RESUMEN
Legumes have the capacity to develop root nodules hosting nitrogen-fixing bacteria, called rhizobia. For the plant, the benefit of the symbiosis is important in nitrogen-deprived conditions, but it requires hosting and feeding massive numbers of rhizobia. Recent studies suggest that innate immunity is reduced or suppressed within nodules [1-10]; this likely maintains viable rhizobial populations. To evaluate the potential consequences and risks associated with an altered immuni`ty in the symbiotic organ, we developed a tripartite system with the model legume Medicago truncatula [11, 12], its nodulating symbiont of the genus Sinorhizobium (syn. Ensifer) [13, 14], and the pathogenic soil-borne bacterium Ralstonia solanacearum [15-18]. We show that nodules are frequent infection sites where pathogen multiplication is comparable to that in the root tips and independent of nodule ability to fix nitrogen. Transcriptomic analyses indicate that, despite the presence of the hosted rhizobia, nodules are able to develop weak defense reactions against pathogenic R. solanacearum. Nodule defense response displays specificity compared to that activated in roots. In agreement with nodule innate immunity, optimal R. solanacearum growth requires pathogen virulence factors. Finally, our data indicate that the high susceptibility of nodules is counterbalanced by the existence of a diffusion barrier preventing pathogen spreading from nodules to the rest of the plant.
Asunto(s)
Medicago truncatula/microbiología , Enfermedades de las Plantas/microbiología , Ralstonia solanacearum/fisiología , Nódulos de las Raíces de las Plantas/microbiología , Sinorhizobium meliloti/fisiología , Sinorhizobium/fisiología , Medicago truncatula/inmunología , Inmunidad de la Planta , Nódulos de las Raíces de las Plantas/inmunologíaRESUMEN
Legumes form root nodules and fix atmospheric nitrogen by establishing symbiosis with rhizobia. However, excessive root nodules are harmful to plants because of the resulting overconsumption of energy from photosynthates. The delay of an inoculation of the soybean super-nodulation mutant NOD1-3 with Bradyrhizobium diazoefficiens USDA110T by 5 d after an inoculation with several soil bacteria confirmed that one bacterial group significantly decreased root nodules throughout the study period. Moreover, no significant changes were observed in nitrogen fixation by root nodules between an inoculation with USDA 110T only and co-inoculation treatments. To clarify the potential involvement of PR proteins in the restriction of nodule formation in the plants tested, the relative expression levels of PR-1, PR-2, PR-5, and PDF1.2 in NOD1-3 roots were measured using real-time PCR. One group of soil bacteria (Gr.3), which markedly reduced nodule numbers, significantly induced the expression of PR-1, PR-5 and PDF1.2 genes by day 5 after the inoculation. By days 7, 10, and 20 after the inoculation, the expression levels of PR-2 and PR-5 were lower than those with the uninoculated treatment. Inoculations with this group of soil bacteria resulted in lower root nodule numbers than with other tested soil bacteria exerting weak inhibitory effects on nodulation, and were accompanied by the induction of plant defense-related genes. Thus, PR genes appear to play important roles in the mechanisms that suppresses nodule formation on soybean roots.
Asunto(s)
Fenómenos Fisiológicos Bacterianos , Bradyrhizobium/fisiología , Regulación de la Expresión Génica de las Plantas , Glycine max/inmunología , Proteínas de Plantas/genética , Nodulación de la Raíz de la Planta/inmunología , Mutación , Fijación del Nitrógeno , Nodulación de la Raíz de la Planta/genética , Nódulos de las Raíces de las Plantas/inmunología , Nódulos de las Raíces de las Plantas/microbiología , Microbiología del Suelo , Glycine max/microbiología , SimbiosisRESUMEN
By evolving the dual capacity of intracellular survival and symbiotic nitrogen fixation in legumes, rhizobia have achieved an ecological and evolutionary success that has reshaped our biosphere. Despite complex challenges, including a dual lifestyle of intracellular infection separated by a free-living phase in soil, rhizobial symbiosis has spread horizontally to hundreds of bacterial species and geographically throughout the globe. This symbiosis has also persisted and been reshaped through millions of years of history. Here, we summarize recent advances in our understanding of the molecular mechanisms, ecological settings, and evolutionary pathways that are collectively responsible for this symbiotic success story. We offer predictions of how this symbiosis can evolve under new influences and for the benefit of a burgeoning human population.
