Evidence for the nitrate-dependent spatial regulation of the nitrate reductase gene in chicory roots

Young chicory plants (Cichorium intybus L. var. Witloof) show a tenfold higher nitrate reductase NR activity in roots compared to leaves. Northern analysis revealed, besides the nitrate inducibility of the nitrate reductase gene (nia), a higher level of expression in the roots. By modifying the external nitrate concentration the NR activity in the leaves remained negligible whereas a maximal activity was observed in the roots when grown in the presence of 5 mM nitrate. Surprisingly, variation of the external nitrate concentration induced changes in the spatial regulation of nia within the root. In-situ hybridization mainly localized nia mRNA in the cortical cells of roots grown at low nitrate concentrations (0.2 mM). At high nitrate concentrations (5 mM), nia mRNA was more abundant in the vascular tissues. The root apex revealed a strong signal under both conditions. The isolation and characterization of the NR structural gene from chicory is also presented. Southern blot analysis revealed the presence of a single nia gene per haploid genome of chicory.     

Auxin Induced Lateral Root Formation in Chicory

The supply of auxins [2,4-dichlorophenoxy acetic acid (2,4D),indole-3 acetic acid (1AA) and -naphthaleneacetic acid (NAA)]to excised chicory roots induced the formation of lateral rootmeristems mainly located close to the pre-existing apical rootmeristem. Lateral root growth induced in non-excised roots requiredhigher auxin concentrations. Inhibition of root elongation andconcomittant enlargement of the apices was also observed. SupplyingIAA induced the formation of lateral meristems earlier thanNAA, but subsequently favoured root elongation. Conversely,in the presence of 2,4D, reactivation of pericycle cells wasvery intense, but conversion of primordia to laterals was inhibited.Regardless of the auxin used, the responsive area in which lateralmeristems appeared was located a maximum of 4 mm away from theapical meristem. This region remained devoid of any lateralroot formation under control conditions. Pericycle cells oppositethe xylem poles in the diarch stele regained meristematic activityand divided transversally, giving rise to shorter cells. Thesecells subsequently divided periclinally, forming pairs of cellson the same transverse level. The root primordium extruded throughcortical cells and was surrounded by a lacuna formed to thedetriment of cortical cells.Copyright 1998 Annals of BotanyCompany Auxins,Cichorium intybus, chicory, lateral root, root elongation.     

A putative transporter is essential for integrating nutrient and hormone signaling with lateral root growth and nodule development in Medicago truncatula

Legume root architecture involves not only elaboration of the root system by the formation of lateral roots but also the formation of symbiotic root nodules in association with nitrogen‐fixing soil rhizobia. The Medicago truncatula LATD/NIP gene plays an essential role in the development of both primary and lateral roots as well as nodule development. We have cloned the LATD/NIP gene and show that it encodes a member of the NRT1(PTR) transporter family. LATD/NIP is expressed throughout the plant. pLATD/NIP‐GFP promoter–reporter fusions in transgenic roots establish the spatial expression of LATD/NIP in primary root, lateral root and nodule meristems and the surrounding cells. Expression of LATD/NIP is regulated by hormones, in particular by abscisic acid which has been previously shown to rescue the primary and lateral root meristem arrest of latd mutants. latd mutants respond normally to ammonium but have defects in responses of the root architecture to nitrate. Taken together, these results suggest that LATD/NIP may encode a nitrate transporter or transporter of another compound.     

Expression of a chimeric nitrate reductase gene in transgenic lettuce reduces nitrate in leaves

 Transgenic plants of four glasshouse-grown lettuce cultivars ('Cortina', 'Evola', 'Flora' and 'Luxor') were obtained by co-cultivating excised cotyledons with Agrobacterium tumefaciens. The Agrobacterium strain LBA4404 contained the binary vector pBCSL16, which carried a nitrate reductase (nia) cDNA linked to CaMV promoter and terminator sequences, and the neomycin phosphotransferase II (nptII) gene. Transformed shoots were selected by their ability to root on medium containing kanamycin sulphate, by a positive NPTII assay and by PCR analysis. The presence of the nia cDNA in transgenic lettuce was confirmed by nitrate reductase (NR) enzymatic assay, a reduction in the nitrate content of leaves and by Southern hybridisation. PCR analysis of cDNA fragments from transgenic plants confirmed that both nia and nptII genes were expressed in first seed-generation (T1) lettuce plants. The commercial importance of reduced nitrate concentrations in lettuce is discussed. Received: 7 January 1998 / Revision received: 24 February 1998 / Accepted: 22 March 1999     

The Mechanism of Nitrate Accumulation in Pakchoi [Brassica Campestris L.ssp. Chinensis(L.)]

