Podcasts about rhizobium

Matters Microbial #90: Using Soil Microbiomes in Sustainable Agriculture May 8, 2025 Today, Dr. Francisco Dini Andreote, Assistant Professor of Phytobiomes at Penn State, joins the #QualityQuorum to tell us about the microbiome of plants and the soil, and how understanding that relationship can improve agriculture. Host: Mark O. Martin Guest: Francisco Dini Andreote Subscribe: Apple Podcasts, Spotify Become a patron of Matters Microbial! Links for this episode An overview of the Type 6 Secretory System of bacteria—almost like a microbial switchblade knife. A wonderful video of the T6SS made by a student in my own microbiology course some time ago. A video introduction to the Rhizobium-legume symbiosis and why you should care about it (by my PhD advisor from long ago, Dr. Sharon Long). A more comprehensive review article on the Rhizobium-legume symbiosis.  The chemical signal of geosmin, and how it might be used by other organisms. Ecological succession in the development of sauerkraut. A must read essay by Carl Zimmer likening the human body to a number of ecological niches. The developing field of agroecology.  A reminder about the “One Health” concept. Mycorrhizae and plant nutrition. Chemical communication within the soil. A fun remembrance of Norman Borlaug, who urged us to “listen” to plants. An overview of the root microbiome. The “superorganism” concept versus the “holobiome” concept.. Striga, a parasite of crop plants. Chemical communication and Striga.  An interesting and relevant publication from Dr. Dini Andreote's research group, describing how the root microbiome could help agriculture.  Dr. Dini Andreote's faculty website. Dr. Dini Andreote's very wonderful research team website. Intro music is by Reber Clark Send your questions and comments to mattersmicrobial@gmail.com

I have been feeling a little bit distant lately. Like some sort of anxious attachment distant. Avoidant even. While trying to not be too clingy or handsy with the land, I have slipped into a disconnection, being one that just observes but doesn't participate in the ways that brought me into relationship with so many plants in the first place. I have been feeling this disconnect, and recognizing something had to be done. Then along comes Red Clover. After attending a workshop on edible and medicinal plants I felt called by the Red Clover (Trifolium pratense). Here was a plant that I felt I could harvest without much impact on the populations, or harm to local species who depend on T. pratense. It felt like I could relearn relationships with the broader landscape, incorporating components of taking and consuming - components of relationship making with plants that I have felt conflicted on recently - and therefore helping to heal that separation which has been sneaking in. Since harvesting, I have also been doing deep dives into Red Clover natural history, and ecofunction. It has been a gift from this special plant to learn from them, harvest them, teach about them and drink the tea made from the flowers. That's what this week's show is all about. To learn more :The ROM Field Guide to Wildflowers of Ontario by Timothy Dickinson, Deborah Metsger, Jenny Bull, and Richard Dickinson. ROM, 2004.The Book of Field and Roadside by John Eastman and Amelia Hansen. Stackpole Books, 2003.American Wildlife & Plants : A Guide to Wildlife Food Habits by Alexander C Martin, Herbert S Zim, Arnold L. Nelson. Dover, 1951.Incredible Wild Edibles by Samuel Thayer. Forager's Harvest, 2017.Held By The Land by Leigh Joseph. Wellfleet Press, 2023.The Earthwise Herbal vol. 1 by Matthew Wood. North Atlantic Books, 2008.Rhizobium leguminosarum wikipedia page

Welcome to SciSection! Joining us in today's interview is our special guest Dr. Rebecca Batstone! Dr. Batstone is an assistant professor in the Department of Biology at McMaster