Asunto(s)
Fijación del Nitrógeno/fisiología , Raíces de Plantas/microbiología , Rhizobium/fisiología , Simbiosis/fisiología , Fijación del Nitrógeno/genética , Nódulos de las Raíces de las Plantas/inmunología , Simbiosis/genéticaRESUMEN
Medicago truncatula belongs to the legume family and forms symbiotic associations with nitrogen fixing bacteria, the rhizobia. During these interactions, the plants develop root nodules in which bacteria invade the plant cells and fix nitrogen for the benefit of the plant. Despite massive infection, legume nodules do not develop visible defence reactions, suggesting a special immune status of these organs. Some factors influencing rhizobium maintenance within the plant cells have been previously identified, such as the M. truncatula NCR peptides whose toxic effects are reduced by the bacterial protein BacA. In addition, DNF2, SymCRK, and RSD are M. truncatula genes required to avoid rhizobial death within the symbiotic cells. DNF2 and SymCRK are essential to prevent defence-like reactions in nodules after bacteria internalization into the symbiotic cells. Herein, we used a combination of genetics, histology and molecular biology approaches to investigate the relationship between the factors preventing bacterial death in the nodule cells. We show that the RSD gene is also required to repress plant defences in nodules. Upon inoculation with the bacA mutant, defence responses are observed only in the dnf2 mutant and not in the symCRK and rsd mutants. In addition, our data suggest that lack of nitrogen fixation by the bacterial partner triggers bacterial death in nodule cells after bacteroid differentiation. Together our data indicate that, after internalization, at least four independent mechanisms prevent bacterial death in the plant cell. These mechanisms involve successively: DNF2, BacA, SymCRK/RSD and bacterial ability to fix nitrogen.
Asunto(s)
Proteínas Bacterianas/genética , Medicago truncatula/inmunología , Inmunidad de la Planta , Proteínas de Plantas/genética , Sinorhizobium meliloti/fisiología , Proteínas Bacterianas/metabolismo , Citoplasma/metabolismo , Medicago truncatula/citología , Medicago truncatula/genética , Medicago truncatula/metabolismo , Mutación , Nitrógeno/metabolismo , Fijación del Nitrógeno , Fenotipo , Proteínas de Plantas/metabolismo , Raíces de Plantas/citología , Raíces de Plantas/genética , Raíces de Plantas/inmunología , Raíces de Plantas/metabolismo , Nódulos de las Raíces de las Plantas/citología , Nódulos de las Raíces de las Plantas/genética , Nódulos de las Raíces de las Plantas/inmunología , Nódulos de las Raíces de las Plantas/metabolismo , SimbiosisRESUMEN
Rhizobia and legumes establish symbiotic interactions leading to the production of root nodules, in which bacteria fix atmospheric nitrogen for the plant's benefit. This symbiosis is efficient because of the high rhizobia population within nodules. Here, we investigated how legumes accommodate such bacterial colonization. We used a reverse genetic approach to identify a Medicago truncatula gene, SymCRK, which encodes a cysteine-rich receptor-like kinase that is required for rhizobia maintenance within the plant cells, and performed detailed phenotypic analyses of the corresponding mutant. The Medicago truncatula symCRK mutant developed nonfunctional and necrotic nodules. A nonarginine asparate (nonRD) motif, typical of receptors involved in innate immunity, is present in the SymCRK kinase domain. Similar to the dnf2 mutant, bacteroid differentiation defect, defense-like reactions and early senescence were observed in the symCRK nodules. However, the dnf2 and symCRK nodules differ by their degree of colonization, which is higher in symCRK. Furthermore, in contrast to dnf2, symCRK is not a conditional mutant. These results suggest that in M. truncatula at least two genes are involved in the symbiotic control of immunity. Furthermore, phenotype differences between the two mutants suggest that two distinct molecular mechanisms control suppression of plant immunity during nodulation.