Decreased nitrate in vegetables can improve crop nitrogen utilization efficiency and lessen the human health risk caused by the reduction of nitrate to nitrite in vegetables. This paper studied the mechanisms of differences in nitrate accumulation and distribution within organs of two cultivars of pakchoi (Brassica campestris L.ssp. Chinensis (L.) previously screened in hydroponic experiments from 12 cultivars popularly grown in China at present. The two typical cultivars used in this experiment were Shanghaiqing with low nitrate accumulation and Liangbaiye 1 with high nitrate accumulation. There was no significant difference of total nitrate uptake but a significant difference in nitrate content existed between the two cultivars. Compared with Liangbaiye 1, Shanghaiqing showed a significantly higher photosynthetic rate and nitrate reductase activity. Determination of nitrate concentration (activity) in vacuoles with double-barrelled nitrate-selective microelectrodes showed that Shanghaiqing had lower vacuolar nitrate activity than Liangbaiye 1. Two putative nitrate reductase genes, nia1 and nia2, were amplified from the leaf blades of these two cultivars. Nia1 mRNA fragments (887 bp, accession numbers DQ082868 and DQ082869) were amplified using degenerate primer and nia2 mRNA fragment was amplified using one pair of generate primers designed according to DQ001901. Sequence analysis of DQ082868 and DQ082869 both showed 97% and 87% similarity with two nitrate reductase mRNA sequences of Brassica napus, accession numbers D38219 and D38220, respectively. The results of real time PCR to compare the relative expression of the putative nitrate reductase genes (nia1 and nia2) showed that Shanghaiqing had significantly higher expression level than Liangbaiye 1 and nia2 was significantly higher than nia1 in leaf blade and petiole. Both the nitrate reductase activity and the relative expression level of nia1 were in the order of leaf blade > root > petiole, while that of nia2 was leaf blade > petiole > root. There was no statistically significant difference of nitrate activity stored in vacuoles between the different organs of the two cultivars. It can be concluded that Shanghaiqing took up slightly less nitrate, but had significantly higher nitrate reductase activity in cytosol and had a higher relative expression of the putative nitrate reductase genes than Liangbaiye 1; this leads to the fact that Shanghaiqing has a lower nitrate content than Liangbaiye 1.     

Effect of time of inoculation on in vitro ectomycorrhizal colonization and nodule initiation in Acacia holosericea seedlings

The complex interactions that occur in systems with more than one type of symbiosis were studied using one isolate of Bradyrhizobium sp. and the ectomycorrhizal fungus Pisolithus tinctorius (Pers.) Coker and Couch inoculated on to the roots of Acacia holosericea A. Cunn. ex G. Don in vitro. After a single inoculation with Bradyrhizobium sp., bacteria typically entered the roots by forming infection threads in the root hair cells via the curling point of the root hair and/ or after intercellular penetration. Sheath formation and intercellular penetration were observed on Acacia roots after a single inoculation with Pisolithus tinctorius but no radial elongation of epidermal cells. Simultaneous inoculation with both microorganisms resulted in nodules and ectomycorrhiza on the root system, occasionally on the same lateral root. On lateral roots bearing nodules and ectomycorrhiza, the nodulation site was characterized by the presence of a nodule meristem and the absence of an infection thread; sheath formation and Hartig net development occurred regularly in the region of the roots adjacent to nodules. Prior inoculation with Bradyrhizobium sp. did not inhibit ectomycorrhizal colonization in root segments adjacent to nodules in which nodule meristems and infection threads were clearly present. Conversely, in ectomycorrhizae inoculated by bacteria, the nodule meristem and the infection thread were typically absent. These results show that simultaneous inoculation with both microorganisms inhibits infection thread development, thus conferring an advantage on fungal hyphae in the competition for infection sites. This suggests that fungal hyphae can modify directly and/or indirectly the recognition factors leading to nodule meristem initiation and infection thread development.     