Las Rhizobium son unas bacterias que viven en simbiosis con determinadas plantas como las leguminosas. Se alojan en sus raíces y van a lograr fijar el nitrógeno atmosférico, necesario para que la planta viva. En 1991 una científica mexicana, Esperanza Martínez Romero, descubrió una rhizobium fuera de serie, la rhizobium tropici. RFI conversó con ella sobre esta microbiota que está revolucionando el mundo agrícola. Es un mundo insospechado, las bacterias fijadoras de nitrógeno que se alojan en las raíces de las plantas o árboles, las bacterias rhizobium. Este tipo de simbiosis es un proceso biológico fundamental para el desarrollo de las plantas. El trébol, por ejemplo, tiene bolitas en sus raíces. Estas bolitas tienen en su interior bacterias fijadoras de nitrógeno que interactúan con esta hierba en particular. Cada especie tiene sus propias bacterias. La soja, el frijol, el nogal o la parota, un árbol gigante que produce unas vainas y que fertiliza de manera natural los suelos.   Rhizobium Tropici, una bacteria fuera de serie En 1991 la doctora Esperanza Martínez Romero descubrió una bacteria fuera de serie, la Rhizobium Tropici capaz de suministrar altas dosis de nitrógeno en leguminosas, como el frijol, incluso en condiciones adversas como altas temperaturas, acidez en los suelos o contaminados con metales pesados. El uso de este tipo de bacterias reduce el uso de fertilizantes químicos, que tanto contaminan el medio ambiente. Y actualmente se está tratando de adaptar estas bacterias para el cultivo de cereales. Esta científica mexicana es investigadora titular del Centro de Ciencias Genómicas de la Universidad Nacional Autónoma de México (UNAM) en Cuernavaca. Su trayectoria científica fue recompensada por el premio L'Oréal Unesco 2020 para América Latina. Debido a la pandemia, tuvo que esperar dos años para poder recibir su premio en París. RFI aprovechó para conversar con la científica sobre estas bacterias que están revolucionando el mundo agrícola: Escuche aquí el programa sobre la trayectoria científica de Esperanza Martínez Romero: Premio L'Oréal UNESCO 2020: Esperanza Martínez Romero y la genética funcional

The family Fabaceae is one of the most ecologically successful and diverse plant families in the world, especially in arid and subtropical regions. In this episode we talk Legumes - their ecology, floral morphology and evolution - with Marty Wojciechowski at ASU. We talk about the 50kb inversion, psychoactive and poisonous secondary chemistry, subfamily classifications elucidated by molecular phylogenetics, how mimosoids lack Rhizobium root affiliations (bummer) and a bunch more interesting sh#t. Plant in the thumbnail photo is Schotia afra. 

Hello Interactors,This has been an eventful week, but also a week of more extreme heat and smoke. Just when climatologists warned of the certainty of more extreme weather patterns. I’m ready for fall and we’re barely halfway through summer. My plants are struggling too. Does anybody out there know how we’re going to adapt?As interactors, you’re special individuals self-selected to be a part of an evolutionary journey. You’re also members of an attentive community so I welcome your participation.Please leave your comments below or email me directly.Now let’s go…THE RIGHT TURNS LEFT FOR RIGHTSMonday of this week, August 9th, was International Day of the World’s Indigenous Peoples. Did you know that? What about Tuesday, August 10th. That was the anniversary of the Pueblo Revolt in what we now call New Mexico. In 1680, the Pueblo people forced 2000 Spanish colonial settlers off their land. Given this was the first example of American people rejecting European rule, some consider this to be America’s first Revolutionary War – nearly 100 years before the more popular version. Oh, and on Wednesday, August 11th my wife and I celebrated our 25th wedding anniversary. But even fewer people know about that historical date.The International Day of the World’s Indigenous Peoples was created by the United Nations in 1994. The date honors August 9th, 1982; the first day of meetings for the UN Working Group on Indigenous Populations. This group’s mandate was to: Promote and protect the human rights and fundamental freedoms of Indigenous peoples;Give attention to the evolution of international standards concerning Indigenous rights.August 9th celebrates the achievements and contributions Indigenous people have made, and continue to make, to governance, stewardship of the environment, and knowledge systems aimed at improving many of the challenges our world’s environment’s face today.Indigenous people make up 5% of the world’s population and use one quarter of its habitable surface. But, they protect in reciprocity 80% of the world’s biodiversity. The UN defines Indigenous People as: “Inheritors and practitioners of unique cultures and ways of relating to people and the environment.”The United Nations’ recognition of the sovereign rights of Indigenous people stems from the International Indian Treaty Council which grew out of the