Shoot control of root development and nodulation is mediated by a receptor-like kinase

Common and divergent shoot-root signalling in legume symbioses

The role of shoot-root signals in the control of nodulation and arbuscular mycorrhizal (AM) development were examined in the divergent legume species pea and blue lupin. These species were chosen as pea can host both symbionts, whereas lupin can nodulate but has lost the ability to form AM. Intergeneric grafts between lupin and pea enabled examination of key long-distance signals in these symbioses. The role of strigolactones, auxin and elements of the autoregulation of nodulation (AON) pathway were investigated. Grafting studies were combined with loss-of-function mutants to monitor symbioses (nodulation, AM) and hormone effects (levels, gene expression and application studies). Lupin shoots suppress AM colonization in pea roots, in part by downregulating strigolactone exudation involving reduced expression of the strigolactone biosynthesis gene PsCCD8. By contrast, lupin shoots enhance pea nodulation, independently of strigolactones, possibly due to a partial incompatibility in AON shoot-root signalling between pea and lupin. This study highlights that nodulation and AM symbioses can be regulated independently and this may be due to long-distance signals, a phenomenon we were able to uncover by working with divergent legumes. We also identify a role for strigolactone exudation in determining the status of non-AM hosts.

Comprehensive Comparative Genomic and Transcriptomic Analyses of the Legume Genes Controlling the Nodulation Process

Nitrogen is one of the most essential plant nutrients and one of the major factors limiting crop productivity. Having the goal to perform a more sustainable agriculture, there is a need to maximize biological nitrogen fixation, a feature of legumes. To enhance our understanding of the molecular mechanisms controlling the interaction between legumes and rhizobia, the symbiotic partner fixing and assimilating the atmospheric nitrogen for the plant, researchers took advantage of genetic and genomic resources developed across different legume models (e.g., Medicago truncatula, Lotus japonicus, Glycine max, and Phaseolus vulgaris) to identify key regulatory protein coding genes of the nodulation process. In this study, we are presenting the results of a comprehensive comparative genomic analysis to highlight orthologous and paralogous relationships between the legume genes controlling nodulation. Mining large transcriptomic datasets, we also identified several orthologous and paralogous genes characterized by the induction of their expression during nodulation across legume plant species. This comprehensive study prompts new insights into the evolution of the nodulation process in legume plant and will benefit the scientific community interested in the transfer of functional genomic information between species.

The Scion/Rootstock Genotypes and Habitats Affect Arbuscular Mycorrhizal Fungal Community in Citrus

Citrus roots have rare root hairs and thus heavily depend on arbuscular mycorrhizal fungi (AMF) for mineral nutrient uptake. However, the AMF community structure of citrus is largely unknown. By using 454-pyrosequencing of 18S rRNA gene fragment, we investigated the genetic diversity of AMF colonizing citrus roots, and evaluated the impact of habitats and rootstock and scion genotypes on the AMF community structure. Over 7,40,000 effective sequences were obtained from 77 citrus root samples. These sequences were assigned to 75 AMF virtual taxa, of which 66 belong to Glomus, highlighting an absolute dominance of this AMF genus in symbiosis with citrus roots. The citrus AMF community structure is significantly affected by habitats and host genotypes. Interestingly, our data suggests that the genotype of the scion exerts a greater impact on the AMF community structure than that of the rootstock where the physical root-AMF association occurs. This study not only provides a comprehensive assessment for the community composition of the AMF in citrus roots under different conditions, but also sheds novel insights into how the AMF community might be indirectly influenced by the spatially separated yet metabolically connected partner—the scion—of the grafted citrus tree.

Full-length de novo assembly of RNA-seq data in pea (Pisum sativum L.) provides a gene expression atlas and gives insights into root nodulation in this species

Next-generation sequencing technologies allow an almost exhaustive survey of the transcriptome, even in species with no available genome sequence. To produce a Unigene set representing most of the expressed genes of pea, 20 cDNA libraries produced from various plant tissues harvested at various developmental stages from plants grown under contrasting nitrogen conditions were sequenced. Around one billion reads and 100 Gb of sequence were de novo assembled. Following several steps of redundancy reduction, 46 099 contigs with N50 length of 1667 nt were identified. These constitute the 'Caméor' Unigene set. The high depth of sequencing allowed identification of rare transcripts and detected expression for approximately 80% of contigs in each library. The Unigene set is now available online (http://bios.dijon.inra.fr/FATAL/cgi/pscam.cgi), allowing (i) searches for pea orthologs of candidate genes based on gene sequences from other species, or based on annotation, (ii) determination of transcript expression patterns using various metrics, (iii) identification of uncharacterized genes with interesting patterns of expression, and (iv) comparison of gene ontology pathways between tissues. This resource has allowed identification of the pea orthologs of major nodulation genes characterized in recent years in model species, as a major step towards deciphering unresolved pea nodulation phenotypes. In addition to a remarkable conservation of the early transcriptome nodulation apparatus between pea and Medicago truncatula, some specific features were highlighted. The resource provides a reference for the pea exome, and will facilitate transcriptome and proteome approaches as well as SNP discovery in pea.

Genes and networks regulating root anatomy and architecture

The root is an excellent model for studying developmental processes that underlie plant anatomy and architecture. Its modular structure, the lack of cell movement and relative accessibility to microscopic visualization facilitate research in a number of areas of plant biology. In this review, we describe several examples that demonstrate how cell type-specific developmental mechanisms determine cell fate and the formation of defined tissues with unique characteristics. In the last 10 yr, advances in genome-wide technologies have led to the sequencing of thousands of plant genomes, transcriptomes and proteomes. In parallel with the development of these high-throughput technologies, biologists have had to establish computational, statistical and bioinformatic tools that can deal with the wealth of data generated by them. These resources provide a foundation for posing more complex questions about molecular interactions, and have led to the discovery of new mechanisms that control phenotypic differences. Here we review several recent studies that shed new light on developmental processes, which are involved in establishing root anatomy and architecture. We highlight the power of combining large-scale experiments with classical techniques to uncover new pathways in root development.

CLE peptide signaling in plants - the power of moving around

The CLAVATA3 (CLV3)/EMBRYO SURROUNDING REGION (ESR)-RELATED (CLE) gene family encodes small secreted peptide ligands in plants. These peptides function non-cell autonomously through interactions with plasma membrane-associated LEUCINE-RICH REPEAT RECEPTOR-LIKE KINASEs (LRR-RLKs). These interactions are critical for cell-to-cell communications and control a variety of developmental and physiological processes in plants, such as regulation of stem cell proliferation and differentiation in the meristems, embryo and endosperm development, vascular development and autoregulation of nodulation. Here, we review the current knowledge in the field of CLE polypeptide signaling.

