Relationship between gibberellin, ethylene and nodulation in Pisum sativum

Phenoxyacetic acid enhances nodulation symbiosis during the rapid growth stage of soybean

Root exudates are known signaling agents that influence legume root nodulation, but the molecular mechanisms for nonflavonoid molecules remain largely unexplored. The number of soybean root nodules during the initial growth phase shows substantial discrepancies at distinct developmental junctures. Using a combination of metabolomics analyses on root exudates and nodulation experiments, we identify a pivotal role for certain root exudates during the rapid growth phase in promoting nodulation. Phenoxyacetic acid (POA) was found to activate the expression of GmGA2ox10 and thereby facilitate rhizobial infection and the formation of infection threads. Furthermore, POA exerts regulatory control on the miR172c-NNC1 module to foster nodule primordia development and consequently increase nodule numbers. These findings collectively highlight the important role of POA in enhancing nodulation during the accelerated growth phase of soybeans.

Cell-layer specific roles for gibberellins in nodulation and root development

Gibberellins (GA) have a profound influence on the formation of lateral root organs. However, the precise role this hormone plays in the cell-specific events during lateral root formation, rhizobial infection and nodule organogenesis, including interactions with auxin and cytokinin (CK), is not clear. We performed epidermal- and endodermal-specific complementation of the severely GA-deficient na pea (Pisum sativum) mutant with Agrobacterium rhizogenes. Gibberellin mutants were used to examine the spatial expression pattern of CK (TCSn)- and auxin (DR5)-responsive promoters and hormone levels. We found that GA produced in the endodermis promote lateral root and nodule organogenesis and can induce a mobile signal(s) that suppresses rhizobial infection. By contrast, epidermal-derived GA suppress infection but have little influence on root or nodule development. GA suppress the CK-responsive TCSn promoter in the cortex and are required for normal auxin activation during nodule primordia formation. Our findings indicate that GA regulate the checkpoints between infection thread (IT) penetration of the cortex and invasion of nodule primordial cells and promote the subsequent progression of nodule development. It appears that GA limit the progression and branching of IT in the cortex by restricting CK response and activate auxin response to promote nodule primordia development.

Effects of Elevated Temperature on Pisum sativum Nodule Development: II-Phytohormonal Responses

High temperature is one of the most important factors limiting legume productivity. We have previously shown the induction of senescence in the apical part of nodules of the pea SGE line, formed by Rhizobium leguminosarum bv. viciae strain 3841, when they were exposed to elevated temperature (28 °C). In this study, we analyzed the potential involvement of abscisic acid (ABA), ethylene, and gibberellins in apical senescence in pea nodules under elevated temperature. Immunolocalization revealed an increase in ABA and 1-aminocyclopropane-1-carboxylic acid (ACC, the precursor of ethylene biosynthesis) levels in cells of the nitrogen fixation zone in heat-stressed nodules in 1 day of exposure compared to heat-unstressed nodules. Both ABA and ethylene appear to be involved in the earliest responses of nodules to heat stress. A decrease in the gibberellic acid (GA3) level in heat-stressed nodules was observed. Exogenous GA3 treatment induced a delay in the degradation of the nitrogen fixation zone in heat-stressed nodules. At the same time, a decrease in the expression level of many genes associated with nodule senescence, heat shock, and defense responses in pea nodules treated with GA3 at an elevated temperature was detected. Therefore, apical senescence in heat-stressed nodules is regulated by phytohormones in a manner similar to natural senescence. Gibberellins can be considered as negative regulators, while ABA and ethylene can be considered positive regulators.

Gibberellic Acid Inhibits Dendrobium nobile-Piriformospora Symbiosis by Regulating the Expression of Cell Wall Metabolism Genes

Orchid seeds lack endosperms and depend on mycorrhizal fungi for germination and nutrition acquisition under natural conditions. Piriformospora indica is a mycorrhizal fungus that promotes seed germination and seedling development in epiphytic orchids, such as Dendrobium nobile. To understand the impact of P. indica on D. nobile seed germination, we examined endogenous hormone levels by using liquid chromatography-mass spectrometry. We performed transcriptomic analysis of D. nobile protocorm at two developmental stages under asymbiotic germination (AG) and symbiotic germination (SG) conditions. The result showed that the level of endogenous IAA in the SG protocorm treatments was significantly higher than that in the AG protocorm treatments. Meanwhile, GA3 was only detected in the SG protocorm stages. IAA and GA synthesis and signaling genes were upregulated in the SG protocorm stages. Exogenous GA3 application inhibited fungal colonization inside the protocorm, and a GA biosynthesis inhibitor (PAC) promoted fungal colonization. Furthermore, we found that PAC prevented fungal hyphae collapse and degeneration in the protocorm, and differentially expressed genes related to cell wall metabolism were identified between the SG and AG protocorm stages. Exogenous GA3 upregulated SRC2 and LRX4 expression, leading to decreased fungal colonization. Meanwhile, GA inhibitors upregulated EXP6, EXB16, and EXP10-2 expression, leading to increased fungal colonization. Our findings suggest that GA regulates the expression of cell wall metabolism genes in D. nobile, thereby inhibiting the establishment of mycorrhizal symbiosis.

Calcium/calmodulin-mediated microbial symbiotic interactions in plants

Cytoplasmic calcium (Ca²⁺) transients and nuclear Ca²⁺ oscillations act as hubs during root nodulation and arbuscular mycorrhizal symbioses. Plants perceive bacterial Nod factors or fungal signals to induce the Ca²⁺ oscillation in the nucleus of root hair cells, and subsequently activate calmodulin (CaM) and Ca²⁺/CaM-dependent protein kinase (CCaMK). Ca²⁺ and CaM-bound CCaMK phosphorylate transcription factors then initiate down-stream signaling events. In addition, distinct Ca²⁺ signatures are activated at different symbiotic stages: microbial colonization and infection; nodule formation; and mycorrhizal development. Ca²⁺ acts as a key signal that regulates a complex interplay of downstream responses in many biological processes. This short review focuses on advances in Ca²⁺ signaling-regulated symbiotic events. It is meant to be an introduction to readers in and outside the field of bacterial and fungal symbioses. We summarize the molecular mechanisms underlying Ca²⁺/CaM-mediated signaling in fine-tuning both local and systemic symbiotic events.

Nitrate Leaching Mitigation Options in Two Dairy Pastoral Soils and Climatic Conditions in New Zealand

This lysimeter study investigated the effect of late-autumn application of dicyandiamide (DCD), co-poly acrylic-maleic acid (PA-MA), calcium lignosulphonate (LS), a split-application of calcium lignosulphonate (2LS), and a combination of gibberellic acid (GA) and LS (GA + LS) to reduce N leaching losses during May 2020 to December 2020 in lysimeter field sites in Manawatu (Orthic Pumice soil) and Canterbury (Pallic Orthic Brown soil), New Zealand. In a second application, urine-only, GA only and GA + LS treatments were applied during July 2020 in mid-winter on both sites. Results showed that late-autumn application of DCD, 2LS and GA + LS reduced mineral N leaching by 8%, 16%, and 35% in the Manawatu site and by 34%, 11%, and 35% in the Canterbury site, respectively when compared to urine-only. There was no significant increase in cumulative herbage N uptake and yield between urine-treated lysimeters in both sites. Mid-winter application of GA and GA + LS reduced mineral N leaching by 23% and 20%, respectively in the Manawatu site relative to urine-only treated lysimeters, but no significant reduction was observed in the Canterbury site. Our results demonstrated the potential application of these treatments in different soils under different climate and management conditions.

Right time, right place: The dynamic role of hormones in rhizobial infection and nodulation of legumes

Many legume plants form beneficial associations with rhizobial bacteria that are hosted in new plant root organs, nodules, in which atmospheric nitrogen is fixed. This association requires the precise coordination of two separate programs, infection in the epidermis and nodule organogenesis in the cortex. There is extensive literature indicating key roles for plant hormones during nodulation, but a detailed analysis of the spatial and temporal roles of plant hormones during the different stages of nodulation is required. This review analyses the current literature on hormone regulation of infection and organogenesis to reveal the differential roles and interactions of auxin, cytokinin, brassinosteroids, ethylene, and gibberellins during epidermal infection and cortical nodule initiation, development, and function. With the exception of auxin, all of these hormones suppress infection events. By contrast, there is evidence that all of these hormones promote nodule organogenesis, except ethylene, which suppresses nodule initiation. This differential role for many of the hormones between the epidermal and cortical programs is striking. Future work is required to fully examine hormone interactions and create a robust model that integrates this knowledge into our understanding of nodulation pathways.

