Stem-Nodulating Legume-Rhizobium Symbiosis and Its Agronomic Use in Lowland Rice

Occurrence and diversity of stem nodulation in Aeschynomene and Sesbania legumes from wetlands of Madagascar

Legumes have the ability to establish a nitrogen-fixing symbiosis with soil rhizobia that they house in specific organs, the nodules. In most rhizobium-legume interactions, nodulation occurs on the root. However, certain tropical legumes growing in wetlands possess a unique trait: the capacity to form rhizobia-harbouring nodules on the stem. Despite the originality of the stem nodulation process, its occurrence and diversity in waterlogging-tolerant legumes remains underexplored, impeding a comprehensive analysis of its genetics and biology. Here, we aimed at filling this gap by surveying stem nodulation in legume species-rich wetlands of Madagascar. Stem nodulation was readily observed in eight hydrophytic species of the legume genera, Aeschynomene and Sesbania, for which significant variations in stem nodule density and morphology was documented. Among these species, A. evenia, which is used as genetic model to study the rhizobial symbiosis, was found to be frequently stem-nodulated. Two other Aeschynomene species, A. cristata and A. uniflora, were evidenced to display a profuse stem-nodulation as occurs in S. rostrata. These findings extend our knowledge on legumes species that are endowed with stem nodulation and further indicate that A. evenia, A. cristata, A. uniflora and S. rostrata are of special interest for the study of stem nodulation. As such, these legume species represent opportunities to investigate different modalities of the nitrogen-fixing symbiosis and this knowledge could provide cues for the engineering of nitrogen-fixation in non-legume crops.

Biological nitrogen fixation and prospects for ecological intensification in cereal-based cropping systems

The demand for nitrogen (N) for crop production increased rapidly from the middle of the twentieth century and is predicted to at least double by 2050 to satisfy the on-going improvements in productivity of major food crops such as wheat, rice and maize that underpin the staple diet of most of the world's population. The increased demand will need to be fulfilled by the two main sources of N supply - biological nitrogen (gas) (N2) fixation (BNF) and fertilizer N supplied through the Haber-Bosch processes. BNF provides many functional benefits for agroecosystems. It is a vital mechanism for replenishing the reservoirs of soil organic N and improving the availability of soil N to support crop growth while also assisting in efforts to lower negative environmental externalities than fertilizer N. In cereal-based cropping systems, legumes in symbiosis with rhizobia contribute the largest BNF input; however, diazotrophs involved in non-symbiotic associations with plants or present as free-living N2-fixers are ubiquitous and also provide an additional source of fixed N. This review presents the current knowledge of BNF by free-living, non-symbiotic and symbiotic diazotrophs in the global N cycle, examines global and regional estimates of contributions of BNF, and discusses possible strategies to enhance BNF for the prospective benefit of cereal N nutrition. We conclude by considering the challenges of introducing in planta BNF into cereals and reflect on the potential for BNF in both conventional and alternative crop management systems to encourage the ecological intensification of cereal and legume production.

Biochemical and Anatomical Investigation of Sesbania herbacea (Mill.) McVaugh Nodules Grown under Flooded and Non-Flooded Conditions

Sesbania herbacea, a native North American fast-growing legume, thrives in wet and waterlogged conditions. This legume enters into symbiotic association with rhizobia, resulting in the formation of nitrogen-fixing nodules on the roots. A flooding-induced anaerobic environment imposes a challenge for the survival of rhizobia and negatively impacts nodulation. Very little information is available on how S. herbacea is able to thrive and efficiently fix N2 in flooded conditions. In this study, we found that Sesbania plants grown under flooded conditions were significantly taller, produced more biomass, and formed more nodules when compared to plants grown on dry land. Transmission electron microscopy of Sesbania nodules revealed bacteroids from flooded nodules contained prominent polyhydroxybutyrate crystals, which were absent in non-flooded nodules. Gas and ion chromatography mass spectrometry analysis of nodule metabolites revealed a marked decrease in asparagine and an increase in the levels of gamma aminobutyric acid in flooded nodules. 2-D gel electrophoresis of nodule bacteroid proteins revealed flooding-induced changes in their protein profiles. Several of the bacteroid proteins that were prominent in flooded nodules were identified by mass spectrometry to be members of the ABC transporter family. The activities of several key enzymes involved in nitrogen metabolism was altered in Sesbania flooded nodules. Aspartate aminotransferase (AspAT), an enzyme with a vital role in the assimilation of reduced nitrogen, was dramatically elevated in flooded nodules. The results of our study highlight the potential of S. herbacea as a green manure and sheds light on the morphological, structural, and biochemical adaptations that enable S. herbacea to thrive and efficiently fix N2 in flooded conditions.

