Organic or Inorganic Nitrogen and Rhizobia Inoculation Provide Synergistic Growth Response of a Leguminous Forb and Tree

Peculiarity of the early metabolomic response in tomato after urea, ammonium or nitrate supply

Nitrogen (N) is the nutrient most applied in agriculture as fertilizer (as nitrate, Nit; ammonium, A; and/or urea, U, forms) and its availability strongly constrains the crop growth and yield. To investigate the early response (24 h) of N-deficient tomato plants to these three N forms, a physiological and molecular study was performed. In comparison to N-deficient plants, significant changes in the transcriptional, metabolomic and ionomic profiles were observed. As a probable consequence of N mobility in plants, a wide metabolic modulation occurred in old leaves rather than in young leaves. The metabolic profile of U and A-treated plants was more similar than Nit-treated plant profile, which in turn presented the lowest metabolic modulation with respect to N-deficient condition. Urea and A forms induced some changes at the biosynthesis of secondary metabolites, amino acids and phytohormones. Interestingly, a specific up-regulation by U and down-regulation by A of carbon synthesis occurred in roots. Along with the gene expression, data suggest that the specific N form influences the activation of metabolic pathways for its assimilation (cytosolic GS/AS and/or plastidial GS/GOGAT cycle). Urea induced an up-concentration of Cu and Mn in leaves and Zn in whole plant. This study highlights a metabolic reprogramming depending on the N form applied, and it also provide evidence of a direct relationship between urea nutrition and Zn concentration. The understanding of the metabolic pathways activated by the different N forms represents a milestone in improving the efficiency of urea fertilization in crops.

Use of Trichoderma in the Production of Forest Seedlings

Forest production has great relevance in the Brazilian economy, characterized by several production sectors, including the production of seedlings. With the focus on maximizing the capacity of survival, development, and adaptation of seedlings, Trichoderma is highlighted as a potentially useful genus of microorganisms for promoting growth and higher product quality. In this sense, this review aims to describe the main mechanisms of fungi action in forest seedlings' production. The different species of the genus Trichoderma have specific mechanisms of action, and the current scenario points to more advances in the number of species. The interaction process mediated by different mechanisms of action begins in the communication with plants, from the colonization process. After the interaction, chemical dialogues allow the plant to develop better because, from colonization, the forest seedlings can maximize height and increase shoot and root development. Fungi promote solubilization and availability of nutrients to seedlings, which show numerous benefits to the development. The use of beneficial microorganisms, such as fungi of the genus Trichoderma, has become a sustainable strategy to enhance seedling development, reducing the use of agrochemicals and industrial fertilizers.

Consequences of excess urea application on photosynthetic characteristics and nitrogen metabolism of Robinia pseudoacacia seedlings

Urea is the most frequently used nitrogen (N) fertilizer worldwide. However, the mechanisms in plants to cope with excess urea are largely unknown, especially for woody legumes that can meet their N demand by their own N2-fixation capacity. Here, we studied the immediate consequences of different amounts of urea application and exposure duration on photosynthesis, N metabolism, and the activity of antioxidative enzymes of Robinia pseudoacacia seedlings. For this purpose, seedlings were grown for 3 months under normal N availability with rhizobia inoculation and, subsequently, 50 mg N kg-1 was applied to the soil twice with urea as additional N source. Our results show that excess urea application significantly promoted photosynthesis, which increased by 80.3% and 84.7% compared with CK after the 1st and 2nd urea applications, respectively. The increase in photosynthesis translated into an increase in root and nodule biomass of 88.7% and 82.0%, respectively, while leaf biomass decreased by 4.8% after the first application of urea. The N content in leaves was 92.6% higher than in roots, but excess urea application increased the N content of protein and free amino acids in roots by 25.0%, and 43.3%, respectively. Apparently, enhanced root growth and N storage in the roots constitute mechanisms to prevent the negative consequences of excess N in the shoot upon urea application. Nitrate reductase (NR) activity of leaves and roots increased by 74.4% and 26.3%, respectively. Glutathione reductase (GR) activity in leaves and roots was enhanced by 337% and 34.0%, respectively, but then decreased rapidly to the initial level before fertilization. This result shows that not only N metabolism, but also antioxidative capacity was transiently promoted by excess urea application. Apparently, excess urea application initially poses oxidative stress to the plants that is immediately counteracted by enhanced scavenging of reactive oxygen species via enhanced GR activity.