Constitutive overexpression of GsIMaT2 gene from wild soybean enhances rhizobia interaction and increase nodulation in soybean (Glycine max)

BACKGROUND

Since the root nodules formation is regulated by specific and complex interactions of legume and rhizobial genes, there are still too many questions to be answered about the role of the genes involved in the regulation of the nodulation signaling pathway.

RESULTS

The genetic and biological roles of the isoflavone-7-O-beta-glucoside 6″-O-malonyltransferase gene GsIMaT2 from wild soybean (Glycine soja) in the regulation of nodule and root growth in soybean (Glycine max) were examined in this work. The effect of overexpressing GsIMaT2 from G. soja on the soybean nodulation signaling system and strigolactone production was investigated. We discovered that the GsIMaT2 increased nodule numbers, fresh nodule weight, root weight, and root length by boosting strigolactone formation. Furthermore, we examined the isoflavone concentration of transgenic G. max hairy roots 10 and 20 days after rhizobial inoculation. Malonyldaidzin, malonylgenistin, daidzein, and glycitein levels were considerably higher in GsMaT2-OE hairy roots after 10- and 20-days of Bradyrhizobium japonicum infection compared to the control. These findings suggest that isoflavones and their biosynthetic genes play unique functions in the nodulation signaling system in G. max.

CONCLUSIONS

Finally, our results indicate the potential effects of the GsIMaT2 gene on soybean root growth and nodulation. This study provides novel insights for understanding the epistatic relationship between isoflavones, root development, and nodulation in soybean.

HIGHLIGHTS

* Cloning and Characterization of 7-O-beta-glucoside 6″-O-malonyltransferase (GsIMaT2) gene from wild soybean (G. soja). * The role of GsIMaT2 gene in the regulation of root nodule development. *Overexpression of GsMaT2 gene increases the accumulation of isoflavonoid in transgenic soybean hairy roots. * This gene could be used for metabolic engineering of useful isoflavonoid production.

Overexpression of Terpenoid Biosynthesis Genes Modifies Root Growth and Nodulation in Soybean (Glycine max)

Root nodule formation in many leguminous plants is known to be affected by endogen ous and exogenous factors that affect formation, development, and longevity of nodules in roots. Therefore, it is important to understand the role of the genes which are involved in the regulation of the nodulation signaling pathway. This study aimed to investigate the effect of terpenoids and terpene biosynthesis genes on root nodule formation in Glycine max. The study aimed to clarify not only the impact of over-expressing five terpene synthesis genes isolated from G. max and Salvia guaranitica on soybean nodulation signaling pathway, but also on the strigolactones pathway. The obtained results revealed that the over expression of GmFDPS, GmGGPPS, SgGPS, SgFPPS, and SgLINS genes enhanced the root nodule numbers, fresh weight of nodules, root, and root length. Moreover, the terpene content in the transgenic G. max hairy roots was estimated. The results explored that the monoterpenes, sesquiterpenes and diterpenes were significantly increased in transgenic soybean hairy roots in comparison with the control. Our results indicate the potential effects of terpenoids and terpene synthesis genes on soybean root growth and nodulation. The study provides novel insights for understanding the epistatic relationship between terpenoids, root development, and nodulation in soybean.

Production of the plant hormone gibberellin by rhizobia increases host legume nodule size

Plant-associated microbes have evolved the ability to independently produce gibberellin (GA) phytohormones as a mechanism to influence their host. Indeed, GA was first discovered as a metabolite from the fungal rice pathogen Gibberella fujikuroi, which uses it as a virulence factor. Though some bacterial plant pathogens similarly use GA to promote infection, symbiotic nitrogen-fixing bacteria (rhizobia), which inhabit the root nodules of legumes, also can produce GA, suggesting a role in symbiosis. The bacterial GA biosynthetic operon has been identified, but in rhizobia this typically no longer encodes the final metabolic gene (cyp115), so that these symbionts can only produce the penultimate intermediate GA9. Here, we demonstrate that soybean (Glycine max) expresses functional GA 3-oxidases (GA3ox) within its nodules, which have the capability to convert GA9 produced by the enclosed rhizobial symbiont Bradyrhizobium diazoefficiens to bioactive GA4. This rhizobia-derived GA is demonstrated to cause an increase in nodule size and decrease in the number of nodules. The increase in individual nodule size correlates to greater numbers of bacterial progeny within a nodule, thereby providing a selective advantage to rhizobia that produce GA during the rhizobia-legume symbiosis. The expression of GA3ox in nodules and resultant nodulation effects of the GA product suggests that soybean has co-opted control of bioactive GA production, and thus nodule size, for its own benefit. Thus, our results suggest rhizobial GA biosynthesis has coevolved with host plant metabolism for cooperative production of a phytohormone that influences nodulation in a mutually beneficial manner.

Progress in the Self-Regulation System in Legume Nodule Development-AON (Autoregulation of Nodulation)

The formation and development of legumes nodules requires a lot of energy. Legumes must strictly control the number and activity of nodules to ensure efficient energy distribution. The AON system can limit the number of rhizobia infections and nodule numbers through the systemic signal pathway network that the aboveground and belowground parts participate in together. It can also promote the formation of nodules when plants are deficient in nitrogen. The currently known AON pathway includes four parts: soil NO₃⁻ signal and Rhizobium signal recognition and transmission, CLE-SUNN is the negative regulation pathway, CEP-CRA2 is the positive regulation pathway and the miR2111/TML module regulates nodule formation and development. In order to ensure the biological function of this important approach, plants use a variety of plant hormones, polypeptides, receptor kinases, transcription factors and miRNAs for signal transmission and transcriptional regulation. This review summarizes and discusses the research progress of the AON pathway in Legume nodule development.

Spatiotemporal changes in gibberellin content are required for soybean nodulation

The plant hormone gibberellin (GA) is required at different stages of legume nodule development, with its spatiotemporal distribution tightly regulated. Transcriptomic and bioinformatic analyses established that several key GA biosynthesis and catabolism enzyme encoding genes are critical to soybean (Glycine max) nodule formation. We examined the expression of several GA oxidase genes and used a Förster resonance energy transfer-based GA biosensor to determine the bioactive GA content of roots inoculated with DsRed-labelled Bradyrhizobium diazoefficiens. We manipulated the level of GA by genetically disrupting the expression of GA oxidase genes. Moreover, exogenous treatment of soybean roots with GA3 induced the expression of key nodulation genes and altered infection thread and nodule phenotypes. GmGA20ox1a, GmGA3ox1a, and GmGA2ox1a are upregulated in soybean roots inoculated with compatible B. diazoefficiens. GmGA20ox1a expression is predominately localized to the transient meristem of soybean nodules and coincides with the spatiotemporal distribution of bioactive GA occurring throughout nodule organogenesis. GmGA2ox1a exhibits a nodule vasculature-specific expression pattern, whereas GmGA3ox1a can be detected throughout the nodule and root. Disruptions in the level of GA resulted in aberrant rhizobia infection and reduced nodule numbers. Collectively, our results establish a central role for GAs in root hair infection by symbiotic rhizobia and in nodule organogenesis.

Interactions of Gibberellins with Phytohormones and Their Role in Stress Responses

Gibberellins are amongst the main plant growth regulators. Discovered over a century ago, the interest in gibberellins research is growing due to their current and potential applications in crop production and their role in the responses to environmental stresses. In the present review, the current knowledge on gibberellins’ homeostasis and modes of action is outlined. Besides this, the complex interrelations between gibberellins and other plant growth regulators are also described, providing an intricate network of interactions that ultimately drives towards precise and specific gene expression. Thus, genes and proteins identified as being involved in gibberellin responses in model and non-model species are highlighted. Furthermore, the molecular mechanisms governing the gibberellins’ relation to stress responses are also depicted. This review aims to provide a comprehensive picture of the state-of-the-art of the current perceptions of the interactions of gibberellins with other phytohormones, and their responses to plant stresses, thus allowing for the identification of the specific mechanisms involved. This knowledge will help us to improve our understanding of gibberellins’ biology, and might help increase the biotechnological toolbox needed to refine plant resilience, particularly under a climate change scenario.

