The rhizosphere is a complex environment where roots interact with physical, chemical and biological properties of soil. Structural and functional characteristics of roots contribute to rhizosphere processes and both have significant influence on the capacity of roots to acquire nutrients. Roots also interact extensively with soil microorganisms which further impact on plant nutrition either directly, by influencing nutrient availability and uptake, or indirectly through plant (root) growth promotion. In this paper, features of the rhizosphere that are important for nutrient acquisition from soil are reviewed, with specific emphasis on the characteristics of roots that influence the availability and uptake of phosphorus and nitrogen. The interaction of roots with soil microorganisms, in particular with mycorrhizal fungi and non-symbiotic plant growth promoting rhizobacteria, is also considered in relation to nutrient availability and through the mechanisms that are associated with plant growth promotion.

Acquisition of phosphorus and nitrogen in the rhizosphere and plant growth promotion by microorganisms

The mineral nutrient needed in greatest abundance by plants is nitrogen. Plants, however, must compete for nitrogen in the soil with abiotic and biotic processes such as erosion, leaching, and microbial consumption. Soil nitrogen is also lost when crops are harvested and plant material is removed from the soil. To be competitive, plants have evolved several mechanisms to acquire nitrogen at low concentrations and to use a variety of forms of nitrogen. Plants can assimilate inorganic forms, such as nitrate and ammonia, and organic forms, such as urea. Some plants, including legumes, can fix dinitrogen gas in association with symbiotic bacteria (see Mylona et al., 1995, this issue). This review focuses on the assimilation of nitrate, a transient yet critical form for plants. Two key questions arise when considering nitrate assimilation. First, how do plants acquire optimal quantities of nitrate when the soil nitrate concentration can vary from 10 pM to 100 mM? Second, how is nitrate assimilation integrated into the overall metabolic program of plants so that optimal amounts of energy and carbon are provided? What is known about nitrate assimilation at the cellular leve1 is summarized in Figure 1. Once taken up, nitrate is either stored in the vacuole or reduced to nitrite by nitrate reductase (NR). Nitrite enters the chloroplast (or plastid in the root) and is then reduced to ammonja by nitrite reductase (NiR). Ammonia can then be fixed into carbon by the action of glutamine synthetase (see Lam et al., 1995, this issue). Reducing energy is provided in the form of NAD(P)H for NR and reduced ferredoxin for NiR. Glutamate provides the immediate carbon skeleton. Because nitrate assimilation produces several hydroxide ions, organic acids are also required for pH homeostasis. Coordinating these steps is a regulatory network that is responsive to both interna1 and external signals, some of which are diagrammed in Figure 1. Ultimately, it is the understanding of this regulatory network that will answer the aforementioned key questions. Genetic analysis of nitrate assimilation has been possible because mutants blocked in the pathway can be rescued by providing ammonia as a source of nitrogen. Such mutants are obtained either by screening directly with an in vitro assay for plants defective in nitrate reduction or by selecting plants that are resistant to chlorate, the chlorine analog of nitrate. When chlorate is taken up and reduced by NR, toxic chlorite is produced. In higher plants, chlorate-resistant mutants are impaired in nitratelchlorate reduction because they have either a defective nitrate reductase structural gene (NIA) or a defective molybdenum cofactor gene (CNX). The one exception is the chll mutant of Arabidopsis, which is defective in nitrate/chlorate uptake. Only in fungi and algae have regulatory mutants been descri bed. There are many excellent recent reviews of the nitrate assimilation field (Pelsy and Caboche, 1992; Warner and Kleinhofs, 1992; Cove, 1993; Crawford and Arst, 1993; Marzluf, 1993; Crawford, 1994; Hoff et al., 1994; Huppe and Turpin, 1994). This review provides an update on recent molecular advances that have uncovered genes and mechanisms responsible for nitrate uptake, reduction, and regulation.

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Nitrate: nutrient and signal for plant growth.

AGPase, ADP glucose pyrophosphorylase
GS, glutamine synthetase
GOGAT, glutamate : oxoglutarate amino transferase
NADP-ICDH, NADP-dependent isocitrate dehydrogenase
NR, nitrate reductase
OPPP, oxidative pentose phosphate pathway
3PGA, glycerate-3-phosphate
PEPCase, phosphoenolpyruvate carboxylase
Rubisco, ribulose-1,5-bisphosphate carboxylase/oxygenase
SPS, sucrose phosphate-synthase

