Iron-induced nitric oxide leads to an increase in the expression of ferritin during the senescence of Lotus japonicus nodules

Dynamics of hemoglobins during nodule development, nitrate response, and dark stress in Lotus japonicus

Legume nodules express multiple leghemoglobins (Lbs) and non-symbiotic hemoglobins (Glbs), but how they are regulated is unclear. Here, we study the regulation of all Lbs and Glbs of Lotus japonicus in different physiologically relevant conditions and mutant backgrounds. We quantified hemoglobin expression, localized reactive oxygen species (ROS) and nitric oxide (NO) in nodules, and deployed mutants deficient in Lbs and in the transcription factors NLP4 (associated with nitrate sensitivity) and NAC094 (associated with senescence). Expression of Lbs and class 2 Glbs was suppressed by nitrate, whereas expression of class 1 and 3 Glbs was positively correlated with external nitrate concentrations. Nitrate-responsive elements were found in the promoters of several hemoglobin genes. Mutant nodules without Lbs showed accumulation of ROS and NO and alterations of antioxidants and senescence markers. NO accumulation occurred by a nitrate-independent pathway, probably due to the virtual disappearance of Glb1-1 and the deficiency of Lbs. We conclude that hemoglobins are regulated in a gene-specific manner during nodule development and in response to nitrate and dark stress. Mutant analyses reveal that nodules lacking Lbs experience nitro-oxidative stress and that there is compensation of expression between Lb1 and Lb2. They also show modulation of hemoglobin expression by NLP4 and NAC094.

Proteomic and Genomic Studies of Micronutrient Deficiency and Toxicity in Plants

Micronutrients are essential for plants. Their growth, productivity and reproduction are directly influenced by the supply of micronutrients. Currently, there are eight trace elements considered to be essential for higher plants: Fe, Zn, Mn, Cu, Ni, B, Mo, and Cl. Possibly, other essential elements could be discovered because of recent advances in nutrient solution culture techniques and in the commercial availability of highly sensitive analytical instrumentation for elemental analysis. Much remains to be learned about the physiology of micronutrient absorption, translocation and deposition in plants, and about the functions they perform in plant growth and development. With the recent advancements in the proteomic and molecular biology tools, researchers have attempted to explore and address some of these questions. In this review, we summarize the current knowledge of micronutrients in plants and the proteomic/genomic approaches used to study plant nutrient deficiency and toxicity.

Characteristics and Research Progress of Legume Nodule Senescence

Delaying the nodule senescence of legume crops can prolong the time of nitrogen fixation and attenuate the lack of fertilizer in the later stage of legume crop cultivation, resulting in improved crop yield and reduced usage of nitrogen fertilizer. However, effective measures to delay the nodule senescence of legume crops in agriculture are relatively lacking. In the present review, we summarized the structural and physiological characteristics of nodule senescence, as well as the corresponding detection methods, providing technical support for the identification of nodule senescence phenotype. We then outlined the key genes currently known to be involved in the regulation of nodule senescence, offering the molecular genetic information for breeding varieties with delayed nodule senescence. In addition, we reviewed various abiotic factors affecting nodule senescence, providing a theoretical basis for the interaction between molecular genetics and abiotic factors in the regulation of nodule senescence. Finally, we briefly prospected research foci of nodule senescence in the future.

Effect of ferritin on nitrogen fixation in Lotus japonicus nodules under various iron concentrations

In the nitrogen fixation process, iron plays a vital role by being part of many symbiotic proteins, such as nitrogenase and leghemoglobin, in an active symbiosis. Excess or insufficient iron in active nitrogen fixation negatively affects the entire process. In Lotus japonicus nodules, ferritin is expressed at the initial stages of nodule development and increases at the nodule senescence stage to mobilize iron release during that stage. In this study, we investigated the effects of overexpressing and suppressing ferritin on nitrogen fixation. Acetylene reduction activity revealed that nitrogen fixation is affected by the overexpression of ferritin at high iron concentrations, but at low iron concentrations, higher nitrogen fixation was observed in ferritin-suppressed plants. qRT-PCR data indicated that suppression of ferritin in nodules induces antioxidant genes, such as superoxide dismutase, dehydroascorbate reductase and ascorbate peroxidase, to detoxify reactive oxygen species. Our data suggest that suppressing ferritin in the nodules is effective for higher nitrogen fixation under iron deficient conditions. Overaccumulated ferritin in nodule is effective under the higher iron conditions, such as senescence state.

Recent progress in the knowledge on the alleviating effect of nitric oxide on heavy metal stress in plants

Recently, nitric oxide (NO), a redox-related signaling molecule, is considered to be a key regulator in plant growth and development as well as response to abiotic stresses. Heavy metal (HM) stress is one of the most serious threats to affect crop growth and production. HM stress attributes to the production of reactive oxygen species (ROS), leading to oxidative stress in plants. Thus, to minimize the toxic effects of HM stress, plants directly or indirectly activate different ROS-scavenging mechanisms comprised antioxidative enzymes and non-enzymatic antioxidants. Understanding the roles of NO is essential to elucidate how NO activates the appropriate set of responses to HM stress. Moreover, the regulation of key genes or proteins is very important in response to stress stimuli. Therefore, here we focus on the recent knowledge concerning the alleviating effect of NO on HM stress, covering from HM iron accumulation to antioxidant system to related gene and protein expression.

