Developmental biology of legume nodulation (original) (raw)
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Tansley Review No. 40. Developmental biology of legume nodulation
New phytologist, 1992
Many legumes respond to Rhizobium inoculation by developing unique structures known as nodules on their roots. The development of a legume nodule in which rhizobia convert atmospheric N.j into ammonia is a finely tuned process. Gene expression from both partners of the symbiosis must be temporally and spatially coordinated. Exactly how this coordination takes place is an area of intense study. Nodule morphogenesis appears to be elicited by at least two distinct signals: one from Rhizobium, a product of the nod genes (Nod factor), and a second signal, which is generated within plant tissues after treatment with Nod factor. The identity of the second signal is unknown but changes in the balance of endogenous plant hormones or the sensitivity of plant tissues to these hormones are likely to be involved. These hormonal changes may be triggered by endogenous flavonoids produced by the root in response to inoculation with Rhizobium. There is some controversy as to whether the legume nodule is an organ sui generis or a highly derived lateral root. A resolution of this question may become more critical as attempts to induce nodules on non-legume hosts, such as rice or maize, increase in number and scope.
Hormonal Control of Lateral Root and Nodule Development in Legumes
Plants, 2015
Many plants can establish symbioses with nitrogen-fixing bacteria, some of which lead to nodulation, including legumes. Indeed, in the rhizobium/legume symbiosis, new root organs, called nodules, are formed by the plant in order to host the rhizobia in protective conditions, optimized for nitrogen fixation. In this way, these plants can benefit from the reduction of atmospheric dinitrogen into ammonia by the hosted bacteria, and in exchange the plant provides the rhizobia with a carbon source. Since this symbiosis is costly for the plant it is highly regulated. Both legume nodule and lateral root organogenesis involve divisions of the root inner tissues, and both developmental programs are tightly controlled by plant hormones. In fact, most of the major plant hormones, such as auxin, cytokinins, abscisic acid, and strigolactones, control both lateral root formation and nodule organogenesis, but often in an opposite manner. This suggests that the sensitivity of legume plants to some phytohormones could be linked to the antagonism that exists between the processes of nodulation and lateral root formation. Here, we will review the implication of some major phytohormones in lateral root formation in legumes, compare them with their roles in nodulation, and discuss specificities and divergences from non-legume eudicot plants such as Arabidopsis thaliana.
Plant signaling & behavior, 2009
Combined nitrogen (N) sources are known to strongly affect initiation, development and functioning of Nitrogen-Fixing-Nodules whose formation is triggered by lipochitin-oligosaccharide signals secreted in the rhizospere by the Rhizobium partner. The rapid effects of N supply on nodule initiation have been mainly described when N sources are present at the moment of Rhizobium inoculation or purified Nod Factors addition. We recently reported that high ammonium nitrate growth conditions might also strongly affect the nodulation competence of Lotus japonicus plants, prior to the Rhizobium inoculation. This is a long-term effect, which suggests a change of the general nutritional status as the signal controlling the reduced nodulation capacities. The mechanisms underlying these inhibitory pathways are apparently different and the identification of the molecular actors involved may provide new insights into the linkage between N environmental changes and root organogenesis programs.
No Home without Hormones: How Plant Hormones Control Legume Nodule Organogenesis
Plant Communications, 2020
The establishment of symbiotic nitrogen fixation requires the coordination of both nodule development and infection events. Despite the evolution of a variety of anatomical structures, nodule organs serve a common purpose in establishing a localized area that facilitates efficient nitrogen fixation. As in all plant developmental processes, the establishment of a new nodule organ is regulated by plant hormones. During nodule initiation, regulation of plant hormone signaling is one of the major targets of symbiotic signaling. We review the role of major developmental hormones in the initiation of the nodule organ and argue that the manipulation of plant hormones is a key requirement for engineering nitrogen fixation in non-legumes as the basis for improved food security and sustainability.
