Biological and genomic analysis of a symbiotic nitrogen fixation defective mutant in Medicago truncatula

Medicago truncatula has been selected as one of the model legume species for gene functional studies. To elucidate the functions of the very large number of genes present in plant genomes, genetic mutant resources are very useful and necessary tools. Fast Neutron (FN) mutagenesis is effective in inducing deletion mutations in genomes of diverse species. Through this method, we have generated a large mutant resource in M. truncatula. This mutant resources have been used to screen for different mutant using a forward genetics methods. We have isolated and identified a large amount of symbiotic nitrogen fixation (SNF) deficiency mutants. Here, we describe the detail procedures that are being used to characterize symbiotic mutants in M. truncatula. In recent years, whole genome sequencing has been used to speed up and scale up the deletion identification in the mutant. Using this method, we have successfully isolated a SNF defective mutant FN007 and identified that it has a large segment deletion on chromosome 3. The causal deletion in the mutant was confirmed by tail PCR amplication and sequencing. Our results illustrate the utility of whole genome sequencing analysis in the characterization of FN induced deletion mutants for gene discovery and functional studies in the M. truncatula. It is expected to improve our understanding of molecular mechanisms underlying symbiotic nitrogen fixation in legume plants to a great extent.

On the Inverse Correlation of Protein and Oil: Examining the Effects of Altered Central Carbon Metabolism on Seed Composition Using Soybean Fast Neutron Mutants

Protein and oil levels measured at maturity are inversely correlated across soybean lines; however, carbon is in limited supply during maturation resulting in tradeoffs for the production of other reserves including oligosaccharides. During the late stages of seed development, the allocation of carbon for storage reserves changes. Lipid and protein levels decline while concentrations of indigestible raffinose family oligosaccharides (RFOs) increase, leading to a decreased crop value. Since the maternal source of carbon is diminished during seed maturation stages of development, carbon supplied to RFO synthesis likely comes from an internal, turned-over source and may contribute to the reduction in protein and lipid content in mature seeds. In this study, fast neutron (FN) mutagenized soybean populations with deletions in central carbon metabolic genes were examined for trends in oil, protein, sugar, and RFO accumulation leading to an altered final composition. Two lines with concurrent increases in oil and protein, by combined 10%, were identified. A delayed switch in carbon allocation towards RFO biosynthesis resulted in extended lipid accumulation and without compromising protein. Strategies for future soybean improvement using FN resources are described.

Physical Mutagenesis in Medicago truncatula Using Fast Neutron Bombardment (FNB) for Symbiosis and Developmental Biology Studies

Medicago truncatula has been selected as a model species for legume molecular genetics and functional genomics studies. With the completion of the Medicago truncatula cv. Jemalong A17 genome sequencing, a major challenge is to determine the function of the large number of genes in the genome. Development of diverse mutant resources is crucial for gene functional studies. In the past years, M2 seeds from over 150,000 Medicago truncatula mutant lines in the Jemalong A17 background have been generated coordinately at the Noble Research Institute, USA, and the John Innes Centre, UK, using fast neutron bombardment (FNB) mutagenesis. These mutant resources have been used in screening and characterization of different categories of mutants including symbiotic nitrogen fixation, nodule development, and growth and patterning of leaf, stem, and root system architecture in the legume system. Here, we describe the detail procedure that has been used for screening of mutants derived from fast neutron bombardment mutagenesis in Medicago truncatula.

