The Populations of Two Differently Medicine-Used Plants of Hedyotis diffusa and Hedyotis corymbosa Shoot-Assembling Rich Bacterial and Fungal Communities with Varied Compositions but Conserved Structures.

The plant-colonized microbial communities have closely micro-ecological effects on host plant growth and health. There are many medicinal plants in the genus Hedyotis, but it is yet unclear about the shoot-assembled bacterial and fungal communities (SBFC) of Hedyotis plants. Hence, eight plant populations of Hedyotis diffusa (HD) and H. corymbosa (HC) were evaluated with 16S rRNA gene and ITS sequences, for comparing the types, abundance, or/and potential functions of SBFC at plant species- and population levels. In tested HD- and HC-SBFC, 682 fungal operational taxonomic units and 1,329 bacterial zero-radius operational taxonomic units were identified, with rich species compositions and varied alpha diversities. Notably, the SBFC compositions of HD and HC plant populations were exhibited with partly different types and abundances at phylum and genus levels but without significantly different beta diversities at plant species and population levels. Typically, the SBFC of HD and HC plant populations were presented with abundance-different biomarkers, such as Frankiaceae and Bryobacteraceae, and with similar micro-ecological functions of microbial metabolisms of lipids, terpenoids,and xenobiotics. Taken together, HD- and HC-SBFC possessed with varied rich compositions, conservative taxonomic structures, and similar metabolic functions, but with small-scale type and abundance differences at plant species- and population- levels.

Rhizosphere priming promotes plant nitrogen acquisition by microbial necromass recycling.

Nitrogen availability in the rhizosphere relies on root-microorganism interactions, where root exudates trigger soil organic matter (SOM) decomposition through the rhizosphere priming effect (RPE). Though microbial necromass contribute significantly to organically bound soil nitrogen (N), the role of RPEs in regulating necromass recycling and plant nitrogen acquisition has received limited attention. We used 15N natural abundance as a proxy for necromass-N since necromass is enriched in 15N compared to other soil-N forms. We combined studies using the same experimental design for continuous 13CO2 labelling of various plant species and the same soil type, but considering top- and subsoil. RPE were quantified as difference in SOM-decomposition between planted and unplanted soils. Results showed higher plant N uptake as RPEs increased. The positive relationship between 15N-enrichment of shoots and roots and RPEs indicated an enhanced necromass-N turnover by RPE. Moreover, our data revealed that RPEs were saturated with increasing carbon (C) input via rhizodeposition in topsoil. In subsoil, RPEs increased linearly within a small range of C input indicating a strong effect of root-released C on decomposition rates in deeper soil horizons. Overall, this study confirmed the functional importance of rhizosphere C input for plant N acquisition through enhanced necromass turnover by RPEs.

Spatial IMA1 regulation restricts root iron acquisition on MAMP perception.

Iron is critical during host-microorganism interactions1-4. Restriction of available iron by the host during infection is an important defence strategy, described as nutritional immunity5. However, this poses a conundrum for externally facing, absorptive tissues such as the gut epithelium or the plant root epidermis that generate environments that favour iron bioavailability. For example, plant roots acquire iron mostly from the soil and, when iron deficient, increase iron availability through mechanisms that include rhizosphere acidification and secretion of iron chelators6-9. Yet, the elevated iron bioavailability would also be beneficial for the growth of bacteria that threaten plant health. Here we report that microorganism-associated molecular patterns such as flagellin lead to suppression of root iron acquisition through a localized degradation of the systemic iron-deficiency signalling peptide Iron Man 1 (IMA1) in Arabidopsis thaliana. This response is also elicited when bacteria enter root tissues, but not when they dwell on the outer root surface. IMA1 itself has a role in modulating immunity in root and shoot, affecting the levels of root colonization and the resistance to a bacterial foliar pathogen. Our findings reveal an adaptive molecular mechanism of nutritional immunity that affects iron bioavailability and uptake, as well as immune responses.

Monocotyledonous plants graft at the embryonic root-shoot interface.