American Indian Movement in the 1960s and 70s. The United Nations recognized the rights of Indigenous people before the United States did. In fact, when the United Nations put the Declaration on the Rights of Indigenous Peoples to vote in 2007, the United States, Canada, New Zealand, and Australia voted against the declaration. They have since reversed this vote, but the American Indian Movement had long recognized the United States was in violation of treaties signed over the last 300 years. So acting as sovereign nations – that happen to reside within a larger, dominant, and controlling nation – they turned to the United Nations for recognition. Much of the legally binding language used in the Declaration on the Rights of Indigenous Peoples comes from the legal language written into the original treaties by the United States. Which is why the conservative originalist from the West, Supreme Court Judge Neil Gorsuch, sided with liberals last year in a landmark ruling over McGirt v. Oklahoma. The Supreme Court determined that much of that state was legally ceded to Indigenous people by the United States Federal government two centuries ago and it was high time the country obeyed their own laws. The year prior, Gorsuch did the same in the state of Wyoming. Oddly, the recently deceased Justice, Ruth Bader Ginsberg, a darling of the left, has a mixed record voting in favor of Indigenous people. A 2021 article from Cornell University states,“During Justice Ginsburg’s first 15 years on the court, 38 Indian law cases were argued. The rights of Indigenous nations prevailed in only seven of those cases. Indigenous nations lost in eight of nine Indian law cases for which she wrote the court’s decision.” After the Oklahoma ruling, John Echohawk from the Native American Rights Fund – an organization that has spent 50 years fighting for Indigenous rights – was quoted as saying, “This [case] brings these issues into public consciousness a little bit more…That’s one of the biggest problems we have, is that most people don’t know very much about us.” It seems Ruth Bader Ginsberg was one of those people. John Echohawk is following in the footsteps of those who kicked off the American Indian Movement back in 1968, drawing attention to Indigenous rights. Their focus was on the systematic poverty and police brutality toward Urban Indian’s who had been forced off of their land and into cities for generations. This Indigenous grassroots movement rose out of the city that was recently put the international map for its display of obvious police brutality – Minneapolis, Minnesota.GRANDMA KILLS A CHICKENI was not yet three years old when the American Indian Movement was born. I grew up about 250 miles due south of Minneapolis, in Norwalk, Iowa. It’s a suburb of Des Moines surrounded by farmland – much of which is being converted to housing developments. We didn’t live on a farm, but we always had a garden. I wasn’t that keen on gardening as a kid, but I wasn’t shy about eating the beans, corn, and potatoes that Iowa’s rich soil and climate yielded. My Mom’s surefire way to get me motivated to weed the garden or pick beans was to say, “Ok, you’re going to want to eat these beans once their picked, so maybe you should be the one picking them.”My parents learned to garden from their parents. My Grandma on my Mom’s side always had a big garden. It ran the width of her backyard and was flanked by a dirt alley on one side and a shed on the other. Off to the side of the yard was a rusty barrel I remember being as tall as me. That’s where we’d burn her garbage; now that was a job I enjoyed. I’d haul a bag full of stuff to the barrel, step up on a log nestled next to it, dump in the combustible waste, and drop a fiery wooden match on top of it. Poof. Those trips to the barrel also included carrying a bucket of kitchen scraps into the garden. We’d dig a hole with a shovel, dump the smelly scraps into the hole, and cover it up. Direct injection composting. My grandparents also kept chickens in the backyard. Our trips to grandma’s house on Sundays usually included a fresh chicken from her yard and vegetables from her garden. She’d walk out back, chase down a chicken, wring its neck, chop its head off, and get to pluckin’. Occasionally, my uncle Bud would show up with a pheasant or two (or three) strung out in his trunk, shot with his shotgun on his way to grandma’s house. I was always careful to avoid eating the lead shot