Molecular mapping and genomics of soybean seed protein: a review and perspective for the future

KEY MESSAGE

Genetic improvement of soybean protein meal is a complex process because of negative correlation with oil, yield, and temperature. This review describes the progress in mapping and genomics, identifies knowledge gaps, and highlights the need of integrated approaches. Meal protein derived from soybean [Glycine max (L) Merr.] seed is the primary source of protein in poultry and livestock feed. Protein is a key factor that determines the nutritional and economical value of soybean. Genetic improvement of soybean seed protein content is highly desirable, and major quantitative trait loci (QTL) for soybean protein have been detected and repeatedly mapped on chromosomes (Chr.) 20 (LG-I), and 15 (LG-E). However, practical breeding progress is challenging because of seed protein content's negative genetic correlation with seed yield, other seed components such as oil and sucrose, and interaction with environmental effects such as temperature during seed development. In this review, we discuss rate-limiting factors related to soybean protein content and nutritional quality, and potential control factors regulating seed storage protein. In addition, we describe advances in next-generation sequencing technologies for precise detection of natural variants and their integration with conventional and high-throughput genotyping technologies. A syntenic analysis of QTL on Chr. 15 and 20 was performed. Finally, we discuss comprehensive approaches for integrating protein and amino acid QTL, genome-wide association studies, whole-genome resequencing, and transcriptome data to accelerate identification of genomic hot spots for allele introgression and soybean meal protein improvement.

Fast neutron-induced structural rearrangements at a soybean NAP1 locus result in gnarled trichomes

KEY MESSAGE

Three adjacent and distinct sequence rearrangements were identified at a NAP1 locus in a soybean mutant. Genetic dissection and validation revealed the function of this gene in soybean trichome development. A soybean (Glycine max (L.) Merr.) gnarled trichome mutant, exhibiting stunted trichomes compared to wild-type, was identified in a fast neutron mutant population. Genetic mapping using whole genome sequencing-based bulked segregant analysis identified a 26.6 megabase interval on chromosome 20 that co-segregated with the phenotype. Comparative genomic hybridization analysis of the mutant indicated that the chromosome 20 interval included a small structural variant within the coding region of a soybean ortholog (Glyma.20G019300) of Arabidopsis Nck-Associated Protein 1 (NAP1), a regulator of actin nucleation during trichome morphogenesis. Sequence analysis of the candidate allele revealed multiple rearrangements within the coding region, including two deletions (approximately 1-2 kb each), a translocation, and an inversion. Further analyses revealed that the mutant allele perfectly co-segregated with the phenotype, and a wild-type soybean NAP1 transgene functionally complemented an Arabidopsis nap1 mutant. In addition, mapping and exon sequencing of NAP1 in a spontaneous soybean gnarled trichome mutant (T31) identified a frame shift mutation resulting in a truncation of the coding region. These data indicate that the soybean NAP1 gene is essential for proper trichome development and show the utility of the soybean fast neutron population for forward genetic approaches for identifying genes.

Soybean (Glycine max) Mutant and Germplasm Resources: Current Status and Future Prospects

Genetic bottlenecks during domestication and modern breeding limited the genetic diversity of soybean (Glycine max (L.) Merr.). Therefore, expanding and diversifying soybean genetic resources is a major priority for the research community. These resources, consisting of natural and induced genetic variants, are valuable tools for improving soybean and furthering soybean biological knowledge. During the twentieth century, researchers gathered a wealth of genetic variation in the forms of landraces, Glycine soja accessions, Glycine tertiary germplasm, and the U.S. Department of Agriculture (USDA) Type and Isoline Collections. During the twenty-first century, soybean researchers have added several new genetic and genomic resources. These include the reference genome sequence, genotype data for the USDA soybean germplasm collection, next-generation mapping populations, new irradiation and transposon-based mutagenesis populations, and designer nuclease platforms for genome engineering. This paper briefly surveys the publicly accessible soybean genetic resources currently available or in development and provides recommendations for developing such genetic resources in the future. © 2016 by John Wiley & Sons, Inc.