Grafting is possible in both animals and plants. Although in animals the process requires surgery and is often associated with rejection of non-self, in plants grafting is widespread, and has been used since antiquity for crop improvement1. However, in the monocotyledons, which represent the second largest group of terrestrial plants and include many staple crops, the absence of vascular cambium is thought to preclude grafting2. Here we show that the embryonic hypocotyl allows intra- and inter-specific grafting in all three monocotyledon groups: the commelinids, lilioids and alismatids. We show functional graft unions through histology, application of exogenous fluorescent dyes, complementation assays for movement of endogenous hormones, and growth of plants to maturity. Expression profiling identifies genes that unify the molecular response associated with grafting in monocotyledons and dicotyledons, but also gene families that have not previously been associated with tissue union. Fusion of susceptible wheat scions to oat rootstocks confers resistance to the soil-borne pathogen Gaeumannomyces graminis. Collectively, these data overturn the consensus that monocotyledons cannot form graft unions, and identify the hypocotyl (mesocotyl in grasses) as a meristematic tissue that allows this process. We conclude that graft compatibility is a shared ability among seed-bearing plants.

Light-induced mobile factors from shoots regulate rhizobium-triggered soybean root nodulation.

Symbiotic nitrogen fixation is an energy-expensive process, and the light available to plants has been proposed to be a primary influencer. We demonstrate that the light-induced soybean TGACG-motif binding factor 3/4 (GmSTF3/4) and FLOWERING LOCUS T (GmFTs), which move from shoots to roots, interdependently induce nodule organogenesis. Rhizobium-activated calcium- and calmodulin-dependent protein kinase (CCaMK) phosphorylates GmSTF3, triggering GmSTF3­GmFT2a complex formation, which directly activates expression of nodule inception (NIN) and nuclear factor Y (NF-YA1 and NF-YB1). Accordingly, the CCaMK­STF­FT module integrates aboveground light signals with underground symbiotic signaling, ensuring that the host plant informs its roots that the aboveground environment is prepared to sustainably supply the carbohydrate necessary for symbiosis. These results suggest approaches that could enhance the balance of carbon and nitrogen in the biosphere.

A microbiota-root-shoot circuit favours Arabidopsis growth over defence under suboptimal light.

Bidirectional root-shoot signalling is probably key in orchestrating stress responses and ensuring plant survival. Here, we show that Arabidopsis thaliana responses to microbial root commensals and light are interconnected along a microbiota-root-shoot axis. Microbiota and light manipulation experiments in a gnotobiotic plant system reveal that low photosynthetically active radiation perceived by leaves induces long-distance modulation of root bacterial communities but not fungal or oomycete communities. Reciprocally, microbial commensals alleviate plant growth deficiency under low photosynthetically active radiation. This growth rescue was associated with reduced microbiota-induced aboveground defence responses and altered resistance to foliar pathogens compared with the control light condition. Inspection of a set of A. thaliana mutants reveals that this microbiota- and light-dependent growth-defence trade-off is directly explained by belowground bacterial community composition and requires the host transcriptional regulator MYC2. Our work indicates that aboveground stress responses in plants can be modulated by signals from microbial root commensals.

Mechanistic understanding of interspecific interaction between a C4 grass and a C3 legume via arbuscular mycorrhizal fungi, as influenced by soil phosphorus availability using a 13 C and 15 N dual-labelled organic patch.