Elucidation of the molecular responses to waterlogging in Sesbania cannabina roots by transcriptome profiling

Sesbania cannabina, a multipurpose leguminous crop, is highly resistant to waterlogging stress. However, the scant genomic resources in the genus Sesbania have greatly hindered further exploration of the mechanisms underlying its waterlogging tolerance. Here, the genetic basis of flooding tolerance in S. cannabina was examined by transcriptome-wide gene expression changes using RNA-Seq in seedlings exposed to short-term (3 h) and long-term (27 h) waterlogging. After de- novo assembly, 213990 unigenes were identified, of which 145162 (79.6%) were annotated. Gene Ontology and pathway enrichment analyses revealed that the glycolysis and fermentation pathways were stimulated to produce ATP under hypoxic stress conditions. Energy-consuming biosynthetic processes were dramatically repressed by short and long term waterlogging, while amino acid metabolism was greatly induced to maintain ATP levels. The expression pattern of 10 unigenes involved in phenylpropanoid biosynthesis, glycolysis, and amino acid metabolism revealed by qRT-PCR confirmed the RNA-Seq data. The present study is a large-scale assessment of genomic resources of Sesbania and provides guidelines for probing the molecular mechanisms underlying S. cannabina waterlogging tolerance.

Nitrogen-fixing symbiosis between photosynthetic bacteria and legumes

Rhizobia having photosynthetic systems form nitrogen-fixing nodules on the stem and/or root of some species of the legumes Aeschynomene and Lotononis. This review is focused on the recent knowledge about the physiology, genetics and role of the photosystem in these bacteria. Photosynthetic electron transport seems to involve reaction centers, soluble cytochrome c2 and cytochrome bc1. Anaerobically, the electron transport system becomes over-reduced. The photosynthesis genes have been partially characterized; their organization is classical but their regulation is unusual as it is activated by far-red light via a bacteriophytochrome. This original mechanism of regulation seems well adapted to promote photosynthesis during stem symbiosis. Photosynthesis plays a major role in the efficiency of stem nodulation. It is also observed that infrared light stimulates nitrogen fixation in nodules containing photosynthetic bacteroids, suggesting that photosynthesis may additionally provides energy for nitrogen fixation, allowing for more efficient plant growth. Other aspects of these bacteria are discussed, in particular their taxonomic position and nodulation ability, the role of carotenoids and the potential for application of photosynthetic rhizobia in rice culture.

Oxygen dependence of photosynthetic electron transport in a bacteriochlorophyll-containing rhizobium

Bacteriochlorophyll-containing rhizobia, which form nitrogen-fixing nodules on the stems and roots of the legume Aeschynomene, grow photosynthetically only in the presence of oxygen or auxiliary electron acceptors. We show that, in whole cells of the Rhizobium strain BTAi 1, a single-turnover excitation flash photooxidized c-type cytochrome under aerobic but not anaerobic conditions. Light-induced fluorescence yield changes show that under anaerobic conditions, the primary acceptor quinone, Q(A), is predominantly in the reduced state and so unable to accept electrons. Thus, as is the case for the aerobic photosynthetic bacterium Roseobacter denitrificans, over-reduction of Q(A) likely prohibits photosynthesis under anaerobic conditions.

The Sesbania Root Symbionts Sinorhizobium saheli and S. teranga bv. sesbaniae Can Form Stem Nodules on Sesbania rostrata, although They Are Less Adapted to Stem Nodulation than Azorhizobium caulinodans

Sesbania species can establish symbiotic interactions with rhizobia from two taxonomically distant genera, including the Sesbania rostrata stem-nodulating Azorhizobium sp. and Azorhizobium caulinodans and the newly described Sinorhizobium saheli and Sinorhizobium teranga bv. sesbaniae, isolated from the roots of various Sesbania species. A collection of strains from both groups were analyzed for their symbiotic properties with different Sesbania species. S. saheli and S. teranga bv. sesbaniae strains were found to effectively stem nodulate Sesbania rostrata, showing that stem nodulation is not restricted to Azorhizobium. Sinorhizobia and azorhizobia, however, exhibited clear differences in other aspects of symbiosis. Unlike Azorhizobium, S. teranga bv. sesbaniae and S. saheli did not induce effective stem nodules on plants previously inoculated on the roots, although stem nodulation was arrested at different stages. For Sesbania rostrata root nodulation, Sinorhizobium appeared more sensitive than Azorhizobium to the presence of combined nitrogen. S. saheli and S. teranga bv. sesbaniae were effective symbionts with all Sesbania species tested, while Azorhizobium strains fixed nitrogen only in symbiosis with Sesbania rostrata. In a simple screening test, S. saheli and S. teranga bv. sesbaniae were incapable of asymbiotic nitrogenase activity. Thus, Azorhizobium can easily be distinguished from Sinorhizobium among Sesbania symbionts on the basis of symbiotic and free-living nitrogen fixation. The ability of Azorhizobium to overcome the systemic plant control appears to be a stem adaptation function. This last property, together with its host-specific symbiotic nitrogen fixation, makes Azorhizobium highly specialized for stem nodulation of the aquatic legume Sesbania rostrata.

