Nodulating Legumes Are Distinguished by a Sensitivity to Cytokinin in the Root Cortex Leading to Pseudonodule Development

Symbiont effector-guided mapping of proteins in plant networks to improve crop climate stress resilience: Symbiont effectors inform highly interconnected plant protein networks and provide an untapped resource for crop climate resilience strategies

There is an urgent need for novel protection strategies to sustainably secure crop production under changing climates. Studying microbial effectors, defined as microbe-derived proteins that alter signalling inside plant cells, has advanced our understanding of plant immunity and microbial plant colonisation strategies. Our understanding of effectors in the establishment and beneficial outcome of plant symbioses is less well known. Combining functional and comparative interaction assays uncovered specific symbiont effector targets in highly interconnected plant signalling networks and revealed the potential of effectors in beneficially modulating plant traits. The diverse functionality of symbiont effectors differs from the paradigmatic immuno-suppressive function of pathogen effectors. These effectors provide solutions for improving crop resilience against climate stress by their evolution-driven specification in host protein targeting and modulation. Symbiont effectors represent stringent tools not only to identify genetic targets for crop breeding, but to serve as applicable agents in crop management strategies under changing environments.

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.

Light-sensitive short hypocotyl genes confer symbiotic nodule identity in the legume Medicago truncatula

Legumes produce specialized root nodules that are distinct from lateral roots in morphology and function, with nodules intracellularly hosting nitrogen-fixing bacteria. We have previously shown that a lateral root program underpins nodule initiation, but there must be additional developmental regulators that confer nodule identity. Here, we show two members of the LIGHT-SENSITIVE SHORT HYPOCOTYL (LSH) transcription factor family, predominantly known to define shoot meristem complexity and organ boundaries, function as regulators of nodule organ identity. In parallel to the root initiation program, LSH1/LSH2 recruit a program into the root cortex that mediates the divergence into nodules, in particular with cell divisions in the mid-cortex. This includes regulation of auxin and cytokinin, promotion of NODULE ROOT1/2 and Nuclear Factor YA1, and suppression of the lateral root program. A principal outcome of LSH1/LSH2 function is the production of cells able to accommodate nitrogen-fixing bacteria, a key feature unique to nodules.

Innovations in two genes kickstarted the evolution of nitrogen-fixing nodules

The root nodule symbiosis between plants and nitrogen-fixing bacteria is a fascinating trait limited to several plant species. Given the agronomic potential of transferring this symbiosis to nonleguminous crops, the symbiosis has attracted researchers' attention for over a century. The origins of this symbiosis can be traced back to a single ancestor, around 110 million years ago. Recent findings have uncovered that adaptations in a receptor complex and the recruitment of the transcription factor Nodule Inception (NIN) are among the first genetic adaptations that allowed this ancestor to respond to its microsymbiont. Understanding the consequences of recruiting these genes provides insights into the start of this complex genetic trait.

NIN-at the heart of NItrogen-fixing Nodule symbiosis

Legumes and actinorhizal plants establish symbiotic relationships with nitrogen-fixing bacteria, resulting in the formation of nodules. Nodules create an ideal environment for nitrogenase to convert atmospheric nitrogen into biological available ammonia. NODULE INCEPTION (NIN) is an indispensable transcription factor for all aspects of nodule symbiosis. Moreover, NIN is consistently lost in non-nodulating species over evolutions. Here we focus on recent advances in the signaling mechanisms of NIN during nodulation and discuss the role of NIN in the evolution of nitrogen-fixing nodule symbiosis.

The PtdIns3P phosphatase MtMP promotes symbiotic nitrogen fixation via mitophagy in Medicago truncatula

Symbiotic nitrogen fixation is a complex process in which legumes interact with rhizobia under nitrogen starvation. In this study, we found that myotubularin phosphatase (MtMP) is mainly expressed in roots and nodules in Medicago truncatula. MtMP promotes autophagy by dephosphorylating PtdIns3P on autophagosomes. The mp mutants inoculated with rhizobia showed a significant reduction in nitrogenase activity and significantly higher number of mitochondria than those of wild-type plants under nitrogen starvation, indicating that MtMP is involved in mitophagy of the infection zone. Mitophagy may provide carbon skeletons and nitrogen for the development of bacteroids and the reprogramming of infected cells. In conclusion, we found, for the first time, that myotubularin phosphatase is involved in autophagy in plants. MtMP-involved autophagy plays an active role in symbiotic nitrogen fixation. These results deepen our understanding of symbiotic nitrogen fixation.

