Argonaute5 and its associated small RNAs modulate the transcriptional response during the rhizobia-Phaseolus vulgaris symbiosis.

Both plant- and rhizobia-derived small RNAs play an essential role in regulating the root nodule symbiosis in legumes. Small RNAs, in association with Argonaute proteins, tune the expression of genes participating in nodule development and rhizobial infection. However, the role of Argonaute proteins in this symbiosis has been overlooked. In this study, we provide transcriptional evidence showing that Argonaute5 (AGO5) is a determinant genetic component in the root nodule symbiosis in Phaseolus vulgaris. A spatio-temporal transcriptional analysis revealed that the promoter of PvAGO5 is active in lateral root primordia, root hairs from rhizobia-inoculated roots, nodule primordia, and mature nodules. Transcriptional analysis by RNA sequencing revealed that gene silencing of PvAGO5 affected the expression of genes involved in the biosynthesis of the cell wall and phytohormones participating in the rhizobial infection process and nodule development. PvAGO5 immunoprecipitation coupled to small RNA sequencing revealed the small RNAs bound to PvAGO5 during the root nodule symbiosis. Identification of small RNAs associated to PvAGO5 revealed miRNAs previously known to participate in this symbiotic process, further supporting a role for AGO5 in this process. Overall, the data presented shed light on the roles that PvAGO5 plays during the root nodule symbiosis in P. vulgaris.

Control of the Rhizobia Nitrogen-Fixing Symbiosis by Common Bean MADS-Domain/AGL Transcription Factors.

Plants MADS-domain/AGL proteins constitute a large transcription factor (TF) family that controls the development of almost every plant organ. We performed a phylogeny of (ca. 500) MADS-domain proteins from Arabidopsis and four legume species. We identified clades with Arabidopsis MADS-domain proteins known to participate in root development that grouped legume MADS-proteins with similar high expression in roots and nodules. In this work, we analyzed the role of AGL transcription factors in the common bean (Phaseolus vulgaris) - Rhizobium etli N-fixing symbiosis. Sixteen P. vulgaris AGL genes (PvAGL), out of 93 family members, are expressed - at different levels - in roots and nodules. From there, we selected the PvAGL gene denominated PvFUL-like for overexpression or silencing in composite plants, with transgenic roots and nodules, that were used for phenotypic analysis upon inoculation with Rhizobium etli. Because of sequence identity in the DNA sequence used for RNAi-FUL-like construct, roots, and nodules expressing this construct -referred to as RNAi_AGL- showed lower expression of other five PvAGL genes highly expressed in roots/nodules. Contrasting with PvFUL-like overexpressing plants, rhizobia-inoculated plants expressing the RNAi_AGL silencing construct presented affection in the generation and growth of transgenic roots from composite plants, both under non-inoculated or rhizobia-inoculated condition. Furthermore, the rhizobia-inoculated plants showed decreased rhizobial infection concomitant with the lower expression level of early symbiotic genes and increased number of small, ineffective nodules that indicate an alteration in the autoregulation of the nodulation symbiotic process. We propose that the positive effects of PvAGL TF in the rhizobia symbiotic processes result from its potential interplay with NIN, the master symbiotic TF regulator, that showed a CArG-box consensus DNA sequence recognized for DNA binding of AGL TF and presented an increased or decreased expression level in roots from non-inoculated plants transformed with OE_FUL or RNAi_AGL construct, respectively. Our work contributes to defining novel transcriptional regulators for the common bean - rhizobia N-fixing symbiosis, a relevant process for sustainable agriculture.

Early Molecular Dialogue Between Legumes and Rhizobia: Why Are They So Important?

Legume-rhizobia symbiosis has a considerable ecological relevance because it replenishes the soil with fixed-nitrogen (e.g., ammonium) for other plants. Because of this benefit to the environment, the exploitation of the legume-rhizobia symbiosis can contribute to the development of the lower input, sustainable agriculture, thereby, reducing dependency on synthetic fertilizers. To achieve this goal, it is necessary to understand the different levels of regulation of this symbiosis to enhance its nitrogen-fixation efficiency. A different line of evidence attests to the relevance of early molecular events in the establishment of a successful symbiosis between legumes and rhizobia. In this chapter, we will review the early molecular signaling in the legume-rhizobia symbiosis. We will focus on the early molecular responses that are crucial for the recognition of the rhizobia as a potential symbiont.

A Novel OmpR-Type Response Regulator Controls Multiple Stages of the Rhizobium etli - Phaseolus vulgaris N2-Fixing Symbiosis.

OmpR, is one of the best characterized response regulators families, which includes transcriptional regulators with a variety of physiological roles including the control of symbiotic nitrogen fixation (SNF). The Rhizobium etli CE3 genome encodes 18 OmpR-type regulators; the function of the majority of these regulators during the SNF in common bean, remains elusive. In this work, we demonstrated that a R. etli mutant strain lacking the OmpR-type regulator RetPC57 (ΔRetPC57), formed less nodules when used as inoculum for common bean. Furthermore, we observed reduced expression level of bacterial genes involved in Nod Factors production (nodA and nodB) and of plant early-nodulation genes (NSP2, NIN, NF-YA and ENOD40), in plants inoculated with ΔRetPC57. RetPC57 also contributes to the appropriate expression of genes which products are part of the multidrug efflux pumps family (MDR). Interestingly, nodules elicited by ΔRetPC57 showed increased expression of genes relevant for Carbon/Nitrogen nodule metabolism (PEPC and GOGAT) and ΔRetPC57 bacteroids showed higher nitrogen fixation activity as well as increased expression of key genes directly involved in SNF (hfixL, fixKf, fnrN, fixN, nifA and nifH). Taken together, our data show that the previously uncharacterized regulator RetPC57 is a key player in the development of the R. etli - P. vulgaris symbiosis.

