The activity of nodules of the supernodulating mutant Mtsunn is not limited by photosynthesis under optimal growth conditions

Compartmentalisation: A strategy for optimising symbiosis and tradeoff management

Plant root architecture is developmentally plastic in response to fluctuating nutrient levels in the soil. Part of this developmental plasticity is the formation of dedicated root cells and organs to host mutualistic symbionts. Structures like nitrogen-fixing nodules serve as alternative nutrient acquisition strategies during starvation conditions. Some root systems can also form myconodules-globular root structures that can host mycorrhizal fungi. The myconodule association is different from the wide-spread arbuscular mycorrhization. This range of symbiotic associations provides different degrees of compartmentalisation, from the cellular to organ scale, which allows the plant host to regulate the entry and extent of symbiotic interactions. In this review, we discuss the degrees of symbiont compartmentalisation by the plant host as a developmental strategy and speculate how spatial confinement mitigates risks associated with root symbiosis.

Differential responses of the sunn4 and rdn1-1 super-nodulation mutants of Medicago truncatula to elevated atmospheric CO2

BACKGROUND AND AIMS

Nitrogen fixation in legumes requires tight control of carbon and nitrogen balance. Thus, legumes control nodule numbers via an autoregulation mechanism. 'Autoregulation of nodulation' mutants super-nodulate are thought to be carbon-limited due to the high carbon-sink strength of excessive nodules. This study aimed to examine the effect of increasing carbon supply on the performance of super-nodulation mutants.

METHODS

We compared the responses of Medicago truncatula super-nodulation mutants (sunn-4 and rdn1-1) and wild type to five CO2 levels (300-850 μmol mol-1). Nodule formation and nitrogen fixation were assessed in soil-grown plants at 18 and 42 d after sowing.

KEY RESULTS

Shoot and root biomass, nodule number and biomass, nitrogenase activity and fixed nitrogen per plant of all genotypes increased with increasing CO2 concentration and reached a maximum at 700 μmol mol-1. While the sunn-4 mutant showed strong growth retardation compared with wild-type plants, elevated CO2 increased shoot biomass and total nitrogen content of the rdn1-1 mutant up to 2-fold. This was accompanied by a 4-fold increase in nitrogen fixation capacity in the rdn1-1 mutant.

CONCLUSIONS

These results suggest that the super-nodulation phenotype per se did not limit growth. The additional nitrogen fixation capacity of the rdn1-1 mutant may enhance the benefit of elevated CO2 for plant growth and N2 fixation.

Ammonium acts systemically while nitrate exerts an additional local effect on Medicago truncatula nodules

Symbiotic nitrogen fixation (SNF) has a high energetic cost for legume plants; legumes thus reduce SNF when soil N is available. The present study aimed to increase our understanding regarding the impacts of the two principal forms of available N in soils (ammonium and nitrate) on SNF. We continuously measured the SNF of Medicago truncatula under controlled conditions. This permitted nodule sampling for comparative transcriptome profiling at points connected to the nodules' reaction following ammonium or nitrate applications. The N component of both ions systemically induced a rhythmic pattern of SNF, while the activity in control plants remained constant. This rhythmic activity reduced the per-day SNF. The nitrate ion had additional local effects; the more pronounced were a strong downregulation of leghaemoglobin, nodule cysteine-rich (NCR) peptides and nodule-enhanced nicotianamine synthase (neNAS). The neNAS has proven to be of importance for nodule functioning. Although other physiological impacts of nitrate on nodules were observed (e.g. nitrosylation of leghaemoglobin), the main effect was a rapid ion-specific and organ-specific change in gene expression levels. Contrastingly, during the first hours after ammonium applications, the transcriptome remained virtually unaffected. Therefore, nitrate-induced genes could be key for increasing the nitrate tolerance of SNF.