Biosynthesis and beneficial effects of microbial gibberellins on crops for sustainable agriculture

Soil microbes promote plant growth through several mechanisms such as secretion of chemical compounds including plant growth hormones. Among the phytohormones, auxins, ethylene, cytokinins, abscisic acid and gibberellins are the best understood compounds. Gibberellins were first isolated in 1935 from the fungus Gibberella fujikuroi and are synthesized by several soil microbes. The effect of gibberellins on plant growth and development has been studied, as has the biosynthesis pathways, enzymes, genes and their regulation. This review revisits the history of gibberellin research highlighting microbial gibberellins and their effects on plant health with an emphasis on the early discoveries and current advances that can find vital applications in agricultural practices.

Gibberellins Inhibit Flavonoid Biosynthesis and Promote Nitrogen Metabolism in Medicago truncatula

Bioactive gibberellic acids (GAs) are diterpenoid plant hormones that are biosynthesized through complex pathways and control various aspects of growth and development. Although GA biosynthesis has been intensively studied, the downstream metabolic pathways regulated by GAs have remained largely unexplored. We investigated Tnt1 retrotransposon insertion mutant lines of Medicago truncatula with a dwarf phenotype by forward and reverse genetics screening and phylogenetic, molecular, biochemical, proteomic and metabolomic analyses. Three Tnt1 retrotransposon insertion mutant lines of the gibberellin 3-beta-dioxygenase 1 gene (GA3ox1) with a dwarf phenotype were identified, in which the synthesis of GAs (GA₃ and GA₄) was inhibited. Phenotypic analysis revealed that plant height, root and petiole length of ga3ox1 mutants were shorter than those of the wild type (Medicago truncatula ecotype R108). Leaf size was also much smaller in ga3ox1 mutants than that in wild-type R108, which is probably due to cell-size diminution instead of a decrease in cell number. Proteomic and metabolomic analyses of ga3ox1/R108 leaves revealed that in the ga3ox1 mutant, flavonoid isoflavonoid biosynthesis was significantly up-regulated, while nitrogen metabolism was down-regulated. Additionally, we further demonstrated that flavonoid and isoflavonoid biosynthesis was induced by prohexadione calcium, an inhibitor of GA3ox enzyme, and inhibited by exogenous GA₃. In contrast, nitrogen metabolism was promoted by exogenous GA₃ but inhibited by prohexadione calcium. The results of this study further demonstrated that GAs play critical roles in positively regulating nitrogen metabolism and transport and negatively regulating flavonoid biosynthesis through GA-mediated signaling pathways in leaves.

A Nod factor- and type III secretion system-dependent manner for Robinia pseudoacacia to establish symbiosis with Mesorhizobium amorphae CCNWGS0123

Under nitrogen-limiting conditions, symbiotic nodulation promotes the growth of legume plants via the fixation of atmospheric nitrogen to ammonia by rhizobia in root nodules. The rhizobial Nod factor (NF) and type III secretion system (T3SS) are two key signaling pathways for establishing the legume-rhizobium symbiosis. However, whether NF signaling is involved in the nodulation of Robinia pseudoacacia and Mesorhizobium amorphae CCNWGS0123, and its symbiotic differences compared with T3SS signaling remain unclear. Therefore, to elucidate the function of NF signaling in nodulation, we mutated nodC in M. amorphae CCNWGS0123, which aborted NF synthesis. Compared with the plants inoculated with the wild type strain, the plants inoculated with the NF-deficient strain exhibited shorter shoots with etiolated leaves. These phenotypic characteristics were similar to those of the plants inoculated with the T3SS-deficient strain, which served as a Nod- (non-effective nodulation) control. The plants inoculated with both the NF- and T3SS-deficient strains formed massive root hair swellings, but no normal infection threads were detected. Sections of the nodules showed that inoculation with the NF- and T3SS-deficient strains induced small, white bumps without any rhizobia inside. Analyzing the accumulation of 6 plant hormones and the expression of 10 plant genes indicated that the NF- and T3SS-deficient strains activated plant defense reactions while suppressing plant symbiotic signaling during the perception and nodulation processes. The requirement for NF signaling appeared to be conserved in two other leguminous trees that can establish symbiosis with M. amorphae CCNWGS0123. In contrast, the function of the T3SS might differ among species, even within the same subfamily (Faboideae). Overall, this work demonstrated that nodulation of R. pseudoacacia and M. amorphae CCNWGS0123 was both NF and T3SS dependent.

Response of Resistant and Susceptible Bayberry Cultivars to Infection of Twig Blight Pathogen by Histological Observation and Gibberellin Related Genes Expression

Bayberry is an important fruit tree native to the subtropical regions of China. However, a systematic twig blight disease caused by Pestalotiopsis versicolor and P. microspora, resulted in the death of the whole tree of bayberry. The main variety Dongkui is highly sensitive to the twig blight disease, but the variety Zaojia is very highly resistant to the disease. Therefore, it is very necessary to clear the difference between resistant and susceptible varieties in response to the fungal infection. In this paper, we investigated the response of resistant and susceptible bayberry cultivars to infection of twig blight pathogen by histological observation and gibberellin signaling pathway-related genes expression. Microscopic observation revealed the difference in the infection process between resistant and susceptible varieties. The results of frozen scanning electron microscopy showed that the Pestalotiopsis conidia were shrunk, the mycelium was shriveled and did not extend into the cells of resistant cultivars, while the conidia were full and the top was extended, the mycelia was normal and continued to extend to the cells of a susceptible cultivar. Indeed, the medulla cells were almost intact in resistant cultivar, but obviously damaged in susceptible cultivar after inoculation of the main fungal pathogen P. versicolor conidia, which is earlier germinated on sterile glass slide than that of a hard plastic slide. The quantitative real-time PCR results showed a significant difference between resistant and susceptible cultivars in the expression of gibberellin signaling pathway-related genes in leaves and stems of bayberry, which is closely related to infection time, the type of genes and varieties. Overall, this study provides a clue for our understanding of the resistance mechanism of bayberry against the twig blight disease.

Endogenous gibberellins affect root nodule symbiosis via transcriptional regulation of NODULE INCEPTION in Lotus japonicus

Legumes and nitrogen-fixing rhizobial bacteria establish root nodule symbiosis, which is orchestrated by several plant hormones. Exogenous addition of biologically active gibberellic acid (GA) is known to inhibit root nodule symbiosis. However, the precise role of GA has not been elucidated because of the trace amounts of these hormones in plants and the multiple functions of GAs. Here, we found that GA signaling acts as a key regulator in a long-distance negative-feedback system of root nodule symbiosis called autoregulation of nodulation (AON). GA biosynthesis is activated during nodule formation in and around the nodule vascular bundles, and bioactive GAs accumulate in the nodule. In addition, GA signaling induces expression of the symbiotic transcription factor NODULE INCEPTION (NIN) via a cis-acting region on the NIN promoter. Mutants with deletions of this cis-acting region have increased susceptibility to rhizobial infection and reduced GA-induced CLE-RS1 and CLE-RS2 expression, suggesting that the inhibitory effect of GAs occurs through AON. This is supported by the GA-insensitive phenotypes of an AON-defective mutant of HYPERNODULATION ABERRANT ROOT FORMATION1 (HAR1) and a reciprocal grafting experiment. Thus, endogenous GAs induce NIN expression via its GA-responsive cis-acting region, and subsequently the GA-induced NIN activates the AON system to regulate nodule formation.