dotting the glistening meat like embedded peppercorns. In the summer, dinner ended with a bowl of fresh berries and cream from a cow just down the road. But most of the time, it was pie. My grandma made a pie – using lard for the crust – almost everyday until the day she died. My grandparents on my Dad’s side had a garden and a few apple trees too. My Dad was born in the depression into a family with 11 siblings in the same town my Mom was born. He and his brothers and sisters lived off of the eggs from the chickens they kept. In the dead of winter, they’d hunt squirrels and hang them from the clothesline in the backyard where they’d freeze stiff; more protein to feed hungry mouths during Iowa’s harsh winters. My grandma Weed made a loaf of bread everyday to feed all those hungry tummies.I am one generation removed from that lifestyle and I’m having trouble keeping a single pepper plant alive. My parents were not farmers, and we did not hunt, but they had learned how to grow and hunt enough food to keep a family alive. Sure their childhood tables were also augmented with store-bought foods, but there was a concerted effort to grow, eat, can, and store as much food as possible. That desire and knowledge seems to get lost with every generation. Many of the techniques my parents and grandparents used to grow food was taught to them by their European ancestry – knowledge that was passed down from generation to generation. Settlers settling farms and homesteads across America brought with them agricultural methods taught to them in their European homeland. One such convention are rows of segregated crops; a row of beans, a row of squash, and a row of corn, for example. But that’s not how those crops were being grown by people they found here already farming this land.THREE SISTERS SHAREColonial settlers were clueless as to what to do with corn when they first arrived. The locals did teach them to farm corn, a plant first domesticated 10,000 years ago by the Indigenous people in what we now call Mexico. But, in return, some puritanical settlers thought they could show these folks a thing or two about farming. Dismayed by the untidiness made from the climbing clumps of squash at the base of corn stalks gently strangled by spiraling bean vines, the settlers went about mansplaining how to properly plant plants in neat tidy rows – one for corn, one for beans, and one for squash.But it turns out planting each of these crops to grow alone yields fewer ears of corn, beans, and squash. What the native farmers had learned over those 10,000 years is that when you plant these three plants next to one another, they uniquely help each other above and below ground to grow and prosper. Native people call this method of planting The Three Sisters and it was often planted in waffle-like gardens that create gridded microclimates.The first sister born is corn. It peaks its head out of the soil in the spring and shoots up straight like a pole. With enough growth to stand on its own, sister bean is born. Bean vines quickly start swirling in circles in search of something to cling on to – like a blindfolded kid playing pin the tail on the donkey. It latches onto the knees of it’s older sister, corn, and they grow toward the sun together. Then comes baby sister squash, crawling along the ground eager to choose its own path in the shadows of its older siblings. The baby sister, with its broad abundant leaves, helps shade the soil trapping water destined for the three sister’s roots in its water retaining waffle divot. It also keeps sun from tempting pesky weeds from popping up. All three sisters need nitrogen to grow, but lack the ability to siphon it from the air – despite the fact our atmosphere is made up of 78% nitrogen gas. What these Indigenous people learned over centuries of ecological observation and experimentation is that beans are the secret to providing the missing nitrogen. And Western science has proved it by providing the tools necessary to observe and understand the microscopic biological mechanisms that allow this genesis to unfold. Indigenous people knew it to be true, and Western science allowed it to be seen and described in consistent, repeatable, mathematical, and physical terms that transcend languages, cultures, and geographical boundaries.What we now know is that nitrogen comes from a fastidious underground bacteria called Rhizobium. It loves to make nitrogen, but only under special conditions. For