Mesorhizobium loti: Difference between revisions

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==Application to Biotechnology==
==Application to Biotechnology==
Does this organism produce any useful compounds or enzymes?  What are they and how are they used?
Does this organism produce any useful compounds or enzymes?  What are they and how are they used?
Root nodules occur on the roots of plants that associate with symbiotic bacteria.
Under nitrogen limiting conditions, plants from the pea family Fabaceae form a symbiotic relationship with a host-specific strain of bacteria known as rhizobia.
Within legume nodules, nitrogen gas from the atmosphere is converted into ammonia, which then is assimilated by the plant to form the basis for amino acids (the building blocks of proteins), nucleotides (the building blocks of DNA and RNA as well as the important energy molecule ATP), and other cellular constituents such as vitamins, flavones, and hormones. The nitrogen fixation property makes legumes an ideal agricultural organism as their requirement for nitrogen fertiliser is reduced. Indeed high nitrogen content blocks nodule development. The energy for splitting the nitrogen gas in the nodule comes from sugar that is translocated from the leaf (a product of photosynthesis). Malate as a breakdown product of sucrose is the direct carbon source for the bacteroid. Nitrogen fixation in the nodule is very oxygen sensitive. Legume nodules harbor an iron containing protein called hemoglobin, closely related to animal myoglobin, to facilitate the conversion of nitrogen gas to ammonia.
Legumes release compounds called flavonoids from their roots, which trigger the production of nod factors by the bacteria. When the nod factor is sensed by the root, a number of biochemical and morphological changes happen: cell division is triggered in the root to create the nodule, and the root hair growth is redirected to wind around the bacteria multiple times until it fully encapsulates 1 or more bacteria. The bacteria encapsulated divide multiple times, forming a microcolony. From this microcolony, the bacteria enter the developing nodule through a structure called an infection thread, which grows through the root hair into the basal part of the epidermis cell, and onwards into the root cortex; they are then surrounded by a plant-derived membrane and differentiate into bacteroids that fix nitrogen.
Nodulation is controlled by a variety of processes, both external (heat, acidic soils, drought, nitrate) and internal (autoregulation of nodulation, ethylene). Autoregulation of nodulation controls nodule numbers per plant thorugh a systemic process involving the leaf. Leaf tissue senses the early nodulation events in the root through an unknown chemical signal, then restricts further nodule development in newly develing root tissue. The Leucine rich repeat (LRR) receptor kinases (NARK in soybean (Glycine max); HAR1 in Lotus japonicus, SUNN in Medicago truncatula) are essential for autoregulation of nodulation (AON). Mutation leading to loss of function in these AON receptor kinases leads to supernodulation or hypernodulation. Often root growth abnormalities accompany the loss of AON receptor kinase activity, suggesting that nodule growth and root development are functionally linked.


==Current Research==
==Current Research==

Revision as of 19:58, 28 August 2007

A Microbial Biorealm page on the genus Mesorhizobium loti

Classification

Higher order taxa

(Domain)Bacteria; (Phylum) Proteobacteria; (Class) Alphaproteobacteria; (Order) Rhizobiales; (family) Phyllobacteriaceae; (genus) Mesorhizobium; (species) loti) [Link for M. loti - use [1] NCBI link to find]

Species

Mesorhizobium loti, formally known as Rhizobium loti

Description and significance

Though this bacteria was vaguely studied starting from 1982 by B.D. Jarvis, it was finally sequenced by the Kazusa DNA Research institute in 2000. The Mesorhizobium species was studied in order to further understand the process of symbiotic nitrogen fixation as well as horizontal gene transfer among natural microsymbionts. It was isolated from the rhizosphere of the legume, Lotus corniculatus in New Zealand, in which the growth of the plants were studied and compared to other plants in surrounding fields. Several plants were obtained and the bacteria found was knocked out and compared to a control plant. In this way, strain MAFF303099: Mesorhizobium loti was discovered.