An emerging role of heterotrimeric G-proteins in nodulation and nitrogen sensing

MAIN CONCLUSION

A comprehensive understanding of nitrogen signaling cascades involving heterotrimeric G-proteins and their putative receptors can assist in the production of nitrogen-efficient plants. Plants are immobile in nature, so they must endure abiotic stresses including nutrient stress. Plant development and agricultural productivity are frequently constrained by the restricted availability of nitrogen in the soil. Non-legume plants acquire nitrogen from the soil through root membrane-bound transporters. In depleted soil nitrogen conditions, legumes are naturally conditioned to fix atmospheric nitrogen with the aid of nodulation elicited by nitrogen-fixing bacteria. Moreover, apart from the symbiotic nitrogen fixation process, nitrogen uptake from the soil can also be a significant secondary source to satisfy the nitrogen requirements of legumes. Heterotrimeric G-proteins function as molecular switches to help plant cells relay diverse stimuli emanating from external stress conditions. They are comprised of Gα, Gβ and Gγ subunits, which cooperate with several downstream effectors to regulate multiple plant signaling events. In the present review, we concentrate on signaling mechanisms that regulate plant nitrogen nutrition. Our review highlights the potential of heterotrimeric G-proteins, together with their putative receptors, to assist the legume root nodule symbiosis (RNS) cascade, particularly during calcium spiking and nodulation. Additionally, the functions of heterotrimeric G-proteins in nitrogen acquisition by plant roots as well as in improving nitrogen use efficiency (NUE) have also been discussed. Future research oriented towards heterotrimeric G-proteins through genome editing tools can be a game changer in the enhancement of the nitrogen fixation process. This will foster the precise manipulation and production of plants to ensure global food security in an era of climate change by enhancing crop productivity and minimizing reliance on external inputs.

The Medicago SymCEP7 hormone increases nodule number via shoots without compromising lateral root number

Legumes acquire soil nutrients through nitrogen-fixing root nodules and lateral roots. To balance the costs and benefits of nodulation, legumes negatively control root nodule number by autoregulatory and hormonal pathways. How legumes simultaneously coordinate root nodule and lateral root development to procure nutrients remains poorly understood. In Medicago (Medicago truncatula), a subset of mature C-TERMINALLY ENCODED PEPTIDE (CEP) hormones can systemically promote nodule number, but all CEP hormones tested to date negatively regulate lateral root number. Here we showed that Medicago CEP7 produces a mature peptide, SymCEP7, that promotes nodulation from the shoot without compromising lateral root number. Rhizobial inoculation induced CEP7 in the susceptible root nodulation zone in a Nod factor-dependent manner, and, in contrast to other CEP genes, its transcription level was elevated in the ethylene signaling mutant sickle. Using mass spectrometry, fluorescence microscopy and expression analysis, we demonstrated that SymCEP7 activity requires the COMPACT ROOT ARCHITECTURE 2 receptor and activates the shoot-to-root systemic effector, miR2111. Shoot-applied SymCEP7 rapidly promoted nodule number in the pM to nM range at concentrations up to five orders of magnitude lower than effects mediated by root-applied SymCEP7. Shoot-applied SymCEP7 also promoted nodule number in White Clover (Trifolium repens) and Lotus (Lotus japonicus), which suggests that this biological function may be evolutionarily conserved. We propose that SymCEP7 acts in the Medicago shoot to counter balance the autoregulation pathways induced rapidly by rhizobia to enable nodulation without compromising lateral root growth, thus promoting the acquisition of nutrients other than nitrogen to support their growth.

Photolysis of caged cytokinin in single cells of Arabidopsis thaliana

BACKGROUND

Cytokinins are a class of phytohormone that play a crucial role in the development of plants. They are involved in the regulation of nearly every aspect of plant growth, from germination to senescence. The role of cytokinins in many developmental programs is complex and varies both spatially and temporally. Current techniques used to investigate the functions of cytokinins in plant development lack this spatial and temporal resolution required to observe cell-type specific effects.

RESULTS

To this end, we present a method of activating a caged cytokinin in single cells. A caged benzyladenine was synthesized, along with caged adenine as a negative control. In vitro testing confirmed ultraviolet light-mediated uncaging, and subsequent root growth assays demonstrated that uncaging produced a cytokinin phenotype. This uncaging was confined to single cells using multiphoton confocal microscopy. Using an Arabidopsis thaliana cytokinin reporter line expressing TCSn::GFP, the resulting GFP expression was confined to the uncaging region, including in single cells. This study presents a novel cell-targeted method of cytokinin delivery, which has the potential to elucidate a broad range of processes in plant development.

CONCLUSIONS

We combined multiphoton confocal microscopy and a caged cytokinin treatment, allowing cell type-specific uncaging of a cytokinin in Arabidopsis roots.

MAX2-dependent competence for callus formation and shoot regeneration from Arabidopsis thaliana root explants

Although the division of the pericycle cells initiates both lateral root development and root-derived callus formation, these developmental processes are affected differently in the strigolactone and karrikin/KARRIKIN INSENSITIVE 2 (KAI2) ligand signalling mutant more axillary growth 2 (max2). Whereas max2 produces more lateral roots than the wild type, it is defective in the regeneration of shoots from root explants. We suggest that the decreased shoot regeneration of max2 originates from delayed formation of callus primordium, yielding less callus material to regenerate shoots. Indeed, when incubated on callus-inducing medium, the pericycle cell division was reduced in max2 and the early gene expression varied when compared with the wild type, as determined by a transcriptomics analysis. Furthermore, the expression of the LATERAL ORGAN BOUNDARIES DOMAIN genes and of callus-induction genes was modified in correlation with the max2 phenotype, suggesting a role for MAX2 in the regulation of the interplay between cytokinin, auxin, and light signalling in callus initiation. Additionally, we found that the in vitro shoot regeneration phenotype of max2 might be caused by a defect in KAI2, rather than in DWARF14, signalling. Nevertheless, the shoot regeneration assays revealed that the strigolactone biosynthesis mutants max3 and max4 also play a minor role.

