Changes in the Common Bean Transcriptome in Response to Secreted and Surface Signal Molecules of Rhizobium etli

TETRASPANIN 8-1 from Phaseolus vulgaris plays a key role during mutualistic interactions

Arbuscular mycorrhizal (AM) fungi and rhizobia form two of the most important plant-microbe associations for the assimilation of phosphorus (P) and nitrogen (N). Symbiont-derived signals are able to coordinate the infection process by triggering multiple responses in the plant root, such as calcium influxes and oscillations, increased reactive oxygen species (ROS), cytoskeletal rearrangements and altered gene expression. An examination was made of the role of tetraspanins, which are transmembrane proteins that self-organize into tetraspanin web regions, where they recruit specific proteins into platforms required for signal transduction, membrane fusion, cell trafficking, and ROS generation. In plant cells, tetraspanins are scaffolding proteins associated with root radial patterning, biotic and abiotic stress responses, cell fate determination, plasmodesmata and hormonal regulation. Some plant tetraspanins, such as Arabidopsis thaliana TETRASPANIN 8 and TETRASPANIN 9 (AtTET8 and AtTET9) are associated with exosomes during inter-kingdom communication. In this study, a homolog of AtTET8, PvTET8-1, in common bean (Phaseolus vulgaris L. var. Negro Jamapa) was examined in roots during interactions with Rhizobium tropici and Rhizophagus irregularis. The promoter of PvTET8-1 contained several cis-acting regulatory DNA elements potentially related to mutualistic interactions, and PvTET8-1 was transcriptionally activated during AM fungal and rhizobial associations. Silencing it decreased the size and number of nodules, nitrogen fixation, and mycorrhizal arbuscule formation, whereas overexpressing it increased the size and number of nodules, and mycorrhizal arbuscule formation but decreased nitrogen fixation. PvTET8-1 appears to be an important element in both of these mutualistic interactions, perhaps through its interaction with NADPH oxidase and the generation of ROS during the infection processes.

Identification of conserved and new miRNAs that affect nodulation and strain selectivity in the Phaseolus vulgaris-Rhizobium etli symbiosis through differential analysis of host small RNAs

Phaseolus vulgaris plants from the Mesoamerican centre of genetic diversification establish a preferential and more efficient root nodule symbiosis with sympatric Rhizobium etli strains. This is mediated by changes in host gene expression, which might occur either at the transcriptional or at the post-transcriptional level. However, the implication of small RNA (sRNA)-mediated control of gene expression in strain selectivity has remained elusive. sRNA sequencing was used to identify host microRNAs (miRNAs) differentially regulated in roots at an early stage of the symbiotic interaction, which were further characterized by applying a reverse genetic approach. In silico analysis identified known and new miRNAs that accumulated to a greater extent in the preferential and more efficient interaction. One of them, designated as Pvu-miR5924, participates in the mechanisms that determine the selection of R. etli strains that will colonize the nodules. In addition, the functional analysis of Pvu-miR390b verified that this miRNA is a negative modulator of nodule formation and bacterial infection. This study not only extended the list of miRNAs identified in P. vulgaris but also enabled the identification of miRNAs that play relevant functions in nodule formation, rhizobial infection and the selection of the rhizobial strains that will occupy the nodule.

Plant circadian clock control of Medicago truncatula nodulation via regulation of nodule cysteine-rich peptides

Legumes house nitrogen-fixing endosymbiotic rhizobia in specialized polyploid cells within root nodules, which undergo tightly regulated metabolic activity. By carrying out expression analysis of transcripts over time in Medicago truncatula nodules, we found that the circadian clock enables coordinated control of metabolic and regulatory processes linked to nitrogen fixation. This involves the circadian clock-associated transcription factor LATE ELONGATED HYPOCOTYL (LHY), with lhy mutants being affected in nodulation. Rhythmic transcripts in root nodules include a subset of nodule-specific cysteine-rich peptides (NCRs) that have the LHY-bound conserved evening element in their promoters. Until now, studies have suggested that NCRs act to regulate bacteroid differentiation and keep the rhizobial population in check. However, these conclusions came from the study of a few members of this very large gene family that has complex diversified spatio-temporal expression. We suggest that rhythmic expression of NCRs may be important for temporal coordination of bacterial activity with the rhythms of the plant host, in order to ensure optimal symbiosis.

