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1.  Spontaneous symbiotic reprogramming of plant roots triggered by receptor-like kinases 
eLife  2014;3:e03891.
Symbiosis Receptor-like Kinase (SYMRK) is indispensable for the development of phosphate-acquiring arbuscular mycorrhiza (AM) as well as nitrogen-fixing root nodule symbiosis, but the mechanisms that discriminate between the two distinct symbiotic developmental fates have been enigmatic. In this study, we show that upon ectopic expression, the receptor-like kinase genes Nod Factor Receptor 1 (NFR1), NFR5, and SYMRK initiate spontaneous nodule organogenesis and nodulation-related gene expression in the absence of rhizobia. Furthermore, overexpressed NFR1 or NFR5 associated with endogenous SYMRK in roots of the legume Lotus japonicus. Epistasis tests revealed that the dominant active SYMRK allele initiates signalling independently of either the NFR1 or NFR5 gene and upstream of a set of genes required for the generation or decoding of calcium-spiking in both symbioses. Only SYMRK but not NFR overexpression triggered the expression of AM-related genes, indicating that the receptors play a key role in the decision between AM- or root nodule symbiosis-development.
eLife digest
Like all plants, crop plants need nutrients such as nitrogen and phosphate to grow. Often these essential elements are in short supply, and so millions of tons of fertiliser are applied to agricultural land each year to maintain crop yields. Another way for plants to gain access to scarce nutrients is to form symbiotic relationships with microorganisms that live in the soil. Plants pass on carbon-containing compounds—such as sugars—to the microbes and, in return, certain fungi provide minerals—such as phosphates—to the plants. Some plants called legumes (such as peas, beans, and clovers) can also form relationships with bacteria that convert nitrogen from the air into ammonia, which the plants then use to make molecules such as DNA and proteins.
To establish these symbiotic relationships with plants, nitrogen-fixing bacteria release chemical signals that are recognized via receptor proteins, called NFR1 and NFR5, found on the surface of the plant root cells. These signals trigger a cascade of events that ultimately lead to the plant forming an organ called ‘root nodule’ to house and nourish the nitrogen-fixing bacteria. A similar signalling mechanism is thought to take place during the establishment of symbiotic relationships between plants and certain soil fungi.
A plant protein called Symbiosis Receptor-like Kinase (or SYMRK for short) that is also located on the root cell surface is required for both bacteria–plant and fungi–plant associations to occur. However, the exact role of this protein in these processes was unclear. Ried et al. have now investigated this by taking advantage of a property of cell surface receptor proteins: if some of these proteins are made in excessive amounts they activate their signalling cascades even when the initial signal is not present.
Ried et al. engineered plants called Lotus japonicus to produce high levels of SYMRK, NFR1, or NFR5. Each of these changes was sufficient to trigger the plants to develop root nodules in the absence of microbes. Genes associated with the activation of the signalling cascade involved the formation of root nodules were also switched on when each of the three proteins was produced in large amounts. In contrast, only an excess of SYMRK could activate genes related to fungi–plant associations. Ried et al. also found that, while SYMRK can function in the absence of the NFRs, NFR1 and NFR5 need each other to function. These data suggest that the receptor proteins play a key role in the decision between the establishment of an association with a bacterium or a fungus.
As an excess of symbiotic receptors caused plants to form symbiotic structures, Ried et al. propose that this strategy could be used to persuade plants that usually do not form symbioses with nitrogen-fixing bacteria to do so. If this is possible, it might lead us to engineer crop plants to form symbiotic interactions with nitrogen-fixing bacteria; this would help increase crop yields and enable crops to be grown in nitrogen-poor environments without the addition of extra fertiliser.
PMCID: PMC4243133  PMID: 25422918
Lotus japonicus; plant root symbiosis; receptor-like kinases; signal transduction; plant development; gene regulation; other
2.  A Modular Plasmid Assembly Kit for Multigene Expression, Gene Silencing and Silencing Rescue in Plants 
PLoS ONE  2014;9(2):e88218.