The article has been written in view of the importance attached to symbiotic N2-fixation taking place in unique organs called legume root nodules under tropics and subtropics. Nodules on legume roots are formed in interaction with soil bacteria a and b rhizobia. Symbiotic interactions between compatible legume host plant and rhizobia involve a fine tuned, molecular communication between the two partners. Calcium has been reported to playa crucial role in symbiotic signaling. Nod factors are central to the initial establishment of legume -rhizobial symbiosis. Production of these signaling molecules is activated by the release of plant phenolics, mainly flavonoids, in the rhizosphere, where they induce a set of nod genes in the appropriate rhizobial strain, leading to synthesis of Nod factors. The nature of both flavonoid signal from plant and Nod factor from the microbial partner are central to the maintenance of specificity in this symbiosis, ensuring that plant accommodates only the useful bacterium. Generally the invasion of plant root occurs through an invagination of root hair cell, called infection thread, at the primary site of interaction. The infection thread spans through the entire root cortex allowing rhizobial invasion into dividing cells of nodule primordium. Rhizobia are released from infection thread into membrane enclosed compartments, where they convert from free living form to N2-fixing form named the bacteroids. Development of functional nodules requires differentiation of both plant and microbial cells. Transcriptomics and proteomics reveal truly great extent of plant and microbial differentiation. Symbiotic N2-fixation is a finely regulated process that involves carbon and energy metabolism of the host plant significantly. The process is also under regulation by N-feedback and °2 supply within the nodules. Redox balance and antioxidant defense system play important roles in establishment of legume-rhizobial symbiosis as well as nodule functioning. Longevity and N2-fixing efficiency of nodules are hugely dependent on environmental conditions prevailing in tropical and subtropical conditions.
Molecular Analysis of Legume Nodule Development and Autoregulation
Journal of Integrative Plant Biology, 2010
Legumes are highly important food, feed and biofuel crops. With few exceptions, they can enter into an intricate symbiotic relationship with specific soil bacteria called rhizobia. This interaction results in the formation of a new root organ called the nodule in which the rhizobia convert atmospheric nitrogen gas into forms of nitrogen that are useable by the plant. The plant tightly controls the number of nodules it forms, via a complex root-to-shoot-to-root signaling loop called autoregulation of nodulation (AON). This regulatory process involves peptide hormones, receptor kinases and small metabolites. Using modern genetic and genomic techniques, many of the components required for nodule formation and AON have now been isolated. This review addresses these recent findings, presents detailed models of the nodulation and AON processes, and identifies gaps in our understanding of these process that have yet to be fully explained. Ferguson BJ, Indrasumunar A, Hayashi S, Lin MH, Lin YH, Reid DE, Gresshoff PM (2010) Molecular analysis of legume nodule development and autoregulation.
Molecular dissection and improvement of the nodule symbiosis in legumes
Field Crops Research, 1997
The initiation and development of nitrogen (N 2) fixing nodules in the roots of leguminous plants occurs by the induction of cell division and redifferentiation in the root cortex, followed by the formation of a meristem and progressive differentiation of specialized cells and tissues. During this process, competent rhizobia invade the root and become specialized N2-fixing endosymbionts. The onset of the symbiosis is largely mediated by an exchange of diffusible signals, bacterial lipo-oligosaccharides being the main determinants of specificity and the initial inducers of plant responses. It is however the host which controls most facets of the nodulation process, including nodule morphology, efficiency, specificity and function. The dissection of plant mechanisms underlying signal-transduction during nodulation may be crucial to understand and then manipulate the symbiosis. Positional cloning or gene targeting offer strategies that promise the identification of crucial plant genes determining nodulation. The search for the nts -1 gene that controls nodulation in soybean illustrates the challenges and limitations of positional cloning. It also shows how biotechnology can offer tools to help in the breeding of plant traits important to agriculture. Molecular dissection of the symbiosis will ultimately be used to improve N 2 fixation by molecular breeding and genetic engineering in legumes. © 1997 Elsevier Science B.V.
Roles of plant hormones in legume nodulation
Biotechnology, 2008
Legumes play a crucial role in agricultural and ecosystems based on their ability to convert atmospheric nitrogen gas into plant-available ammonium in symbiosis with soil bacteria commonly called 'rhizobia'. This nitrogen-fixing ability makes legumes attractive as an agricultural, economical and environmentally friendly crop as their requirement for nitrogen fertilizer is reduced, leading to a reduction of fertilizer runoff into ground and surface waters. Such contamination may lead to human health and environmental quality effects. In recent years, the cost of industrial nitrogen fertilizers has increased dramatically due to the rising cost of fossil fuel. This makes legumes more attractive as they provide alternatives to nitrogen fertilizer in a sustainable production system for manufacture of both food and fuel.