Argonaute Proteins: Why Are They So Important for the Legume-Rhizobia Symbiosis?

Unlike most other land plants, legumes can fulfill their nitrogen needs through the establishment of symbioses with nitrogen-fixing soil bacteria (rhizobia). Through this symbiosis, fixed nitrogen is incorporated into the food chain. Because of this ecological relevance, the genetic mechanisms underlying the establishment of the legume-rhizobia symbiosis (LRS) have been extensively studied over the past decades. During this time, different types of regulators of this symbiosis have been discovered and characterized. A growing number of studies have demonstrated the participation of different types of small RNAs, including microRNAs, in the different stages of this symbiosis. The involvement of small RNAs also indicates that Argonaute (AGO) proteins participate in the regulation of the LRS. However, despite this obvious role, the relevance of AGO proteins in the LRS has been overlooked and understudied. Here, we discuss and hypothesize the likely participation of AGO proteins in the regulation of the different steps that enable the establishment of the LRS. We also briefly review and discuss whether rhizobial symbiosis induces DNA damages in the legume host. Understanding the different levels of LRS regulation could lead to the development of improved nitrogen fixation efficiency to enhance sustainable agriculture, thereby reducing dependence on inorganic fertilizers.

A Novel Positive Regulator of the Early Stages of Root Nodule Symbiosis Identified by Phosphoproteomics.

Signals and signaling pathways underlying the symbiosis between legumes and rhizobia have been studied extensively over the past decades. In a previous phosphoproteomic study on the Medicago truncatula-Sinorhizobium meliloti symbiosis, we identified plant proteins that are differentially phosphorylated upon the perception of rhizobial signals, called Nod factors. In this study, we provide experimental evidence that one of these proteins, Early Phosphorylated Protein 1 (EPP1), is required for the initiation of this symbiosis. Upon inoculation with rhizobia, MtEPP1 expression was induced in curled root hairs. Down-regulation of MtEPP1 in M. truncatula roots almost abolished calcium spiking, reduced the expression of essential symbiosis-related genes (MtNIN, MtNF-YB1, MtERN1 and MtENOD40) and strongly decreased nodule development. Phylogenetic analyses revealed that orthologs of MtEPP1 are present in legumes and specifically in plant species able to host arbuscular mycorrhizal fungi, suggesting a possible role in this association too. Short chitin oligomers induced the phosphorylation of MtEPP1 like Nod factors. However, the down-regulation of MtEPP1 affected the colonization of M. truncatula roots by arbuscular mycorrhizal fungi only moderately. Altogether, these findings indicate that MtEPP1 is essential for the establishment of the legume-rhizobia symbiosis but might plays a limited role in the arbuscular mycorrhizal symbiosis.

Phosphate Deficiency Negatively Affects Early Steps of the Symbiosis between Common Bean and Rhizobia.

Phosphate (Pi) deficiency reduces nodule formation and development in different legume species including common bean. Despite significant progress in the understanding of the genetic responses underlying the adaptation of nodules to Pi deficiency, it is still unclear whether this nutritional deficiency interferes with the molecular dialogue between legumes and rhizobia. If so, what part of the molecular dialogue is impaired? In this study, we provide evidence demonstrating that Pi deficiency negatively affects critical early molecular and physiological responses that are required for a successful symbiosis between common bean and rhizobia. We demonstrated that the infection thread formation and the expression of PvNSP2, PvNIN, and PvFLOT2, which are genes controlling the nodulation process were significantly reduced in Pi-deficient common bean seedlings. In addition, whole-genome transcriptional analysis revealed that the expression of hormones-related genes is compromised in Pi-deficient seedlings inoculated with rhizobia. Moreover, we showed that regardless of the presence or absence of rhizobia, the expression of PvRIC1 and PvRIC2, two genes participating in the autoregulation of nodule numbers, was higher in Pi-deficient seedlings compared to control seedlings. The data presented in this study provides a mechanistic model to better understand how Pi deficiency impacts the early steps of the symbiosis between common bean and rhizobia.

Gene Silencing of Argonaute5 Negatively Affects the Establishment of the Legume-Rhizobia Symbiosis.

The establishment of the symbiosis between legumes and nitrogen-fixing rhizobia is finely regulated at the transcriptional, posttranscriptional and posttranslational levels. Argonaute5 (AGO5), a protein involved in RNA silencing, can bind both viral RNAs and microRNAs to control plant-microbe interactions and plant physiology. For instance, AGO5 regulates the systemic resistance of Arabidopsis against Potato Virus X as well as the pigmentation of soybean (Glycine max) seeds. Here, we show that AGO5 is also playing a central role in legume nodulation based on its preferential expression in common bean (Phaseolus vulgaris) and soybean roots and nodules. We also report that the expression of AGO5 is induced after 1 h of inoculation with rhizobia. Down-regulation of AGO5 gene in P. vulgaris and G. max causes diminished root hair curling, reduces nodule formation and interferes with the induction of three critical symbiotic genes: Nuclear Factor Y-B (NF-YB), Nodule Inception (NIN) and Flotillin2 (FLOT2). Our findings provide evidence that the common bean and soybean AGO5 genes play an essential role in the establishment of the symbiosis with rhizobia.