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.

Are legumes different? Origins and consequences of evolving nitrogen fixing symbioses

Nitrogen fixing symbioses between plants and bacteria are ancient and, while not numerous, are formed in diverse lineages of plants ranging from microalgae to angiosperms. One symbiosis stands out as the most widespread one is that between legumes and rhizobia, leading to the formation of nitrogen-fixing nodules. The legume family is one of the largest and most diverse group of plants and legumes have been used by humans since the beginning of agriculture, both as high nitrogen food, as well as pastures and rotation crops. One open question is whether their ability to form a nitrogen-fixing symbiosis has contributed to legumes' success, and whether legumes have any unique characteristics that have made them more diverse and widespread than other groups of plants. This review examines the evolutionary journey that has led to the diversification of legumes, in particular its nitrogen-fixing symbiosis, and asks four questions to investigate which legume traits might have contributed to their success: 1. In what ways do legumes differ from other plant groups that have evolved nitrogen-fixing symbioses? In order to answer this question, the characteristics of the symbioses, and efficiencies of nitrogen fixation are compared between different groups of nitrogen fixing plants. 2. Could certain unique features of legumes be a reason for their success? This section examines the manifestations and possible benefits of a nitrogen-rich 'lifestyle' in legumes. 3. If nitrogen fixation was a reason for such a success, why have some species lost the symbiosis? Formation of symbioses has trade-offs, and while these are less well known for non-legumes, there are known energetic and ecological reasons for loss of symbiotic potential in legumes. 4. What can we learn from the unique traits of legumes for future crop improvements? While exploiting some of the physiological properties of legumes could be used to improve legume breeding, our increasing molecular understanding of the essential regulators of root nodule symbioses raise hope of creating new nitrogen fixing symbioses in other crop species.

A rulebook for peptide control of legume-microbe endosymbioses

Plants engage in mutually beneficial relationships with microbes, such as arbuscular mycorrhizal fungi or nitrogen-fixing rhizobia, for optimized nutrient acquisition. In return, the microbial symbionts receive photosynthetic carbon from the plant. Both symbioses are regulated by the plant nutrient status, indicating the existence of signaling pathways that allow the host to fine-tune its interactions with the beneficial microbes depending on its nutrient requirements. Peptide hormones coordinate a plethora of developmental and physiological processes and, recently, various peptide families have gained special attention as systemic and local regulators of plant-microbe interactions and nutrient homeostasis. In this review, we identify five 'rules' or guiding principles that govern peptide function during symbiotic plant-microbe interactions, and highlight possible points of integration with nutrient acquisition pathways.

Drought Stress Exacerbates Fungal Colonization and Endodermal Invasion and Dampens Defense Responses to Increase Dry Root Rot in Chickpea

Drought plays a central role in increasing the incidence and severity of dry root rot (DRR) disease in chickpea. This is an economically devastating disease, compromising chickpea yields particularly severely in recent years due to erratic rainfall patterns. Macrophomina phaseolina (formerly Rhizoctonia bataticola) is the causal agent of DRR disease in the chickpea plant. The infection pattern in chickpea roots under well-watered conditions and drought stress are poorly understood at present. This study provides detailed disease symptomatology and the characteristics of DRR fungus at morphological and molecular levels. Using microscopy techniques, the infection pattern of DRR fungus in susceptible chickpea roots was investigated under well-watered and drought-stress conditions. Our observations suggested that drought stress intensifies the progression of already ongoing infection by weakening the endodermal barrier and overall defense. Transcriptomic analysis suggested that the plant's innate immune defense program is downregulated in infected roots when subjected to drought stress. Furthermore, genes involved in hormonal regulation are differentially expressed under drought stress. These findings provide hints in terms of potential chickpea genes to target in crop improvement programs to develop climate-change-resilient cultivars.[Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

At the Root of Nodule Organogenesis: Conserved Regulatory Pathways Recruited by Rhizobia

The interaction between legume plants and soil bacteria rhizobia results in the formation of new organs on the plant roots, symbiotic nodules, where rhizobia fix atmospheric nitrogen. Symbiotic nodules represent a perfect model to trace how the pre-existing regulatory pathways have been recruited and modified to control the development of evolutionary "new" organs. In particular, genes involved in the early stages of lateral root development have been co-opted to regulate nodule development. Other regulatory pathways, including the players of the KNOX-cytokinin module, the homologues of the miR172-AP2 module, and the players of the systemic response to nutrient availability, have also been recruited to a unique regulatory program effectively governing symbiotic nodule development. The role of the NIN transcription factor in the recruitment of such regulatory modules to nodulation is discussed in more details.