The Control of Auxin Transport in Parasitic and Symbiotic Root-Microbe Interactions

Most field-grown plants are surrounded by microbes, especially from the soil. Some of these, including bacteria, fungi and nematodes, specifically manipulate the growth and development of their plant hosts, primarily for the formation of structures housing the microbes in roots. These developmental processes require the correct localization of the phytohormone auxin, which is involved in the control of cell division, cell enlargement, organ development and defense, and is thus a likely target for microbes that infect and invade plants. Some microbes have the ability to directly synthesize auxin. Others produce specific signals that indirectly alter the accumulation of auxin in the plant by altering auxin transport. This review highlights root-microbe interactions in which auxin transport is known to be targeted by symbionts and parasites to manipulate the development of their host root system. We include case studies for parasitic root-nematode interactions, mycorrhizal symbioses as well as nitrogen fixing symbioses in actinorhizal and legume hosts. The mechanisms to achieve auxin transport control that have been studied in model organisms include the induction of plant flavonoids that indirectly alter auxin transport and the direct targeting of auxin transporters by nematode effectors. In most cases, detailed mechanisms of auxin transport control remain unknown.

Enhanced resistance to soybean cyst nematode Heterodera glycines in transgenic soybean by silencing putative CLE receptors

CLE peptides are small extracellular proteins important in regulating plant meristematic activity through the CLE-receptor kinase-WOX signalling module. Stem cell pools in the SAM (shoot apical meristem), RAM (root apical meristem) and vascular cambium are controlled by CLE signalling pathways. Interestingly, plant-parasitic cyst nematodes secrete CLE-like effector proteins, which act as ligand mimics of plant CLE peptides and are required for successful parasitism. Recently, we demonstrated that Arabidopsis CLE receptors CLAVATA1 (CLV1), the CLAVATA2 (CLV2)/CORYNE (CRN) heterodimer receptor complex and RECEPTOR-LIKE PROTEIN KINASE 2 (RPK2), which transmit the CLV3 signal in the SAM, are required for perception of beet cyst nematode Heterodera schachtii CLEs. Reduction in nematode infection was observed in clv1, clv2, crn, rpk2 and combined double and triple mutants. In an effort to develop nematode resistance in an agriculturally important crop, orthologues of Arabidopsis receptors including CLV1, CLV2, CRN and RPK2 were identified from soybean, a host for the soybean cyst nematode Heterodera glycines. For each of the receptors, there are at least two paralogues in the soybean genome. Localization studies showed that most receptors are expressed in the root, but vary in their level of expression and spatial expression patterns. Expression in nematode-induced feeding cells was also confirmed. In vitro direct binding of the soybean receptors with the HgCLE peptide was analysed. Knock-down of the receptors in soybean hairy roots showed enhanced resistance to SCN. Our findings suggest that targeted disruption of nematode CLE signalling may be a potential means to engineer nematode resistance in crop plants.

Phytosulfokine Is Involved in Positive Regulation of Lotus japonicus Nodulation

Phytosulfokine (PSK) is a tyrosine-sulfated peptide that is widely distributed in plants, participating in cell proliferation, differentiation, and innate immunity. The potential role of PSK in nodulation in legumes has not been reported. In this work, five PSK precursor genes were identified in Lotus japonicas, designated as LjPSK1 to LjPSK5. Three of them (LjPSK1, LjPSK4, and LjPSK5) were found to be expressed in nitrogen-fixing root nodules. LjPSK1 and LjPSK4 were not induced at the early stage of nodulation. Interestingly, while the expression of LjPSK4 was also found in spontaneous nodules without rhizobial colonization, LjPSK1 was not induced in these pseudo nodules. Promoter-β-glucuronidase analysis revealed that LjPSK1 was highly expressed in enlarged symbiotic cells of nodules. Exogenous addition of 1 1M synthetic PSK peptide resulted in increased nodule numbers per plant. Consistently, the number of mature nodules but not the events of rhizobial infection and nodule initiation was increased by overexpressing LjPSK1 in transgenic hairy roots, in which the expression of jasmonate-responsive genes was found to be repressed. These results suggest that PSK is a new peptide signal that regulates nodulation in legumes, probably through cross-talking with other phytohormones.

Multiple Autoregulation of Nodulation (AON) Signals Identified through Split Root Analysis of Medicago truncatula sunn and rdn1 Mutants

Nodulation is energetically costly to the host: legumes balance the nitrogen demand with the energy expense by limiting the number of nodules through long-distance signaling. A split root system was used to investigate systemic autoregulation of nodulation (AON) in Medicago truncatula and the role of the AON genes RDN1 and SUNN in the regulatory circuit. Developing nodule primordia did not trigger AON in plants carrying mutations in RDN1 and SUNN genes, while wild type plants had fully induced AON within three days. However, despite lacking an early suppression response, AON mutants suppressed nodulation when roots were inoculated 10 days or more apart, correlated with the maturation of nitrogen fixing nodules. In addition to correlation between nitrogen fixation and suppression of nodulation, suppression by extreme nutrient stress was also observed in all genotypes and may be a component of the observed response due to the conditions of the assay. These results suggest there is more than one systemic regulatory circuit controlling nodulation in M. truncatula. While both signals are present in wild type plants, the second signal can only be observed in plants lacking the early repression (AON mutants). RDN1 and SUNN are not essential for response to the later signal.

Leguminous plants: inventors of root nodules to accommodate symbiotic bacteria

Legumes and a few other plant species can establish a symbiotic relationship with nitrogen-fixing rhizobia, which enables them to survive in a nitrogen-deficient environment. During the course of nodulation, infection with rhizobia induces the dedifferentiation of host cells to form primordia of a symbiotic organ, the nodule, which prepares plants to accommodate rhizobia in host cells. While these nodulation processes are known to be genetically controlled by both plants and rhizobia, recent advances in studies on two model legumes, Lotus japonicus and Medicago truncatula, have provided great insight into the underlying plant-side molecular mechanism. In this chapter, we review such knowledge, with particular emphasis on two key processes of nodulation, nodule development and rhizobial invasion.

Shoot HAR1 mediates nitrate inhibition of nodulation in Lotus japonicus

Nitrate is a major environmental factor in the inhibition of nodulation. In a model legume Lotus japonicus, a CLV1-like receptor kinase, HAR1, mediates nitrate inhibition and autoregulation of nodulation. Autoregulation of nodulation involves root-to-shoot-to-root long-distance communication, and HAR1 functions in shoots. However, it remains elusive where HAR1 functions in the nitrate inhibition of nodulation. We performed grafting experiments with the har1 mutant under various nitrate conditions, and found that shoot HAR1 is critical for the inhibition of nodulation at 10 mM nitrate. Combined with our recent finding that the nitrate-induced CLE-RS2 glycopeptide binds directly to the HAR1 receptor, this result suggests that CLE-RS2/HAR1 long-distance signaling plays an important role in the both nitrate inhibition and the autoregulation of nodulation.

