Expression of nodulin genes in plant-determined ineffective nodules of pea

Isolation and differential expression of β-1,3-glucanase messenger RNAs, SrGLU3 and SrGLU4, following inoculation of Sesbania rostrata

We report here the isolation and characterisation of two new β-1,3-glucanase cDNAs, SrGLU3 and SrGLU4, from a tropical legume Sesbania rostrata Bremek. & Oberm., which form N₂-fixing nodules on the stem after infection by Azorhizobium caulinodans. SrGLU3 was characterised as being grouped in a branch with tobacco class I β-1,3-glucanases, where the isoforms were reported to be induced by either pathogen infection or ethylene treatment. SrGLU4 was characterised as separate from other classes, and we propose this new branch as a new class (Class VI). The SrGLU3 gene was constitutively expressed in normal stem nodules induced by the wild type strain of A. caulinodans (ORS571), and also even in immature stem nodules induced by a mutant (ORS571-C1), which could not form mature stem-nodules. In contrast, the transcript accumulation of SrGLU4 was hardly detectable in immature nodules inoculated by the ORS571-C1 mutant. We suggest that S. rostrata makes use of SrGLU4 to discriminate between symbionts and non-symbionts (mutants) in developing nodules. We propose the SrGLU4 gene as a new nodulin during nodulation.

A novel fix- symbiotic mutant of Lotus japonicus, Ljsym105, shows impaired development and premature deterioration of nodule infected cells and symbiosomes

Nitrogen-fixing symbiosis between legume plants and rhizobia is established through complex interactions between two symbiotic partners. To identify the host legume genes that play crucial roles in such interactions, we isolated a novel Fix- mutant, Ljsym105, from a model legume Lotus japonicus MG-20. The Ljsym105 plants displayed nitrogen-deficiency symptoms after inoculation with Mesorhizobium loti under nitrogen-free conditions, but their growth recovered when supplied with nitrogen-rich nutrients. Ljsym105 was recessive and monogenic and mapped on the upper portion of chromosome 4. The mutant Ljsym105 formed an increased number of small and pale-pink nodules. Nitrogenase (acetylene reduction) activity per nodule fresh weight was low but retained more than 50% of that of the wild-type nodules. Light and electron microscopic observations revealed that the Ljsym105 nodule infected cells were significantly smaller than those of wild-type plants, contained enlarged symbiosomes with multiple bacteroids, and underwent deterioration of the symbiosomes prematurely as well as disintegration of the whole infected cell cytoplasm. These results indicate that the ineffectiveness of the Ljsym105 nodules is primarily due to impaired growth of infected cells accompanied with the premature senescence induced at relatively early stages of nodule development. These symbiotic phenotypes are discussed in respect to possible functions of the LjSym105 locus in the symbiotic interactions required for establishment of the nitrogen-fixing symbiosis.

cDNA macroarray analysis of gene expression in ineffective nodules induced on the Lotus japonicus sen1 mutant

The Lotus japonicus sen1 mutant forms ineffective nodules in which development is arrested at the stage of bacterial differentiation into nitrogen-fixing bacteroids. Here, we used cDNA macroarray systems to compare gene expression in ineffective nodules induced on the sen1 mutant with gene expression in wild-type nodules, in order to identify the host plant genes that are involved in nitrogen fixation. Macroarray analysis coupled with Northern blot analysis revealed that the expression of 18 genes was significantly enhanced in ineffective sen1 nodules, whereas the expression of 30 genes was repressed. Many of the enhanced genes encoded hydrolase enzymes, such as cysteine proteinase and asparaginase, that might function in the early senescence of sen1 nodules. By contrast, the repressed genes encoded nodulins, enzymes that are involved in carbon and nitrogen metabolism, membrane transporters, enzymes involved in phytohormone metabolism and secondary metabolism, and regulatory proteins. These proteins might have a role in the establishment of nitrogen fixation. In addition, we discovered two novel genes that encoded glutamate-rich proteins and were localized in the vascular bundles of the nodules. The expression of these genes was repressed in the ineffective nodules, which had lower levels of nitrogenase activity.

The Lotus japonicus Sen1 gene controls rhizobial differentiation into nitrogen-fixing bacteroids in nodules

A Lotus japonicus mutant, Ljsym75, which forms ineffective symbiotic nodules and defines a new locus involved in the process of nitrogen fixation, was characterized in detail in order to identify the stage of developmental arrest of the nodules. No nitrogen-fixing activity was detectable in Ljsym75 nodules at any stage during plant development, and plant growth was markedly retarded. Ljsym75 plants formed twice as many nodules as the wild-type Gifu, and this phenotype was not influenced by the application of low concentrations of nitrate. Although the ineffective nodules formed on Ljsym75 were anatomically similar to effective Gifu nodules, Ljsym75 nodules senesced prematurely. Microscopic examination revealed that bacteria endocytosed into Ljsym75 nodules failed to differentiate into bacteroids. Moreover, the bacteria contained no nitrogenase proteins, whereas leghemoglobin was detected in the cytosol of the nodules. These results indicate that Ljsym75 is required for bacterial differentiation into nitrogen-fixing bacteroids in nodules, and thus the Ljsym75 gene was renamed sen1 (for stationary endosymbiont nodule). Linkage analysis using DNA markers showed that Sen1 is located on chromosome 4.