Transcriptomic analysis of Mesoamerican and Andean Phaseolus vulgaris accessions revealed mRNAs and lncRNAs associated with strain selectivity during symbiosis

Legume plants establish a nitrogen-fixing symbiosis with soil bacteria known as rhizobia. Compatibility between legumes and rhizobia is determined at species-specific level, but variations in the outcome of the symbiotic process are also influenced by the capacity of the plant to discriminate and select specific strains that are better partners. We compared the transcriptional response of two genetically diverse accessions of Phaseolus vulgaris from Mesoamerica and South Andes to Rhizobium etli strains that exhibit variable degrees of symbiotic affinities. Our results indicate that the plant genotype is the major determinant of the transcriptional reprogramming occurring in roots at early stages of the symbiotic interaction. Differentially expressed genes (DEGs) regulated in the Mesoamerican and the Andean accessions in response to specific strains are different, but they belong to the same functional categories. The common and strain-specific transcriptional responses to rhizobia involve distinct transcription factors  and cis-elements present in the promoters of DEGs in each accession, showing that diversification and domestication of common bean at different geographic regions influenced the evolution of symbiosis differently in each genetic pool. Quantitative PCR analysis validated our transcriptional datasets, which constitute a valuable source of coding and non-coding candidate genes to further unravel the molecular determinants governing the mechanisms by which plants select bacterial strains that produce a better symbiotic outcome.

Legume-rhizobium dance: an agricultural tool that could be improved?

The specific interaction between rhizobia and legume roots leads to the development of a highly regulated process called nodulation, by which the atmospheric nitrogen is converted into an assimilable plant nutrient. This capacity is the basis for the use of bacterial inoculants for field crop cultivation. Legume plants have acquired tools that allow the entry of compatible bacteria. Likewise, plants can impose sanctions against the maintenance of nodules occupied by rhizobia with low nitrogen-fixing capacity. At the same time, bacteria must overcome different obstacles posed first by the environment and then by the legume. The present review describes the mechanisms involved in the regulation of the entire legume-rhizobium symbiotic process and the strategies and tools of bacteria for reaching the nitrogen-fixing state inside the nodule. Also, we revised different approaches to improve the nodulation process for a better crop yield.

Involvement of Arachis hypogaea Jasmonate ZIM domain/TIFY proteins in root nodule symbiosis

Jasmonate ZIM domain (JAZ) proteins are the key negative regulators of jasmonate signaling, an important integrator of plant-microbe relationships. Versatility of jasmonate signaling outcomes are maintained through the multiplicity of JAZ proteins and their definitive functionalities. How jasmonate signaling influences the legume-Rhizobium symbiotic relationship is still unclear. In Arachis hypogaea (peanut), a legume plant, one JAZ sub-family (JAZ1) gene and one TIFY sequence containing protein family member (TIFY8) gene show enhanced expression in the early stage and late stage of root nodule symbiosis (RNS) respectively. In plants, JAZ sub-family proteins belong to a larger TIFY family. Here, this study denotes the first attempt to reveal in planta interactions of downstream jasmonate signaling regulators through proteomics and mass spectrometry to find out the mode of jasmonate signaling participation in the RNS process of A. hypogaea. From 4-day old Bradyrhizobium-infected peanut roots, the JAZ1-protein complex shows its contribution towards the rhizobial entry, nodule development, autoregulation of nodulation and photo-morphogenesis during the early stage of symbiosis. From 30-day old Bradyrhizobium infected roots, the TIFY8-protein complex reveals repressor functionality of TIFY8, suppression of root jasmonate signaling, modulation of root circadian rhythm and nodule development. Cellular localization and expression level of the interaction partners during the nodulation process further substantiate the in planta interaction pairs. This study provides a comprehensive insight into the jasmonate functionality in RNS through modulation of nodule number and development, during the early stage and root circadian rhythm during the late stage of nodulation, through the protein complexes of JAZ1 and TIFY8 respectively in A. hypogaea.