Secretion systems and signal exchange between nitrogen-fixing rhizobia and legumes

The formation of symbiotic nitrogen-fixing nodules on the roots and/or stem of leguminous plants involves a complex signal exchange between both partners. Since many microorganisms are present in the soil, legumes and rhizobia must recognize and initiate communication with each other to establish symbioses. This results in the formation of nodules. Rhizobia within nodules exchange fixed nitrogen for carbon from the legume. Symbiotic relationships can become non-beneficial if one partner ceases to provide support to the other. As a result, complex signal exchange mechanisms have evolved to ensure continued, beneficial symbioses. Proper recognition and signal exchange is also the basis for host specificity. Nodule formation always provides a fitness benefit to rhizobia, but does not always provide a fitness benefit to legumes. Therefore, legumes have evolved a mechanism to regulate the number of nodules that are formed, this is called autoregulation of nodulation. Sequencing of many different rhizobia have revealed the presence of several secretion systems - and the Type III, Type IV, and Type VI secretion systems are known to be used by pathogens to transport effector proteins. These secretion systems are also known to have an effect on host specificity and are a determinant of overall nodule number on legumes. This review focuses on signal exchange between rhizobia and legumes, particularly focusing on the role of secretion systems involved in nodule formation and host specificity.

Does SUNN-SYMRK Crosstalk occur in Medicago truncatula for regulating nodule organogenesis?

Recently we reported that overexpression of intracellular kinase domain of Symbiosis Receptor Kinase (SYMRK-kd) hyperactivated spontaneous nodulation in Medicago truncatula indicating the importance of SYMRK ectodomain in restricting nodule number. To clarify whether sunn and sickle pathways were overcome by SYMRK-kd for hyperactivation of nodule organogenesis, we overexpressed SYMRK-kd in these mutants and analyzed for spontaneous nodulation in absence of rhizobia. Spontaneous nodulation in skl/SYMRK-kd roots was 2-fold higher than A17/SYMRK-kd roots indicating nodule organogenesis induced by SYMRK-kd to be ethylene sensitive. Intriguingly, sunn/SYMRK-kd roots failed to generate any spontaneous nodule which directly indicate the LRR-RLK SUNN to have a role in SYMRK-kd mediated nodule development under non-symbiotic conditions. We hypothesize a crosstalk between SUNN and SYMRK receptors for activation as well as restriction of nodule development.

CLE Peptide Signaling and Crosstalk with Phytohormones and Environmental Stimuli

The CLE (CLAVATA3/Endosperm surrounding region-related) peptide family is one of the best-studied secreted peptide families in plants. Accumulated data have revealed that CLE genes play vital roles on stem cell homeostasis in different types of meristems. Additionally, CLE genes have been found to perform various biological roles in plant growth and development, and in response to environmental stimuli. With recent advances on our understanding of CLE peptide function, it is showing that the existence of potential crosstalks of CLE peptides with phytohormones and external stimuli. Complex interactions exist in which CLE petides coordinate with hormones to regulate plant growth and development, and in response to external stimuli. In this article, we present recent advances in cell-cell communication that is mediated by CLE peptides combining with phytohormones and external stimuli, and suggest additional Arabidopsis CLE genes that are likely to be controlled by hormones and environmental cues.

The value of biodiversity in legume symbiotic nitrogen fixation and nodulation for biofuel and food production

Much of modern agriculture is based on immense populations of genetically identical or near-identical varieties, called cultivars. However, advancement of knowledge, and thus experimental utility, is found through biodiversity, whether naturally-found or induced by the experimenter. Globally we are confronted by ever-growing food and energy challenges. Here we demonstrate how such biodiversity from the food legume crop soybean (Glycine max L. Merr) and the bioenergy legume tree Pongamia (Millettia) pinnata is a great value. Legume plants are diverse and are represented by over 18,000 species on this planet. Some, such as soybean, pea and medics are used as food and animal feed crops. Others serve as ornamental (e.g., wisteria), timber (e.g., acacia/wattle) or biofuel (e.g., Pongamia pinnata) resources. Most legumes develop root organs (nodules) after microsymbiont induction that serve as their habitat for biological nitrogen fixation. Through this, nitrogen fertiliser demand is reduced by the efficient symbiosis between soil Rhizobium-type bacteria and the appropriate legume partner. Mechanistic research into the genetics, biochemistry and physiology of legumes is thus strategically essential for future global agriculture. Here we demonstrate how molecular plant science analysis of the genetics of an established food crop (soybean) and an emerging biofuel P. pinnata feedstock contributes to their utility by sustainable production aided by symbiotic nitrogen fixation.

Dynamics of long-distance signaling via plant vascular tissues

Plant vascular systems are constructed by specific cell wall modifications through which cells are highly specialized to make conduits for water and nutrients. Xylem vessels are formed by thickened cell walls that remain after programmed cell death, and serve as water conduits from the root to the shoot. In contrast, phloem tissues consist of a complex of living cells, including sieve tube elements and their neighboring companion cells, and translocate photosynthetic assimilates from mature leaves to developing young tissues. Intensive studies on the content of vascular flow fluids have unveiled that plant vascular tissues transport various types of gene product, and the transport of some provides the molecular basis for the long-distance communications. Analysis of xylem sap has demonstrated the presence of proteins in the xylem transpiration stream. Recent studies have revealed that CLE and CEP peptides secreted in the roots are transported to above ground via the xylem in response to plant-microbe interaction and soil nitrogen starvation, respectively. Their leucine-rich repeat transmembrane receptors localized in the shoot phloem are required for relaying the signal from the shoot to the root. These findings well-fit to the current scenario of root-to-shoot-to-root feedback signaling, where peptide transport achieves the root-to-shoot signaling, the first half of the signaling process. Meanwhile, it is now well-evidenced that proteins and a range of RNAs are transported via the phloem translocation system, and some of those can exert their physiological functions at their destinations, including roots. Thus, plant vascular systems may serve not only as conduits for the translocation of essential substances but also as long-distance communication pathways that allow plants to adapt to changes in internal and external environments at the whole plant level.

Local and systemic regulation of plant root system architecture and symbiotic nodulation by a receptor-like kinase

In plants, root system architecture is determined by the activity of root apical meristems, which control the root growth rate, and by the formation of lateral roots. In legumes, an additional root lateral organ can develop: the symbiotic nitrogen-fixing nodule. We identified in Medicago truncatula ten allelic mutants showing a compact root architecture phenotype (cra2) independent of any major shoot phenotype, and that consisted of shorter roots, an increased number of lateral roots, and a reduced number of nodules. The CRA2 gene encodes a Leucine-Rich Repeat Receptor-Like Kinase (LRR-RLK) that primarily negatively regulates lateral root formation and positively regulates symbiotic nodulation. Grafting experiments revealed that CRA2 acts through different pathways to regulate these lateral organs originating from the roots, locally controlling the lateral root development and nodule formation systemically from the shoots. The CRA2 LRR-RLK therefore integrates short- and long-distance regulations to control root system architecture under non-symbiotic and symbiotic conditions.