Detection of induced mutations in CaFAD2 genes by next-generation sequencing leading to the production of improved oil composition in Crambe abyssinica

Crambe abyssinica is a hexaploid oil crop for industrial applications. An increase of erucic acid (C22:1) and reduction of polyunsaturated fatty acid (PUFA) contents in crambe oil is a valuable improvement. An increase in oleic acid (C18:1), a reduction in PUFA and possibly an increase in C22:1 can be obtained by down-regulating the expression of fatty acid desaturase2 genes (CaFAD2), which code for the enzyme that converts C18:1 into C18:2. We conducted EMS-mutagenesis in crambe, followed by Illumina sequencing, to screen mutations in three expressed CaFAD2 genes. Two novel analysis strategies were used to detect mutation sites. In the first strategy, mutation detection targeted specific sequence motifs. In the second strategy, every nucleotide position in a CaFAD2 fragment was tested for the presence of mutations. Seventeen novel mutations were detected in 1100 one-dimensional pools (11 000 individuals) in three expressed CaFAD2 genes, including non-sense mutations and mis-sense mutations in CaFAD2-C1, -C2 and -C3. The homozygous non-sense mutants for CaFAD2-C3 resulted in a 25% higher content of C18:1 and 25% lower content of PUFA compared to the wild type. The mis-sense mutations only led to small changes in oil composition. Concluding, targeted mutation detection using NGS in a polyploid was successfully applied and it was found that a non-sense mutation in even a single CaFAD2 gene can lead to changes in crambe oil composition. Stacking the mutations in different CaFAD2 may gain additional changes in C18:1 and PUFA contents.

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.

Genome resilience and prevalence of segmental duplications following fast neutron irradiation of soybean

Fast neutron radiation has been used as a mutagen to develop extensive mutant collections. However, the genome-wide structural consequences of fast neutron radiation are not well understood. Here, we examine the genome-wide structural variants observed among 264 soybean [Glycine max (L.) Merrill] plants sampled from a large fast neutron-mutagenized population. While deletion rates were similar to previous reports, surprisingly high rates of segmental duplication were also found throughout the genome. Duplication coverage extended across entire chromosomes and often prevailed at chromosome ends. High-throughput resequencing analysis of selected mutants resolved specific chromosomal events, including the rearrangement junctions for a large deletion, a tandem duplication, and a translocation. Genetic mapping associated a large deletion on chromosome 10 with a quantitative change in seed composition for one mutant. A tandem duplication event, located on chromosome 17 in a second mutant, was found to cosegregate with a short petiole mutant phenotype, and thus may serve as an example of a morphological change attributable to a DNA copy number gain. Overall, this study provides insight into the resilience of the soybean genome, the patterns of structural variation resulting from fast neutron mutagenesis, and the utility of fast neutron-irradiated mutants as a source of novel genetic losses and gains.

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.

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.

Towards the discovery of novel genetic component involved in stress resistance in Arabidopsis thaliana

The exposure of plants to high concentrations of trace metallic elements such as copper involves a remodeling of the root system, characterized by a primary root growth inhibition and an increase in the lateral root density. These characteristics constitute easy and suitable markers for screening mutants altered in their response to copper excess. A forward genetic approach was undertaken in order to discover novel genetic factors involved in the response to copper excess. A Cu(2+) -sensitive mutant named copper modified resistance1 (cmr1) was isolated and a causative mutation in the CMR1 gene was identified by using positional cloning and next-generation sequencing. CMR1 encodes a plant-specific protein of unknown function. The analysis of the cmr1 mutant indicates that the CMR1 protein is required for optimal growth under normal conditions and has an essential role in the stress response. Impairment of the CMR1 activity alters root growth through aberrant activity of the root meristem, and modifies potassium concentration and hormonal balance (ethylene production and auxin accumulation). Our data support a putative role for CMR1 in cell division regulation and meristem maintenance. Research on the role of CMR1 will contribute to the understanding of the plasticity of plants in response to changing environments.

Never too many? How legumes control nodule numbers

Restricted availability of nitrogen compounds in soils is often a major limiting factor for plant growth and productivity. Legumes circumvent this problem by establishing a symbiosis with soil-borne bacteria, called rhizobia that fix nitrogen for the plant. Nitrogen fixation and nutrient exchange take place in specialized root organs, the nodules, which are formed by a coordinated and controlled process that combines bacterial infection and organ formation. Because nodule formation and nitrogen fixation are energy-consuming processes, legumes develop the minimal number of nodules required to ensure optimal growth. To this end, several mechanisms have evolved that adapt nodule formation and nitrogen fixation to the plant's needs and environmental conditions, such as nitrate availability in the soil. In this review, we give an updated view on the mechanisms that control nodulation.