An Auxin-Inducible F-Box Protein CEGENDUO Negatively Regulates Auxin-Mediated Lateral Root Formation in Arabidopsis

Previously, we characterized 92 Arabidopsis genes (AtSFLs) similar to the S-locus F-box genes involved in S-RNase-based self-incompatibility and found that they likely play diverse roles in Arabidopsis. In this study, we investigated the role of one of these genes, CEGENDUO (CEG, AtSFL61), in the lateral root formation. A T-DNA insertion in CEG led to an increased lateral root production, which was complemented by transformation of the wild-type gene. Its downregulation by RNAi also produced more lateral roots in transformed Arabidopsis plants whereas its overexpression generated less lateral roots compared to wild-type, indicating that CEG acts as a negative regulator for the lateral root formation. It was found that CEG was expressed abundantly in vascular tissues of the primary root, but not in newly formed lateral root primordia and the root meristem, and induced by exogenous auxin NAA (α-naphthalene acetic acid). In addition, the ceg mutant was hyposensitive to NAA, IAA (indole-3-acetic acid) and 2,4-D (2,4-dichlorophenoxyacetic acid), as well as the auxin transport inhibitor TIBA (3,3,5-triiodobenzoic acid), showing that CEG is an auxin-inducible gene. Taken together, our results show that CEG is a novel F-box protein negatively regulating the auxin-mediated lateral root formation in Arabidopsis. Electronic supplementary material Electronic supplementary material is available for this article at and accessible for authorised users.     

Non-coordinate expression of the neighbouring genes rplL and rpoB,C of Escherichia coli.

Summary Two types of nitrate reductase-deficient mutant cell lines (nia and cnx) of Nicotiana tabacum have been used for in vitro reconstitution of NADH-nitrate reductase. The cnx mutants simultaneously lack NADH-,FADH₂-, red benzyl viologen-nitrate reductase, and xanthine dehydrogenase activities, but retain the nitrate reductase-associated NADH-cytochrome c reductase activity. These mutants are interpreted to be defective in the molybdenum-containing cofactor necessary for nitrate reductase activity. In the nia lines xanthine dehydrogenase activity is unaffected, and the loss of NADH-nitrate reductase is accompanied by a loss of all partial activities of nitrate reductase, including NADH-cytochrome c reductase. When cnx cells (induced by nitrate) were homogenized together with nia cells (induced by nitrate or uninduced), NADH-nitrate reductase activity was detectable in the cell extract. No nitrate reductase was observed when the cnx mutants were homogenized together, or after cohomogenization of the nia mutants. Thus, the inactive nitrate reductase molecule formed in the cnx mutants has been complemented in vitro with the molybdenum-containing cofactor supplied by nia extracts, thus giving rise to NADH-nitrate reductase activity. This result gives additional support to the interpretation that the active nitrate reductase of Nicotiana tabacum is composed of at least the NADH-cytochrome c reductase moiety and a molybdenum-containing cofactor which is formed by the action of the cnx gene product(s).     

Control of root architecture and nodulation by the LATD/NIP transporter

The Medicago truncatula LATD/NIP gene is essential for the development of lateral and primary root and nitrogen-fixing nodule meristems as well as for rhizobial invasion of nodules. LATD/NIP encodes a member of the NRT1(PTR1) nitrate and di-and tri-peptide transporter family, suggesting that its function is to transport one of these or another compound(s). Because latd/nip mutants can have their lateral and primary root defects rescued by ABA, ABA is a potential substrate for transport. LATD/NIP expression in the root meristem was demonstrated to be regulated by auxin, cytokinin and abscisic acid, but not by nitrate. LATD/NIP''s potential function and its role in coordinating root architecture and nodule formation are discussed.Key words: nodule development, lateral root development, root architecture, symbiotic nitrogen fixation, Medicago truncatula, NRT1(PTR) gene familyUnlike most other plants, legumes form two kinds of lateral root organs: lateral roots and nitrogen-fixing root nodules that form in conjunction with compatible symbiotic rhizobium bacteria. Although the morphology and function of these two root organs is distinct, both require the function of the LATD/NIP gene, indicating shared genetic components for these two developmental processes and providing support for a model in which legume nodules evolved from a lateral root blueprint. Both lateral roots and nodules initiate in previously differentiated root cells in response to environmental and developmental cues mediated by hormones. Interestingly, regulation of nodules and lateral roots by hormones is often opposite, allowing formation of one organ or another depending on the conditions.     