Despite the essential functions of roots in plant access to water and nutrients, root system architecture has not been directly considered for crop breeding improvement, but it is now considered key for a “second green revolution.” In this study, we aimed to decipher integrated molecular mechanisms coordinating lateral organ development in legume roots: lateral roots and nitrogen-fixing symbiotic nodules. The compact root architecture 2 (cra2) mutant form an increased number of lateral roots and a reduced number of symbiotic nitrogen-fixing nodules. This mutant is affected in a CLAVATA1-like Leucine-Rich Repeat Receptor-Like Kinase (LRR-RLK) that has not previously been linked to root development. Grafting experiments showed that CRA2 negatively controls lateral root formation and positively controls nodule development through local and systemic pathways, respectively. Overall, our results can be integrated in the framework of regulatory pathways controlling the symbiotic nodule number, the so-called “Autoregulation of Nodulation” (AON), involving another LRR-RLK that also acts systemically from the shoots, SUNN (Super Numeric Nodules). A coordinated function of the CRA2 and SUNN LRR-RLKs may thereby permit the dynamic fine tuning of the nodule number depending on the environmental conditions.

Nitrate inhibits soybean nodulation by regulating expression of CLE genes

Nitrogen compounds such as nitrate act as a potential inhibitor for legume nodulation. In this study, we isolated a new CLE gene, GmNIC2, from nitrate-treated roots, which shares high sequence homology with nitrate-induced CLE gene GmNIC1. Similar to GmNIC1, the expression level of GmNIC2 was not significantly altered in roots by rhizobial inoculation and was much higher in young nodules than in roots. In addition, overexpression of GmNIC2 led to similar nodulation inhibition of transgenic hairy roots to that of GmNIC1, which occurred in GmNARK-dependent manner and at the local level. By analyzing GmNARK loss-of-function mutant, SS2-2, it was found that expression levels of GmNIC1 and GmNIC2 in the SS2-2 roots were lower than in the wild type (WT) roots in response to nitrate. In contrast to GmNIC1 and GmNIC2, expressions of GmRIC1 and GmRIC2 genes that are related to the autoregulation of nodulation (AON) were strongly suppressed both of the soybeans during all periods of nitrate treatment and even were not induced by additional inoculation with rhizobia. Taken together, the results of this study suggest that GmNIC2, as an active homologous gene located in chromosome 13, acts locally to suppress nodulation, like GmNIC1, and nitrate inhibition of nodulation is led by fine-tuned regulation of both nitrate-induced CLEs and rhizobia-induced CLEs.

Soybean miR172c targets the repressive AP2 transcription factor NNC1 to activate ENOD40 expression and regulate nodule initiation

MicroRNAs are noncoding RNAs that act as master regulators to modulate various biological processes by posttranscriptionally repressing their target genes. Repression of their target mRNA(s) can modulate signaling cascades and subsequent cellular events. Recently, a role for miR172 in soybean (Glycine max) nodulation has been described; however, the molecular mechanism through which miR172 acts to regulate nodulation has yet to be explored. Here, we demonstrate that soybean miR172c modulates both rhizobium infection and nodule organogenesis. miR172c was induced in soybean roots inoculated with either compatible Bradyrhizobium japonicum or lipooligosaccharide Nod factor and was highly upregulated during nodule development. Reduced activity and overexpression of miR172c caused dramatic changes in nodule initiation and nodule number. We show that soybean miR172c regulates nodule formation by repressing its target gene, Nodule Number Control1, which encodes a protein that directly targets the promoter of the early nodulin gene, ENOD40. Interestingly, transcriptional levels of miR172c were regulated by both Nod Factor Receptor1α/5α-mediated activation and by autoregulation of nodulation-mediated inhibition. Thus, we established a direct link between miR172c and the Nod factor signaling pathway in addition to adding a new layer to the precise nodulation regulation mechanism of soybean.

The soybean (Glycine max) nodulation-suppressive CLE peptide, GmRIC1, functions interspecifically in common white bean (Phaseolus vulgaris), but not in a supernodulating line mutated in the receptor PvNARK

Legume plants regulate the number of nitrogen-fixing root nodules they form via a process called the Autoregulation of Nodulation (AON). Despite being one of the most economically important and abundantly consumed legumes, little is known about the AON pathway of common bean (Phaseolus vulgaris). We used comparative- and functional-genomic approaches to identify central components in the AON pathway of common bean. This includes identifying PvNARK, which encodes a LRR receptor kinase that acts to regulate root nodule numbers. A novel, truncated version of the gene was identified directly upstream of PvNARK, similar to Medicago truncatula, but not seen in Lotus japonicus or soybean. Two mutant alleles of PvNARK were identified that cause a classic shoot-controlled and nitrate-tolerant supernodulation phenotype. Homeologous over-expression of the nodulation-suppressive CLE peptide-encoding soybean gene, GmRIC1, abolished nodulation in wild-type bean, but had no discernible effect on PvNARK-mutant plants. This demonstrates that soybean GmRIC1 can function interspecifically in bean, acting in a PvNARK-dependent manner. Identification of bean PvRIC1, PvRIC2 and PvNIC1, orthologues of the soybean nodulation-suppressive CLE peptides, revealed a high degree of conservation, particularly in the CLE domain. Overall, our work identified four new components of bean nodulation control and a truncated copy of PvNARK, discovered the mutation responsible for two supernodulating bean mutants and demonstrated that soybean GmRIC1 can function in the AON pathway of bean.

Nodule Inception creates a long-distance negative feedback loop involved in homeostatic regulation of nodule organ production

Autoregulatory negative-feedback loops play important roles in fine-balancing tissue and organ development. Such loops are composed of short-range intercellular signaling pathways via cell-cell communications. On the other hand, leguminous plants use a long-distance negative-feedback system involving root-shoot communication to control the number of root nodules, root lateral organs that harbor symbiotic nitrogen-fixing bacteria known as rhizobia. This feedback system, known as autoregulation of nodulation (AON), consists of two long-distance mobile signals: root-derived and shoot-derived signals. Two Lotus japonicus CLAVATA3/endosperm surrounding region (CLE)-related small peptides, CLE root signal1 (CLE-RS1) and CLE-RS2, function as root-derived signals and are perceived by a shoot-acting AON factor, the hypernodulation aberrant root formation1 (HAR1) receptor protein, an ortholog of Arabidopsis CLAVATA1, which is responsible for shoot apical meristem homeostasis. This peptide-receptor interaction is necessary for systemic suppression of nodulation. How the onset of nodulation activates AON and how optimal nodule numbers are maintained remain unknown, however. Here we show that an RWP-RK-containing transcription factor, nodule inception (NIN), which induces nodule-like structures without rhizobial infection when expressed ectopically, directly targets CLE-RS1 and CLE-RS2. Roots constitutively expressing NIN systemically repress activation of endogenous NIN expression in untransformed roots of the same plant in a HAR1-dependent manner, leading to systemic suppression of nodulation and down-regulation of CLE expression. Our findings provide, to our knowledge, the first molecular evidence of a long-distance autoregulatory negative-feedback loop that homeostatically regulates nodule organ formation.