Arbuscular mycorrhizal fungi (AMF) can improve plant nutrient acquisition, either by directly supplying nutrients to plants or by promoting soil organic matter mineralization, thereby affecting interspecific plant relationships in natural communities. We examined the mechanism by which the addition of P affects interspecific interactions between a C4 grass (Bothriochloa ischaemum, a dominant species in natural grasslands) and a C3 legume (Lespedeza davurica, a subordinate species in natural grasslands) via AMF and plant growth, by continuous 13 C and 15 N labelling, combined with soil enzyme analyses. The results of 15 N labelling revealed that P addition affected the shoot uptake of N via AMF by B. ischaemum and L. davurica differently. Specifically, the addition of P significantly increased the shoot uptake of N via AMF by B. ischaemum but significantly decreased that by L. davurica. Interspecific plant interactions via AMF significantly facilitated the plant N uptake via AMF by B. ischaemum but significantly inhibited that by L. davurica under P-limited soil conditions, whereas the opposite effect was observed in the case of excess P. This was consistent with the impact of interspecific plant interaction via AMF on arbuscular mycorrhizal (AM) benefit for plant growth. Our data indicate that the capability of plant N uptake via AMF is an important mechanism that influences interspecific relationships between C4 grasses and C3 legumes. Moreover, the effect of AMF on the activities of the soil enzymes responsible for N and P mineralization substantially contributed to the consequence of interspecific plant interaction via AMF for plant growth.

Volatile organic compound patterns predict fungal trophic mode and lifestyle.

Fungi produce a wide variety of volatile organic compounds (VOCs), which play central roles in the initiation and regulation of fungal interactions. Here we introduce a global overview of fungal VOC patterns and chemical diversity across phylogenetic clades and trophic modes. The analysis is based on measurements of comprehensive VOC profiles of forty-three fungal species. Our data show that the VOC patterns can describe the phyla and the trophic mode of fungi. We show different levels of phenotypic integration (PI) for different chemical classes of VOCs within distinct functional guilds. Further computational analyses reveal that distinct VOC patterns can predict trophic modes, (non)symbiotic lifestyle, substrate-use and host-type of fungi. Thus, depending on trophic mode, either individual VOCs or more complex VOC patterns (i.e., chemical communication displays) may be ecologically important. Present results stress the ecological importance of VOCs and serve as prerequisite for more comprehensive VOCs-involving ecological studies.

Systemic reprogramming of phytohormone profiles and metabolic traits by virulent Diplodia infection in its pine (Pinus sylvestris L.) host.

The widespread ascomycetous fungus Diplodia pinea is a latent, necrotrophic pathogen in Pinus species causing severe damages and world-wide economic losses. However, the interactions between pine hosts and virulent D. pinea are largely not understood. In the present study, systemic defence responses were investigated in non-inoculated, asymptomatic needles and roots of D. pinea infected saplings of two P. sylvestris provenances under controlled greenhouse conditions. Here, we show that D. pinea infection induced a multitude of systemic responses of the phytohormone profiles and metabolic traits. Shared systemic responses of both pine provenances in needles and roots included increased abscisic acid and jasmonic acid levels. Exclusively in the roots of both provenances, enhanced salicylic acid and reduced indole-3-acetic acid levels, structural biomass, and elevated activities of anti-oxidative enzymes were observed. Despite these similarities, the two pine provenances investigated different significantly in the systemic responses of both, phytohormone profiles and metabolic traits in needles and roots. However, the different systemic responses did not prevent subsequent destruction of non-inoculated needles, but rather prevented damage to the roots. Our results provide a detailed view on systemic defence mechanisms of pine hosts that are of particular significance for the selection of provenances with improved defence capacity.

Genomics assisted functional characterization of Paenibacillus polymyxa HK4 as a biocontrol and plant growth promoting bacterium.

The diseases caused by phytopathogens account for huge economic losses in the agricultural sector. Paenibacillus polymyxa is one of the agriculturally important biocontrol agents and plant growth promoting bacterium. This study describes the antifungal potential of P. polymyxa HK4 against an array of fungal phytopathogens and its ability to stimulate seed germination of cumin and groundnut under in vitro conditions. The cumin and groundnut seeds bacterized with HK4 exhibited enhanced germination efficiency in comparison to controls. The use of HK4 as a soil inoculant significantly promoted the shoot length and fresh weight of groundnut plants in pot studies. The draft genome analysis of HK4 revealed the genetic attributes for motility, root colonization, antagonism, phosphate solubilization, siderophore production and production of volatile organic compounds. The bacterium HK4 harnessed several hydrolytic enzymes that may assist its competence in the rhizosphere. The PCR amplification and sequence analysis of the conserved region of the fusA gene amplicon revealed the ability of HK4 to produce fusaricidin. Furthermore, the LC-ESI-MS/MS of crude cell pellet extract of HK4 confirmed the presence of fusaricidin as a major antifungal metabolite. This study demonstrated the potential of HK4 as a biocontrol agent and a plant growth promoter.