Symbiotic Regulatory Genes Controlling Nodule Development in Pisum sativum L

Analyses of natural variation and the use of mutagenesis and molecular-biological approaches have revealed 50 symbiotic regulatory genes in pea (Pisum sativum L.). Studies of genomic synteny using model legumes, such as Medicago truncatula Gaertn. and Lotus japonicus (Regel) K. Larsen, have identified the sequences of 15 symbiotic regulatory genes in pea. These genes encode receptor kinases, an ion channel, a calcium/calmodulin-dependent protein kinase, transcription factors, a metal transporter, and an enzyme. This review summarizes and describes mutant alleles, their phenotypic manifestations, and the functions of all identified symbiotic regulatory genes in pea. Some examples of gene interactions are also given. In the review, all mutant alleles in genes with identified sequences are designated and still-unidentified symbiotic regulatory genes of great interest are considered. The identification of these genes will help elucidate additional components involved in infection thread growth, nodule primordium development, bacteroid differentiation and maintenance, and the autoregulation of nodulation. The significance of symbiotic mutants of pea as extremely fruitful genetic models for studying nodule development and for comparative cell biology studies of legume nodules is clearly demonstrated. Finally, it is noted that many more sequences of symbiotic regulatory genes remain to be identified. Transcriptomics approaches and genome-wide sequencing could help address this challenge.

Systematic Analysis of Gibberellin Pathway Components in Medicago truncatula Reveals the Potential Application of Gibberellin in Biomass Improvement

Gibberellins (GAs), a class of phytohormones, act as an essential natural regulator of plant growth and development. Many studies have shown that GA is related to rhizobial infection and nodule organogenesis in legume species. However, thus far, GA metabolism and signaling components are largely unknown in the model legume Medicago truncatula. In this study, a genome-wide analysis of GA metabolism and signaling genes was carried out. In total 29 components, including 8 MtGA20ox genes, 2 MtGA3ox genes, 13 MtGA2ox genes, 3 MtGID1 genes, and 3 MtDELLA genes were identified in M. truncatula genome. Expression profiles revealed that most members of MtGAox, MtGID1, and MtDELLA showed tissue-specific expression patterns. In addition, the GA biosynthesis and deactivation genes displayed a feedback regulation on GA treatment, respectively. Yeast two-hybrid assays showed that all the three MtGID1s interacted with MtDELLA1 and MtDELLA2, suggesting that the MtGID1s are functional GA receptors. More importantly, M. truncatula exhibited increased plant height and biomass by ectopic expression of the MtGA20ox1, suggesting that enhanced GA response has the potential for forage improvement.

No Home without Hormones: How Plant Hormones Control Legume Nodule Organogenesis

The establishment of symbiotic nitrogen fixation requires the coordination of both nodule development and infection events. Despite the evolution of a variety of anatomical structures, nodule organs serve a common purpose in establishing a localized area that facilitates efficient nitrogen fixation. As in all plant developmental processes, the establishment of a new nodule organ is regulated by plant hormones. During nodule initiation, regulation of plant hormone signaling is one of the major targets of symbiotic signaling. We review the role of major developmental hormones in the initiation of the nodule organ and argue that the manipulation of plant hormones is a key requirement for engineering nitrogen fixation in non-legumes as the basis for improved food security and sustainability.

DELLA family duplication events lead to different selective constraints in angiosperms

Gibberellic acid (GA) is a major plant hormone involved in several biological processes from the flowering to the symbiosis with microorganisms. Thus, the GA regulation is crucial for plant biology. This regulation occurs via the DELLA proteins that belong to the GRAS transcription factor family. DELLA proteins are characterised by a DELLA N-terminal and a GRAS C-terminal domains. It is well known that DELLA activity appears after the bryophytes divergence and then evolved in the vascular plant lineages. Here we present the phylogeny of DELLA across 75 species belonging to various lineages from algae, liverworts and angiosperms. Our study confirmed two main duplication events, the first occurring before the angiosperms divergence and the other specific to the eudicots lineage. Comparative analysis of DELLA subclades in angiosperms revealed the loss in Poaceae and strong alteration in other species of the DELLA functional domain in the DELLA2 clade. In addition, molecular evolution analysis suggests that each of the clades (named DELLA1.1, DELLA1.2 and DELLA2) evolved differently but copies of each subclade are under strong purifying selection. This also suggests that, although the DELLA functional domain is altered in DELLA2, DELLA2 orthologs are still functional and operate in a different way compared to DELLA1 copies. In angiosperms, additional duplication events occurred and led to duplicate copies in species, genus or family such as in the Fabaceae subfamily Papilionoideae. This duplication led to the formation of additional paralogs in the DELLA1.2 subclade (DELLA1.2.1 and DELLA1.2.2). Interestingly, both copies appeared to be under relaxing selection revealing different evolutionary fate of the DELLA duplicated copies.

The intensity of ethylene release by soybean plants under the influence of fungicides in the early stages of legume-rhizobial symbiosis

The effect of pre-sowing treatment of soybean seeds with fungicides on the intensity of ethylene release, the processes of nodulation and nitrogen fixation in different symbiotic systems in the early stages of ontogenesis were investigated. The objects of the study were selected symbiotic systems formed with the participation of soybean (Glycine max (L.) Merr.) Diamond variety, strains Bradyrhizobium japonicum 634b (active, virulent) and 604k (inactive, highly virulent) and fungicides Maxim XL 035 PS (fludioxonil, 25 g/L, metalaxyl, 10 g/L), and Standak Top (fipronil, 250 g/L, thiophanate methyl, 225 g/L, piraclostrobin, 25 g/L). Before sowing, the seeds of soybean were treated with solutions of fungicides, calculated on the basis of one rate of expenditure of the active substance of each preparation indicated by the producer per ton of seed. One part of the seeds treated with fungicides was inoculated with rhizobium culture for 1 h (the titre of bacteria was 107 cells/mL). To conduct the research we used microbiological, physiological, biochemical methods, gas chromatography and spectrophotometry. It is found that, regardless of the effectiveness of soybean rhizobial symbiosis, the highest level of ethylene release by plants was observed in the stages of primordial leaf and first true leaf. This is due to the initial processes of nodulation – the laying of nodule primordia and the active formation of nodules on the roots of soybeans. The results show that with the participation of fungicides in different symbiotic systems, there are characteristic changes in phytohormone synthesis in the primordial leaf stage, when the nodule primordia are planted on the root system of plants. In particular, in the ineffective symbiotic system, the intensity of phytohormone release decreases, while in the effective symbiotic system it increases. At the same time, a decrease in the number of nodules on soybean roots inoculated with an inactive highly virulent rhizobia 604k strain due to the action of fungicides and an increase in their number in variants with co-treatment of fungicides and active virulent strain 634b into the stage of the second true leaf were revealed. It was shown that despite a decrease in the mass of root nodules, there is an increase in their nitrogen-fixing activity in an effective symbiotic system with the participation of fungicides in the stage of the second true leaf. The highest intensity of ethylene release in both symbiotic systems was recorded in the stage of the first true leaf, which decreased in the stage of the second true leaf and was independent of the nature of the action of the active substances of fungicides. The obtained data prove that the action of fungicides changes the synthesis of ethylene by soybean plants, as well as the processes of nodulation and nitrogen fixation, which depend on the efficiency of the formed soybean-rhizobial systems and their ability to realize their symbiotic potential under appropriate growing conditions.

Brassinosteroids Induce Strong, Dose-Dependent Inhibition of Etiolated Pea Seedling Growth Correlated with Ethylene Production

We have recently discovered that brassinosteroids (BRs) can inhibit the growth of etiolated pea seedlings dose-dependently in a similar manner to the 'triple response' induced by ethylene. We demonstrate here that the growth inhibition of etiolated pea shoots strongly correlates with increases in ethylene production, which also responds dose-dependently to applied BRs. We assessed the biological activities of two natural BRs on pea seedlings, which are excellent material as they grow rapidly, and respond both linearly and uni-phasically to applied BRs. We then compared the BRs' inhibitory effects on growth, and induction of ethylene and ACC (1-aminocyclopropane-1-carboxylic acid) production, to those of representatives of other phytohormone classes (cytokinins, auxins, and gibberellins). Auxin induced ca. 50-fold weaker responses in etiolated pea seedlings than brassinolide, and the other phytohormones induced much weaker (or opposite) responses. Following the optimization of conditions for determining ethylene production after BR treatment, we found a positive correlation between BR bioactivity and ethylene production. Finally, we optimized conditions for pea growth responses and developed a new, highly sensitive, and convenient bioassay for BR activity.