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.

Comparative investigation on carbon-based moving bed biofilm reactor (MBBR) for synchronous removal of phenols and ammonia in treating coal pyrolysis wastewater at pilot-scale

By regulating the extraction solvent and alkali in pretreatment, two carbon-based MBBRs were compared in pilot-scale to synchronously remove phenols and ammonia of coal pyrolysis wastewater (CPW) under fluctuant phenols-ammonia loadings. It revealed that lignite activated coke (LAC)-based MBBR performed more stable with phenols increasing (250-550 mg/L), and reached higher tolerance limit to ammonia (>320 mg/L) than activated carbon (AC)-based MBBR under fluctuant ammonia loadings. During the phenols-ammonia synchronous removal process, the LAC provided the firm basis for shock resistance due to superior resilient adsorption capacity, enhanced sludge property and microbial cooperation. Furthermore, microbial analysis revealed that the strengthened collaboration between archaea and facultative bacteria played the primary role in phenols-ammonia synchronous degradation. Specifically, the heterotrophic bacteria consumed phenols-ammonia by partial nitrification process and ammonia assimilation, following by denitrifying process to further eliminate phenols. The multifunctional Comamonas was the critical genus participating in all procedures.

The PvNF-YA1 and PvNF-YB7 Subunits of the Heterotrimeric NF-Y Transcription Factor Influence Strain Preference in the Phaseolus vulgaris-Rhizobium etli Symbiosis

Transcription factors of the Nuclear Factor Y (NF-Y) family play essential functions in plant development and plasticity, including the formation of lateral root organs such as lateral root and symbiotic nodules. NF-Ys mediate transcriptional responses by acting as heterotrimers composed of three subunits, NF-YA, NF-YB, and NF-YC, which in plants are encoded by relatively large gene families. We have previously shown that, in the Phaseolus vulgaris × Rhizobium etli interaction, the PvNF-YC1 subunit is involved not only in the formation of symbiotic nodules, but also in the preference exhibited by the plant for rhizobial strains that are more efficient and competitive in nodule formation. PvNF-YC1 forms a heterotrimer with the PvNF-YA1 and PvNF-YB7 subunits. Here, we used promoter:reporter fusions to show that both PvNF-YA1 and PvNF-YB7 are expressed in symbiotic nodules. In addition, we report that knock-down of PvNF-YA1 and its close paralog PvNF-YA9 abolished nodule formation by either high or low efficient strains and arrested rhizobial infection. On the other hand, knock-down of PvNF-YB7 only affected the symbiotic outcome of the high efficient interaction, suggesting that other symbiotic NF-YB subunits might be involved in the more general mechanisms of nodule formation. More important, we present functional evidence supporting that both PvNF-YA1 and PvNF-YB7 are part of the mechanisms that allow P. vulgaris plants to discriminate and select those bacterial strains that perform better in nodule formation, most likely by acting in the same heterotrimeric complex that PvNF-YC1.

Analysis of the common bean (Phaseolus vulgaris L.) transcriptome regarding efficiency of phosphorus use

Common bean is a highly important food in tropical regions, where most production occurs on small farms with limited use of technology and, consequently, greater vulnerability to abiotic stresses such as nutritional stress. Usually phosphorus (P) is the most limiting nutrient for crop growth in these regions. The aim of this study was to characterize the gene expression profiles of the genotypes of common bean IAC Imperador (P-responsive) and DOR 364 (P-unresponsive) under different P concentrations using RNA-seq transcriptome sequencing technology. Plants were grown hydroponically, with application of two P concentrations (4.00 mg L^-1 restrictive level and 8.00 mg L^-1 control level). Differential expression analyses, annotation, and functional classification were performed comparing genotypes within each P rate administered and comparing each genotype response to the different P levels. Considering differential expression analyses within genotypes, IAC Imperador exhibited 1538 up-regulated genes under P restriction and 1679 up-regulated genes in the control, while DOR 364 exhibited 13 up-regulated genes in the control and only 2 up-regulated genes under P restriction, strongly corroborating P-unresponsiveness of this genotype. Genes related to phosphorus restriction were identified among the differentially expressed genes, including transcription factors such as WRKY, ERF, and MYB families, phosphatase related genes such as pyrophosphatase, acid phosphatase, and purple acid phosphatase, and phosphate transporters. The enrichment test for the P restriction treatment showed 123 enriched gene ontologies (GO) for IAC Imperador, while DOR 364 enriched only 24. Also, the enriched GO correlated with P metabolism, compound metabolic processes containing phosphate, nucleoside phosphate binding, phosphorylation, and also response to stresses. Thus, this study proved to be informative to phosphorus limitation in common bean showing global changes at transcript level.