Molecular mechanisms controlling legume autoregulation of nodulation

BACKGROUND

High input costs and environmental pressures to reduce nitrogen use in agriculture have increased the competitive advantage of legume crops. The symbiotic relationship that legumes form with nitrogen-fixing soil bacteria in root nodules is central to this advantage.

SCOPE

Understanding how legume plants maintain control of nodulation to balance the nitrogen gains with their energy needs and developmental costs will assist in increasing their productivity and relative advantage. For this reason, the regulation of nodulation has been extensively studied since the first mutants exhibiting increased nodulation were isolated almost three decades ago.

CONCLUSIONS

Nodulation is regulated primarily via a systemic mechanism known as the autoregulation of nodulation (AON), which is controlled by a CLAVATA1-like receptor kinase. Multiple components sharing homology with the CLAVATA signalling pathway that maintains control of the shoot apical meristem in arabidopsis have now been identified in AON. This includes the recent identification of several CLE peptides capable of activating nodule inhibition responses, a low molecular weight shoot signal and a role for CLAVATA2 in AON. Efforts are now being focused on directly identifying the interactions of these components and to identify the form that long-distance transport molecules take.

Targeted mutagenesis of duplicated genes in soybean with zinc-finger nucleases

We performed targeted mutagenesis of a transgene and nine endogenous soybean (Glycine max) genes using zinc-finger nucleases (ZFNs). A suite of ZFNs were engineered by the recently described context-dependent assembly platform--a rapid, open-source method for generating zinc-finger arrays. Specific ZFNs targeting dicer-like (DCL) genes and other genes involved in RNA silencing were cloned into a vector under an estrogen-inducible promoter. A hairy-root transformation system was employed to investigate the efficiency of ZFN mutagenesis at each target locus. Transgenic roots exhibited somatic mutations localized at the ZFN target sites for seven out of nine targeted genes. We next introduced a ZFN into soybean via whole-plant transformation and generated independent mutations in the paralogous genes DCL4a and DCL4b. The dcl4b mutation showed efficient heritable transmission of the ZFN-induced mutation in the subsequent generation. These findings indicate that ZFN-based mutagenesis provides an efficient method for making mutations in duplicate genes that are otherwise difficult to study due to redundancy. We also developed a publicly accessible Web-based tool to identify sites suitable for engineering context-dependent assembly ZFNs in the soybean genome.

Phenotypic and genomic analyses of a fast neutron mutant population resource in soybean

Mutagenized populations have become indispensable resources for introducing variation and studying gene function in plant genomics research. In this study, fast neutron (FN) radiation was used to induce deletion mutations in the soybean (Glycine max) genome. Approximately 120,000 soybean seeds were exposed to FN radiation doses of up to 32 Gray units to develop over 23,000 independent M2 lines. Here, we demonstrate the utility of this population for phenotypic screening and associated genomic characterization of striking and agronomically important traits. Plant variation was cataloged for seed composition, maturity, morphology, pigmentation, and nodulation traits. Mutants that showed significant increases or decreases in seed protein and oil content across multiple generations and environments were identified. The application of comparative genomic hybridization (CGH) to lesion-induced mutants for deletion mapping was validated on a midoleate x-ray mutant, M23, with a known FAD2-1A (for fatty acid desaturase) gene deletion. Using CGH, a subset of mutants was characterized, revealing deletion regions and candidate genes associated with phenotypes of interest. Exome resequencing and sequencing of PCR products confirmed FN-induced deletions detected by CGH. Beyond characterization of soybean FN mutants, this study demonstrates the utility of CGH, exome sequence capture, and next-generation sequencing approaches for analyses of mutant plant genomes. We present this FN mutant soybean population as a valuable public resource for future genetic screens and functional genomics research.

