Molecular and Genetic Insights into Shoot Control of Nodulation in Soybean
Peter M. GresshoffKeishi SenooJaime PadillaDebbie Landau-EllisAnatoli FilatovAlexander KolchinskyGustavo Caetano‐Anollés
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Lateral root
In the course of the symbiotic development of free-living rhizobia to mature bacteroids in alfalfa nodules, mRNA transcripts of the nitrogenase structural genes are first detected when the rhizobia are inside the infection threads. These transcripts are most abundant in the mature bacteroids of young (6 to 8 weeks) root nodules and are present in a much lower level in bacteroids from senescent root nodules (13 weeks). Translation of these and related rhizobial genes essential for the symbiotic association apparently occurs very early in the symbiosis because the protein pattern of the "infection thread rhizobia" is very similar to that of the mature bacteroids and is different from that of the cultured vegetative rhizobia. Although bacteroids have a higher DNA content than the vegetative bacteria and are very different in the nucleoid organization, no specific amplification or rearrangement of the nitrogenase genes is detected in bacteroids.
Sinorhizobium meliloti
Leghemoglobin
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Root hair
Nodule (geology)
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We examined a range of responses of root cortical cells to Rhizobium sp. inoculation to investigate why rhizobia preferentially nodulate legume roots in the zone of emerging root hairs, but generally fail to nodulate the mature root. We tested whether the inability to form nodules in the mature root is due to a lack of plant flavonoids to induce the bacterial genes required for nodulation or a failure of mature cortical cells to respond to Rhizobium spp. When rhizobia were inoculated in the zone of emerging root hairs, changes in β-glucuronidase (GUS) expression from an auxin-responsive promoter (GH3), expression from three chalcone synthase promoters, and the accumulation of specific flavonoid compounds occurred in cortical cells prior to nodule formation. Rhizobia failed to induce these responses when inoculated in the mature root, even when co-inoculated with nod gene-inducing flavonoids. However, mature root hairs remained responsive to rhizobia and could support infection thread formation. This suggests that a deficiency in signal transduction is the reason for nodulation failure in the mature root. However, nodules could be initiated in the mature root at sites of lateral root emergence. A comparison between lateral root and nodule formation showed that similar patterns of GH3:gusA expression, chalcone synthase gene expression, and accumulation of a particular flavonoid compound occurred in the cortical cells involved in both processes. The results suggest that rhizobia can“ hijack” cortical cells next to lateral root emergence sites because some of the early responses required for nodule formation have already been activated by the plant in those cells.
Root hair
Lateral root
Nod factor
Primordium
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Many relict and endemic legumes are often rare and endangered species, located in remote and almost impenetrable areas. This makes it difficult to study their symbiosis with nodule bacteria. The aim of this work was to obtain root nodules under laboratory conditions of Hedysarum zundukii, Oxytropis popoviana, Oxytropis triphylla, Oxytropis tragacanthoides, Astragalus chorinensis and Glycyrrhiza uralensis belonging to the Miocene-Pliocene relicts, usually growing in restricted habitats of the Baikal Lake region. A pot experiment was conducted with soils and seeds collected in distribution areas of these legumes. The collected soils had low content of N and P, suggesting importance of nitrogen-fixing symbiosis for successful maintenance of plant populations. The symbiotic nodules were observed on roots of all plant species tested and 52 bacterial strains assumed as root nodule bacteria (rhizobia) were isolated from nodules. The proposed method avoids collection of native plants and prevents decrease in plant populations. The isolated bacteria will be used to search for ancestral genes responsible for the specificity and efficiency of rhizobia-legume symbiosis and can contribute to the conservation of rare and endangered species of relict legumes in the Baikal region.
Glycyrrhiza uralensis
Nodule (geology)
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Primordium
Pericycle
Lateral root
Apex (geometry)
Apical cell
Root cap
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Root-nodule bacteria (rhizobia) are of great importance for nitrogen acquisition through symbiotic nitrogen fixation in a wide variety of leguminous plants. These bacteria differ from most other soil microorganisms by taking dual forms, i.e. a free-living form in soils and a symbiotic form inside of host legumes. Therefore, they should have a versatile strategy for survival, whether inhabiting soils or root nodules formed through rhizobia-legume interactions. Rhizobia generally contain large amounts of the biogenic amine homospermidine, an analog of spermidine which is an essential cellular component in most living systems. The external pH, salinity and a rapid change in osmolarity are thought to be significant environmental factors affecting the persistence of rhizobia. The present review describes the regulation of homospermidine biosynthesis in response to environmental stress and its possible functional role in rhizobia. Legume root nodules, an alternative habitat of rhizobia, usually contain a variety of biogenic amines besides homospermidine and the occurrence of some of these amines is closely associated with rhizobial infections. In the second half of this review, novel biogenic amines found in certain legume root nodules and the mechanism of their synthesis involving cooperation between the rhizobia and host legume cells are also described.
