Legume growth-promoting rhizobia: An overview on the

Mesorhizobium genus by Laranji et al Microbiological Research 169 (2014) 2–17 

 The term “rhizobia” was originally used to name bacteria belonging to the genus Rhizobium. However, nowadays, only a small fraction of rhizobia belong to this genus and other genera were later identified, such as Bradyrhizobium (Jordan, 1982), Sinorhizobium (Chen et al., 1988) and Mesorhizobium (Jarvis et al., 1997). Until recently, rhizobia were thought to belong exclusively to the Alphaproteobacteria, namely the order Rhizobiales, which also includes many species that are not legume microsymbionts. However, some unexpected findings from studying microsymbionts of wild legumes, resulted in the identifi- cation of rhizobia from the  etaproteobacteria class (Moulin et al., 2001).

For the establishment of an effective symbiosis two main classes of bacterial symbiosis genes are needed: nodulation and nitrogen fixation genes . Nodulation genes (e.g. nodABC) encode enzymes responsible for the biosynthesis and secretion of Nod factors, which are host determinant lipochitooligosaccharides (LCOs) that interact with the plant flavonoids (Downie, 1998). The structure of LCOs varies in the oligosaccharide backbone length, the type of fatty acid at the non-reducing terminus and the number and types of substituent groups carried by the molecule. Nitrogen fixation genes (nif and fix)include the structural genes for the nitrogenase (nifHDK), the enzyme responsible for atmospheric nitrogen fixation (Kaminski et al., 1998).

The mechanisms by which rhizobia and legumes choose their partners are not fully understood. Some authors suggest that the evolution of rhizobia-legumes associations may be host driven (Kiers et al., 2003; Sachs et al., 2011). However, legumes can be  nodulated by rhizobia even if the association results in low symbiotic nitrogen fixation (Den Herder and Parniske, 2009). Most legumes are promiscuous meaning that they can be nodulated by different rhizobia and particularly by broad host range rhizobia strains (e.g. Sinorhizobium sp. NGR234). . Chickpea is only nodulated by symbiovar ciceri strains, which on the other hand are not known to nodulate any other legumes. This specificity is far from being understood, but it may be associated with the variety of flavonoid nod gene inducers released by the legume and also with the variety of Nod factors

released by the rhizobium.

Promiscuous symbioses seem to be more ancestral than restrictive ones and they are widely spread (Pueppke and Broughton, 1999); probably the first rhizobia were broad host range strains ancestrals fromSinorhizobium sp.NGR234 (Perret et al., 2000). Thus, ancestral Nod factors would have been simpler chitin oligomers, able to interact with many plants; in the course of evolution additional substituents have been added to narrow the host range (Pueppke and Broughton, 1999).

Investigation on native chickpea mesorhizobia has contributed to clarify the molecular mechanisms of stress response in Mesorhizobium, in particular the involvement of molecular chaperones in heat (Alexandre and Oliveira, 2011), salt (Brígido et al., 2012a) and acid (Brígido and Oliveira, 2013) stress tolerance. Using tolerant and sensitive isolates, a relationship was found between higher transcriptional induction of major chaperone genes and higher tolerance of mesorhizobia to heat and acidity stress.

PDF is enclosed for further reading...

Rhizobium are species specific endophytes.

Thank you very much Anoop Kumar Srivastava & Kuldeep Patel.