Evolutionary duplication of lipo-oligochitin-like receptor genes in soybean differentiates their function in cell division and cell invasion

Gene duplication in evolution has long been viewed as a mechanism for functional divergence. We recently cloned two related lipo-oligo-chitin receptor genes (GmNFR1α and GmNFR1β) in Glycine max(soybean) that allowed the distinction of two nodulation factor (NF) responses during early legume nodule ontogeny, namely invasion of the root hair and concomitant cortical cell divisions. Root-controlled GmNFR1αmutants nod49 and rj1 failed to form curled root hairs, infection threads and nodules but develop subepidermal cortical cell divisions (CCD) and mycorrhizal associations. In contrast GmNFR1β mutant PI437.654 had full symbiotic abilities. However, GmNFR1α mutants formed normal nodules at reduced frequency when inoculated with high Bradyrhizobium titers. The mutation was complemented in Agrobacterium rhizogenes K599 transformed roots using both CaMV 35S and the native GmNFR1promoters. GmNFR1α may encode a high affinity NF receptor responsible for the entire nodulation cascade while GmNFR1β with lower affinity to NF suffices to induce cell divisions but not early infection events.

Functional genomics of soybean for improvement of productivity in adverse conditions

Global soybean production is frequently impacted by various stresses, including both abiotic and biotic stresses. To develop soybean plants with enhanced tolerance to different stressors, functional genomics of soybean and a comprehensive understanding of available biotechnological resources and approaches are essential. In this review, we will discuss recent advances in soybean functional genomics which provide unprecedented opportunities to understand global patterns of gene expression, gene regulatory networks, various physiological, biochemical, and metabolic pathways as well as their association with the development of specific phenotypes. Soybean functional genomics, therefore, will ultimately enable us to develop new soybean varieties with improved productivity under adverse conditions by genetic engineering.

Physiological and molecular approaches to improve drought resistance in soybean

Drought stress is a major constraint to the production and yield stability of soybean [Glycine max (L.) Merr.]. For developing high yielding varieties under drought conditions, the most widely employed criterion has traditionally been direct selection for yield stability over multiple locations. However, this approach is time consuming and labor intensive, because yield is a highly quantitative trait with low heritability, and influenced by differences arising from soil heterogeneity and environmental factors. The alternative strategy of indirect selection using secondary traits has succeeded only in a few crops, due to problems with repeatability and lack of phenotyping strategies, especially for root-related traits. Considerable efforts have been directed towards identifying traits associated with drought resistance in soybean. With the availability of the whole genome sequence, physical maps, genetics and functional genomics tools, integrated approaches using molecular breeding and genetic engineering offer new opportunities for improving drought resistance in soybean. Genetic engineering for drought resistance with candidate genes has been reported in the major food crops, and efforts for developing drought-resistant soybean lines are in progress. The objective of this review is to consolidate the current knowledge of physiology, molecular breeding and functional genomics which may be influential in integrating breeding and genetic engineering approaches for drought resistance in soybean.

Autoregulation of nodulation (AON) in Pisum sativum (pea) involves signalling events associated with both nodule primordia development and nitrogen fixation

To define the signalling events required for the activation of AON, we utilised approach grafts between wild-type pea plants and their mutants defective at successive stages of nodule formation. AON signalling strength was monitored by prior inoculation of mutant root portions (as so-called 'sensor') and quantifying nodule formation on connected roots of delayed inoculated wild type (the 'reporter'). Detectable AON sensing and associated signal exchange between root and shoot started after root hair curling but before the initiation of visible cortical and pericycle cell divisions. The strength of AON signalling was correlated with the stage of nodule development and size of nodule, with mature nitrogen-fixing nodules possessing the strongest AON-inducing signal. We demonstrated that the pea supernodulating mutant nod3 may function pre-NARK in the root. A model for the activation of AON signalling and its potential relationship with cell division, nitrogen fixation and/or cytokinin signal transduction are presented.

