Cessation of photosynthesis in Lotus japonicus leaves leads to reprogramming of nodule metabolism

Primary and Secondary Metabolites in Lotus japonicus

Lotus japonicus is a leguminous model plant used to gain insight into plant physiology, stress response, and especially symbiotic plant-microbe interactions, such as root nodule symbiosis or arbuscular mycorrhiza. Responses to changing environmental conditions, stress, microbes, or insect pests are generally accompanied by changes in primary and secondary metabolism to account for physiological needs or to produce defensive or signaling compounds. Here we provide an overview of the primary and secondary metabolites identified in L. japonicus to date. Identification of the metabolites is mainly based on mass spectral tags (MSTs) obtained by gas chromatography linked with tandem mass spectrometry (GC-MS/MS) or liquid chromatography-MS/MS (LC-MS/MS). These MSTs contain retention index and mass spectral information, which are compared to databases with MSTs of authentic standards. More than 600 metabolites are grouped into compound classes such as polyphenols, carbohydrates, organic acids and phosphates, lipids, amino acids, nitrogenous compounds, phytohormones, and additional defense compounds. Their physiological effects are briefly discussed, and the detection methods are explained. This review of the exisiting literature on L. japonicus metabolites provides a valuable basis for future metabolomics studies.

Selenium-binding Protein 1 (SBD1): A stress response regulator in Chlamydomonas reinhardtii

Selenium-binding proteins (SBPs) represent a ubiquitous protein family implicated in various environmental stress responses, although the exact molecular and physiological role of the SBP family remains elusive. In this work, we report the identification and characterization of CrSBD1, an SBP homolog from the model microalgae Chlamydomonas reinhardtii. Growth analysis of the C. reinhardtii sbd1 mutant strain revealed that the absence of a functional CrSBD1 resulted in increased growth under mild oxidative stress conditions, although cell viability rapidly declined at higher hydrogen peroxide (H2O2) concentrations. Furthermore, a combined global transcriptomic and metabolomic analysis indicated that the sbd1 mutant exhibited a dramatic quenching of the molecular and biochemical responses upon H2O2-induced oxidative stress when compared to the wild-type. Our results indicate that CrSBD1 represents a cell regulator, which is involved in the modulation of C. reinhardtii early responses to oxidative stress. We assert that CrSBD1 acts as a member of an extensive and conserved protein-protein interaction network including Fructose-bisphosphate aldolase 3, Cysteine endopeptidase 2, and Glutaredoxin 6 proteins, as indicated by yeast two-hybrid assays.

Rhizobia as a Source of Plant Growth-Promoting Molecules: Potential Applications and Possible Operational Mechanisms

The symbiotic interaction between rhizobia and legumes that leads to nodule formation is a complex chemical conversation involving plant release of nod-gene inducing signal molecules and bacterial secretion of lipo-chito-oligossacharide nodulation factors. During this process, the rhizobia and their legume hosts can synthesize and release various phytohormones, such as IAA, lumichrome, riboflavin, lipo-chito-oligossacharide Nod factors, rhizobitoxine, gibberellins, jasmonates, brassinosteroids, ethylene, cytokinins and the enzyme 1-aminocyclopropane-1-carboxylate (ACC) deaminase that can directly or indirectly stimulate plant growth. Whereas these attributes may promote plant adaptation to various edapho-climatic stresses including the limitations in nutrient elements required for plant growth promotion, tapping their full potential requires understanding of the mechanisms involved in their action. In this regard, several N2-fixing rhizobia have been cited for plant growth promotion by solubilizing soil-bound P in the rhizosphere via the synthesis of gluconic acid under the control of pyrroloquinoline quinone (PQQ) genes, just as others are known for the synthesis and release of siderophores for enhanced Fe nutrition in plants, the chelation of heavy metals in the reclamation of contaminated soils, and as biocontrol agents against diseases. Some of these metabolites can enhance plant growth via the suppression of the deleterious effects of other antagonistic molecules, as exemplified by the reduction in the deleterious effect of ethylene by ACC deaminase synthesized by rhizobia. Although symbiotic rhizobia are capable of triggering biological outcomes with direct and indirect effects on plant mineral nutrition, insect pest and disease resistance, a greater understanding of the mechanisms involved remains a challenge in tapping the maximum benefits of the molecules involved. Rather than the effects of individual rhizobial or plant metabolites however, a deeper understanding of their synergistic interactions may be useful in alleviating the effects of multiple plant stress factors for increased growth and productivity.

Mechanisms in plant growth-promoting rhizobacteria that enhance legume-rhizobial symbioses

Nitrogen fixation is an important biological process in terrestrial ecosystems and for global crop production. Legume nodulation and N₂ fixation have been improved using nodule-enhancing rhizobacteria (NER) under both regular and stressed conditions. The positive effect of NER on legume-rhizobia symbiosis can be facilitated by plant growth-promoting (PGP) mechanisms, some of which remain to be identified. NER that produce aminocyclopropane-1-carboxylic acid deaminase and indole acetic acid enhance the legume-rhizobia symbiosis through (i) enhancing the nodule induction, (ii) improving the competitiveness of rhizobia for nodulation, (iii) prolonging functional nodules by suppressing nodule senescence and (iv) upregulating genes associated with legume-rhizobia symbiosis. The means by which these processes enhance the legume-rhizobia symbiosis is the focus of this review. A better understanding of the mechanisms by which PGP rhizobacteria operate, and how they can be altered, will provide opportunities to enhance legume-rhizobial interactions, to provide new advances in plant growth promotion and N₂ fixation.

