Rhizobium Ecology,Nodule Bacteria,legumes Soil,Rhizobia Decline,Infection,Symbiotic Nitrogen Fixation,Arachis

Rhizobium Ecology

	Ecology Nodule bacteria occur in soil and in the root region of legumes as well as non-legumes. In the absence of legumes soil population of rhizobia decline. Rhizobia present in the soil may be antagonised by other bacteria, fungi or phages. Rhizobium is very acid sensitive. Soil acidity may eliminate Rhizobium. Generally Rhizobium can tolerate high soil temperature.
Infection A functional root nodule is a site of symbiotic nitrogen fixation. It is developed by cooperative interactions between the legume host and Rhizobium. Thus an effectively nodulate legume is fully autotrophic for nitrogen, a limiting factor in plant nutrition. Specific group of legumes are receptive to a particular species of Rhizobium. Initiation of nodule development requires the successful entry of Rhizobium into the host root cell which generally takes place through the root hair.
The first visible plant response to Rhizobium is the curling or contorted growth of the hair at the portion of infection followed by the development of a tube-like structure called the infection thread (IT). The subsequent growth of the IT and pattern of infection of root cortical cells is determined by the host. In legumes such as Arachis (groundnut), where an IT is not observed, it appears that infection proceeds through intracellular spaces.
Rhizobium enters into the root hair either through intracellular gaps or through the formation of IT.