Shoot-derived cytokinins systemically regulate root nodulation

Legumes establish symbiotic associations with nitrogen-fixing bacteria (rhizobia) in root nodules to obtain nitrogen. Legumes control nodule number through long-distance communication between roots and shoots, maintaining the proper symbiotic balance. Rhizobial infection triggers the production of mobile CLE-RS1/2 peptides in Lotus japonicus roots; the perception of the signal by receptor kinase HAR1 in shoots presumably induces the production of an unidentified shoot-derived inhibitor (SDI) that translocates to roots and blocks further nodule development. Here we show that, CLE-RS1/2-HAR1 signalling activates the production of shoot-derived cytokinins, which have an SDI-like capacity to systemically suppress nodulation. In addition, we show that LjIPT3 is involved in nodulation-related cytokinin production in shoots. The expression of LjIPT3 is activated in an HAR1-dependent manner. We further demonstrate shoot-to-root long-distance transport of cytokinin in L. japonicus seedlings. These findings add essential components to our understanding of how legumes control nodulation to balance nutritional requirements and energy status.

Unexpectedly low nitrogen acquisition and absence of root architecture adaptation to nitrate supply in a Medicago truncatula highly branched root mutant

To complement N2 fixation through symbiosis, legumes can efficiently acquire soil mineral N through adapted root architecture. However, root architecture adaptation to mineral N availability has been little studied in legumes. Therefore, this study investigated the effect of nitrate availability on root architecture in Medicago truncatula and assessed the N-uptake potential of a new highly branched root mutant, TR185. The effects of varying nitrate supply on both root architecture and N uptake were characterized in the mutant and in the wild type. Surprisingly, the root architecture of the mutant was not modified by variation in nitrate supply. Moreover, despite its highly branched root architecture, TR185 had a permanently N-starved phenotype. A transcriptome analysis was performed to identify genes differentially expressed between the two genotypes. This analysis revealed differential responses related to the nitrate acquisition pathway and confirmed that N starvation occurred in TR185. Changes in amino acid content and expression of genes involved in the phenylpropanoid pathway were associated with differences in root architecture between the mutant and the wild type.

A Positive Regulator of Nodule Organogenesis, NODULE INCEPTION, Acts as a Negative Regulator of Rhizobial Infection in Lotus japonicus

Legume-rhizobium symbiosis occurs in specialized root organs called nodules. To establish the symbiosis, two major genetically controlled events, rhizobial infection and organogenesis, must occur. For a successful symbiosis, it is essential that the two phenomena proceed simultaneously in different root tissues. Although several symbiotic genes have been identified during genetic screenings of nonsymbiotic mutants, most of the mutants harbor defects in both infection and organogenesis pathways, leading to experimental difficulty in investigating the molecular genetic relationships between the pathways. In this study, we isolated a novel nonnodulation mutant, daphne, in Lotus japonicus that shows complete loss of nodulation but a dramatically increased numbers of infection threads. Characterization of the locus responsible for these phenotypes revealed a chromosomal translocation upstream of NODULE INCEPTION (NIN) in daphne. Genetic analysis using a known nin mutant revealed that daphne is a novel nin mutant allele. Although the daphne mutant showed reduced induction of NIN after rhizobial infection, the spatial expression pattern of NIN in epidermal cells was broader than that in the wild type. Overexpression of NIN strongly suppressed hyperinfection in daphne, and daphne phenotypes were partially rescued by cortical expression of NIN. These observations suggested that the daphne mutation enhanced the role of NIN in the infection pathway due to a specific loss of the role of NIN in nodule organogenesis. Based on these results, we provide evidence that the bifunctional transcription factor NIN negatively regulates infection but positively regulates nodule organogenesis during the course of the symbiosis.

The potential roles of strigolactones and brassinosteroids in the autoregulation of nodulation pathway

BACKGROUND AND AIMS

The number of nodules formed on a legume root system is under the strict genetic control of the autoregulation of nodulation (AON) pathway. Plant hormones are thought to play a role in AON; however, the involvement of two hormones recently described as having a largely positive role in nodulation, strigolactones and brassinosteroids, has not been examined in the AON process.

METHODS

A genetic approach was used to examine if strigolactones or brassinosteroids interact with the AON system in pea (Pisum sativum). Double mutants between shoot-acting (Psclv2, Psnark) and root-acting (Psrdn1) mutants of the AON pathway and strigolactone-deficient (Psccd8) or brassinosteroid-deficient (lk) mutants were generated and assessed for various aspects of nodulation. Strigolactone production by AON mutant roots was also investigated.

KEY RESULTS

Supernodulation of the roots was observed in both brassinosteroid- and strigolactone-deficient AON double-mutant plants. This is despite the fact that the shoots of these plants displayed classic strigolactone-deficient (increased shoot branching) or brassinosteroid-deficient (extreme dwarf) phenotypes. No consistent effect of disruption of the AON pathway on strigolactone production was found, but root-acting Psrdn1 mutants did produce significantly more strigolactones.

CONCLUSIONS

No evidence was found that strigolactones or brassinosteroids act downstream of the AON genes examined. While in pea the AON mutants are epistatic to brassinosteroid and strigolactone synthesis genes, we argue that these hormones are likely to act independently of the AON system, having a role in the promotion of nodule formation.

The role of symbiotic nitrogen fixation in sustainable production of biofuels

With the ever-increasing population of the world (expected to reach 9.6 billion by 2050), and altered life style, comes an increased demand for food, fuel and fiber. However, scarcity of land, water and energy accompanied by climate change means that to produce enough to meet the demands is getting increasingly challenging. Today we must use every avenue from science and technology available to address these challenges. The natural process of symbiotic nitrogen fixation, whereby plants such as legumes fix atmospheric nitrogen gas to ammonia, usable by plants can have a substantial impact as it is found in nature, has low environmental and economic costs and is broadly established. Here we look at the importance of symbiotic nitrogen fixation in the production of biofuel feedstocks; how this process can address major challenges, how improving nitrogen fixation is essential, and what we can do about it.

The activity of nodules of the supernodulating mutant Mtsunn is not limited by photosynthesis under optimal growth conditions

Legumes match the nodule number to the N demand of the plant. When a mutation in the regulatory mechanism deprives the plant of that ability, an excessive number of nodules are formed. These mutants show low productivity in the fields, mainly due to the high carbon burden caused through the necessity to supply numerous nodules. The objective of this study was to clarify whether through optimal conditions for growth and CO2 assimilation a higher nodule activity of a supernodulating mutant of Medicago truncatula (M. truncatula) can be induced. Several experimental approaches reveal that under the conditions of our experiments, the nitrogen fixation of the supernodulating mutant, designated as sunn (super numeric nodules), was not limited by photosynthesis. Higher specific nitrogen fixation activity could not be induced through short- or long-term increases in CO2 assimilation around shoots. Furthermore, a whole plant P depletion induced a decline in nitrogen fixation, however this decline did not occur significantly earlier in sunn plants, nor was it more intense compared to the wild-type. However, a distinctly different pattern of nitrogen fixation during the day/night cycles of the experiment indicates that the control of N2 fixing activity of the large number of nodules is an additional problem for the productivity of supernodulating mutants.

