A comprehensive analysis of root morphological changes and nitrogen allocation in maize in response to low nitrogen stress

The plasticity of root architecture is crucial for plants to acclimate to unfavourable environments including low nitrogen (LN) stress. How maize roots coordinate the growth of axile roots and lateral roots (LRs), as well as longitudinal and radial cell behaviours in response to LN stress, remains unclear. Maize plants were cultivated hydroponically under control (4 mm nitrate) and LN (40 μm ) conditions. Temporal and spatial samples were taken to analyse changes in the morphology, anatomical structure and carbon/nitrogen (C/N) ratio in the axile root and LRs. LN stress increased axile root elongation, reduced the number of crown roots and decreased LR density and length. LN stress extended cell elongation zones and increased the mature cell length in the roots. LN stress reduced the cell diameter and total area of vessels and increased the amount of aerenchyma, but the number of cell layers in the crown root cortex was unchanged. The C/N ratio was higher in the axile roots than in the LRs. Maize roots acclimate to LN stress by optimizing the anatomical structure and N allocation. As a result, axile root elongation is favoured to efficiently find available N in the soil.     

Ethylene‐dependent aerenchyma formation in adventitious roots is regulated differently in rice and maize

In roots of gramineous plants, lysigenous aerenchyma is created by the death and lysis of cortical cells. Rice (Oryza sativa) constitutively forms aerenchyma under aerobic conditions, and its formation is further induced under oxygen‐deficient conditions. However, maize (Zea mays) develops aerenchyma only under oxygen‐deficient conditions. Ethylene is involved in lysigenous aerenchyma formation. Here, we investigated how ethylene‐dependent aerenchyma formation is differently regulated between rice and maize. For this purpose, in rice, we used the reduced culm number1 (rcn1) mutant, in which ethylene biosynthesis is suppressed. Ethylene is converted from 1‐aminocyclopropane‐1‐carboxylic acid (ACC) by the action of ACC oxidase (ACO). We found that OsACO5 was highly expressed in the wild type, but not in rcn1, under aerobic conditions, suggesting that OsACO5 contributes to aerenchyma formation in aerated rice roots. By contrast, the ACO genes in maize roots were weakly expressed under aerobic conditions, and thus ACC treatment did not effectively induce ethylene production or aerenchyma formation, unlike in rice. Aerenchyma formation in rice roots after the initiation of oxygen‐deficient conditions was faster and greater than that in maize. These results suggest that the difference in aerenchyma formation in rice and maize is due to their different mechanisms for regulating ethylene biosynthesis.     

Early root growth plasticity in seedlings of three Mediterranean woody species

Using a 141 F₂ population generated from maize inbred B64 × teosinte Zea nicaraguensis cross, quantitative trait loci (QTLs) controlling aerenchyma formation in roots under non-flooding drained soil conditions were identified. Seedlings of Z. nicaraguensis formed clear aerenchyma in the cortex of adventitious roots in non-flooding conditions, whereas the maize inbred line B64 did not. In the F₂ population, the capacity to develop aerenchyma exhibited wide and continuous variation, suggesting the trait was controlled by multiple genes. A linkage map was developed using 85 SSR markers, covering 1,224 cM across all ten chromosomes. Composite interval mapping analysis revealed that four QTLs for aerenchyma formation under non-flooding conditions were located to two regions of chromosome 1 (identified as Qaer1.02-3 and Qaer1.07), chromosome 5 (Qaer5.09) and chromosome 8 (Qaer8.06-7), and these explained 46.5% of the total phenotypic variance. The multiple interval mapping approach identified additional QTLs on chromosomes 1 (Qaer1.01) and 5 (Qaer5.01). Using these results, it may be possible to use SSR markers linked to aerenchyma formation in a marker assisted selection approach to introduce aerenchyma formation in drained soil conditions into maize for the eventual development of flooding tolerant maize hybrids.     