Evolutionary and functional analysis of two-component system in chickpea reveals CaRR13, a TypeB RR, as positive regulator of symbiosis

The critical role of cytokinin in early nodulation in legumes is well known. In our study, exogenous cytokinin application to roots of the important crop legume, chickpea (Cicer arietinum L.), led to the formation of pseudo-nodules even in the absence of rhizobia. Hence, a genome-wide analysis of the cytokinin signalling, two-component system (TCS) genes, was conducted in chickpea, Medicago and Cajanus cajan. The integrated phylogenetic, evolutionary and expression analysis of the TCS genes was carried out, which revealed that histidine kinases (HKs) were highly conserved, whereas there was diversification leading to neofunctionalization at the level of response regulators (RRs) especially the TypeB RRs. Further, the functional role of the CaHKs in nodulation was established by complementation of the sln1Δ mutant of yeast and cre1 mutants of (Medicago) which led to restoration of the nodule-deficient phenotype. Additionally, the highest expressing TypeB RR of chickpea, CaRR13, was functionally characterized. Its localization in the nucleus and its Y1H assay-based interaction with the promoter of the early nodulation gene CaNSP2 indicated its role as a transcription factor regulating early nodulation. Overexpression, RNAi lines and complementation of cre1 mutants with CaRR13 revealed its critical involvement as an important signalling molecule regulating early events of nodule organogenesis in chickpea.

Role of hydroxymethylglutaryl-coenzyme A (HMG-CoA) reductase 1 in nodule development of soybean

Autoregulation of nodulation (AON) plays a central role in nodulation by inhibiting the formation of excess number of legume root nodules. In this study, the effect of hydroxymethylglutaryl-coenzyme A reductase 1 (GmHMGR1) gene expression on nodulation and the AON system in Glycine max (L.) Merr was investigated. Wild-type soybean (cultivar Bragg) and its near-isogenic supernodulating mutant (nitrate tolerant symbiotic) nts1007 were selected to identify the expression pattern of this gene in rootlets after inoculation by its microsymbiont Bradyrhizobium. For further analysis, the full length of GmHMGR1 and its promoter were cloned after amplification by inverse-PCR and BAC library screening. Also, we constructed an intron hairpin RNA interference (ihpRNAi) and a GmHMGR1 promoter: β-glucuronidase fusion constructs, consequently for suppression of GmHMGR1 and histochemical analysis in transgenic soybean hairy roots induced by Agrobacterium rhizogenes strain K599. The GmHMGR1 gene was functional during the early stages of nodulation with the AON system having a negative effect on GmHMGR1 expression and nodule formation in wild-type rootlets. GmHMGR1 was particularly expressed in the developing phloem within the root, nodules and nodule lenticels. Expression of GmHMGR1 in transgenic hairy roots was suppressed by RNAi silencing approximately 85% as compared to empty vector controls. This suggests that the GmHMGR1 gene has an important role in triggering nodule formation as its suppression caused a reduction of nodule formation in nts mutant lines with a deficient AON system.

Evolutionary and expression dynamics of LRR-RLKs and functional establishment of KLAVIER homolog in shoot mediated regulation of AON in chickpea symbiosis

Chickpea shoot exogenously treated with cytokinin showed stunted phenotype of root, shoot and significantly reduced nodule numbers. Genome-wide identification of LRR-RLKs in chickpea and Medicago resulted in 200 and 371 genes respectively. Gene duplication analysis revealed that LRR-RLKs family expanded through segmental duplications in chickpea and tandem duplications in Medicago. Expression profiling of LRR-RLKs revealed their involvement in cytokinin signaling and plant organ development. Overexpression of KLAVIER ortholog of chickpea, Ca_LRR-RLK147, in roots revealed its localization in the membrane but showed no effect on root nodulation despite increased cle peptide levels. Two findings (i) drastic effect on nodule number by exogenous cytokinin treatment to only shoot and restoration to normal nodulation by treatment to both root and shoot tissue and (ii) no effect on nodule number by overexpression of Ca_LRR-RLK147 establishes the fact that despite presence of cle peptides in root, the function of Ca_LRR-RLK147 was shoot mediated during AON.

From Microns to Meters: Exploring Advances in Legume Microbiome Diversity for Agroecosystem Benefits

Legumes are of primary importance for agroecosystems because they provide protein-rich foods and enhance soil fertility through fixed atmospheric nitrogen. The legume-rhizobia symbiosis that makes this possible has been extensively studied, from basic research on biochemical signaling to practical applications in cropping systems. While rhizobia are the most-studied group of associated microorganisms, the functional benefit they confer to their legume hosts by fixing nitrogen is not performed in isolation. Indeed, non-rhizobia members of the rhizosphere and nodule microbiome are now understood to contribute in multiple ways to nodule formation, legume fitness, and other agroecosystem services. In this review, we summarize advances contributing to our understanding of the diversity and composition of bacterial members of the belowground legume microbiome. We also highlight applied work in legume food and forage crops that link microbial community composition with plant functional benefits. Ultimately, further research will assist in the development of multi-species microbial inoculants and cropping systems that maximize plant nutrient benefits, while reducing sources of agricultural pollution.

