Abstract
The interaction of bacteria with plants can result in either a positive, negative, or neutral association. The rhizobium-legume interaction is a well-studied model system of a process that is considered a positive interaction. This process has evolved to require a complex signal exchange between the host and the symbiont. During this process, rhizobia are subject to several stresses, including low pH, oxidative stress, osmotic stress, as well as growth inhibiting plant peptides. A great deal of work has been carried out to characterize the bacterial response to these stresses. Many of the responses to stress are also observed to have key roles in symbiotic signaling. We propose that stress tolerance responses have been co-opted by the plant and bacterial partners to play a role in the complex signal exchange that occurs between rhizobia and legumes to establish functional symbiosis. This review will cover how rhizobia tolerate stresses, and how aspects of these tolerance mechanisms play a role in signal exchange between rhizobia and legumes.
Highlights
Rhizobia-legume symbiosis is a well-studied interaction which results in the formation of a plant derived organelle for the purposes of symbiotic nitrogen fixation
These observations from early signaling involving the interplay of flavonoids and Nod factor production are a clear example of how a potential stress, flavonoids, induce a bacterial signal, Nod factor, that have become a key component for symbiotic interaction
The establishment of the rhizobium – legume symbiotic interaction is often described as a direct complex signal exchange between both the bacteria and the plant, with emphasis placed on how a molecule from one induces changes in the other or invokes a signaling response
Summary
Rhizobia-legume symbiosis is a well-studied interaction which results in the formation of a plant derived organelle for the purposes of symbiotic nitrogen fixation. Iso-flavonoids were originally thought to be involved in the defense response against fungi and were shown to have toxic effects on some isolated bacteria Flavones and isoflavones were found to induce transcription of the nod genes in other rhizobia, and recognition of specific flavonoids was shown to play a role in plant-host specificity during symbiosis. It seems that the initial role for flavonoids and iso-flavonoids secreted by plants was to be an anti-microbial (Cowan, 1999). Rhizobia have been able to utilize very specific portions of flavonoids, the iso-flavonoids, as a signal indicating the presence of a compatible host and respond through the production of Nod factor to initiate symbiotic signaling
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