Metagenomics of two gnotobiotically grown aromatic rice cultivars reveals genotype-dependent and tissue-specific colonization of endophytic bacterial communities attributing multiple plant growth promoting traits.

Exploration of community structures, habitations, and potential plant growth promoting (PGP) attributes of endophytic bacteria through next generation sequencing (NGS) is a prerequisite to culturing PGP endophytic bacteria for their application in sustainable agriculture. The present study unravels the taxonomic abundance and diversity of endophytic bacteria inhabiting in vitro grown root, shoot and callus tissues of two aromatic rice cultivars through 16S rRNA gene-based Illumina NGS. Wide variability in the number of bacterial operational taxonomic units (OTUs) and genera was observed between the two samples of the root (55, 14 vs. 310, 76) and shoot (26, 12 vs. 276, 73) but not between the two callus samples (251, 61 vs. 259, 51), indicating tissue-specific and genotype-dependent bacterial community distribution in rice plant, even under similar gnotobiotic growth conditions. Sizes of core bacteriomes of the selected two rice genotypes varied from 1 to 15 genera, with Sphingomonas being the only genus detected in all six samples. Functional annotation, based upon the abundance of bacterial OTUs, revealed the presence of several PGP trait-related genes having variable relative abundance in tissue-specific and genotype-dependent manners. In silico study also documented a higher abundance of certain genes in the same biochemical pathway, such as nitrogen fixation, phosphate solubilization and indole acetic acid production; implying their crucial roles in the biosynthesis of metabolites leading to PGP. New insights on utilizing callus cultures for isolation of PGP endophytes aiming to improve rice crop productivity are presented, owing to constancy in bacterial OTUs and genera in callus tissues of both the rice genotypes.

Natural Bacterial Assemblages in Arabidopsis thaliana Tissues Become More Distinguishable and Diverse during Host Development.

To study the spatial and temporal dynamics of bacterial colonization under field conditions, we planted and sampled Arabidopsis thaliana during 2 years at two Michigan sites and surveyed colonists by sequencing 16S rRNA gene amplicons. Mosaic and dynamic assemblages revealed the plant as a patchwork of tissue habitats that differentiated with age. Although assemblages primarily varied between roots and shoots, amplicon sequence variants (ASVs) also differentiated phyllosphere tissues. Increasing assemblage diversity indicated that variants dispersed more widely over time, decreasing the importance of stochastic variation in early colonization relative to tissue differences. As tissues underwent developmental transitions, the root and phyllosphere assemblages became more distinct. This pattern was driven by common variants rather than those restricted to a particular tissue or transiently present at one developmental stage. Patterns also depended critically on fine phylogenetic resolution: when ASVs were grouped at coarse taxonomic levels, their associations with host tissue and age weakened. Thus, the observed spatial and temporal variation in colonization depended upon bacterial traits that were not broadly shared at the family level. Some colonists were consistently more successful at entering specific tissues, as evidenced by their repeatable spatial prevalence distributions across sites and years. However, these variants did not overtake plant assemblages, which instead became more even over time. Together, these results suggested that the increasing effect of tissue type was related to colonization bottlenecks for specific ASVs rather than to their ability to dominate other colonists once established.IMPORTANCE Developing synthetic microbial communities that can increase plant yield or deter pathogens requires basic research on several fronts, including the efficiency with which microbes colonize plant tissues, how plant genes shape the microbiome, and the microbe-microbe interactions involved in community assembly. Findings on each of these fronts depend upon the spatial and temporal scales at which plant microbiomes are surveyed. In our study, phyllosphere tissues housed increasingly distinct microbial assemblages as plants aged, indicating that plants can be considered collections of tissue habitats in which microbial colonists-natural or synthetic-are established with differing success. Relationships between host genes and community diversity might vary depending on when samples are collected, given that assemblages grew more diverse as plants aged. Both spatial and temporal trends weakened when colonists were grouped by family, suggesting that functional rather than taxonomic profiling will be necessary to understand the basis for differences in colonization success.