Stem and root nodules on the tropical wetland legume Aeschynomene fluminensis

Aeschynomene fluminensis Veil., originally obtained from flooded areas of the Pantanal Matogrossense region of Brazil, was grown under stem-flooded or non-flooded conditions for 70 d after inoculation with isolates of photosynthetic stem nodule rhizobia obtained from native A. fluminensis. Stem nodules formed only on submerged stems of flooded plants (mean of 25 per plant), and did not form on aerial parts, although they were capable of growing and fixing N₂ after drainage of the stems. Root nodules formed on both non-flooded and flooded plants but were usually decreased in number by flooding (from means of 124 to 51 per plant, respectively). Flooding (and stem-nodulation) resulted in an increase in shoot (and a decrease in root) dry weight, regardless of rhizobial isolate. Stem nodules were attached by a wide collar of aerenchymatous tissue at the base of the nodule. There were large air spaces in the stem where nodules were subtended and these were continuous with nodule aerenchyma/outer cortex. In addition, aerenchyma and spongy tissue at the base of the nodule connected both flooded and non-flooded root nodules to large intercellular spaces in the root cortex. The stem and root nodules were ovoid in shape, and essentially aeschynomenoid in type, i.e. the central infected tissue was without uninfected, interstitial cells. Root nodules had a similar structure to stem nodules (although stem nodules were generally larger), and flooded root nodules were approximately twice the size of non-flooded nodules. The infected tissue of root and stem nodules consisted of spherical, bacteroid-containing cells containing one or two rod-shaped bacteroids per peribacteroid unit and prominent organelles. Infection threads were observed in root but not in stem nodules. The cortex of stem and root nodules had an apparent oxygen diffusion barrier, consisting of concentric layers of small cells with interlocking cell walls and few intercellular spaces. Cell layers external to these consisted of larger cells and intercellular spaces, with some spaces being occluded with an electron-dense material that contained a glycoprotein recognized by the monoclonal antibodies MAC236 and MAC265. The amount of glycoprotein occlusions did not appear to differ between nodule types or treatments, although stem nodules contained intracellular glycoprotein vesicles adjacent to cell walls. The exterior of the nodules consisted of an epidermis of thin flattened cells with occasional lenticels. Amyloplasts were common in lower stem and hypocotyl nodules, but fewer in flooded or non-flooded root nodules. Upper stem nodules (i.e. those within 6 cm of the water surface) differed from more profoundly submerged stem nodules by having chloroplasts throughout the cortex. Root nodules did not contain chloroplasts, and undifferentiated plastids were found mainly in lower stem nodules.

Nitrogen-fixing stem nodules of the Legume, Discolobium pulchellum Benth

We report on the structure of N₂ -fixing nodules formed on the stem of Discolobium pulchellum Benth., an aquatic legume in the subfamily Pupilionoideae, tribe Aeschynomeneae, from the Hooded areas in the 'Pantanal Matogrossense' region of Brazil. The stern (and root) nodules were obligately aquatic, requiring permanent submergence in water or flooded soil, and receive oxygen via profuse aerenchyma covering The lower stem. Of the 69 isolates of rhizobia isolated from stem and root nodules, 70% were fast-growing acid producers and 38% were slow growers. The rhizobia were not photosynthetic. Nodules were connected to the stem, and the vascular system from the stem branched throughout the nodule, penetrating the infected, tissue within finger-like ingrowths of cortex. In both stem and root nodules, infected tissue was aeschynomenoid or desmodioid, that is, without uninfected (interstitial) cells. The infected cells in stem nodules were vacuolate, with visible infection threads. The inner cortex was rich in amyloplasts and contained the components of an oxygen diffusion barrier (a boundary cell layer without intercellular spaces and glycoprotein occlusions of intercellular spaces in other cell layers). The mid-cortex, external to the boundary layer, consisted of loosely-packed cells and these were continuous with stem aerenchyma. The outer part of the nodules was made up of phellogen-derived cells forming a periderm, or 'corky' layer of cells. The periderm formed large lenticels above cortical vascular bundles. These lenticels also connected with the stem aerenchyma. Root nodules differed only in that infected cells were not vacuolate, bacteroids were larger and contained more poly-β-hydroxybutyrate (PHB) and there was less aerenchyma/lenticellular tissue. Stem and root nodule structure is discussed in terms of adaptations to O₂ constraints in an aquatic environment.