Root Signal Research Articles

Early action of pea symbiotic gene NOD3 is confirmed by adventitious root phenotype

A supernodulating and Nts (nitrate-tolerant symbiosis) symbiotic mutation of pea ( Pisum sativum L.) line RisfixC was found to retain its expression in the distant genetic background of pea lines Afghanistan L1268, Zhodino E900, and cv. Arvika. This finding allowed for reliable scoring for the trait in mapping crosses. The RisfixC mutation was localized 8.2 cM apart from SYM2 and cosegregated with molecular markers for SYM2– NOD3 region Psc923 and OA-1. Grafting experiments showed that supernodulation is root-determined, consistently with mutants in the NOD3 locus. Therefore, the mutation of RisfixC can be localized in gene NOD3. Like in other published nod3 alleles, the RisfixC mutation determines supernodulation when it is expressed in the root but not in the shoot. Supernodulated adventitious roots that are spontaneously formed in the wild-type scions on mutant rootstocks indicate that the descending systemic signal, which is inhibitory to nodule formation, is absent in this type of chimeric plants. Since the descending signal production in the wild-type shoot reflects the presence of the ascending root signal, the nod3-associated lesion must be located in the beginning of the systemic circuit regulating nodule number.

Selection of entomopathogenic nematodes for enhanced responsiveness to a volatile root signal helps to control a major root pest

The efficacy of natural enemies as biological control agents against insect pests can theoretically be enhanced by artificial selection for high responsiveness to foraging cues. The recent discovery that maize roots damaged by the western corn rootworm (WCR) emit a key attractant for insect-killing nematodes has opened the way to explore whether a selection strategy can improve the control of root pests. The compound in question, (E)-beta-caryophyllene, is only weakly attractive to Heterorhabditis bacteriophora, one of the most infectious nematodes against WCR. To overcome this drawback, we used a six-arm below-ground olfactometer to select for a strain of H. bacteriophora that is more readily attracted to (E)-beta-caryophyllene. After six generations of selection, the selected strain responded considerably better and moved twice as rapidly towards a (E)-beta-caryophyllene source than the original strain. There was a minor trade-off between this enhanced responsiveness and nematode infectiveness. Yet, in subsequent field tests, the selected strain was significantly more effective than the original strain in reducing WCR populations in plots with a maize variety that releases (E)-beta-caryophyllene, but not in plots with a maize variety that does not emit this root signal. These results illustrate the great potential of manipulating natural enemies of herbivores to improve biological pest control.

Root and pre-constant signals of the 1D Teager-Kaiser operator

We give an input/output characterization of the Teager-Kaiser operator, then we classify the root signals (those that remain invariant) and the pre-constant signals (those that produce a constant output) of the operator. With the gained knowledge, a simple detector of sinusoidal signals and of their corresponding frequencies is designed. The methods used allow for a certain amount of generalization providing a paradigm of sorts for the design of quadratic and other polynomial filters.

The interaction between H2O2 and NO, Ca2+, cGMP, and MAPKs during adventitious rooting in mung bean seedlings

Hydrogen peroxide (H2O2), an active oxygen species, is widely generated in many biological systems and mediates various physiological and biochemical processes in plants. In the present study, we present a signaling network involving H2O2, nitric oxide (NO), calcium (Ca2+), cyclic guanosine monophosphate (cGMP), and the mitogen-activated protein kinase (MAPK) cascade during adventitious rooting in mung bean seedlings. Both exogenous H2O2 and the NO donor sodium nitroprussiate were capable of promoting the formation and development of adventitious roots. H2O2 and NO signaling pathways were elicited in parallel in auxin-induced adventitious rooting. Cytosolic Ca2+ was required for adventitious rooting, and Ca2+ served as a downstream component of H2O2, as well as cGMP or MAPK, signaling cascades. cGMP and MAPK cascades function downstream of H2O2 signaling and depend on auxin responses in adventitious root signaling processes.