Temporal assessment of root and shoot colonization of elephant grass (Pennisetum purpureum Schum.) host seedlings by Gluconacetobacter diazotrophicus strain LP343.

Gluconacetobacter diazotrophicus is a species of great agronomic potential due to its growth-promotion traits. Its colonization process in different plants has been reported. However, there have been no studies regarding its structural colonization in elephant grass. This is a fast-growing C4-Poaceae plant, and its application in Brazil is mainly aimed at feeding dairy cattle, due to its high nutritional value. Also, in the last decade, this grass has been applied in the production of biofuels. The present study aimed to monitor the colonization process of strain LP343 of G. diazotrophicus inoculated in elephant grass seedlings of PCEA genotype, by using a mCherry-tagged bacterium. Samples of roots and shoots collected at different periods were visualized by confocal laser-scanning microscopy. The colony-counting assay was used to compare the number of cells recovered in different niches and a qPCR was performed for the quantification of endophytic cells in root and shoot tissues. Results suggested that the strain LP343 quickly recognized the PCEA roots as host, attached to the elephant grass roots at 6 h, and 7 days after inoculation were able to colonize the xylem vessels of roots and shoots of elephant grass. This study advances our knowledge about the colonization process of G. diazotrophicus species in elephant grass, contributing to future studies involving the plant-bacteria interaction cultivated under gnotobiotic conditions.

Piperazic acid containing peptides produced by an endophytic Streptomyces sp. isolated from the medicinal plant Atropa belladonna.

The culture broth of endophytic Streptomyces sp. AB100, isolated from the shoots of medicinal plant Atropa belladonna (L.) was investigated for the presence of antibacterial compounds. After initial testing followed by bioactivity-guided fractionation, six new piperazic acid (PA)-containing congeners of two known peptides, JBIR-39 and JBIR-40, were identified by HR-MS/MS and NMR analyses. Only the dehydroxylated hexapeptidic derivatives with unusual incorporation of four PA moieties exhibited weak antibacterial activity against Gram-positive test organism Bacillus subtilis. A 16S rDNA-based phylogenetic tree of known Streptomyces spp. producing PA-containing hexapeptides isolated from different habitats and endophyte Streptomyces AB100 showed considerable diversity, suggesting that these metabolites may play an important environmental role beyond their antibacterial activity.

Inhibition of multiple defense responsive pathways by CaWRKY70 transcription factor promotes susceptibility in chickpea under Fusarium oxysporum stress condition.

BACKGROUND: Suppression and activation of plant defense genes is comprehensively regulated by WRKY family transcription factors. Chickpea, the non-model crop legume suffers from wilt caused by Fusarium oxysporum f. sp. ciceri Race1 (Foc1), defense response mechanisms of which are poorly understood. Here, we attempted to show interaction between WRKY70 and several downstream signaling components involved in susceptibility/resistance response in chickpea upon challenge with Foc1. RESULTS: In the present study, we found Cicer arietinum L. WRKY70 (CaWRKY70) negatively governs multiple defense responsive pathways, including Systemic Acquired Resistance (SAR) activation in chickpea upon Foc1 infection. CaWRKY70 is found to be significantly accumulated at shoot tissues of susceptible (JG62) chickpea under Foc1 stress and salicylic acid (SA) application. CaWRKY70 overexpression promotes susceptibility in resistant chickpea (WR315) plants to Foc1 infection. Transgenic plants upon Foc1 inoculation demonstrated suppression of not only endogenous SA concentrations but expression of genes involved in SA signaling. CaWRKY70 overexpressing chickpea roots exhibited higher ion-leakage and Foc1 biomass accumulation compared to control transgenic (VC) plants. CaWRKY70 overexpression suppresses H2O2 production and resultant reactive oxygen species (ROS) induced cell death in Foc1 infected chickpea roots, stem and leaves. Being the nuclear targeted protein, CaWRKY70 suppresses CaMPK9-CaWRKY40 signaling in chickpea through its direct and indirect negative regulatory activities. Protein-protein interaction study revealed CaWRKY70 and CaRPP2-like CC-NB-ARC-LRR protein suppresses hyper-immune signaling in chickpea. Together, our study provides novel insights into mechanisms of suppression of the multiple defense signaling components in chickpea by CaWRKY70 under Foc1 stress. CONCLUSION: CaWRKY70 mediated defense suppression unveils networking between several immune signaling events negatively affecting downstream resistance mechanisms in chickpea under Foc1 stress.