Response of peanut plant and soil N-fixing bacterial communities to conventional and biodegradable microplastics

Microplastics (MPs) occur and distribute widely in agroecosystems, posing a potential threat to soil-plant systems. However, little is known about their effects on legumes and N-fixing microbes. Here, we explored the effects of high-density polyethylene (HDPE), polystyrene (PS), and polylactic acid (PLA) on the growth of peanuts and soil N-fixing bacterial communities. All MPs treatments showed no phytotoxic effects on plant biomass, and PS and PLA even increased plant height, especially at the high dose. All MPs changed soil NO3--N and NH4+-N contents and the activities of urease and FDAse. Particularly, high-dose PLA decreased soil NO3--N content by 97% and increased soil urease activity by 104%. In most cases, MPs negatively affected plant N content, and high-dose PLA had the most pronounced effects. All MPs especially PLA changed soil N-fixing bacterial community structure. Symbiotic N-fixer Rhizoboales were greatly enriched by high-dose PLA, accompanied by the emergence of root nodulation, which may represent an adaptive strategy for peanuts to overcome N deficiency caused by PLA MPs pollution. Our findings indicate that MPs can change peanut-N fixing bacteria systems in a type- and dose-dependent manner, and biodegradable MPs may have more profound consequences for N biogeochemical cycling than traditional MPs.

Cell Mutagenic Autopolyploidy Enhances Salinity Stress Tolerance in Leguminous Crops

Salinity stress affects plant growth and development by causing osmotic stress and nutrient imbalances through excess Na+, K+, and Cl- ion accumulations that induce toxic effects during germination, seedling development, vegetative growth, flowering, and fruit set. However, the effects of salt stress on growth and development processes, especially in polyploidized leguminous plants, remain unexplored and scantly reported compared to their diploid counterparts. This paper discusses the physiological and molecular response of legumes towards salinity stress-based osmotic and ionic imbalances in plant cells. A multigenic response involving various compatible solutes, osmolytes, ROS, polyamines, and antioxidant activity, together with genes encoding proteins involved in the signal transduction, regulation, and response mechanisms to this stress, were identified and discussed. This discussion reaffirms polyploidization as the driving force in plant evolution and adaptation to environmental stress constraints such as drought, feverish temperatures, and, in particular, salt stress. As a result, thorough physiological and molecular elucidation of the role of gene duplication through induced autopolyploidization and possible mechanisms regulating salinity stress tolerance in grain legumes must be further studied.

Study on diversity, nitrogen-fixing capacity, and heavy metal tolerance of culturable Pongamia pinnata rhizobia in the vanadium-titanium magnetite tailings

Introduction

The diversity, nitrogen-fixing capacity and heavy metal tolerance of culturable rhizobia in symbiotic relationship with Pongamia pinnata surviving in vanadium (V) - titanium (Ti) magnetite (VTM) tailings is still unknown, and the rhizobia isolates from the extreme barren VTM tailings contaminated with a variety of metals would provide available rhizobia resources for bioremediation.

Methods

P. pinnata plants were cultivated in pots containing the VTM tailings until root nodules formed, and then culturable rhizobia were isolated from root nodules. The diversity, nitrogen-fixing capacity and heavy metal tolerance of rhizobia were performed.

Results

Among 57 rhizobia isolated from these nodules, only twenty strains showed different levels of tolerance to copper (Cu), nickel (Ni), manganese (Mn) and zinc (Zn), especially strains PP1 and PP76 showing high tolerance against these four heavy metals. Based on the phylogenetic analysis of 16S rRNA and four house-keeping genes (atpD, recA, rpoB, glnII), twelve isolates were identified as Bradyrhizobium pachyrhizi, four as Ochrobactrum anthropic, three as Rhizobium selenitireducens and one as Rhizobium pisi. Some rhizobia isolates showed a high nitrogen-fixing capacity and promoted P. pinnata growth by increasing nitrogen content by 10%-145% in aboveground plant part and 13%-79% in the root. R. pachyrhizi PP1 showed the strongest capacity of nitrogen fixation, plant growth promotion and resistance to heavy metals, which provided effective rhizobia strains for bioremediation of VTM tailings or other contaminated soils. This study demonstrated that there are at least three genera of culturable rhizobia in symbiosis with P. pinnata in VTM tailings.

