Rhizobium is a genus of gram-negative bacteria that form symbiotic relationships with legumes to fix atmospheric nitrogen in root nodules, providing essential nutrients to the plants. The document outlines various genera and species within the Rhizobiaceae family, describing their characteristics, classification, and the beneficial interactions between rhizobia and leguminous plants. It also discusses the formation, structure, and physiology of nitrogen-fixing nodules and the mechanisms of nodulation influenced by both external and internal factors.

1. Mr. Sandesh Pawar
Dept. of Plant Pathology
Dr. PDKV., Akola

2. Rhizobium is a genus of Gram negative soil bacteria that fix
nitrogen.Rhizobium forms an endosymbiotic nitrogen
fixing association with roots of legumes and Parasponia.

3. The bacteria colonize plant cells within root
nodules where they convert atmospheric nitrogen
into ammonia and then provide organic nitrogenous
compounds such as glutamine or ureides to the plant.
The plant in turn provides the bacteria with organic
compounds made by photosynthesis.
This mutually beneficial relationship is true of all of
the rhizobia, of which the Rhizobium genus is a typical
example.

4. Kingdom: Bacteria
Phylum: Proteobacteria
Class: Alphaproteobacteria
Order: Rhizobiales
Family: Rhizobiaceae
Genus:
Rhizobium
Frank 1889
Beijerinck in the Netherlands was the first to isolate and cultivate
a microorganism from the nodules of legumes in 1888. He named itBacillus
radicicola, which is now placed in Bergey's Manual of Determinative
Bacteriology under the genus Rhizobium.

5. Culture of Rhizobium on agar plate

6. Rhizobia are nitrogen-fixing bacteria that form root nodules on legume
plants.
Most of these bacterial species are in the Rhizobiacae family in the alpha-
proteobacteria and are in either the Rhizobium, Mesorhizobium, Ensifer,
or Bradyrhizobium genera.
However recent research has shown that there are many other rhizobial
species in addition to these.
In some cases these new species have arisen through lateral gene transfer
of symbiotic genes. The list below is not 'official' by any means, and it is
merely my compilation and interpretation of the literature.
An alternative list is maintained by the "ICSP Subcommittee on the
taxonomy of Rhizobium and Agrobacterium".

7. Class: Alphaproteobacteria
Order: Rhizobiales
Family: Rhizobiaceae
Rhizobium alamii Rhizobium freirei
Rhizobium alkalisoli Rhizobium galegae
Rhizobium azibense Rhizobium gallicum
Rhizobium calliandrae Rhizobium giardinii
Rhizobium cauense Rhizobium grahamii
Rhizobium cellulosilyticum Rhizobium hainanense
Rhizobium daejeonense Rhizobium halophytocola
Rhizobium endophyticum Rhizobium herbae
Rhizobium etli Rhizobium huautlense
Rhizobium fabae Rhizobium indigoferae
The genus Rhizobium (Frank 1889) was the first named (from
Latin meaning 'root living'), and for many years this was a
'catch all' genus for all rhizobia. Some species were later moved
in to new genera based on phylogenetic analyses. It currently
consists of 49 rhizobial species (and 11 non rhizobial species).

8. Rhizobium jaguaris Rhizobium phaseoli
Rhizobium laguerreae Rhizobium pisi
Rhizobium leguminosarum Rhizobium tibeticum
Rhizobium leucaenae Rhizobium sophorae
Rhizobium loessense Rhizobium sophoriradicis
Rhizobium lusitanum Rhizobium sphaerophysae
Rhizobium mayense Rhizobium sullae
Rhizobium mesoamericanum Rhizobium taibaishanense
Rhizobium mesosinicum Rhizobium tropici
Rhizobium miluonense Rhizobium tubonense
Rhizobium mongolense Rhizobium undicola
Rhizobium multihospitium Rhizobium vallis
Rhizobium oryzae Rhizobium vignae
Rhizobium paranaense Rhizobium yanglingense
Rhizobium petrolearium

9. Class: Alphaproteobacteria
Order: Rhizobiales
Family: Phyllobacteriaceae
The genus Mesorhizobium was described by Jarvis et al.
in 1997. Several Rhizobium species were transferred to
this genus. It currently consists of 21 rhizobia species
(and 1 non-rhizobial species).
Mesorhizobium abyssinicae Mesorhizobium chacoense
Mesorhizobium albiziae Mesorhizobium ciceri
Mesorhizobium alhagi Mesorhizobium erdmanii
Mesorhizobium amorphae Mesorhizobium gobiense
Mesorhizobium australicum Mesorhizobium hawassense
Mesorhizobium camelthorni Mesorhizobium huakuii
Mesorhizobium caraganae Mesorhizobium jarvisii