This review first summarizes the numerous studies that have described the interaction between the nitrogen supply and the response of photosynthesis, metabolism and growth to elevated [CO2]. The initial stimulation of photosynthesis in elevated [CO2] is often followed by a decline of photosynthesis, that is typically accompanied by a decrease of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), an accumulation of carbohydrate especially starch, and a decrease of the nitrogen concentration in the plant. These changes are particularly marked when the nitrogen supply is low, whereas when the nitrogen supply is adequate there is no acclimation of photosynthesis, no major decrease in the internal concentration of nitrogen or the levels of nitrogen metabolites, and growth is stimulated markedly. Second, emerging evidence is discussed that signals derived from nitrate and nitrogen metabolites such as glutamine act to regulate the expression of genes involved in nitrate and ammonium uptake and assimilation, organic acid synthesis and starch accumulation, to modulate the sugar-mediated repression of the expression of genes involved in photosynthesis, and to modulate whole plant events including shoot–root allocation, root architecture and flowering. Third, increased rates of growth in elevated [CO2] will require higher rates of inorganic nitrogen uptake and assimilation. Recent evidence is discussed that an increased supply of sugars can increase the rates of nitrate and ammonium uptake and assimilation, the synthesis of organic acid acceptors, and the synthesis of amino acids. Fourth, interpretation of experiments in elevated [CO2] requires that the nitrogen status of the plants is monitored. The suitability of different criteria to assess the plant nitrogen status is critically discussed. Finally the review returns to experiments with elevated [CO2] and discusses the following topics: is, and if so how, are nitrate and ammonium uptake and metabolism stimulated in elevated [CO2], and does the result depend on the nitrogen supply? Is acclimation of photosynthesis the result of sugar-mediated repression of gene expression, end-product feedback of photosynthesis, nitrogen-induced senescence, or ontogenetic drift? Is the accumulation of starch a passive response to increased carbohydrate formation, or is it triggered by changes in the nutrient status? How do changes in sugar production and inorganic nitrogen assimilation interact in different conditions and at different stages of the life history to determine the response of whole plant growth and allocation to elevated [CO2]?

https://www.onlinelibrary.wiley.com/doi/pdf/10.1046/j.1365-3040.1999.00386.x

The interaction between elevated carbon dioxide and nitrogen nutrition: the physiological and molecular background

A plasma membrane-enriched fraction prepared from barley roots was analyzed by two-dimensional gel electrophoresis. Four methods of sample solubilization were assessed on silver stained gels. When membranes were solubilized with 2% sodium dodecyl sulfate followed by addition of Nonidet P-40, gels had high background staining and few proteins because of incomplete solubilization. Gels of membranes solubilized in urea and Nonidet P-40 had a greater number of proteins but proteins with molecular weights greater than 85,000 were absent and proteins with low molecular weights were diffuse. High molecular weight proteins were present in gels of membranes solubilized in 4% sodium dodecyl sulfate followed by acetone precipitation but background staining and streaking remained a problem. Gels of the best quality were obtained when membrane proteins were extracted with phenol and precipitated with ammonium acetate in methanol; background staining and streaking were diminished and proteins were clearly resolved. This method makes possible the resolution required for meaningful qualitative and quantitative comparisons of protein patterns on two-dimensional gels of plant membrane proteins.

http://www.plantphysiol.org/content/plantphysiol/81/3/802.full.pdf

Solubilization of Plant Membrane Proteins for Analysis by Two-Dimensional Gel Electrophoresis

Molecular and physiological aspects of nitrate uptake in plants

Nitrate and NO2- transport by roots of 8-day-old uninduced and induced intact barley (Hordeum vulgare L. var CM 72) seedlings were compared to kinetic patterns, reciprocal inhibition of the transport systems, and the effect of the inhibitor, p-hydroxymercuribenzoate. Net uptake of NO3- and NO2- was measured by following the depletion of the ions from the uptake solutions. The roots of uninduced seedlings possessed a low concentration, saturable, low Km, possibly a constitutive uptake system, and a linear system for both NO3- and NO2-. The low Km system followed Michaelis-Menten kinetics and approached saturation between 40 and 100 micromolar, whereas the linear system was detected between 100 and 500 micromolar. In roots of induced seedlings, rates for both NO3- and NO2- uptake followed Michaelis-Menten kinetics and approached saturation at about 200 micromolar. In induced roots, two kinetically identifiable transport systems were resolved for each anion. At the lower substrate concentrations, less than 10 micromolar, the apparent low Kms of NO3- and NO2- uptake were 7 and 9 micromolar, respectively, and were similar to those of the low Km system in uninduced roots. At substrate concentrations between 10 and 200 micromolar, the apparent high Km values of NO3- uptake ranged from 34 to 36 micromolar and of NO2- uptake ranged from 41 to 49 micromolar. A linear system was also found in induced seedlings at concentrations above 500 micromolar. Double reciprocal plots indicated that NO3- and NO2- inhibited the uptake of each other competitively in both uninduced and induced seedlings; however, Ki values showed that NO3- was a more effective inhibitor than NO2-. Nitrate and NO2- transport by both the low and high Km systems were greatly inhibited by p-hydroxymercuribenzoate, whereas the linear system was only slightly inhibited.