Transcriptome analyses suggest that changes in fungal endophyte lifestyle could be involved in grapevine bud necrosis.

Bud necrosis (BN) is a common disorder that affects Vitis vinifera L. and reduces its potential yield. To minimize the losses caused by BN, the double pruning management was applied in Brazilian Southeast vineyards. In this management strategy plants are pruned at the winter to promote a vegetative cycle and then, at summer, to promote the reproductive cycle at optimal environmental conditions. To investigate the relationship of BN and the double pruning management RNA-seq libraries were sequenced from healthy and necrotic tissues at four different stages of the year. The comparison of differentially expressed genes in necrotic and non-necrotic tissues showed an enhanced expression of genes related to cell death possibly induced by endophytic microorganisms in the necrotic tissues. The de novo assembly, characterization and quantification of transcripts within the RNA-seq libraries showed that genes from the endophytic fungus Alternaria alternata, responsible for the production of toxic compounds were highly expressed under BN. Here we propose a model in which unfavorable conditions and reduced carbohydrate levels in buds can promote the switch from a biotrophic lifestyle to a necrotrophic lifestyle in the endophytic fungi, which seems to be involved in the development of BN.

Assessing the Efficacy of Broad-Spectrum Antibiotics in Controlling Bacterial Contamination in the In Vitro Micropropagation of Ginger (Zingiber officinale Rosc).

Ginger (Zingiber officinale Rosc) (Zingiberaceae) is a livelihood and commercial crop in Ethiopia. But, the availability of clean and healthy planting materials has become a problem due to wilt disease, caused by Ralstonia solanacearum Biovar 3 Race 4. This problem obliged growers to seek for tens of millions of vigorous and disease-free planting materials very quickly via in vitro micropropagation of shoot tip explants. For this purpose, protocols of sterilizing shoot tip explants and controlling bacterial contamination of one Ethiopian ginger cultivar called Deribo were tested. Hence, this article reports the finding of a study that aimed at testing the (a) effectiveness of three sterilization agents, namely, 0.25% w/v RBK (composed of ridomile, bayleton, and kocide at 1 : 1 : 1 ratio), 0.50% v/v NaOCl, and 70% v/v ethanol at three different treatment times in combination with 0.25% HgCl2; (b) efficacy of four broad-spectrum antibiotics and their combinations in controlling bacterial contaminants of ginger shoot tip explants and in vitro micropropagation media; and (c) effects of the antibiotics on the shooting performances of the explants of the cultivar. A 0.50% v/v NaOCl at exposure time of 20 min followed by 0.25% HgCl2 has resulted in 80% contamination-free and 70% live explants after three weeks of incubation. Likewise, cefotaxime at 50, 100, and 200 mg/L and cefotaxime plus streptomycin at 25, 50, and 100 mg/L yielded 87 to 93% contamination-free microshoots after three weeks of culturing. The number of explants killed by the antibiotics increased with increasing the concentration of the antibiotics. Cefotaxime at 50 mg/L and cefotaxime plus streptomycin at 25 mg/L yielded significantly highest mean microshoots per explant (7.10 ± 0.36 and 7.51 ± 0.27, respectively) and mean shoot length (4.2 ± 0.26 and 3.56 ± 0.17 cm, respectively). Some of the microshoots showed some yellowing. But, they turned green and grew normal after subcultured into fresh, antibiotics-free culture media. These findings are important foundations towards developing more optimized protocols of sterilizing explants and controlling bacterial contaminants for large-scale in vitro micropropagation of the Deribo ginger cultivar.