Molecular and physiological characterization of the monosaccharide transporters gene family in Medicago truncatula

Monosaccharide transporters (MSTs) represent key components of the carbon transport and partitioning mechanisms in plants, mediating the cell-to-cell and long-distance distribution of a wide variety of monosaccharides. In this study, we performed a thorough structural, molecular, and physiological characterization of the monosaccharide transporter gene family in the model legume Medicago truncatula. The complete set of MST family members was identified with a novel bioinformatic approach. Prolonged darkness was used as a test condition to identify the relevant transcriptomic and metabolic responses combining MST transcript profiling and metabolomic analysis. Our results suggest that MSTs play a pivotal role in the efficient partitioning and utilization of sugars, and possibly in the mechanisms of carbon remobilization in nodules upon photosynthate-limiting conditions, as nodules are forced to acquire a new role as a source of both C and N.

Bacterial IAA-Delivery into Medicago Root Nodules Triggers a Balanced Stimulation of C and N Metabolism Leading to a Biomass Increase

Indole-3-acetic acid (IAA) is the main auxin acting as a phytohormone in many plant developmental processes. The ability to synthesize IAA is widely associated with plant growth-promoting rhizobacteria (PGPR). Several studies have been published on the potential application of PGPR to improve plant growth through the enhancement of their main metabolic processes. In this study, the IAA-overproducing Ensifer meliloti strain RD64 and its parental strain 1021 were used to inoculate Medicago sativa plants. After verifying that the endogenous biosynthesis of IAA did not lead to genomic changes during the initial phases of the symbiotic process, we analyzed whether the overproduction of bacterial IAA inside root nodules influenced, in a coordinated manner, the activity of the nitrogen-fixing apparatus and the photosynthetic function, which are the two processes playing a key role in legume plant growth and productivity. Higher nitrogen-fixing activity and a greater amount of total nitrogen (N), carbon (C), Rubisco, nitrogen-rich amino acids, soluble sugars, and organic acids were measured for RD64-nodulated plants compared to the plants nodulated by the wild-type strain 1021. Furthermore, the RD64-nodulated plants showed a biomass increase over time, with the highest increment (more than 60%) being reached at six weeks after infection. Our findings show that the RD64-nodulated plants need more substrate derived from photosynthesis to generate the ATP required for their increased nitrogenase activity. This high carbohydrate demand further stimulates the photosynthetic function with the production of molecules that can be used to promote plant growth. We thus speculate that the use of PGPR able to stimulate both C and N metabolism with a balanced C/N ratio represents an efficient strategy to obtain substantial gains in plant productivity.

Comparative Analysis of the Combined Effects of Different Water and Phosphate Levels on Growth and Biological Nitrogen Fixation of Nine Cowpea Varieties

Water deficit and phosphate (Pi) deficiency adversely affect growth and biological nitrogen fixation (BNF) of legume crops. In this study, we examined the impact of interaction between soil water conditions and available soil-Pi levels on growth, nodule development and BNF potential of nine cowpea varieties grown on dry savanna soils. In our experimental design, soils with different available soil-Pi levels, i.e., low, moderate, and high soil-Pi levels, collected from various farming fields were used to grow nine cowpea varieties under well-watered and water-deficit conditions. Significant and severe water deficit-damaging effects on BNF, nodulation, growth, levels of plant-nitrogen (N) and -phosphorus (P), as well as shoot relative water content and chlorophyll content of cowpea plants were observed. Under well-watered and high available soil-Pi conditions, cowpea varieties IT07K-304-9 and Dan'Ila exhibited significantly higher BNF potential and dry biomass, as well as plant-N and -P contents compared with other tested ones. Significant genotypic variations among the cowpeas were recorded under low available soil-Pi and water-deficit conditions in terms of the BNF potential. Principal component (PC) analysis revealed that varieties IT04K-339-1, IT07K-188-49, IT07K-304-9, and IT04K-405-5 were associated with PC1, which was better explained by performance for nodulation, plant biomass, plant-N, plant-P, and BNF potential under the combined stress of water deficit and Pi deficiency, thereby offering prospects for development of varieties with high growth and BNF traits that are adaptive to such stress conditions in the region. On another hand, variety Dan'Ila was significantly related to PC2 that was highly explained by the plant shoot/root ratio and chlorophyll content, suggesting the existence of physiological and morphological adjustments to cope with water deficit and Pi deficiency for this particular variety. Additionally, increases in soil-Pi availability led to significant reductions of water-deficit damage on dry biomass, plant-N and -P contents, and BNF potential of cowpea varieties. This finding suggests that integrated nutrient management strategies that allow farmers to access to Pi-based fertilizers may help reduce the damage of adverse water deficit and Pi deficiency caused to cowpea crop in the regions, where soils are predominantly Pi-deficient and drought-prone.

