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Nodulation of Aeschynomene afraspera and A. indica by Photosynthetic Bradyrhizobium Sp. Strain ORS285: The Nod-Dependent Versus the Nod-Independent Symbiotic Interaction

¹IRD, Laboratoire des Symbioses Tropicales et Méditerranéennes (LSTM), UMR IRD/SupAgro/INRA/UM2/CIRAD, F-34398 Montpellier, France; ²UM2, F-34095 Montpellier, France; ³CIRAD, UMR BGPI INRA/CIRAD/SUP AGRO, Campus International de Baillarguet, 34398 Montpellier cedex 5, France; ⁴CIRAD, LSTM, UMR IRD/SupAgro/INRA/UM2/CIRAD, F-34398 Montpellier, France; ⁵Department of Plant Systems Biology, VIB, and ⁶Department of Plant Biotechnology and Genetics, Ghent University, Technologiepark 927, B-9052 Gent, Belgium

Here, we present a comparative analysis of the nodulation processes of Aeschynomene afraspera and A. indica that differ in their requirement for Nod factors (NF) to initiate symbiosis with photosynthetic bradyrhizobia. The infection process and nodule organogenesis was examined using the green fluorescent protein–labeled Bradyrhizobium sp. strain ORS285 able to nodulate both species. In A. indica, when the NF-independent strategy is used, bacteria penetrated the root intercellularly between axillary root hairs and invaded the subepidermal cortical cells by invagination of the host cell wall. Whereas the first infected cortical cells collapsed, the infected ones immediately beneath kept their integrity and divided repeatedly to form the nodule. In A. afraspera, when the NF-dependent strategy is used, bacteria entered the plant through epidermal fissures generated by the emergence of lateral roots and spread deeper intercellularly in the root cortex, infecting some cortical cells during their progression. Whereas the infected cells of the lower cortical layers divided rapidly to form the nodule, the infected cells of the upper layers gave rise to an outgrowth in which the bacteria remained enclosed in large tubular structures. Together, two distinct modes of infection and nodule organogenesis coexist in Aeschynomene legumes, each displaying original features.