starters, it needs to be free of oxygen. Given soil is filled with oxygen, it needs to find a suitable host willing to provide an oxygen free environment. As sister bean sends her many roots in all directions it invariably encounters the lingering Rhizobium nodules. Through microscopic chemical communications, the two strike a deal. In exchange for the much needed nitrogen, the bean root provides an oxygen-free nitrogen manufacturing facility for the bacteria; the benefactors of this underground nitrogen source are not only the beans, but her sisters, corn and squash, as well.I learned all this from Robin Wall Kimmerer, a Potawatomi tribal member as well as the Distinguished Professor of Biology and Director of the Center for Native Peoples and the Environment, at the State University of New York. She sums up this symphony of familial biological reciprocity in her landmark book, Braiding Sweetgrass, with a lesson for us all – not just plants. A lesson taught and practiced by Indigenous people for generations. She writes,“The most important thing each of us can know is our unique gift and how to use it in the world. Individuality is cherished and nurtured, because, in order for the whole to flourish, each of us has to be strong in who we are and carry our gifts with conviction, so they can be shared with others. Being among the sisters provides a visible manifestation of what a community can become when its members understand and share their gifts. In reciprocity, we fill our spirits as well as our bellies.”There was one more big event this week from another UN organization called the Intergovernmental Panel on Climate Change (IPCC). This is a team of climate researchers from around the world and they came out this week to report what they’ve been saying all along about climate change, but this time with an unequivocal warning. The extreme weather events we’re experiencing is indisputably caused by humans. Oh, that’s us. Past reports have used words like may and could but scientists have tossed away their gloves and came out swinging this week. We’re in trouble and it may not be reversible.Three years ago I ripped out my lawn and planted drought tolerant succulents. Well, the raccoons had the idea first I just went along with it. When the Northwest had its hottest June on record, the sun sucked the life out of plants that are naturally equipped to withstand prolonged heat. Some of the leaves didn’t just shrivel, they nearly evaporated. My backdoor neighbor’s peppers looked like they had roasted on the vine. On Wednesday night I was talking to a restoration ecologist who works for the City of Kirkland. He organizes teams of volunteers across the city to help eradicate invasive species and plant natives in their place. When I asked him about one park filled with tall lush cedars and firs along Interstate 405 that also features a mining pit at one end where the state dug for gravel to build the freeway, he talked of the struggles getting plants to grow on this compromised soil. He went on to explain how they’ve decided to pick a species that can handle not only the rocky soil, but also the increasing temperatures in Western Washington. So they’re trying a tree more commonly found on the more arid side of Washington state, the ponderosa pine. One of the big takeaways in listening to Robin Wall Kimmerer’s book is that while we humans have a way of beating ourselves up over the damage we’ve caused the environment, we also have the capacity (and the obligation) to help heal it. When we care for the earth, it cares for us in return in a symbiotic act of reciprocity. Indigenous people figured this out eons ago and the hubris of “Enlightened” European colonial settlers regarded their ways as “savage”. I’m not advocating for some romantic pastoral nirvana where we all trade our homes for huts, tend to our own chickens, and live off the land. But I do believe we live among millions of people who possess ancestral knowledge that, when paired with modern science and technology, could yield a more fruitful outcome. Many cultures living together on the same soil exchanging nutrients and knowledge in an act of reciprocity that benefits us all as individuals and as a global community faced with few alternatives for survival. Subscribe at interplace.io

Rhizobium is a tiny microbe with a big dollar impact. Ensuring this beneficial soil bacteria has an optimal effect requires a number of key conditions.