Redox states of glutathione and ascorbate in root tips of poplar (Populus tremulaxP. alba) depend on phloem transport from the shoot to the roots

Glutathione (GSH) and ascorbate (ASC) are important antioxidants that are involved in stress defence and cell proliferation of meristematic root cells. In principle, synthesis of ASC and GSH in the roots as well as ASC and GSH transport from the shoot to the roots by phloem mass flow is possible. However, it is not yet known whether the ASC and/or the GSH level in roots depends on the supply from the shoot. This was analysed by feeding mature leaves with [14C]ASC or [35S]GSH and subsequent detection of the radiolabel in different root fractions. Quantitative dependency of root ASC and GSH on shoot-derived ASC and GSH was investigated with poplar (Populus tremula×P. alba) trees interrupted in phloem transport. [35S]GSH is transported from mature leaves to the root tips, but is withdrawn from the phloem along the entire transport path. When phloem transport was interrupted, the GSH content in root tips halved within 3 d. [14C]ASC is also transported from mature leaves to the root tips but, in contrast to GSH, ASC is not removed from the phloem along the transport path. Accordingly, ASC accumulates in root tips. Interruption of phloem transport disturbed the level and the ASC redox state within the entire root system. Diminished total ASC levels were attributed mainly to a decline of dehydroascorbate (DHA). As the redox state of ASC is of particular significance for root growth and development, it is concluded that phloem transport of ASC may constitute a shoot to root signal to coordinate growth and development at the whole plant level.

Silver Ions Increase Auxin Efflux Independently of Effects on Ethylene Response

Silver nitrate and aminoethoxyvinylglycine (AVG) are often used to inhibit perception and biosynthesis, respectively, of the phytohormone ethylene. In the course of exploring the genetic basis of the extensive interactions between ethylene and auxin, we compared the effects of silver nitrate (AgNO(3)) and AVG on auxin responsiveness. We found that although AgNO(3) dramatically decreased root indole-3-acetic acid (IAA) responsiveness in inhibition of root elongation, promotion of DR5-beta-glucuronidase activity, and reduction of Aux/IAA protein levels, AVG had more mild effects. Moreover, we found that that silver ions, but not AVG, enhanced IAA efflux similarly in root tips of both the wild type and mutants with blocked ethylene responses, indicating that this enhancement was independent of ethylene signaling. Our results suggest that the promotion of IAA efflux by silver ions is independent of the effects of silver ions on ethylene perception. Although the molecular details of this enhancement remain unknown, our finding that silver ions can promote IAA efflux in addition to blocking ethylene signaling suggest that caution is warranted in interpreting studies using AgNO(3) to block ethylene signaling in roots.

Correction for Degenhardt et al., Restoring a maize root signal that attracts insect-killing nematodes to control a major pest

Correction for Degenhardt et al., Restoring a maize root signal that attracts insect-killing nematodes to control a major pest

The rhizosphere: biochemistry and organic substances at the soil–plant interface. 2nd edn.