Promoters of orthologous Glycine max and Lotus japonicus nodulation autoregulation genes interchangeably drive phloem-specific expression in transgenic plants

The nodule autoregulation receptor kinase (GmNARK) of soybean (Glycine max) is essential for the systemic autoregulation of nodulation. Based on quantitative reverse-transcriptase polymerase chain reaction, GmNARK is ex-pressed to varying levels throughout the plant; the transcript was detected at high levels in mature leaves and roots but to a lesser extent in young leaves, shoot tips, and nodules. The transcript level was not significantly affected by Bradyrhizobium japonicum during the first week following inoculation. In addition, the activities of the promoters of GmNARK and Lotus japonicus HARI, driving a beta-glucuronidase (GUSPlus) reporter gene, were examined in stably transformed L. japonicus and transgenic hairy roots of soybean. Histochemical GUS activity in L. japonicus plants carrying either a 1.7-kb GmNARKpr::GUS or 2.0-kb LjHAR1pr::GUS construct was clearly localized to living cells within vascular bundles, especially phloem cells in leaves, stems, roots, and nodules. Phloem-specific expression also was detected in soybean hairy roots carrying these constructs. Our study suggests that regulatory elements required for the transcription of these orthologous genes are conserved. Moreover, rapid amplification of 5' cDNA ends (5' rapid amplification of cDNA ends) revealed two major transcripts of GmNARK potentially originating from two TATA boxes. Further analysis of the GmNARK promoter has confirmed that these two TATA boxes are functional. Deletion analysis also located a region controlling phloem-specific expression to a DNA sequence between 908 bp and 1.7 kb upstream of the translation start site of GmNARK.

Nodulation deficiency caused by fast neutron mutagenesis of the model legume Lotus japonicus

Fast neutron mutagenesis of Lotus japonicus wild-type genotype Gifu resulted in the isolation of a stable mutant (FNN5-2) unable to form nitrogen-fixing nodules in symbiosis with Mesorhizobium loti, though being infected by mycorrhizal fungi. The mutation behaves as a loss-of-function recessive, and has no other apparent phenotypic effects. Molecular characterization indicates a partial loss of the lysin motif domain (LysM) type receptor kinase gene (LjNFR1). Additionally part of the LjNIN gene (encoding a putative transcription factor needed for nodulation) is also missing. Transcript levels for both genes are severely reduced. As LjNIN and LjNFR1 are in the same chromosomal region we tested whether this terminal portion is lacking. DNA polymerase chain reaction analysis confirms that genes within the relevant interval (such as LjPAL1 (encoding phenylalanine ammonia lyase) and LjEIL2 (encoding an ethylene insensitive-like response regulator)) are present, suggesting that the mutational event induced by the fast neutrons was either a double hit coincidently involving two nodulation-related genes, a major genome rearrangement, or a major segmental inversion.

Differentially expressed genes related to symbiotic association in a supernodulating soybean mutant and its wild-type

SUMMARY To understand the molecular basis of symbiotic association, a cDNA-AFLP technique was used to identify differentially expressed transcripts between a supernodulating soybean mutant, SS2-2, and its wild-type, Sinpaldalkong 2. As sources of cDNA-AFLP templates, trifoliates of 2-week-old plants were collected 1 week after Bradyrhizobium japonicum inoculation. A total of 147 bands out of 4000 amplicons were recognized as differentially expressed fragments, with 40 transcript-derived fragments (TDFs) in SS2-2 and 65 TDFs in Sinpaldalkong 2. Qualitative and quantitative real-time RT-PCR assays suggested that the expression patterns of genes in both genotypes were clearly differentiated. TDFs homologous to nodulin (65S2) and a putative senescence-associated protein (9S1) were up-regulated in SS2-2, whereas Sinpaldalkong 2 showed up-regulation of a receptor-like kinase (48sin1) and a kinase-like protein (17sin1). This indicates that different genes may be involved in regulation of the symbiotic programme that distinguishes SS2-2 from its wild-type. A TDF showing a change in a single base from A (Sinpaldalkong 2) to T (SS2-2) in this study was identified as a Glycine max nodule autoregulation receptor-like protein kinase precursor, previously identified by map-based cloning. These results demonstrate that cDNA-AFLP is a powerful technique to detect interesting genes without prior assumptions about the nature of the genes. The differentially expressed genes between Sinpaldalkong 2 and SS2-2 suggest that different signal transduction pathways for symbiosis may be involved in the two soybean genotypes.