Aerenchyma Formed Under Phosphorus Deficiency Contributes to the Reduced Root Hydraulic Conductivity in Maize Roots

Root hydraulic conductivity has been shown to decrease under phosphorus （P） deficiency. This study Investigated how the formation of aerenchyma is related to this change. Root anatomy, as well as root hydraulic conductivity was studied In maize （Zea mays L.） roots under different phosphorus nutrition conditions. Plant roots under P stress showed enhanced degradation of cortical cells and the aerenchyma formation was associated with their reduced root hydraulic conductivity, supporting our hypothesis that air spaces that form in the cortex of phosphorusstressed roots Impede the radial transport of water in a root cylinder. Further evidence came from the variation In aerenchyma formation due to genotypic differences. Five maize inbred lines with different porosity in their root cortex showed a significant negative correlation with their root hydraulic conductivity. Shoot relative water content was also found lower In P-deficient maize plants than that in P-sufficient ones when such treatment was prolonged enough, suggesting a limitation of water transport due to lowered root hydraulic conductivity of P-deficient plants.     

A novel morphological response of maize (Zea mays) adult roots to heterogeneous nitrate supply revealed by a split‐root experiment

Approximately 35–55% of total nitrogen (N) in maize plants is taken up by the root at the reproductive stage. Little is known about how the root of an adult plant responds to heterogeneous nutrient supply. In this study, root morphological and physiological adaptations to nitrate‐rich and nitrate‐poor patches and corresponding gene expression of ZmNrt2.1 and ZmNrt2.2 of maize seedlings and adult plants were characterized. Local high nitrate (LoHN) supply increased both lateral root length (LRL) and density of the treated nodal roots of adult maize plants, but only increased LRL of the treated primary roots of seedlings. LoHN also increased plant total N acquisition but not N influx rate of the treated roots, when expressed as per unit of root length. Furthermore, LoHN markedly increased specific root length (m g^?1) of the treated roots but significantly inhibited the growth of the lateral roots outside of the nitrate‐rich patches, suggesting a systemic carbon saving strategy within a whole root system. Surprisingly, local low nitrate (LoLN) supply stimulated nodal root growth of adult plants although LoLN inhibited growth of primary roots of seedlings. LoLN inhibited the N influx rate of the treated roots and did not change plant total N content. The gene expression of ZmNrt2.1 and ZmNrt2.2 of the treated roots of seedlings and adult plants was inhibited by LoHN but enhanced by LoLN. In conclusion, maize adult roots responded to nitrate‐rich and nitrate‐poor patches by adaptive morphological alterations and displayed carbon saving strategies in response to heterogeneous nitrate supply.     

Influence of NO3 - and NH4 + nutrition on fermentation,nitrate reductase activity and adenylate energy charge of roots of Carex pseudocyperus L. and Carex sylvatica Huds. exposed to anaerobic nutrient solutions