Root Patterning: Tuning SHORT ROOT Function Creates Diversity in Form

Roots have a fundamental role in plant growth and adaptation to different environments. Diversity in root morphology and architecture enables plants to acquire water and nutrients in contrasting substrate conditions, resist biotic and abiotic stress, and develop symbiotic associations. At its most fundamental level, morphology is determined by discrete changes in tissue patterning. Differences in the number and arrangement of the cell layers in the root can change tissue structure, as well as root length and girth, affecting important productivity traits. Therefore, understanding the molecular mechanisms controlling variation in developmental patterning is an important goal in biology. The ground tissue (GT) system is an ideal model to study the genetic basis of morphological diversity because it displays great interspecific variability in cell layer number. In addition, the genetic circuit controlling GT patterning in Arabidopsis thaliana has been well described, although little is known about species with more complex root anatomies. In this review, we will describe the Arabidopsis model for root radial patterning and present recent progress in elucidating the genetic circuitry controlling GT patterning in monocots and the legume Medicago truncatula (Mt), species that develop roots with more complex anatomies and multilayered cortex.

GmPIN-dependent polar auxin transport is involved in soybean nodule development

To overcome nitrogen deficiency, legume roots establish symbiotic interactions with nitrogen-fixing rhizobia that are fostered in specialized organs (nodules). Similar to other organs, nodule formation is determined by a local maximum of the phytohormone auxin at the primordium site. However, how auxin regulates nodule development remains poorly understood. Here, we found that in soybean, (Glycine max), dynamic auxin transport driven by PIN-FORMED (PIN) transporter GmPIN1 is involved in nodule primordium formation. GmPIN1 was specifically expressed in nodule primordium cells and GmPIN1 was polarly localized in these cells. Two nodulation regulators, (iso)flavonoids trigger expanded distribution of GmPIN1b to root cortical cells, and cytokinin rearranges GmPIN1b polarity. Gmpin1abc triple mutants generated with CRISPR-Cas9 showed the impaired establishment of auxin maxima in nodule meristems and aberrant divisions in the nodule primordium cells. Moreover, overexpression of GmPIN1 suppressed nodule primordium initiation. GmPIN9d, an ortholog of Arabidopsis thaliana PIN2, acts together with GmPIN1 later in nodule development to acropetally transport auxin in vascular bundles, fine-tuning the auxin supply for nodule enlargement. Our findings reveal how PIN-dependent auxin transport modulates different aspects of soybean nodule development and suggest that the establishment of auxin gradient is a prerequisite for the proper interaction between legumes and rhizobia.

Soil nutrient status of KwaZulu-Natal savanna and grassland biomes causes variation in cytokinin functional groups and their levels in above-ground and underground parts of three legumes

Cytokinins (CKs) are involved in several developmental stages in the life-cycle of plants. The CK content in plants and their respective organs are susceptible to changes under different environmental conditions. In the current study, we profiled the CK content in the above and underground organs of three legumes (Lessertia frutescens, Mucuna pruriens and Pisum sativum) grown in soils collected from four locations (Ashburton, Bergville, Hluhluwe and Izingolweni) in KwaZulu-Natal province, South Africa. The quantified CK contents in the three legumes were categorized on the basis of their side chains (isoprenoid, aromatic and furfural) and modifications (e.g. free bases and glucosides). Legume and soil types as well as their interaction significantly influenced the concentrations of CKs. Lessertia frutescens, Mucuna pruriens and Pisum sativum had CK content that ranged from 124-653, 170-670 and 69-595 pmol/g DW, respectively. Substantial quantity (> 600 pmol/g DW) of CK were observed in plants grown in Bergville (above-ground part of Lessertia frutescens) and Izingolweni (underground part of Mucuna pruriens) soils. A total of 28 CK derivatives observed in the legumes comprised of isoprenoid (22), aromatic (5) and furfural (1) side-chain CKs. However, the 16 CK derivatives in Mucuna pruriens were isoprenoid-type based on the side-chain. Generally, a higher ratio of cis-zeatin (cZ) relative to the trans-zeatin (tZ) was evident in the above-ground part of Lessertia frutescens and Pisum sativum for the four soil treatments. In terms of functional and physiological importance of the CKs, the free bases (active form) and ribosides (translocation form) were the most abundant CK in Lessertia frutescens and Pisum sativum. However, N-glucoside, a deactivation/detoxicification product was the most dominant CK in Mucuna pruriens from Hluhluwe and Izingolweni soils. The total CKs in the underground parts of the legumes had a positive significant correlation with the total phosphorus and nitrogen content in the plant as well as the soil nitrogen. Overall, the CK profiles of the legumes were strongly influenced by the soil types.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s12298-021-01021-2.