Asunto(s)
Medicago truncatula/enzimología , Medicago truncatula/inmunología , Proteínas de Plantas/metabolismo , Proteínas Quinasas/metabolismo , Nódulos de las Raíces de las Plantas/inmunología , Simbiosis/inmunología , Secuencia de Aminoácidos , Regulación de la Expresión Génica de las Plantas , Genes de Plantas , Medicago truncatula/genética , Medicago truncatula/microbiología , Datos de Secuencia Molecular , Fijación del Nitrógeno/genética , Inmunidad de la Planta/genética , Proteínas de Plantas/química , Proteínas de Plantas/genética , Proteínas Quinasas/química , Proteínas Quinasas/genética , Nódulos de las Raíces de las Plantas/genética , Nódulos de las Raíces de las Plantas/microbiología , Sinorhizobium melilotiRESUMEN
Legume plants are able to engage in root nodule symbiosis with nitrogen-fixing soil bacteria, collectively called rhizobia. This mutualistic association is highly specific, such that each rhizobial species/strain interacts with only a specific group of legumes, and vice versa. Symbiosis specificity can occur at multiple phases of the interaction, ranging from initial bacterial attachment and infection to late nodule development associated with nitrogen fixation. Genetic control of symbiosis specificity is complex, involving fine-tuned signal communication between the symbiotic partners. Here we review our current understanding of the mechanisms used by the host and bacteria to choose their symbiotic partners, with a special focus on the role that the host immunity plays in controlling the specificity of the legume - rhizobial symbiosis.
Asunto(s)
Fabaceae/microbiología , Rhizobiaceae/fisiología , Proteínas Bacterianas/fisiología , Fabaceae/inmunología , Fabaceae/metabolismo , Fijación del Nitrógeno , Inmunidad de la Planta , Lectinas de Plantas/fisiología , Polisacáridos Bacterianos/fisiología , Rhizobiaceae/inmunología , Rhizobiaceae/metabolismo , Nódulos de las Raíces de las Plantas/inmunología , Nódulos de las Raíces de las Plantas/microbiología , Especificidad de la Especie , SimbiosisRESUMEN
Legume root nodules are sites of intense biochemical activity and consequently are at high risk of damage as a result of the generation of reactive oxygen species (ROS) and reactive nitrogen species (RNS). These molecules can potentially give rise to oxidative and nitrosative damage but, when their concentrations are tightly controlled by antioxidant enzymes and metabolites, they also play positive roles as critical components of signal transduction cascades during nodule development and stress. Thus, recent advances in our understanding of ascorbate and (homo)glutathione biosynthesis in plants have opened up the possibility of enhancing N(2) fixation through an increase of their concentrations in nodules. It is now evident that antioxidant proteins other than the ascorbate-glutathione enzymes, such as some isoforms of glutathione peroxidases, thioredoxins, peroxiredoxins, and glutathione S-transferases, are also critical for nodule activity. To avoid cellular damage, nodules are endowed with several mechanisms for sequestration of Fenton-active metals (nicotianamine, phytochelatins, and metallothioneins) and for controlling ROS/RNS bioactivity (hemoglobins). The use of 'omic' technologies has expanded the list of known antioxidants in plants and nodules that participate in ROS/RNS/antioxidant signaling networks, although aspects of developmental variation and subcellular localization of these networks remain to be elucidated. To this end, a critical point will be to define the transcriptional and post-transcriptional regulation of antioxidant proteins.
Asunto(s)
Antioxidantes/metabolismo , Fabaceae/inmunología , Nódulos de las Raíces de las Plantas/inmunología , Fabaceae/metabolismo , Estrés Oxidativo , Proteínas de Plantas/metabolismo , Transducción de SeñalRESUMEN
Water-tolerant nodulation is an adaptation of legumes that grow in wet or temporarily flooded habitats. This nodulation mode takes place at lateral root bases via intercellular bacterial invasion in cortical infection pockets. The tropical legume Sesbania rostrata has become a model for the study of the molecular basis of crack entry nodulation compared with root hair curl nodulation. For intercellular invasion, Nodulation Factor (NF) signalling recruits an ethylene-dependent, common Sym gene-independent pathway, leading to local cell death. The NF structure requirements are less stringent than for intracellular invasion in root hairs, which is correlated with a very specific NF-induced calcium spiking signature, presumably necessary for correct gene expression to assemble a functional entry complex in the epidermis.