Bradyrhizobium
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Abstract Rhizobia are soil bacteria that enter into a symbiosis with leguminous plants, producing root outgrowths (nodules) within which the bacteria convert atmospheric nitrogen into ammonia, that is made available to the plants. Legume nodules develop as a result of signal exchange between the symbiotic partners and the rhizobia infect the nodules via specialized infection threads. The bacteria are released into plant cells within the nodule and develop into a specialized form which is primarily dedicated to the production of ammonia via the action of an enzyme called nitrogenase.
Nodule (geology)
Symbiotic bacteria
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Background: Plant growth promoting rhizobia play an important in agricultural ecosystem through symbiotic association with a wide range of leguminous plants. Legume-rhizobia association is a host specific symbiosis hence the need to identify the strains and the diversity of rhizobia associated with specific type of legume for better bio-prospecting of the associated benefits. A wide range of variation is present among the distribution of rhizobia and greatly affected by geographical locality, edaphic factors and environmental variation with time and space. Molecular techniques have been developed to aid the traditional phenotypic and morpho-cultural techniques in distinguishing the different microbial genera, species and strains. The present study aimed at isolation and morpho-molecular characterization of rhizobia from root nodule of green gram cultivated in central part of India.Methods: In the field-laboratory investigation during 2018-2019, various locations of central India with contrasting agro climatic conditions were surveyed and rhizobia trapping done. A total of 40 rhizobia were retrieved from nodules of green gram and characterized in laboratory based on morphological, biochemical and molecular techniques and results validated for taxonomic identification.Result: In our investigation all forty isolated rhizobia were found phosphate solubilizers, 38 IAA producers and 37 ammonia excretors and grew well at 28oC and 37oC. In carbohydrate fermentation test 34 isolates changed the broth colour from red to yellow with gas formation in durum tubes. Twenty diverse rhizobia isolates were selected with respect to their multifunctional properties and studied for molecular characterization. Based on 16S rRNA gene sequencing, these bacterial strains were identified under two genera: Rhizobium and Brady Rhizobium. Phylogenetic analysis divided 20 rhizobia isolates into two clusters. Major group included 12 strains and minor group included 8 strains.
Radiata
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Recently, some research reports showed varied endophytic bacteria in the root nodules of some legumes, which attracts great interest in research field of rhizobia. Here, we reviewed the discovery, identification of some endophytic bacteria (Agrobacterium, non-symbiotic rhizobia, and other bacteria) in root nodules and their influence on symbiosis or plant growth, to understand the microecosystem of root nodule and to extend the field of rhizobia research.
Root (linguistics)
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Abstract Nodulation in legumes involves the coordination of epidermal infection by rhizobia with cell divisions in the underlying cortex. During nodulation, rhizobia are entrapped within curled root hairs to form an infection pocket. Transcellular tubes called infection threads then develop from the pocket and become colonized by rhizobia. The infection thread grows toward the developing nodule primordia and rhizobia are taken up into the nodule cells, where they eventually fix nitrogen. The epidermal and cortical developmental programs are synchronized by a yet-to-be-identified signal that is transmitted from the outer to the inner cell layers of the root. Using a new allele of the Medicago truncatula mutant Lumpy Infections, lin-4, which forms normal infection pockets but cannot initiate infection threads, we show that infection thread initiation is required for normal nodule development. lin-4 forms nodules with centrally located vascular bundles similar to that found in lateral roots rather than the peripheral vasculature characteristic of legume nodules. The same phenomenon was observed in M. truncatula plants inoculated with the Sinorhizobium meliloti exoY mutant, and the M. truncatula vapyrin-2 mutant, all cases where infections arrest. Nodules on lin-4 have reduced expression of the nodule meristem marker MtCRE1 and do not express root-tip markers. In addition, these mutant nodules have altered patterns of gene expression for the cytokinin and auxin markers CRE1 and DR5. Our work highlights the coordinating role that bacterial infection exerts on the developing nodule and allows us to draw comparisons with primitive actinorhizal nodules and rhizobia-induced nodules on the nonlegume Parasponia andersonii.
Medicago truncatula
Root hair
Nodule (geology)
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