The adenylate energy charge, production of ethanol and lactate, and nitrate reductase activity were determined in order to study the influence of different nitrogen sources on the metabolic responses of roots of Carex pseudocyperus L. and Carex sylvatica HUDS. exposed to anaerobic nutrient solutions. Determination of adenylates was carried out by means of a modified HPLC technique. Total quantity of adenylates was higher in Carex pseudocyperus than in Carex sylvatica under all conditions. In contrast, the adenylate energy charge was only slightly different between the species and decreased more or less in relation to the applied nitrogen source under oxygen deficiency. The adenylate energy charge in roots of plants under nitrate nutrition showed a smaller decrease under anaerobic environmental conditions than plants grown with ammonium or nitrate/ammonium. Roots of nitrate-fed plants showed a lower ethanol and lactate production than ammonium/nitrate- and ammonium-fed plants. Ethanol production was higher in C. pseudocyperus, formation of lactate was lower compared to that in Carex sylvatica. The activity of enzymes involved in fermentation processes (ADH, LDH and PDC) was enhanced significantly after 24 hours of exposure to anaerobic nutrient solutions in roots of both species. The induction of these enzymes was only slightly influenced by different nitrogen supply. In vivo nitrate reductase activity increased almost 3-fold compared to the aerobic treatment in both species and overcompensated loss of NADH reoxidation capacity caused by decrease of ethanol and lactate development. Induction of in vitro nitrate reductase activity was enhanced 313% in C. pseudocyperus and 349% in C. sylvatica under anaerobic environmental conditions and nitrate supply. These results indicate that nitrate may serve as an alternative electron acceptor in anaerobic plant root metabolism and that the nitrate-supported energy charge may be due to an accelerated glycolytic flux resulting from a more effective NADH reoxidation capacity by nitrate reduction plus fermentation than by fermentation alone.Abbreviations ADH alcohol dehydrogenase - AEC adenylate energy charge - DMSO dimethyl sulfoxide - EDTA ethylen diamine tetraacetic acid - HPLC high performance liquid chromatography - LDH lactate dehydrogenase - NRA nitrate reductase activity - PCA perchloric acid - PDC pyruvate decarboxylase - PVP polyvinylpyrrolidone - PVPP polyvinylpolypyrrolidone - TCA trichloroacetic acid, Tris-tris(hydroxymethyl)aminomethane     

Evidence for nitrate reductase expression during initiation of lateral roots by NAA in chicory

The supply of -naphthaleneacetic acid (NAA), to excised chicory roots induced the formation of lateral root meristems mainly localized proximal to the pre-existing apical root meristem, in a region which does not initiate any lateral roots in control conditions. Inhibition of root elongation and concomitant enlargement of the apices were also observed. Quantification of NAA and cytokinin levels showed that the most reproducible and significant changes occurring after the NAA treatment consisted of a decrease in the level of zeatin-O-glucoside conjugates. Hydrolysis of these conjugates might deliver free zeatin-type compounds which were consumed during the lateral root growth. After 5 d, control excised roots contained a high level of amino acids, mainly as asparagine and arginine, probably issued from proteolysis associated to a senescent-like process. Conversely, in the presence of NAA, neither accumulation of amino acids nor a decrease of the total protein content of the tissue could be detected. Newly initiated meristems expressed the nia gene which encodes nitrate reductase, the first enzyme of the nitrate assimilatory pathway. Thus the increased expression of nitrate reductase which was observed in excised roots of chicory supplied with NAA (Vuylsteker et al., 1997b) may be ascribed to lateral root formation and development. The reinduction of nitrate reduction activity was driven by the increased demand for reduced nitrogen. Thus, the nia gene is one of the genes expressed during the early stages of root meristem formation.Keywords: Auxins, chicory, in situ hybridization, lateral root, nitrate reductase.      

Ideotype root architecture for efficient nitrogen acquisition by maize in intensive cropping systems

The use of nitrogen (N) fertilizers has contributed to the production of a food supply sufficient for both animals and humans despite some negative environmental impact. Sustaining food production by increasing N use efficiency in intensive cropping systems has become a major concern for scientists, environmental groups, and agricultural policymakers worldwide. In high-yielding maize systems the major method of N loss is nitrate leaching. In this review paper, the characteristic of nitrate movement in the soil, N uptake by maize as well as the regulation of root growth by soil N availability are discussed. We suggest that an ideotype root architecture for efficient N acquisition in maize should include (i) deeper roots with high activity that are able to uptake nitrate before it moves downward into deep soil; (ii) vigorous lateral root growth under high N input conditions so as to increase spatial N availability in the soil; and (iii) strong response of lateral root growth to localized nitrogen supply so as to utilize unevenly distributed nitrate especially under limited N conditions.     

