Abstract
Leguminous plants possess the almost unique ability to enter symbiosis with soil-resident, nitrogen fixing bacteria called rhizobia. During this symbiosis, the bacteria physically colonize specialized organs on the roots of the host plant called nodules, where they reduce atmospheric nitrogen into forms that can be assimilated by the host plant and receive photosynthates in return. In order for nodule development to occur, there is extensive chemical cross-talk between both parties during the formative stages of the symbiosis. The vast majority of the legume family are capable of forming root nodules and typically rhizobia are only able to fix nitrogen within the context of this symbiotic association. However, many legume species only enter productive symbiosis with a few, or even single rhizobial species or strains, and vice-versa. Permitting symbiosis with only rhizobial strains that will be able to fix nitrogen with high efficiency is a crucial strategy for the host plant to prevent cheating by rhizobia. This selectivity is enforced at all stages of the symbiosis, with partner choice beginning during the initial communication between the plant and rhizobia. However, it can also be influenced even once nitrogen-fixing nodules have developed on the root. This review sets out current knowledge about the molecular mechanisms employed by both parties to influence host range during legume-rhizobia symbiosis.
Highlights
The legume family is almost unique amongst plants in that its members are able to interact with soil dwelling bacteria called rhizobia
Given the resemblance of the role of Effector-trigger immunity (ETI) during legume-rhizobia symbiosis to what occurs during some pathogen interactions, it could be speculated that rhizobial strains targeted by ETI in this way could interfere with Rgene mediated recognition of their effectors through the use of additional effectors and continue to infect a legume host, this has not yet been demonstrated
These results suggest that the bacA gene of S. meliloti has evolved to interact with M. sativa. This is supported by phylogenetic analysis indicating that the bacA gene of S. meliloti Rm2011 has undergone rapid evolution and its sequence resembles the bacA gene of pathogenic genera Klebsiella, Brucella and Escherichia more closely than it resembles many other rhizobial bacA orthologs. This suggests that BacA and BacA-like proteins possessed by rhizobia may be a determinant of host range when infecting legumes belonging to the inverted repeat-lacking clade (IRLC) and that this is likely mediated by interactions with host nodule-specific cysteine-rich (NCR) peptides
Summary
The legume family is almost unique amongst plants in that its members are able to interact with soil dwelling bacteria called rhizobia. Inoculation of M. truncatula Nfr1+/Nfr5+ with strains of the L. japonicus symbionts M. loti or R. leguminosarum DZL modified to constitutively express NodD ( producing Nod factors independently of flavonoid signaling) led to nodule formation, which does not occur in wild type M. truncatula.
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