Mesorhizobium loti is a member of Rhizobia. Rhizobia is a collective name for the genera of Rhizobium, Sinorhizobium, Mesorhizobium, and Bradyrhizobium; rhiza, greek for the word root, and bios, greek for the word life. M. loti are soil and rhizosphere bacteria that undergo nitrogen-fixing symbiosis with leguminous plants (such as clover, beans, and soy). It is motile, a gram-negative bacteria with non-sporulating rods and cannot perform nitrogen fixation without a plant host.


Nitrogen fixation begins at the root hair. When entered, the bacteria travels down through an anoxic center of the root hair cell, then proliferating, forming a nodule. Here, the bacteria differentiates into a bacteroid (in definition: a rod-shaped or branched bacteria in the root nodules of nitrogen-fixing plants). M. loti fixes the nitrogen from the atmosphere into ammonium (NH4+) with the enzyme nitrogenase. This is described as a symbiotic relationship because this nitrogen is fixed to a plant-usable form; in return, the plant supplies M.loti with carbohydrates, proteins, and oxygen.


Mesorhizobium loti is also very useful for research anaylsis of other species of the same genus as well as in the same family. The quest for better understanding horizontal gene transfer and the evolutionary changes that occur over the years is still being investigated. Also, comparative sequence analysis of the genomic similarities between Mesorhizobium loti and Escherichia coli in order to better understand other bacteria. Since Mesorhizobium lives in a beneficial symbiotic relationship with other plants, bacteria which have a plant pathology detrimental to plants can be studied and compared to see what exactly makes that bacteria harmful.


Describe the appearance, habitat, etc. of the organism, and why it is important enough to have its genome sequenced. Describe how and where it was isolated. Include a picture or two (with sources) if you can find them.

Genome structure

The complete nucleotide sequence of the genome of a symbiotic bacterium Mesorhizobium loti strain MAFF303099 was determined. The genome of M. loti consisted of a single chromosome (7,036,071 bp) and two plasmids, designated as pMLa (351,911 bp) and pMLb (208, 315 bp). The chromosome comprises 6752 potential protein-coding genes, two sets of rRNA genes and 50 tRNA genes representing 47 tRNA species. Fifty-four percent of the potential protein genes showed sequence similarity to genes of known function, 21% to hypothetical genes, and the remaining 25% had no apparent similarity to reported genes. A 611-kb DNA segment, a highly probable candidate of a symbiotic island, was identified, and 30 genes for nitrogen fixation and 24 genes for nodulation were assigned in this region. Codon usage analysis suggested that the symbiotic island as well as the plasmids originated and were transmitted from other genetic systems. The genomes of two plasmids, pMLa and pMLb, contained 320 and 209 potential protein-coding genes, respectively, for a variety of biological functions. These include genes for the ABC-transporter system, phosphate assimilation, two-component system, DNA replication and conjugation, but only one gene for nodulation was identified. Describe the size and content of the genome. How many chromosomes? Circular or linear? Other interesting features? What is known about its sequence? Does it have any plasmids? Are they important to the organism's lifestyle?

Cell structure and metabolism

Describe any interesting features and/or cell structures; how it gains energy; what important molecules it produces.

Mesorhizobium loti gains energy through oxidative phosphorylation, carbon fixation, CO2 fixation, methane metabolism, nitrogen metabolism ands ulfur metabolism.

Ecology

Describe any interactions with other organisms (included eukaryotes), contributions to the environment, effect on environment, etc.

Pathology

How does this organism cause disease? Human, animal, plant hosts? Virulence factors, as well as patient symptoms.

Application to Biotechnology

Does this organism produce any useful compounds or enzymes? What are they and how are they used? Root nodules occur on the roots of plants that associate with symbiotic bacteria.

Under nitrogen limiting conditions, plants from the pea family Fabaceae form a symbiotic relationship with a host-specific strain of bacteria known as rhizobia.