Comparative Kinetics and Reciprocal Inhibition of Nitrate and Nitrite Uptake in Roots of Uninduced and Induced Barley (Hordeum vulgare L.) Seedlings

Remotely sensed electromagnetic reflectance data can provide at relatively low cost a set of detailed, spatially
distributed data on plant growth and development. Vegetation indices based on algebraic combinations of different
wavelength bands are especially useful in summarizing reflectance data. One of the most commonly used vegetation
indices is the normalized difference vegetation index, or NDVI. The objective of this study was to determine whether
measurements based on the NDVI could provide information useful for site-specific management of cotton. Aerial
photographs were taken of replicated Acala cotton field experiments in California in which the treatment was water or
nitrogen stress level. NDVI integrated over time showed a significant correlation with lint yield in those experiments in
which there was a significant stress effect on yield. The spatiotemporal pattern of NDVI reflected stress factors and was
approximately coincident with the onset of measurable water stress. NDVI tended to indicate the presence of nitrogen
stress even in those cases where the stress did not result in a significant yield reduction. In a study of the correlation of
NDVI with late season plant mapping indices NDVI was correlated with nodes above white flower and strongly correlated
with nodes above cracked boll. An alternative vegetation index, the relative nitrogen vegetation index, was not better than
NDVI as an indicator of nitrogen stress.

Relationships between remotely sensed reflectance data and cotton growth and yield

Relationships between remotely sensed reflectance data, Acala cotton growth and yield.

Acala (Gossypium hirsutum L.) and Pima (G. barbadense L.) cotton growth, lint yield, and fiber quality responses to N in the San Joaquin Valley, CA were evaluated. Numerous reasons, including adaptation of N fertilization guidelines to modern production practices, recent increases in energy costs, and growing concerns about NO3 contamination of ground water, led to the initiation of this study. Acala was grown for 3 yr on a Panoche day loam [fine-loamy, mixed (calcareous), thermic Typic Torriorthents] and a Wasco sandy loam (coarseloamy, mixed, nonacid, thermic Typic Torriorthents). Pima was grown for 2 yr on the Panoche clay loam. Four N treatments were established in a randomized complete block design: 56, 112, 168, and 224 kg N ha -1 . Three-year average aboveground dry matter production of Acala was 7800 and 12 600 kg ha -1 on the Panoche clay loam and 8500 and 11 900 kg ha -1 on the Wasco sandy loam for the 56 and 168 kg N ha -1 treatments, respectively. The equivalent 2-yr averages for Pima were 7600 (56 kg N ha -1 ) and 10 800 kg ha -1 (168 kg N ha -1 ). Linear increases in lint yield with increased N fertility level occurred for Acala on Panoche clay loam in every year. Maximum lint yield averaged over 3 yr was 1842 kg ha -1 in the 224 kg N ha -1 treatment. The response of Acala lint yield to N management on the Wasco sandy loam was smaller than on Panoche clay loam, with a maximum lint yield of 1666 kg ha -1 (224 kg N ha -1 , 3-yr average). Pima lint yield responded to N management in a quadratic fashion with maximum yields in the 168 kg N ha -1 treatment in both years (1638 kg ha -1 , 2-yr average). Acala gin turnouts were greater at the Panoche than at the Wasco site. Decreases in gin turnout with increasing N were significant on the Panoche clay loam (Acala and Pima) but not on the Wasco sandy loam (Acala). There was a generally positive relationship between increasing N fertilization and yield; however, efficient N management should include an assessment of available soil residual N, soil type, and yearly climatic conditions.

Response of Irrigated Acala and Pima Cotton to Nitrogen Fertilization

The disappearance of nitrate reductase activity in leaves of Hordeum vulgare L. during darkness was inhibited by cycloheximide, actinomycin D, and low temperature. Thus, protein synthesis was probably required for the disappearance of nitrate reductase in the dark. Since chloramphenicol did not affect the rate of loss of activity, the degradation or inactivation apparently required protein synthesis by the cytoplasmic ribosomal system. Consistent with this observation, nitrate reductase is also reportedly located in the cytoplasm. Thus, the amount of nitrate reductase activity present in leaves of barley may be controlled by a balance between activating and inactivating systems.

Robert L. Travis

Papers

Comparative Kinetics and Reciprocal Inhibition of Nitrate and Nitrite Uptake in Roots of Uninduced and Induced Barley (Hordeum vulgare L.) Seedlings

Relationships between remotely sensed reflectance data and cotton growth and yield

Relationships between remotely sensed reflectance data, Acala cotton growth and yield.

Response of Irrigated Acala and Pima Cotton to Nitrogen Fertilization

Evidence for an Inactivating System of Nitrate Reductase in Hordeum vulgare L. during Darkness That Requires Protein Synthesis.