This book reviews rhizosphere research from approximately the middle of last century to the present. It is a comprehensive coverage of root exudates, specific root–microbial interactions, and nutritional processes in the rhizosphere. The main strength of this second edition is that it is on the topic of the rhizosphere, and despite the many research questions associated with this topic, there are few competing books and it is still an emerging field. Because the field is growing, I suspect it will have a wide readership among root and rhizosphere researchers. Certain chapters to this edition are outstanding, and these include Chapter 1 (Uren) and Chapter 7 (Crowley and Kraemer) because they present critical analyses of rhizosphere processes in field soil. Uren's chapter begins the book with a thorough review of root exudates and their possible functions in soil. It forces the rhizosphere researcher to think about the processes in soil that affect the locations, longevities and activities of exudates, and if they can be expected to function in the field as they do in the laboratory. The phytosiderophore analysis in Chapter 7 is excellent and a good use of a very intensely studied rhizosphere process to illustrate the challenges in identifying such processes that are important in the field. Although not new to this edition, the topics of these chapters are still highly relevant and challenging to the rhizosphere researcher. This second edition includes interesting chapters not present in the first edition. For example, Chapter 14 (Mercier et al.) about gene flow in the rhizosphere is particularly novel and is unique to this book, to my knowledge, and I found it had the most new information. Chapter 5 (Schmidt and Linke) reviews the area of root morphology and architecture, and how they are regulated by nutritional and hormonal signals. In Chapter 6, Varanini and Pinton present an informative coverage of root membrane transporters related to nutrient uptake. This area is expanding rapidly due to gene sequence and protein structure information from model and crop species, and hopefully the third edition will include membrane transporters and receptors related to rhizosphere microbial and root signals. Chapter 11 (Perry et al.) reviews such chemical signals between roots (including allelopathy) and between roots and microorganisms, and this is clearly an exciting area of rhizosphere research. My main criticism about the content of the book, and something the editors may want to think about for a new edition, was that, aside from Chapter 14, the topics of the chapters were essentially ‘traditional’ rhizosphere topics. There are new areas that would each warrant their own chapter, such as biofilms in the rhizosphere, rhizosphere soil structure, water uptake and the spatial and temporal processes that can now be quantified non-invasively with imaging technologies, and the microbial ecology of the rhizosphere determined using DNA-based methods that have dominated soil ecology since the early 1990s. These methods repeatedly suggest a microbial complexity in the rhizosphere that must be addressed by groups working on single organisms in simple, controlled conditions. My more minor criticism is that a couple of the chapters do not present sufficiently challenging and critical views of the topics. The images are in black and white, very poor quality (eg. Fig. 7.4) and are poorly labelled. This is a great shame given, for example, the exciting fluorescent images that are available of root–microbial associations throughout the literature. The publishers should consider a website with colour images to accompany this and future editions if colour printed plates are too expensive. There are typographical errors in the text; these are minor but distracting to the high quality of the content of the book. This book is highly suitable for researchers of roots and rhizospheres, including graduate students. It is a specialized text, and may not initially attract scientists who are not specialized in these areas. However; whole-plant scientists and ecologists interested to see links among rhizosphere processes and plant productivity in managed and natural systems should consider reading this edition. Hopefully this broader, important audience will be more directly reached in the third edition. This review sustains the plant scientist's amazement at all there is yet to discover about the rhizosphere, particularly in real field soils.

Restoring a maize root signal that attracts insect-killing nematodes to control a major pest

When attacked by herbivorous insects, plants emit volatile compounds that attract natural enemies of the insects. It has been proposed that these volatile signals can be manipulated to improve crop protection. Here, we demonstrate the full potential of this strategy by restoring the emission of a specific belowground signal emitted by insect-damaged maize roots. The western corn rootworm induces the roots of many maize varieties to emit (E)-beta-caryophyllene, which attracts entomopathogenic nematodes that infect and kill the voracious root pest. However, most North American maize varieties have lost the ability to emit (E)-beta-caryophyllene and may therefore receive little protection from the nematodes. To restore the signal, a nonemitting maize line was transformed with a (E)-beta-caryophyllene synthase gene from oregano, resulting in constitutive emissions of this sesquiterpene. In rootworm-infested field plots in which nematodes were released, the (E)-beta-caryophyllene-emitting plants suffered significantly less root damage and had 60% fewer adult beetles emerge than untransformed, nonemitting lines. This demonstration that plant volatile emissions can be manipulated to enhance the effectiveness of biological control agents opens the way for novel and ecologically sound strategies to fight a variety of insect pests.

The short-rooted vitamin B6-deficient mutant pdx1 has impaired local auxin biosynthesis

The phytohormone auxin regulates many aspects of plant growth and development. Auxin often acts distantly from the site of its biosynthesis and this long-distance-transported auxin is well known to play a critical role in eliciting physiological responses including regulating root development. Auxin can be produced in roots, yet the function of locally synthesized auxin in root growth is unclear. The major auxin in plants, indole 3-acetic acid (IAA), is mainly synthesized through tryptophan (Trp)-dependent pathways that require pyridoxal phosphate (an active form of vitamin B(6))-dependent enzymes. We previously reported that the Arabidopsis vitamin B(6) biosynthesis mutant pdx1 has stunted root growth although the underlying cause is unknown. Here we showed that the pdx1 root is deficient in auxin biosynthesis. By reciprocal grafting of pdx1 and the wild type, we demonstrated that the stunted root growth in pdx1 is caused by a locally generated signal(s) in roots. To test whether auxin might be one such signal, the auxin responsive DR5::GUS reporter was introduced into the mutant. The DR5::GUS activity in pdx1 root tips was greatly reduced compared with that in the wild type although the auxin response was unaltered. pdx1 also suppresses the root hair growth defects in the auxin overproduction mutant yucca. These data indicate that pdx1 is impaired in Trp-dependent auxin biosynthesis, which may contribute to the short-root phenotype of pdx1. We suggest that locally synthesized auxin may play a critical role in postembryonic root growth.