Nitric oxide signaling, metabolism and toxicity in nitrogen-fixing symbiosis

Interactions between legumes and rhizobia lead to the establishment of a symbiotic relationship characterized by the formation of a new organ, the nodule, which facilitates the fixation of atmospheric nitrogen (N2) by nitrogenase through the creation of a hypoxic environment. Significant amounts of nitric oxide (NO) accumulate at different stages of nodule development, suggesting that NO performs specific signaling and/or metabolic functions during symbiosis. NO, which regulates nodule gene expression, accumulates to high levels in hypoxic nodules. NO accumulation is considered to assist energy metabolism within the hypoxic environment of the nodule via a phytoglobin-NO-mediated respiration process. NO is a potent inhibitor of the activity of nitrogenase and other plant and bacterial enzymes, acting as a developmental signal in the induction of nodule senescence. Hence, key questions concern the relative importance of the signaling and metabolic functions of NO versus its toxic action and how NO levels are regulated to be compatible with nitrogen fixation functions. This review analyses these paradoxical roles of NO at various stages of symbiosis, and highlights the role of plant phytoglobins and bacterial hemoproteins in the control of NO accumulation.

Nitric oxide in plants: pro- or anti-senescence

Senescence is a regulated process of tissue degeneration that can affect any plant organ and consists of the degradation and remobilization of molecules to other growing tissues. Senescent organs display changes at the microscopic level as well as modifications to internal cellular structure and differential gene expression. A large number of factors influencing senescence have been described including age, nutrient supply, and environmental interactions. Internal factors such as phytohormones also affect the timing of leaf senescence. A link between the senescence process and the production of nitric oxide (NO) in senescing tissues has been known for many years. Remarkably, this link can be either a positive or a negative correlation depending upon the organ. NO can be both a signaling or a toxic molecule and is known to have multiple roles in plants; this review considers the duality of NO roles in the senescence process of two different plant organs, namely the leaves and root nodules.

Nitric oxide and plant mineral nutrition: current knowledge

Plants under conditions of essential mineral deficiency trigger signaling mechanisms that involve common components. Among these components, nitric oxide (NO) has been identified as a key participant in responses to changes in nutrient availability. Usually, nutrient imbalances affect the levels of NO in specific plant tissues, via modification of its rate of synthesis or degradation. Changes in the level of NO affect plant morphology and/or trigger responses associated with nutrient homeostasis, mediated by its interaction with reactive oxygen species, phytohormones, and through post-translational modification of proteins. NO-related events constitute an exciting field of research to understand how plants adapt and respond to conditions of nutrient shortage. This review summarizes the current knowledge on NO as a component of the multiple processes related to plant performance under conditions of deficiency in mineral nutrients, focusing on macronutrients such as nitrogen, phosphate, potassium, and magnesium, as well as micronutrients such as iron and zinc.

Inoculation with Efficient Nitrogen Fixing and Indoleacetic Acid Producing Bacterial Microsymbiont Enhance Tolerance of the Model Legume Medicago truncatula to Iron Deficiency

The aim of this study was to assess the effect of symbiotic bacteria inoculation on the response of Medicago truncatula genotypes to iron deficiency. The present work was conducted on three Medicago truncatula genotypes: A17, TN8.20, and TN1.11. Three treatments were performed: control (C), direct Fe deficiency (DD), and induced Fe deficiency by bicarbonate (ID). Plants were nitrogen-fertilized (T) or inoculated with two bacterial strains: Sinorhizobium meliloti TII7 and Sinorhizobium medicae SII4. Biometric, physiological, and biochemical parameters were analyzed. Iron deficiency had a significant lowering effect on plant biomass and chlorophyll content in all Medicago truncatula genotypes. TN1.11 showed the highest lipid peroxidation and leakage of electrolyte under iron deficiency conditions, which suggest that TN1.11 was more affected than A17 and TN8.20 by Fe starvation. Iron deficiency affected symbiotic performance indices of all Medicago truncatula genotypes inoculated with both Sinorhizobium strains, mainly nodules number and biomass as well as nitrogen-fixing capacity. Nevertheless, inoculation with Sinorhizobium strains mitigates the negative effect of Fe deficiency on plant growth and oxidative stress compared to nitrogen-fertilized plants. The highest auxin producing strain, TII7, preserves relatively high growth and root biomass and length when inoculated to TN8.20 and A17. On the other hand, both TII7 and SII4 strains improve the performance of sensitive genotype TN1.11 through reduction of the negative effect of iron deficiency on chlorophyll and plant Fe content. The bacterial inoculation improved Fe-deficient plant response to oxidative stress via the induction of the activities of antioxidant enzymes.