Physiological, Hormonal and Metabolic Responses of two Alfalfa Cultivars with Contrasting Responses to Drought

Alfalfa (Medicago sativa L.) is frequently constrained by environmental conditions such as drought. Within this context, it is crucial to identify the physiological and metabolic traits conferring a better performance under stressful conditions. In the current study, two alfalfa cultivars (San Isidro and Zhong Mu) with different physiological strategies were selected and subjected to water limitation conditions. Together with the physiological analyses, we proceeded to characterize the isotopic, hormone, and metabolic profiles of the different plants. According to physiological and isotopic data, Zhong Mu has a water-saver strategy, reducing water lost by closing its stomata but fixing less carbon by photosynthesis, and therefore limiting its growth under water-stressed conditions. In contrast, San Isidro has enhanced root growth to replace the water lost through transpiration due to its more open stomata, thus maintaining its biomass. Zhong Mu nodules were less able to maintain nodule N2 fixing activity (matching plant nitrogen (N) demand). Our data suggest that this cultivar-specific performance is linked to Asn accumulation and its consequent N-feedback nitrogenase inhibition. Additionally, we observed a hormonal reorchestration in both cultivars under drought. Therefore, our results showed an intra-specific response to drought at physiological and metabolic levels in the two alfalfa cultivars studied.

MtGA2ox10 encoding C20-GA2-oxidase regulates rhizobial infection and nodule development in Medicago truncatula

Gibberellin (GA) plays a controversial role in the legume-rhizobium symbiosis. Recent studies have shown that the GA level in legumes must be precisely controlled for successful rhizobial infection and nodule organogenesis. However, regulation of the GA level via catabolism in legume roots has not been reported to date. Here, we investigate a novel GA inactivating C20-GA2-oxidase gene MtGA2ox10 in Medicago truncatula. RNA sequencing analysis and quantitative polymerase chain reaction revealed that MtGA2ox10 was induced as early as 6 h post-inoculation (hpi) of rhizobia and reached peak transcript abundance at 12 hpi. Promoter::β-glucuronidase fusion showed that the promoter activity was localized in the root infection/differentiation zone during the early stage of rhizobial infection and in the vascular bundle of the mature nodule. The CRISPR/Cas9-mediated deletion mutation of MtGA2ox10 suppressed infection thread formation, which resulted in reduced development and retarded growth of nodules on the Agrobacterium rhizogenes-transformed roots. Over-expression of MtGA2ox10 in the stable transgenic plants caused dwarfism, which was rescued by GA₃ application, and increased infection thread formation but inhibition of nodule development. We conclude that MtGA2ox10 plays an important role in the rhizobial infection and the development of root nodules through fine catabolic tuning of GA in M. truncatula.

The DELLA Proteins Influence the Expression of Cytokinin Biosynthesis and Response Genes During Nodulation

The key event that initiates nodule organogenesis is the perception of bacterial signal molecules, the Nod factors, triggering a complex of responses in epidermal and cortical cells of the root. The Nod factor signaling pathway interacts with plant hormones, including cytokinins and gibberellins. Activation of cytokinin signaling through the homeodomain-containing transcription factors KNOX is essential for nodule formation. The main regulators of gibberellin signaling, the DELLA proteins are also involved in regulation of nodule formation. However, the interaction between the cytokinin and gibberellin signaling pathways is not fully understood. Here, we show in Pisum sativum L. that the DELLA proteins can activate the expression of KNOX and BELL transcription factors involved in regulation of cytokinin metabolic and response genes. Consistently, pea la cry-s (della1 della2) mutant showed reduced ability to upregulate expression of some cytokinin metabolic genes during nodulation. Our results suggest that DELLA proteins may regulate cytokinin metabolism upon nodulation.

Gibberellins Inhibit Nodule Senescence and Stimulate Nodule Meristem Bifurcation in Pea (Pisum sativum L.)

The development of nitrogen-fixing nodules formed during Rhizobium-legume symbiosis is strongly controlled by phytohormones. In this study, we investigated the effect of gibberellins (GAs) on senescence of pea (Pisum sativum) symbiotic nodules. Pea wild-type line SGE, as well as corresponding mutant lines SGEFix--1 (sym40), SGEFix--2 (sym33), SGEFix--3 (sym26), and SGEFix--7 (sym27), blocked at different stages of nodule development, were used in the study. An increase in expression of the GA2ox1 gene, encoding an enzyme involved in GA deactivation (GA 2-oxidase), and a decrease in the transcript abundance of the GA20ox1 gene, encoding one of the enzymes involved in GA biosynthesis (GA 20-oxidase), were observed in analyzed genotypes during nodule aging. A reduction in the amount of bioactive GA3 was demonstrated by immunolocalization in the early senescent mutant and wild-type lines during aging of symbiotic nodules. Down-regulated expression of senescence-associated genes encoding cysteine proteases 1 and 15a, thiol protease, bZIP transcription factor, 1-aminocyclopropane-1-carboxylate (ACC) synthase, ACC oxidase, and aldehyde oxidase was observed in the nodules of wild-type plants treated with exogenous GA3 relative to the untreated plants. GA3-treated plants also showed increases in nodule size and the nitrogen fixation zone, and decreases in the number of nodules and the senescence zone. Immunogold localization revealed higher levels of GA3 in the peribacteroid spaces in symbiosomes than in the matrix of infection threads. Furthermore, a decrease in GA3 label in mature and senescent symbiosomes in comparison with juvenile symbiosomes was observed. These results suggest a negative effect of GAs on the senescence of the pea symbiotic nodule and possible involvement of GAs in functioning of the mature nodule. Simultaneously, GA3 treatment led to nodule meristem bifurcation, indicating a possible role of GAs in nodule meristem functioning.

Cysteine-Rich Receptor-Like Kinase Gene Family Identification in the Phaseolus Genome and Comparative Analysis of Their Expression Profiles Specific to Mycorrhizal and Rhizobial Symbiosis

Receptor-like kinases (RLKs) are conserved upstream signaling molecules that regulate several biological processes, including plant development and stress adaptation. Cysteine (C)-rich receptor-like kinases (CRKs) are an important class of RLK that play vital roles in disease resistance and cell death in plants. Genome-wide analyses of CRK genes have been carried out in Arabidopsis and rice, while functional characterization of some CRKs has been carried out in wheat and tomato in addition to Arabidopsis. A comprehensive analysis of the CRK gene family in leguminous crops has not yet been conducted, and our understanding of their roles in symbiosis is rather limited. Here, we report the comprehensive analysis of the PhaseolusCRK gene family, including identification, sequence similarity, phylogeny, chromosomal localization, gene structures, transcript expression profiles, and in silico promoter analysis. Forty-six CRK homologs were identified and phylogenetically clustered into five groups. Expression analysis suggests that PvCRK genes are differentially expressed in both vegetative and reproductive tissues. Further, transcriptomic analysis revealed that shared and unique CRK genes were upregulated during arbuscular mycorrhizal and rhizobial symbiosis. Overall, the systematic analysis of the PvCRK gene family provides valuable information for further studies on the biological roles of CRKs in various Phaseolus tissues during diverse biological processes, including Phaseolus-mycorrhiza/rhizobia symbiosis.

Legume nodulation: The host controls the party

Global demand to increase food production and simultaneously reduce synthetic nitrogen fertilizer inputs in agriculture are underpinning the need to intensify the use of legume crops. The symbiotic relationship that legume plants establish with nitrogen-fixing rhizobia bacteria is central to their advantage. This plant-microbe interaction results in newly developed root organs, called nodules, where the rhizobia convert atmospheric nitrogen gas into forms of nitrogen the plant can use. However, the process of developing and maintaining nodules is resource intensive; hence, the plant tightly controls the number of nodules forming. A variety of molecular mechanisms are used to regulate nodule numbers under both favourable and stressful growing conditions, enabling the plant to conserve resources and optimize development in response to a range of circumstances. Using genetic and genomic approaches, many components acting in the regulation of nodulation have now been identified. Discovering and functionally characterizing these components can provide genetic targets and polymorphic markers that aid in the selection of superior legume cultivars and rhizobia strains that benefit agricultural sustainability and food security. This review addresses recent findings in nodulation control, presents detailed models of the molecular mechanisms driving these processes, and identifies gaps in these processes that are not yet fully explained.

