Symbiotic Fungi: Principles and Practice (Soil Biology)

1 Symbiosis: The Art of Living 5
Fig. 1.2 A typical root nodule caused by Rhizobium sp
Rhizobia(a collective name for representatives of the gram-negative genera Azorhizobioum,
Bradyrhizobium, Mesorhizobium, Allorhizobium, Rhizobium, Sinorhizobium)
are found in the soil of crop fields where bacteria and plant both
specifically affect each other’s growth. By and large known as rhizobia, they are
grouped in different taxonomic families of the alpha-proteobacteria classified as
Rhizobiaceae, Phylobacteriaceae and Bradyrhizobiaceae (Gonzalez et al. 2008).
The bacterium which is microsymbiont infects the host root and forms nodules.
Inside the nodules, the bacterium forms bacteroids in which it undergoes morphological
and physiological transformations that can fix atmospheric nitrogen.
Sesbania species can establish symbiotic interactions with rhizobia from two
taxonomically distant genera, including the Sesbania rostrata stem-nodulating
Azorhizobium sp. and Azorhizobium caulinodans and the newly described Sinorhizobium
saheli and Sinorhizobium teranga bv sesbaniae, isolated from the roots
of various Sesbania species. Production of wheat typically requires intensive use of
chemical fertilizers. Reliance on fertilizers may be decreased by exploitation of
plant growth-promoting organisms. The work of Anyia et al. (2004) examines
the responsiveness of Canadian cultivars of Hard Red Spring wheat to inoculation
with Azorhizobium caulinodans, a diazotroph isolated from the legume Sesbania
rostrata. This bacterium has been shown to colonize wheat roots through crack
entry of the lateral roots. Inoculation of wheat cultivar CDC Teal grown in field soil
caused increases in grain yield and total biomass of 34% and 49% respectively.
Inoculated plants produced more tillers and had larger leaf area than un-inoculated
plants.