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.

Is protection against florivory consistent with the optimal defense hypothesis?

BACKGROUND

Plant defense traits require resources and energy that plants may otherwise use for growth and reproduction. In order to most efficiently protect plant tissues from herbivory, one widely accepted assumption of the optimal defense hypothesis states that plants protect tissues most relevant to fitness. Reproductive organs directly determining plant fitness, including flowers and immature fruit, as well as young, productive leaf tissue thus should be particularly well-defended. To test this hypothesis, we quantified the cyanogenic potential (HCNp)-a direct, chemical defense-systemically expressed in vegetative and reproductive organs in lima bean (Phaseolus lunatus), and we tested susceptibility of these organs in bioassays with a generalist insect herbivore, the Large Yellow Underwing (Noctuidae: Noctua pronuba). To determine the actual impact of either florivory (herbivory on flowers) or folivory on seed production as a measure of maternal fitness, we removed varying percentages of total flowers or young leaf tissue and quantified developing fruit, seeds, and seed viability.

RESULTS

We found extremely low HCNp in flowers (8.66 ± 2.19 μmol CN(-) g(-1) FW in young, white flowers, 6.23 ± 1.25 μmol CN(-) g(-1) FW in mature, yellow flowers) and in pods (ranging from 32.05 ± 7.08 to 0.09 ± 0.08 μmol CN(-) g(-1) FW in young to mature pods, respectively) whereas young leaves showed high levels of defense (67.35 ± 3.15 μmol CN(-) g(-1) FW). Correspondingly, herbivores consumed more flowers than any other tissue, which, when taken alone, appears to contradict the optimal defense hypothesis. However, experimentally removing flowers did not significantly impact fitness, while leaf tissue removal significantly reduced production of viable seeds.

CONCLUSIONS

Even though flowers were the least defended and most consumed, our results support the optimal defense hypothesis due to i) the lack of flower removal effects on fitness and ii) the high defense investment in young leaves, which have high consequences for fitness. These data highlight the importance of considering plant defense interactions from multiple angles; interpreting where empirical data fit within any plant defense hypothesis requires understanding the fitness consequences associated with the observed defense pattern.

PII Overexpression in Lotus japonicus Affects Nodule Activity in Permissive Low-Nitrogen Conditions and Increases Nodule Numbers in High Nitrogen Treated Plants

We report here the first characterization of a GLNB1 gene coding for the PII protein in leguminous plants. The main purpose of this work was the investigation of the possible roles played by this multifunctional protein in nodulation pathways. The Lotus japonicus LjGLB1 gene shows a significant transcriptional regulation during the light-dark cycle and different nitrogen availability, conditions that strongly affect nodule formation, development, and functioning. We also report analysis of the spatial profile of expression of LjGLB1 in root and nodule tissues and of the protein's subcellular localization. Transgenic L. japonicus lines overexpressing the PII protein were obtained and tested for the analysis of the symbiotic responses in different conditions. The uncoupling of PII from its native regulation affects nitrogenase activity and nodule polyamine content. Furthermore, our results suggest the involvement of PII in the signaling of the nitrogen nutritional status affecting the legumes' predisposition for nodule formation.

Mechanisms of physiological adjustment of N2 fixation in Cicer arietinum L. (chickpea) during early stages of water deficit: single or multi-factor controls

Drought negatively impacts symbiotic nitrogen fixation (SNF) in Cicer arietinum L. (chickpea), thereby limiting yield potential. Understanding how drought affects chickpea nodulation will enable the development of strategies to biotechnologically engineer chickpea varieties with enhanced SNF under drought conditions. By analyzing carbon and nitrogen metabolism, we studied the mechanisms of physiological adjustment of nitrogen fixation in chickpea plants nodulated with Mesorhizobium ciceri during both drought stress and subsequent recovery. The nitrogenase activity, levels of several key carbon (in nodules) and nitrogen (in both nodules and leaves) metabolites and antioxidant compounds, as well as the activity of related nodule enzymes were examined in M. ciceri-inoculated chickpea plants under early drought stress and subsequent recovery. Results indicated that drought reduced nitrogenase activity, and that this was associated with a reduced expression of the nifK gene. Furthermore, drought stress promoted an accumulation of amino acids, mainly asparagine in nodules (but not in leaves), and caused a cell redox imbalance in nodules. An accumulation of organic acids, especially malate, in nodules, which coincided with the decline of nodulated root respiration, was also observed under drought stress. Taken together, our findings indicate that reduced nitrogenase activity occurring at early stages of drought stress involves, at least, the inhibition of respiration, nitrogen accumulation and an imbalance in cell redox status in nodules. The results of this study demonstrate the potential that the genetic engineering-based improvement of SNF efficiency could be applied to reduce the impact of drought on the productivity of chickpea, and perhaps other legume crops.