The Role of Gibberellins and Brassinosteroids in Nodulation and Arbuscular Mycorrhizal Associations

Plant hormones play key roles in nodulation and arbuscular mycorrhizal (AM) associations. These two agriculturally and ecologically important symbioses enable plants to gain access to nutrients, in particular, nitrogen in the case of nodulation and phosphorous in the case of AM. Work over the past few decades has revealed how symbioses with nitrogen-fixing rhizobia, restricted almost exclusively to legumes, evolved in part from ancient AM symbioses formed by more than 80% of land plants. Although overlapping, these symbiotic programs also have important differences, including the de novo development of a new organ found only in nodulation. One emerging area of research is the role of two plant hormone groups, the gibberellins (GAs) and brassinosteroids (BRs), in the development and maintenance of these symbioses. In this review, we compare and contrast the roles of these hormones in the two symbioses, including potential interactions with other hormones. This not only focuses on legumes, most of which can host both symbionts, but also examines the role of these in AM development in non-legumes. GA acts by suppressing DELLA, and this regulatory module acts to negatively influence both rhizobial and mycorrhizal infection but appears to promote nodule organogenesis. While an overall positive role for BRs in nodulation and AM has been suggested by studies using mutants disrupted in BR biosynthesis or response, application studies indicate that BR may play a more complex role in nodulation. Given the nature of these symbioses, with events regulated both spatially and temporally, future studies should examine in more detail how GAs and BRs may influence precise events during these symbioses, including interactions with other hormone groups.

Hormone modulation of legume-rhizobial symbiosis

Leguminous plants can establish symbiotic associations with diazotropic rhizobia to form nitrogen-fixating nodules, which are classified as determinate or indeterminate based on the persistence of nodule meristem. The formation of nitrogen-fixing nodules requires coordinating rhizobial infection and root nodule organogenesis. The formation of an infection thread and the extent of nodule formation are largely under plant control, but vary with environmental conditions and the physiological state of the host plants. Many achievements in these two areas have been made in recent decades. Phytohormone signaling pathways have gradually emerged as important regulators of root nodule symbiosis. Cytokinin, strigolactones (SLs) and local accumulation of auxin can promote nodule development. Ethylene, jasmonic acid (JA), abscisic acid (ABA) and gibberellic acid (GA) all negatively regulate infection thread formation and nodule development. However, salicylic acid (SA) and brassinosteroids (BRs) have different effects on the formation of these two nodule types. Some peptide hormones are also involved in nodulation. This review summarizes recent findings on the roles of these plant hormones in legume-rhizobial symbiosis, and we propose that DELLA proteins may function as a node to integrate plant hormones to regulate nodulation.

The Art of Self-Control - Autoregulation of Plant-Microbe Symbioses

Plants interact with diverse microbes including those that result in nutrient-acquiring symbioses. In order to balance the energy cost with the benefit gained, plants employ a systemic negative feedback loop to control the formation of these symbioses. This is particularly well-understood in nodulation, the symbiosis between legumes and nitrogen-fixing rhizobia, and is known as autoregulation of nodulation (AON). However, much less is understood about the autoregulation of the ancient arbuscular mycorrhizal symbioses that form between Glomeromycota fungi and the majority of land plants. Elegant physiological studies in legumes have indicated there is at least some overlap in the genes and signals that regulate these two symbioses but there are major gaps in our understanding. In this paper we examine the hypothesis that the autoregulation of mycorrhizae (AOM) pathway shares some elements with AON but that there are also some important differences. By reviewing the current knowledge of the AON pathway, we have identified important directions for future AOM studies. We also provide the first genetic evidence that CLV2 (an important element of the AON pathway) influences mycorrhizal development in a non-legume, tomato and review the interaction of the autoregulation pathway with plant hormones and nutrient status. Finally, we discuss whether autoregulation may play a role in the relationships plants form with other microbes.

Gibberellins promote nodule organogenesis but inhibit the infection stages of nodulation

Leguminous plant roots can form a symbiosis with soil-dwelling nitrogen-fixing rhizobia, leading to the formation of a new root organ, the nodule. Successful nodulation requires co-ordination of spatially separated events in the root, including infection in the root epidermis and nodule organogenesis deep in the root cortex. We show that the hormone gibberellin plays distinct roles in these epidermal and cortical programmes. We employed a unique set of genetic material in pea that includes severely gibberellin-deficient lines and della-deficient lines that enabled us to characterize all stages of infection and nodule development. We confirmed that gibberellin suppresses infection thread formation and show that it also promotes nodule organogenesis into nitrogen-fixing organs. In both cases, this is achieved through the action of DELLA proteins. This study therefore provides a mechanism to explain how both low and high gibberellin signalling can result in reduced nodule number and reveals a clear role for gibberellin in the maturation of nodules into nitrogen-fixing organs. We also demonstrate that gibberellin acts independently of ethylene in promoting nodule development.

RNA sequencing and analysis of three Lupinus nodulomes provide new insights into specific host-symbiont relationships with compatible and incompatible Bradyrhizobium strains

Nitrogen fixation in the legume root-nodule symbiosis has a critical importance in natural and agricultural ecosystems and depends on the proper choice of the symbiotic partners. However, the genetic determinism of symbiotic specificity remains unclear. To study this process, we inoculated three Lupinus species (L. albus, L. luteus, L. mariae-josephae), belonging to the under-investigated tribe of Genistoids, with two Bradyrhizobium strains (B. japonicum, B. valentinum) presenting contrasted degrees of symbiotic specificity depending on the host. We produced the first transcriptomes (RNA-Seq) from lupine nodules in a context of symbiotic specificity. For each lupine species, we compared gene expression between functional and non-functional interactions and determined differentially expressed (DE) genes. This revealed that L. luteus and L. mariae-josephae (nodulated by only one of the Bradyrhizobium strains) specific nodulomes were richest in DE genes than L. albus (nodulation with both microsymbionts, but non-functional with B. valentinum) and share a higher number of these genes between them than with L. albus. In addition, a functional analysis of DE genes highlighted the central role of the genetic pathways controlling infection and nodule organogenesis, hormones, secondary, carbon and nitrogen metabolisms, as well as the implication of plant defence in response to compatible or incompatible Bradyrhizobium strains.

Analysis of nodule senescence in pea (Pisum sativum L.) using laser microdissection, real-time PCR, and ACC immunolocalization

A delay in the senescence of symbiotic nodules could prolong active nitrogen fixation, resulting in improved crop yield and a reduced need for chemical fertilizers. The molecular genetic mechanisms underlying nodule senescence have not been extensively studied with a view to breeding varieties with delayed nodule senescence. In such studies, plant mutants with the phenotype of premature degradation of symbiotic structures are useful models to elucidate the genetic basis of nodule senescence. Using a dataset from transcriptome analysis of Medicago truncatula Gaertn. nodules and previous studies on pea (Pisum sativum L.) nodules, we developed a set of molecular markers based on genes that are known to be activated during nodule senescence. These genes encode cysteine proteases, a thiol protease, a bZIP transcription factor, enzymes involved in the biosynthesis of ethylene (ACS2 for ACC synthase and ACO1 for ACC oxidase) and ABA (AO3 for aldehyde oxidase), and an enzyme involved in catabolism of gibberellins (GA 2-oxidase). We analyzed the transcript levels of these genes in the nodules of two pea wild-types (cv. Sparkle and line Sprint-2) and two mutant lines, one showing premature nodule senescence (E135F (sym13)) and one showing no morphological signs of symbiotic structure degradation (Sprint-2Fix- (sym31)). Real-time PCR analyses revealed that all of the selected genes showed increased transcript levels during nodule aging in all phenotypes. Remarkably, at 4 weeks after inoculation (WAI), the transcript levels of all analyzed genes were significantly higher in the early senescent nodules of the mutant line E135F (sym13) and in nodules of the mutant Sprint-2Fix- (sym31) than in the active nitrogen-fixing nodules of wild-types. In contrast, the transcript levels of the same genes of both wild-types were significantly increased only at 6 WAI. We evaluated the expression of selected markers in the different histological nodule zones of pea cv. Sparkle and its mutant line E135F (sym13) by laser capture microdissection analysis. Finally, we analyzed ACC by immunolocalization in the nodules of both wild-type pea and their mutants. Together, the results indicate that nodule senescence is a general plant response to nodule ineffectiveness.