The Short-Rooted Phenotype of thebrevis radixMutant Partly Reflects Root Abscisic Acid Hypersensitivity

To gain further insight into abscisic acid (ABA) signaling and its role in growth regulation, we have screened for Arabidopsis (Arabidopsis thaliana) mutants hypersensitive to ABA-mediated root growth inhibition. As a result, we have identified a loss-of-function allele of BREVIS RADIX (BRX) in the Columbia background, named brx-2, which shows enhanced response to ABA-mediated inhibition of root growth. BRX encodes a key regulator of cell proliferation and elongation in the root, which has been implicated in the brassinosteroid (BR) pathway as well as in the regulation of auxin-responsive gene expression. Mutants affected in BR signaling that are not impaired in root growth, such as bes1-D, bzr1-D, and bsu1-D, also showed enhanced sensitivity to ABA-mediated inhibition of root growth. Triple loss-of-function mutants affected in PP2Cs, which act as negative regulators of ABA signaling, showed impaired root growth in the absence of exogenous ABA, indicating that disturbed regulation of ABA sensitivity impairs root growth. In agreement with this result, diminishing ABA sensitivity of brx-2 by crossing it with a 35S:HAB1 ABA-insensitive line allowed significantly higher recovery of root growth after brassinolide treatment. Finally, transcriptomic analysis revealed that ABA treatment negatively affects auxin signaling in wild-type and brx-2 roots and that ABA response is globally altered in brx-2. Taken together, our results reveal an interaction between BRs, auxin, and ABA in the control of root growth and indicate that altered sensitivity to ABA is partly responsible for the brx short-root phenotype.

Stomatal Responses in Rainfed Lowland Rice to Partial Soil Drying; Comparison of Two Lines

Previously, we demonstrated that root tips in drying soil communicate with shoots for stomatal closure in rainfed lowland rice, despite further water being available at depth. This study examines variation between two lines in root signals.Rice lines CT9993 and IR62266 were grown in the field, and in the greenhouse with the split-root root-sever wax-layer system, to investigate their responses to mild and severe water deficit by monitoring stomatal conductance (gs), leaf water potential and leaf ABA concentration. In the greenhouse, root systems were divided, withholdingwater from one portion, and in some cases, severing the droughted portion of roots to remove the signal. Wax layers differing in strength were placed at hardpan depth. Roots of CT9993 were better able to penetrate the wax layers. IR62266 exhibited stronger responses than CT9993, with IR62266’s stomatal conductance dropping sharply under water deficit, and recovering at slower rates but less completely, when roots subjected to drying soil were severed. The greater stomatal response in IR62266 was associated with a higher leaf ABA concentration during early water deficit, which in turn was associated with its greater number of roots in drying soil. In the field, a second reduction in gs wasobserved under severe water deficit, with stronger signals in IR62266 associated with more conservative water use as soil drying intensified. To better exploit subsoil water in mild or transient water deficit, selection for reduced root signals might be warranted.

ABA mediation of shoot cytokinin oxidase activity: assessing its impacts on cytokinin status and biomass allocation of nutrient-deprived durum wheat.