Formation of NADPH-nitrate reductase activity in vitro from Aspergillus nidulans niaD and cnx mutants

Summary Mutants of A. nidulans at several loci lack detectable NADPH-nitrate reductase activity. These loci include niaD, the structural gene for the nitrate reductase polypeptide, and five other loci termed cnxABC, E, F, G and H which are presumed to be involved in the formation of a molybdenum-containing component (MCC) necessary for nitrate reductase activity. When frozen mycelia from A. nidulans deletion mutant niaD26 were homogenized in a Ten Broeck homogenizer together with frozen mycelia from either enzA6, cnxE29, cnxF12, enxG4 or cnxH3 strains grown on urea+nitrate as the nitrogen source, nitrate reductase activity was detectable in the extract. Similar results were obtained by co-homogenizing niaD mycelia with Neurospora crassa nit-1 mycelia induced on nitrate. Thus, all A. nidulans cnx mutants are similar to the N. crassa nit-1 strain in their capacity to yield NADPH-nitrate reductase in the presence of the presumed MCC. As judged by the amounts of nitrate reductase formed, niaD26 mycelia grown on urea±nitrate contained much more available MCC than ammonium-grown mycelia. No NADPH-nitrate reductase activity was found in extracts prepared by co-homogenizing mycelia from all five A. nidulans cnx strains. Wild-type A. nidulans NADPH-nitrate reductase acid dissociated by adjustment to pH 2.0–2.5 and re-adjusted to pH 7 could itself re-assemble to form active nitrate reductase and thus was not a sueful source of MCC for these experiments. These results are consistent with the conclusion that the active nitrate reductase complex is composed of polypeptide components which are the niaD gene product, plus the MCC which is formed through the combined action of the cnx gene products. Further, the production of MCC may be regulated in response to the nitrogen nutrition available to the organism.     

In vitro plant regeneration of Hedychium roxburghii blume through rhizome-meristem culture

Full grown plants have been raised successfully in vitro by culturing meristems excised from rhizome of Hedychium roxburghii Blume on Murashige and Skoog's medium supplemented with NAA and kinetin. Zeatin associated with IAA stimulated the development of buds on the basal region of the stem. A low concentration of NAA (0.2 mg.1^-1) enhanced the root formation on young excised buds.     

Cell division activation in the quiescent center of excised maize root tip

The phenomenon of activating cell proliferation in the quiescent center of excised maize roots is described. The root tips were grown on wet filter paper in Petri dishes. This phenomenon was observed in 8 to 14 maize cultivars and was absent in excised Arabidopsis root tips. The distribution of mitoses in meristems greatly varied in individual seedlings roots from the same seed lot and seedlings of different cultivars. Meristem opening was observed after the removal of small root tips not longer than 3 mm and intact seminal roots. Sucrose (2%) and 10⁻⁶–10⁻⁸ M indole-3-acetic acid did not prevent meristem opening. These findings indicate that the state of quiescent center is maintained by a system of intercellular and interorgan relations, which are to be clarified.     

HORMONAL CONTROL OF ORGANOGENESIS IN OPUNTIA POLYACANTHA (CACTACEAE)

An axillary bud (areole) of Opuntia polyacantha is composed of an extremely short axis which bears highly modified leaves (spines). After producing the spines, the bud apical meristem becomes quiescent. When the axillary bud is excised and cultured with benzylaminopurine (BAP), the apical meristem increases in complexity and produces leaves on an elongated axis. Gibberellic acid (GA) induces spine production with no elongation of the axis. Naphthaleneacetic acid (NAA) causes no structural change in the meristem but induces root formation in adjacent tissue. The initiation of roots, spines, or leaves is visible within three days. Explants on medium lacking hormones show no structural change, but by the tenth day are insensitive to BAP or NAA. These aged explants can be made to respond to BAP or NAA by merely rewounding them at the time of hormone application. With concentrations previously found to be optimal for single hormone responses, NAA in combination with BAP prevents shoot formation, or in combination with GA prevents spine production. When all three hormones are combined, spines are produced, indicating that GA is active morphologically while BAP and NAA counteract each other. If the level of NAA is decreased, BAP counteracts the NAA and also overrides the GA and leafy shoots are produced. By varying the proportions of BAP and GA, in the absence of NAA, four types of lateral appendage can be produced: leaves, mildly altered leaves, leaf-spine transition forms, and spines.     

