Intercellular localization of nitrate reductase in roots

Experiments were conducted with segments of corn roots to investigate whether nitrate reductase (NR) is compartmentalized in particular groups of cells that collectively form the root symplastic pathway. A microsurgical technique was used to separate cells of the epidermis, of the cortex, and of the stele. The presence of NR was determined using in vitro and enzyme-linked immunosorbent assays. In roots exposed to 0.2 millimolar NO₃⁻ for 20 hours, NR was detected almost exclusively in epidermal cells, even though substantial amounts of NO₃⁻ likely were being transported through cortical and steler cells during transit to the vascular system. Although NR was present in all cell groups of roots exposed to 20.0 millimolar NO₃⁻, the majority of the NR still was contained in epidermal cells. The results are consistent with previous observations indicating that limited reduction of endogenous NO₃⁻ occurs during uptake and reduction of exogenous NO₃⁻. Several mechanisms are advanced to account for the restricted capacity of cortical and stelar cells to induce NR and reduce NO₃⁻. It is postulated that (a) the biochemical system involved in the induction of NR in the cortex and stele is relatively insensitive to the presence of NO₃⁻, (b) the receptor for the NR induction response and the NR protein are associated with cell plasmalemmae and little NO₃⁻ is taken up by cells of the cortex and stele, and/or (c) NO₃⁻ is compartmentalized during transport through the symplasm, which limits exposure for induction of NR and NO₃⁻ reduction.     

Endogenous Rhythms in Photosynthesis, Sucrose Phosphate Synthase Activity, and Stomatal Resistance in Leaves of Soybean (Glycine max [L.] Merr.)

Experiments were conducted with soybean (Glycine max [L.] Merr. cv `Ransom') plants to determine if diurnal rhythms in net carbon dioxide exchange rate (CER), stomatal resistance, and sucrose-phosphate synthase (SPS) activity persisted in constant environmental conditions (constant light, LL; constant dark DD) and to assess the importance of these rhythms to the production of nonstructural carbohydrates (starch, sucrose, and hexose). Rhythms in CER, stomatal resistance, and SPS activity were observed in constant environmental conditions but the rhythms differed in period length, amplitude, and phase. The results indicated that these photosynthetic parameters are not controlled in a coordinated manner. The activity of UDPG pyrophosphorylase, another enzyme involved in sucrose formation, did not fluctuate rhythmically in constant conditions but increased with time in plants in LL. In LL, the rhythm in CER was correlated positively with fluctuations in total chlorophyll (r = 0.810) and chlorophyll a (r = 0.791) concentrations which suggested that changes in pigment concentration were associated with, but not necessarily the underlying mechanism of, the rhythm in photosynthetic rate. Assimilate export rate, net starch accumulation rate, and leaf sucrose concentration also fluctuated in constant light. No single photosynthetic parameter was closely correlated with fluctuations in assimilate export during LL; thus, assimilate export may have been controlled by interactions among the endogenous rhythms in CER, SPS activity, or other metabolic factors which were not measured in the present study.     

Changes in Starch Formation and Activities of Sucrose Phosphate Synthase and Cytoplasmic Fructose-1,6-bisphosphatase in Response to Source-Sink Alterations

Short term experiments were conducted with vegetative soybean plants (Glycine max L. Merr. `Ransom' or `Arksoy') to determine whether sourcesink manipulations, which rapidly changed the `demand' for sucrose and partitioning of photosynthetically fixed carbon into starch, were associated with alterations in activities of sucrose-P synthase and/or cytoplasmic fructose-1,6-bisphosphatase in leaf extracts. When demand for sucrose from a particular source leaf was increased by defoliation of other source leaves, starch accumulation was restricted and activities of both enzymes were markedly enhanced. When demand for sucrose from source leaves was limited by excision, starch accumulation in the detached leaves was increased while activity of sucrose-P synthase declined sharply. The consistent responsiveness of sucrose-P synthase activity to changes in demand for sucrose supports the contention that regulation of sucrose-P synthase is an integral component of the system which controls sucrose biosynthesis and partitioning of carbon between starch and sucrose biosynthesis in the light.     