10. Mesorhizobium loti Mesorhizobium shangrilense
Mesorhizobium mediterraneum Mesorhizobium shonense
Mesorhizobium metallidurans Mesorhizobium silamurunense
Mesorhizobium muleiense Mesorhizobium septentrionale
Mesorhizobium opportunistum Mesorhizobium tamadayense
Mesorhizobium plurifarium Mesorhizobium tarimense
Mesorhizobium qingshengii Mesorhizobium temperatum
Mesorhizobium robiniae Mesorhizobium tianshanense
Mesorhizobium sangaii

11. class: Alphaproteobacteria
order: Rhizobiales
family: Rhizobiaceae
The Sinorhizobium genus was described by Chen et al. in 1988.
However some recent studies show that Sinorhizobium and the
genus Ensifer (Casida, 1982) belong to a single taxon. Ensiferis the
earlier heterotypic synonym (it was named first) and thus takes
priority (Young, 2003). This means that all Sinorhizobium spp. must
be renamed as Ensifer spp. according to the Bacteriological code.
There has been some discussion in the literature
if Ensifer or Sinorhizobium is correct Young (2010), this website
follows the opinion of the Judicial Commission, and will use Ensifer.
The genus currently consists of 17 species.

12. Ensifer abri Ensifer meliloti
Sinorhizobium americanum Ensifer mexicanus
Ensifer arboris 'Sinorhizobium morelense'
Ensifer fredii Ensifer adhaerens
Ensifer garamanticus Ensifer numidicus
Ensifer indiaense Ensifer saheli
Ensifer kostiensis Ensifer sojae
Ensifer kummerowiae Ensifer terangae
Ensifer medicae

13. class: Alphaproteobacteria
order: Rhizobiales
family: Bradyrhizobiaceae
The Bradyrhizobium genus was described by Jordan
in 1982. It currently consists of 9 rhizobia species
(and 1 non rhizobial species).
Bradyrhizobium canariense Bradyrhizobium japonicum
Bradyrhizobium cytisi Bradyrhizobium jicamae
Bradyrhizobium denitrificans Bradyrhizobium liaoningense
Bradyrhizobium elkanii Bradyrhizobium pachyrhizi
Bradyrhizobium iriomotense Bradyrhizobium yuanmingense

14. class: Betaproteobacteria
order: Burkholderiales
family: Burkholderiaceae
The Burkholderia genus currently contains
seven named rhizobial members and others
as Burkholderia sp.
Burkholderia caribensis
Burkholderia cepacia
Burkholderia mimosarum
Burkholderia nodosa
Burkholderia phymatum
Burkholderia sabiae
Burkholderia tuberum

15. class: Alphaproteobacteria
order: Rhizobiales
family: Phyllobacteriaceae
The Phyllobacterium genus currently contains
three rhizobial species.
Phyllobacterium trifolii
Phyllobacterium ifriqiyense
Phyllobacterium leguminum

16. class: Alphaproteobacteria
order: Rhizobiales
family: Methylobacteriaceae
The Azorhizobium genus was described by Dreyfus et al.
in 1988. It currently consists of 2 species.
Azorhizobium caulinodans
Azorhizobium doebereinerae

17. class: Alphaproteobacteria
order: Rhizobiales
family: Brucellaceae
The Ochrobactrum genus currently contains
two rhizobial species.
Ochrobactrum cytisi
Ochrobactrum lupini

18. class: Alphaproteobacteria
order: Rhizobiales
family: Methylobacteriaceae
Methylobacterium nodulans
class: Betaproteobacteria
order: Burkholderiales
family: Burkholderiaceae
Cupriavidus taiwanensis

19. class: Alphaproteobacteria
order: Rhizobiales
family: Hyphomicrobiaceae
Devosia neptuniae
class: Alphaproteobacteria
order: Rhizobiales
family: Rhizobiacea
Shinella kummerowiae

20. Root nodules occur on the roots of plants (primarily Fabaceae) that
associate with symbiotic nitrogen-fixing bacteria.
Undernitrogen-limiting conditions, capable plants form a symbiotic
relationship with a host-specific strain of bacteria known as rhizobia.
This process has evolved multiple times within the Fabaceae, as well
as in other species found within the Rosidclade.
The Fabaceae include legume crops such as beans and peas.
Within legume nodules, nitrogen gas from the atmosphere is
converted into ammonia, which is then assimilated into 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.

21. Cross section though a soybean (Glycine max'Essex') root nodule. The
bacterium, Bradyrhizobium japonicum, colonizes the roots and
establishes a nitrogen fixing symbiosis. This high magnification image
shows part of a cell with single bacteroids within their symbiosomes. In
this image, endoplasmic reticulum, dictysome and cell wall can be seen.

22. Their ability to fix gaseous nitrogen makes legumes an ideal
agricultural organism as their requirement for nitrogen
fertilizer is reduced.
Indeed high nitrogen content blocks nodule development as
there is no benefit for the plant of forming the symbiosis. 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 leghaemoglobin, closely related to animal myoglobin, to
facilitate the conversion of nitrogen gas to ammonia.