Lateral root development and saponin accumulation as affected by IBA or NAA in adventitious root cultures of <Emphasis Type="Italic">Panax ginseng</Emphasis> C.A. Meyer

Summary The effect of indole-3-butyric acid (IBA) and 1-naphthaleneacetic acid (NAA) on lateral root formation was investigated in adventitious root culture of Panax ginseng. Lateral root formation was affected by IBA (24.6 μM) or NAA (9.8 μM). Lateral root primordia emerged from the explant root pericycle after about 7 d of culture when the roots were cultured on Schenk and Hildebrandt (SH) medium supplemented with 24.6 μM IBA or 9.8 μM NAA. However, no changes were observed in the explant root pericycle on auxin-free medium. The IBA treatment was more effective for lateral root induction and root growth compared to NAA. In morphological and histological aspects, the lateral roots formed under IBA treatment developed normally, while NAA-treated roots exhibited abnormal growth. The accumulation of total saponin was greater in roots treated with IBA than with NAA.     

2种菊苣再生体系及遗传转化效率的比较

以普那菊苣和将军菊苣子叶为材料,通过植物组织培养的方法,探讨了不同激素浓度配比对二者愈伤组织诱导、芽分化以及根再生的影响,并通过农杆菌介导法将编码獐茅液泡膜Na+/H+逆向转运蛋白基因(AlNHX)导入菊苣中,比较普那菊苣和将军菊苣的遗传转化效率。结果表明:不同基因型的菊苣愈伤组织诱导和芽分化条件不同,普那菊苣最佳培养基为MS+1.5mg/L 6-BA+0.2mg/L IBA;将军菊苣最佳培养基为MS+1.0mg/L 6-BA+0.5mg/L NAA;二者最佳生根培养基均为1/2MS+0.1mg/L NAA。获得的抗性芽经PCR检测,初步证实AlNHX已插入到菊苣基因组中,且普那菊苣转化效率为10.0%,将军菊苣转化效率为13.3%。     

Role of sugars in nitrate utilization by roots of dwarf bean

Nitrate uptake and in vivo, nitrate reductase activity (NRA) in roots of Phaseolus vulgaris, L. cv. Witte Krombek were measured in nitrogen-depleted plants of varying sugar status, Variation in sugar status was achieved at the start of nitrate nutrition by excision, ringing, darkness or administration of sugars to the root medium. The shape of the apparent induction pattern of nitrate uptake was not influenced by the sugar status of the absorbing tissue. When measured after 6 h of nitrate nutrition (0.1 mol m^?3), steady state nitrate uptake and root NRA were in the order intact>dark>ringed>excised. Exogenous sucrose restored NRA in excised roots to the level of intact plants. The nitrate uptake rate of excised roots, however, was not fully restored by sucrose (0.03–300 mol m^?3). When plants were decapitated after an 18 h NO₃^? pretreatment, the net uptake rate declined gradually to become negative after three hours. This decline was slowed down by exogenous fructose, whilst glucose rapidly (sometimes within 5 min) stimulated NG^?₃ uptake. Presumably due to a difference in NO₃^? due to a difference in NO₃^? uptake, the NRA of excised roots was also higher in the presence of glucose than in the presence of fructose after 6 h of nitrate nutrition. The sugar-stimulation of, oxygen consumption as well as the release of ¹⁴CO₂ from freshly absorbed (U-¹⁴C) sugar was the same for glucose and fructose. Therefore, we propose a glucose-specific effect on NO₃^? uptake that is due to the presence of glucose rather than to its utilization in root respiration. A differential glucose-fructose effect on nitrate reductase activity independent of the effect on NO₃^? uptake was not indicated. A constant level of NRA occurred in roots of NO₃^? induced plants. Removal of nutrient nitrate from these plants caused an exponential NRA decay with an approximate half-life of 12 h in intact plants and 5.5 h in excised roots. The latter value was also found in roots that were excised in the presence of nitrate, indicating that the sugar status primarily determines the apparent rate of nitrate reductase decay in excised roots.     

