Aeschynomene evenia, a Model Plant for Studying the Molecular Genetics of the Nod-Independent Rhizobium-Legume Symbiosis

Abstract

Research on the nitrogen-fixing symbiosis has been focused, thus far, on two model legumes, Medicago truncatula and Lotus japonicus, which use a sophisticated infection process involving infection thread formation. However, in 25% of the legumes, the bacterial entry occurs more simply in an intercellular fashion. Among them, some Aeschynomene spp. are nodulated by photosynthetic Bradyrhizobium spp. that do not produce Nod factors. This interaction is believed to represent a living testimony of the ancestral state of the rhizobium-legume symbiosis. To decipher the mechanisms of this Nod-independent process, we propose Aeschynomene evenia as a model legume because it presents all the characteristics required for genetic and molecular analysis. It is a short-perennial and autogamous species, with a diploid and relatively small genome (2n=20; 460 Mb/1C). A. evenia 'IRFL6945' is nodulated by the well-characterized photosynthetic Bradyrhizobium sp. strain ORS278 and is efficiently transformed by Agrobacterium rhizogenes. Aeschynomene evenia is genetically homozygous but polymorphic accessions were found. A manual hybridization procedure has been set up, allowing directed crosses. Therefore, it should be relatively straightforward to unravel the molecular determinants of the Nod-independent process in A. evenia. This should shed new light on the evolution of rhizobium-legume symbiosis and could have important agronomic implications.

The Nod factor-independent nodulation in Aeschynomene evenia required the common plant-microbe symbiotic "toolkit"

“…Aeschynomene evenia seeds used in this study came from line IRFL6945 (U.S. Department of Agriculture; Arrighi et al, 2012). Seeds were scarified with concentrated (95% [v/v]) sulfuric acid for 40 min.…”

“…Aeschynomene evenia recently emerged as a new model legume for the study of legume-rhizobium interactions (Arrighi et al, 2012). Most studies of rhizobium-legume symbioses up to now were those in temperate climates, which are far from representing the diversity of such interactions, particularly the range of infection processes.…”

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

“…For example, within the Aeschynomene genus, A. afraspera has E-type bacteroids but A. indica or A. evenia have S-type bacteroids (Bonaldi et al, 2011;Arrighi et al, 2012). The bacteroid morphogenesis in the Aeschynomene genus is also under plant control, since the Bradyrhizobium spp.…”

“…The bacteroid morphogenesis in the Aeschynomene genus is also under plant control, since the Bradyrhizobium spp. strain ORS285 transforms into E-morphotype bacteroids in A. afraspera nodules and in S-morphotype bacteroids in A. indica and A. evenia nodules (Bonaldi et al, 2011;Arrighi et al, 2012). The Aeschynomene symbiosis with Bradyrhizobium spp.…”

“…The Aeschynomene symbiosis with Bradyrhizobium spp. has several other particular features, including the formation of nodules on both roots and stems, photosynthesis by the Bradyrhizobium symbionts that is required for optimal stem nodulation (Giraud et al, 2000), and the use of a Nod factor-independent nodulation pathway in some Aeschynomene spp., including A. indica and A. evenia (Giraud et al, 2007;Arrighi et al, 2012). Furthermore, in contrast to classical determinate and indeterminate nodules, aeschynomenoid nodules originate from the successive divisions of only one or a few root cortical cells initially infected by an infection threadindependent mechanism (Bonaldi et al, 2011).…”