Interaction of Symbiotic Rhizobia and Parasitic Root-Knot Nematodes in Legume Roots: From Molecular Regulation to Field Application

Legumes form two types of root organs in response to signals from microbes, namely, nodules and root galls. In the field, these interactions occur concurrently and often interact with each other. The outcomes of these interactions vary and can depend on natural variation in rhizobia and nematode populations in the soil as well as abiotic conditions. While rhizobia are symbionts that contribute fixed nitrogen to their hosts, parasitic root-knot nematodes (RKN) cause galls as feeding structures that consume plant resources without a contribution to the plant. Yet, the two interactions share similarities, including rhizosphere signaling, repression of host defense responses, activation of host cell division, and differentiation, nutrient exchange, and alteration of root architecture. Rhizobia activate changes in defense and development through Nod factor signaling, with additional functions of effector proteins and exopolysaccharides. RKN inject large numbers of protein effectors into plant cells that directly suppress immune signaling and manipulate developmental pathways. This review examines the molecular control of legume interactions with rhizobia and RKN to elucidate shared and distinct mechanisms of these root-microbe interactions. Many of the molecular pathways targeted by both organisms overlap, yet recent discoveries have singled out differences in the spatial control of expression of developmental regulators that may have enabled activation of cortical cell division during nodulation in legumes. The interaction of legumes with symbionts and parasites highlights the importance of a comprehensive view of root-microbe interactions for future crop management and breeding strategies.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Leguminous nodule symbiosis involves recruitment of factors contributing to lateral root development

Legumes and several plant species in the monophyletic nitrogen-fixing clade produce root nodules that function as symbiotic organs and establish mutualistic relationships with nitrogen-fixing bacteria. The modes of nodule organogenesis are distinct from those of lateral root development and also differ among different types of nodules formed in legumes and actinorhizal plants. It is considered that the evolution of new organs occurs through rearrangement of molecular networks interposed by certain neo-functionalized factors. Accumulating evidence has suggested that root nodule organogenesis involves root or lateral root developmental pathways. This review describes the current knowledge about the factors/pathways acquired by the common ancestor of the nitrogen-fixing clade in order to control nodule organogenesis.

Lotus japonicus Nuclear Factor YA1, a nodule emergence stage-specific regulator of auxin signalling

Organogenesis of legume root nodules begins with the nodulation factor-dependent stimulation of compatible root cells to initiate divisions, signifying an early nodule primordium formation event. This is followed by cellular differentiation, including cell expansion and vascular bundle formation, and we previously showed that Lotus japonicus NF-YA1 is essential for this process, presumably by regulating three members of the SHORT INTERNODES/STYLISH (STY) transcription factor gene family. In this study, we used combined genetics, genomics and cell biology approaches to characterize the role of STY genes during root nodule formation and to test a hypothesis that they mediate nodule development by stimulating auxin signalling. We show here that L. japonicus STYs are required for nodule emergence. This is attributed to the NF-YA1-dependent regulatory cascade, comprising STY genes and their downstream targets, YUCCA1 and YUCCA11, involved in a local auxin biosynthesis at the post-initial cell division stage. An analogous NF-YA1/STY regulatory module seems to operate in Medicago truncatula in association with the indeterminate nodule patterning. Our data define L. japonicus and M. truncatula NF-YA1 genes as important nodule emergence stage-specific regulators of auxin signalling while indicating that the inductive stage and subsequent formation of early nodule primordia are mediated through an independent mechanism(s).

Rhizobia as a Source of Plant Growth-Promoting Molecules: Potential Applications and Possible Operational Mechanisms

The symbiotic interaction between rhizobia and legumes that leads to nodule formation is a complex chemical conversation involving plant release of nod-gene inducing signal molecules and bacterial secretion of lipo-chito-oligossacharide nodulation factors. During this process, the rhizobia and their legume hosts can synthesize and release various phytohormones, such as IAA, lumichrome, riboflavin, lipo-chito-oligossacharide Nod factors, rhizobitoxine, gibberellins, jasmonates, brassinosteroids, ethylene, cytokinins and the enzyme 1-aminocyclopropane-1-carboxylate (ACC) deaminase that can directly or indirectly stimulate plant growth. Whereas these attributes may promote plant adaptation to various edapho-climatic stresses including the limitations in nutrient elements required for plant growth promotion, tapping their full potential requires understanding of the mechanisms involved in their action. In this regard, several N2-fixing rhizobia have been cited for plant growth promotion by solubilizing soil-bound P in the rhizosphere via the synthesis of gluconic acid under the control of pyrroloquinoline quinone (PQQ) genes, just as others are known for the synthesis and release of siderophores for enhanced Fe nutrition in plants, the chelation of heavy metals in the reclamation of contaminated soils, and as biocontrol agents against diseases. Some of these metabolites can enhance plant growth via the suppression of the deleterious effects of other antagonistic molecules, as exemplified by the reduction in the deleterious effect of ethylene by ACC deaminase synthesized by rhizobia. Although symbiotic rhizobia are capable of triggering biological outcomes with direct and indirect effects on plant mineral nutrition, insect pest and disease resistance, a greater understanding of the mechanisms involved remains a challenge in tapping the maximum benefits of the molecules involved. Rather than the effects of individual rhizobial or plant metabolites however, a deeper understanding of their synergistic interactions may be useful in alleviating the effects of multiple plant stress factors for increased growth and productivity.