DELLA-mediated gibberellin signalling regulates Nod factor signalling and rhizobial infection

Legumes develop symbiotic interactions with rhizobial bacteria to form nitrogen-fixing nodules. Bacterial Nod factors (NFs) and plant regulatory pathways modulating NF signalling control rhizobial infections and nodulation efficiency. Here we show that gibberellin (GA) signalling mediated by DELLA proteins inhibits rhizobial infections and controls the NF induction of the infection marker ENOD11 in Medicago truncatula. Ectopic expression of a constitutively active DELLA protein in the epidermis is sufficient to promote ENOD11 expression in the absence of symbiotic signals. We show using heterologous systems that DELLA proteins can interact with the nodulation signalling pathway 2 (NSP2) and nuclear factor-YA1 (NF-YA1) transcription factors that are essential for the activation of NF responses. Furthermore, MtDELLA1 can bind the ERN1 (ERF required for nodulation 1) promoter and positively transactivate its expression. Overall, we propose that GA-dependent action of DELLA proteins may directly regulate the NSP1/NSP2 and NF-YA1 activation of ERN1 transcription to regulate rhizobial infections.

DELLA proteins are common components of symbiotic rhizobial and mycorrhizal signalling pathways

Legumes form symbiotic associations with either nitrogen-fixing bacteria or arbuscular mycorrhizal fungi. Formation of these two symbioses is regulated by a common set of signalling components that act downstream of recognition of rhizobia or mycorrhizae by host plants. Central to these pathways is the calcium and calmodulin-dependent protein kinase (CCaMK)-IPD3 complex which initiates nodule organogenesis following calcium oscillations in the host nucleus. However, downstream signalling events are not fully understood. Here we show that Medicago truncatula DELLA proteins, which are the central regulators of gibberellic acid signalling, positively regulate rhizobial symbiosis. Rhizobia colonization is impaired in della mutants and we provide evidence that DELLAs can promote CCaMK-IPD3 complex formation and increase the phosphorylation state of IPD3. DELLAs can also interact with NSP2-NSP1 and enhance the expression of Nod-factor-inducible genes in protoplasts. We show that DELLA is able to bridge a protein complex containing IPD3 and NSP2. Our results suggest a transcriptional framework for regulation of root nodule symbiosis.

Rhizobial gibberellin negatively regulates host nodule number

In legume-rhizobia symbiosis, the nodule number is controlled to ensure optimal growth of the host. In Lotus japonicus, the nodule number has been considered to be tightly regulated by host-derived phytohormones and glycopeptides. However, we have discovered a symbiont-derived phytohormonal regulation of nodule number in Mesorhizobium loti. In this study, we found that M. loti synthesized gibberellic acid (GA) under symbiosis. Hosts inoculated with a GA-synthesis-deficient M. loti mutant formed more nodules than those inoculated with the wild-type form at four weeks post inoculation, indicating that GA from already-incorporated rhizobia prevents new nodule formation. Interestingly, the genes for GA synthesis are only found in rhizobial species that inhabit determinate nodules. Our findings suggest that the already-incorporated rhizobia perform GA-associated negative regulation of nodule number to prevent delayed infection by other rhizobia.

Down-regulated Lotus japonicus GCR1 plants exhibit nodulation signalling pathways alteration

G Protein Coupled Receptor (GPCRs) are integral membrane proteins involved in various signalling pathways by perceiving many extracellular signals and transducing them to heterotrimeric G proteins, which further transduce these signals to intracellular downstream effectors. GCR1 is the only reliable plant candidate as a member of the GPCRs superfamily. In the legume/rhizobia symbiotic interaction, G proteins are involved in signalling pathways controlling different steps of the nodulation program. In order to investigate the putative hierarchic role played by GCR1 in these symbiotic pathways we identified and characterized the Lotus japonicus gene encoding the seven transmembrane GCR1 protein. The detailed molecular and topological analyses of LjGCR1 expression patterns that are presented suggest a possible involvement in the early steps of nodule organogenesis. Furthermore, phenotypic analyses of independent transgenic RNAi lines, showing a significant LjGCR1 expression down regulation, suggest an epistatic action in the control of molecular markers of nodulation pathways, although no macroscopic symbiotic phenotypes could be revealed.

Interactions between ethylene, gibberellins, and brassinosteroids in the development of rhizobial and mycorrhizal symbioses of pea

The regulation of arbuscular mycorrhizal development and nodulation involves complex interactions between the plant and its microbial symbionts. In this study, we use the recently identified ethylene-insensitive ein2 mutant in pea (Pisum sativum L.) to explore the role of ethylene in the development of these symbioses. We show that ethylene acts as a strong negative regulator of nodulation, confirming reports in other legumes. Minor changes in gibberellin1 and indole-3-acetic acid levels in ein2 roots appear insufficient to explain the differences in nodulation. Double mutants produced by crosses between ein2 and the severely gibberellin-deficient na and brassinosteroid-deficient lk mutants showed increased nodule numbers and reduced nodule spacing compared with the na and lk single mutants, but nodule numbers and spacing were typical of ein2 plants, suggesting that the reduced number of nodules innaandlkplants is largely due to the elevated ethylene levels previously reported in these mutants. We show that ethylene can also negatively regulate mycorrhizae development when ethylene levels are elevated above basal levels, consistent with a role for ethylene in reducing symbiotic development under stressful conditions. In contrast to the hormone interactions in nodulation, ein2 does not override the effect of lk or na on the development of arbuscular mycorrhizae, suggesting that brassinosteroids and gibberellins influence this process largely independently of ethylene.

Ethylene, a Hormone at the Center-Stage of Nodulation

Nodulation is the result of a beneficial interaction between legumes and rhizobia. It is a sophisticated process leading to nutrient exchange between the two types of symbionts. In this association, within a nodule, the rhizobia, using energy provided as photosynthates, fix atmospheric nitrogen and convert it to ammonium which is available to the plant. Nodulation is recognized as an essential process in nitrogen cycling and legume crops are known to enrich agricultural soils in nitrogenous compounds. Furthermore, as they are rich in nitrogen, legumes are considered important as staple foods for humans and fodder for animals. To tightly control this association and keep it mutualistic, the plant uses several means, including hormones. The hormone ethylene has been known as a negative regulator of nodulation for almost four decades. Since then, much progress has been made in the understanding of both the ethylene signaling pathway and the nodulation process. Here I have taken a large view, using recently obtained knowledge, to describe in some detail the major stages of the process. I have not only reviewed the steps most commonly covered (the common signaling transduction pathway, and the epidermal and cortical programs), but I have also looked into steps less understood (the pre-infection step with the plant defense response, the bacterial release and the formation of the symbiosome, and nodule functioning and senescence). After a succinct review of the ethylene signaling pathway, I have used the knowledge obtained from nodulation- and ethylene-related mutants to paint a more complete picture of the role played by the hormone in nodule organogenesis, functioning, and senescence. It transpires that ethylene is at the center of this effective symbiosis. It has not only been involved in most of the steps leading to a mature nodule, but it has also been implicated in host immunity and nodule senescence. It is likely responsible for the activation of other hormonal signaling pathways. I have completed the review by citing three studies which makes one wonder whether knowledge gained on nodulation in the last decades is ready to be transferred to agricultural fields.

