Role of Bio-fertilizers in Crop Improvement - Crop Improvement: New Approaches and Modern Techniques - page 192

associated with root of legumes. It enters into plants through the root system then
it forms nodule. The name Rhizobium was established by Frank in 1889. Seven
distinct species of rhizobium has so far being discovered on the basis of “Cross
Inoculation Group Concept” and more than twenty cross-inoculations groups have
been established so far. Out of this, merely seven are most important. One group of
rhizobia are very slow growing and are known as Bradyrhizobium while as other
group is rhizobia is fast growing and is known as Rhizobium. Both slow growing as
well as fast growing rhizobia has ability to fix atmospheric nitrogen. They create a
symbiotic association with legumes and some non-legumes like Parasponia . Rhizo-
bium legume symbiosis is very host-specific process and it fixes N in particular host
plant only, this host specifity is mediated by plant compounds such as flavonoids
(Goethals et al. 1992 ). Flavonoid activates the nod genes present in Rhizobium. The
communication of rhizobia and legumes begins with signal exchange and recogni-
tion of the symbiotic partners, which is followed by attachment of the rhizobia to
the plant root hairs. After infection, the root hair starts deforming, and the bacteria
invades the plant by a newly produced infection thread growing through it at the
same time, cortical cells which are mitotically activated, give rise to the nodule pri-
mordium. Infection threads grow toward the primordium, and the bacteria are then
released into the cytoplasm of the host cells, surrounded by a plant derived perib-
acteroid membrane (PBM) (Van Workum et al. 1998 ). In the course of process the
nodule primordium develops into a mature nodule, while the bacteria differentiate
into their endosymbiotic form that is called as the bacteroid. The effective nodules
are filled with pink sap called leghaemoglobin pigment. Leghaemoglobin regulates
the supply of oxygen to the bacteria and helps the activity of nitrogenase enzyme
and other regulatory enzymes (Choudhury and Kennedy 2004 ). The nitrogenase is
responsible for reduction of nitrogen to ammonia in the process of nitrogen fixation.
Bacteroids, together with the surrounding PBMs, are called symbiosomes. When
symbiosomes are developed, bacteria synthesize nitrogenase, which catalyzes
the reduction of nitrogen (Mylona et al. 1995 ). The product of nitrogen fixation,
ammonia, is then exported to the plant. The plant provides all immediate nutrients
and energy for the bacteria and just in a week small bead like structures i.e., nodules
are formed. The root nodules act as a micro fermentor for biological N fixation
where they can convert atmospheric N into ammonia. Rhizobium is able to induce
the shoot and root growth in rice plants. (Yanni and El-Fattah 1999 ). Nodules occur
in many shapes such as in Alfalfa and clover, nodules are fingerlike, round in Lentil ,
palm shaped in Cicer , though the entire nodule is generally less than 1/2 in in di-
ameter during favorable conditions. Since the Nitrogen fixed is not free so the plant
must contribute a considerable amount of energy in the form of photosynthates and
other essential nutritional factors which are important for the bacteria. Rhizobium
plays a key role and is the maximum researched bio-fertilizer (Mishra and Dadhich
2010 ). Currently the legume-rhizobia symbiosis has been extended to economically
essential food crops or cereals and certain rhizobia that are competent of crack entry
into ruptured epidermis during emergence of lateral rootlets in cereal crops (Kalia
and Gupta 2002 ) the process can be improved by the addition of phytohormones
(Kannaiyan et al. 2001 ) or use of signal chemicals (Amutha and Kannaiyan 2000 ).