Hormones as go-betweens in plant microbiome assembly

The interaction of plants with complex microbial communities is the result of co-evolution over millions of years and contributed to plant transition and adaptation to land. The ability of plants to be an essential part of complex and highly dynamic ecosystems is dependent on their interaction with diverse microbial communities. Plant microbiota can support, and even enable, the diverse functions of plants and are crucial in sustaining plant fitness under often rapidly changing environments. The composition and diversity of microbiota differs between plant and soil compartments. It indicates that microbial communities in these compartments are not static but are adjusted by the environment as well as inter-microbial and plant-microbe communication. Hormones take a crucial role in contributing to the assembly of plant microbiomes, and plants and microbes often employ the same hormones with completely different intentions. Here, the function of hormones as go-betweens between plants and microbes to influence the shape of plant microbial communities is discussed. The versatility of plant and microbe-derived hormones essentially contributes to the creation of habitats that are the origin of diversity and, thus, multifunctionality of plants, their microbiota and ultimately ecosystems.

An SHR-SCR module specifies legume cortical cell fate to enable nodulation

Legumes, unlike other plants, have the ability to establish symbiosis with nitrogen-fixing rhizobia. It has been theorized that a unique property of legume root cortical cells enabled the initial establishment of rhizobial symbiosis^1-3. Here we show that a SHORTROOT-SCARECROW (SHR-SCR) stem cell program in cortical cells of the legume Medicago truncatula specifies their distinct fate. Regulatory elements drive the cortical expression of SCR, and stele-expressed SHR protein accumulates in cortical cells of M. truncatula but not Arabidopsis thaliana. The cortical SHR-SCR network is conserved across legume species, responds to rhizobial signals, and initiates legume-specific cortical cell division for de novo nodule organogenesis and accommodation of rhizobia. Ectopic activation of SHR and SCR in legumes is sufficient to induce root cortical cell division. Our work suggests that acquisition of the cortical SHR-SCR module enabled cell division coupled to rhizobial infection in legumes. We propose that this event was central to the evolution of rhizobial endosymbiosis.

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.

Evolution of NIN and NIN-like Genes in Relation to Nodule Symbiosis

Legumes and actinorhizal plants are capable of forming root nodules symbiosis with rhizobia and Frankia bacteria. All these nodulating species belong to the nitrogen fixation clade. Most likely, nodulation evolved once in the last common ancestor of this clade. NIN (NODULE INCEPTION) is a transcription factor that is essential for nodulation in all studied species. Therefore, it seems probable that it was recruited at the start when nodulation evolved. NIN is the founding member of the NIN-like protein (NLP) family. It arose by duplication, and this occurred before nodulation evolved. Therefore, several plant species outside the nitrogen fixation clade have NLP(s), which is orthologous to NIN. In this review, we discuss how NIN has diverged from the ancestral NLP, what minimal changes would have been essential for it to become a key transcription controlling nodulation, and which adaptations might have evolved later.

Infection of Medicago truncatula by the Root-Knot Nematode Meloidogyne javanica Does Not Require Early Nodulation Genes

Because of the developmental similarities between root nodules induced by symbiotic rhizobia and root galls formed by parasitic nematodes, we investigated the involvement of nodulation genes in the infection of Medicago truncatula by the root knot nematode (RKN), Meloidogyne javanica. We found that gall formation, including giant cell formation, pericycle and cortical cell division, as well as egg laying, occurred successfully in the non-nodulating mutants nfp1 (nod factor perception1), nin1 (nodule inception1) and nsp2 (nodulation signaling pathway2) and the cytokinin perception mutant cre1 (cytokinin receptor1). Gall and egg formation were significantly reduced in the ethylene insensitive, hypernodulating mutant skl (sickle), and to a lesser extent, in the low nodulation, abscisic acid insensitive mutant latd/nip (lateral root-organ defective/numerous infections and polyphenolics). Despite its supernodulation phenotype, the sunn4 (super numeric nodules4) mutant, which has lost the ability to autoregulate nodule numbers, did not form excessive numbers of galls. Co-inoculation of roots with nematodes and rhizobia significantly reduced nodule numbers compared to rhizobia-only inoculated roots, but only in the hypernodulation mutant skl. Thus, this effect is likely to be influenced by ethylene signaling, but is not likely explained by resource competition between galls and nodules. Co-inoculation with rhizobia also reduced gall numbers compared to nematode-only infected roots, but only in the wild type. Therefore, the protective effect of rhizobia on nematode infection does not clearly depend on nodule number or on Nod factor signaling. Our study demonstrates that early nodulation genes that are essential for successful nodule development are not necessary for nematode-induced gall formation, that gall formation is not under autoregulation of nodulation control, and that ethylene signaling plays a positive role in successful RKN parasitism in M. truncatula.

A Roadmap toward Engineered Nitrogen-Fixing Nodule Symbiosis

In the late 19th century, it was discovered that legumes can establish a root nodule endosymbiosis with nitrogen-fixing rhizobia. Soon after, the question was raised whether it is possible to transfer this trait to non-leguminous crops. In the past century, an ever-increasing amount of knowledge provided unique insights into the cellular, molecular, and genetic processes controlling this endosymbiosis. In addition, recent phylogenomic studies uncovered several genes that evolved to function specifically to control nodule formation and bacterial infection. However, despite this massive body of knowledge, the long-standing objective to engineer the nitrogen-fixing nodulation trait on non-leguminous crop plants has not been achieved yet. In this review, the unsolved questions and engineering strategies toward nitrogen-fixing nodulation in non-legume plants are discussed and highlighted.