Early vigour improves phosphate uptake in wheat

Quantitative trait loci (QTLs) for shoot biomass were identified in wheat grown on a soil high in total phosphorus (P) but low in plant-available P. The two populations screened included recombinant inbred lines (RILs) from Chuan-Mai 18/Vigour 18 and doubled-haploid lines from Kukri/Janz. Glasshouse-grown plants were harvested at the five-leaf stage. Seven QTLs for shoot biomass were identified in the RILs, with the largest on chromosome 7A accounting for 7.4% of the phenotypic variance. RILs from the upper tail had larger embryos than RILs from the lower tail. Tail lines were then grown in non-limiting P and the results indicated that early vigour and the capacity to access P contributed to the initial distribution. The influence of early vigour on P nutrition was examined further with advanced vigour lines (AVLs). The AVLs accumulated more shoot biomass, maintained lower shoot P concentrations, and showed greater P-acquisition efficiency than Vigour 18. Nine QTLs for shoot biomass were identified in the Kukri/Janz population. Two on chromosomes 4B and 4D accounted for 24.8% of the variance. Candidates underlying these QTLs are the Rht genes. We confirmed the influence of these genes using near-isogenic lines with different Rht alleles. The dwarf and semi-dwarf alleles affected shoot and root biomass at high and low P but not the efficiency of P acquisition. We conclude that early vigour contributed to the distributions in both populations. Early vigour can increase plant growth at suboptimal P and some sources can also improve the efficiency of P acquisition.

Hormonal Control of Lateral Root and Nodule Development in Legumes

Many plants can establish symbioses with nitrogen-fixing bacteria, some of which lead to nodulation, including legumes. Indeed, in the rhizobium/legume symbiosis, new root organs, called nodules, are formed by the plant in order to host the rhizobia in protective conditions, optimized for nitrogen fixation. In this way, these plants can benefit from the reduction of atmospheric dinitrogen into ammonia by the hosted bacteria, and in exchange the plant provides the rhizobia with a carbon source. Since this symbiosis is costly for the plant it is highly regulated. Both legume nodule and lateral root organogenesis involve divisions of the root inner tissues, and both developmental programs are tightly controlled by plant hormones. In fact, most of the major plant hormones, such as auxin, cytokinins, abscisic acid, and strigolactones, control both lateral root formation and nodule organogenesis, but often in an opposite manner. This suggests that the sensitivity of legume plants to some phytohormones could be linked to the antagonism that exists between the processes of nodulation and lateral root formation. Here, we will review the implication of some major phytohormones in lateral root formation in legumes, compare them with their roles in nodulation, and discuss specificities and divergences from non-legume eudicot plants such as Arabidopsis thaliana.

Role of gibberellins during arbuscular mycorrhizal formation in tomato: new insights revealed by endogenous quantification and genetic analysis of their metabolism in mycorrhizal roots

Gibberellins (GAs) are key regulators of plant growth and development and recent studies suggest also a role during arbuscular mycorrhizal (AM) formation. Here, complementary approaches have been used to obtain a clearer picture that correlates AM fungal development inside roots with GA metabolism. An extensive analysis of genes associated with GA metabolism as well as a quantification of GA content in roots was made. Application of GA3 and its biosynthesis inhibitor prohexadione calcium (PrCa) combined with a GA-constitutive response mutant (procera) were used to determine whether fungal colonization is altered by the level of these hormones or by changes in the GA-signaling pathway. The increased levels of specific GAs from the 13-hydroxylation pathway in mycorrhizal roots correlate closely with the increased expression of genes coding enzymes from the GA biosynthetic trail. The imbalance of GAs in tomato roots caused by exogenous applications of GA3 or PrCa affects arbuscules in both negative and positive ways, respectively. In addition, procera plants were adversely affected by the mycorrhization process. Our findings demonstrate that an imbalance in favor of an increased amount of GAs negatively affects the frequency of mycorrhization and particularly the arbuscular abundance in tomato mycorrhizal roots and the results point out that AM formation is associated with a change in the 13-hydroxylation pathway of GAs.

Gibberellins interfere with symbiosis signaling and gene expression and alter colonization by arbuscular mycorrhizal fungi in Lotus japonicus

Arbuscular mycorrhiza is a mutualistic plant-fungus interaction that confers great advantages for plant growth. Arbuscular mycorrhizal (AM) fungi enter the host root and form symbiotic structures that facilitate nutrient supplies between the symbionts. The gibberellins (GAs) are phytohormones known to inhibit AM fungal infection. However, our transcriptome analysis and phytohormone quantification revealed GA accumulation in the roots of Lotus japonicus infected with AM fungi, suggesting that de novo GA synthesis plays a role in arbuscular mycorrhiza development. We found pleiotropic effects of GAs on the AM fungal infection. In particular, the morphology of AM fungal colonization was drastically altered by the status of GA signaling in the host root. Exogenous GA treatment inhibited AM hyphal entry into the host root and suppressed the expression of Reduced Arbuscular Mycorrhization1 (RAM1) and RAM2 homologs that function in hyphal entry and arbuscule formation. On the other hand, inhibition of GA biosynthesis or suppression of GA signaling also affected arbuscular mycorrhiza development in the host root. Low-GA conditions suppressed arbuscular mycorrhiza-induced subtilisin-like serine protease1 (SbtM1) expression that is required for AM fungal colonization and reduced hyphal branching in the host root. The reduced hyphal branching and SbtM1 expression caused by the inhibition of GA biosynthesis were recovered by GA treatment, supporting the theory that insufficient GA signaling causes the inhibitory effects on arbuscular mycorrhiza development. Most studies have focused on the negative role of GA signaling, whereas our study demonstrates that GA signaling also positively interacts with symbiotic responses and promotes AM colonization of the host root.

Role of ethylene in responses of plants to nitrogen availability

Ethylene is a plant hormone involved in several physiological processes and regulates the plant development during the whole life. Stressful conditions usually activate ethylene biosynthesis and signaling in plants. The availability of nutrients, shortage or excess, influences plant metabolism and ethylene plays an important role in plant adaptation under suboptimal conditions. Among the plant nutrients, the nitrogen (N) is one the most important mineral element required for plant growth and development. The availability of N significantly influences plant metabolism, including ethylene biology. The interaction between ethylene and N affects several physiological processes such as leaf gas exchanges, roots architecture, leaf, fruits, and flowers development. Low plant N use efficiency (NUE) leads to N loss and N deprivation, which affect ethylene biosynthesis and tissues sensitivity, inducing cell damage and ultimately lysis. Plants may respond differently to N availability balancing ethylene production through its signaling network. This review discusses the recent advances in the interaction between N availability and ethylene at whole plant and different organ levels, and explores how N availability induces ethylene biology and plant responses. Exogenously applied ethylene seems to cope the stress conditions and improves plant physiological performance. This can be explained considering the expression of ethylene biosynthesis and signaling genes under different N availability. A greater understanding of the regulation of N by means of ethylene modulation may help to increase NUE and directly influence crop productivity under conditions of limited N availability, leading to positive effects on the environment. Moreover, efforts should be focused on the effect of N deficiency or excess in fruit trees, where ethylene can have detrimental effects especially during postharvest.

Mechanistic action of gibberellins in legume nodulation

Legume plants are capable of entering into a symbiotic relationship with rhizobia bacteria. This results in the formation of novel organs on their roots, called nodules, in which the bacteria capture atmospheric nitrogen and provide it as ammonium to the host plant. Complex molecular and physiological changes are involved in the formation and establishment of such nodules. Several phytohormones are known to play key roles in this process. Gibberellins (gibberellic acids; GAs), a class of phytohormones known to be involved in a wide range of biological processes (i.e., cell elongation, germination) are reported to be involved in the formation and maturation of legume nodules, highlighted by recent transcriptional analyses of early soybean symbiotic steps. Here, we summarize what is currently known about GAs in legume nodulation and propose a model of GA action during nodule development. Results from a wide range of studies, including GA application, mutant phenotyping, and gene expression studies, indicate that GAs are required at different stages, with an optimum, tightly regulated level being key to achieve successful nodulation. Gibberellic acids appear to be required at two distinct stages of nodulation: (i) early stages of rhizobia infection and nodule primordium establishment; and (ii) later stages of nodule maturation.