Biomass production and nitrogen contents of the CAM plants Kalanchoe daigremontiana and K. tubiflora in cultures with different nitrogen and water supply

Summary Two CAM plant species (Kalanchoe daigremontiana and K. tubiflora) were cultivated in pure cultures with two different levels of nitrogen and water supply. A comparison of the plant dry weight showed that the productivity was severely reduced under low nitrogen or/and water conditions. Additionally the proportion of the different organs contributing to the total biomass, and thus the ratio of root/shoot dry weight shifted substantially. At the same time the production of leaf buds in ratio to leaf biomass was increased. Concentrations of organic nitrogen and nitrate in the shoots were drastically reduced under low nitrogen or/and water conditions, but organic nitrogen concentration in the roots remained relatively high. The leaf conductance over a day was investigated only for K. daigremontiana, and decreased with reduced water and nitrogen supply. The results indicate that CAM plants do not have the predicted high nitrogen use efficiency. Under environmental stress they change from biomass production towards provisions for life preservation.     

Ammonium and nitrate nutrition inPlantago lanceolata L. andPlantago major L. ssp.major

With the aims (1) to test whether the different natural occurrence of twoPlantago species in grasslands is explained by a different preference of the species for nitrate or ammonium; (2) to test whether the different occurrence is explained by differences in the flexibility of the species towards changes in the nitrogen form; (3) to find suitable parameters as a tool to study ammonium and nitrate utilization of these species at the natural sites in grasslands, plants ofPlantago lanceolata andP. major ssp.major were grown with an abundant supply of nitrate, ammonium or nitrate+ammonium as the nitrogen source (0.5 mM). The combination of ammonium and nitrate gave a slightly higher final plant weight than nitrate or ammonium alone. Ammonium lowered the shoot to root ratio inP. major. Uptake of nitrate per g root was faster than that of ammonium, but from the mixed source ammonium and nitrate were taken up at the same rate. In vivo nitrate reductase activity (NRA) was present in both shoot and roots of plants receiving nitrate. When ammonium was applied in addition to nitrate, NRA of the shoot was not affected, but in the root the activity decreased. Thus, a larger proportion of total NRA was present in the shoot than with nitrate alone. In vitro glutamate dehydrogenase activity (GDHA) was enhanced by ammonium, both in the shoot and in the roots.In vitro glutamine synthetase activity (GSA) was highest in roots of plants receiving ammonium. Both GDHA and GSA were higher inP. lanceolata than inP. major. The concentration of ammonium in the roots increased with ammonium, but it did not accumulate in the shoot. The concentration of amino acids in the roots was also enhanced by ammonium. Protein concentration was not affected by the form of nitrogen. Nitrate accumulated in both the shoot and the roots of nitrate grown plants. When nitrate in the solution was replaced by ammonium, the nitrate concentration in the roots decreased rapidly. It also decreased in the shoot, but slowly. It is concluded that the nitrogen metabolism of the twoPlantago species shows a similar response to a change in the form of the nitrogen source, and that differences in natural occurrence of these species are not related to a differential adaptation of nitrogen metabolism towards the nitrogen form. Suitable parameters for establishing the nitrogen source in the field are thein vivo NRA, nitrate concentrations in tissues and xylem exudate, and the fraction of total reduced nitrogen in the roots that is in the soluble form, and to some extent thein vitro GDHA and GSA of the roots. Grassland Species Research Group. Publ. no 118.     

Induction of Nitrate Reductase and Peroxidase Activity in the Seedlings of Normal and High Lysine Maize

Steady state levels of in vivo nitrate reductase activity in the endosperm, scutella, roots and shoots of maize seedlings were higher in normal maize than those in high lysine maize. Activity of peroxidase in the roots, however, was higher in the high lysine cultivar. The nitrate reductase activity increased with the supply of nitrate in all parts of the seedlings of both cultivars, although the rates of increment in the endosperm were lower than those in scutella, roots and shoots. In relation to substrate concentration, a saturation was achieved at 5 to 10 mM of nitrate except in the endosperm, where activity increased slowly up to 100 mM at least. Final levels of enzyme activity were significantly higher in the scutella of normal than in that of high lysine seedlings. In vitro enzyme activity in the roots also increased with the supply of nitrate in both cultivars, reaching maximum at 5 to 10 mM nitrate.     