Root-derived CLE glycopeptides control nodulation by direct binding to HAR1 receptor kinase

Leguminous plants establish a symbiosis with rhizobia to enable nitrogen fixation in root nodules under the control of the presumed root-to-shoot-to-root negative feedback called autoregulation of nodulation. In Lotus japonicus, autoregulation is mediated by CLE-RS genes that are specifically expressed in the root, and the receptor kinase HAR1 that functions in the shoot. However, the mature functional structures of CLE-RS gene products and the molecular nature of CLE-RS/HAR1 signalling governed by these spatially distant components remain elusive. Here we show that CLE-RS2 is a post-translationally arabinosylated glycopeptide derived from the CLE domain. Chemically synthesized CLE-RS glycopeptides cause significant suppression of nodulation and directly bind to HAR1 in an arabinose-chain and sequence-dependent manner. In addition, CLE-RS2 glycopeptide specifically produced in the root is found in xylem sap collected from the shoot. We propose that CLE-RS glycopeptides are the long sought mobile signals responsible for the initial step of autoregulation of nodulation.

Nitrate regulates rhizobial and mycorrhizal symbiosis in common bean (Phaseolus vulgaris L.)

Nitrogen-limited conditions are considered to be a prerequisite for legume-rhizobial symbiosis, but the effects of nitrate-rich conditions on symbiotic status remain poorly understood. We addressed this issue by examining rhizobial (Rhizobim tropici) and arbusclar mycorrhizal (Glomus intraradices) symbiosis in Phaseolus vulgaris L. cv. Negro Jamapa under nitrate pre-incubation and continuous nitrate conditions. Our results indicate that nitrate pre-incubation, independent of the concentration, did not affect nodule development. However, the continuous supply of nitrate at high concentrations impaired nodule maturation and nodule numbers. Low nitrate conditions, in addition to positively regulating nodule number, biomass, and nitrogenase activity, also extended the span of nitrogen-fixing activity. By contrast, for arbuscular mycorrhizae, continuous 10 and 50 mmol/L nitrate increased the percent root length colonization, concomitantly reduced arbuscule size, and enhanced ammonia transport without affecting phosphate transport. Therefore, in this manuscript, we have proposed the importance of nitrate as a positive regulator in promoting both rhizobial and mycorrhizal symbiosis in the common bean.

A Comparative Study of Phase States of the Peribacteroid Membrane from Yellow Lupin and Broad Bean Nodules

A comparative study of the lipid bilayer phase status and structure of the outer membrane of free-living Bradyrhizobium strain 359a (Nod⁺Fix⁺) and 400 (Nod⁺FixL) or Rhizobium leguminosarum 97 (Nod⁺Fix⁺, effective) and 87 (Nod⁺FixL, ineffective) has been carried out. Also, the effect of the symbiotic pair combination on the lipid bilayer structure of the bacteroid outer membrane and peribacteroid membrane, isolated from the nodules of Lupinus luteus L. or Vicia faba L., has been studied. As a result, it is shown that the lipid bilayer status of the bacteroid outer membrane is mainly determined by microsymbiont, but not the host plant. In the contrast, the lipid bilayer status of the peribacteroid membrane and, as a consequence, its properties depend on interaction of both symbiotic partners.

Posttranslationally modified small-peptide signals in plants

Cell-to-cell signaling is essential for many processes in plant growth and development, including coordination of cellular responses to developmental and environmental cues. Cumulative studies have demonstrated that peptide signaling plays a greater-than-anticipated role in such intercellular communication. Some peptides act as signals during plant growth and development, whereas others are involved in defense responses or symbiosis. Peptides secreted as signals often undergo posttranslational modification and proteolytic processing to generate smaller peptides composed of approximately 10 amino acid residues. Such posttranslationally modified small-peptide signals constitute one of the largest groups of secreted peptide signals in plants. The location of the modification group incorporated into the peptides by specific modification enzymes and the peptide chain length defined by the processing enzymes are critical for biological function and receptor interaction. This review covers 20 years of research into posttranslationally modified small-peptide signals in plants.

Transcript profiles of maize embryo sacs and preliminary identification of genes involved in the embryo sac-pollen tube interaction

The embryo sac, the female gametophyte of flowering plants, plays important roles in the pollination and fertilization process. Maize (Zea mays L.) is a model monocot, but little is known about the interactions between its embryo sac and the pollen tube. In this study, we compared the transcript profiles of mature embryo sacs, mature embryo sacs 14-16 h after pollination, and mature nucelli. Comparing the transcript profiles of the embryo sacs before and after the entry of the pollen tube, we identified 3467 differentially expressed transcripts (3382 differentially expressed genes; DEGs). The DEGs were grouped into 22 functional categories. Among the DEGs, 221 genes were induced upon the entry of the pollen tube, and many of them encoded proteins involved in RNA binding, processing, and transcription, signaling, miscellaneous enzyme family processes, and lipid metabolism processes. Genes in the DEG dataset were grouped into 17 classes in a gene ontology enrichment analysis. The DEGs included many genes encoding proteins involved in protein amino acid phosphorylation and protein ubiquitination, implying that these processes might play important roles in the embryo sac-pollen tube interaction. Additionally, our analyses indicate that the expression of 112 genes encoding cysteine-rich proteins (CRPs) is induced during pollination and fertilization. The CRPs likely regulate pollen tube guidance and embryo sac development. These results provide important information on the genes involved in the embryo sac-pollen tube interaction in maize.

The identification of novel loci required for appropriate nodule development in Medicago truncatula

BACKGROUND

The formation of functional symbiotic nodules is the result of a coordinated developmental program between legumes and rhizobial bacteria. Genetic analyses in legumes have been used to dissect the signaling processes required for establishing the legume-rhizobial endosymbiotic association. Compared to the early events of the symbiotic interaction, less attention has been paid to plant loci required for rhizobial colonization and the functioning of the nodule. Here we describe the identification and characterization of a number of new genetic loci in Medicago truncatula that are required for the development of effective nitrogen fixing nodules.

RESULTS

Approximately 38,000 EMS and fast neutron mutagenized Medicago truncatula seedlings were screened for defects in symbiotic nitrogen fixation. Mutant plants impaired in nodule development and efficient nitrogen fixation were selected for further genetic and phenotypic analysis. Nine mutants completely lacking in nodule formation (Nod-) represented six complementation groups of which two novel loci have been identified. Eight mutants with ineffective nodules (Fix-) represented seven complementation groups, out of which five were new monogenic loci. The Fix- M. truncatula mutants showed symptoms of nitrogen deficiency and developed small white nodules. Microscopic analysis of Fix- nodules revealed that the mutants have defects in the release of rhizobia from infection threads, differentiation of rhizobia and maintenance of persistence of bacteria in nodule cells. Additionally, we monitored the transcriptional activity of symbiosis specific genes to define what transcriptional stage of the symbiotic process is blocked in each of the Fix- mutants. Based on the phenotypic and gene expression analysis a functional hierarchy of the FIX genes is proposed.