This episode: A global estimate of plants and their root fungi shows how agriculture may have greatly affected soil carbon storage over time! Download Episode (5.7 MB, 8.3 minutes) Show notes: Microbe of the episode: Rhizobium virus RHEph4 News item Takeaways Even small organisms can have a big effect on the climate of the planet if there are enough of them. This includes trees, which are small relative to the planet, and also includes the fungi that attach to the roots of trees and other plants. These mycorrhizal fungi thread subtly through the soil, some occasionally popping up mushrooms, and transfer valuable nutrients they gather to the trees in exchange for carbon fixed from the air. Knowing how big an effect a given kind of organism has requires knowing how much of it is around. This study collates data from various surveys of global plant populations and the fungi that interact with their roots, to estimate a global picture of the fungi below our feet. It estimates that a kind of fungus that stores more carbon in the soil may have been replaced in many areas with fungi that store less, or no fungi at all, due to the transformation of land from wild areas to farmland. Journal Paper: Soudzilovskaia NA, van Bodegom PM, Terrer C, Zelfde M van’t, McCallum I, Luke McCormack M, Fisher JB, Brundrett MC, de Sá NC, Tedersoo L. 2019. Global mycorrhizal plant distribution linked to terrestrial carbon stocks. Nat Commun 10:1–10. Other interesting stories: Phages bind to fish mucosal surfaces and protect from infection Examining how archaea affect meteorites   Email questions or comments to bacteriofiles at gmail dot com. Thanks for listening! Subscribe: Apple Podcasts, Google Podcasts, Android, or RSS. Support the show at Patreon, or check out the show at Twitter or Facebook.

Ciclo #8MCiencia | Entrevista: Ing. Elena Beyhaut - YVY Life Sciences: “Trabajar con cannabis, un desconocido global, es sumamente motivante; es una oportunidad increíble para quienes investigamos en estos temas" Elena Beyhaut es ingeniera agrónoma, vinculada tanto al sector público -a través del Instituto Nacional de Investigación Agropecuaria (INIA)-, como también al sector privado en la empresa YVY Life Sciences, que se dedica específicamente al rubro del cannabis medicinal. Beyhaut se ha especializado en el estudio de microorganismos que componen el suelo e influyen en el crecimiento de las plantas. La ingeniera destaca a estos microorganismos como “la vida del suelo y el eje central de lo que éste hace por nosotros”; por ejemplo, alimentarnos o vestirnos. En este sentido señala el buen trabajo que ha realizado nuestro país en el uso de inoculantes con base Rhizobium para las leguminosas. “Permite que los cultivos que pertenecen a esta familia (leguminosas), como puede ser la soja, la alfalfa o los tréboles, utilicen el nitrógeno del aire en lugar de el del suelo”.

Vincent, Michael, and Michele discuss how soil-dwelling bacteria induce the formation of root nodules on legumes via a protein called CYCLOPS. 

The aim of this doctoral thesis was to investigate the factors relevant in plant interaction of two plant growth promoting rhizobacteria (PGPR). For this, the strain Acidovorax sp. N35 isolated from surface sterilized wheat roots and the two strains F4 and F7 of Rhizobium radiobacter, a bacterium associated with the plant growth promoting fungus Piriformospora indica, were chosen. First of all, the isolate N35 was characterized using phylogenetic and taxonomic methods. The 16S rRNA gene sequence analysis showed that strain N35 has the closest sequence similarities (98.2, 98.5 and 99.0 %) to the environmental Acidovorax species A. delafieldii, A. facilis and A. defluvii. The DNA-DNA hybridization values clearly separated the isolate from these three species. Additionally, phenotypic properties, such as substrate metabolization profiles as determined by a Biolog GN2 assay and cell wall fatty acid profiles concerning the fatty acids C16:0, C16:1ω7cis/trans, C17:0cyclo and C18:0cyclo and C19:0cyclo, facilitated the differentiation of the newly isolated strain N35 from its closest