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.

Compatibility between Legumes and Rhizobia for the Establishment of a Successful Nitrogen-Fixing Symbiosis

The root nodule symbiosis established between legumes and rhizobia is an exquisite biological interaction responsible for fixing a significant amount of nitrogen in terrestrial ecosystems. The success of this interaction depends on the recognition of the right partner by the plant within the richest microbial ecosystems on Earth, the soil. Recent metagenomic studies of the soil biome have revealed its complexity, which includes microorganisms that affect plant fitness and growth in a beneficial, harmful, or neutral manner. In this complex scenario, understanding the molecular mechanisms by which legumes recognize and discriminate rhizobia from pathogens, but also between distinct rhizobia species and strains that differ in their symbiotic performance, is a considerable challenge. In this work, we will review how plants are able to recognize and select symbiotic partners from a vast diversity of surrounding bacteria. We will also analyze recent advances that contribute to understand changes in plant gene expression associated with the outcome of the symbiotic interaction. These aspects of nitrogen-fixing symbiosis should contribute to translate the knowledge generated in basic laboratory research into biotechnological advances to improve the efficiency of the nitrogen-fixing symbiosis in agronomic systems.

Comparative phylogenetic and expression analysis of small GTPases families in legume and non-legume plants

BACKGROUND

Small monomeric GTPases act as molecular switches in several processes that involve polar cell growth, participating mainly in vesicle trafficking and cytoskeleton rearrangements. This gene superfamily has largely expanded in plants through evolution as compared with other Kingdoms, leading to the suggestion that members of each subfamily might have acquired new functions associated to plant-specific processes. Legume plants engage in a nitrogen-fixing symbiotic interaction with rhizobia in a process that involves polar growth processes associated with the infection throughout the root hair. To get insight into the evolution of small GTPases associated with this process, we use a comparative genomic approach to establish differences in the Ras GTPase superfamily between legume and non-legume plants.

RESULTS

Phylogenetic analyses did not show clear differences in the organization of the different subfamilies of small GTPases between plants that engage or not in nodule symbiosis. Protein alignments revealed a strong conservation at the sequence level of small GTPases previously linked to nodulation by functional genetics. Interestingly, one Rab and three Rop proteins showed conserved amino acid substitutions in legumes, but these changes do not alter the predicted conformational structure of these proteins. Although the steady-state levels of most small GTPases do not change in response to rhizobia, we identified a subset of Rab, Rop and Arf genes whose transcript levels are modulated during the symbiotic interaction, including their spatial distribution along the indeterminate nodule.

CONCLUSIONS

This study provides a comprehensive study of the small GTPase superfamily in several plant species. The genetic program associated to root nodule symbiosis includes small GTPases to fulfill specific functions during infection and formation of the symbiosomes. These GTPases seems to have been recruited from members that were already present in common ancestors with plants as distant as monocots since we failed to detect asymmetric evolution in any of the subfamily trees. Expression analyses identified a number of legume members that can have undergone neo- or sub-functionalization associated to the spatio-temporal transcriptional control during the onset of the symbiotic interaction.