CONCLUSIONS

The new symbiotic loci of M. truncatula isolated in this study provide the foundation for further characterization of the mechanisms underpinning nodulation, in particular the later stages associated with bacterial release and nodule function.

Nitrogen modulation of legume root architecture signaling pathways involves phytohormones and small regulatory molecules

Nitrogen, particularly nitrate is an important yield determinant for crops. However, current agricultural practice with excessive fertilizer usage has detrimental effects on the environment. Therefore, legumes have been suggested as a sustainable alternative for replenishing soil nitrogen. Legumes can uniquely form nitrogen-fixing nodules through symbiotic interaction with specialized soil bacteria. Legumes possess a highly plastic root system which modulates its architecture according to the nitrogen availability in the soil. Understanding how legumes regulate root development in response to nitrogen availability is an important step to improving root architecture. The nitrogen-mediated root development pathway starts with sensing soil nitrogen level followed by subsequent signal transduction pathways involving phytohormones, microRNAs and regulatory peptides that collectively modulate the growth and shape of the root system. This review focuses on the current understanding of nitrogen-mediated legume root architecture including local and systemic regulations by different N-sources and the modulations by phytohormones and small regulatory molecules.

Evolutionarily conserved CLE peptide signaling in plant development, symbiosis, and parasitism

Small polypeptides are widely used as signaling molecules in cell-to-cell communication in animals and plants. The CLAVATA3/EMBRYO SURROUNDING REGION-RELATED (CLE) gene family is composed of numerous genes that contain conserved CLE domains in various plant species and plant-parasitic nematodes. Here, we review recent progress in our understanding of CLE signaling during stem cell maintenance in Arabidopsis and grasses. We also summarize the roles of CLE signaling in the legume-Rhizobium symbiosis and infection by plant-parasitic nematodes. CLE signaling is important for diverse aspects of cell-to-cell signaling and long-distance communication, which are critical for survival, and the basic components of the CLE signaling pathway are evolutionarily conserved in both plants and animals.

Two distinct EIN2 genes cooperatively regulate ethylene signaling in Lotus japonicus

Leguminous plants establish a mutualistic symbiosis with bacteria, collectively referred to as rhizobia. Host plants positively and negatively regulate the symbiotic processes to keep the symbiosis at an appropriate level. Although the plant hormone ethylene is known as a negative regulator of symbiotic processes, the molecular mechanisms of ethylene signaling remain unresolved, especially in the model plant Lotus japonicus. Here, we identified two genes, LjEIN2-1 and LjEIN2-2, from L. japonicus. These genes share moderate similarity in their amino acid sequences, are located on different chromosomes and are composed of different numbers of exons. Suppression of either LjEIN2-1 or LjEIN2-2 expression significantly promoted the root growth of transformed plants on plates containing 1-amino-cyclopropane-carboxylic acid (ACC), the biosynthetic precursor of ethylene. Simultaneous suppression of both LjEIN2-1 and LjEIN2-2 markedly increased the ethylene insensitivity of transgenic roots and resulted in an increased nodulation phenotype. These results indicate that LjEIN2-1 and LjEIN2-2 concertedly regulate ethylene signaling in L. japonicus. We also observed that Nod factor (NF) induced the expression of the ethylene-responsive gene LjACO2, and simultaneous treatment with NF and ACC markedly increases its transcript level compared with either NF or ACC alone. Because LjACO2 encodes ACC oxidase, which is a key enzyme in ethylene biosynthesis, this result suggests the existence of an NF-triggered negative feedback mechanism through ethylene signaling.

Vascular gene expression: a hypothesis

The phloem is the conduit through which photoassimilates are distributed from autotrophic to heterotrophic tissues and is involved in the distribution of signaling molecules that coordinate plant growth and responses to the environment. Phloem function depends on the coordinate expression of a large array of genes. We have previously identified conserved motifs in upstream regions of the Arabidopsis genes, encoding the homologs of pumpkin phloem sap mRNAs, displaying expression in vascular tissues. This tissue-specific expression in Arabidopsis is predicted by the overrepresentation of GA/CT-rich motifs in gene promoters. In this work we have searched for common motifs in upstream regions of the homologous genes from plants considered to possess a "primitive" vascular tissue (a lycophyte), as well as from others that lack a true vascular tissue (a bryophyte), and finally from chlorophytes. Both lycophyte and bryophyte display motifs similar to those found in Arabidopsis with a significantly low E-value, while the chlorophytes showed either a different conserved motif or no conserved motif at all. These results suggest that these same genes are expressed coordinately in non-vascular plants; this coordinate expression may have been one of the prerequisites for the development of conducting tissues in plants. We have also analyzed the phylogeny of conserved proteins that may be involved in phloem function and development. The presence of CmPP16, APL, FT, and YDA in chlorophytes suggests the recruitment of ancient regulatory networks for the development of the vascular tissue during evolution while OPS is a novel protein specific to vascular plants.

Down-regulation of NSP2 expression in developmentally young regions of Lotus japonicus roots in response to rhizobial inoculation

During the early 1980s, Bauer and associates reported that nodulation potential in primary roots of soybean seedlings following inoculation with rhizobia was significantly reduced in developmentally younger regions. They suggested that this phenomenon might be due to a fast-acting regulatory mechanism in the host that prevented excessive nodulation. However, the molecular mechanism of this fast-acting regulatory response remains uncertain. Here, we sought to elucidate components of this regulatory mechanism by investigating the expression of the NSP1 and NSP2 genes that encode a GRAS transcription factor required for nodule initiation. First, we confirmed that younger regions of Lotus japonicus roots also show a reduction in nodule numbers in response to Mesorhizobium loti. Then, we compared the expression levels of NSP1 and NSP2 in developmentally younger regions of primary roots. After inoculation with M. loti, expression of NSP1 was transiently induced whereas that of NSP2 was significantly down-regulated 1 d after inoculation. This result implicates that down-regulation of NSP2 might cause a fast-acting regulatory mechanism to prevent further nodulation. Next we overexpressed NSP2 in wild-type plants. Overexpression resulted in the clustering of nodules in the upper region of the root but strong suppression of nodulation in the lower region. In contrast, overexpression of NSP2 in har1 hypernodulating mutants resulted in an increased number of nodule primordia even in the root tip region. These results indicate that HAR1 negatively regulates NSP2-induced excessive nodule formation in the developmentally younger regions of roots.

