Transcriptome Profiles of Nod Factor-independent Symbiosis in the Tropical Legume Aeschynomene evenia

Djamel Gully,Jean-François Arrighi,Eric Giraud,Cyrille Longin,Fabienne Cartieaux,Philippe François,Frédéric Mahé,Stéphane Cruveiller,Sabine Nidelet,Stéphanie Rialle,David Vallenet,Maitrayee Dasgupta,Pierre Czernic

doi:10.1038/s41598-018-29301-0

Abstract

Nod factors (NF) were assumed to be indispensable for the establishment of a rhizobium-legume symbiosis until the discovery that certain Bradyrhizobium strains interacting with certain Aeschynomene species lack the canonical nodABC genes required for their synthesis. So far, the molecular dialogue between Aeschynomene and its symbionts remains an open question. Here we report a time course transcriptional analysis of Aeschynomene evenia in response to inoculation with Bradyrhizobium ORS278. The NF-independent symbiotic process was monitored at five time points between bacterial infection and nodule maturity. The five time points correspond to three specific events, root infection by crack entry, nodule organogenesis, and the establishment of the nitrogen fixing process. During the third stage, about 80 NCR-like genes and eight symbiotic genes known to be involved in signaling, bacterial infection or nodulation regulation were highly expressed. Comparative gene expression analyses at the five time points also enabled the selection of genes with an expression profile that makes them promising markers to monitor early plant responses to bacteria. Such markers could be used in bioassays to identify the nature of the bacterial signal(s). Our data represent valuable resources for investigation of this Nod factor-independent symbiosis.

Highlights

Legumes and cereals are among the earliest plants domesticated by human beings
Legumes first produce flavonoids that are perceived by rhizobia and induce the expression of nod genes involved in the synthesis and secretion of lipochitooligosaccharide signals
Thanks to studies of the genetic diversity of Aeschynomene species that use an Nod factors (NF)-independent process, Aeschynomene evenia has become a new model legume for the purpose of deciphering the molecular mechanisms of the NF-independent symbiosis[7]. Using this new model species, we recently demonstrated that two common symbiotic genes[8,9] (SYMRK and CCaMK) and a cytokinin receptor involved in nodule organogenesis[10] (HK1) were recruited for efficient nodulation in A. evenia[11]

Summary

OPEN Transcriptome Profiles of Nod

Djamel Gully[1], Pierre Czernic[2], Stéphane Cruveiller[3], Frédéric Mahé[1], Cyrille Longin[3], David Vallenet 3, Philippe François[1], Sabine Nidelet[4], Stéphanie Rialle[4], Eric Giraud[1], Jean-François Arrighi[1], Maitrayee DasGupta5 & Fabienne Cartieaux[1]. In a few rare cases, the symbiotic interaction may even take place despite the absence of Nod factors This is the case of some Aeschynomene species that are nodulated by photosynthetic Bradyrhizobium strains lacking the canonical nodABC genes required for NF synthesis[5]. This NF independent infection process is initiated when the root are penetrated through epidermal cracks at the lateral root emergence sites[6]. Identification of differentially expressed genes (DEGs) at the different time points, including genes known to be involved in the symbiotic process of model legumes, was consistent with the general process of nodule organogenesis, development and functioning. Special focus was placed on genes whose early stable induction following inoculation makes them suitable for a reporter gene strategy

Results and Discussion

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Methods

Author Contributions

Additional Information