Natural rhizobial diversity helps to reveal genes and QTLs associated with biological nitrogen fixation in common bean

Common bean is one of the most important crops for human feed, and the most important legume for direct consumption by millions of people, especially in developing countries. It is a promiscuous host legume in terms of nodulation, able to associate with a broad and diverse range of rhizobia, although the competitiveness for nodulation and the nitrogen fixation capacity of most of these strains is generally low. As a result, common bean is very inefficient for symbiotic nitrogen fixation, and nitrogen has to be supplied with chemical fertilizers. In the last years, symbiotic nitrogen fixation has received increasing attention as a sustainable alternative to nitrogen fertilizers, and also as a more economic and available one in poor countries. Therefore, optimization of nitrogen fixation of bean-rhizobia symbioses and selection of efficient rhizobial strains should be a priority, which begins with the study of the natural diversity of the symbioses and the rhizobial populations associated. Natural rhizobia biodiversity that nodulates common bean may be a source of adaptive alleles acting through phenotypic plasticity. Crosses between accessions differing for nitrogen fixation may combine alleles that never meet in nature. Another way to discover adaptive genes is to use association genetics to identify loci that common bean plants use for enhanced biological nitrogen fixation and, in consequence, for marker assisted selection for genetic improvement of symbiotic nitrogen fixation. In this review, rhizobial biodiversity resources will be discussed, together with what is known about the loci that underlie such genetic variation, and the potential candidate genes that may influence the symbiosis' fitness benefits, thus achieving an optimal nitrogen fixation capacity in order to help reduce reliance on nitrogen fertilizers in common bean.

How legumes recognize rhizobia

Legume plants have developed the capacity to establish symbiotic interactions with soil bacteria (known as rhizobia) that can convert N2 to molecular forms that are incorporated into the plant metabolism. The first step of this relationship is the recognition of bacteria by the plant, which allows to distinguish potentially harmful species from symbiotic partners. The main molecular determinant of this symbiotic interaction is the Nod Factor, a diffusible lipochitooligosaccharide molecule produced by rhizobia and perceived by LysM receptor kinases; however, other important molecules involved in the specific recognition have emerged over the years. Secreted exopolysaccharides and the lipopolysaccharides present in the bacterial cell wall have been proposed to act as signaling molecules, triggering the expression of specific genes related to the symbiotic process. In this review we will briefly discuss how transcriptomic analysis are helping to understand how multiple signaling pathways, triggered by the perception of different molecules produced by rhizobia, control the genetic programs of root nodule organogenesis and bacterial infection. This knowledge can help to understand how legumes have evolved to recognize and establish complex ecological relationships with particular species and strains of rhizobia, adjusting gene expression in response to identity determinants of bacteria.

A Phylogenetically Conserved Group of Nuclear Factor-Y Transcription Factors Interact to Control Nodulation in Legumes

The endosymbiotic association between legumes and soil bacteria called rhizobia leads to the formation of a new root-derived organ called the nodule in which differentiated bacteria convert atmospheric nitrogen into a form that can be assimilated by the host plant. Successful root infection by rhizobia and nodule organogenesis require the activation of symbiotic genes that are controlled by a set of transcription factors (TFs). We recently identified Medicago truncatula nuclear factor-YA1 (MtNF-YA1) and MtNF-YA2 as two M. truncatula TFs playing a central role during key steps of the Sinorhizobium meliloti-M. truncatula symbiotic interaction. NF-YA TFs interact with NF-YB and NF-YC subunits to regulate target genes containing the CCAAT box consensus sequence. In this study, using a yeast two-hybrid screen approach, we identified the NF-YB and NF-YC subunits able to interact with MtNF-YA1 and MtNF-YA2. In yeast (Saccharomyces cerevisiae) and in planta, we further demonstrated by both coimmunoprecipitation and bimolecular fluorescence complementation that these NF-YA, -B, and -C subunits interact and form a stable NF-Y heterotrimeric complex. Reverse genetic and chromatin immunoprecipitation-PCR approaches revealed the importance of these newly identified NF-YB and NF-YC subunits for rhizobial symbiosis and binding to the promoter of MtERN1 (for Ethylene Responsive factor required for Nodulation), a direct target gene of MtNF-YA1 and MtNF-YA2. Finally, we verified that a similar trimer is formed in planta by the common bean (Phaseolus vulgaris) NF-Y subunits, revealing the existence of evolutionary conserved NF-Y protein complexes to control nodulation in leguminous plants. This sheds light on the process whereby an ancient heterotrimeric TF mainly controlling cell division in animals has acquired specialized functions in plants.