Too much love, a novel Kelch repeat-containing F-box protein, functions in the long-distance regulation of the legume-Rhizobium symbiosis

The interaction of legumes with N2-fixing bacteria collectively called rhizobia results in root nodule development. The number of nodules formed is tightly restricted through the systemic negative feedback control by the host called autoregulation of nodulation (AON). Here, we report the characterization and gene identification of TOO MUCH LOVE (TML), a root factor that acts during AON in a model legume Lotus japonicus. In our genetic analyses using another root-regulated hypernodulation mutant, plenty, the tml-1 plenty double mutant showed additive effects on the nodule number, whereas the tml-1 har1-7 double mutant did not, suggesting that TML and PLENTY act in different genetic pathways and that TML and HAR1 act in the same genetic pathway. The systemic suppression of nodule formation by CLE-RS1/RS2 overexpression was not observed in the tml mutant background, indicating that TML acts downstream of CLE-RS1/RS2. The tml-1 Snf2 double mutant developed an excessive number of spontaneous nodules, indicating that TML inhibits nodule organogenesis. Together with the determination of the deleted regions in tml-1/-2/-3, the fine mapping of tml-4 and the next-generation sequencing analysis, we identified a nonsense mutation in the Kelch repeat-containing F-box protein. As the gene knockdown of the candidate drastically increased the number of nodules, we concluded that it should be the causative gene. An expression analysis revealed that TML is a root-specific gene. In addition, the activity of ProTML-GUS was constitutively detected in the root tip and in the nodules/nodule primordia upon rhizobial infection. In conclusion, TML is a root factor acting at the final stage of AON.

Structure-function analysis of the GmRIC1 signal peptide and CLE domain required for nodulation control in soybean

Legumes control the nitrogen-fixing root nodule symbiosis in response to external and internal stimuli, such as nitrate, and via systemic autoregulation of nodulation (AON). Overexpression of the CLV3/ESR-related (CLE) pre-propeptide-encoding genes GmNIC1 (nitrate-induced and acting locally) and GmRIC1 (Bradyrhizobium-induced and acting systemically) suppresses soybean nodulation dependent on the activity of the nodulation autoregulation receptor kinase (GmNARK). This nodule inhibition response was used to assess the relative importance of key structural components within and around the CLE domain sequences of these genes. Using a site-directed mutagenesis approach, mutants were produced at each amino acid within the CLE domain (RLAPEGPDPHHN) of GmRIC1. This approach identified the Arg1, Ala3, Pro4, Gly6, Pro7, Asp8, His11, and Asn12 residues as critical to GmRIC1 nodulation suppression activity (NSA). In contrast, none of the mutations in conserved residues outside of the CLE domain showed compromised NSA. Chimeric genes derived from combinations of GmRIC1 and GmNIC1 domains were used to determine the role of each pre-propeptide domain in NSA differences that exist between the two peptides. It was found that the transit peptide and CLE peptide regions of GmRIC1 significantly enhanced activity of GmNIC1. In contrast, the comparable GmNIC1 domains reduced the NSA of GmRIC1. Identification of these critical residues and domains provides a better understanding of how these hormone-like peptides function in plant development and regulation.

Grafting analysis indicates that malfunction of TRICOT in the root causes a nodulation-deficient phenotype in Lotus japonicus

Leguminous plants develop root nodules in symbiosis with soil rhizobia. Nodule formation occurs following rhizobial infection of the host root that induces dedifferentiation of some cortical cells and the initiation of a new developmental program to form nodule primordia. In a recent study, we identified a novel gene, TRICOT (TCO), that acts as a positive regulator of nodulation in Lotus japonicus. In addition to its role in nodulation, tco mutant plants display pleiotropic defects including abnormal shoot apical meristem formation. Here, we investigated the effect of the tco mutation on nodulation using a grafting approach. The results strongly indicate that the nodulation-deficient phenotype of the mutant results from malfunction of the TCO gene in the root.

Strigolactones: Internal and external signals in plant symbioses?

As the newest plant hormone, strigolactone research is undergoing an exciting expansion. In less than five years, roles for strigolactones have been defined in shoot branching, secondary growth, root growth and nodulation, to add to the growing understanding of their role in arbuscular mycorrhizae and parasitic weed interactions. Strigolactones are particularly fascinating as signaling molecules as they can act both inside the plant as an endogenous hormone and in the soil as a rhizosphere signal. Our recent research has highlighted such a dual role for strigolactones, potentially acting as both an endogenous and exogenous signal for arbuscular mycorrhizal development. There is also significant interest in examining strigolactones as putative regulators of responses to environmental stimuli, especially the response to nutrient availability, given the strong regulation of strigolactone production by nitrate and phosphate observed in many species. In particular, the potential for strigolactones to mediate the ecologically important response of mycorrhizal colonization to phosphate has been widely discussed. However, using a mutant approach we found that strigolactones are not essential for phosphate regulation of mycorrhizal colonization or nodulation. This is consistent with the relatively mild impairment of phosphate control of seedling root growth observed in Arabidopsis strigolactone mutants. This contrasts with the major role for strigolactones in phosphate control of shoot branching of rice and Arabidopsis and indicates that the integration of strigolactones into our understanding of nutrient response will be complex. New data presented here, along with the recent discovery of phosphate specific CLE peptides, indicates a potential role for PsNARK, a component of the autoregulation of nodulation pathway, in phosphate control of nodulation.

TRICOT encodes an AMP1-related carboxypeptidase that regulates root nodule development and shoot apical meristem maintenance in Lotus japonicus

During the course of evolution, mainly leguminous plants have acquired the ability to form de novo structures called root nodules. Recent studies on the autoregulation and hormonal controls of nodulation have identified key mechanisms and also indicated a possible link to other developmental processes, such as the formation of the shoot apical meristem (SAM). However, our understanding of nodulation is still limited by the low number of nodulation-related genes that have been identified. Here, we show that the induced mutation tricot (tco) can suppress the activity of spontaneous nodule formation 2, a gain-of-function mutation of the cytokinin receptor in Lotus japonicus. Our analyses of tco mutant plants demonstrate that TCO positively regulates rhizobial infection and nodule organogenesis. Defects in auxin regulation are also observed during nodule development in tco mutants. In addition to its role in nodulation, TCO is involved in the maintenance of the SAM. The TCO gene was isolated by a map-based cloning approach and found to encode a putative glutamate carboxypeptidase with greatest similarity to Arabidopsis ALTERED MERISTEM PROGRAM 1, which is involved in cell proliferation in the SAM. Taken together, our analyses have not only identified a novel gene for regulation of nodule organogenesis but also provide significant additional evidence for a common genetic regulatory mechanism in nodulation and SAM formation. These new data will contribute further to our understanding of the evolution and genetic basis of nodulation.

The roles of peptide hormones during plant root development

Peptide hormones are a key mechanism that plants use for cell-cell interactions; these interactions function to coordinate development, growth, and environmental responses among different cells. Peptide signals are produced by one cell and received by receptors in neighboring cells. It has previously been reported that peptide hormones regulate various aspects of plant development. The mechanism of action of peptides in the shoot is well known. However, the function of peptides in the root has been relatively uncharacterized. Recent studies have discovered important roles for peptide hormones in the development of the root meristem, lateral roots, and nodules. In this review, we focus on current findings regarding the function of peptide hormones in root development.