MESOCOTYL ROOT FORMATION IN ECHINOCHLOA PHYLLOPOGON (POACEAE) IN RELATION TO ROOT ZONE AERATION

Echinochloa phyllopogon was grown hydroponically under four root zone gassing treatments to determine aeration effects on the growth and development of the plant root system. Although mesocotyl growth and the number of nodal roots were unaffected by the treatments, other aspects of plant growth were altered. Shoot growth was reduced by hypoxic (5 kPa partial pressure O₂ in nitrogen gas) and anoxic conditions (O₂ free nitrogen gas), but not by ethylene (0.1 ppm in air). Seminal root growth was unaffected by hypoxia or ethylene treatments, but was reduced under anoxia. Hypoxic environments stimulated the emergence of roots along the length of the mesocotyl when compared to aerobic controls; anoxic and ethylene treatments had no significant effects. Mesocotyl roots elongated from primordia that were produced de novo in response to the hypoxic treatment. Under hypoxic conditions, aerenchyma was present in the cortex of nodal roots and to a lesser extent in seminal roots, but mesocotyl roots were devoid of aerenchyma under these conditions. The results are compared with the literature concerning flooding and aeration effects on growth and development in other species.     

Inhibition of maize root growth by high nitrate supply is correlated with reduced IAA levels in roots

The plant root system is highly sensitive to nutrient availability and distribution in the soil. For instance, root elongation is inhibited when grown in high nitrate concentrations. To decipher the mechanism underlying the nitrate-induced inhibition of root elongation, the involvement of the plant hormone auxin in nitrate-dependent root elongation of maize was investigated. Root growth, nitrogen and nitrate concentrations, and indole-3-acetic acid (IAA) concentrations in roots and in phloem exudates of maize grown under varying nitrate concentrations were analyzed. Total N and nitrate concentrations in shoots and roots increased and elongation of primary, seminal and crown roots were inhibited with increasing external nitrate from 0.05 to 5 mM. High nitrate-inhibited root growth resulted primarily from the reduced cell elongation and not from changes in meristem length. IAA concentrations in phloem exudates reduced with higher nitrate supply. Inhibition of root growth by high nitrate was closely related to the reduction of IAA levels in roots, especially in the sections close to root tips. Exogenous NAA and IAA restored primary root growth in high nitrate concentrations. It is concluded that the inhibitory effect of high nitrate concentrations on root growth may be partly attributed to the decrease in auxin concentrations of roots.     

Nitrate regulation of nitrate uptake and nitrate reductase expression in barley grown at different nitrate:ammonium ratios at constant relative nitrogen addition rate

Barley (Hordeum vulgare L. cv. Golf) was cultured using the relative addition rate technique, where nitrogen is added in a fixed relation to the nitrogen already bound in biomass. The relative rate of total nitrogen addition was 0.09 day^?1 (growth limiting by 35%), while the nitrate addition was varied by means of different nitrate: ammonium ratios. In 3- to 4-week-old plants, these ratios of nitrate to ammonium supported nitrate fluxes ranging from 0 to 22 μmol g^?1 root dry weight h^?1, whereas the total N flux was 21.8 ± 0.25 μmol g^?1 root dry weight h^?1 for all treatments. The external nitrate concentrations varied between 0.18 and 1.5 μM. The relative growth rate, root to total biomass dry weight ratios, as well as Kjeldahl nitrogen in roots and shoots were unaffected by the nitrate:ammonium ratio. Tissue nitrate concentration in roots were comparable in all treatments. Shoot nitrate concentration increased with increasing nitrate supply, indicating increased translocation of nitrate to the shoot. The apparent V_max for net nitrate uptake increased with increased nitrate fluxes. Uptake activity was recorded also after growth at zero nitrate addition. This activity may have been induced by the small, but detectable, nitrate concentration in the medium under these conditions. In contrast, nitrate reductase (NR) activity in roots was unaffected by different nitrate fluxes, whereas NR activity in the shoot increased with increased nitrate supply. NR-mRNA was detected in roots from all cultures and showed no significant response to the nitrate flux, corroborating the data for NR activity. The data show that an extremely low amount of nitrate is required to elicit expression of NR and uptake activity. However, the uptake system and root NR respond differentially to increased nitrate flux at constant total N nutrition. It appears that root NR expression under these conditions is additionally controlled by factors related to the total N flux or the internal N status of the root and/or plant. The method used in this study may facilitate separation of nitrate-specific responses from the nutritional effect of nitrate.     