The Golden Gate (GG) modular assembly approach offers a standardized, inexpensive and reliable way to ligate multiple DNA fragments in a pre-defined order in a single-tube reaction. We developed a GG based toolkit for the flexible construction of binary plasmids for transgene expression in plants. Starting from a common set of modules, such as promoters, protein tags and transcribed regions of interest, synthetic genes are assembled, which can be further combined to multigene constructs. As an example, we created T-DNA constructs encoding multiple fluorescent proteins targeted to distinct cellular compartments (nucleus, cytosol, plastids) and demonstrated simultaneous expression of all genes in Nicotiana benthamiana, Lotus japonicus and Arabidopsis thaliana.
We assembled an RNA interference (RNAi) module for the construction of intron-spliced hairpin RNA constructs and demonstrated silencing of GFP in N. benthamiana. By combination of the silencing construct together with a codon adapted rescue construct into one vector, our system facilitates genetic complementation and thus confirmation of the causative gene responsible for a given RNAi phenotype. As proof of principle, we silenced a destabilized GFP gene (dGFP) and restored GFP fluorescence by expression of a recoded version of dGFP, which was not targeted by the silencing construct.
PMCID: PMC3923767  PMID: 24551083
3.  Analysis of the Lotus japonicus nuclear pore NUP107-160 subcomplex reveals pronounced structural plasticity and functional redundancy 
Mutations in the Lotus japonicus nucleoporin genes, NUP85, NUP133, and NENA (SEH1), lead to defects in plant-microbe symbiotic signaling. The homologous proteins in yeast and vertebrates are part of the conserved NUP84/NUP107-160 subcomplex, which is an essential component of the nuclear pore scaffold and has a pivotal role in nuclear pore complex (NPC) assembly. Loss and down-regulation of NUP84/NUP107-160 members has previously been correlated with a variety of growth and molecular defects, however, in L. japonicus only surprisingly specific phenotypes have been reported. We investigated whether Lotus nup85, nup133, and nena mutants exhibit general defects in NPC composition and distribution. Whole mount immunolocalization confirmed a typical nucleoporin-like localization for NUP133, which was unchanged in the nup85-1 mutant. Severe NPC clustering and aberrations in the nuclear envelope have been reported for Saccharomyces cerevisiae nup85 and nup133 mutants. However, upon transmission electron microscopy analysis of L. japonicus nup85, nup133 and nena, we detected only a slight reduction in the average distances between neighboring NPCs in nup133. Using quantitative immunodetection on protein-blots we observed that loss of individual nucleoporins affected the protein levels of other NUP107–160 complex members. Unlike the single mutants, nup85/nup133 double mutants exhibited severe temperature dependent growth and developmental defects, suggesting that the loss of more than one NUP107–160 member affects basal functions of the NPC.
PMCID: PMC3897872  PMID: 24478780
nuclear pore complex; nucleoporins; Lotus japonicus; NUP107-160 subcomplex; plant nucleus
4.  A Set of Lotus japonicus Gifu × Lotus burttii Recombinant Inbred Lines Facilitates Map-based Cloning and QTL Mapping 
Model legumes such as Lotus japonicus have contributed significantly to the understanding of symbiotic nitrogen fixation. This insight is mainly a result of forward genetic screens followed by map-based cloning to identify causal alleles. The L. japonicus ecotype ‘Gifu’ was used as a common parent for inter-accession crosses to produce F2 mapping populations either with other L. japonicus ecotypes, MG-20 and Funakura, or with the related species L. filicaulis. These populations have all been used for genetic studies but segregation distortion, suppression of recombination, low polymorphism levels, and poor viability have also been observed. More recently, the diploid species L. burttii has been identified as a fertile crossing partner of L. japonicus. To assess its qualities in genetic linkage analysis and to enable quantitative trait locus (QTL) mapping for a wider range of traits in Lotus species, we have generated and genotyped a set of 163 Gifu × L. burttii recombinant inbred lines (RILs). By direct comparisons of RIL and F2 population data, we show that L. burttii is a valid alternative to MG-20 as a Gifu mapping partner. In addition, we demonstrate the utility of the Gifu × L. burttii RILs in QTL mapping by identifying an Nfr1-linked QTL for Sinorhizobium fredii nodulation.