Characterization of the Lotus japonicus symbiotic mutant lot1 that shows a reduced nodule number and distorted trichomes

We isolated a recessive symbiotic mutant of Lotus japonicus that defines a genetic locus, LOT1 (for low nodulation and trichome distortion). The nodule number per plant of the mutant was about one-fifth of that of the wild type. The lot1 mutant showed a moderate dwarf phenotype and distorted trichomes, but its root hairs showed no apparent differences to those of the wild type. Infection thread formation after inoculation of Mesorhizobium loti was repressed in lot1 compared to that in the wild type. The nodule primordia of lot1 did not result in any aborted nodule-like structure, all nodules becoming mature and exhibiting high nitrogen fixation activity. The mutant was normally colonized by mycorrhizal fungi. lot1 also showed higher sensitivity to nitrate than the wild type. The grown-up seedlings of lot1 were insensitive to any ethylene treatments with regard to nodulation, although the mutant showed normal triple response on germination. It is conceivable that a nodulation-specific ethylene signaling pathway is constitutively activated in the mutant. Grafting experiments with lot1 and wild-type seedlings suggested that the root genotype mainly determines the low nodulation phenotype of the mutant, while the trichome distortion is regulated by the shoot genotype. Grafting of har1-4 shoots to lot1 roots resulted in an intermediate nodule number, i.e. more than that of lot1 and less than that of har1-4. Putative double mutants of lot1 and har1 also showed intermediate nodulation. Thus, it was indicated that LOT1 is involved in a distinct signal transduction pathway independent of HAR1.

SNP identification and SNAP marker development for a GmNARK gene controlling supernodulation in soybean

Supernodulation in soybean (Glycine max L. Merr.) is an important source of nitrogen supply to subterranean ecological systems. Single nucleotide-amplified polymorphism (SNAP) markers for supernodulation should allow rapid screening of the trait in early growth stages, without the need for inoculation and phenotyping. The gene GmNARK (Glycine max nodule autoregulation receptor kinase), controlling autoregulation of nodulation, was found to have a single nucleotide polymorphism (SNP) between the wild-type cultivar Sinpaldalkong 2 and its supernodulating mutant, SS2-2. Transversion of A to T at the 959-bp position of the GmNARK sequence results in a change of lysine (AAG) to a stop codon (TAG), thus terminating its translation in SS2-2. Based on the identified SNP in GmNARK, five primer pairs specific to each allele were designed using the WebSnaper program to develop a SNAP marker for supernodulation. One A-specific primer pair produced a band present in only Sinpaldalkong 2, while two T-specific pairs showed a band in only SS2-2. Both complementary PCRs, using each allele-specific primer pair were performed to genotype supernodulation against F2 progeny of Sinpaldalkong 2 x SS2-2. Among 28 individuals with the normal phenotype, eight individuals having only the A-allele-specific band were homozygous and normal, while 20 individuals were found to be heterozygous at the SNP having both A and T bands. Twelve supernodulating individuals showed only the band specific to the T allele. This SNAP marker for supernodulation could easily be analyzed through simple PCR and agarose gel electrophoresis. Therefore, use of this SNAP marker might be faster, cheaper, and more reproducible than using other genotyping methods, such as a cleaved amplified polymorphic sequence marker, which demand of restriction enzymes.