Nitrate reductase-deficient mutant cell lines of Nicotiana tabacum

Summary The wild-type line and 14 nitrate reductase-deficient mutant cell lines of Nicotiana tabacum were tested for the presence of nitrate reductase partial activities, and for nitrite reductase and xanthine dehydrogenase activity. Data characterizing the electron donor specificity of nitrate reductase (EC 1.6.6.1., NADH:nitrate oxidoreductase) and nitrite reductase (EC 1.7.7.1., ferredoxin:nitrite oxidoreductase) of the wild-type line are presented. Three lines (designated cnx) simultaneously lack NADH-, FADH₂-, red. benzyl viologen-nitrate reductase, and xanthine dehydrogenase activities, but retain the nitrate reductase-associated NADH-cytochrome c reductase activity. These mutants are, therefore, interpreted to be impaired in gene functions essential for the synthesis of an active molybdenum-containing cofactor. For cnx-68 and cnx-101, the sedimentation coefficient of the defective nitrate reductase molecules does not differ from that of the wild-type enzyme (7.6S). In 11 lines (designated nia) xanthine dehydrogenase activity is unaffected, and the loss of NADH-nitrate reductase is accompanied by a loss of all partial activities, including NADH-cytochrome c reductase. However, one line (nia-95) was found to possess a partially active nitrate reductase molecule, retaining its FADH₂- and red. benzyl viologen nitrate reductase activity. It is likely that nia-95 is a mutation in the structural gene for the apoprotein. Both, the nia and cnx mutant lines exhibit nitrite reductase activity, being either nitrate-inducible or constitutive. Evidence is presented that, in Nicotiana tabacum, nitrate, without being reduced to nitrite, is an inducer of the nitrate assimilation pathway.     

Influence of auxin and mycorrhizal fungi on thein vitro formation and growth ofPinus pinaster roots

Summary Experiments, performed withPinus pinaster cloned shoots submitted to an auxin treatment (NAA 10^–6 M, 18 days), demonstrated that rooting abilityin vitro persists over 5 successive induction cycles (through out a 9-month period). Rooting ability needs a permanent synthesis of auxin synergists which activate the metabolism of cell dedifferentiation and root primordium initiation. Agar culture permitted intense meristem initiation, but prevented active root elongation. In the presence of a mycorrhizal fungus,Pisolithus tinctorius orHebeloma cylindrosporum, roots resumed growth and short lateral root formation was stimulated. These two phenomena induced by fungal association improve the quality of the root systems required to facilitate successful transplantation from test-tubes to field conditions.     

Nitric oxide enhances development of lateral roots in tomato (Solanum lycopersicum L.) under elevated carbon dioxide

Elevated carbon dioxide (CO₂) has been shown to enhance the growth and development of plants, especially of roots. Amongst them, lateral roots play an important role in nutrient uptake, and thus alleviate the nutrient limitation to plant growth under elevated CO₂. This paper examined the mechanism underlying CO₂ elevation-induced lateral root formation in tomato. The endogenous nitric oxide (NO) in roots was detected by the specific probe 4-amino-5-methylamino-2′,7′-difluorofluorescein diacetate (DAF-FM DA). We suggest that CO₂ elevation-induced NO accumulation was important for lateral root formation. Elevated CO₂ significantly increased the activity of nitric oxide synthase in roots, but not nitrate reductase activity. Moreover, the pharmacological evidence showed that nitric oxide synthase rather than nitrate reductase was responsible for CO₂ elevation-induced NO accumulation. Elevated CO₂ enhanced the activity of nitric oxide synthase and promoted production of NO, which was involved in lateral root formation in tomato under elevated CO₂.     

Lateral root frequency decreases when nitrate accumulates in tobacco transformants with low nitrate reductase activity: consequences for the regulation of biomass partitioning between shoots and root1

Accumulation of nitrate in the shoot of low-nitrate reductase tobacco transformants leads to an increase of the shoot:root ratio to higher values than in nitrogen-sufficient wild-type plants, even though the transformants are severely deficient in organic nitrogen. In the present paper, wild-type plants and low- nitrate reductase transformants were grown on vertical agar plates to investigate whether this inhibition of root growth by internal nitrate (i) can be reversed by adding sugars to the roots and (ii) is due to slower growth of the main roots or to a decreased number of lateral roots. When grown with a low nitrate supply, the transformants resembled wild-type plants with respect to amino acid and protein levels, shoot-root allocation, lateral root frequency, and rates of growth. When the transformants were grown with a high nitrate supply in the absence of sucrose they grew more slowly and had lower levels of amino acids and protein than wild-type plants, but accumulated more nitrate and developed a high shoot:root ratio. Root length was not affected, but the number of lateral roots per plant decreased. The slower root growth was accompanied by an increase of the concentration of sugars in the roots. Addition of 2% sucrose to the medium partially reversed the high shoot:root ratio in the transformants, but did not increase the frequency of lateral roots. It is concluded that nitrate accumulation in the plant leads to decreased root growth via (i) changes in carbon allocation leading to decreased allocation of sugars to root growth, and (ii) a decrease in the number of lateral roots and a shift in the sensitivity with which root growth responds to the sugar supply. This revised version was published online in June 2006 with corrections to the Cover Date.