Although nutrient deprivation alters the concentrations of several plant hormones, the role of each in decreasing shoot-to-root ratio is not clear. A 10-fold dilution of the nutrient concentration supplied to hydroponically-grown 7-day-old durum wheat (Triticum turgidum L. ssp. durum Desf.) plants decreased shoot growth, shoot-to-root ratio and shoot and root cytokinin concentrations, increased shoot ABA concentration and shoot cytokinin oxidase activity, but had no effect on xylem sap ABA and cytokinin concentrations. Nutrient deprivation also increased xylem concentrations of conjugated ABA. The role of ABA in these responses was addressed by adding 11.4 µm ABA to the nutrient solution of well fertilised plants, or 1.2 mm fluridone (an inhibitor of ABA biosynthesis) to the nutrient solution of nutrient-deprived plants. The former induced similar changes in shoot-to-root ratio (by inhibiting shoot growth), shoot ABA concentration, shoot and root cytokinin concentrations and shoot cytokinin oxidase activity as nutrient deprivation. Conversely, fluridone addition to nutrient-deprived plants restored shoot-to-root ratio (by inhibiting root growth), shoot ABA concentration, shoot and root cytokinin concentrations to levels similar to well fertilised plants. Although root growth maintenance during nutrient deprivation depends on a threshold ABA concentration, shoot growth inhibition is independent of shoot ABA status. Although fluridone decreased shoot cytokinin oxidase activity of nutrient-deprived plants, it was still 1.7-fold greater than well fertilised plants, implying that nutrient deprivation could also activate shoot cytokinin oxidase independently of ABA. These data question the root signal basis of cytokinin action, but demonstrate that changes in ABA status can regulate shoot cytokinin concentrations via altering their metabolism.

Essential role of MYB transcription factor: PvPHR1 and microRNA: PvmiR399 in phosphorus‐deficiency signalling in common bean roots

Phosphorus (P), an essential element for plants, is one of the most limiting nutrients for plant growth. A few transcription factor (TF) genes involved in P-starvation signalling have been characterized for Arabidopsis thaliana and rice. Crop production of common bean (Phaseolus vulgaris L.), the most important legume for human consumption, is often limited by low P in the soil. Despite its agronomic importance, nothing is known about transcriptional regulation in P-deficient bean plants. We functionally characterized the P-deficiency-induced MYB TF TC3604 (Dana Farber Cancer Institute, Common Bean Gene Index v.2.0), ortholog to AtPHR1 (PvPHR1). For its study, we applied RNAi technology in bean composite plants. PvPHR1 is a positive regulator of genes implicated in P transport, remobilization and homeostasis. Although there are no reports on the regulatory roles of microRNAs (miRNA) in bean, we demonstrated that PvmiR399 is an essential component of the PvPHR1 signalling pathway. The analysis of DICER-like1 (PvDCL1) silenced bean composite plants suppressed for accumulation of PvmiR399 and other miRNAs suggested that miR399 is a negative regulator of the ubiquitin E2 conjugase: PvPHO2 expression. Our results set the basis for understanding the signalling for P-starvation responses in common bean and may contribute to crop improvement.

Root signals and stomatal closure in relation to photosynthesis, chlorophyll a fluorescence and adventitious rooting of flooded tomato plants

An investigation was carried out to determine whether stomatal closure in flooded tomato plants (Solanum lycopersicum) results from decreased leaf water potentials (psi(L)), decreased photosynthetic capacity and attendant increases in internal CO(2) (C(i)) or from losses of root function such as cytokinin and gibberellin export. Pot-grown plants were flooded when 1 month old. Leaf conductance was measured by diffusion porometry, the efficiency of photosystem II (PSII) was estimated by fluorimetry, and infrared gas analysis was used to determine C(i) and related parameters. Flooding starting in the morning closed the stomata and increased psi(L) after a short-lived depression of psi(L). The pattern of closure remained unchanged when psi(;L) depression was avoided by starting flooding at the end rather than at the start of the photoperiod. Raising external CO(2) concentrations by 100 micromol mol(-1) also closed stomata rapidly. Five chlorophyll fluorescence parameters [F(q)'/F(m)', F(q)'/F(v)', F(v)'/F(m)', non-photochemical quenching (NPQ) and F(v)/F(m)] were affected by flooding within 12-36 h and changes were linked to decreased C(i). Closing stomata by applying abscisic acid or increasing external CO(2) substantially reproduced the effects of flooding on chlorophyll fluorescence. The presence of well-aerated adventitious roots partially inhibited stomatal closure of flooded plants. Allowing adventitious roots to form on plants flooded for >3 d promoted some stomatal re-opening. This effect of adventitious roots was not reproduced by foliar applications of benzyl adenine and gibberellic acid. Stomata of flooded plants did not close in response to short-lived decreases in psi(L) or to increased C(i) resulting from impaired PSII photochemistry. Instead, stomatal closure depressed C(i) and this in turn largely explained subsequent changes in chlorophyll fluorescence parameters. Stomatal opening was promoted by the presence of well-aerated adventitious roots, implying that loss of function of root signalling contributes to closing of stomata during flooding. The possibility that this involves inhibition of cytokinin or gibberellin export was not well supported.