Root Development and Absorption of Ammonium and Nitrate from the Rhizosphere

Plant roots operate in an environment that is extremely heterogeneous, both spatially and temporally. Nonetheless, under conditions of limited diffusion and against intense competition from soil microorganisms, plant roots locate and acquire vital nitrogen resources. Several factors influence the mechanisms by which roots respond to ammonium and nitrate. Nitrogen that is required for cell division and expansion derives primarily from the apex itself absorbing rhizosphere ammonium and nitrate. Root density and extension are greater in nutrient solutions containing ammonium than in those containing nitrate as the sole nitrogen source. Root nitrogen acquisition alters rhizosphere pH and redox potential, which in turn regulate root cell proliferation and mechanical properties. The net result is that roots proliferate in soil zones rich in nitrogen. Moreover, plants develop thinner and longer roots when ammonium is the primary nitrogen source, an appropriate strategy for a relatively immobile nitrogen form. Present address of Alison R. Taylor: The Marine Biological Association, The Laboratory, Citadel Hill, Plymouth PL1 2PB, UK.     

Ethylene-Induced Activation of Xylanase in Adventitious Roots of Maize as a Response to the Stress Effect of Root Submersion

Submersion of roots of ten-day-old maize (Zea maysL.) seedlings was accompanied by a decrease in pO₂and an increase in pCO₂of the medium adjacent to the roots. These changes stimulated ethylene evolution in intact plants. Enhanced biosynthesis of ethylene was accompanied by xylanase activation in adventitious roots. As a result, an enhanced formation of aerenchyma was observed in the cortex of adventitious roots. Therefore, these processes resulted in the development of a ventilation system by which O₂can reach the root system exposed to hypoxia. The volume of aerenchyma was assessed by the volume of gas cavities (porosity). In contrast to the main root, the growth of adventitious roots was not inhibited under these conditions. Enlargement of the stem base and increase in the number of aerenchymatous adventitious roots facilitated the oxygen supply to the submerged organs of the plants.     

Nitrogen and Nitrate Accumulation in Species Having Different Relationships Between Nitrate Uptake and Reduction

Species differ in the relationship of nitrate reductase activityto nitrate uptake. In Capsicum annuum different diurnal patternsof leaf nitrate reductase activity and nitrate uptake have beenreported. As a consequence, the relationship of free nitratein the plant to nitrate supplied has a higher level of significancethan has reduced nitrogen to nitrate supplied. In Zea mays ithas been reported that leaf nitrate reductase activity respondsdirectly to nitrate translocation to the leaf and in this speciesthe relationship of greatest significance is reduced nitrogencontent to nitrate supplied. In both species, and also in Cucumis melo, the proportion oftotal plant free nitrate and reduced nitrogen in the roots decreases,and in the stem increases, with increasing nitrate supplied. The accumulation of free nitrate in leaves is accompanied bya quantitatively different relationship between reduced nitrogenand dry weight compared to leaves not accumulating nitrate. Capsicum annuum. L., Cucumis melo L., melon, Zea mays L., maize, sweet corn, nitrate reductase, nitrate uptake