Adaptation of the symbiotic Mesorhizobium-chickpea relationship to phosphate deficiency relies on reprogramming of whole-plant metabolism

Low inorganic phosphate (Pi) availability is a major constraint for efficient nitrogen fixation in legumes, including chickpea. To elucidate the mechanisms involved in nodule acclimation to low Pi availability, two Mesorhizobium-chickpea associations exhibiting differential symbiotic performances, Mesorhizobium ciceri CP-31 (McCP-31)-chickpea and Mesorhizobium mediterranum SWRI9 (MmSWRI9)-chickpea, were comprehensively studied under both control and low Pi conditions. MmSWRI9-chickpea showed a lower symbiotic efficiency under low Pi availability than McCP-31-chickpea as evidenced by reduced growth parameters and down-regulation of nifD and nifK These differences can be attributed to decline in Pi level in MmSWRI9-induced nodules under low Pi stress, which coincided with up-regulation of several key Pi starvation-responsive genes, and accumulation of asparagine in nodules and the levels of identified amino acids in Pi-deficient leaves of MmSWRI9-inoculated plants exceeding the shoot nitrogen requirement during Pi starvation, indicative of nitrogen feedback inhibition. Conversely, Pi levels increased in nodules of Pi-stressed McCP-31-inoculated plants, because these plants evolved various metabolic and biochemical strategies to maintain nodular Pi homeostasis under Pi deficiency. These adaptations involve the activation of alternative pathways of carbon metabolism, enhanced production and exudation of organic acids from roots into the rhizosphere, and the ability to protect nodule metabolism against Pi deficiency-induced oxidative stress. Collectively, the adaptation of symbiotic efficiency under Pi deficiency resulted from highly coordinated processes with an extensive reprogramming of whole-plant metabolism. The findings of this study will enable us to design effective breeding and genetic engineering strategies to enhance symbiotic efficiency in legume crops.

Long-term non-invasive and continuous measurements of legume nodule activity

Symbiotic nitrogen fixation is a process of considerable economic, ecological and scientific interest. The central enzyme nitrogenase reduces H(+) alongside N2 , and the evolving H2 allows a continuous and non-invasive in vivo measurement of nitrogenase activity. The objective of this study was to show that an elaborated set-up providing such measurements for periods as long as several weeks will produce specific insight into the nodule activity's dependence on environmental conditions and genotype features. A system was developed that allows the air-proof separation of a root/nodule and a shoot compartment. H2 evolution in the root/nodule compartment can be monitored continuously. Nutrient solution composition, temperature, CO2 concentration and humidity around the shoots can concomitantly be maintained and manipulated. Medicago truncatula plants showed vigorous growth in the system when relying on nitrogen fixation. The set-up was able to provide specific insights into nitrogen fixation. For example, nodule activity depended on the temperature in their surroundings, but not on temperature or light around shoots. Increased temperature around the nodules was able to induce higher nodule activity in darkness versus light around shoots for a period of as long as 8 h. Conditions that affected the N demand of the shoots (ammonium application, Mg or P depletion, super numeric nodules) induced consistent and complex daily rhythms in nodule activity. It was shown that long-term continuous measurements of nodule activity could be useful for revealing special features in mutants and could be of importance when synchronizing nodule harvests for complex analysis of their metabolic status.

Symbiotic nitrogen fixation in legume nodules: metabolism and regulatory mechanisms

The special issue “Symbiotic Nitrogen Fixation in Legume Nodules: Metabolism and Regulatory Mechanisms” aims to investigate the physiological and biochemical advances in the symbiotic process with an emphasis on nodule establishment, development and functioning. The original research articles included in this issue provide important information regarding novel aspects of nodule metabolism and various regulatory pathways, which could have important future implications. This issue also included one review article that highlights the importance of using legume trees in the production of renewable biofuels.

The carbon-nitrogen balance of the nodule and its regulation under elevated carbon dioxide concentration

Legumes have developed a unique way to interact with bacteria: in addition to preventing infection from pathogenic bacteria like any other plant, legumes also developed a mutualistic symbiotic relationship with one gender of soil bacteria: rhizobium. This interaction leads to the development of a new root organ, the nodule, where the differentiated bacteria fix for the plant the atmospheric dinitrogen (atmN₂). In exchange, the symbiont will benefit from a permanent source of carbon compounds, products of the photosynthesis. The substantial amounts of fixed carbon dioxide dedicated to the symbiont imposed to the plant a tight regulation of the nodulation process to balance carbon and nitrogen incomes and outcomes. Climate change including the increase of the concentration of the atmospheric carbon dioxide is going to modify the rates of plant photosynthesis, the balance between nitrogen and carbon, and, as a consequence, the regulatory mechanisms of the nodulation process. This review focuses on the regulatory mechanisms controlling carbon/nitrogen balances in the context of legume nodulation and discusses how the change in atmospheric carbon dioxide concentration could affect nodulation efficiency.