Cloning of nodule-specific cDNAs of Galega orientalis

Differential display was applied in order to clone cDNAs expressed exclusively or predominantly in nodules, compared to uninoculated root tissue of Galega orientalis. Forty-five fragments were unique for nodule RNA. These fragments were reamplified and cloned. Six of them produced a nodule-specific signal on Northern hybridization. These six fragments were sequenced. Five of the sequenced fragments showed homology to nodulin-gene sequences in databases, among them Vicia faba mRNA for protein showing partial homology with Medicago sativa nodulin-25 (Nms25), Pisum sativum PsN466, V. faba CCP2 and CCP4, P. sativum ENOD3, and Maackia amurensis ENOD2. The remaining sequence had no significant homology with sequences in the databanks. Full-size cDNA for the homologue to V. faba mRNA for the protein showing partial homology with M. sativa nodulin-25 (Nms25) and P. sativum PsN466 were cloned and sequenced.

Expression of genes encoding late nodulins characterized by a putative signal peptide and conserved cysteine residues is reduced in ineffective pea nodules

Five nodulin genes, PsN1, PsN6, PsN314, PsN335, and PsN466, with reduced expression in ineffective nodules on the pea (Pisum sativum) mutant E135 (sym13) were characterized. They encode small polypeptides containing a putative signal peptide and conserved cysteine residues and show homology to the nodulins PsENOD3/14 and PsNOD6. For each gene, multiple bands were detected by genomic Southern analysis. Northern analysis showed that all five genes were expressed exclusively in nodules and that their temporal expression patterns were similar to that of the leghemoglobin (Lb) gene during nodule development. Their transcripts were localized predominantly from the interzone II-III to the distal part of nitrogen-fixing zone III in effective nodules, resembling the Lb gene. However, transcripts in ineffective E135 nodules were localized in narrower regions than those in the effective nodules. These results indicate that these nodulins are abundant in pea nodules and that their successive expression during nodule development is associated with nitrogen-fixing activity.

A critical evaluation of differential display as a tool to identify genes involved in legume nodulation: looking back and looking forward

Screening for differentially expressed genes is a straightforward approach to study the molecular basis of a biological system. In the last 10 years, differential screening technology has evolved rapidly and currently high-throughput tools for genome-wide transcript profiling, such as expressed sequence tags and microarray analysis, are becoming widely available. Here, an overview of this (r)evolution is given with emphasis on the differential display method, which for many years has been the preferred technique of scientists in diverse fields of research. Differential display has also been the method of choice for the identification of genes involved in the symbiotic interaction between Azorhizobium caulinodans and Sesbania rostrata. The advantages with respect to tissue specificity of this particular model system for legume nodulation and the results of a screening for early nodulation-related genes have been considered in the context of transcriptome analyses in other rhizobium-legume interactions.

Two types of pea leghemoglobin genes showing different O2-binding affinities and distinct patterns of spatial expression in nodules

Five distinct cDNA clones for leghemoglobin (Lb) were isolated from pea (Pisum sativum) nodules. They were classified into two groups designated PsLbA and PsLbB according to sequence homology, O(2)-binding affinities of the recombinant proteins, and in situ localization of the mRNAs. The PsLbB group was comprised of four cDNA clones: PsLb120-1, -8, -29, and -34. They showed a high similarity of deduced amino acid sequences and O(2)-binding affinities of their recombinant proteins. Among them, the spatial expression pattern of PsLb120-1 was investigated in great detail, indicating that its transcripts were localized in the region from infection zone II to the distal part of nitrogen fixation zone III in effective nodules. PsLb5-10, which is the only cDNA clone of the PsLbA type, differed considerably from the PsLbB type in amino acid sequence, and the recombinant protein had a higher O(2)-binding affinity than those of the PsLbB type. The transcripts of PsLb5-10 were detected throughout the central tissue of effective nodules. However, in ineffective nodules on the pea mutant E135 (sym13), transcripts of PsLb5-10 were restricted to the distal portion of the central tissue as well as those of PsLb120-1. These findings indicate that the pea genome contains two types of Lb genes and suggest that they have different roles in the development of nitrogen-fixing symbiosis in pea nodules.

Widespread occurrence of the homologues of the early nodulin (ENOD) genes in Oryza species and related grasses

Eighty accessions representing 23 species from the genus Oryza were examined for the presence of homologues of early nodulin (ENOD) genes. Southern analyses indicated a widespread distribution of homologues of ENOD genes across all the genomes of rice as well as other monocots. The degree of cross-hybridization of the legume ENOD genes with sequences in the genomes of various species, as revealed by hybridization differentials measured in terms of signal intensities, however, suggests that the homologues of ENOD genes are conserved to varied extents in different Oryza species. The presence of homologues of ENOD genes in a wide variety of plant species denotes that the biological functions of early nodulins may be diverse, and not restricted to nodule organogenesis alone. The fact that ENOD gene homologues exist widely both in dicots and monocots provides evidence that these homologues have arisen from a common ancestral plant.