Localization of Nitrate Absorption and Translocation within Morphological Regions of the Corn Root

The absorption of NO₃⁻ was characterized in six regions of a 7-d-old corn root (Zea mays L. cv W64A × W182E) growing in a complete nutrient solution. Based on changing rates of ¹⁵N accumulation during 15-min time courses, translocation of the concurrently absorbed N through each region of the intact root was calculated and distinguished from direct absorption from the medium. Of the ¹⁵N accumulated in the 5-mm root tip after 15 min, less than 15 and 35% had been absorbed directly from the external solution at 0.1 and 10 mm NO₃⁻ concentration of the external solution, respectively. The characterization of the apical portion of the primary root as a sink for concurrently absorbed N was conconfirmed in a pulse-chase experiment that showed an 81% increase of ¹⁵N in the 5-mm root tip during a 12-min chase (subsequent to a 6-min labeling period). The lateral roots alone accounted for 60% of root influx and 70% of 15-min whole root ¹⁵N accumulation at either 0.1 or 10 mm. NO₃⁻ concentration of the external solution. Because relatively steady rates of ¹⁵N accumulation in the shoot were reached after 6 min, the rapidly exchanging pools in lateral roots must have been involved in supplying ¹⁵N to the shoot. The laterals and the basal primary root also showed large decreases (24 and 17%) in ¹⁵N during the chase experiment, confirming their role in rapid translocation.     

Nitrate Reduction in Roots as Affected by the Presence of Potassium and by Flux of Nitrate through the Roots

Dark-grown, detopped corn seedlings (cv. Pioneer 3369A) were exposed to treatment solutions containing Ca(NO₃)₂, NaNO₃, or KNO₃; KNO₃ plus 50 or 100 millimolar sorbitol; and KNO₃ at root temperatures of 30, 22, or 16 C. In all experiments, the accelerated phase of NO₃⁻ transport had previously been induced by prior exposure to NO₃⁻ for 10 hours. The experimental system allowed direct measurements of net NO₃⁻ uptake and translocation, and calculation of NO₃⁻ reduction in the root. The presence of K⁺ resulted in small increases in NO₃⁻ uptake, but appreciably stimulated NO₃⁻ translocation out of the root. Enhanced translocation was associated with a marked decrease in the proportion of absorbed NO₃⁻ that was reduced in the root. When translocation was slowed by osmoticum or by low root temperatures, a greater proportion of absorbed NO₃⁻ was reduced in the presence of K⁺. Results support the proposition that NO₃⁻ reduction in the root is reciprocally related to the rate of NO₃⁻ transport through the root symplasm.     

Gas exchange and carbon metabolism in two Prosopis species (Fabaceae) from semiarid habitats: effects of elevated CO2, N supply, and N source

Predicting future plant and ecosystem responses to elevated CO(2) also requires an understanding of the role of other factors, especially soil nitrogen. This is particularly challenging for global aridlands where total N and the relative amounts of nitrate and ammonia vary both spatially and seasonally. We measured gas exchange and primary and secondary C metabolites in seedlings of two dominant aridland shrub species (Prosopis flexuosa [S America] and P. glandulosa [N America]) grown at ambient (350 ppm) or elevated (650 ppm) CO(2) and nitrogen at two levels (low [0.8 mM] and high [8.0 mM]) and at either 1 : 1 or 3 : 1 nitrate to ammonia. Whereas elevated CO(2) increased assimilation rate, water use efficiency, and primary carbon metabolites in both species, these increases were strongly contingent upon nitrogen availability. Elevated CO(2) did not increase secondary metabolites (i.e., phenolics). For these important aridland species, the effects of elevated CO(2) are strongly influenced by nitrogen availability and to a lesser extent by the relative amounts of nitrate and ammonia supplied, which underscores the importance of both the amount and chemical composition of soil nitrogen in mediating the potential responses of seedling growth and establishment of aridland plants under future CO(2)-enriched atmospheres.     