Isotope Labeling to Study Phosphorus Uptake in the Arbuscular Mycorrhizal Symbiosis.

Isotope labeling enables the detection and quantification of nutrient fluxes between soil and plants through arbuscular mycorrhizal (AM) fungi. Here we describe the use of radioactive isotopes, 33P and 32P, to study the uptake of P from soil by AM fungal mycelium and its transfer to the host plant through the mycorrhizal pathway.

Arsenic accumulation by a rhizosphere bacterial strain Ochrobactrum tritici reduces rice plant arsenic levels.

Arsenic naturally occurs in the earth's crust and can be introduced in the environment by human activities. Agricultural practices in arsenic-contaminated environments pose a threat to human health. The contamination of crops contributes to the metalloid's introduction in the food chain. This study aims to test the hypotheses that the inoculation of a hyperaccumulator rhizobacterial strain, Ochrobactrum tritici As5, to the rhizosphere of rice plants reduces the arsenic presence inside the tissue of the rice plants and reduces the inhibitory effect of the metalloid on the plant's growth parameters. Inoculation of the hyperaccumulating strain O. tritici As5 showed the lowest concentration of arsenic in the plant's tissue (2.6 fold lower than sterile plants), compared to the unmodified type O. tritici SCII24 and sterile rice plants. The inoculation of the type strain SCII24 also led to a decrease in arsenic concentration in the plant tissue compared with sterile plants (1.6 fold lower than sterile plants). The difference in arsenic presence in shoots was smaller among treatment groups than in the roots, showing a similar trend. The inoculation of the hyperaccumulator As5 strain alleviated some of the toxic effects of arsenic on shoot growth compared to inoculation of the unmodified type strain. All these findings together, contribute to our understanding of the interplay between arsenic pollution, plants and their rhizobacteria, especially the role of bioaccumulation of metal(oids) by rhizobacteria, and provide important information on the prevention of arsenic uptake by crops and the development of phytostabilizers.

Occurrence of Pathogenic and Potentially Pathogenic Bacteria in Microgreens, Sprouts, and Sprouted Seeds on Retail Market in Riga, Latvia.

Microgreens and sprouts have been used for raw consumption for a long time and are generally viewed as a healthy food. However, several serious outbreaks of foodborne illness have been recorded in European countries, Japan, and North America. Many companies in Latvia nowadays are producing this type of products. The aim of this study was to characterize the incidence of Shiga toxin-producing Escherichia coli (STEC), Salmonella spp., and Listeria spp. in microgreens, sprouts, and seeds intended for domestic production of microgreens on retail market in Riga, Latvia, from January to April 2019. The background microflora was identified as well. A total of 45 samples were purchased, including fresh and processed sprouts, microgreens, baby greens, as well as seeds intended for domestic production of microgreens and sprouts. The samples were processed according to the methods set by the International Organization for Standardization (ISO)-ISO/TS 13136:2012 for STEC, ISO 6579-1:2017 for Salmonella spp., and ISO 11290-1:2017 for Listeria spp. Molecular detection of Salmonella spp. was also performed using real-time polymerase chain reaction. The typical and atypical colonies isolated from selective plates were identified with matrix-assisted laser desorption and ionization time-of-flight mass spectrometry. Listeria monocytogenes was not detected in any of the tested samples. However, the presence of Listeria innocua was detected in two (4.4%) of the samples. Three (6.7%) samples of dried sprouts were positive for the STEC virulence genes. Salmonella spp. was detected in one (2.2%) sample of common sunflower seeds. Altogether, 46 different background bacterial species were identified. The majority were environmental bacteria characteristic to soil, water, and plants, including coliform bacteria. The results provide evidence that microgreens and seeds available for Latvian consumers are generally safe, however, attention has to be paid to dried sprouts.