PMCID: PMC3415293  PMID: 22619310
Lotus japonicus; Lotus burttii; RIL; QTL; Sinorhizobium fredii
5.  Functional Domain Analysis of the Remorin Protein LjSYMREM1 in Lotus japonicus 
PLoS ONE  2012;7(1):e30817.
In legumes rhizobial infection during root nodule symbiosis (RNS) is controlled by a conserved set of receptor proteins and downstream components. MtSYMREM1, a protein of the Remorin family in Medicago truncatula, was shown to interact with at least three receptor-like kinases (RLKs) that are essential for RNS. Remorins are comprised of a conserved C-terminal domain and a variable N-terminal region that defines the six different Remorin groups. While both N- and C-terminal regions of Remorins belonging to the same phylogenetic group are similar to each other throughout the plant kingdom, the N-terminal domains of legume-specific group 2 Remorins show exceptional high degrees of sequence divergence suggesting evolutionary specialization of this protein within this clade. We therefore identified and characterized the MtSYMREM1 ortholog from Lotus japonicus (LjSYMREM1), a model legume that forms determinate root nodules. Here, we resolved its spatio-temporal regulation and showed that over-expression of LjSYMREM1 increases nodulation on transgenic roots. Using a structure-function approach we show that protein interactions including Remorin oligomerization are mainly mediated and stabilized by the Remorin C-terminal region with its coiled-coil domain while the RLK kinase domains transiently interact in vivo and phosphorylate a residue in the N-terminal region of the LjSYMREM1 protein in vitro. These data provide novel insights into the mechanism of this putative molecular scaffold protein and underline its importance during rhizobial infection.
PMCID: PMC3264624  PMID: 22292047
6.  Identification of candidate genome regions controlling disease resistance in Arachis 
BMC Plant Biology  2009;9:112.
Worldwide, diseases are important reducers of peanut (Arachis hypogaea) yield. Sources of resistance against many diseases are available in cultivated peanut genotypes, although often not in farmer preferred varieties. Wild species generally harbor greater levels of resistance and even apparent immunity, although the linkage of agronomically un-adapted wild alleles with wild disease resistance genes is inevitable. Marker-assisted selection has the potential to facilitate the combination of both cultivated and wild resistance loci with agronomically adapted alleles. However, in peanut there is an almost complete lack of knowledge of the regions of the Arachis genome that control disease resistance.
In this work we identified candidate genome regions that control disease resistance. For this we placed candidate disease resistance genes and QTLs against late leaf spot disease on the genetic map of the A-genome of Arachis, which is based on microsatellite markers and legume anchor markers. These marker types are transferable within the genus Arachis and to other legumes respectively, enabling this map to be aligned to other Arachis maps and to maps of other legume crops including those with sequenced genomes. In total, 34 sequence-confirmed candidate disease resistance genes and five QTLs were mapped.
Candidate genes and QTLs were distributed on all linkage groups except for the smallest, but the distribution was not even. Groupings of candidate genes and QTLs for late leaf spot resistance were apparent on the upper region of linkage group 4 and the lower region of linkage group 2, indicating that these regions are likely to control disease resistance.
PMCID: PMC2739205  PMID: 19698131
7.  A cytosolic invertase is required for normal growth and cell development in the model legume, Lotus japonicus 
Journal of Experimental Botany  2009;60(12):3353-3365.
Neutral/alkaline invertases are a subgroup, confined to plants and cyanobacteria, of a diverse family of enzymes. A family of seven closely-related genes, LjINV1–LjINV7, is described here and their expression in the model legume, Lotus japonicus, is examined. LjINV1 previously identified as encoding a nodule-enhanced isoform is the predominant isoform present in all parts of the plant. Mutants for two isoforms, LjINV1 and LjINV2, were isolated using TILLING. A premature stop codon allele of LjINV2 had no effect on enzyme activity nor did it show a visible phenotype. For LjINV1, premature stop codon and missense mutations were obtained and the phenotype of the mutants examined. Recovery of homozygous mutants was problematic, but their phenotype showed a severe reduction in growth of the root and the shoot, a change in cellular development, and impaired flowering. The cellular organization of both roots and leaves was altered; leaves were smaller and thicker with extra layers of cells and roots showed an extended and broader zone of cell division. Moreover, anthers contained no pollen. Both heterozygotes and homozygous mutants showed decreased amounts of enzyme activity in nodules and shoot tips. Shoot tips also contained up to a 9-fold increased level of sucrose. However, mutants were capable of forming functional root nodules. LjINV1 is therefore crucial to whole plant development, but is clearly not essential for nodule formation or function.
