenes (enriched primarily in γ -cadinene) led to a reduction in the root growth rate and an increase in P. microcarpus-colonized root tips. These results support a very early putative role of fungal-produced terpenes in the establishment of symbiosis between mycorrhizal fungi and their hosts.

Contribution of plant growth-promoting endophytic bacteria from hyperaccumulator to non-host plant zinc nutrition and health.

Application of microbial agents is a novel strategy to improve the quality and health of plant, which can be used to increase zinc (Zn) uptake and alleviate Zn toxicity. Here, endophytic bacteria with Zn solubilizing and growth-promoting properties were isolated from hyperaccumulating ecotype (HE) of Sedum alfredii Hance and their effects on Zn absorption and accumulation of non-hyperaccumulating ecotype (NHE) were studied. The results showed that most endophytic bacteria of HE have good Zn solubilizing or growth-promoting properties. Under the condition of 20 µM ZnSO4, the biomass of NHE inoculated with SaPS1, SaEN2, SaPR2, SaBA2, SaBA3 was 2.8-3.2 times higher than that of non-inoculation control, and the Zn concentration of shoots was increased by 45.9, 89.0, 53.7, 77.5, and 42.6% after inoculation with SaPA1, SaP1, SaEN2, SaBA1, and SaBA2. Under the condition of 100 µM ZnSO4, inoculation with SaVA1, SaPS3, SaB1, SaPR1, and SaEN3 alleviated Zn stress and significantly reduced Zn concentration of shoots. Therefore, endophytic bacteria can be an effective means of improving plant Zn nutrition quality in the normal condition and benefit plant health in the stress environment.

In this study, endophytes with Zn solubilizing properties were systematically isolated from Zn hyperaccumulator Sedum alfredii Hance. The aim is to identify endophyte resources with good promoting effect on plant growth and Zn uptake, and to provide scientific basis for the development of efficient biofortified agent.