Gene Expression in Nitrogen-Fixing Symbiotic Nodule Cells in Medicago truncatula and Other Nodulating Plants

Root nodules formed by plants of the nitrogen-fixing clade (NFC) are symbiotic organs that function in the maintenance and metabolic integration of large populations of nitrogen-fixing bacteria. These organs feature unique characteristics and processes, including their tissue organization, the presence of specific infection structures called infection threads, endocytotic uptake of bacteria, symbiotic cells carrying thousands of intracellular bacteria without signs of immune responses, and the integration of symbiont and host metabolism. The early stages of nodulation are governed by a few well-defined functions, which together constitute the common symbiosis-signaling pathway (CSSP). The CSSP activates a set of transcription factors (TFs) that orchestrate nodule organogenesis and infection. The later stages of nodule development require the activation of hundreds to thousands of genes, mostly expressed in symbiotic cells. Many of these genes are only active in symbiotic cells, reflecting the unique nature of nodules as plant structures. Although how the nodule-specific transcriptome is activated and connected to early CSSP-signaling is poorly understood, candidate TFs have been identified using transcriptomic approaches, and the importance of epigenetic and chromatin-based regulation has been demonstrated. We discuss how gene regulation analyses have advanced our understanding of nodule organogenesis, the functioning of symbiotic cells, and the evolution of symbiosis in the NFC.

NODULE INCEPTION Recruits the Lateral Root Developmental Program for Symbiotic Nodule Organogenesis in Medicago truncatula

To overcome nitrogen deficiencies in the soil, legumes enter symbioses with rhizobial bacteria that convert atmospheric nitrogen into ammonium. Rhizobia are accommodated as endosymbionts within lateral root organs called nodules that initiate from the inner layers of Medicago truncatula roots in response to rhizobial perception. In contrast, lateral roots emerge from predefined founder cells as an adaptive response to environmental stimuli, including water and nutrient availability. CYTOKININ RESPONSE 1 (CRE1)-mediated signaling in the pericycle and in the cortex is necessary and sufficient for nodulation, whereas cytokinin is antagonistic to lateral root development, with cre1 showing increased lateral root emergence and decreased nodulation. To better understand the relatedness between nodule and lateral root development, we undertook a comparative analysis of these two root developmental programs. Here, we demonstrate that despite differential induction, lateral roots and nodules share overlapping developmental programs, with mutants in LOB-DOMAIN PROTEIN 16 (LBD16) showing equivalent defects in nodule and lateral root initiation. The cytokinin-inducible transcription factor NODULE INCEPTION (NIN) allows induction of this program during nodulation through activation of LBD16 that promotes auxin biosynthesis via transcriptional induction of STYLISH (STY) and YUCCAs (YUC). We conclude that cytokinin facilitates local auxin accumulation through NIN promotion of LBD16, which activates a nodule developmental program overlapping with that induced during lateral root initiation.

Accumulation of and Response to Auxins in Roots and Nodules of the Actinorhizal Plant Datisca glomerata Compared to the Model Legume Medicago truncatula

Actinorhizal nodules are structurally different from legume nodules and show a greater similarity to lateral roots. Because of the important role of auxins in lateral root and nodule formation, auxin profiles were examined in roots and nodules of the actinorhizal species Datisca glomerata and the model legume Medicago truncatula. The auxin response in roots and nodules of both species was analyzed in transgenic root systems expressing a beta-glucuronidase gene under control of the synthetic auxin-responsive promoter DR5. The effects of two different auxin on root development were compared for both species. The auxin present in nodules at the highest levels was phenylacetic acid (PAA). No differences were found between the concentrations of active auxins of roots vs. nodules, while levels of the auxin conjugate indole-3-acetic acid-alanine were increased in nodules compared to roots of both species. Because auxins typically act in concert with cytokinins, cytokinins were also quantified. Concentrations of cis-zeatin and some glycosylated cytokinins were dramatically increased in nodules compared to roots of D. glomerata, but not of M. truncatula. The ratio of active auxins to cytokinins remained similar in nodules compared to roots in both species. The auxin response, as shown by the activation of the DR5 promoter, seemed significantly reduced in nodules compared to roots of both species, suggesting the accumulation of auxins in cell types that do not express the signal transduction pathway leading to DR5 activation. Effects on root development were analyzed for the synthetic auxin naphthaleneacetic acid (NAA) and PAA, the dominant auxin in nodules. Both auxins had similar effects, except that the sensitivity of roots to PAA was lower than to NAA. However, while the effects of both auxins on primary root growth were similar for both species, effects on root branching were different: both auxins had the classical positive effect on root branching in M. truncatula, but a negative effect in D. glomerata. Such a negative effect of exogenous auxin on root branching has previously been found for a cucurbit that forms lateral root primordia in the meristem of the parental root; however, root branching in D. glomerata does not follow that pattern.