PMCID: PMC2724688  PMID: 19474088
Cellular development; legume; Lotus japonicus; neutral/alkaline invertase; mutants; plant development; sucrose metabolism; TILLING
8.  Functional Adaptation of a Plant Receptor- Kinase Paved the Way for the Evolution of Intracellular Root Symbioses with Bacteria 
PLoS Biology  2008;6(3):e68.
Nitrogen-fixing root nodule symbioses (RNS) occur in two major forms—Actinorhiza and legume-rhizobium symbiosis—which differ in bacterial partner, intracellular infection pattern, and morphogenesis. The phylogenetic restriction of nodulation to eurosid angiosperms indicates a common and recent evolutionary invention, but the molecular steps involved are still obscure. In legumes, at least seven genes—including the symbiosis receptor-kinase gene SYMRK—are essential for the interaction with rhizobia bacteria and for the Arbuscular Mycorrhiza (AM) symbiosis with phosphate-acquiring fungi, which is widespread in occurrence and believed to date back to the earliest land plants. We show that SYMRK is also required for Actinorhiza symbiosis of the cucurbit Datisca glomerata with actinobacteria of the genus Frankia, revealing a common genetic basis for both forms of RNS. We found that SYMRK exists in at least three different structural versions, of which the shorter forms from rice and tomato are sufficient for AM, but not for functional endosymbiosis with bacteria in the legume Lotus japonicus. Our data support the idea that SYMRK sequence evolution was involved in the recruitment of a pre-existing signalling network from AM, paving the way for the evolution of intracellular root symbioses with nitrogen-fixing bacteria.
Author Summary
As an adaptation to nutrient limitations in terrestrial ecosystems, most plants form Arbuscular Mycorrhiza (AM), which is a symbiotic relationship between phosphate-delivering fungi and plant roots that dates back to the earliest land plants. More recently, a small group including the legumes and close relatives has evolved the ability to accommodate nitrogen-fixing bacteria intracellularly. The resulting symbiosis is manifested by the formation of specialized root organs, the nodules, and comes in two forms: the interaction of legumes with rhizobia, and the more widespread Actinorhiza symbiosis of mostly woody plants with Frankia bacteria. The symbiosis receptor kinase SYMRK acts in a signalling pathway that legume plants require to trigger the development of nodules and the uptake of fungi or bacteria into their root cells. Here we show that the induction of Actinorhiza nodulation also relies on SYMRK, consistent with the idea that both types of nodulation evolved by recruiting common signalling genes from the pre-existing AM program. We observed that SYMRK from different land plant lineages differs significantly in exon composition, with a “full-length” version in the nodulating clade and shorter SYMRK genes in plants outside this lineage. Only the most complete SYMRK version was fully functional in nodulation, suggesting this gene played a central role in the recruitment event associated with the evolution of intracellular root symbioses with bacteria.
Root nodule symbioses with nitrogen-fixing bacteria provide many plants with a source of nitrogen. This study uncovers evidence that changes in the gene SYMRK were involved in the evolution of this important biological innovation.
PMCID: PMC2270324  PMID: 18318603
9.  The Most Widespread Symbiosis on Earth 
PLoS Biology  2006;4(7):e239.
PMCID: PMC1489982  PMID: 16822096
10.  Chemotaxis and nod Gene Activity of Bradyrhizobium japonicum in Response to Hydroxycinnamic Acids and Isoflavonoids 
For Bradyrhizobium japonicum, the chemotactic and the nod gene-inducing effects of hydroxycinnamic acids and two of their derivatives were compared with those of isoflavonoids. Only the hydroxycinnamic acids were strong chemoattractants, while the other substances tested were chemotactically inactive. Besides the known nod gene induction by isoflavonoids, a weak nod gene induction by coniferyl alcohol, chlorogenic acid, and ferulic acid was found.
PMCID: PMC182705  PMID: 16348401

Results 1-10 (10)