Abstract
Legumes are able to form a symbiotic relationship with nitrogen-fixing soil bacteria called rhizobia. The result of this symbiosis is to form nodules on the plant root, within which the bacteria can convert atmospheric nitrogen into ammonia that can be used by the plant. Establishment of a successful symbiosis requires the two symbiotic partners to be compatible with each other throughout the process of symbiotic development. However, incompatibility frequently occurs, such that a bacterial strain is unable to nodulate a particular host plant or forms nodules that are incapable of fixing nitrogen. Genetic and molecular mechanisms that regulate symbiotic specificity are diverse, involving a wide range of host and bacterial genes/signals with various modes of action. In this review, we will provide an update on our current knowledge of how the recognition specificity has evolved in the context of symbiosis signaling and plant immunity.
Highlights
The legume-rhizobial symbiosis starts with a signal exchange between the host plant and its microsymbiont (Oldroyd, 2013)
Under nitrogen-limiting conditions, legume roots secrete a cocktail of flavonoid compounds into the rhizosphere, and they serve to activate the expression of a group of bacterial nodulation genes, leading to the synthesis of the Nod factor, a lipochitooligosaccharidic signal that is essential for initiating symbiotic development in most legumes (Oldroyd et al, 2011)
The best examples are from the pea-R. leguminosarum symbiosis where bacterial nod gene mutants that lead to changed Nod factor composition or structure exhibited genotype-specific nodulation (Firmin et al, 1993; Bloemberg et al, 1995)
Summary
The legume-rhizobial symbiosis starts with a signal exchange between the host plant and its microsymbiont (Oldroyd, 2013). Under nitrogen-limiting conditions, legume roots secrete a cocktail of flavonoid compounds into the rhizosphere, and they serve to activate the expression of a group of bacterial nodulation (nod) genes, leading to the synthesis of the Nod factor, a lipochitooligosaccharidic signal that is essential for initiating symbiotic development in most legumes (Oldroyd et al, 2011). The best examples are from the pea-R. leguminosarum symbiosis where bacterial nod gene mutants that lead to changed Nod factor composition or structure exhibited genotype-specific nodulation (Firmin et al, 1993; Bloemberg et al, 1995) This alteration of host range corresponds to allelic variations at the Sym2/Sym37/PsK1 locus, an orthologous region of NFR1 that contains a cluster of LysM receptor kinases (Zhukov et al, 2008; Li et al, 2011). Transferring NFR1 and NFR5 of L. japonicus into M. truncatula enables nodulation of the transformants by the L. japonicus symbiont Mesorhizobium loti (Radutoiu et al, 2007)
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