Nitrogen Transfer Between Plants: A 15N Natural Abundance Study with Crop and Weed Species

An increasing amount of evidence indicates that N can be transferred between plants. Nonetheless, a number of fundamental questions remain. A series of experiments was initiated in the field to examine N transfer between N₂-fixing soybean (Glycine max [L.] Merr.) varieties and a non-nodulating soybean, and between N₂-fixing peanut (Arachis hypogaea L.) or soybean and neighboring weed species. The experiments were conducted in soils with low N fertilities and used differences in N accumulation and/or ¹⁵N natural abundance to estimate N transfer. Mixtures of N₂-fixing and non-nod soybean indicated that substantial inter-plant N transfer occurred. Amounts were variable, ranging from negligible levels to 48% of the N found in the non-nod at maturity. Transfer did not appear to strongly penalize the N₂-fixing donor plants. But, in cases where high amounts of N were transferred, N content of donors was noticeably lowered. Differences were evident in the amount of N transferred from different N₂-fixing donor genotypes. Results of experiments with N₂-fixing crops and the weed species prickly sida (Sida spinosa L.) and sicklepod (Senna obtusifolia [L.] Irwin & Barneby) also indicated substantial N transfer occurred over a 60-day period, with amounts accounting for 30–80% of the N present in the weeds. Transfer of N, however, was generally very low in weed species that are known to be non-hosts for arbuscular mycorrhizae (yellow nutsedge, Cyperus esculentus L. and Palmer amaranth, Amaranthus palmeri [S.] Watson). The results are consistent with the view that N transfer occurs primarily through mycorrhizal hyphal networks, and they reveal that N transfer may be a contributing factor to weed problems in N₂-fixing crops in low N fertility conditions.     

Mycorrhizal‐mediated nitrogen acquisition in switchgrass under elevated temperatures and N enrichment

Arbuscular mycorrhizal fungi (AMF) can perform key roles in ecosystem functioning through improving host nutrient acquisition. Nitrogen (N) is an essential nutrient for plant growth, however, anthropogenic N loading (e.g. crop fertilization and deposition from combustion sources) is increasing so that N now threatens ecosystem sustainability around the world by causing terrestrial and aquatic eutrophication and acidification. It is important to better understand the capacity of AMF to directly uptake N from soils and transfer it to host plants because this process may increase N recycling and retention within ecosystems. In addition to understanding the role of AMF in the N cycle in the present day it is important to understand how AMF function may change as global change proceeds. Currently the net effects of N enrichment and elevated temperature predicted with global change on AMF are unknown. In this study, we examined the effects of N enrichment by simulated N‐deposition loading, elevated temperatures expected by future global changes and their interactions on growth and AMF‐mediated N acquisition of switchgrass (Panicum virgatum var. Alamo), an important species for biofuel production. Switchgrass plants were grown in microcosm units that divided mycorrhizal roots from AMF hyphae and organic residues enriched with ¹⁵N by compartments separated by an air gap to reduce N diffusion. While AMF did not enhance switchgrass biomass, mycorrhizas significantly increased ¹⁵N in shoots and total shoot N. Neither N enrichment nor elevated temperatures influenced this mycorrhizal‐mediated N uptake and transfer. Results from this study can aid in developing sustainable bioethanol and switchgrass production practices that are less reliant on synthetic fertilizers and more dependent on internal N recycling from AMF.     

Systems analysis of primary Sjögren's syndrome pathogenesis in salivary glands identifies shared pathways in human and a mouse model

Bone tissue has an exceptional quality to regenerate to native tissue in response to injury. However, the fracture repair process requires mechanical stability or a viable biological microenvironment or both to ensure successful healing to native tissue. An improved understanding of the molecular and cellular events that occur during bone repair and remodeling has led to the development of biologic agents that can augment the biological microenvironment and enhance bone repair. Orthobiologics, including stem cells, osteoinductive growth factors, osteoconductive matrices, and anabolic agents, are available clinically for accelerating fracture repair and treatment of compromised bone repair situations like delayed unions and nonunions. Preclinical and clinical studies using biologic agents like recombinant bone morphogenetic proteins have demonstrated an efficacy similar or better than that of autologous bone graft in acute fracture healing. A lack of standardized outcome measures for comparison of biologic agents in clinical fracture repair trials, frequent off-label use, and a limited understanding of the biological activity of these agents at the bone repair site have limited their efficacy in clinical applications.