Evolution of symbiosis in the legume genus Aeschynomene

Abstract

SummaryLegumes in the genus Aeschynomene form nitrogen-fixing root nodules in association with Bradyrhizobium strains. Several aquatic and subaquatic species have the additional capacity to form stem nodules, and some of them can symbiotically interact with specific strains that do not produce the common Nod factors synthesized by all other rhizobia. The question of the emergence and evolution of these nodulation characters has been the subject of recent debate.We conducted a molecular phylogenetic analysis of 38 different Aeschynomene species. The phylogeny was reconstructed with both the chloroplast DNA trnL intron and the nuclear ribosomal DNA ITS/5.8S region. We also tested 28 Aeschynomene species for their capacity to form root and stem nodules by inoculating different rhizobial strains, including nodABCcontaining strains (ORS285, USDA110) and a nodABC-lacking strain (ORS278).Maximum likelihood analyses resolved four distinct phylogenetic groups of Aeschynomene. We found that stem nodulation may have evolved several times in the genus, and that all Aeschynomene species using a Nod-independent symbiotic process clustered in the same clade.The phylogenetic approach suggested that Nod-independent nodulation has evolved once in this genus, and should be considered as a derived character, and this result is discussed with regard to previous experimental studies.

Convergent Evolution of Endosymbiont Differentiation in Dalbergioid and Inverted Repeat-Lacking Clade Legumes Mediated by Nodule-Specific Cysteine-Rich Peptides

“…using a Nod factor-independent symbiotic process form a monophyletic clade that does not include other species using a Nod factor-dependent process (Supplemental Fig. S1B; Chaintreuil et al, 2013). Therefore, it would be tempting to speculate that the S-morphotype of bacteroids and Nod factor-independent symbiosis are both derived characters, which are correlated.…”

Preferential Association of Endophytic Bradyrhizobia with Different Rice Cultivars and Its Implications for Rice Endophyte Evolution

“…It seems that the phenotypic properties of SUT-PR9 are still ambiguous. Perhaps the ability to form nodules without Nod factor, rather than being driven solely by the plants, has also been dependent on a specific single bacterial evolution/mutation (41). The CalvinBenson-Bassham (CBB) cycle from Bradyrhizobium species plays an important role in chemoautotrophic growth (42), and it is important for efficient symbiosis with A. indica (43) by controlling the oxygen tension of the early stage of symbiosis (44).…”

Extra-slow-growing Tardiphaga strains isolated from nodules of Vavilovia formosa (Stev.) Fed.

“…It remains a mystery why such a Nod factor (NF)-independent system is found only in symbiosis with Aeschynomene plants, which can also be nodulated by bradyrhizobial strains carrying the nodABC genes. Evolution of symbiosis in the legume genus Aeschynomene was studied using the molecular phylogenetic analysis of 38 different species (Chaintreuil et al 2013). All Aeschynomene species forming an efficient symbiosis with strains lacking the common nod genes fell into a single clade in the concatenated ITS-trnL dendrogram.…”