23. Legume family
Plants that contribute to nitrogen fixation include the legume family – Fabaceae –
with taxa such as kudzu, clovers, soybeans, alfalfa, lupines, peanuts, and rooibos.
They containsymbiotic bacteria called rhizobia within the nodules, producing
nitrogen compounds that help the plant to grow and compete with other plants.
When the plant dies, the fixed nitrogen is released, making it available to other
plants and this helps to fertilize the soil.
The great majority of legumes have this association, but a few genera
(e.g.,Styphnolobium) do not.
In many traditional and organic farming practices, fields are rotated through
various types of crops, which usually includes one consisting mainly or entirely of
clover or buckwheat (non-legume family Polygonaceae), which are often referred
to as "green manure".
Inga alley farming relies on the leguminous genus Inga, a small tropical, tough-
leaved, nitrogen-fixing tree.[4]

24. Non-leguminous
Although by far the majority of plants able to form
nitrogen-fixing root nodules are in the legume
family Fabaceae, there are a few exceptions:
•Parasponia, a tropical genus in the Cannabaceae also
able to interact with rhizobia and form nitrogen-fixing
nodules
•Actinorhizal plants such as alder and bayberry can also
form nitrogen-fixing nodules, thanks to a symbiotic
association with Frankiabacteria. These plants belong to
25 genera[6] distributed among 8 plant families.
The ability to fix nitrogen is far from universally present
in these families. For instance, of 122 genera in
the Rosaceae, only 4 genera are capable of fixing
nitrogen. All these families belong to
the orders Cucurbitales, Fagales, and Rosales, which
together with the Fabalesform a clade of eurosids.
A sectioned alder tree
root nodule.

25. Two main types of nodule have been described: determinate and
indeterminate.
Determinate nodules are found on certain tribes of tropical legume such as
those of the genera Glycine (soybean), Phaseolus (common bean),
and Vigna. and on some temperate legumes such as Lotus.
These determinate nodules lose meristematic activity shortly after initiation,
thus growth is due to cell expansion resulting in mature nodules which are
spherical in shape.
Another types of determinate nodule is found in a wide range of herbs,
shrubs and trees, such as Arachis (peanut).
These are always associated with the axils of lateral or adventitious roots and
are formed following infection via cracks where these roots emerge and not
using root hairs. Their internal structure is quite different from those of the
soybean type of nodule.

26. Indeterminate nodules are found in the majority of legumes from
all three sub-families, whether in temperate regions or in the
tropics.
They can be seen in papilioinoid legumes such
as Pisum (pea), Medicago (alfalfa), Trifolium (clover),
and Vicia (vetch) and all mimosoid legumes such as acacias
(mimosas), the few nodulated caesalpinioid legumes such as
partridge pea they earned the name "indeterminate" because
they maintain an active apical meristem that produces new cells
for growth over the life of the nodule.
This results in the nodule having a generally cylindrical shape,
which may be extensively branched.
Because they are actively growing, indeterminate nodules
manifest zones which demarcate different stages of
development/symbiosis.

27. Indeterminate nodules growing on the roots of Medicago italica

28. Diagram illustrating the different zones of an
indeterminate root nodule

29. Zone I—the active meristem. This is where new nodule tissue is formed
which will later differentiate into the other zones of the nodule.
Zone II—the infection zone. This zone is permeated with infection
threads full of bacteria. The plant cells are larger than in the
previous zone and cell division is halted.
Interzone II–III—Here the bacteria have entered the plant cells, which
contain amyloplasts. They elongate and begin terminally
differentiating into symbiotic, nitrogen-fixing bacteroids.
Zone III—the nitrogen fixation zone. Each cell in this zone contains a
large, central vacuole and the cytoplasm is filled with fully
differentiated bacteroids which are actively fixing nitrogen. The
plant provides these cells with leghemoglobin, resulting in a distinct
pink color.

30. Zone IV—the senescent zone. Here plant cells and their
bacteroid contents are being degraded.
The breakdown of the heme component of leghemoglobin
results in a visible greening at the base of the nodule.
This is the most widely studied type of nodule, but the details
are quite different in nodules of peanut and relatives and some
other important crops such as lupins where the nodule is
formed following direct infection of rhizobia through the
epidermis and where infection threads are never formed.
Nodules grow around the root, forming a collar-like structure.
In these nodules and in the peanut type the central infected
tissue is uniform, lacking the uninfected ells seen in nodules of
soybean and many indeterminate types such as peas and
clovers.

31. 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.

33. 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 through 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 developing 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. Investigations into the mechanisms of nodule formation
showed that theENOD40 gene, coding for a 12–13 amino acid protein [41], is
up-regulated during nodule formation [3].