relatives. Thus, the strain N35 was classified as representative of a new species within the genus Acidovorax, and the name Acidovorax radicis sp. nov. is suggested. “Cand. A. radicis” N35 undergoes an irreversible phenotypic variation, resulting in different colony shapes on agar plate. In soil system, both phenotypes showed a plant growth promoting effect both on barley roots and shoots. The wild type N35 (rough colony type) had a better plant growth promoting effect on barley in comparison with phenotype variant N35v (smooth colony type). Wild type and phenotype variant cells of “cand. A. radicis” N35 were labeled with GFP and/or YFP and their separate and co-colonization behavior was investigated in a monoxenic system and a soil system using a CLSM for detection. Both types of N35 could endophytically colonize barley roots after 12 weeks inoculation in the soil system. Competitive root colonization behavior was observed after co-inoculation with differentially labeled wild type N35 and phenotype variant N35v bacteria, where the wild type showed dominant colonization of barley roots compared to the phenotype variant. Moreover, the variant N35v lost its motility due to missing flagella and swarming ability. The differences of both types at genetic level were investigated using whole genome sequence data obtained from 454 pyrosequencing (Roche) using the GS FLX Titanium chemistry. As only difference in the genome sequence, a 16 nucleotides deletion was identified in the mutL gene, which encodes for the mismatch repair protein MutL. In phenotype N35v, the frame shift caused by this deletion leads to the formation of a stop codon in the coding gene, resulting in a truncated MutL protein with a missing functional MutL C-terminal domain. This mutation occurred in exactly the same way in all investigated phenotype variants. These results suggest that MutL might be directly or indirectly responsible for the phenotypic variation in “cand. A. radicis” N35. Quorum sensing signaling molecules produced by “cand. A. radicis” N35 were identified using biosensors as well as Fourier transform ion cyclotron resonance - mass spectrometry (FT-ICR-MS) and ultra performance liquid chromatography (UPLC). Both types of “cand. A. radicis” N35 possess the same AraI/AraR quorum sensing system, which belongs to the LuxI/LuxR type. The two N35 phenotypes produced nearly the same amount of 3-OH-C10-HSL in the exponential growth phase. A co-inoculation experiment of AHL producing wild type N35 and a constructed AHL negative mutant N35 ΔaraI showed that wild type N35 had a dominant colonization behavior compared to the AHL negative mutant on barley roots in a monoxenic system. These data indicate that quorum sensing is involved in regulation of root colonization by “cand. A. radicis” N35. The second examined PGPR, R. radiobacter, which occurs naturally as endofungal bacterium in the plant growth promoting fungus P. indica, was demonstrated to colonize the surface of barley roots with fluorescence in situ hybridization (FISH) in a monoxenic system. The interaction of P. indica harboring R. radiobacter with other rhizobacteria was investigated using plate confrontation assays. Antibiotics and lipopeptides produced and excreted by the plant growth enhancing rhizobacterium Bacillus amyloliquefaciens FZB42 and the biocontrol rhizobacterium Pseudomonas fluorescens SS101 were shown to be responsible for the observed inhibition of P. indica by these bacteria. R. radiobacter F4 and F7 were able to synthesize a variety of oxo- and hydroxyl-C8- to C12-HSL compounds. In addition, both strains also produced coumaroyl-HSL when coumaric acid was supplied in the medium. The lactonase expressing transformants F4 NM13 and F7 NM13, which are the AHL negative phenotypes, abolished the lipase and siderophore activity. Considering this, quorum sensing influences the production of metabolites including lipase and siderophores in R. radiobacter F4 and F7. Further work should be directed to the question whether quorum sensing also plays a role in the interaction of the bacterium with fungus and/or plant.