Is cortical root colonization required for carbon transfer to arbuscular mycorrhizal fungi? Evidence from colonization phenotypes and spore production in the reduced mycorrhizal colonization (rmc) mutant of tomato

For the first time, the phenotypes formed in the reduced mycorrhizal colonization (rmc) Solanum lycopersicum L. (tomato) mutant with different arbuscular mycorrhizal (AM) fungi were used to explore the potential of different fungal structures to support development of external fungal mycelium and spores. The life cycle of AM fungi with rmc was followed for up to 24 weeks. Results showed that production of external mycelium was slight and transitory for those fungi that did not penetrate the roots of rmc (Pen–) ( Glomus intraradices DAOM181602 and Glomus etunicatum ). For fungi that penetrated the root epidermis and hypodermis (Coi–, Glomus coronatum and Scutellospora calospora ) the mycelium produced varied in size, but was always smaller than with the wild-type 76R. Spores were formed by these fungi with 76R but not with rmc. The only fungus forming a Myc+ phenotype with rmc, G. intraradices WFVAM23, produced as much mycelium with rmc as with 76R. We observed lipid accumulation in hyphae and vesicles in both plant genotypes with this fungus. Mature spores were formed with 76R. However, with rmc, spores remained small and (presumably) immature for up to 24 weeks. We conclude that significant carbon transfer from plant to fungus can occur in Coi– interactions with rmc in which no cortical colonization occurs. We speculate that both carbon transfer and root signals are required for mature spores to be produced.

Comparative effects of deficit irrigation (DI) and partial rootzone drying (PRD) on soil water distribution, water use, growth and yield in greenhouse grown hot pepper

This study was conducted to compare two water-saving practices, deficit irrigation (DI) and partial rootzone drying (PRD), and examine how they affected soil water distribution, water use, growth and yield of greenhouse grown hot pepper compared to commercial irrigation (CI). Control (CI) in which irrigation water was applied to both sides of the system when soil water content was lower by 80% of field capacity; deficit irrigation (DI50, DI75) in which 50% and 75% irrigation water of CI supplied to both sides of the root system; 1PRD with half of the root system exposed to soil drying and other half kept well-watered with 50% irrigation water of CI, and 2PRD with 50% irrigation water of CI supplied, half to fixed side of the root system. The results showed mean soil volumetric water content of DI75, DI50, 1PRD and 2PRD were lower by 21.06%, 28.32%, 24.48% and 34.76%, respectively than that of CI after starting the experiment. Water consumption showed some significant effect of irrigation treatments during the growing period of drought stress application, and therefore decreased in DI75, DI50, 1PRD and 2PRD to a level around 75% and 50% of CI. All the DI and PRD treatments resulted in a reduction of total dry mass of 7.29–44.10%, shoot biomass of 24.97–47.72% compared to CI, but an increase in the root–shoot ratio of 12.50–35.42% compared to the control and with significant differences between 2PRD, 1PRD, DI50 and CI. The yield of 1PRD was significantly reduced by 23.98% compared to CI (19,566 kg hm −2) over a period of 109 days after transplanting. However, the 1PRD treatment had 17.21% and 24.54% additional yield over the DI50 and 2PRD treatments and had 52.05% higher irrigation water use efficiency (IWUE) than CI treatment. At harvest, although there was a significant difference recorded as single fruit weight and single fruit volume were reduced under the DI and PRD treatments, total soluble solids concentration of fruit harvested under the water-deficit treatments were higher compared to CI. Stomatal conductance measured in fresh leaf was the lowest under 1PRD treatment relative to CI and other treatments. The low stomatal conductance of fresh-leaf issue observed in the work supported the root signaling mechanism reported earlier in plants having undergone partial root drying cycles.