Within legume nodules, nitrogen gas from the atmosphere is converted into ammonia, which then is assimilated by the plant to form the basis for amino acids (the building blocks of proteins), nucleotides (the building blocks of DNA and RNA as well as the important energy molecule ATP), and other cellular constituents such as vitamins, flavones, and hormones. The nitrogen fixation property makes legumes an ideal agricultural organism as their requirement for nitrogen fertiliser is reduced. Indeed high nitrogen content blocks nodule development. The energy for splitting the nitrogen gas in the nodule comes from sugar that is translocated from the leaf (a product of photosynthesis). Malate as a breakdown product of sucrose is the direct carbon source for the bacteroid. Nitrogen fixation in the nodule is very oxygen sensitive. Legume nodules harbor an iron containing protein called hemoglobin, closely related to animal myoglobin, to facilitate the conversion of nitrogen gas to ammonia.

Legumes release compounds called flavonoids from their roots, which trigger the production of nod factors by the bacteria. When the nod factor is sensed by the root, a number of biochemical and morphological changes happen: cell division is triggered in the root to create the nodule, and the root hair growth is redirected to wind around the bacteria multiple times until it fully encapsulates 1 or more bacteria. The bacteria encapsulated divide multiple times, forming a microcolony. From this microcolony, the bacteria enter the developing nodule through a structure called an infection thread, which grows through the root hair into the basal part of the epidermis cell, and onwards into the root cortex; they are then surrounded by a plant-derived membrane and differentiate into bacteroids that fix nitrogen.

Nodulation is controlled by a variety of processes, both external (heat, acidic soils, drought, nitrate) and internal (autoregulation of nodulation, ethylene). Autoregulation of nodulation controls nodule numbers per plant thorugh a systemic process involving the leaf. Leaf tissue senses the early nodulation events in the root through an unknown chemical signal, then restricts further nodule development in newly develing root tissue. The Leucine rich repeat (LRR) receptor kinases (NARK in soybean (Glycine max); HAR1 in Lotus japonicus, SUNN in Medicago truncatula) are essential for autoregulation of nodulation (AON). Mutation leading to loss of function in these AON receptor kinases leads to supernodulation or hypernodulation. Often root growth abnormalities accompany the loss of AON receptor kinase activity, suggesting that nodule growth and root development are functionally linked.

Current Research

Enter summaries of the most recent research here--at least three required

Currently there are studies in bacterial genome research in order to learn more about bacteria–plant interactions. At the Protection and Food Research Centre in London there are undergoing studies about the invasion of Lotus japonicus root hairless 1 by M. loti. In many legumes, including Lotus japonicus and Medicago truncatula, susceptible root hairs are the primary sites for the initial signal perception and physical contact between the host plant and the compatible nitrogen-fixing bacteria that leads to the initiation of root invasion and nodule organogenesis. However, diverse mechanisms of nodulation have been described in a variety of legume species that do not rely on root hairs. To clarify the significance of root hairs during the L. japonicus-Mesorhizobium loti symbiosis, we have isolated and performed a detailed analysis of four independent L. japonicus root hair developmental mutants. We show that although important for the efficient colonization of roots, the presence of wild-type root hairs is not required for the initiation of nodule primordia (NP) organogenesis and the colonization of the nodule structures. In the genetic background of the L. japonicus root hairless 1 mutant, the nodulation factor-dependent formation of NP provides the structural basis for alternative modes of invasion by M. loti. Surprisingly, one mode of root colonization involves nodulation factor-dependent induction of NP-associated cortical root hairs and epidermal root hairs, which, in turn, support bacterial invasion. In addition, entry of M. loti through cracks at the cortical surface of the NP is described. These novel mechanisms of nodule colonization by M. loti explain the fully functional, albeit significantly delayed, nodulation phenotype of the L. japonicus ROOT HAIRLESS mutant.

Lehrstuhl für Genetik, Universität Bielefeld, 33594, Bielefeld, Germany

http://jxb.oxfordjournals.org/cgi/content/full/56/416/1643#SEC4

References

[Sample reference] Takai, K., Sugai, A., Itoh, T., and Horikoshi, K. "Palaeococcus ferrophilus gen. nov., sp. nov., a barophilic, hyperthermophilic archaeon from a deep-sea hydrothermal vent chimney". International Journal of Systematic and Evolutionary Microbiology. 2000. Volume 50. p. 489-500.

Edited by student of Rachel Larsen