The aim of this study was to increase the understanding of diversity and activity of dominant bacterial populations in the rhizospheres of three economically important grain legumes (Vicia faba, Lupinus albus and Pisum sativum). A cultivation-independent approach was employed to achieve this aim bearing in mind the limitation of cultivation-dependent technique that only 10% of bacteria present in rhizosphere can be cultured. PCR amplification of 16S rDNA and subsequent separation of the amplicons by DGGE was used in an initial screening of replicates for experimental variation and for the first characterization of bacterial community composition of the three rhizospheres under study. Specific profiles generated by the three legumes, derived by both 16S rDNA and rRNA, emphasized the need to perform detailed analysis of the communities present in these rhizospheres. Clone libraries for PCR and RT-PCR products were generated for representative samples of all the three legumes. Firmicutes were found to be the most dominant in all the legumes, both in DNA- and RNA-derived libraries, indicating them to be the most active group as well. A plant-dependent rhizosphere effect was reflected by the absence of ?-subdivision members in Pisum and ?-subdivision members of proteobacteria in Vicia rhizosphere. High numbers of as yet unclassified bacteria were also obtained. With this experimental set-up, using the same soil material but three different legumes and a uniform inoculation with Rhizobium sp., it became evident that plant roots influence the development of bacterial communities in the rhizosphere in a plant-specific manner. The extent of the rhizosphere effect could vary in natural field conditions as the present study was performed under controlled conditions in green house using soil from agricultural site. Extraction and analysis of rRNA has enabled identification of active taxa in the present study. Fingerprints were obtained for total RNA using two different primers. The profiles generated revealed marked differences between the three rhizospheres of the three legumes under study, indicating differences between the metabolic status of the bacterial communities present in the rhizospheres of these three legumes. To address the question of functional diversity, mRNA extraction and subsequent RT-PCR were performed for various genes important in nutrient cycling. The presence of chitinase genes could be established by specific PCR amplification using DNA extracted from the three rhizospheres. However, no expression of the gene could be detected by RT-PCR. Enzyme assays confirmed no or very low levels of the chitinase protein in the rhizospheres. Analysis of proteolytic enzymes (serine and neutral metallopeptidases) showed the presence and activity of serine peptidase in the three rhizospheres. Neutral metallopeptidase gene was also present in the three rhizospheres but no expression could be detected in the Lupinus rhizosphere. This was a confirmation of plant-dependent effect at the level of functioning of the bacterial communities. Genes for nitrite reductase (nirK and nirS), which may lead to removal of nitrogen from the system by denitrification, were targeted to gain an understanding of the importance of this enzyme in a nitrogen-enriching environment. The presence of nirS was not detected in any of the legume rhizospheres, but both the presence and activity of nirK was established for the three rhizospheres. The diversity of this gene was investigated by generating clone libraries with the RT-PCR products from the three plant rhizospheres. The observation of distinct differences in the distribution of phylotypes of expressed nirK gene in the three legume rhizospheres confirmed a plant specific effect on the functions of the rhizosphere bacterial communities. The present study revealed a hitherto unknown diversity of rhizospheric bacteria associated with grain legumes. Entirely cultivation-independent approaches to characterize the structure and function of the bacterial community of the rhizosphere of the three grain legumes clearly revealed plant-dependent rhizosphere effect on bacterial community structure and function.

A new comprehensive communication concept in the Rhizobium/Bradyrhizobium legume symbiosis was developed. It includes a root zone specific flavonoid exudation, the differential activity of phenylpropane/acetate pathway derivatives on chemotaxis, nod-gene inducing activity and phytoalexin resistance induction on the microsymbiont side (Bradyrhizobium). Nod factor production from the microsymbiont affects the host plant in root hair curling and meristem induction. Phytoalexin production in the host plant is also an early response, however repressed to a low level after a few hours. Another strategy of the microsymbiont to overcome phytoalexin effects is degradation of phytoalexins in Rhizobium leguminosarum bv. vicieae. Competitiveness within the same infection group of the microsymbiont was studied with gus-gene fusion, using the blue coloured nodules to easily discriminate marked strains from unmarked competitors. New exopolysaccharide (EPS) mutants of Bradyrhizobium japonicum were reconstructed homologous with a DNA region to exoB gene of Rhizobium meliloti. Their clearly reduced competitiveness of nodulation, demonstrates that exopolysaccharides of Bradyrhizohium japonicum also have an important function during the early stages of this symbiotic interaction.