Discussion

Abundant culturable rhizobia with the capacity of nitrogen fixation, plant growth promotion and resistance to heavy metals survived in VTM tailings, indicating more valuable functional microbes could be isolated from extreme soil environments such as VTM tailings.

Nursery Cultural Techniques Facilitate Restoration of Acacia koa Competing with Invasive Grass in a Dry Tropical Forest

Anthropogenic activity has caused persistent and prominent losses of forest cover in dry tropical forests. Natural regeneration of forest trees in grazed areas often fails due to lack of seed sources and consumption by ungulates. To address this, the effective restoration of such sites often requires fencing and outplanting nursery-grown seedlings. In the degraded, dry forests of tropical Hawaii, USA, an additional challenge to restoration of native forest trees is the introduced kikuyu grass (Cenchrus clandestinus). This invasive, rapidly growing rhizomatous plant forms deep, dense mats. We studied the use of nursery cultural techniques to facilitate the establishment of koa (Acacia koa) seedlings outplanted amidst well-established kikuyu grass on a volcanic cinder cone on the dry, western side of Hawaii Island. Seedlings were grown four months in three container sizes (49, 164, 656 cm3) and with four rates (0, 4.8, 7.2, and 9.6 kg m−3) of 15–9–12 (NPK) controlled-release fertilizer incorporated into media prior to sowing. After 16 months in the field, seedling survival was > 80% for all treatments with two exceptions: the non-fertilized 49 cm3 (78%) and 164 cm3 (24%) containers. After 10 years, only these two treatments had significantly lower survival (35% and 10%, respectively) than the other treatments. One year following planting, none of the non-fertilized seedlings had transitioned to phyllodes from juvenile true leaves, regardless of container size. For the fertilized 656 cm3 container treatment, 78%–85% of seedlings had phyllodes, with mean values increasing by fertilizer rate. Phyllodes are known to confer greater drought resistance than true leaves in koa, which may help to explain the improved survival of fertilized trees on this relatively dry site. Overall, nursery fertilization was more influential on seedling height and diameter response than container size after outplanting. However, the largest container (656 cm3) with the addition of fertilizer, produced significantly larger trees than all other treatments during the early regeneration phase; early growth differences tended to fade at 10 years due to inter-tree canopy competition. Although koa is able to fix atmospheric nitrogen through rhizobium associations, our data confirm the importance of nursery fertilization in promoting regeneration establishment. Nursery cultural techniques may play an important role in forest restoration of dry tropical sites invaded by exotic vegetation.

Interaction of Biochar Type and Rhizobia Inoculation Increases the Growth and Biological Nitrogen Fixation of Robinia pseudoacacia Seedlings

Adding biochar to soil can change soil properties and subsequently affect plant growth, but this effect can vary because of different feedstocks and methods (e.g., pyrolysis or gasification) used to create the biochar. Growth and biological nitrogen fixation (BNF) of leguminous plants can be improved with rhizobia inoculation that fosters nodule development. Thus, this factorial greenhouse study examined the effects of two types of biochar (i.e., pyrolysis and gasification) added at a rate of 5% (v:v) to a peat-based growth substrate and rhizobia inoculation (yes or no) on Robinia pseudoacacia (black locust) seedlings supplied with 15NH415NO3. Seedling and nodule growth, nitrogen (N) content, and δ15N × 1000 were evaluated after 3 months. While addition of biochar without inoculation had no effect on seedling growth, inoculation with rhizobia increased seedling growth, BNF, and N status. Inoculated seedlings had reduced δ15N, indicating that N provided via fertilization was being diluted by N additions through BNF. Biochar type and inoculation interacted to affect seedling growth. Combining inoculation with either biochar type increased seedling leaf, stem, and total biomass, whereas gasifier biochar and inoculation improved all seedling growth variables and nodule biomass.