Expression and Assembly of Cholera Toxin B Subunit (CTB) in Transgenic Carrot (Daucus carota L.)

We expressed the cholera toxin B subunit (CTB) fused to an endoplasmic reticulum retention signal (SEKDEL) in carrot roots using an Agrobacterium-mediated transformation method. Fourteen independent transgenic lines were regenerated via somatic embryogenesis after 6 months of culture. The sCTB gene was detected in the genomic DNA of transgenic carrot by PCR amplification. Expressions and assembly of sCTB protein into oligomeric structures of pentameric size were observed in transgenic plant extracts by Western blot analysis. The sCTB produced by transgenic carrot roots demonstrated strong affinity for GM1-ganglioside, suggesting that the sCTB conserved the antigenic sites for binding and proper folding of the pentameric sCTB structure. The expression level of sCTB comprised approximately 0.48% of total soluble protein (TSP) in root of transgenic carrot.

Arginase-negative mutants of Arabidopsis exhibit increased nitric oxide signaling in root development.

Mutation of either arginase structural gene (ARGAH1 or ARGAH2 encoding arginine [Arg] amidohydrolase-1 and -2, respectively) resulted in increased formation of lateral and adventitious roots in Arabidopsis (Arabidopsis thaliana) seedlings and increased nitric oxide (NO) accumulation and efflux, detected by the fluorogenic traps 3-amino,4-aminomethyl-2',7'-difluorofluorescein diacetate and diamino-rhodamine-4M, respectively. Upon seedling exposure to the synthetic auxin naphthaleneacetic acid, NO accumulation was differentially enhanced in argah1-1 and argah2-1 compared with the wild type. In all genotypes, much 3-amino,4-aminomethyl-2',7'-difluorofluorescein diacetate fluorescence originated from mitochondria. The arginases are both localized to the mitochondrial matrix and closely related. However, their expression levels and patterns differ: ARGAH1 encoded the minor activity, and ARGAH1-driven beta-glucuronidase (GUS) was expressed throughout the seedling; the ARGAH2::GUS expression pattern was more localized. Naphthaleneacetic acid increased seedling lateral root numbers (total lateral roots per primary root) in the mutants to twice the number in the wild type, consistent with increased internal NO leading to enhanced auxin signaling in roots. In agreement, argah1-1 and argah2-1 showed increased expression of the auxin-responsive reporter DR5::GUS in root tips, emerging lateral roots, and hypocotyls. We propose that Arg, or an Arg derivative, is a potential NO source and that reduced arginase activity in the mutants results in greater conversion of Arg to NO, thereby potentiating auxin action in roots. This model is supported by supplemental Arg induction of adventitious roots and increased NO accumulation in argah1-1 and argah2-1 versus the wild type.

Roots, Nitrogen Transformations, and Ecosystem Services

This review considers some of the mechanistic processes that involve roots in the soil nitrogen (N) cycle, and their implications for the ecological functions that retain N within ecosystems: 1) root signaling pathways for N transport systems, and feedback inhibition, especially for NO(3)(-) uptake; 2) dependence on the mycorrhizal and Rhizobium/legume symbioses and their tradeoffs for N acquisition; 3) soil factors that influence the supply of NH(4)(+) and NO(3)(-) to roots and soil microbes; and 4) rhizosphere processes that increase N cycling and retention, such as priming effects and interactions with the soil food web. By integrating information on these plant-microbe-soil N processes across scales and disciplinary boundaries, we propose ideas for better manipulating ecological functions and processes by which the environment provides for human needs, i.e., ecosystem services. Emphasis is placed on agricultural systems, effects of N deposition in natural ecosystems, and ecosystem responses to elevated CO(2) concentrations. This shows the need for multiscale approaches to increase human dependence on a biologically based N supply.